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Use of Prefabrication, Construction and Demolition Wastes as an Aggregate in Vibropressed Precast Concrete Blocks Production

The aim of current study was to determine the recycled concrete aggregate (RCA) applicability in the production of concrete mixture for vibropressed concrete blocks. The experiments were focused especially on the crushed waste material from the same concrete elements producing plant.  For this type of precast elements only some finer fractions can be implemented and the “earth-moist” consistency of fresh mixture is required. The series of samples was prepared in which the mixture of natural aggregates was partially or totally substituted by recycled concrete aggregate. The 0/4 RCA fraction, which is usually rejected in ready mix concrete technology, plays a role of 0/2 sand.  The substitution of sand fraction was from 20% to 100% respectively. The substitution of the coarser aggregate fractions by 4/16 RCA was also done. The standard properties of vibropressed elements, such as the degree of densification, the density of material, the compressive and splitting tensile strength and the water absorption capacity according to the relevant standards were determined. The parameters of materials with the natural aggregate substitution by RCA are affected by the ratio of recycled concrete aggregate. In most cases the results do not decline specially from those for reference samples, when only the natural sand (0/2) fraction is substituted by the 0/4 recycled aggregate. As one could expect, as lower the substitution, as better the test results. The partial substitution of natural aggregate by coarser fractions requires experimental verification; over 20% substitution of natural aggregate by 4/8, 8/16 or 0/16 RCA should be excluded.

The properties of preplaced aggregate concrete technology contain the industrial waste-material and the various shapes and sizes of coarse aggregate

Abstract The success of preplaced aggregate concrete technology depends on two main factors which are potential grout and coarse aggregate. This research was conducted experimentally to determine the effect of using two different fly ash sources as an alternative for the partial replacement of cement and several size and shapes of coarse aggregate on the compressive and tensile strength of PAC specimens. This involved the use of seven concrete mixes with a low water-cement ratio of 0.4 and cement to sand ratio of 1:0.75 to produce standard cylinder specimens of concrete containing rounded and crush aggregate. Moreover, fly ash was added at a dosage of 5% and 10% of cement weight while three shapes and sizes of a rounded and crushed aggregate at 20 mm, 30 mm, and a mixture of the two were also applied. The results showed the compressive strength of specimens with different sizes or a mix of rounded aggregate in PAC exhibited a similar performance with 30 mm of crushed coarse aggregate. Furthermore, the specimen with a higher content of calcium fly ash demonstrated a more rapid strength at an early age of seven days than those with lower content. Therefore, the partial replacement of cement with industrial waste material in the form of fly ash in preplaced aggregate concrete has the ability to save up to 10% of cement and also produce certain environmental benefits.

3D printing-A Review of Materials, Applications, and Challenges

Abstract: Now a days 3-Dimensional Printing (3DP) technology is used world widely and it can actually print each and every thing with the desired computer program. In Construction engineering the challenges are like availability of skilled man power, time constraint, cost effectiveness and complicated shapes etc. But with the help of an automated machine, the 3D printing technology, has huge potential to have faster and more accurate construction of complex and more laborious works. This technology can build three-dimensional (3D) objects by connecting layers of materials and can be applied to convert waste and by-products into new materials. The 3DP in concrete construction is increasing thanks to its freedom in geometry, rapidness, formwork-less printing, low waste generation, eco-friendliness, cost-saving nature and safety. This paper attempts to review the digital printing technology introduced in the construction industry and the also highlights the impact on concrete technology. It also discusses about the materials used in 3DP, mix design, various applications and challenges in the construction industry. Keywords: 3D printing, Concrete, 3DCP, Mix design.

Novel Fuzzy-Based Optimization Approaches for the Prediction of Ultimate Axial Load of Circular Concrete-Filled Steel Tubes

An accurate estimation of the axial compression capacity of the concrete-filled steel tubular (CFST) column is crucial for ensuring the safety of structures containing them and preventing related failures. In this article, two novel hybrid fuzzy systems (FS) were used to create a new framework for estimating the axial compression capacity of circular CCFST columns. In the hybrid models, differential evolution (DE) and firefly algorithm (FFA) techniques are employed in order to obtain the optimal membership functions of the base FS model. To train the models with the new hybrid techniques, i.e., FS-DE and FS-FFA, a substantial library of 410 experimental tests was compiled from openly available literature sources. The new model’s robustness and accuracy was assessed using a variety of statistical criteria both for model development and for model validation. The novel FS-FFA and FS-DE models were able to improve the prediction capacity of the base model by 9.68% and 6.58%, respectively. Furthermore, the proposed models exhibited considerably improved performance compared to existing design code methodologies. These models can be utilized for solving similar problems in structural engineering and concrete technology with an enhanced level of accuracy.

Design of Cold-Mixed High-Toughness Ultra-Thin Asphalt Layer towards Sustainable Pavement Construction

Ultra-thin asphalt overlay has become the mainstream measure of road preventive maintenance due to its good economic benefits and road performance. However, hot mix asphalt concrete technology is widely used at present, which is not the most ideal way to promote energy saving and emission reduction in the field of road maintenance. At the same time, the ultra-thin friction course based on cold mix technology, such as slurry seal layer, micro-surface, and other technologies, are still far behind the hot mix friction course in terms of crack resistance. In this research, by establishing an integrated design of materials and structures, a cold paving technology called “high-toughness cold-mixed ultra-thin pavement (HCUP)” is proposed. The high-viscosity emulsified bitumen prepared by using high-viscosity and high-elasticity modified bitumen is used as the binder and sticky layer of HCUP. The thickness of HCUP is 0.8–2.0 cm, the typical thickness is 1.2 cm, and the nominal maximum size of the coarse aggregate is 8 mm. Indoor tests show that HCUP-8 has water stability, anti-skid performance, high temperature performance, peeling resistance, and crack resistance that are not weaker than traditional hot-mixed ultra-thin wear layers such as AC-10, Novachip, and GT-8. At the same time, the test road paving further proved that HCUP-8 has excellent road performance with a view to providing new ideas for low-carbon and environmentally friendly road materials.

Unspoken Modernity: Bamboo-Reinforced Concrete, China 1901-40

Abstract Engineering science in the China of 1901-40 had unique characteristics that disrupt the idea of a universal approach to its history.1 The following case study describes the ideas and trials of introducing bamboo into the seemingly globalised technology of reinforced concrete—an innovation developed across the borders of mechanical, naval, civil, and aeronautical engineering. The article showcases a way of knowing and working by twentieth century engineers that has not been fully acknowledged, and is not only a phenomenon of China. While bamboo was a complicated and somewhat marginal object for engineering, it did make the European concrete technology more viable in the construction sites of China, and stimulate engineers’ experimental and resourceful spirit in mobilising both craft and scientific knowledge. It also opened up a challenge to engineering science of the time.

Evaluation of Rapid Repair of Concrete Pavements Using Precast Concrete Technology: A Sustainable and Cost-Effective Solution

Abstract Concrete and asphalt are the two competitive materials for a highway. In Sweden, the predominant material for the highway system is asphalt. But under certain conditions, concrete pavements are competitive alternatives. For example, concrete pavements are suitable for high-traffic volume roads, roads in tunnels, concentrated loads (e.g., bus stops and industrial pavement). Besides the load-carrying capacity, the concrete pavement has many advantages such as durability (wear resistance), resistance against frost heave, environment (pollution, recycling, and low rolling resistance leading to fuel savings), fire resistance, noise limitations, brightness, evenness and aesthetics. Concrete pavements are long-lasting but need final repair. Single slabs may crack in the jointed concrete pavement due to various structural and non-structural factors. Repair and maintenance operations are, therefore, necessary to increase the service life of the structures. To avoid extended lane closures, prevent traffic congestions, and expedite the pavement construction process, precast concrete technology is a recent innovative construction method that can meet the requirement of rapid construction and rehabilitation of the pavement. This paper evaluates rapid repair techniques of concrete pavement using precast concrete technology by analysing three case studies on jointed precast concrete pavements. The study showed that the required amount of time to re-open the pavement to traffic is dramatically reduced with jointed precast concrete panels.

Water Absorption of Incorporating Sustainable Quarry Dust in Self-Compacting Concrete

Abstract In construction industry nowadays, self-compacting concrete (SCC) is a concrete technology innovation which gives more benefits over conventional concrete. SCC was invented to improve concrete durability without using any vibrator while placing it into formwork. In order to conserve natural sand, quarry dust (QD) as a waste and sustainable material has been incorporated to replace fine aggregate in SCC. In this study, conventional concrete and quarry dust in self-compacting concrete (QDSCC) mixes consist of 0%, 10%, 20%, 30%, 40% and 50% QD were prepared. The workability test was conducted to determine the performance of fresh concrete and ensuring all the QDSCC properties follow the acceptance criteria for SCC. Meanwhile, the hardened concrete specimens were water cured for 7, 28 and 60 days to conduct water absorption test. This research aim is to determine water absorption of incorporating sustainable QDSCC. Thus, it resulted that 50% of QDSCC has achieved the lowest water absorption of QDSCC as compared to other dosages. Finally, sustainability in concrete technology can be promoted by incorporating QDSCC.

Application of electro-hydraulic shock in concrete technology

Abstract The aspects, related to the influence of the electrohydraulic shock method use in a water-cement slurry passing in a closed chamber (activation reactor) with a pre-applied pressure to the system under various processing modes are highlighted in the article. In order to test the effect of this method on water-cement slurry, an installation was developed, consisting of: a high-voltage source, a high-voltage diode, capacitor banks, a closing element and an activation reactor. The necessary experiments were carried out on the completed installation. The procedure for conducting experiments is described in the work, shows a schematic diagram of the installation for performing activation, a diagram of the reactor, and the processing modes. Several activation modes were considered, depending on: the number of pulses (1-4), pulse energy (0.5-8 kJ), water-cement ratio (0.2-0.35), time intervals for starting treatment from the moment the cement was mixed with water (0 -120 minutes), volume and shape of the container (activation reactor), holding temperature (20-60°C), etc. According to the results of the data obtained, it was experimentally established that the use of electric pulse treatment of water-cement suspension has a positive effect on strength (cup compressive strength) indicators, obtained as a result of processing cement stone samples at different times of hardening (1-3 days). The compressive strength of the treated specimens’ increases in comparison with the untreated specimens, increase in strength reaches up to 45%, depending on the activation mode. The resulting effect was achieved due to many factors (high pressure, magnetic, temperature, energy, ultrasonic and other influences), which were applied in the most optimal period of time (stage) of the cement grain hydration process.

Built Infrastructure Renewal and Climate Change Mitigation Can Both Find Solutions in CO2

From technology to policy, the US is thinking about construction differently. The federal government is motivated to address the aging infrastructure across the country, and policy proposals are surfacing that seek green methods of performing this construction. This paper reviews the current status of concrete technology and policy to provide insight into the current state of the art. The scale of CO2 emissions from concrete production and use is elucidated. Current embodied emissions reduction methods show that action can be taken today in small and large projects alike. Additionally, developing concrete technologies offers pathways to reuse and rely on concrete for longer service lifetimes and reduce their lifetime embodied emissions. These concrete technologies must be implemented, and public procurement proves a unique tool to develop a nationwide demand signal for low embodied carbon building materials. Local governments closely interact with concrete producers, state governments oversee large infrastructure projects, and the federal government invests massively in construction. All three levels of government must coordinate for the effective rollout of low embodied carbon construction practices. Disparate policy approaches show successes and pitfalls to developing an effective construction policy that is aligned with climate. Importantly, approaches to addressing the twin challenge of climate change and crumbling infrastructure must consider the whole lifetime of the concrete. Throughout this paper, we examine the sector to highlight current practices and provide a vision for effective implementation.

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Review article, digital transformation of concrete technology—a review.

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  • Building Materials, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, Luleå, Sweden

Digital transformation of concrete technology is one of the current “hot topics” tackled by both academia and industry. The final goal is to fully integrate the already existing advanced concrete technologies with novel sensors, virtual reality, or Internet of things to create self-learning and highly automated platforms controlling design, production, and long-term usage and maintenance of concrete and concrete structures. The digital transformation should ultimately enhance sustainability, elongate service life, and increase technological and cost efficiencies. This review article focuses on up-to-date developments. It explores current pathways and directions seen in research and industrial practices. It indicates benefits, challenges, and possible opportunities related to the digital transformation of concrete technology.

Introduction

Digitization refers to transfer of data stored in traditional documents to binary forms, while digital transformation is defined as a process of changing existing methods and models by utilizing latest IT technologies to produce real-time information for fast decision making ( Parusheva, 2019 ; Zeltser et al., 2019 ; Daniotti et al., 2020 ; Papadonikolaki et al., 2020 ). For cement and concrete industries, it facilitates the process of data acquisition, their analysis, and utilization ( Walther, 2018 ). Production of concrete starts with material characterization, mix design, and actual mixing followed by its transportation to a building site ( Tomek, 2017 ). A significant amount of data created can be digitalized and used to control that process ( Rasmussen and Beliatis, 2019 ). The digital transformation is expected to produce a more efficient process, improving the working environment and sustainability of concrete products ( Phang et al., 2020 ). However, a number of challenges still need to be addressed, for example, methods for reliable prediction of early-age properties, modeling of hardening processes, and development of strength or durability ( Wangler et al., 2019 ).

Concrete structures can be cast directly on a building site or prefabricated in advanced in a factory. The cast-on-site technique is preferable for monolithic, large-size structures including foundations, beams, columns, slabs, retaining walls, tunnels, and bridges ( Liu et al., 2020 ). Concrete is transported from a ready-mix plant to the building site and then placed using pumps or dumpers. In the case of precast technology, concrete elements are cast in production halls and after achieving sufficient strength, transported to the building site. The cast-in-place technology offers more flexibility and adaptability ( Simonsson and Emborg, 2009 ). Weakness includes sensitivity to weather, that is, extreme temperatures, wind, and precipitation. The current industrialization degree of concrete technology is relatively high, but it still requires several improvements in the quality of work, optimization of the process, and enhanced sustainability. It is foreseen that there is a possibility to expedite the process using the latest digitalization techniques and technological advancements ( Wangler et al., 2016 ). Self-compacting concrete (SCC) is increasingly used, especially for the cast-in-place technique, which, due to the exclusion of vibration, offers a faster construction process and better working conditions ( Ouchi, 2000 ). The main advantages include high casting rate and passability in congested reinforcement ( De Schutter et al., 2008 ). The main challenge while using SCC is a need to use a new casting technology ( Ferrara et al., 2007 ).

The digitalization process starts by merging material properties and construction techniques into an integrated digital environment. It includes digitalizing of fresh concrete properties, hardening processes, strength development, and durability using data collected from either manual measurements or installed sensors. The integration of measured parameters and digital technology enables to enhance the quality of concrete. However, it requires a strengthened collaboration between research and industry ( Courard et al., 2014 ). Data collected from sensors can be integrated into a monitoring system, building information models, and controlling software. This process is expected to introduce a safer and error-free process and improve the productivity. The site supervisor has real-time access to data, which should facilitate the decision-making process related to, for example, the optimum casting speed, safe demolding time, or the required curing routine.

Research has been on going in the field of digital concrete, which refers to the digital fabrication of concrete, for example, 3D printing and robotics in digital fabrication ( Wangler et al., 2016 ; Wangler et al., 2019 ; Van Damme, 2020 ). Those studies have explored the methods of fabrication and construction. The basic properties, mix design parameters, and their associated information need to be addressed. Commonly, these data are obtained in the laboratory, and the question remains open about the possibility of transforming the information acquisition into a digital process. This article reviews previous research dealing with digital transformation in concrete technology, and it focuses on latest developments with a special emphasis on disadvantages and limitations. It also indicates areas that need further improvements. This article is part of a project where attempts are made to develop a system that can help integrate all the available technologies into one smart decision-making system that enables engineers to foresee and expect the outcome of the mix design based on the inputs of material properties either physically or chemically related.

Material Characterizations and Mix Design

Advanced technologies such as virtual reality, 3D printing, Internet of things, smart sensors, and autonomous robots and vehicles have already been used in various industries. However, the concrete industry is clearly behind due to the lack of acceptance, related cost, current regulations, and new required expertise. Concrete itself has gone a tremendous development path over the past few decades. Cement has been partially or fully replaced with several types of by-products to enhance some properties and to increase its sustainability. At the same time, casting technology has remained rather unchanged ( Ferrara et al., 2007 ).

Concrete consists of binder, coarse and fine aggregate, water, admixtures, and various types of dry and wet additives. These materials are characterized by chemical composition, surface area, shape, texture, and amount of intermixed fine and coarse aggregates. These properties affect the mix design and behavior of concrete during mixing and casting and later determine hardened state properties and, often, also durability ( Polat, 2013 ). The following sections will review currently used methods which are/or could be used to digitalize the properties of concrete ingredients.

Aggregates used in concrete include gravel, crushed stone, sand, slag, recycled concrete, and geosynthetic aggregates. They occupy up of 70–80 vol.% of concrete mix and affect most of its physical and mechanical properties. Aggregates should be clean, hard, and free of chemical and biological contaminants ( Babu, 2014 ). Their quality and properties are quantified by several indicators, including shape, texture, air content, particle size distribution, water content, specific gravity, or density. Some of these indicators have already been successfully digitalized. For example, volume, angularity, and gradation have been determined using analysis of images obtained from video cameras. The obtained results have been in good agreement with manual measurements ( Rao and Tutumluer, 2000 ). 3D mathematical analysis of particle shape has been successfully combined with X-ray tomography and spherical harmonics to determine particle shapes ( Garboczi, 2002 ). Others used the same technique but supplemented it with a virtual reality modeling language. This approach enabled to obtain 3D images of aggregate particles ( Erdogan et al., 2006 ). The surface texture has been determined using imaging techniques coupled with wavelet analysis of grey images. Unfortunately, results were strongly affected by the angularity and form of aggregates ( Al-Rousan et al., 2007 ). The shape index and morphological features of coarse aggregates have been assessed by a digital processing approach, which established a correlation between the shape of aggregate and mechanical properties of asphalt concrete ( Arasan et al., 2011 ). The shape of aggregates affected the required cement content, as well as the mechanical properties and durability of the produced concrete. Content of air voids in aggregates can be directly linked to the observed water demand. It has been determined by a feed-forward neural network with the error back-propagation algorithm using artificial neural networks (ANNs) and multiple linear regression with specific toolkits such as NTR2003 and WEKA ( Zavrtanik et al., 2016 ). Digitalization of other properties, that is, water content, specific gravity, and density, appears to be still at a very early stage. A summary of research related to the digitalization of aggregate properties is shown in Table 1 .

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TABLE 1 . Digitalization of aggregate properties.

Selection of cement type and its amount must ensure achieving the targeted fresh and hardened state properties. The decision-making process is usually strongly regulated and depends, for example, on the exposure conditions or planned service life of the structure. Potentially, it could be automated through digitalization by utilizing research data collected over the last few decades combined with regulations and practical observations. As it will be shown later, most methods used in the current practice provide digital data which could be implemented into IT platforms. For example, Hughes et al. (1995 ) used Fourier-transform infrared (FTIR) spectroscopy to determine the cement composition, while Hamza et al. (2017) established the impact of the cement type on the resistance of concrete to sulfate attack. Suryani et al. (2020 ) determined the structural and optical properties of cement with the aid of X-ray diffraction (XRD). It included crystal size, microstrain, energy deformation, and stress.

The specific surface area of cement is a crucial parameter when selecting the cement type. Larger surface enhances the hydration process ( Neville and Brooks, 1987 ). This parameter has been determined by various techniques, for example, neutron scattering, gas sorption, small-angle scattering, nuclear magnetic resonance imaging, X-ray scattering, and mercury intrusion porosimetry ( Winslow and Diamond, 1974 ; Olek et al., 1990 ; Thomas et al., 1998 ). Unfortunately, none is digitalized and require additional manual work to transform collected data into a usable digital format ( Thomas et al., 1999 ). Ferraris and Garboczi (2013 ) measured the particle size and specific surface area by laser diffraction X-ray computed microtomography, which enabled to determine particles as small as 45 μm. Another method is laser diffraction spectrometry, which determines the particle size by spreading the light around the particle’s contours ( Hackley, 2004 ). It is able to detect particles having diameters in the range between 10 μm and 1 mm ( Bowen, 2002 ). Erdogan (2010 ) used the X-ray microcomputed tomography technique incorporated with spherical harmonic analysis to determine the 3D shape of cement particles for characterizing cement, based on particle shape and chemical composition. In that case, the used spherical harmonic analysis enabled to determine the particle length, width, and thickness. The average shape of cement particles has been correlated with the volume fraction of belite and alite. A summary of digitalization of cement properties is given in Table 2 .

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TABLE 2 . Digitalization of cement properties.

Concrete Mix Design

The concrete mix design establishes the proportions and type of its constituents, that is, binder or binders, aggregates, fillers, water, chemical additives, admixtures, and possible fibers. The concrete mix design along with other factors, especially including, casting technology, curing procedure, and environmental conditions, determines the ultimate workability, strength, or durability of concrete. The concept of digitalizing the concrete mix design has been used for a relatively long time already. For example, the water-to-cement ratio has been determined using a near-field microwave technique with an open-ended rectangular waveguide probe radiating into OPC materials at 5 GHz (G-band) and 10 GHz (X-band) ( Bois et al., 1998 ). The same concept has been also applied to determine the coarse aggregate-to-cement (ca/c) ratio ( Bois et al., 2000 ). A real-time, on-site evaluation of the water-to-cement ratio (w/c) used microwave non-destructive testing ( Mubarak et al., 2001 ). A monopole antenna probe, operating at 3 GHz with a reflectometer, has been also used to efficiently determine the w/c ratio ( Providakis et al., 2011 ). The concrete mix design has been also optimized by artificial neural networks (ANNs) using various input data, for example, workability or compressive strength ( Ji-Zong et al., 1999 ; Yeh, 1999 ; Ji et al., 2006 ; Ziolkowski and Niedostatkiewicz, 2019 ). The method enabled estimation of dosage of materials, choice of the type of cement, and effects of chemical and mineral admixtures ( Ji-Zong et al., 1999 ). The same concept but with different design algorithms has been used to estimate nominal and equivalent w/c ratios, fly ash (FA)-to-binder ratio, and aggregate size ( Ji et al., 2006 ). Others used a set of concrete recipes to optimize the mix design based on maximum aggregate size, slump, fineness modulus, and compressive strength by incorporating an adaptive neural fuzzy inference system ( Neshat, 2012 ). Recently, a machine learning algorithm has been used to optimize the mix ( Ziolkowski and Niedostatkiewicz, 2019 ). Concrete mixes for 3D printing were designed to obtain the required extrudability, buildability, workability, and open time ( Lediga and Kruger, 2017 ). A summary of digitalized methods and tools used in the concrete mix design is shown in Table 3 .

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TABLE 3 . Digitalization of the mix design.

Concrete Properties

Concrete temperature.

The temperature of fresh concrete and the ambient temperature are very important parameters while designing concrete mix composition, or planning, transporting, casting, and curing ( Shoukry et al., 2011 ). Generally, high temperature accelerates the hydration process, which might require addition of retarders, decreasing the amount of cement, or addition of certain secondary cementitious materials (SCMs) ( Gamil et al., 2019 ). On the contrary, a lower temperature slows down the hydration process and delays strength development ( Ma et al., 2015 ). To counteract these effects, accelerators can be used in combination with, for example, rapid hardening cement and heat curing ( Alhozaimy, 2009 ; Fang et al., 2018 ). Most standards limit the maximum concrete temperature to prevent cracking, lower strength, and delayed ettringite formation ( Hale et al., 2005 ).

Digitalization of concrete temperature measurement is rather advanced ( Wong et al., 2007 ; Norris et al., 2008 ; Barroca et al., 2013 ; Chen and Wu, 2015 ; Kim et al., 2015 ; Liu et al., 2017 ). State-of-the-art technologies with embedded sensors have been used. One common technology used to monitor the temperature is thermal imaging using infrared thermography. This technology is non-destructive, but it is applicable only to concrete not exposed to sunlight ( Tran et al., 2017 ). Other techniques include, for example, fiber Bragg grating sensors, which are used to monitor temperature and shrinkage at the same time ( Wong et al., 2007 ). Embedded nanotechnology/microelectromechanical systems (MEMS) sensors have been used to monitor moisture and temperature of concrete at the same time. Unfortunately, issues with repeatability and signal processing have been faced ( Norris et al., 2008 ). Embedded thermal sensors have been used for temperature monitoring, but the thermography sensors must be in visual contact with the monitored concrete. It might be difficult to achieve due to, for example, form covers or other materials present on the concrete surface ( Azenha et al., 2011 ). To overcome this drawback, automatic wireless sensors were used, but a 5 °C discrepancy was observed between actual and experimental values ( Barroca et al., 2013 ). Another example is the so-called passive wireless surface acoustic wave (SAW) sensor combined with orthogonal frequency coding (OFC). The main constraints were related to the effect of propagation loss and isotropic radiation loss ( Kim et al., 2015 ). Sensors utilizing passive radio frequency identification (RFID) and radio frequency integrated circuit (RFIC) ( Chen and Wu, 2015 ; Liu et al., 2017 ) enabled short-range remote sensing and achieved the detection resolution of 0.25 °C ( Chen and Wu, 2015 ; Liu et al., 2017 ). Their main shortcoming was the signal instability and a lack of electronic protection ( Chang and Hung, 2012 ). A summary of methods and tool for digitalization of concrete temperature monitoring is given in Table 4 .

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TABLE 4 . Digital transformation of concrete temperature monitoring.

Workability

Workability is an essential technological property of concrete controlling the casting process and affecting the quality of produced concrete elements or structures. It can be measured, for example, by slump or slump flow combined with T50 time in the case of self-compacting concrete ( Fares, 2015 ). A number of digitalizing solutions have been introduced, and artificial neural networks (ANNs) is one the examples ( Bai, 2003 ; Yeh, 2006a ; Oztas, 2006 ; Yeh, 2009 ; Kim and Park, 2018 ). They produce a more accurate prediction of workability than the non-linear regression analysis ( Yeh, 2006a ), and it has the ability to model the slump for any mix design ( Yeh, 2009 ). Another example method is based on 3D depth sensors ( Kim and Park, 2018 ). Rheological properties of concrete described by the yield stress and the plastic viscosity are crucial for designing self-compacting concrete mixes ( Wallevik, 2003 ; Roussel, 2011 ) ( Ferraris et al., 2012 ). An effective device called 4C-Rheometer was developed by the Danish Technological Institute ( Danish Technological Institute and C.C, 2020 ). It enabled to determine rheology based on automated measurements of slump flow and flow time. A summary of digitalization of workability measurements is given in Table 5 .

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TABLE 5 . Digital transformation of workability measurement in concrete.

Setting Time and Hydration Rate

Initial and final setting times of cement are used to monitor the hardening rate. The initial setting time indicates how long concrete mix maintains its plasticity. It indicates the allowable time to cast the concrete. The final setting time indicates the time after which concrete loses its plasticity, and it is especially useful for planning surface finishing processes. Both times are related to the hydration process, which can be monitored using calorimetry and measuring the evolved heat ( Mostafa and Brown, 2005 ; Xu, 2011 ; Gawlicki et al., 2010 ). Parameters affecting the degree of cement hydration are summarized in Figure 1 ( Xu et al., 2010 ).

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FIGURE 1 . Parameters and process used to characterize cement hydration (summarized from the work of Xu et al., 2010 ).

Several attempts were made to digitalize the assessment of the setting time. For example, Rizzo et al. (2014 ) used a non-destructive setup measuring strength development by sensors detecting the propagation of highly non-linear solitary waves (HNSWs). The waves were reflected at the sensor interface and transmitted to the monitored concrete. The transmission time and the reflection from the interface were measured and compared with the hydration time. These parameters were then correlated with initial and final setting times measured by using the Vicat apparatus. The hydration rate has been also monitored using the Fabry–Perot fiber optic temperature sensor. The concrete temperature depended on the water-to-cement ratio ( Zou et al., 2012 ). Yet another effective method to digitalize the hydration rate is the monitoring of the crack formation ( Yang et al., 2010 ). The hydration degree was also assessed by the thermogravimetric analysis ( Deboucha et al., 2017 ). The method estimated the ultimate amount of bound water, which was verified by isothermal calorimetry combined with the assessment of compressive strength. The differential thermal and thermogravimetric analysis was also used to estimate the degree of hydration. In that case, the degree of hydration was calculated using experimental results. A good agreement between results based on differential thermal and thermogravimetric analysis was observed ( Monteagudo et al., 2014 ).

The hydration process can also be measured using other methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), or non-contact impedance measurement (NCIM) ( Tang et al., 2016 ). For example, XRD was combined with calorimetry to monitor the hydration of cement blended with fly ash for the first 44 h. It enabled estimating the effects of fly ash (FA) ( Dittrich et al., 2014 ).

Concrete Maturity

Maturity is an indicator used to predict strength development depending on the curing temperature ( Chengju, 1989 ; McCullough and Rasmussen, 1999 ; Topçu and Toprak, 2005 ; Zhang et al., 2008 ; Yikici and Chen, 2015 ). The required ( Ballim and Graham, 2009 ; Lee and Hover, 2015 ) systems based on that concept have been developed. For example, high-performance concrete paving (HIPERPAV) software utilized temperature data and the maturity concept to estimate the concrete strength at an early age ( Ruiz, 2001 ). Another system developed by Giatec Scientific Inc.is based on wireless temperature sensors integrated with a special smartphone application. It enables live monitoring, but the maximum allowable distance between the sensor and the monitored concrete surface is limited ( De Carufel, 2018 ).

Mechanical Properties

The compressive strength of concrete is certainly the most commonly used indicator of mechanical properties ( Damineli et al., 2010 ; Yang et al., 2010 ; Ma et al., 2015 ). It is usually determined using a cube compression test, which is a time-consuming process. Consequently, several models have been created to reliably predict the strength without the need of physical testing. The ANNs method, described earlier, has been used in several studies ( Lee, 2003 ; Kim et al., 2004 ; Yeh, 2006b ; Prasad et al., 2009 ). It could estimate the compressive strength taking into account slump, air content, and fly ash amount as indicators in PreConS (intelligent system of strength). Unfortunately, the system showed a lower reliability at variable curing temperatures ( Lee, 2003 ). Others used the ANN approach but based on different concrete mix proportions ( Kim et al., 2004 ). In that case, literature data were used to estimate the compressive strength of SCC and high-performance concrete (HPC) taking into account the volume of fly ash and the water-to-cement ratio ( Prasad et al., 2009 ). ANNs were also combined with the image processing technique and design of experiments to estimate the strength ( Dogan et al., 2017 ; Waris et al., 2020 ). It enabled prediction of various mechanical properties, including compressive strength, modulus of elasticity, and maximum deformation, reaching 98.65% accuracy. ANNs were also efficiently incorporated in an approach based on utilizing data obtained from ultrasonic pulse velocity (UPV) measurements ( Kewalramani and Gupta, 2006 ). Similarly, a neural expert system was used to predict the strength based on results from testing a total of 864 concrete specimens. The applied ANN model used a back-propagation learning algorithm, and the results were compared to a built-in expert system, which enabled prediction of the strength using rule-based knowledge representation techniques ( Gupta et al., 2006 ). Both compressive and tensile strength of high-performance concrete were determined using a modified firefly algorithm–artificial neural network expert system. A good correlation between actual and predicted results was achieved ( Bui et al., 2018 ).

A deep learning prediction method has been applied to predict the compressive strength of recycled aggregate concrete. The model used the water-to-cement ratio and the recycled aggregate replacement percentage as input parameters. Tests were performed on 74 concrete blocks. The achieved precision was higher than that of a traditional neural network ( Deng et al., 2018 ). A machine learning approach has been utilized to predict the compressive strength at different ages for concrete with high fly ash content. The water cycle algorithm and the genetic algorithm showed a good correlation between the variation of fly ash content and compressive strength ( Naseri, 2020 ).

Real-time prediction of the compressive strength has been carried out using data obtained using novel types of sensors ( Providakis et al., 2011 ; Tareen et al., 2019 ; John et al., 2020 ). The early-age concrete strength was effectively estimated using data obtained from the active wireless sensing system ( John et al., 2020 ). It used an electromechanical impedance measuring chip and a piezoelectric transducer installed in a Teflon-based ( Providakis et al., 2011 ). Other approaches to predict the early strength used smart temperature (SmartRock) and PZT (piezoelectric) sensors with ultrasonic wave propagation combined with the concrete maturity concept ( Tareen et al., 2019 ). Recently, the technology of Internet of things (IoT) was utilized to estimate the compressive strength using temperature sensors and Wi-Fi microcontrollers. The technology enabled real-time monitoring of strength ( John et al., 2020 ). A summary of digitalization methods for prediction of compressive strength is shown in Table 6 .

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TABLE 6 . Digital transformation of the compressive strength of concrete.

Crack Monitoring

Crack monitoring remains a major concern in the concrete industry, and it is crucial for safety and maintenance costs ( Omondi et al., 2016 ). Concrete cracks are caused by two effects, that is, extrinsic and intrinsic ( Li et al., 2018a ). The former is induced by the application of excessive loads. The intrinsic effects are related to the hardening process and are considered as non-structural. Intrinsic cracks are controlled by the mix design, mixing method, ambient temperature, and humidity ( Bolleni, 2009 ). Automated crack detection and monitoring are still in the developmental stage, and various approaches have been considered. Digital image processing is certainly one of the most used methods ( Dare et al., 2002 ; Chen et al., 2006 ; Nagy, 2014 ; Gehri et al., 2020 ). An automated image processing technique with multitemporal crack measurements detected the extrinsic cracks in concrete. The automatic method accurately delineated cracks even when using poor-quality images ( Dare et al., 2002 ). The same method was applied to study the relationship between the crack width and its expansion with multitemporal image processing. In that case, images were taken every 2 weeks with a high-resolution scanner. The method enabled automatic crack tracing and showed a good correlation between the estimated width and the manual measurement ( Chen et al., 2006 ). Crack width was also measured by two emerging technologies, that is, the image digitalizing method and the digital image processing (DIP) method combined with a digital microscope that enabled mapping the tortuosity of cracks ( Nagy, 2014 ). An example process of transforming crack monitoring data into a digital form is shown in Figure 2 . The process starts by taking an image of the crack followed by adjustment and cropping of the crack line. In the next step, pixel coordinates are used to determine the crack width ( Nagy, 2014 ).

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FIGURE 2 . Example of crack width measurement using digital image processing ( Nagy, 2014 ).

The same technology has been used to monitor the crack behavior and the crack orientation by extracting images with the digital image correlation (DIC) method ( Gehri et al., 2020 ). The obtained results were limited only to closely spaced cracks. DIC has been also used to study the fracture behavior of concrete interfaces ( Shah and Chandra Kishen, 2011 ). The used optical and non-contact measurement tool analyzed the displacement of the surface using images obtained before and after the displacement occurred. Another application of DIC has been monitoring and measuring deformation developing in compression ( Choi and Shah, 1997 ). Results showed a well-balanced image rate for both lateral and axial deformation after the peak load.

More advanced methods were applied to determine the crack width and length using a digital camera embedded in a calibrated cylindrical attachment. The crack width could be estimated reliably, but the obtained results strongly depended on the operator ( Dare et al., 2002 ).

Other new technologies that have been used to detect and monitor cracking of concrete include thermography ( Bolleni, 2009 ), combined acoustic emission and digital image correlation techniques ( Omondi et al., 2016 ), local binarization algorithm ( Li et al., 2018b ), and ultrasound-excited thermography ( Jia et al., 2019 ). Thermography uses a thermal camera based on the infrared radiation, and it does not require a direct access to concrete layers to detect the damage ( Bolleni, 2009 ). This method has also been combined with the ultrasound-excited thermography and enabled detection of microcracks having width between 0.01 and 0.09 mm ( Jia et al., 2019 ). DIC has been successfully combined with acoustic emission technology to detect cracks and determine their orientation ( Omondi et al., 2016 ). Yet another tested approach is a technology based on a local binarization. The color of the image is transferred into a binary image that has two colors, typically black and white. The image is then processed to detect the surface and cross-sectional area of present cracks ( Li et al., 2018b ). A summary of digitalization of crack formation in concrete is shown in Table 7 .

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TABLE 7 . Concrete crack monitoring using digital technology.

More pieces of information were involved in the production of concrete, such as raw material characterizations, mix design, and properties of ready concrete, which are essential parameters used to envisage the quality of the end-product. Mostly, this information is acquitted manually in the laboratory. This process is time consuming, and technical experts need time to make quick judgments about modifying the mix design or developing the mix for specific use and environment condition. To save time and produce favorable and good-quality concrete, transforming information acquisition to real-time updates using digital technologies is preferred. The possibility of digital transformation of these essentials seems to be valid and possible; perhaps, more integration of different technologies can work efficiently to develop a system to obtain and communicate concrete information. The information needed from the source of raw materials at the quarry sites and the cement production plant by the engineer who develops the mix is surface area, specific gravity, shape, gradation, etc. Having this information on time will allow the mix design developer to adjust the proportions for the specific needs. Then, during the casting process, engineers need to monitor the concrete temperature, workability, formwork pressure, which is not discussed in this article, casting rate, maturity of the concrete to decide on the formwork removal time, mechanical properties, and crack monitoring. The question comes about merging all this information in one complete system using emerged technologies with embedded sensors and IoT for instantaneous communication, Figure 3 . Extensive research has been carried out, as discussed in this article, to gradually transform the data acquisition into a digital form. Still, not all the attempts have been applied at the jobsite. There are reasons and challenges for low acceptance, and the process involves consideration of a multitude of stages. The first is the availability and accessibility of technology. Then, the question comes about the acceptance and confidence from the side of construction stakeholders of the technology, and that incurs some cost and expertise; these restrains need to be addressed through intensive research and full-scale experiments. It is suggested for future development to integrate the current technologies and applications into one integrated system for possible information acquisitions and instant communication.

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FIGURE 3 . Digital transformation of concrete properties.

Digitalization can be defined as converting information into a digital format and using these data to control, for example, the production and usage of concrete. Digital transformation enables us to save time and cost, facilitates access to information, and increases efficiency and readiness. In the concrete industry, the digital transformation of concrete properties and production helps to create a more consistent and faster construction process. Availability of real-time data enables engineers to follow and control the entire production process more efficiently and with higher reliability. Access to data is facilitated by, for example, cloud storage platforms. For example, the construction process can be accelerated and made safer by more accurate prediction of the formwork removal timing. In the current era, more advanced digital concrete has been introduced, and that technology needs to be coupled with the digitalized process of concrete data acquisition.

The real-time data assist engineers and managers in the decision-making process. The decision can be related, for example, to optimizing the mix design by reducing the usage of raw materials, thus leading to enhanced sustainability. On the negative side, the digital transformation, in the case of concrete technology, is a complex process due to not yet fully understood basic processes controlling, for example, hydration of Portland cement. An even worse situation is faced in the case of new ecological binders. Only for these reasons, it is extremely difficult to develop reliable models. Models which could be used to design concrete mixes predict strength development, crack formation, or deterioration due to various types of exposures. Another set of problems is related to the acceptance of the concrete and construction industry as well as compliance with current regulations and standards. There is also a need to ensure that the acquired data are communicated and stored correctly, analyzed, and interpreted by the responsible personnel. Other challenges include proper installation of sensors, data collection and storage devices, and data safety or data transmission.

There is still a significant amount of work to be completed before benefits of digitalization could be fully utilized in concrete technology. Problems to be solved are related not only to basic phenomena, for example, hydration of cement, but also to full-scale real-life applications with a number of factors not being present in laboratory settings.

Author Contributions

YG has established the concept of the article, collected and analyze the data while AC has reviewed and supervise the work. He also contributed to proofreading and revising the article critically for important intellectual content.

This research was funded by the Development Fund of the Swedish Construction Industry (SBUF) and NCC construction company.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors acknowledge the financial support from the funding agencies of the project and Lulea University of Technology for the research material support.

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Keywords: digital transformation, concrete properties, concrete technology, sustainability, advanced technology, monitoring

Citation: Gamil Y and Cwirzen A (2022) Digital Transformation of Concrete Technology—A Review. Front. Built Environ. 8:835236. doi: 10.3389/fbuil.2022.835236

Received: 14 December 2021; Accepted: 24 January 2022; Published: 11 March 2022.

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Copyright © 2022 Gamil and Cwirzen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Yaser Gamil, [email protected]

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Automation and Artificial Intelligence in Construction and Management of Civil Infrastructure

Application-oriented fundamental research on concrete and reinforced concrete structures: selected findings from an Austro-Chinese research project

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  • Volume 231 , pages 2231–2255, ( 2020 )

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  • Eva Binder 1 , 2 ,
  • Hui Wang 1 , 2 , 3 ,
  • Jiao-Long Zhang 1 , 2 ,
  • Thomas Schlappal 2 ,
  • Yong Yuan 1 ,
  • Herbert A. Mang 1 , 2 &
  • Bernhard L. A. Pichler   ORCID: orcid.org/0000-0002-6468-1840 2  

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In this paper, the significance of application-oriented fundamental research on concrete and reinforced concrete structures for progress regarding practical applications to structural design is addressed based on four examples. They were treated in a joint research project of Vienna University of Technology and Tongji University. The first topic refers to sudden heating or cooling of concrete structures, the second one to high-dynamic strength of specimens made of cementitious materials, the third one to structural analysis of segmental tunnel rings used in mechanized tunneling, and the fourth one to serviceability and ultimate limit states of concrete hinges used in integral bridge construction. The first two topics deal with exceptional load cases. Results from the fundamental research call for improvements of state-of-the-art simulation approaches used in civil engineering design. The last two topics refer to reinforced concrete hinges used in mechanized tunneling and integral bridge construction, respectively. Integrative research has led to progress regarding the verification of serviceability and ultimate limit states. In all four examples, results from fundamental research are used to scrutinize state-of-the-art approaches used in practical structural design of civil engineering structures. This allows for identifying interesting directions for the future development of design guidelines and standards.

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1 Introduction

This paper is focused on four examples of fundamental research carried out in the field of engineering mechanics of concrete and reinforced concrete structures. The problems concerned were treated in a recently completed research project, called “Bridging the Gap by Means of Multiscale Structural Analyses” [ 1 ]. This project was a cooperative effort of Vienna University of Technology, in Austria, Europe, and Tongji University, in Shanghai, China. It focused on the added value resulting from the use of modern multiscale material models for concrete in the framework of structural analysis of reinforced concrete structures. The first two examples refer to exceptional load cases, and the remaining two to the regular service of civil engineering infrastructure.

Sudden heating or cooling is the topic of the first example. A multiscale poromechanics model was used to explain why the macroscopic thermal expansion coefficient of mature cement paste is a nonlinear function of the internal relative humidity [ 2 ]. At the macrostructural scale, thermal eigenstrains, resulting from transient heat conduction, were analyzed [ 3 , 4 ]. Herein, the question is discussed whether or not it is possible to define equivalent temperature gradients, such that a structural analysis can be carried out with commercial simulation software, such as recommended by the state-of-the-art guideline [ 5 ]. In addition, durability issues, resulting from recurrent cycles of temperature and relative humidity, are discussed.

The high-dynamic strength of cementitious materials is the topic of the second example. Fundamental research in this area is based on the theory of propagation of stress waves and cracks through linear-elastic, isotropic media. An engineering-mechanics approach was used to develop a model for the high-dynamic compressive and tensile strength of specimens made of cement pastes, mortars, and concretes [ 3 , 6 , 7 , 8 ]. Herein, the question is discussed whether or not it is possible to analyze concrete structures subjected to dynamic loading by means of commercial simulation software for quasi-static analysis, simply by introducing higher values of the strength.

Structural analysis of real-scale tests of segmental tunnel rings is the topic of the third example. Bearing capacity tests were analyzed with the help of transfer relations in the form of analytical solutions of the first-order theory of thin circular arches. In the developed hybrid analysis method, transfer relations and measured relative rotations of neighboring segments were combined [ 9 , 10 ]. Herein, the discussion focuses on the convergences, in the vertical and the horizontal direction, governing the serviceability limits, and on the development of plastic hinges at the interfaces, defining the bearing capacity of segmental tunnel rings.

Recommendations for verification of serviceability and ultimate limit states of reinforced concrete hinges are the topic of the fourth example. An engineering-mechanics model was established to determine acceptable relative rotations as a function of the normal force transmitted across the neck [ 11 , 12 ]. The usefulness of corresponding dimensionless diagrams was checked by means of experimental data. Herein, the discussion focuses on similarities of serviceability limit states of concrete hinges in integral bridge construction and mechanized tunneling and on differences between the two.

The main aim of the present paper is to use results from fundamental research in the civil engineering science as the basis for scrutinizing state-of-the-art approaches used in practical structural design of civil engineering structures. This allows for identifying interesting directions for the future development of design guidelines and standards.

The paper is structured as follows: one section each is dedicated to the aforementioned four examples, see Sects.  2 – 5 . Overall conclusions are drawn in Sect.  6 .

2 Multiscale thermoelastic analysis of concrete and concrete structures

Concrete structures, such as pavement plates made of plain concrete and tunnel segments made of reinforced concrete, are generally subjected to recurrent cycles of temperature and relative humidity. They may have to sustain exceptional loadings, such as sudden heating in case of fire disasters, or sudden cooling in case of hail showers. The resulting thermomechanical loading may lead to damage of these structures. Thus, it is a threat to their long-term durability and safety.

The thermal expansion coefficient of concrete governs the thermal deformations and stresses of concrete structures subjected to thermal loading. This coefficient is a nonlinear function of the internal relative humidity RH of the material, because the thermal expansion coefficient of the cement paste is an asymmetrical bell-shaped function of RH , see Fig.  1 . Its maximum value occurs at \(RH \approx 65\%\) , which is virtually twice as large as its minimum value at \(RH = 100\%\) . This provided the motivation for fundamental scientific research regarding the thermal expansion coefficient of cement paste and concrete.

figure 1

Dependence of the thermal expansion coefficient of cement paste on the internal relative humidity: experimental results of Meyers [ 13 ], Mitchell [ 14 ], and Dettling [ 15 ]; curve obtained by Emanuel and Hulsey [ 16 ]

At the structural scale of pavement plates, heat conduction mainly concerns the thickness direction. When it comes to transient heat conduction, the temperature and the associated thermal eigenstrains are time dependent. Their distributions over the thickness of a plate are nonlinear. However, there still exist state-of-the-art guidelines for quantification of thermal stresses of pavement plates recommending the assumption of a linear distribution of the temperature along the thickness of the plate, see, e.g., [ 5 ]. This provided the motivation for research regarding the contribution of the nonlinear part of the temperature profile to the thermal stresses.

2.1 Results from recent fundamental research

This Subsection is organized in three parts. At first, a thermoporoelastic multiscale material model is used to provide insight into the question why thermal expansion coefficients of cementitious materials depend on the internal relative humidity. Subsequently, a structural analysis of a pavement plate subjected to solar heating followed by a suddenly starting hail shower is discussed. Finally, methods available for top-down quantification of stresses experienced by the concrete constituents (the cement paste, the aggregates, and the interfacial transition zones) are briefly summarized.

Concrete is a hierarchically organized microheterogeneous material, see Fig.  2 . The microstructure of concrete consists of aggregate inclusions embedded in a matrix of cement paste. The microstructure of cement paste consists of unhydrated cement clinker inclusions embedded in a matrix of hydrate foam. The microstructure of the hydrate foam consists of capillary pores embedded in a matrix of hydrate gel. Finally, the microstructure of the hydrate gel consists of nanoscopic gel pores embedded in a matrix of solid hydrates.

figure 2

Multiscale representation of a concrete pavement plate, following [ 17 ]

Subsequent homogenization of the four matrix-inclusion composites in Fig.  2 is used for bridging the scales from the nanoscopic level of the solid hydrates and the gel pores to the macroscopic scale of the concrete. Each representative volume element (RVE), occupying a volume of \(V_{\text {RVE}}\) , consists of a matrix phase with a volume of \(V_m\) and an inclusion phase with a volume of \(V_i\) . Both material phases \(k \in [m, \,i\,]\) exhibit a specific elastic stiffness, \(\mathbb {C}_k\) , a specific eigenstress, \(\varvec{\sigma }_k^e\) , and a specific volume fraction, \(f_k\) ,

These three properties are discussed in the following paragraph.

The solid constituents, i.e., the solid hydrates (shyd), the unhydrated clinker grains (clin), and the aggregates (agg), are considered to be isotropic. Thus, their elastic stiffness can be described based on their bulk and shear moduli [ 2 ]. The gel and capillary pores, in turn, are filled by water or air. Thus, they are characterized by vanishing solid stiffnesses. The eigenstresses of the solid constituents, \(\varvec{\sigma }_{k}^{e}\) , are proportional to the thermal eigenstrains \(\varvec{\varepsilon }_k^e\) , i.e.,

The latter are proportional to the thermal expansion coefficients \(\alpha _{k}\) and the temperature change \(\varDelta T\) ,

The pores are idealized as being connected and spherical, with radii r following exponential distributions:

where gpor stands for gel pores and cpor for capillary pores. The characteristic radius \(R_\mathrm{gpor}\) amounts to \(\approx \) 2 nm, \(R_\mathrm{cpor}\) to \(\approx \) 12 nm, see [ 2 ]. As for homogenization, the eigenstresses of gel and capillary pores are proportional to the average effective pore pressures [ 18 ],

\(p_\mathrm{gpor}\) and \(p_\mathrm{cpor}\) are obtained from averaging the effective pressures p ( r ), referring to individual pores with radius r , over pores of all sizes [ 2 ]:

The effective pressures of individual pores result from the pressure of the fluids filling the pores, either the liquid pressure \(p_\ell \) or the gas pressure \(p_g\) , and from the surface tension prevailing at the interface between the pore and the surrounding solid, either that at solid-liquid interfaces, \(\gamma ^{s \ell }\) , or that at solid-gas interfaces, \(\gamma ^{sg}\) . Because of the latter contributions, the effective pressures are a function of the radius r of the pores,

where t denotes the thickness of the layer of water adsorbed to the surface of the pores. The Kelvin radius \(r_\mathrm{Kelvin}\) discriminates between water-filled and air-filled pores. It is a function of the temperature and the relative humidity [ 2 ]. Setting \(p_g\) equal to zero and considering the Kelvin–Laplace equation, Young’s equation, and Berthelot’s state equation, Eq. ( 7 ) can be reformulated in terms of measurable quantities as well as the universal gas constant R , the absolute temperature T , and the molar volume of water \(\nu _m\) as [ 2 ]

The volume fractions of the solid constituents and the pores are quantified based on Powers’ hydration model [ 2 ]. They are functions of the initial composition of concrete, quantified in terms of the initial aggregate-to-cement mass ratio and the initial water-to-cement mass ratio, and of the maturity of concrete, quantified in terms of the hydration degree.

Bottom-up homogenization of the RVEs in Fig.  2 is carried out in a step-by-step fashion, using methods of continuum micromechanics [ 19 ]. The thermoelastic behavior of homogenized composites follows the generalized Hooke’s law as

where \({\varvec{\Sigma }}_{\hom }\) and \(\mathbf {E}_{\hom }\) denote the macroscopic stress and strain, respectively. The homogenized stiffness, \(\mathbb {C}_{\hom }\) , and eigenstress, \({\varvec{\Sigma }}_{\hom }^e\) , follow as

with \(\mathbb {A}_k\) standing for the strain concentration tensor of phase k . \(\mathbb {A}_k\) are estimated by means of the Mori–Tanaka–Benveniste scheme [ 20 ]. Under consideration of macroscopic stress-free expansion, \({\varvec{\Sigma }}_{\hom }=0\) , the macroscopic strain is, on the one hand, proportional to the homogenized eigenstress, \(\mathbf {E}_{\hom }=-(\mathbb {C}_{\hom })^{-1}:{\varvec{\Sigma }}_{\hom }^e\) . On the other hand, it is proportional to the homogenized thermal expansion coefficient, \(\mathbf {E}_{\hom } = \alpha _{\hom } \, \varDelta T \, \varvec{1}\) . Thus, \(\alpha _{\hom }\) can be calculated.

The described multiscale model allows for explaining the thermal expansion behavior of cement paste (Fig.  1 ), based on the water uptake/release characteristics of the nanoscopic solid hydrates, a formerly unknown material property, see Fig.  3 . The water uptake/release coefficient \(\varDelta \mu / \varDelta T\) is a bell-shaped function of the relative humidity prevailing in the air-filled pores before the temperature change \(\varDelta T\) . Thereby, \(\varDelta \mu \) denotes the change of the mass of the hydrates resulting from the uptake/release of water, divided by the initial mass of the hydrates: \(\varDelta \mu = \varDelta m_\mathrm{hyd} / m_\mathrm{hyd}\) . The uptake/release of water is a rather subtle effect, but still large enough to explain the thermal expansion behavior of cement paste. Provided that the temperature of the material is increased, the solid constituents expand according to their thermal expansion coefficients and the solid hydrates release water. This results in a redistribution of water within the partially saturated pore network and in an increase of the Kelvin radius. Consequently, the internal relative humidity increases and the average effective pore underpressures decrease [ 2 ]. This manifests itself macroscopically as an additional poromechanical contribution to the macroscopic swelling of the cement paste. The opposite effects occur when cement paste is cooled down. The solid constituents shrink according to their coefficients of thermal expansion and the solid hydrates take up water, the Kelvin radius decreases, the internal relative humidity decreases, the average effective pore underpressures increase, and this manifests itself macroscopically as an additional poromechanical contribution to the macroscopic shrinkage of the cement paste. As regards the experimental validation of the described phenomena, the water uptake/release characteristics of the hydrates were observed recently based on proton nuclear magnetic resonance relaxometry tests of cement paste subjected to temperature changes [ 21 ].

figure 3

Specific water uptake/release of hydration products per temperature increase, \(\varDelta \mu / \varDelta T\) , identified in [ 2 ]

The homogenized thermomechanical properties of concrete [ 22 ] are used as input for the structural analysis of a pavement plate subjected to solar heating, followed by a suddenly starting hail shower [ 4 ]. The thickness of the plate amounts to 25 cm, the homogenized Young’s modulus of concrete to 32 GPa, and the homogenized coefficient of thermal expansion to \(11.5\times 10^{-6}/^\circ \mathrm {C}\) . At first, heat conduction in the thickness direction is analyzed. In the initial configuration, the plate is in an isothermal state at \(17\,^\circ \) C. As regards the boundary conditions, the temperature is set constant at the bottom of the plate, and the top surface is subjected to a prescribed temperature evolution. During the first 12 hours of the analysis, solar heating results in an increase of the surface temperature from 17 to \(62\,^\circ \) C. After that, a suddenly starting hail shower reduces the surface temperature instantaneously to \(0\,^\circ \) C. This surface temperature remains constant for several minutes. The described thermal loading results in transient heat conduction. This problem can be solved based on existing series solutions [ 4 ], providing quantitative access to the time-dependent and spatially nonlinear distributions of the temperature. The resulting thermal eigenstrains of concrete are equal to the product of the temperature changes and the homogenized thermal expansion coefficient. Thus, the spatial distribution of the thermal eigenstrains is also nonlinear, see Fig.  4 for the eigenstrains obtained 3 min after the start of the hail shower.

figure 4

a Schematic illustration of a pavement subjected to solar heating followed by a hail shower, and b thermal eigenstrains and their decomposition into the eigenstretch, the eigencurvature, and the eigendistortion, 3 min after the start of the hail shower [ 4 ]

The thermal stresses of the pavement plate are quantified using the theory of thin plates [ 4 ]. It is based on Kirchhoff’s normal hypothesis. It implies that the generators of thin plates remain straight. Thus, the total strains (i.e., the sum of the thermal eigenstrains and the stress-related mechanical strains) are varying linearly in the thickness direction. This provides the motivation for decomposing the nonlinear thermal eigenstrains into a linear and a nonlinear part. The linear part results in an eigenstretch and an eigencurvature of the midplane of the plate, see Fig.  4 . The eigenstretch is free to develop, because of the joints between neighboring pavement plates. The eigencurvature, in turn, is constrained by the Winkler foundation on which the plate is resting. Thus, the eigencurvature results in thermal stresses of the plate. These stresses, however, turn out to be negligible relative to the thermal stresses resulting from the nonlinear part of the eigenstrains [ 4 ]. This nonlinear part of the eigenstrains can be interpreted as an eigendistortion of the generators of the plates. The latter are prevented at the scale of the plate generators, because they must remain straight according to Kirchhoff’s normal hypothesis. Thus, stress-related mechanical strains are activated, which have the same size and distribution, but the opposite sign, as the nonlinear eigenstrains. The mechanical strains result in self-equilibrated thermal stresses, see Fig.  5 , and note the similarity with the distribution of the eigendistortion illustrated in Fig.  4 .

figure 5

Normal stresses induced by the eigendistortions at the time instant 3 min after the start of the hail shower, comparison between the results of the analytical solution presented in [ 4 ] and a Finite Element simulation

Multiscale structural analysis continues with quantifying the average microscopic strains of the cement paste and of the aggregates, based on their thermal eigenstrains and the macroscopic strain of concrete \(\mathbf {E}_\mathrm{con}\) , using the following concentration–influence relation [ 23 ]:

where \(\mathbb {D}_{kj}\) denotes the eigenstrain influence tensor describing the influence of the eigenstrain of material phase j on the total strain of material phase k . The average microscopic stresses of the cement paste and of the aggregates follow from the phase-specific versions of the generalized Hooke’s law: \(\varvec{\sigma }_{k} = \mathbb {C}_{k}:(\varvec{\varepsilon }_{k} - \varvec{\varepsilon }_{k}^e)\) . The microstresses of the cement paste differ from those of the aggregates, because the two material phases have different elastic stiffnesses and different coefficients of thermal expansion [ 22 ]. This raises the interest in quantifying the stresses transmitted across the interfacial transition zones (ITZs) separating the cement paste matrix from the aggregates. The microstress states of the ITZs can be quantified based on the known stress and strain states of the aggregates, continuity conditions regarding traction vectors and displacements across the interface between the aggregates and the surrounding ITZ, as well as the elasticity law of the interfaces, see [ 4 , 24 ] for details. These developments establish the basis required for future sensitivity analysis regarding the influence of the thermal expansion coefficient of cement paste on the thermal stresses of pavement plates subjected to transient heat conduction.

2.2 Implications for civil engineering design

Results from fundamental research provide an interesting view on a durability issue. The described multiscale poromechanics model allows for identification of the water uptake or release process of hydration products when cooled down or heated up. This nanoscopic material phenomenon governs the strong dependence of the macroscopic thermal expansion coefficient of the cement paste on its internal relative humidity. Within the regime of intermediate relative humidity, the thermal expansion coefficient of the cement paste is much larger than that of the aggregates, which gives rise to self-equilibrated internal stresses, as the temperature is changing, see [ 22 ] for details. In view of daily cycles of temperature and relative humidity, these self-equilibrated thermal stresses represent a fatigue problem. It can be a serious threat to the long-term integrity and durability of concrete. This provides high motivation for future research on this important topic.

The temperature inside pavement plates subjected to transient heat conduction is distributed nonlinearly in the thickness direction. State-of-the-art guidelines for quantification of thermal stresses of pavement plates recommend the assumption of an “equivalent” linear distribution of the temperature along the thickness of the plate, see, e.g., [ 5 ]. The presented example underscores that this assumption is unjustified, because the thermal stresses are governed by the nonlinear part of the temperature distribution [ 4 ]. The assumption of an “equivalent” linear temperature distribution was also shown to be unnecessary, because the analysis of a nonlinear temperature distribution is straightforward. At first, the spatially nonlinear temperature changes are multiplied with the thermal expansion coefficient of concrete, in order to compute the corresponding eigenstrains. The latter are subdivided into two parts: (i) the linear part, associated with an eigenstretch and an eigencurvature of the midplane of the plate, and (ii) the nonlinear part, referring to an eigendistortion of the generators of the plate. According to Kirchhoff’s normal hypothesis, these eigendistortions are prevented at the scale of the plate generators. This activates self-equilibrated thermal stresses which can be quantified in a straightforward fashion.

3 High-dynamic strength of specimens made of cementitious materials

Important infrastructure must not only withstand regular service loads but also exceptional loads. Such infrastructure includes, but is not restricted to, schools, hospitals, power plants, tunnels, and bridges subjected to impact and blast loads. These dynamic loads may result, e.g., from cars accidentally crashing into engineering structures, or from detonation of so-called Improvised Explosive Devices. This provided the motivation for fundamental scientific research regarding the high-dynamic strength of specimens made of cementitious materials.

Split Hopkinson Pressure Bars represent a popular test equipment for the experimental determination of high-dynamic strength values. As for high-dynamic compression tests, the setup consists of the serial arrangement of a striker bar, an incident bar, a cylindrical specimen, a transmitter bar, and an adsorber, see Fig.  6 a. The striker bar is shot with a gas gun against the incident bar. The impact results in a compression pulse propagating along the incident bar. When the pulse strikes the interface with the specimen, it is partly transmitted and partly reflected. The transmitted compression pulse propagates along the specimen. When it strikes the interface with the transmitter bar, it is again partly transmitted and partly reflected. The stress and strain histories experienced by the specimen can be computed by means of readings of strain gauges mounted to the incident and transmitter bars [ 25 ]. This allows for quantitative determination of the high-dynamic compressive strength, sustained by the specimen. The focus of this work is on tests of cement paste cylinders by Fischer et al. [ 6 ], mortar cylinders by Zhang et al. [ 26 ], and concrete cylinders by Kühn et al. [ 27 ], see Fig.  7 a.

figure 6

Setup of split Hopkinson pressure bars for testing of high-dynamic strength: a  compression, and b  tension

figure 7

Results from quasi-static and high-dynamic testing of specimens made from cementitious materials: a  compressive strength values [ 6 , 26 , 27 ], and b  tensile strength values [ 28 , 29 ]

As for high-dynamic tension tests, the setup of a Split Hopkinson Pressure Bar is limited to a striker bar, an incident bar, and a specimen, see Fig.  6 b. Testing starts, analogous to high-dynamic compression tests, with shooting the striker bar against the incident bar. The compression pulse transmitted to the specimen is entirely reflected at the free end of the specimen. This creates a tension pulse propagating in the opposite direction. It interferes with the rest of the incoming compression pulse. The effective stress \(\sigma _{\mathrm{eff}}(x,t)\) experienced by the specimen is the sum of the incoming compression pulse \(\sigma _{\mathrm{comp}}(x,t)\) and the reflected tension pulse \(\sigma _{\mathrm{tens}}(x,t)\) ,

where x denotes the position and t the time variable. Regarding quantification of the maximum tensile stress experienced by the specimen, different evaluation procedures are used, see, e.g., [ 28 , 29 ]. They are based on formulae from the theory of wave propagation through isotropic elastic media. The focus of the present paper lies on tests of mortar cylinders by Brara and Klepaczko [ 28 ] and concrete cylinders by Erzar and Forquin [ 29 ], see Fig.  7 a.

3.1 Results from recent fundamental research

The high-dynamic strength \(f_{\mathrm{dyn}}\) is traditionally quantified by multiplying the quasi-static strength \(f_{\mathrm{sta}}\) with the Dynamic Strength Increase Factor (DIF)

This dimensionless quantity is equal to the strength value obtained for a specific speed of loading, divided by the quasi-static strength of the tested material, see Fig.  8 for DIF values obtained from the strength values in Fig.  7 .

figure 8

DIF values as a function of the strain rate; points refer to experimental data taken from Fig.  7 , solid lines to the developed engineering-mechanics model: a  compression tests analyzed in [ 3 , 6 , 7 ], and b  tension tests analyzed in [ 8 ]

In order to explain the high-dynamic strength gain, many models accounting for rate-dependent material properties and/or inertial confinement were proposed. Rate-dependent models are based on the assumption that the mechanical properties of cementitious materials depend on the loading rate, see, e.g., [ 30 , 31 ] for the high-dynamic strength gain in compression and in tension, respectively. Models accounting for the inertial confinement consider that triaxial stress states develop even under high-dynamic uniaxial loading. Under high-dynamic uniaxial compression, triaxial compressive stress states are envisaged. Triaxial compression increases the strength of cementitious materials relative to their uniaxial compressive strength. This is sufficient for describing the high-dynamic strength gain, see, e.g., [ 32 ]. Under high-dynamic uniaxial tension, in turn, triaxial tensile stress states are envisaged. Triaxial tension decreases the strength of cementitious materials relative to their uniaxial tensile strength. This leads to a high-dynamic strength loss , see, e.g., [ 33 ]. In order to overcompensate this effect, such that the experimentally observed strength gain can be modeled, rate-dependent material properties are introduced, see, e.g., [ 34 ]. The discussed models have in common that some of the input parameters cannot be predicted. Thus, they must be identified such that test data are reproduced in the best-possible fashion. This provided the motivation for fundamental research on the present topic. It is aimed at developing better formulae, characterized by reducing the number of fitted parameters to a minimum.

An engineering-mechanics model for the high-dynamic strength was developed by Fischer et al. [ 6 ] for compression tests and extended by Binder et al. [ 8 ] for tension tests. The elasto-brittle model is based on the following hypotheses:

Also for high-dynamic loading, cracking will start when the quasi-static strength of the material is reached.

Cracks are assumed to propagate at a speed that is virtually equal to the velocity of shear waves.

High-dynamic strengthening occurs during the failure process, lasting from the start of crack propagation to disintegration of the specimen.

The model of Fischer et al. [ 6 ] for high-dynamic compressive strength is based on the following processes. The first crack nucleates at the point where the minimum of the strength prevails. The crack starts to propagate in the loading direction, see Fig.  9 a. Because the material on both sides of the crack remains intact, the axial normal stresses inside the specimen can further increase. This results in additional cracks, nucleating at other points inside the specimen. They also propagate predominantly in the direction of loading. The specimen disintegrates when the first crack has propagated through the entire specimen.

The conceptual simplicity of the described model allowed for deriving the following analytical expression for the DIF [ 6 ]:

where E denotes Young’s modulus, \(\dot{\varepsilon }\) the strain rate, \(f_{c}\) the quasi-static uniaxial compressive strength, \(\ell \) the length of crack propagation, \(\mu \) the shear modulus, and \(\varrho \) the mass density. Thus, \(E\,\dot{\varepsilon }\) represents the stress rate, \(\sqrt{\mu /\varrho }\) denotes the speed of shear waves, which is virtually equal to the speed of crack propagation [ 6 ], and \(\ell /\sqrt{\mu /\varrho }\) stands for the duration of the failure process, lasting from the start of crack propagation to disintegration of the specimen.

figure 9

Predominant direction of the crack propagation in a  a high-dynamic compression test, and b  a high-dynamic tension test

The input quantities E , \(f_{c}\) , \(\mu \) , and \(\varrho \) can be quantified by means of standard testing methods. The quantity \(\ell \) , however, is associated with a degree of uncertainty, because the position at which the first crack nucleates cannot be predicted. Still, bounds for \(\ell \) can be derived as follows [ 6 ]:

If the first crack nucleates at one of the interfaces between the specimen and the load plates, the propagating crack edge has to travel along the total axial length h of the specimen. Thus, the upper bound for \(\ell \) is equal to h , see Fig.  10 a.

If the crack nucleates at the center of the specimen, two crack edges are propagating simultaneously. Each of them has to cover a distance equal to h /2. Thus, the lower bound for \(\ell \) is equal to h /2, see Fig.  10 b.

Consequently, realistic values of \(\ell \) must be located in the following interval:

figure 10

Bounds of the length of crack propagation, \(\ell \) , in a compression test: a  upper bound, and b  lower bound

A check was made whether or not high-dynamic strength values from complete testing campaigns can be reproduced, based on Eq. ( 15 ) and on one specific value of \(\ell \) that satisfies ( 16 ). In more detail, \(\ell \) was identified for each test series by minimizing the difference \(\mathcal {E}\) between the modeled DIF values according to Eq. ( 15 ) and the experimentally determined \(\text {DIF}^{\mathrm{exp}}\) values,

where n denotes the number of high-dynamic tests included in the analyzed test series. The corresponding analyses focused on results from three testing campaigns of cylinders, made of cement paste, mortar, and concrete, respectively, illustrated as points in Fig.  8 a. These three investigations have in common that the input quantities E , \(f_{c}\) , \(\mu \) , and \(\varrho \) were provided by the experimenters. They enter Eq. ( 15 ) as known input. The crack propagation length \(\ell \) was optimized according to Eq. ( 17 ). Based on \(\ell = 0.59\,h\) , the high-dynamic strength values of the specimens made of cement paste could be reproduced, see [ 6 ] and Fig.  8 a. The high-dynamic strength values of the specimens made of mortar could be reproduced for \(\ell = 0.74\,h\) , see [ 3 ] and Fig.  8 a. Photographs showing the destroyed concrete cylinders after high-dynamic testing, see Fig.  11 , taken from [ 27 ], suggest that \(\ell = 0.50\,h\) . Based on this value, the corresponding high-dynamic strength tests could be reproduced, see [ 7 ] and Fig.  8 a.

figure 11

Photo showing fragments of specimens after high-dynamic testing by Kühn et al. [ 27 ]

An engineering-mechanics model for the high-dynamic tensile strength was developed by Binder et al. [ 8 ]. It represents a continuation of the previously described line of research. The elasto-brittle model is based on the theory of wave propagation through isotropic, elastic media. The following processes were envisaged to occur during a high-dynamic tension test. The first crack nucleates inside the specific cross-section of the specimen, where the effective stress \(\sigma _\text {eff}\) according to Eq. ( 13 ) reaches the quasi-static uniaxial tensile strength first. The crack starts propagating inside this cross-section, orthogonal to the loading direction, see Fig.  9 b. The speed of crack propagation is again virtually equal to the speed of shear waves. Stress pulses, striking the propagating crack, are reflected. Stress pulses that do not strike the crack pass through the intact part of the cross-section containing the crack. The high-dynamic tensile strength of the tested specimen is equal to the tensile stress, transmitted across the last bridge of material, right before the specimen disintegrates into two pieces. In order to assess the model, Binder et al. [ 8 ] have used it for the analysis of the high-dynamic tensile strength values of mortar and concrete cylinders illustrated as points in Fig  8 b. The test results could be reproduced, see the solid lines in Fig.  8 b.

The described elasto-brittle modeling approach allowed for explaining high-dynamic compression and tension tests. It was corroborated in the framework of five different testing campaigns, carried out in five different laboratories by five different teams of experimenters, using cylindrical specimens made of cement paste, mortar, and concrete.

3.2 Implications for civil engineering design

The design of dynamically loaded structures is a challenging task. Main difficulties are the specification of the design loads and the investigation of the load-carrying behavior. Concerning the latter, practitioners prefer static to dynamic simulations. In order to account for the dynamic nature of the underlying problem, static analyses are based on increased loads and increased values of material strength. The latter are obtained by multiplying the quasi-static strength with DIF values depending on the loading rate. DIF formulae, published in the Model Code 2010 [ 35 ], enjoy great popularity, because this Model Code is the official pre-standard of the International Federation for Structural Concrete (fib).

Recent fundamental research has led to a significantly better insight into high-dynamic strength tests of specimens of cementitious materials. Also under high-dynamic loading, cracking will start when the quasi-static strength is reached. This implies that a structure will be damaged if the dynamic stress exceeds the quasi-static strength, no matter how fast the stress is increased, and no matter how short the stress pulse lasts. In addition, the five examples illustrated in Fig.  8 underline that DIF values obtained from testing with Split Hopkinson Pressure Bars depend on the size of the tested specimens. Thus, DIF values are structural properties of the tested specimens rather than material properties.

Based on the improved understanding of high-dynamic strength tests, quasi-static design approaches for dynamic problems can be scrutinized as follows. Quasi-static design calculations are to be based on the quasi-static strength of cementitious materials. The calculations are realistic provided that the stresses remain smaller than the strength. If the calculated stresses reach the quasi-static strength, the analyzed structure will be damaged at least locally. In order to gain realistic insight into the structural damage, dynamic analysis appears to be indispensable. Footnote 1 Thereby, it is important to explicitly account for a realistic speed of crack propagation. In addition, reliable simulations require realistic crack patterns. This raises the need for advanced models, capable of predicting realistic directions of crack propagation. As regards the material stiffness and strength, in turn, it appears to be realistic to use rate-independent properties. The latter are accessible by means of standard quasi-static material testing.

4 Hybrid and classical analysis of segmental tunnel rings

The linings of tunnels, excavated by boring machines, consist of segmental rings. The individual rings are assembled by precast segments. Hence, these rings contain segment-to-segment interfacial joints. The mechanical behavior of the joints is nontrivial, because of (i) nonlinearities resulting from bending-induced partial segment-from-segment separation, (ii) nonlinearities in consequence of the load-dependent material behavior of concrete and of the existence of steel bolts connecting neighboring segments, and (iii) the time-dependent viscoelastic material behavior of concrete. This renders structural analysis of segmental tunnel rings challenging.

Early analytical models for structural analysis of segmental tunnel rings were restricted to closed rings, without explicit consideration of the joints, see, e.g., [ 36 , 37 ]. In order to improve these simplistic models, “correction factors” of their stiffness were provided. This enabled analytical structural investigations with “equivalent” continuous tunnel rings, see [ 38 ]. Because of its simplicity, this has become the prevailing model for design-oriented structural analysis of segmental tunnel rings.

Lee et al. [ 39 ] are pioneers who developed analytical models for structural analysis of segmental tunnel rings, with explicit consideration of the joints. They assumed that the relative rotation angles at the joints were linear functions of the internal forces transmitted across the joints. These functions were established by means of the unit force method for determination of the internal forces and the displacements resulting from both the external loading and the relative rotation angles. Blom [ 40 ] as well as El Naggar and Hinchberger [ 41 ] simplified this mode of analysis. They argued that the relative rotation angles at the joints result in rigid-body displacements of the segments. This approach allows for a reliable determination of Serviceability Limit States of segmental tunnel rings subjected to imperceptibly anisotropic ground pressure, associated with coefficients of lateral ground pressure around 0.9, see, e.g., Blom [ 40 ].

A series of real-scale bearing capacity tests of segmental tunnel rings, see, e.g., [ 42 , 43 ] and Fig.  12 , have provided experimental evidence that Ultimate Limit States of such tunnel rings are associated with the bearing capacity of the joints. State-of-the-art models of the joints, see, e.g., [ 44 ], suggest that the relative rotation angles increase nonlinearly with increasing loading. This provided the motivation for fundamental research, focusing on the development of methods for structural analysis of segmental tunnel rings up to their bearing capacity. These methods account for the nonlinearities resulting from (i) bending-induced cracking of the segments, (ii) segment-from-segment separation, and (iii) the nonlinear behavior of the materials of the joints.

figure 12

a Photograph and b geometric dimensions of a segmental tunnel ring, tested at Tongji University, according to [ 43 ] and [ 9 ], respectively

4.1 Results from recent fundamental research

Two types of structural analysis of segmental tunnel rings are discussed in the following. The first type refers to the hybrid analysis of a real-scale test in which a segmental tunnel ring was subjected to 24 point loads. The analysis is hybrid because the external loading and the measured relative rotations at the segment-to-segment interfaces are used as input. The second type of structural analysis refers to a classical analysis of a segmental tunnel ring subjected to ground pressure. The analysis is classical because the external loading is used as input, whereas the relative rotations at the segment-to-segment interfaces are computed as functions of the bending moments and normal forces transmitted across these interfaces. Both types of analysis are based on transfer relations, discussed next.

Analytical methods for structural analysis of segmental tunnels, based on transfer relations, were developed by Zhang et al. [ 9 ]. These relations represent analytical solutions of the linear theory of circular arches. The vector of the state variables at an arbitrary cross-section, defined by the angular coordinate \(\varphi \) , is obtained by multiplying the so-called transfer matrix by the vector of the state variables at the initial cross-section (index “ i ”), i.e., at \(\varphi _i = 0\) . The transfer relations read as [ 9 ]

research paper on concrete technology pdf

where u and v stand for the radial and tangential component, respectively, of the displacement of the axis of the ring; \(\theta \) represents the cross-sectional rotation; M , N , and V denote the bending moment, the axial force, and the shear force, respectively; The mathematical expressions for the nonzero elements of the transfer matrix read as [ 9 ]

The top six elements in the last column of the transfer matrix contain so-called load integrals. They represent analytical solutions for dead load [ 9 ], ground pressure [ 45 ], uniform temperature changes [ 46 ], point loads [ 9 ], and discontinuities of the kinematic variables at the joints [ 9 ]. These relations serve as the vehicle for structural analysis of segmental tunnel rings.

A specific mode of such analysis is a hybrid mode. In the given context, it refers to re-analysis of a real-scale bearing capacity test, carried out at Tongji University, see Fig.  12 . The ring was subjected to anisotropic loading, imposed by 24 hydraulic jacks. The vertical and horizontal convergences and the relative rotation angles at the joints were measured. Hybrid structural analysis of the tested ring is based on two types of input: prescribed point loads and measured relative rotation angles at the joints. The proposed approach for such an analysis follows the scientific work by Blom [ 40 ] and by El Naggar and Hinchberger [ 41 ]. They assumed that the relative rotation angles at the joints result in rigid-body displacements of the segments. This allows for subdividing the hybrid analysis into two load cases [ 10 ].

Load case I refers to the point loads. The relative rotation angles are set equal to zero. Thus, the corresponding structural analysis is one of a closed ring without joints, subjected to point loads. In this part of the hybrid analysis, bending-induced tensile cracking of the segments is considered. The segments are subdivided into elements, the characteristic size of which is equal to the distance of neighboring cracks. The elements contain a central crack band and two adjacent undamaged zones, see Fig.  13 . The state of damage in the crack bands is determined by means of a multiscale model for tensile softening of concrete [ 47 ]. Subsequently, the effective bending and extensional stiffnesses of the damaged element are quantified, using the Voigt-Reuss-Hill estimate. The effective stiffnesses of the elements are used for simulations of the segmental tunnel ring, with element-wise constant values of effective bending and extensional stiffnesses. The results from analysis of load case I are the final results from nonlinear hybrid analysis of the segmental tunnel ring, as far as the internal forces and the state of damage of both the segments and the joints are concerned. Figure  14 shows the distributions of the internal forces and of cracks with different depths, associated with the bearing capacity.

figure 13

Characteristic element between two cracks for the hybrid analysis with a central crack band between undamaged zones

Load case II refers to the relative rotation angles at the joints. They are estimated from monitoring data, recorded during the test. The estimation is based on the Bernoulli-Euler hypothesis. Since the validity of this hypothesis is questionable for neck-like joints, the estimated relative rotation angles are post-processed such as to refer to rigid-body displacements of the segments. This includes symmetrization and adding the smallest possible increments such that rigid-body displacements are obtained, see [ 10 ] for details. Finally, the two load cases are superimposed. This is admissible, even though the analysis of load case I is nonlinear, because superposition of load case II only adds rigid-body displacements, and the equilibrium of the structure is formulated in the undeformed configuration [ 10 ].

The simulated convergences agree well with the experimental results, see Fig.  15 . This underlines the usefulness of the chosen simulation strategy. Analyzing the contributions of the two load cases to the overall convergences reveals that rigid-body displacements of the segments account for approximately 95% of the convergences, whereas the deformations of the segments only account for the remaining 5%. Thus, the relative rotation angles at the joints govern the convergences of the analyzed segmental tunnel ring. Because cracking of the segments does not contribute significantly to the convergences, load case I could have been based on the assumption of linear-elastic behavior of the segments.

figure 14

Analysis results: distributions of a  bending moments, b  axial forces, and c  shear forces; d  depths of cracks of segments, associated with the bearing capacity [ 10 ]

figure 15

Comparison of the convergences obtained from simulation and measurements: a  vertical convergence; b  horizontal convergence [ 10 ]

In general, measured relative rotation angles at the segment-to-segment interfaces are not available. Thus, the described hybrid method of analysis of segmental tunnels is not applicable. As a remedy, one needs to resort to a classical mode of analysis of such tunnels. To this end, linear transfer relations are combined with a nonlinear interface law. The latter allows for computing the relative rotation angles at segment-to-segment interfaces as a function of the bending moments and the normal forces transmitted across these interfaces. The used interface law accounts for (i) bending-induced partial segment-from-segment separation, (ii) compressive crushing of the concrete, and (iii) tensile yielding of the steel bolts connecting neighboring segments. Both the developed interface model and the corresponding method for structural analysis of segmental tunnel rings were validated by comparing computed results with measurements from independent bearing capacity tests of bolted joints and of a segmental tunnel ring, see [ 45 ].

In the following, two segmental tunnel rings subjected to ground pressure (Fig.  16 ) are analyzed in a classical fashion, in order to compute serviceability limit states and bearing capacities. The difference between the two analyzed structures is that the first ring contains unreinforced joints, whereas the second ring contains bolts connecting neighboring segments across the joints. Herein, serviceability and ultimate limit states are defined as follows. The serviceability limit will be reached, if either the maximum stress of concrete reaches the compressive strength, or the steel of a bolt starts to yield, or the bending-induced separation of an unreinforced joint extends across more than half of the initial contact area putting the position stability of the interfaces at risk [ 11 ], or if two of these criteria are fulfilled simultaneously. The bearing capacity will be reached, if the structure develops a kinematic mechanism. As regards the investigated symmetric tunnel rings consisting of six segments, subjected to symmetric ground pressure, this mechanism is associated with the development of plastic hinges at two pairs of interfaces [ 45 ].

figure 16

Illustration of a segmental tunnel ring subjected to ground pressure [ 45 ]

A sensitivity analysis with respect to the coefficient of lateral ground pressure is performed in the interval

see Fig.  17 . Results obtained for the ring with unreinforced interfaces are discussed first, see Fig.  17 a . In this case, K must be larger than 0.54. Otherwise the transfer of compressive forces across the unreinforced interfaces of the segments is not sufficient to provide the normal force and the bending moment required for overall structural equilibrium, i.e., the structure is kinematically independent of the intensity of the ground pressure. In the interval \(K \in [0.54; 0.72]\) , bending-induced separation at some of the interfaces extends across more than half of the initial contact area, again independent of the intensity of the ground pressure. Thus, serviceability of the structure is only achieved in case of \(K>0.72\) , see Fig.  17 a. With increasing value of K , both the serviceability limit and the bearing capacity increase. The discussion continues with the results obtained for the ring with bolted interfaces, see Fig.  17 b. Interfacial bolts result in significant structural advantages for \(K<0.72\) , because the bolts ensure the position stability of the joints, compare Fig.  17 a, b. As for the analyzed ring, interfacial bolts are strongly recommended for \(K < 0.72\) , and they are required for \(K<0.54\) . The latter case refers, e.g., to usual soft ground conditions.

figure 17

Comparison of intensities of the ground pressure related to the elastic limit and to the bearing capacity of a segmental tunnel ring with a  unreinforced interfaces and b  bolted interfaces [ 45 ]

4.2 Implications for civil engineering design

In order to assess the safety of segmental tunnel linings in service, it is highly desirable to estimate their internal forces and the external ground pressure acting on their inaccessible outer surface. Typically, the convergences of the linings are known. They serve as input for this assessment. The reported hybrid analysis has shown that the convergences of the investigated segmental tunnel ring are governed by rigid-body displacements of the segments. Hence, there is no unique solution for the internal forces and the external ground pressure, if only the convergences are known. In this case, it is impossible to estimate the loading of segmental tunnel linings.

In tunnel engineering, serviceability limit states (SLS) and ultimate limit states (ULS) of segmental tunnels are typically related to the convergences. If an SLS is surpassed, the convergences may have a negative effect on the traffic in the tunnel, if an ULS is surpassed, they do have such a negative effect, because the clear diameter of the cross-section is reduced beyond a tolerable limit. The described fundamental research has shown that both the convergences and the bearing capacity of segmental tunnel rings are governed by the mechanical behavior of the joints. Hence, it is indispensable to account for their behavior. As regards the prediction of the convergences and the bearing capacity of segmental tunnel rings, the mechanical behavior of the reinforced concrete segments is less important, and the segments may be modeled as linear-elastic. However, consideration of cracking of the segments is of great importance when it comes to the durability assessment of segmental tunnel rings.

The reported fundamental research has clarified the role of the interfacial bolts in the structural behavior of segmental tunnel rings. Such bolts result in a significant increase in the serviceability limit of segmental tunnel rings and in the bearing capacity of such rings in case of markedly anisotropic external loading.

5 Serviceability and ultimate limit states of reinforced concrete hinges

Bridges and tunnels must reach the end of their estimated service life in order to ensure the sustainability of public investments into traffic infrastructure. The long-term durability of reinforced concrete bridges is strongly affected by the performance of structural hinges. They must be durable, because their repair or replacement frequently represents a great challenge. Moreover, the associated temporary loss of serviceability may result in large costs.

Integral bridge construction is an interesting approach. The idea is to build monolithic structures, where beams, columns, and the abutments are connected by means of so-called concrete hinges. They are either unreinforced or just marginally reinforced necks in reinforced concrete bridges. Proposed by Freyssinet in the 1920s, they became quite popular in Europe in the 1960s. Pioneering design guidelines were developed by Leonhardt and Reimann in 1965, see [ 48 ]. However, concrete hinges have lost their popularity at the end of the 1960s, because their long-term behavior was unclear. Nowadays, many existing integral bridges provide evidence that concrete hinges are indeed very durable structural elements. Their regained popularity does not only call for a better scientific understanding of their structural performance, but also for modern design guidelines for verification of serviceability and ultimate limit states in the framework of the semi-probabilistic design concept.

Development of such guidelines and their application to a recently built integral bridge in Austria are the topics of this part of the present work. Figure  18 [ 49 ] shows one half of the longitudinal section of the bridge. Figure  19 illustrates a vertical section of one of the concrete hinges of the integral bridge shown in Fig.  18 . It contains the reinforcement crossing the neck [ 50 ].

figure 18

One half of the longitudinal section of a recently built integral bridge in Austria [ 49 ]

figure 19

Vertical section of a concrete hinge of the integral bridge [ 50 ] shown in Fig.  18

In the past, the structural performance of reinforced concrete hinges was, in general, investigated experimentally, using a test protocol consisting of two steps. At first, a specific compressive normal force was applied and kept constant thereafter. This was followed by imposing a relative rotation, which was also kept constant thereafter. The first type of loading resulted in creep of concrete and the second one in stress relaxation. This form of viscoelastic behavior of concrete provided the motivation to carry out eccentric compression tests and to perform Finite Element (FE) simulations with state-of-the-art software, in order to gain detailed insight into the structural behavior of reinforced concrete hinges. Because the aforementioned design guidelines of Leonhardt and Reimann do not account explicitly for reinforcement bars that cross the neck centrically, an extension of these guidelines was needed.

5.1 Results from recent fundamental research

Combined experimental and theoretical research was carried out in order to study the structural behavior of reinforced concrete hinges [ 51 , 52 ]. The experiments included material tests of plain concrete and structural tests of three nominally identical reinforced concrete hinges. The latter were designed according to the guidelines of Leonhardt and Reimann [ 48 ]. Several types of tests were carried out. First tests were carried out using loads representative for regular service. Finally, the bearing capacity of the specimens was determined in eccentric compression tests, see [ 51 ] and Fig.  20 . The measurement equipment consisted of three components. The load cell, integrated into the testing machine, recorded the normal force N . Multiplication by the eccentricity e delivered the bending moment \(M = N\cdot e\) . Linear Variable Displacement Transducers (LVTD) were mounted to the lateral surfaces of the specimens in order to measure the shortening, \(\varDelta \ell \) , and the relative rotation, \(\varDelta \varphi \) , of the neck, see Fig.  20 . A Digital Image Correlation system was used to observe bending-induced tensile cracking along the roots of the front-side and the back-side notches.

figure 20

a  Schematic illustration of the tested concrete hinges, and b  comparison of measurements from eccentric compression tests with numerical results from three-dimensional FE simulations, see [ 51 , 52 ]

The test results for regular service loads have shown that the magnitude of material creep of concrete is similar to the one of structural creep of reinforced concrete hinges under centric compression. Structural creep under eccentric compression, however, has turned out to be significantly larger. This is a consequence of the strong interaction between this form of creep and cracking [ 51 ]. Bending-induced macrocracks progressively open and propagate under sustained loading. Vice versa, it was concluded that stress relaxation results in progressive crack closure. This is beneficial to the long-term durability of concrete hinges [ 51 ].

As for the bearing capacity tests, the eccentricity of the normal force was set equal to one-third of the width of the neck. According to the guidelines of Leonhardt and Reimann, for this eccentricity, bending-induced tensile macrocracking will extend across one half of the initial cross-section of the neck. Along the remaining compressed ligament, the guidelines suggest a triangular stress distribution, with a vanishing normal stress at the centerline of the neck and the maximum compressive stress at its outer edge. Following this conceptual approach and setting the maximum compressive normal stress equal to the measured uniaxial compressive strength of concrete, i.e., to \(f_c=46.88\,\mathrm {MPa}\) , the ultimate load is expected to amount to \(264\,\mathrm {kN}\) . This is significantly smaller than the measured bearing capacities, amounting to \(699\pm 49\,\mathrm {kN}\) . Furthermore, the tested reinforced concrete hinges failed in a pronounced ductile fashion, see Fig.  20 b. This was the motivation for a detailed numerical analysis of the bearing capacity tests. They were simulated using the state-of-the-art FE software "Atena science” and a nonlinear material model for concrete implemented therein [ 52 ]. Numerical simulations were carried out in order to gain quantitative insight into the stress states activated in the neck region and to find out why reinforced concrete hinges fail in such a ductile fashion.

Results from the FE simulations have shown that triaxial compressive stress states are activated in the region of the neck [ 52 ]. Figure  21 a shows a cross-section of the 3D FE discretization. Figure  21 b illustrates the distribution of the normal stress in the loading direction, along the width of the neck, for different values of external loading up to 35 kN/cm. Figure  21 c, d shows analogous plots of distributions of the normal stress in the thickness direction and in the lateral direction, respectively. The ratio of characteristic values of the compressive normal stresses in the three directions is 1.00 : 0.45 : 0.30, see Fig.  21 and [ 52 ]. Thus, concrete is strongly confined in the neck region. Because of the confinement pressure, both the strength and the ductility of the material are markedly larger than for uniaxial compression.

figure 21

a Cross-section of the FE model of the concrete hinge; distribution of the normal stresses in b  the loading direction, c  the thickness direction, and d  the lateral direction, for different values of external loading [ 52 ]

There is yet another important mechanism that enables concrete hinges to sustain large ultimate loads and to fail in a very ductile fashion. To show this, stress states at the surface of the notch, on the compressed side of the throat, are investigated. Because it is a free surface, a plane stress state prevails. A biaxial compressive stress state is the only possible stress state. The biaxial compressive strength is only slightly larger than the uniaxial compressive strength. This implies that the concrete located at the surface of the notch, on the compressed side of the throat, starts to fail rather early in a bearing capacity test of a concrete hinge. Despite this local material failure, the surface layer stays in place and, thus, does not spall. This enables the building-up of triaxial stress states inside the concrete hinges [ 52 ]. Thus, the ductile failure of concrete under biaxial compression at the surface of the neck root is the mechanism that enables concrete hinges to fail in the experimentally observed ductile fashion.

The performed numerical simulations are a good example for modern multiscale structural analysis. The first three-dimensional FE simulations were based on default input values, derived from the known Young’s modulus and the compressive strength of the concrete. The obtained numerical results overestimated both the initial stiffness and the bearing capacity of the tested concrete hinges. This confirmed one of Leonhard’s expectations that the steel rebars constrain the free autogeneous shrinkage of concrete. It promotes shrinkage-induced damage of concrete. In a classical setting, this would have called for fitting of Young’s modulus, E , of the tensile strength, \(f_t\) , and of the fracture energy, \(G_f\) , of concrete, such that the simulation outputs agree, in the best-possible fashion, with experimental data. Based on a multiscale model for the tensile strength and for softening of concrete [ 47 ], it was possible to reduce the number of fitted parameters from three to one. This model provides quantitative relationships between one damage variable, on the one hand, and E , \(f_t\) , and \(G_f\) , on the other hand. Pre-existing damage was identified in the context of correlated structural sensitivity analyses. The simulated initial stiffness agreed well with the experimental data. In order to adequately simulate the entire bearing capacity test, see Fig.  20 b, the triaxial compressive strength of concrete had to be reduced from the default value proposed by the Menétrey-Willam failure criterion implemented in the state-of-the-art FE software "Atena science.” The reduced value turned out to be consistent with the regulations regarding partially loaded areas, taken from the Eurocode 2 [ 53 ]. Thus, it was concluded that the triaxial strength of concrete can be estimated reliably based on Eurocode-regulations regarding partially loaded areas [ 52 ].

Combined experimental-theoretical research has resulted in increased scientific understanding of the structural performance of reinforced concrete hinges. This provided the motivation to develop design guidelines for reinforced concrete hinges. An important aspect of these guidelines is the specification of the limit of tolerable relative rotations as a function of the compressive normal force transmitted across the neck. This is required for both serviceability and ultimate limit states of reinforced concrete hinges.

An engineering-mechanics model was the basis for the development of new design guidelines for reinforced concrete hinges in integral bridge construction. The Bernoulli–Euler hypothesis was used to describe displacement and strain states in the region of the neck. It was combined with linear-elastic and ideally plastic stress–strain relationships for concrete in compression and steel in tension. The triaxial-to-uniaxial compressive strength factor, F , was estimated on the basis of regulations from the Eurocode 2 [ 53 ], see above. The tensile strength of concrete was set equal to zero. The steel reinforcement was disregarded, if subjected to compression.

Besides application of the described model to integral bridge construction, it was also used for the analysis of bearing capacity tests of segmental tunnel rings. Moreover, it was applied to quantification of the behavior of segment-to-segment interfaces. These interfaces are bolted concrete hinges, see Sect.  4 and [ 10 ].

As for integral bridge construction, analytical formulae for verification of serviceability and ultimate limits of reinforced concrete hinges were derived [ 11 , 12 ]. The serviceability limit states indicate that the maximum compressive normal stress of concrete has reached the triaxial compressive strength and/or that the steel rebars have started to yield. A detailed analysis has led to consideration of four different operating conditions of reinforced concrete hinges. The obtained analytical formulae provide serviceability limits of tolerable relative rotations, \(\varDelta \varphi _\ell \) , as a function of the degree of utilization regarding the normal force transmitted across the hinge,

where \(Ff_{c}\) denotes the triaxial compressive strength of concrete and ab represents the cross-sectional area of the neck, see also Fig.  21 a and the serviceability limit envelope (SLE) in Fig.  22 a. Denoting Young’s modulus of concrete as \(E_{c}\) , Young’s modulus of steel as \(E_s\) , the reinforcement ratio as \(\rho \) , and the yield stress of steel as \(f_y\) , the SLE is based on the following analytical formulae. In “compression-dominated operation,” the entire cross-section of the neck is subjected to compressive stresses. The mathematical formulation of the SLE reads as [ 11 ]

In the operating condition “tensile macrocracking up to one half of the width of the neck,” the reinforcement is subjected to compressive stresses. The mathematical formulation of the SLE reads [ 11 ]

figure 22

Analysis of eccentric compression tests of reinforced concrete hinges: a  identification of serviceability and ultimate limits [ 11 , 12 ]; b  experimental data

In the operating condition “tensile macrocracking beyond one half of the width of the neck, resulting in tensile loading of the reinforcement,” the mathematical formulation of the SLE reads [ 11 ]

Finally, in “tension-dominated operation,” the mathematical formulation of the SLE reads [ 11 ]

The ultimate limit states reveal that the maximum compressive normal strain of concrete and/or the maximum tensile normal strain of the steel rebars has reached the corresponding ultimate limit strain. A detailed analysis has led to six different operating conditions of reinforced concrete hinges [ 12 ]. The obtained analytical formulae provide ultimate limits for \(\varDelta \varphi _\ell \) as a function of \(\nu \) , see the ultimate limit envelope (ULE) in Fig.  22 a.

Graphs, illustrating the test data, see Fig.  22 b, were added to the diagrams in Fig.  22 a, containing the SLE and the ULE. Because the experiments were eccentric compression tests, the bending moment M is directly proportional to the normal force N . Thus, the relation between the relative rotation \(\varDelta \varphi \) and M is affine to the relation between \(\varDelta \varphi \) and \(\nu \) , compare Fig.  22 a, b. The points, at which the graphs of the experimental data intersect the ones of the serviceability and ultimate limit envelopes, represent pairs of limit state values, consisting of a specific normal force and a relative rotation, see the circles and squares in Fig.  22 a. Marking these points in the graphs of the experimental data, see Fig.  22 b, allows for identifying serviceability limit states (SLS) and ultimate limit states (ULS) of the tested concrete hinges. Up to the identified serviceability limits, the concrete hinges behave in a moderately nonlinear fashion. Significant nonlinearities occur beyond the serviceability limits, see Fig.  22 b. Beyond the identified ultimate limits, the bending moment can no longer be increased significantly. The ultimate limits are nonetheless conservative, because the relative rotation may be increased in the experiment to significantly larger values, see Fig.  22 b. As mentioned previously, the new design guidelines were successfully applied to a recently built integral bridge in Austria.

5.2 Implications for civil engineering design

Combined experimental-theoretical research has enabled the development of new design guidelines for verification of serviceability and ultimate limit states of reinforced concrete hinges. Fundamental research has served its ultimate purpose to promote progress in civil engineering design. This was achieved in the framework of interdisciplinary research, carried out in the fields of integral bridge construction and mechanized tunneling, respectively.

The new design guidelines explicitly account for the crossing steel rebars which run across the neck. The rebars provide the required position stability in case bending-induced macrocracking extends beyond one half of the width of the neck.

There are interesting similarities and differences regarding serviceability and ultimate limit states of concrete hinges in the construction of integral bridges and of segmental tunnel rings. Serviceability limit states of concrete hinges in integral bridge construction are associated with elastic limits of concrete and/or steel. Serviceability limit states of segmental tunnel rings, in turn, are related to convergences that have grown so large that the traffic running through the tunnel is negatively affected, because the clear diameter of the cross-section is reduced beyond a tolerable limit. In such cases, it is almost certain that several interfaces of segments have already become plastic hinges [ 45 ]. From the viewpoint of integral bridge construction, the development of the first plastic hinge already indicates an ultimate limit state. This underlines that different engineering structures require different definitions of serviceability and ultimate limit states.

6 Summary, discussion, and conclusions

Scientific research, both fundamental and applied, provides answers to open questions. Consequently, state-of-the-art models are to be scrutinized regularly. If possible, they must be improved. In the present paper, results from fundamental research in the field of engineering mechanics of concrete and reinforced concrete structures were presented. Implications on engineering design were discussed.

Recurrent cycles of temperature and relative humidity are important load cases for assessing the long-term durability of concrete and reinforced concrete structures. A multiscale poromechanics model allowed for top-down identification of a formerly unknown material phenomenon. Nanoscopic calcium-silicate-hydrates release water upon heating and take up water upon cooling in a quasi-instantaneous and reversible fashion. The corresponding changes of the internal relative humidity result in changes of the effective pore underpressures. This explains why the thermal expansion of the cement paste is a function of the internal relative humidity and why the thermal expansion in the range of intermediate relative humidities is virtually twice as large as in fully saturated or fully dried states. As for engineering design, the following conclusions are drawn:

In the regime of intermediate internal relative humidities, the thermal expansion of the cement paste is virtually twice as large as the one of customary aggregates. Thus, temperature changes result in considerable self-equilibrated internal stresses.

Concrete infrastructures exposed to the open air undergo daily cycles of internal stresses. The mentioned self-equilibrated internal stresses are the source of a fatigue problem. It puts at risk on the long-term integrity of the microstructure of concrete and, thus, the long-term durability of the material.

Exceptional loading in terms of sudden heating or cooling presents another serious load case. Sudden changes of temperature result in transient heat conduction. This is associated with time-dependent and spatially nonlinear temperature fields. Thus, also the thermal eigenstrains are distributed in a spatially nonlinear fashion. The linear part results in an eigenstrain and an eigencurvature of the midplane of plates or the axis of beams. The nonlinear part is associated with an eigendistortion of the generators of plates or the cross-sections of beams. The eigendistortion is prevented, however, because of Kirchhoff’s normal hypothesis. Thus, spatially nonlinear mechanical strains, i.e., stress-related strains, are activated. When added to the thermal eigenstrains, they yield linear total strains in the thickness direction of plates. This is compatible with Kirchhoff’s normal hypothesis. As for engineering design, the following conclusions are drawn:

When engineering structures are subjected to transient heat conduction, thermal stresses are inevitably activated, no matter whether the structure is supported in a statically determinate or indeterminate fashion.

The thermal eigenstresses are distributed nonlinearly along the generators of plates (or within the cross-section of beams). They neither contribute to the membrane forces (normal forces) nor to the bending moments. Thus, it is impossible to define equivalent linear temperature fields, notwithstanding that this is still a very popular approach, because it is recommended by state-of-the-art guidelines such as [ 5 ].

Exceptional loading in terms of high-dynamic compression or tension is another important problem. Results from fundamental scientific research suggest that cracking starts, also under high-dynamic loading, when the quasi-static strength of the material is reached. Cracks propagate at a speed which is virtually equal to the velocity of shear waves. High-dynamic strengthening occurs during the failure process, lasting from the initiation of crack propagation to disintegration of the specimen. The duration of the failure process depends on the size of the investigated structure. Thus, DIF values, obtained from tests carried out with a Split Hopkinson Pressure Bar, are structural properties of the tested specimens rather than material properties. As for engineering design, the following conclusions are drawn:

Engineering structures made of concrete will be damaged provided that the dynamic stress exceeds the quasi-static strength, no matter how fast the stress is increased and how short the stress pulse lasts. Hence, conventional quasi-static analysis allows for identifying whether or not the structure will be damaged. The extent of the local damage, however, cannot be assessed.

As for quantifying the local damage resulting from high-dynamic loading, it is indispensable to carry out dynamic simulations, to explicitly account for a realistic speed of crack propagation, and to use advanced models that are capable of predicting realistic directions of crack propagation also in case of multiaxial types of loading. This calls for a change of paradigm in civil engineering design.

Hybrid and classical simulation methods are well-suited for structural analysis of real-scale bearing capacity tests of segmental tunnel rings used in mechanized tunneling. Both types of analysis reported in this work were based on transfer relations, representing analytical solutions of the first-order theory of thin circular arches. Externally imposed loading was used as known input. As for the “hybrid” analysis, measured relative rotations at the interfaces between the segments also entered the analysis as known input. As for the “classical” analysis, however, interface laws had to be used in order to predict the relative rotations resulting from the bending moments and the normal forces transmitted across the interfaces between the segments. The simulations provided valuable insight into the structural behavior of segmental tunnel rings. As for engineering design, the following conclusions are drawn:

Tunnel convergences are governed by rigid-body displacements of the tunnel segments. Thus, it is impossible to estimate the loading of segmental tunnel linings, based on measured convergences.

The bearing capacity of segmental tunnel rings is associated with the development of a kinematic mechanism. It is characterized by the development of plastic hinges at four segment-to-segment interfaces. Bolted interfaces, representing reinforced concrete hinges, significantly increase the bearing capacity of segmental tunnel rings subjected to strongly anisotropic external loading.

Reinforced concrete hinges represent necks in reinforced concrete structures. Because of the throat, three-dimensional compressive stress states are activated in the region of the neck. The resulting confinement significantly increases the strength and the ductility of concrete. The triaxial compressive strength can be estimated based on regulations for partially loaded areas. New recommendations were elaborated for verification of serviceability and ultimate limit states of reinforced concrete hinges. Because the reinforcement was explicitly accounted for, the tolerable limits of the relative rotations are larger than those according to the guidelines of Leonhardt and Reimann. As for engineering design, the following conclusions are drawn.

In order to ensure the position stability of unreinforced concrete hinges, bending-induced tensile macrocracking must be limited to one half of the width of the neck. This limitation can be discarded provided that tensile forces in centrically crossing steel rebars can stabilize the concrete hinge, even when tensile macrocracking extends beyond one half of the width of the neck.

Elastic limits of concrete in compression and/or of steel in tension represent serviceability limit states of reinforced concrete hinges. Ultimate limit states refer to the situation that concrete reaches its ultimate limit strain in compression and/or steel attains its ultimate limit strain in tension.

There are interesting similarities as well as differences regarding serviceability and ultimate limit states of concrete hinges in integral bridge construction, on the one hand, and segmental tunnel rings in mechanized tunneling, on the other hand.

Serviceability limit states of concrete hinges in integral bridge construction are associated with elastic limits of concrete and/or steel. Serviceability limit states of segmental tunnel rings, however, are associated with large convergences. This may have a negative effect on the traffic in the tunnel, because the clear diameter of the cross-section may be reduced beyond a tolerable limit. In such cases, it is very likely that several interfaces between the segments have already developed plastic hinges.

From the viewpoint of integral bridge construction, already the development of the first plastic hinge signals an ultimate limit state. This underlines that different types of structures may be associated with different kinds of serviceability and ultimate limit states.

Finally, it is emphasized that the described fundamental research activities were carried out within an Austro-Chinese research project, bringing together researchers from Vienna University of Technology and from Tongji University, in Shanghai. The common language of the engineering sciences and the strong joint motivation to carry out fundamental research have not only led to valuable insight into important phenomena and processes governing the mechanical behavior of concrete and reinforced concrete structures, but also contributed to the advancement of civil engineering design.

Realistic quasi-static calculations cannot be based on “high-dynamic strength values” obtained from multiplying the quasi-static strength with a DIF value, because the DIF value is a structural rather than a material property.

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Acknowledgements

Open access funding provided by Austrian Science Fund (FWF). Financial support by the Austrian Science Fund (FWF), provided within project P 281 31-N32 “Bridging the Gap by Means of Multiscale Structural Analyses”, is gratefully acknowledged. The second and third authors are also indebted to the China Scholarship Council (CSC) for financial support.

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College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai, China

Eva Binder, Hui Wang, Jiao-Long Zhang, Yong Yuan & Herbert A. Mang

Institute for Mechanics of Materials and Structures, TU Wien - Vienna University of Technology, Karlsplatz 13/202, 1040, Vienna, Austria

Eva Binder, Hui Wang, Jiao-Long Zhang, Thomas Schlappal, Herbert A. Mang & Bernhard L. A. Pichler

Department of Transportation Engineering, School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, China

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Binder, E., Wang, H., Zhang, JL. et al. Application-oriented fundamental research on concrete and reinforced concrete structures: selected findings from an Austro-Chinese research project. Acta Mech 231 , 2231–2255 (2020). https://doi.org/10.1007/s00707-020-02639-1

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Research on Concrete Construction Technology and Innovation Management in Civil Construction

Miao Wang 1 , Chunhui Lan 2 and Xiaowei Dai 3

Published under licence by IOP Publishing Ltd IOP Conference Series: Earth and Environmental Science , Volume 692 , 3. Geological Resources and Mining Engineering Citation Miao Wang et al 2021 IOP Conf. Ser.: Earth Environ. Sci. 692 042005 DOI 10.1088/1755-1315/692/4/042005

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1 School of Civil Engineering and Architecture, East China Jiaotong University, Nanchang, Jiangxi, 330000

2 School of Human Settlement and Civil Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049

3 School of Civil Engineering, Taiyuan University of Technology, Taiyuan, Shanxi, 030000

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With the continuous progress of society, the continuous development of economy and the acceleration of urbanization, the construction industry has also developed vigorously. In the civil building foundation structure, it is inseparable from the use of mass concrete. Only by ensuring that the structural strength, crack resistance and bearing capacity of mass concrete meet the requirements, can we improve the quality of civil engineering buildings. The construction technology of mass concrete structure in civil engineering buildings is analyzed. The society has put forward more and more stringent requirements for the construction technology of civil engineering, and civil engineering construction technology is the core technology in engineering construction. In recent years, although civil engineering construction technology has developed rapidly and achieved certain achievements and results, most of the technologies are imported or referenced by western technologies, which are lack of innovation, and some of them are relatively backward, wasting a lot of manpower, material and financial resources, resulting in waste of resources, unqualified construction quality and other phenomena. Therefore, in order to better apply civil engineering and improve its construction effect and quality, it is necessary to innovate the construction technology. In the actual construction process, it is necessary to innovate the civil engineering construction technology, so that the engineering construction industry can improve productivity and meet the needs of the development of social construction. Promote the good development of society. This paper will discuss and study the civil engineering construction technology, in order to improve the building quality and economic benefits of the construction industry, and bring some enlightenment to the innovation of civil engineering construction technology.

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Journal of Advanced Concrete Technology (JACT) is an open access international journal for publishing high quality articles on concrete materials, concrete structures and other related subjects toward the advancement of concrete engineering field. All articles in JACT are free to access and download. Also, there are no charges for submission and publication of the articles. The topics include the followings: Materials: Material properties of concrete, Fresh concrete, Hardened concrete, High performance concrete, New materials, Fiber reinforcement. Structures: Design and construction of RC and PC Structures, Seismic design, Safety against environmental disasters, Failure mechanism and non-linear analysis/modeling, Composite and mixed structures Maintenance and Rehabilitation: Durability and repair, Strengthening/Rehabilitation, LCC for concrete structures, Environment conscious materials. Others: Monitoring, Aesthetics of concrete structures, Other concrete related topics.

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Project Report On 'High-Strength Concrete Mix Design' Using Various Admixtures and Evaluation of Properties

Profile image of Abhal Gudhka

Concrete is a mixture of Portland cement, water, coarse and fine aggregates and sometimes admixtures. Proportioning a concrete mix for a given purpose is the art of obtaining a suitable ratio of various ingredients of concrete with the required properties at the lower cost. The primary difference between high-strength concrete and normal-strength concrete relates to the compressive strength. High strength concrete has compressive strength of up to 100 MPa as against conventional concrete which has compressive strengths of less than 40 MPa. Low watercement ratio is a crucial aspect which can be achieved by using chemical admixtures such as plasticizers. A mineral admixture introduces favourable behaviour with respect to shrinkage and high evolution of heat of hydration and enhances durability. The project revolves around the development of high compressive strength using proper mix of ingredients.

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This study presents an experimental investigation on self-compacting concrete (SCC) with fine aggregate (sand) replacement of a Quarry Dust (QD) (0%, 25%, 50%, 75%, 100%) and addition of mineral admixtures like Fly Ash (FA) and Silica Fume (SF) & chemical admixtures like super plasticizers (SP). After each mix preparation, 45 cubes specimens and 45 cylinders specimens are cast and cured. The specimens are cured in water for 3, 7 & 28 days. The slump, V-funnel and L-Box test are carried out on the fresh SCC and in harden concrete compressive strength and split tensile strength values are determined. Attempts have been made to study the properties of such SCCs and to investigate the suitability of Quarry Dust to be used as partial replacement materials for sand in SCC.

Subramanian Narayanan

This is the first Chapter of the Book released by Oxford University Press, New Delhi, recently. Design of Reinforced Concrete Structures is designed to meet the requirements of undergraduate students of civil and structural engineering. This book will also be an invaluable reference to postgraduate students and practising engineers and researchers. This book provides an extensive coverage of the design of reinforced concrete structures in accordance with the current Indian codes of practice (IS 456:2000 and IS 13920:1993). As some of the Indian code provisions are outdated, the American code (ACI 318:2011) provisions are used wherever necessary. In addition, discussion on earthquake-resistant design and detailing is provided, as about 60 per cent of India falls under moderate to severe earthquake zones. The text is based on the modern limit states design and covers topics such as the properties of concrete, structural forms, and loadings. Besides, discussion on the behaviour of various structural elements such as compression and tension members, beams, slabs, foundations, walls, and joints and design and detailing for flexure, shear, torsion, bond, tension, compression, uniaxial and biaxial bending and interaction of these forces make it an invaluable guide for students and designers. The book also provides appendices on strut-and-tie-method, properties of soils, design aids, and practical tips and thumb rules that add value to the rich content of book. For interested readers, more than 1300 references and numerous web links are provided in the chapters for further study and research. For more info: http://www.oup.co.in/product/higher-education/engineering-computer-science/civil-engineering/8/design-reinforced-concrete-structures-1e/9780198086949

ajer research

High strength self-compacting concrete (HSSCC) is one of the most significant recent advances in concrete technology. Replacing cement with different supplementary cementitious materials has a good impact on the environment according tothe reducing of the generated amount of carbon dioxide. Supplementary materials can be usednot only as a cement replacement but also as a filler to help in reducing the total voids content in concrete. Replacing cement with supplementary cementitiousmaterialwas affected the workability and strength of concrete and modify the microstructure of the matrix. An experimental investigation wascarried out to study the effect of replacing cement with different contents of silica-fume, fly ash and marble powder or with mix of two types of these supplementary materials. The rapid chloride penetration test (RCPT) was conductedas an indicator for concrete permeability and durability. Inaddition, the micro-structural analysis using SEM helped in confirming the compressive strength results.

IAEME Publication

Green concrete is the latest development in the field of construction technology which offers a sustainable and eco-friendly solution as a building material. The cement used in conventional concrete is responsible for releasing high amount of carbon dioxide which is harmful for the environment. The concept of green concrete renders replacement of cement partially or fully by various materials which are either byproducts in the production process of other materials or recycled waste. In this paper we focuses on replacing a different percentage of the cement with pozzolanic materials and also replacing the coarse and fine aggregate with locally volcanic materials to produce an eco-friendly and sustainable concrete. Thus, Four trail mixes were casted for estimating the concrete materials and proportion, also fifteen mixes were casted with some variables . Two types of coarse aggregate were used (dolomite and volcanic rock) to show the effect of volcanic aggregate on concrete properties. Fly Ash was used with 10% replacement of the cement , Volcanic ash was used with ( 20 % to 80%) replacement of the cement , the water cementatious ratio equal 0.3, Super plasticizer (visocrete-3425) was used with constant ratio 1% of the cement. Ordinary Portland cement was used in all mixes with constant cement content equal 500 kg/m3. Slump test were prepared on concrete in its fresh phase, hardened concrete tests (compression strength, bond strength, and bending strength) were prepared to identify the mechanical properties of concrete, the results show that using volcanic ash as a replacement of the cement nearly does not affect the slump of concrete, but on the other hand enhances the mechanical properties of concrete.

Alwis Deva Kirupa

Geopolymer Concrete (GPC) will be of considerable cure to Global Warming related with construction industry since Ordinary Portland Cement (OPC) production produces substantial CO2 emission. Simultaneously, GPC replaces OPC completely or about 80% with industrial waste products like Fly Ash, Ground Granulated Blast Furnace Slag (GGBS), Silica Fume, Rice-Husk Ash, Metakaoline or Red Mud (RM). In this exploration, GPC will be made up of RM and GGBS incorporating hybrid fibres (Steel fibre, Polypropylene fibre and Banana fibre) in various ratio for the four mixes designated as Mix A, Mix B, Mix C and Mix D. Polymerization will be done by Alkaline solution comprised of Sodium Hydroxide and Sodium Silicate. Copper Slag, a waste material will replace natural sand by 50% which makes the concrete further green. Compressive Strength and Flexural strength are to be investigated by casting cube and prism specimens which are proposed to be cured by hot-curing for 24 hours. Control Mix is casted for M30 grade. Mechanical properties are investigated at the age of 7 days and 28 days. Although there are numerous studies that have assessed the suitability of Alkali Activated Slag (AAS) and fly ash based geopolymer as the binder in concrete, only limited research has been conducted on the chloride penetration and carbonation of these concretes –the main causes of degradation of concrete structures in practice. This study provides new insight into the strength development and the durability performance in terms of its stability. Durability studies including Water Absorption, Acid Attack, Sulphate Attack, Carbonation Depth and Alkalinity are made for Mix D (optimum strength). Results reveal that, among all the mixes, Mix D shows best mechanical properties owing to the incorporation of hybrid fibres and reduction of Red Mud. Also GPC illustrated extensive stability in aggressive environments when compared to control Mix. All together, GPC exhibited strength gain of 23% when contacted with 5% Sulphuric Acid and 17% with 5% Sodium Sulphate which expels that, GPC is highly suitable option in aggressive environments.

IJSTE - International Journal of Science Technology and Engineering

Data from more than 40 recent studies on the high strength physical and mechanical properties of High strength self-compacting concrete (HSSCC) have been analyzed and which materials and admixtures is suitable for my project should be thoroughly studied and selected. The results obtained in much of the data is a consequence of the wide range of materials and mixes used for HSSCC, but clear relationships have been compressive strength, tensile and compressive strengths, and elastic moduli sand compressive strength. In limestone quarries, considerable amounts of limestone powders are being produced as byproducts of stone crushers. It is also clear that limestone powder, a common addition to SCC mixes, makes a substantial contribution to strength gain. It is also used in controlling the segregation potential and deformability of fresh self-compacting concrete (SCC). This paper deals with the utilization of alternative materials, such as quarry dust, for HSSCC applications. If the properties of high strength self-compacting concrete (HSSCC) incorporating SiO2 micro and nanoparticles have been studied. It was concluded that foundry silica-dust material could be used in producing economical HSSCC. The analysis has shown that sufficient data have been obtained to give confidence in the general behavior of HSSCC, and future studies need only be focused on specific or confirmatory materials and data for particular applications.

Shazrul Saruji

The percentage amount of Silica as replacement and additional of cement is very essential to determine the highest compressive strength of SCLFC and also lowest drying shrinkage.

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A generative AI reset: Rewiring to turn potential into value in 2024

It’s time for a generative AI (gen AI) reset. The initial enthusiasm and flurry of activity in 2023 is giving way to second thoughts and recalibrations as companies realize that capturing gen AI’s enormous potential value is harder than expected .

With 2024 shaping up to be the year for gen AI to prove its value, companies should keep in mind the hard lessons learned with digital and AI transformations: competitive advantage comes from building organizational and technological capabilities to broadly innovate, deploy, and improve solutions at scale—in effect, rewiring the business  for distributed digital and AI innovation.

About QuantumBlack, AI by McKinsey

QuantumBlack, McKinsey’s AI arm, helps companies transform using the power of technology, technical expertise, and industry experts. With thousands of practitioners at QuantumBlack (data engineers, data scientists, product managers, designers, and software engineers) and McKinsey (industry and domain experts), we are working to solve the world’s most important AI challenges. QuantumBlack Labs is our center of technology development and client innovation, which has been driving cutting-edge advancements and developments in AI through locations across the globe.

Companies looking to score early wins with gen AI should move quickly. But those hoping that gen AI offers a shortcut past the tough—and necessary—organizational surgery are likely to meet with disappointing results. Launching pilots is (relatively) easy; getting pilots to scale and create meaningful value is hard because they require a broad set of changes to the way work actually gets done.

Let’s briefly look at what this has meant for one Pacific region telecommunications company. The company hired a chief data and AI officer with a mandate to “enable the organization to create value with data and AI.” The chief data and AI officer worked with the business to develop the strategic vision and implement the road map for the use cases. After a scan of domains (that is, customer journeys or functions) and use case opportunities across the enterprise, leadership prioritized the home-servicing/maintenance domain to pilot and then scale as part of a larger sequencing of initiatives. They targeted, in particular, the development of a gen AI tool to help dispatchers and service operators better predict the types of calls and parts needed when servicing homes.

Leadership put in place cross-functional product teams with shared objectives and incentives to build the gen AI tool. As part of an effort to upskill the entire enterprise to better work with data and gen AI tools, they also set up a data and AI academy, which the dispatchers and service operators enrolled in as part of their training. To provide the technology and data underpinnings for gen AI, the chief data and AI officer also selected a large language model (LLM) and cloud provider that could meet the needs of the domain as well as serve other parts of the enterprise. The chief data and AI officer also oversaw the implementation of a data architecture so that the clean and reliable data (including service histories and inventory databases) needed to build the gen AI tool could be delivered quickly and responsibly.

Our book Rewired: The McKinsey Guide to Outcompeting in the Age of Digital and AI (Wiley, June 2023) provides a detailed manual on the six capabilities needed to deliver the kind of broad change that harnesses digital and AI technology. In this article, we will explore how to extend each of those capabilities to implement a successful gen AI program at scale. While recognizing that these are still early days and that there is much more to learn, our experience has shown that breaking open the gen AI opportunity requires companies to rewire how they work in the following ways.

Figure out where gen AI copilots can give you a real competitive advantage

The broad excitement around gen AI and its relative ease of use has led to a burst of experimentation across organizations. Most of these initiatives, however, won’t generate a competitive advantage. One bank, for example, bought tens of thousands of GitHub Copilot licenses, but since it didn’t have a clear sense of how to work with the technology, progress was slow. Another unfocused effort we often see is when companies move to incorporate gen AI into their customer service capabilities. Customer service is a commodity capability, not part of the core business, for most companies. While gen AI might help with productivity in such cases, it won’t create a competitive advantage.

To create competitive advantage, companies should first understand the difference between being a “taker” (a user of available tools, often via APIs and subscription services), a “shaper” (an integrator of available models with proprietary data), and a “maker” (a builder of LLMs). For now, the maker approach is too expensive for most companies, so the sweet spot for businesses is implementing a taker model for productivity improvements while building shaper applications for competitive advantage.

Much of gen AI’s near-term value is closely tied to its ability to help people do their current jobs better. In this way, gen AI tools act as copilots that work side by side with an employee, creating an initial block of code that a developer can adapt, for example, or drafting a requisition order for a new part that a maintenance worker in the field can review and submit (see sidebar “Copilot examples across three generative AI archetypes”). This means companies should be focusing on where copilot technology can have the biggest impact on their priority programs.

Copilot examples across three generative AI archetypes

  • “Taker” copilots help real estate customers sift through property options and find the most promising one, write code for a developer, and summarize investor transcripts.
  • “Shaper” copilots provide recommendations to sales reps for upselling customers by connecting generative AI tools to customer relationship management systems, financial systems, and customer behavior histories; create virtual assistants to personalize treatments for patients; and recommend solutions for maintenance workers based on historical data.
  • “Maker” copilots are foundation models that lab scientists at pharmaceutical companies can use to find and test new and better drugs more quickly.

Some industrial companies, for example, have identified maintenance as a critical domain for their business. Reviewing maintenance reports and spending time with workers on the front lines can help determine where a gen AI copilot could make a big difference, such as in identifying issues with equipment failures quickly and early on. A gen AI copilot can also help identify root causes of truck breakdowns and recommend resolutions much more quickly than usual, as well as act as an ongoing source for best practices or standard operating procedures.

The challenge with copilots is figuring out how to generate revenue from increased productivity. In the case of customer service centers, for example, companies can stop recruiting new agents and use attrition to potentially achieve real financial gains. Defining the plans for how to generate revenue from the increased productivity up front, therefore, is crucial to capturing the value.

Upskill the talent you have but be clear about the gen-AI-specific skills you need

By now, most companies have a decent understanding of the technical gen AI skills they need, such as model fine-tuning, vector database administration, prompt engineering, and context engineering. In many cases, these are skills that you can train your existing workforce to develop. Those with existing AI and machine learning (ML) capabilities have a strong head start. Data engineers, for example, can learn multimodal processing and vector database management, MLOps (ML operations) engineers can extend their skills to LLMOps (LLM operations), and data scientists can develop prompt engineering, bias detection, and fine-tuning skills.

A sample of new generative AI skills needed

The following are examples of new skills needed for the successful deployment of generative AI tools:

  • data scientist:
  • prompt engineering
  • in-context learning
  • bias detection
  • pattern identification
  • reinforcement learning from human feedback
  • hyperparameter/large language model fine-tuning; transfer learning
  • data engineer:
  • data wrangling and data warehousing
  • data pipeline construction
  • multimodal processing
  • vector database management

The learning process can take two to three months to get to a decent level of competence because of the complexities in learning what various LLMs can and can’t do and how best to use them. The coders need to gain experience building software, testing, and validating answers, for example. It took one financial-services company three months to train its best data scientists to a high level of competence. While courses and documentation are available—many LLM providers have boot camps for developers—we have found that the most effective way to build capabilities at scale is through apprenticeship, training people to then train others, and building communities of practitioners. Rotating experts through teams to train others, scheduling regular sessions for people to share learnings, and hosting biweekly documentation review sessions are practices that have proven successful in building communities of practitioners (see sidebar “A sample of new generative AI skills needed”).

It’s important to bear in mind that successful gen AI skills are about more than coding proficiency. Our experience in developing our own gen AI platform, Lilli , showed us that the best gen AI technical talent has design skills to uncover where to focus solutions, contextual understanding to ensure the most relevant and high-quality answers are generated, collaboration skills to work well with knowledge experts (to test and validate answers and develop an appropriate curation approach), strong forensic skills to figure out causes of breakdowns (is the issue the data, the interpretation of the user’s intent, the quality of metadata on embeddings, or something else?), and anticipation skills to conceive of and plan for possible outcomes and to put the right kind of tracking into their code. A pure coder who doesn’t intrinsically have these skills may not be as useful a team member.

While current upskilling is largely based on a “learn on the job” approach, we see a rapid market emerging for people who have learned these skills over the past year. That skill growth is moving quickly. GitHub reported that developers were working on gen AI projects “in big numbers,” and that 65,000 public gen AI projects were created on its platform in 2023—a jump of almost 250 percent over the previous year. If your company is just starting its gen AI journey, you could consider hiring two or three senior engineers who have built a gen AI shaper product for their companies. This could greatly accelerate your efforts.

Form a centralized team to establish standards that enable responsible scaling

To ensure that all parts of the business can scale gen AI capabilities, centralizing competencies is a natural first move. The critical focus for this central team will be to develop and put in place protocols and standards to support scale, ensuring that teams can access models while also minimizing risk and containing costs. The team’s work could include, for example, procuring models and prescribing ways to access them, developing standards for data readiness, setting up approved prompt libraries, and allocating resources.

While developing Lilli, our team had its mind on scale when it created an open plug-in architecture and setting standards for how APIs should function and be built.  They developed standardized tooling and infrastructure where teams could securely experiment and access a GPT LLM , a gateway with preapproved APIs that teams could access, and a self-serve developer portal. Our goal is that this approach, over time, can help shift “Lilli as a product” (that a handful of teams use to build specific solutions) to “Lilli as a platform” (that teams across the enterprise can access to build other products).

For teams developing gen AI solutions, squad composition will be similar to AI teams but with data engineers and data scientists with gen AI experience and more contributors from risk management, compliance, and legal functions. The general idea of staffing squads with resources that are federated from the different expertise areas will not change, but the skill composition of a gen-AI-intensive squad will.

Set up the technology architecture to scale

Building a gen AI model is often relatively straightforward, but making it fully operational at scale is a different matter entirely. We’ve seen engineers build a basic chatbot in a week, but releasing a stable, accurate, and compliant version that scales can take four months. That’s why, our experience shows, the actual model costs may be less than 10 to 15 percent of the total costs of the solution.

Building for scale doesn’t mean building a new technology architecture. But it does mean focusing on a few core decisions that simplify and speed up processes without breaking the bank. Three such decisions stand out:

  • Focus on reusing your technology. Reusing code can increase the development speed of gen AI use cases by 30 to 50 percent. One good approach is simply creating a source for approved tools, code, and components. A financial-services company, for example, created a library of production-grade tools, which had been approved by both the security and legal teams, and made them available in a library for teams to use. More important is taking the time to identify and build those capabilities that are common across the most priority use cases. The same financial-services company, for example, identified three components that could be reused for more than 100 identified use cases. By building those first, they were able to generate a significant portion of the code base for all the identified use cases—essentially giving every application a big head start.
  • Focus the architecture on enabling efficient connections between gen AI models and internal systems. For gen AI models to work effectively in the shaper archetype, they need access to a business’s data and applications. Advances in integration and orchestration frameworks have significantly reduced the effort required to make those connections. But laying out what those integrations are and how to enable them is critical to ensure these models work efficiently and to avoid the complexity that creates technical debt  (the “tax” a company pays in terms of time and resources needed to redress existing technology issues). Chief information officers and chief technology officers can define reference architectures and integration standards for their organizations. Key elements should include a model hub, which contains trained and approved models that can be provisioned on demand; standard APIs that act as bridges connecting gen AI models to applications or data; and context management and caching, which speed up processing by providing models with relevant information from enterprise data sources.
  • Build up your testing and quality assurance capabilities. Our own experience building Lilli taught us to prioritize testing over development. Our team invested in not only developing testing protocols for each stage of development but also aligning the entire team so that, for example, it was clear who specifically needed to sign off on each stage of the process. This slowed down initial development but sped up the overall delivery pace and quality by cutting back on errors and the time needed to fix mistakes.

Ensure data quality and focus on unstructured data to fuel your models

The ability of a business to generate and scale value from gen AI models will depend on how well it takes advantage of its own data. As with technology, targeted upgrades to existing data architecture  are needed to maximize the future strategic benefits of gen AI:

  • Be targeted in ramping up your data quality and data augmentation efforts. While data quality has always been an important issue, the scale and scope of data that gen AI models can use—especially unstructured data—has made this issue much more consequential. For this reason, it’s critical to get the data foundations right, from clarifying decision rights to defining clear data processes to establishing taxonomies so models can access the data they need. The companies that do this well tie their data quality and augmentation efforts to the specific AI/gen AI application and use case—you don’t need this data foundation to extend to every corner of the enterprise. This could mean, for example, developing a new data repository for all equipment specifications and reported issues to better support maintenance copilot applications.
  • Understand what value is locked into your unstructured data. Most organizations have traditionally focused their data efforts on structured data (values that can be organized in tables, such as prices and features). But the real value from LLMs comes from their ability to work with unstructured data (for example, PowerPoint slides, videos, and text). Companies can map out which unstructured data sources are most valuable and establish metadata tagging standards so models can process the data and teams can find what they need (tagging is particularly important to help companies remove data from models as well, if necessary). Be creative in thinking about data opportunities. Some companies, for example, are interviewing senior employees as they retire and feeding that captured institutional knowledge into an LLM to help improve their copilot performance.
  • Optimize to lower costs at scale. There is often as much as a tenfold difference between what companies pay for data and what they could be paying if they optimized their data infrastructure and underlying costs. This issue often stems from companies scaling their proofs of concept without optimizing their data approach. Two costs generally stand out. One is storage costs arising from companies uploading terabytes of data into the cloud and wanting that data available 24/7. In practice, companies rarely need more than 10 percent of their data to have that level of availability, and accessing the rest over a 24- or 48-hour period is a much cheaper option. The other costs relate to computation with models that require on-call access to thousands of processors to run. This is especially the case when companies are building their own models (the maker archetype) but also when they are using pretrained models and running them with their own data and use cases (the shaper archetype). Companies could take a close look at how they can optimize computation costs on cloud platforms—for instance, putting some models in a queue to run when processors aren’t being used (such as when Americans go to bed and consumption of computing services like Netflix decreases) is a much cheaper option.

Build trust and reusability to drive adoption and scale

Because many people have concerns about gen AI, the bar on explaining how these tools work is much higher than for most solutions. People who use the tools want to know how they work, not just what they do. So it’s important to invest extra time and money to build trust by ensuring model accuracy and making it easy to check answers.

One insurance company, for example, created a gen AI tool to help manage claims. As part of the tool, it listed all the guardrails that had been put in place, and for each answer provided a link to the sentence or page of the relevant policy documents. The company also used an LLM to generate many variations of the same question to ensure answer consistency. These steps, among others, were critical to helping end users build trust in the tool.

Part of the training for maintenance teams using a gen AI tool should be to help them understand the limitations of models and how best to get the right answers. That includes teaching workers strategies to get to the best answer as fast as possible by starting with broad questions then narrowing them down. This provides the model with more context, and it also helps remove any bias of the people who might think they know the answer already. Having model interfaces that look and feel the same as existing tools also helps users feel less pressured to learn something new each time a new application is introduced.

Getting to scale means that businesses will need to stop building one-off solutions that are hard to use for other similar use cases. One global energy and materials company, for example, has established ease of reuse as a key requirement for all gen AI models, and has found in early iterations that 50 to 60 percent of its components can be reused. This means setting standards for developing gen AI assets (for example, prompts and context) that can be easily reused for other cases.

While many of the risk issues relating to gen AI are evolutions of discussions that were already brewing—for instance, data privacy, security, bias risk, job displacement, and intellectual property protection—gen AI has greatly expanded that risk landscape. Just 21 percent of companies reporting AI adoption say they have established policies governing employees’ use of gen AI technologies.

Similarly, a set of tests for AI/gen AI solutions should be established to demonstrate that data privacy, debiasing, and intellectual property protection are respected. Some organizations, in fact, are proposing to release models accompanied with documentation that details their performance characteristics. Documenting your decisions and rationales can be particularly helpful in conversations with regulators.

In some ways, this article is premature—so much is changing that we’ll likely have a profoundly different understanding of gen AI and its capabilities in a year’s time. But the core truths of finding value and driving change will still apply. How well companies have learned those lessons may largely determine how successful they’ll be in capturing that value.

Eric Lamarre

The authors wish to thank Michael Chui, Juan Couto, Ben Ellencweig, Josh Gartner, Bryce Hall, Holger Harreis, Phil Hudelson, Suzana Iacob, Sid Kamath, Neerav Kingsland, Kitti Lakner, Robert Levin, Matej Macak, Lapo Mori, Alex Peluffo, Aldo Rosales, Erik Roth, Abdul Wahab Shaikh, and Stephen Xu for their contributions to this article.

This article was edited by Barr Seitz, an editorial director in the New York office.

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