Temperature Stress Testing Machine Research Articles

Effect of temperature history and restraint degree on cracking behavior of early-age concrete

This study experimentally investigated the effect of temperature history and restraint degree on cracking behavior of early-age concrete, including cracking temperature, cracking stress/strength, creep/free deformation and cracking potential. The deformation, temperature and stress were measured with two different temperature histories and three different restraint degrees via a temperature stress testing machine (TSTM). The results show that, under different temperature histories, the magnitude of cracking potential of concrete was found to vary significantly. The concrete with a high-low cooling rate at early-age had better anti-cracking performance (improved by at least 13.1%) due to the more effective utilization of creep on reducing restrained stress. The high-low cooling rate history showed more outstanding effect on lowering cracking risk of concrete under higher restraint degree. Furthermore, an analytical method for establishing the relationship between the restraint degree and temperature difference was recommended for assessing cracking risk of concrete.

Research on the Crack Risk of Early-Age Concrete under the Temperature Stress Test Machine.

A new temperature stress test machine (TSTM) was developed to improve temperature control accuracy and efficiency. As there is no uniform standard for the threshold value of TSTM, quite different threshold values are used in the test, which results in the influences of the threshold value on the evaluation of thermal stresses and crack risk of concrete not being determined. To illustrate the importance of an appropriate threshold value, different threshold values were used to evaluate the influences on the thermal stresses and cracking resistance of concrete specimens under a complete restraint test with semi-adiabatic temperature development. The results show that the maximum compressive stress of a concrete specimen with a threshold value 3 με was slightly larger than that of 1.5 με when the growth rate of tensile stress of the concrete specimen with the threshold value 3 με was slightly greater than that of concrete specimen with the threshold value 1.5 με. Based on the combination of crack risk coefficient and restraint degree, a new system for evaluating the crack resistance of concrete was proposed, in which different threshold values were used to estimate their influences on the crack risk of concrete. Thus, an appropriate range of threshold values could be determined.

Influence of Barchip fiber on early-age cracking potential of high performance concrete under restrained condition

High performance concrete (HPC), which is a new-tech concrete, has been widely applied for its good workability, lasting durability, low permeability, and high strength. However, the high temperature rise and high autogenous shrinkage induced by the low water-to-cement (w/c) ratio would increase the early-age cracking potential in HPC. In order to reduce the cracking potential of HPC, Barchip fibers are applied to strengthen the early-age properties. Although the influence of Barchip fibers on the mechanical properties of concrete has been studied, investigations on the influence of Barchip fibers on the early-age cracking potential of HPC under adiabatic condition at uniaxial constant restraint degree are still lacking. The early-age cracking potential of HPC reinforced with different amounts of Barchip fibers (0, 4, 8, and 12 kg/m3) was tested by Temperature Stress Test Machine under adiabatic condition at full restraint degree in present study. Test results and corresponding analysis indicated that (1) the compressive strength and splitting tensile strength were enhanced by the addition of Barchip fibers in HPC; (2) Barchip fibers reduced autogenous shrinkage, restrained tensile stress rate and increased cracking age, specific tensile creep of early-age HPC; (3) the integrated criterion utilized to evaluate the cracking potential decreased first and then increased with increasing amount of Barchip fibers; (4) the optimal Barchip fiber amount of 8 kg/m3 was recommended for that poor dispersion and fiber clumping may occur in HPC with excessive addition of Barchip fibers.

Double Feedback Control Method for Determining Early-Age Restrained Creep of Concrete Using a Temperature Stress Testing Machine

Early-age restrained creep influences the cracking properties of concrete. However, conventional creep measurements require a large number of tests to predict the restrained creep as it is influenced by the combined effects of variable temperature, creep recovery, and varying compression and tension stresses. In this work, a double feedback control method for temperature stress testing was developed to measure the early-age restrained creep of concrete. The results demonstrate that the conventional single feedback control method neglects the effect of restrained elastic deformation, thus providing a larger-than-actual creep measurement. The tests found that the double feedback control method eliminates the influence of restrained elastic deformation. The creep results from the double feedback method match well with results from the single feedback method after compensation for the effects of restrained elastic deformation is accounted for. The difference in restrained creep between the single and double feedback methods is significant for concrete with a low modulus of elasticity but can be neglected in concrete with a high modulus of elasticity. The ratio between creep and free deformation was found to be 40–60% for low, moderate, and high strength concretes alike. The double feedback control method is therefore recommended for determining the restrained creep using a temperature stress testing machine.

Tensile creep and cracking potential of high performance concrete internally cured with super absorbent polymers at early age

High performance concrete (HPC) has been extensively applied in practice. However, the water-to-cement (w/c) ratio of this concrete is low, high temperature rise and high self-desiccation will occur, and both of which can increase the cracking potential of HPC at early age and then decrease the service life of concrete structures. Therefore, super absorbent polymers (SAPs) are applied in HPC as an internal curing (IC) agent. Although the autogenous shrinkage, relative humidity, and stress relaxation of concrete internally cured with SAPs have been studied, investigations on the influence of SAPs on the tensile creep and cracking potential of HPC are still lacking. In present study, the influence of SAPs as an IC agent on the temperature, autogenous shrinkage, restrained stress, basic tensile creep, and cracking potential of HPC were simultaneously studied by Temperature Stress Test Machine. Test results and corresponding analysis showed that: (1) the adiabatic temperature rise of HPC was 27.6, 29.3, 31.0, and 34.9 °C and increased with the increase of amount of SAPs; (2) the rate of restrained tensile stress of HPC was 1.7, 1.5, 1.4, and 1.2 MPa/day and decreased with the increase of amount of SAPs; (3) the specific basic tensile creep of HPC at the age when the restrained specimen of mixture SAP-0 cracked was 45, 23, 13, and 7 με/MPa and decreased with the increase of amount of SAPs; (4) a model for predicting the basic tensile creep compliance function of HPC considering the influence of amount of SAPs was proposed; (5) the cracking potential of HPC which was based on the integrated criterion decreased with the increase of amount of SAPs.

Calibration of Material Models against TSTM Test for Crack Risk Assessment of Early‐Age Concrete Containing Fly Ash

Making reliable cracking risk assessment involves experimental testing and advanced modeling of the time‐ and temperature‐dependent behavior of the properties, the restraint conditions of the structure, and the external environmental conditions. Mineral additives such as silica fume (SF), blast furnace slag (BFS), and fly ash (FA) have been used extensively in production of high performance concrete in the last decades. The mineral additives such as fly ash and blast furnace slag will reduce the hydration heat during the hardening phase, and the mineral additives also have significant influence on the development of mechanic and viscoelastic properties at early age. Within the NOR‐CRACK project, extensive test programs were performed to investigate the material properties related to cracking risk of early‐age concrete containing mineral additives. In current paper, the advanced modeling of the heat of hydration, volume changes (autogenous shrinkage and thermal dilation) during hardening, the development of mechanical properties (E‐modulus, compressive strength, and tensile strength), and creep/relaxation properties are discussed. Tests were performed in “temperature stress testing machine” (TSTM) to measure the restraint stress, and well‐documented material models were verified by performing 1‐D analysis of restraint stress development in the TSTM (Ji, 2008).

Tensile creep and cracking resistance of concrete with different water-to-cement ratios at early age

With the widely use of high-performance concrete, the lower and lower water-to-cement (w/c) ratio is applied in practice. This low w/c ratio can offer high strength and low permeability, however, coming with other drawbacks, such as high self-desiccation and high temperature rise, both of which can increase the early-age cracking potential of concrete. Creep is important to evaluate the early-age cracking resistance of concrete. Although studies on the influence of w/c ratio on the creep of mature concrete have been conducted, investigations on the influence of w/c ratio on the early-age tensile creep remain lacking. Therefore, investigations on the influence of w/c ratio on the tensile creep and cracking resistance of early-age concrete must be conducted. Experimental studies on the influence of w/c ratio (0.33, 0.40, and 0.50) on the temperature change, autogenous shrinkage, restrained stress, tensile creep, and cracking resistance of early-age concrete under adiabatic condition and at full restraint degree were conducted using Temperature Stress Test Machine. The test results indicate that (1) the temperature rise under adiabatic condition increased by 32.6%, and 74.2% when w/c ratio decreased from 0.50 to 0.40, and 0.33, respectively; (2) the restrained tensile stress rate under adiabatic condition and at full restraint degree increased by 16.7%, and 33.3% when w/c ratio decreased from 0.50 to 0.40, and 0.33, respectively; (3) the early-age specific basic tensile creep and basic tensile creep/shrinkage under adiabatic condition and at full restraint degree increased with the decrease of w/c ratio; (4) the early-age cracking potential under adiabatic condition and at full restraint degree increased with the decrease of w/c ratio according to the integrated criterion.

Self-Developed Testing System for Determining the Temperature Behavior of Concrete.

Cracking due to temperature and restraint in mass concrete is an important issue. A temperature stress testing machine (TSTM) is an effective test method to study the mechanism of temperature cracking. A synchronous closed loop federated control TSTM system has been developed by adopting the design concepts of a closed loop federated control, a detachable mold design, a direct measuring deformation method, and a temperature deformation compensation method. The results show that the self-developed system has the comprehensive ability of simulating different restraint degrees, multiple temperature and humidity modes, and closed-loop control of multi-TSTMs during one test period. Additionally, the direct measuring deformation method can obtain a more accurate deformation and restraint degree result with little local damage. The external temperature deformation affecting the concrete specimen can be eliminated by adopting the temperature deformation compensation method with different considerations of steel materials. The concrete quality of different TSTMs can be guaranteed by being vibrated on the vibrating stand synchronously. The detachable mold design and assembled method has greatly overcome the difficulty of eccentric force and deformation.

Comparison of tensile and compressive creep of fly ash concretes in the hardening phase

Tensile and compressive creep tests of fly ash concretes in the hardening phase have been performed in the Temperature-Stress Testing Machine (TSTM) at NTNU. All tests were performed under nominally identical conditions, e.g. dimensions, curing conditions, measurement set-up and load-regime. Two concretes with 17% and 33% fly ash (as % by weight of cement and fly ash content) were investigated. Contradictorily to most of the compressive versus tensile creep comparisons found in the literature, both the investigated concretes showed similar creep behaviour in compression and tension throughout the hardening phase. It was also seen that for the investigated time-span, the specific creep development over time was found to increase (i.e. soften) with increasing amount of fly ash.

Early-age tensile creep and cracking potential of concrete internally cured with pre-wetted lightweight aggregate

High-performance concrete (HPC) is widely used in practice due to its potential long-term benefits, such as high strength and low permeability. However, high self-desiccation and high temperature rise occur due to the low water-to-cement (w/c) ratio of HPC, both of which would increase the cracking potential of concrete at early age. Although the creep and cracking potential of early-age HPC have been investigated, studies on the tensile creep and cracking potential of internally cured concrete with pre-wetted lightweight aggregates (LWAs) at early age under adiabatic condition at various w/c ratios remain lacking. In present study, the tensile creep and cracking potential of concrete at early age were experimentally investigated under adiabatic condition using the temperature stress test machine. Test results and corresponding analysis showed that: (1) a model for predicting the compressive strength of concrete was presented in consideration of the influence of pre-wetted LWAs; (2) the adding of pre-wetted LWAs reduced the autogenous shrinkage of concrete with different basic w/c ratios; (3) the basic tensile creep/shrinkage and absolute value of basic tensile creep of internally cured concrete were lower than that of normal concrete when the basic w/c ratios were the same; (4) the basic tensile creep/shrinkage and absolute value of specific basic tensile creep of normal and internally cured concrete at the age of cracking both increased with the decrease of basic w/c ratio; (5) the cracking potential of normal and internally cured concrete both increased with the decrease of basic w/c ratio and the adding of pre-wetted LWAs reduced the cracking potential.

Creep testing of concrete since setting time by means of permanent and repeated minute-long loadings

Monitoring the evolution of early age properties of concrete is necessary to provide input data to the models of prediction of the behavior of concrete structures since setting time. The experimental challenge lies in the fact that this monitoring must be fully automatic since the earliest age because the hardening process of the concrete takes place continuously over a period counted in hours and even in days after the casting time. This research paper presents a new methodology developed at ULB and IFSTTAR to monitor the creep and relaxation of an ordinary concrete chosen as reference concrete since setting time. Compressive creep rigs and two test devices were used: at ULB, a Temperature Stress Testing Machine (TSTM) specifically designed for testing concrete since setting time under free and restraint conditions and at IFSTTAR, a test set up called BTJASPE developed to monitor, in compression, the modulus of elasticity, the creep and the relaxation of a concrete since very early age. For the sake of the study, the same concrete has been used in the two laboratories. In addition, this study is performed at a constant temperature to exclude this parameter, at this step of the study. The methodology is based on experimental measurements with two kinds of test. A classical creep test with permanent loading during one week and a repeated minute-long loading test for which every 30 min, a loading is applied and kept constant during 5 min and finally totally removed. The classical test is used to characterize the non-aging creep function and the repeated minute-long loading test is used to quantify the aging creep functions.

Influence of curing temperature on autogenous shrinkage and cracking resistance of high-performance concrete at an early age

High-performance concrete (HPC) is widely used in practice. The water-to-cement ratio of HPC is low, and self-desiccation occurs which will induce marked autogenous shrinkage. Autogenous shrinkage usually increases the risk of cracking if the concrete is restrained from shrinking freely at early age. The autogenous shrinkage and cracking resistance of early-age concrete is influenced by curing temperature. However, the effect of curing temperature on autogenous shrinkage of early-age concrete is not in consistency and how the curing temperature affects the cracking resistance of concrete remains lacking. Thus, investigation on the effect of curing temperature on cracking resistance of early age concrete must be further studied. In this study, experimental studies on early-age cracking of concrete under 100% restraint and different curing temperatures were carried out using Temperature Stress Test Machine (TSTM). The present study investigated autogenous shrinkage of early-age HPC cured at different curing temperatures. The experimental results indicate that (1) the ratios of cracking stress to tensile strength for HPC specimens were all lower than 1.0; (2) the autogenous shrinkage of HPC increased with the increase of curing temperature; (3) a prediction model for autogenous shrinkage of HPC was presented considering the effect of curing temperature; (4) cracking temperatures and stress reserves were selected as the main cracking evaluation indicators of TSTM, and the HPC specimen cured at isothermal 20°C showed better cracking resistance than that at isothermal 45°C and adiabatic condition.

Influence of prewetted lightweight aggregates on the behavior and cracking potential of internally cured concrete at an early age

High-performance concrete (HPC) is widely used in practice. The water-to-cement ratio of HPC is low, and self-desiccation and high temperature rise occur. Internal curing (IC) with prewetted lightweight aggregates (LWAs) is applied to enhance the early-age behavior of concrete structures in terms of temperature, shrinkage, creep deformation, and stress, thus resulting in low cracking potential. Although tests on the effect of prewetted LWAs on the cracking potential of concrete under semi-adiabatic or isothermal conditions have been conducted, studies on the influence of the amount of prewetted LWAs on the cracking potential of internally cured concrete under adiabatic conditions remain lacking. In this study, the cracking potential of internally cured concrete with prewetted LWAs was experimentally investigated with a temperature stress testing machine. The effect of prewetted LWAs in reducing the cracking potential in HPC was investigated with different amounts of LWAs (0%, 10%, 30%, and 50%) in four pairs of large prismatic HPC specimens tested simultaneously under free and fully restrained shrinkage. Test results showed that (1) both temperature rise and drop of concrete occurred as prewetted LWAs increased; (2) the autogenous shrinkage and cracking stress of concrete decreased with the increase in the amount of prewetted LWAs; (3) the tensile stress rate of concrete with prewetted LWAs decreased; (4) the ratio of cracking stress to the tensile strength of concrete increased with the increase in the amount of prewetted LWAs; (5) the cracking age of concrete with the LWAs increased; (6) the specific tensile creep of concrete with prewetted LWAs decreased; and (7) the cracking potential of concrete with prewetted LWAs was reduced, as obtained from the integrated criterion. Internally cured concrete with prewetted clay LWAs is more robust for construction at early ages.

Effect of Reinforcement on Early-Age Concrete Temperature Stress: Preliminary Experimental Investigation and Analytical Simulation

For concrete under short-term loading, effect of reinforcement on concrete crack resistance capability is usually negligible; however, recent research results show that extension of this viewpoint to concrete under long-term loading (temperature variation) may be unsuitable. In order to investigate this phenomenon, this paper presents the experimental and analytical results of early-age reinforced concrete temperature stress development under uniaxial restraint. The experiments were carried out on a temperature stress testing machine (TSTM). Experimental results show that the coupling of reinforcement and concrete creep behavior influenced the concrete temperature stress development, and nearly 16% of concrete stress was reduced in the current research. Moreover, the cracking time of reinforced concrete was also delayed. Finally, based on the principle of superposition, analytical simulations of effect of reinforcement on concrete temperature stress have been performed.

Thermal stress analyses and reinforcement design of massive RC structures

Massive reinforced concrete (RC) structure is widely used in long-term damp environments. To improve the concrete durability and reduce the reinforcing steel corrosion, crack width control is more rigorous in massive RC structure. In this paper, the short-term (30 days) temperatures of concrete are measured by the distributed temperature system (DTS) to inverse the thermal parameters using the genetic algorithm (GA), and the long-term (300 days) temperature is predicted by using the calculated thermal parameters. Moreover, thermal stress field considering the creep is calculated based on the eight-parameters-equation, and the parameters were modified by the results of temperature stress test machine (TSTM). Furthermore, a quantitative reinforcement configuration method is proposed on the basis of the predicted thermal stress field, and its feasibility and effectiveness are verified by a sluice pier structure. Finally, the optimal reinforcement scheme obtained was selected by the comparisons of the crack width, reinforcement stress and the total cross-sectional area of reinforcement. This article provides a new methodology to design reinforcement configuration for massive RC structure.

Determination of Tensile Creep and Stress Relaxation of Concrete by Ring Test

The understanding of stress relaxation and tensile creep behavior is extremely important in accurate stress analysis and crack prediction of early-age concrete. The free shrinkage deformations of concrete with different strength grade were examined. The early-age tensile elastic modulus of concrete was investigated through temperature-stress testing machine. The tensile creep and shrinkage stress were obtained through the modified restrained ring test. The results indicate that the development of free shrinkage coordinates well with the inner strain of steel ring. Tensile creep decreases as water-binder ratio increases. Creep counteracts tensile stress of concrete by 28%~40% , decreases the possibility of cracking of concrete at early ages.

Experimental Study on Early-Age Crack of RC Using TSTM

Thermal stress is a major cause of early-age crack of massive concrete structures. In order to analyze the influencing factors of concrete crack under thermal loads, a series of tests were conducted using the improved Temperature Stress Testing Machine (TSTM). Effects of temperature on crack resistance of concrete were studied on different concrete placing temperatures and curing temperatures. Meanwhile, the roles of reinforcement on concrete crack resistance and crack-width limitation were quantitative analyzed, which compare cracks of plain concrete and reinforced concrete with the same mix proportion. The results indicate that reinforcement can improve the crack resistance of the structures by approximately twenty percents, which against the engineering experience. After concrete cracks, the cracks photos show that reinforcement can induce the smaller cracks formation, and the crack width of reinforced concrete is about 1/10 of the plain concrete crack width.

Experimental Study on Early-Age Crack of Mass Concrete under the Controlled Temperature History

Thermal deformation under restrained conditions often leads to early-age cracking and durability problems in mass concrete structures. It is crucial to monitor accurately the evolution of temperature and thermal stresses. In this paper, experimental studies using temperature stress testing machine (TSTM) are carried out to monitor the generated thermal cracking in mass concrete. Firstly, components and working principle of TSTM were introduced. Cracking temperatures and stress reserves are selected as the main cracking evaluation indicators of TSTM. Furthermore, effects of temperature controlling measures on concrete cracking were quantitatively studied, which consider the concrete placing temperature (before cooling) and cooling rates (after cooling). Moreover, the influence of reinforcement on early-age cracking has been quantitatively analyzed using the TSTM. The experimental results indicate that the crack probability of reinforced concrete (RC) is overestimated. Theoretical calculations proved that the internal stress can transfer from concrete to reinforcement due to creep effect. Finally, the experimental results indicate that the reinforcement can improve the crack resistance of concrete by nearly 30% in the TSTM tests, and the ultimate tensile strain of RC is approximately 105% higher than that of plain concrete with the same mix proportions.

How to monitor the modulus of elasticity of concrete, automatically since the earliest age?

Monitoring the evolution of an early age set of parameters on concrete is necessary to predict the early age behaviour of structures. The difficulty lies in the fact that this monitoring must be automatic because the concrete hardening process takes place over a long period after the casting. This paper presents a new methodology and an apparatus, specifically designed at IFSTTAR, to monitor the hardening process of a concrete. Mainly, the Young’s modulus can be monitored in compression. Measurements start soon after having cast the concrete and the sample temperature is completely controlled so that the concrete maturity is well mastered. The performances of this apparatus, obtained on an ordinary concrete, are compared to more classical measurements using an extensometer mounted on the sample just after the setting time and to ultrasonic measurements. In these cases, the temperatures were not controlled and results have to be expressed in equivalent time. A comparison with another method developed and used at ULB by using the same concrete, in the frame of a joined cooperation between our two laboratories is achieved. This test set up is based on the so called Temperature Stress Testing Machine (TSTM). This device has been specifically designed with a control of the concrete maturity by the use of a dummy specimen only submitted to free deformations (thermal, shrinkage). The TSTM allows compressive and tensile testing starting just after the setting time. In addition, concrete properties, such as compressive and tensile strength, have been characterized at early age. These values have been used for the design of the loading histories applied in the automatic tests. The heat released by the cement hydration has also been measured in order to express the results on a maturity scale.

Influence of Aggregates on Cracking Resistance of Concrete at Early Age

In order to investigate influence of aggregates on cracking resistance of concrete at early age, four kinds of aggregates, i.e. syenite, basalt, marble and sandstone, were used for the test on cracking resistance of hydraulic concretes by the temperature stress testing machine. Analogy analysis was carried out with test results of concretes with two gradings of aggregates. The results show that aggregates affect elastic modulus, thermal expansion coefficients and tensile creep behavior of concrete at early age. However, the temperature rise of concrete was slightly affected by various types and gradings of aggregate. Moreover, the cracking temperature is reliably to be used to quantitatively evaluate the influence of aggregates on cracking resistance of concrete at early age.