Early-age Thermal Cracking Research Articles

In-situ measurements of temperature and strain fields of large-diameter concrete-filled steel tube columns and the refined finite element modeling

Concrete-filled steel tube (CFST) columns are prevalent in transmission towers and arch bridges due to their cost-effectiveness and superior mechanical performance. The growing megastructures lead to large-diameter CFST components, which are exposed to the risk of early-age thermal concrete cracking. In this study, temperature and strain fields of two 2.1 m-dia. CFST columns were monitored in-situ. Within 30 h, the peak temperature and tensile strain at the centroid reached 97.0°C and 400 με, respectively. Meanwhile, temperature fields of eight scaled CFST columns were also monitored and compared. In addition, ultrasonic pulse velocity test was performed to detect the early-age thermal cracking. To predict the temperature and strain evolutions accurately, a refined numerical model is proposed, considering coupled mechanical, hydration and thermal processes. It is revealed that the model could accurately predict the temperature and strain evolutions of CFST columns. Furthermore, experiments and numerical simulations illustrate that thermal cracking would likely be triggered in CFST components with diameter greater than 1.2 m.

Finite element modeling of early-age temperature development of in-situ concrete under variable ambient temperatures

The early-age temperature rises in in-situ concrete, caused by the exothermic hydration of cement, poses a significant risk of thermal cracking in concrete structures. This issue is particularly critical in large-scale structures, where thermal cracking can lead to serious structural integrity issues. Therefore, the accurate prediction of the concrete hydration temperature development is paramount for predicting and mitigating early thermal cracking in concrete. However, one of the main challenges in predicting in-situ concrete temperatures is the significant effect of variable ambient temperature on cement hydration rates and concrete heat dissipation. To address this challenge, this paper presents a finite element modeling (FEM) approach that accurately predicts the early-age temperature development of in-situ concrete by adequately considering the effect of variable ambient temperatures. The model integrates the heat of hydration of the cement, the impact of ambient temperature variations, and the heat transfer through the concrete and insulation mould. The cement hydration rate under real-life ambient temperatures is simulated using an Arrhenius-based method, which mathematically adjusts the isothermal calorimetry curves to reflect the actual conditions. Additionally, the study investigates the inhibitory effect of Ground Granulated Blast-furnace Slag (GGBS) on the early hydration temperature of concrete, with experimental tests and FEM simulations conducted on concrete mixes where up to 70% of the cement was replaced with GGBS. The validity of the FEM model was confirmed through comparison with concrete semi-adiabatic tests, demonstrating that the model provides accurate predictions of the temperature development of in-situ concrete. Importantly, the results indicate that the adjusted isothermal calorimetry curves successfully capture the ambient temperature effect on the cement hydration rate. The results provide valuable insights for the design of concrete structures, particularly in preventing early-age thermal cracking.

Utilizing artificial neural networks to anticipate early-age thermal parameters in concrete piers

Recently, researches have been used Artificial Neural Network (ANN) to predict the early-age thermal cracking of rectangle piers. But ANN has not resulted for different types of concrete piers. This article presents an evaluation of the early-age thermal characteristics of mass concrete piers with four distinct cross-sectional shapes. A finite element (FE) model was employed to estimate the maximum temperature, thermal stress, and cracking potential of the concrete pier at its early age. To investigate the impact of various pier geometries on the thermal cracking potential, different pier geometries were considered. In this study, an ANN model was utilized to predict the maximum temperature and decrease the risk of cracking in mass concrete piers at early age. The database of thermal mass concrete piers used in this study comprises 128 results obtained from the FE model. The results of the analysis indicate that the ANN model can predict early-age thermal parameters, and cracking risk in early-age concrete piers with good accuracy and help to the designer to choose the appropriate size in minimizing cracks on the pier concrete.

Equivalent Convective Heat Transfer Coefficient for Boundary Conditions in Temperature Prediction of Early-Age Concrete Elements Using FD and PSO

Engineering infrastructure includes a large number of concrete structures. The main binder used in concrete is Portland cement, which reacts with water when mixing. During the chemical reaction (cement hydration) heat is released causing non-uniform temperature distribution and subsequent internal stresses that may crack the concrete. Therefore, accurate prediction of concrete temperature evolution in design and control of concrete temperature in construction are crucial tasks for engineers. This paper presents the development of a finite difference (FD) model for predicting thermal behavior of concrete structures during cement hydration. A formwork or insulation layer that covers the outer of the concrete was accounted for in the model. The temperature profiles calculated by the proposed model show close agreement with those measured from a concrete bridge footing and a concrete cube. The Particle Swarm Optimization (PSO) algorithm was adopted to compute an “equivalent convection coefficient” for the convection boundary condition in the simplified model for several common insulation materials. The results show that using the simplified model along with an equivalent convection coefficient can give a significant reduction in the computation time while the accuracy is still maintained. In addition, an equation for estimating an equivalent convection coefficient for a given insulation material was developed for design and practical purposes. The proposed method can be used in the design of concrete structures and control of concrete temperature to prevent early-age thermal cracking thus ensuring the expected durability and service life. From the obtained results, varying equivalent convection values are also recommended to be used for different insulation materials.

Exploring the effect of low-heat cement on early-age thermal cracking resistance of roller-compacted concrete

In this paper, the early-age thermal cracking behavior of roller-compacted concrete prepared with low-heat cement (LHC) was investigated using a concrete temperature stress testing machine. The test results show that under the same restraint degrees, the concrete cracking temperature difference increased at a diminishing rate with the increasing concrete curing age; when loaded at the same curing age, the cracking temperature difference of the concrete increased with the decreasing concrete restraint degree. Under a fully restraint condition (100% restraint degree), the LHC roller-compacted concrete was able to resist cracking under sudden decreases in temperature in the range of 2.96–4.68 °C at curing ages of 3–14 d. A numerical simulation and theoretical derivation were used to verify and extend the analysis of the cracking temperature difference of the LHC roller-compacted concrete under different restraint conditions.

Early age thermal cracking resistance of basalt fiber-reinforced concrete for mass concrete structures under restraint condition

This study investigated the early-age thermal cracking behavior of basalt fiber (BF)-reinforced concrete exposed to rapid temperature changes (0.5 °C/h) at different curing ages of 3, 7, and 14 d, at fiber volume dosages of 0 %, 0.1 %, and 0.5 % using a developed temperature stress testing machine (TSTM). The material properties testing results demonstrated that as the BF dosage increased from 0 % to 0.1 %, and 0.5 %, the greatest reductions in the elastic modulus of BF-reinforced concrete relative to ordinary concrete were 18.9 % and 8.3 %, respectively, and the greatest increases in the axial tensile strength were 5.6 % and 13.5 %, respectively. The thermal stress test results indicated that the thermal cracking risk (cracking temperature difference) of BF-reinforced concrete vary nonlinearly with the increasing basalt fiber dosage. As the BF dosage increased from 0 % to 0.1 % and 0.5 %, the maximum increases in the temperature drop were 42.9 % and 18.3 %, respectively. Additionally, the stress relaxation of concrete subjected to a rapid temperature change appeared gradually after 9 h loading, and the concrete with a fiber dosage of 0.1 % showed the greatest degree of relaxation (i.e., 0.216), which was greater than that of reference concrete by 42.1 %. In regard to the thermal cracking resistance, the optimal BF dosage obtained in the current research was 0.1 % (vol.). The relationship established between the cracking temperature differences and BF dosages can help engineers to effectively assess the risk of surface temperature cracking in harsh environments during the construction period of mass concrete structure.

Assessment of cracking performance in edge restrained RC walls

AbstractExcessive cracking due to restraint of thermal and shrinkage strains is a widespread problem in the concrete construction industry. In design, restraint induced cracking is managed by the provision of reinforcement intended to distribute internal strains in such a way as to control the cracking pattern and limit crack widths. The area of secondary (horizontal) reinforcement required in members such as retaining walls and water tanks is often governed by the need to control early age thermal cracking. This paper presents results from four edge restrained walls tested at Imperial College London and the University of Leeds as part of an Engineering and Physical Sciences Research Council funded project into restraint induced cracking. The paper describes the development of volumetric strain and cracking in the tested walls. The cracking performance is assessed by comparing the restrained strain with the tensile strain capacity of concrete.

Numerical simulation of risk mitigation strategies for early-age thermal cracking and DEF in concrete

Early-age thermal cracking and delayed ettringite formation (DEF) are short-term and long-term major issues to the durability of mass concrete and require a high level of consideration in the development of a suitable concrete mix design in placement and curing strategies, particularly in mixes of high cement content. Where DEF risk proves to be significant, contractors are required to adopt risk mitigation strategies including lowering concrete’s temperature during placement by adding cool water or ice, for example, to the mix, embedding cooling pipes in the cast element and/or sequential placement of the concrete. These strategies can be implemented at different levels, as needed, to minimise the risk of cracking and DEF. Determining the most suitable and cost-efficient risk mitigation strategy, however, together with its implementation, is a complex task that requires consideration of each complex strategy under project-specific conditions. While a significant research effort has been invested to develop an improved understanding of the effectiveness of different early age thermal cracking and DEF risk mitigation strategies, the lack of reliable models capable of simulating and comparing the effectiveness of all scenarios remains elusive and considerably limits the ability of the practitioners in identifying the optimal approach. This paper presents a Multiphysics model developed to predict the effect of early-age thermal cracking and DEF risk mitigation scenarios. The model is validated using four real-life project Case Studies and is demonstrated to provide reliable results. In addition, the paper studies the comparative effectiveness of key risk mitigation strategies and the important parameters influencing their effectiveness.

Evaluation of early-age thermal cracking resistance of high w/b, high volume fly ash (HVFA) concrete using temperature stress testing machine

Fly ash (FA) is an environmentally hazardous industrial solid waste. High FA replacement levels on properties of concrete with low w/b have been widely investigated, however, studies on the thermal cracking behavior of HVFA concrete with high w/b are lacking. This paper investigated the effect of high volume fly ash (HVFA) on mechanical properties and thermal cracking resistance of concrete with a high w/b of 0.45 using temperature stress testing machine (TSTM). An adiabatic curing condition during the temperature rise stage and a rapid cooling during the temperature drop stage were applied on test specimens. The effectiveness of concrete with different FA replacement levels (0%, 20%, 50% and 80%) on the thermal cracking potential of concrete under a restraint degree of 100% has been addressed. Results showed that the mechanical properties of concrete mixture decreased with the increasing FA replacement level; the ratio of creep-to-free strain (i.e., relaxation behavior) of concrete mixture decreased when the FA replacement level reached 20%, and converted to increase for the FA replacement level ranging from 50% to 80%; FA replacement level with 50% showed a desirable effect on the thermal cracking resistance enhancement of high w/b concrete mixture regarding restrained cracking control.

Minimising risk of early-age thermal cracking and delayed ettringite formation in concrete – A hybrid numerical simulation and genetic algorithm mix optimisation approach

Early-age thermal cracking and delayed ettringite formation (DEF) are major durability risks associated with mass concrete casting with high cement content. Among various strategies to mitigate the risk of early-age thermal cracking and DEF, concrete mix optimisation is favoured by the industry due to its minimal required operational adjustments and insignificant implementation cost. The existing concrete mix design optimization methods are, however, incapable of accounting for the risk of early-age thermal cracking and DEF; and have been designed solely to meet the mechanical performance targets. Mitigating the risk of DEF could be challenging mainly because limiting the temperature rise in concrete requires moderating the hydration heat/rate to minimize the risk of early-age thermal cracking and DEF which could contradict the traditional objectives associated with the concrete’s mechanical performance. Further, early-age thermal cracking can be influenced by project-specific factors including the dimensions of the concrete element, ambient conditions, and external restraints which do not come into the picture in a conventional mix design approach. These issues add to the complexity of the mix design problem. This paper presents a mix design approach based on integrated numerical simulation of heat transfer in concrete and genetic algorithm optimisation that minimises the risk of early-age cracking within the limits specified for the mechanical performance of concrete. The results of the optimization framework applied to three case studies show a significant decrease in peak temperature, as well as the ratio of induced tensile stresses to developed tensile strength for optimized concrete mixes.

Thermal cracking assessment for nuclear containment buildings using high-strength concrete

To shorten the construction times of nuclear facility structures, three high-strength concrete mixtures were developed with specific consideration given to their curing temperatures, their economic efficiency, and the practicality of their quality control. This study was conducted to examine the temperature rise profiles of these three concrete mixtures and the potential for early-age thermal cracking in the primary containment vessel of a nuclear reactor with a wall thickness of 1200 mm. The one-layer placement height of the concrete for the primary containment vessel was increased from the conventional 3 m to 3.5 m. A nonlinear finite element analysis (FEA) was conducted using the thermal properties of concrete determined from the isothermal hydration and adiabatic hydration tests, and tuned through comparisons made with temperature rise profiles obtained for 1200-mm-thick mock-up wall specimens cured at temperatures of 5, 20, and 35oC. The hydration heat performance of the three concrete mixtures and their potential to produce thermal cracking in nuclear facilities indicate that the mixtures have considerable potential for practical application to the primary containment vessel of a nuclear reactor at various curing temperatures, fulfilling the minimum requirements of the ACI 301 and minimizing the likelihood of the occurrence of thermal cracks.

Enhanced massivity index based on evidence from case studies: Towards a robust pre-design assessment of early-age thermal cracking risk and practical recommendations

Tensile stresses resulting from a combination of thermal volumetric changes, due to the heat of hydration and ambient conditions, autogenous deformations and boundary restraints, often induce a significant intrinsic load on massive concrete structures. Whenever such stresses attain the concrete tensile strength, cracking occurs, which may in turn impair the serviceability and durability of the structure. This study is an output of Working Group 7 of RILEM Technical Committee 254-CMS: Thermal cracking in massive concrete structures and is presenting case studies of early-age thermal cracking in massive concrete structures where internal restraining conditions often prevail, such as thick blocks, armour units, footings, dams and spillways, and large-sized columns. It covers the analysis of causes of this type of cracking together with the lessons learned from the collected evidence along with best mitigation practices (often resulting from forensic investigations by means of computer-based simulations). Based on the evidence retrieved from the analysed case studies, the concept of massivity used to indicate potential thermal crack proneness in massive concrete structures is significantly improved to account for binder type and content as well as casting and fresh concrete temperature, in addition to the geometrical characteristics of the element under investigation. The use of such an indicator may lead to a more robust pre-design assessment of the likelihood for thermal cracking occurrence in massive concrete elements, advising designers and contractors dealing with such structures whether more complex analyses should be performed already at the design stage.

A combined finite difference and finite element model for temperature and stress predictions of cast-in-place cap beam on precast columns

In this study, a combined finite difference and finite element model was developed to predict the temperature development, thermally induced stresses and associated cracking risk in the concrete of a cast-in-place cap beam cast on precast columns of a bridge. The numerical model considers degree of hydration dependent heat rate, Young’s modulus development, strength development and early age tensile and compressive creep behavior. The temperature and stress analyses were performed on two sections of a cast-in-place cap beam (with a cross section of 1.6 m × 2.1 m): one at mid-span of the cap beam and the other on top of the precast column. The results show that the section of the cap beam at the column had high tensile stresses at the mid-sides which exceeded the early age concrete tensile strength when not covered with insulation blankets during construction. Additionally, the use of insulation materials, reduction of initial concrete temperature and proper choice of casting time can significantly mitigate the thermal stress and cracking risk of the cap beam. The model can be conveniently programmed and be a useful tool to help engineers control concrete temperature and take measures to minimize the risk of early-age thermal cracking for cast-in-place pier caps and its connection to a precast column, or other similar concrete members/connections, thus accelerating construction schedules for bridge projects.

Examining the effects of microencapsulated phase change materials on early-age temperature evolutions in realistic pavement geometries

This study examines experimentally and numerically the effects of microencapsulated phase change material (PCM) additions on temperature rise and cool-down rate, and the associated impact on reducing the risk of thermal cracking in concrete pavements. Specimens representative of a realistic pavement geometry were exposed to diurnal temperature cycling at early ages while internal temperature evolutions were monitored. In spite of the fact that the lower thermal conductivity of the PCM inhibited heat dissipation, the results showed the presence of PCMs can reduce considerably the temperature rise and cool-down rate in the first 24 h following placement provided the PCM melting temperature is chosen suitably. A transient one-dimensional thermal model of a pavement section suitably captured experimental temperature evolutions and thereby offers the ability to inform the design of concrete pavements containing PCMs that feature early-age thermal cracking resistance.

Precision of cement hydration heat models in capturing the effects of SCMs and retarders

A major input to numerical simulation models used to predict the risk of early-age thermal cracking in concrete is the hydration heat estimation. The precision of hydration heat estimation models has been extensively verified for different cement compositions in previous studies. However, little has been done to investigate the accuracy of such models for concrete mixes containing supplementary cementitious materials and retarders. This paper presents the results of a series of isothermal calorimetry tests conducted first to investigate the effects of Class F fly ash, ground-granulated blast-furnace slag (GGBFS) and three commonly used retarders (namely, retarder N, sucrose and citrate) on the heat of hydration profile of Australian general-purpose cement under different curing temperatures of 10, 23 and 30°C, and second to evaluate the precision of the two most commonly used hydration heat models in capturing the effects of fly ash, GGBFS, retarders and curing temperature on the hydration profile. The results reveal the possibility of considerable errors in estimating the hydration heat of concrete mixes containing supplementary cementitious materials and retarders under different curing temperatures, highlighting the need for re-calibration of the existing models for locally used materials to avoid misleading errors in numerical simulation of early-age thermal cracking.

Learning from failures in an emblematic building in Valencia, Spain

Abstract On many occasions advances have been made in science and engineering thanks to the knowledge gained from failures. In the particular case of structural engineering, the study of actual failures makes it possible to advance and define new theories, concepts and designs. Some examples are the changes and improvements that appeared after some of the “classical” failures such as the Ronan Point building, Quebec Bridge, Murrah Federal Building, Tacoma Narrows Bridge and the World Trade Center. This paper describes a teaching method used with structural engineering students at the Universitat Politecnica de Valencia based on the study of cases of damage to buildings in Valencia, Spain. Due to its special characteristics, one of the buildings studied is the Principe Felipe Science Museum. Some of its main characteristics are: 1) it is one of Valencia's emblematic buildings, 2) its considerable dimensions required huge quantities of concrete, 3) it has a complex structure and an innovative architectural design, 4) the wide variation in the type of damage detected, which make it a particularly valuable teaching aid. The most important damage detected has been classified and described during the visits to the Principe Felipe Science Museum. The damage mechanisms are widely diverse and include: those due to the behaviour of the concrete itself (e.g. shrinkage and early age thermal cracking), those due to the presence of damp, those whose origin can be traced back to the construction phase, and others due to corroded reinforcement and to the loads acting on the structure. The paper has a double value since on one hand it describes a highly successful teaching aid for the training of experts in structural engineering, while on the other it classifies and describes the existing damage in one of the most important modern buildings in Spain and perhaps in Europe.

Effect of Aggregate Mineralogy and Concrete Microstructure on Thermal Expansion and Strength Properties of Concrete

Aggregate type and mineralogy are critical factors that influence the engineering properties of concrete. Temperature variations result in internal volume changes could potentially cause a network of micro-cracks leading to a reduction in the concrete’s compressive strength. The study specifically studied the effect of the type and mineralogy of fine and coarse aggregates in the normal strength concrete properties. As performance measures, the coefficient of thermal expansion (CTE) and compressive strength were tested with concrete specimens containing different types of fine aggregates (manufactured and natural sands) and coarse aggregates (dolomite and granite). Petrographic examinations were then performed to determine the mineralogical characteristics of the aggregate and to examine the aggregate and concrete microstructure. The test results indicate the concrete CTE increases with the silicon (Si) volume content in the aggregate. For the concrete specimens with higher CTE, the micro-crack density in the interfacial transition zone (ITZ) tended to be higher. The width of ITZ in one of the concrete specimens with a high CTE displayed the widest core ITZ (approx. 11 µm) while the concrete specimens with a low CTE showed the narrowest core ITZ (approx. 3.5 µm). This was attributed to early-age thermal cracking. Specimens with higher CTE are more susceptible to thermal stress.

Causes of Early-Age Thermal Cracking of Concrete Foundation Slabs and their Reinforcement to Control the Cracking

Abstract This paper focuses on the causes and consequences of early-age cracking of mass concrete foundation slabs due to restrained volume changes. Considering the importance of water leaking through cracks in terms of the serviceability, durability and environmental impact of watertight concrete structures, emphasis is placed on the effect of temperature loads on foundation slabs. Foundation slabs are usually restrained to some degree externally or internally. To evaluate the effect of external restraints on foundation slabs, friction and interaction models are introduced. The reinforcement of concrete cannot prevent the initiation of cracking, but when cracking has occurred, it may act to reduce the spacing and width of cracks. According to EN 1992-1-1, results of calculating crack widths with local variations included in National Annexes (NAs) vary considerably. A comparison of the required reinforcement areas according to different NAs is presented.

Tensile creep and early-age concrete cracking due to restrained shrinkage

Concrete structures are prone to cracking due to restraint provided to early age autogenous and drying shrinkage. In addition, the risk of early-age thermal cracking is increased by increasing the degree of restraint to early-age thermal contraction. At this early-age, tensile creep plays a key role in relaxing shrinkage induced tensile stresses and delaying the time to cracking. However, limited data are available concerning tensile creep of concrete and the magnitude and rate of development of the early-age shrinkage of concrete. As a consequence, restraint to shrinkage is often poorly modelled in structural design. In order to accurately quantify the early-age shrinkage and tensile creep of concrete, a comprehensive experimental program is being conducted at the UNSW Centre for Infrastructure Engineering and Safety. Tensile creep is measured on dog-bone shaped specimens subjected to constant sustained tensile stress, while shrinkage is measured on identical unloaded specimens. Restrained ring tests were also performed to validate the tensile creep coefficients calculated from dog-bone specimens. A simple analytical procedure to accurately predict the degree of restraint and the tensile stresses in concrete induced by shrinkage is described for the restrained ring specimens.

Effect of Mineral Admixture (Alccofine-1203) on Durability of Hybrid Fiber Reinforced Concrete

Objective: To estimate effect of mineral admixture (Alccofine-1203) on durability of Hybrid Fiber Reinforced Concrete. Methods/Analysis: This paper presents effect of hybrid fibers and alccofine-1203 on estimation of durability of hybrid fiber reinforced concrete with different combinations. The final combination is used as fiber volume fraction - 1.5% (80% steel fibers and 20% polypropylene fibers) by volume of concrete and Alcoffine-1203 with 7.5% by weight of cement. Findings: The study presents behavior of hybrid fiber (steel fibers and polypropylene fibers) reinforced concrete including mineral admixture like Alccofine-1203. The hybrid fibers help to reduce the cracks due to shrinkage, creep and early age thermal cracking along with increase in ductility, toughness and fatigue resistance. The mineral admixture (Alccofine-1203) enhances strength of concrete, modulus of elasticity along with workability, durability and reduction in segregation. Novelty/Improvement: Addition of Hybrid fibers and mineral admixture (Alccofine-1203) increases the ductility, crack resistance strength and durability of concrete.