Temperature Stress Testing Machine Research Articles

Harnessing Limestone powder to enhance the thermal crack resistance of manufactured sand.

Manufactured sand concrete (MSC) has increasingly applied in engineering. However, how to enhance the thermal crack resistance of MSC has not clearly studied yet, which limits the wide applications of manufactured sand in engineering. We have adopted the method of mixing limestone powder (LP) to make MSC with four mass fractions (wt.%) of 5%, 10%, 15%, and 20% of LP content, designed the mix proportion of concrete, tested their mechanical properties, observed their microstructure by scanning electron microscopy (SEM) and investigated the effect of LP on anti-cracking performance using temperature stress testing machine (TSTM). These findings reveal that the mechanical properties and thermal crack resistance of 15% LP MSC are the best. Our results also provide a research basis for the MSC to be widely used in engineering.

Efficiently assessing the early-age cracking risk of cementitious materials with a mini temperature stress testing machine

Temperature Stress Testing Machine (TSTM) is a universal testing tool for many properties relevant to early-age cracking of cementitious materials. However, the complexity of TSTMs require heavy lab work and thus hinders a more thorough parametric study on a range of cementitious materials. This study presents the development and validation of a Mini-TSTM for efficiently testing the autogenous deformation (AD), viscoelastic properties, and their combined results, the early-age stress (EAS). The setup was validated through systematic tests of EAS, AD, elastic modulus, and creep. Besides, the heating/cooling capability of the setup was examined by tests of coefficient of thermal expansion by temperature cycles. The results of EAS correspond well to that of AD, which qualitatively validates the developed setup. To quantitatively validate the setup, a classical viscoelastic model was built, based on the scenario of a 1-D uniaxial restraint test, to predict the EAS results with the tested AD, elastic modulus, and creep of the same cementitious material as the input. The predicted EAS matched the testing results of Mini-TSTM with good accuracy in 6 different cases. The viscoelastic model also provided quantitative explanations for why variations in early AD do not influence the EAS results. The testing and modelling results together validate the developed Mini-TSTM setup as an efficient tool for studying early-age cracking of cementitious materials. At the end, the potential limitations of the Mini-TSTM are discussed and its applicability for concrete with aggregate size up to 22 mm is demonstrated.

Evaluation of early crack resistance performance of concrete mixed with ternary minerals using temperature stress testing machine (TSTM)

As industrial by-products, mineral admixtures can improve the crack resistance of concrete. However, the early crack resistance performance of concrete mixed with ternary minerals (steel slag powder, mineral powder and fly ash) was not fully understood. In this study, the mechanical and crack resistance performance of each group was preliminarily investigated using the slab method. Then G52 group and the control group were selected for a comparative analysis based on temperature stress testing machine (TSTM) and analytic hierarchy process (AHP) method. The results show that the ternary minerals admixtures reduced the early strength and enhanced the 28d strength of concrete. Meanwhile, they reduced the early adiabatic temperature rise, the maximum compressive stress and the influence of creep on the crack resistance performance of concrete, thereby enhancing the tensile stress at cracking of concrete. Overall, this study provided new insights into the mix proportions of ternary minerals concrete.

Early-age thermal cracking performance of carbon nanotube modified face slab concrete

The thermal cracking of face slab concrete (FSC) seriously affects the durability of face rockfill dams. The mechanical properties of carbon nanotubes such as high strength, excellent thermal conductivity provide great potential to improve the temperature cracking of the FSC. In this paper, the effects of CNTs on the thermal cracking resistance of the FSC at early ages were investigated in terms of mixture workability, pore structure, tensile modulus, and temperature stress testing machine test. It was demonstrated that the incorporation of 0.1 wt% CNTs significantly improved the pore structure of FSC and increased the tensile modulus of FSC at early ages. Besides, a model for the coefficient of thermal expansion (CTE) of FSC at early ages was established based on the temperature stress testing machine test, and the cracking comprehensive index, the cracking core index, and the cracking refinement index were compared. The incorporation of CNTs effectively reduced the CTE and the cracking temperature of FSC at early ages, and improved the ability of FSC to resist cracking in harsh environments. Therefore, CNTs exhibited a great potential to enhance the early-age cracking resistance performance of FSC.

Expansion and creep of concrete with expansive agents at variable temperature

Extensive research has shown that expansive agents can reduce engineering cracks of concrete, while temperature and creep significantly affect the ability of the expansive agent to compensate shrinkage of concrete. In order to study the expansion and creep of concrete mixed with expansive agents under variable temperature, Series of parameters of concrete mixed with MgO expansive agent (MEA) and CaO-calcium sulfoaluminate expansive agent (CEA) were tested by temperature stress testing machine (TSTM), and the elastic modulus, expansion and creep were calculated according to the maturity theory. The results showed that CEA had a larger free expansion but smaller constrained expansion compared to MEA, and the expansion energy of CEA almost completely released within 36 h of the temperature-rise stage. MEA had a smaller compressive stress creep in the temperature-rise stage and larger tensile stress creep in the cooling stage, which was beneficial to resistance to cracking. The proposed expansion kinetics model showed a remarkable level of consistency with the test results and held significant importance in guiding the application of MEA in practical engineering.

Experimentally informed modeling of the early‐age stress evolution in cementitious materials using exponential conversion from creep to relaxation

AbstractThis study presents comprehensive numerical modeling methods for simulating early‐age stress (EAS) relaxation in cementitious materials, based on the autogenous deformation (AD), elastic modulus, creep, and stress continuously tested by a mini temperature stress testing machine (Mini‐TSTM) and a mini AD testing machine from a very early age (i.e., from a few hours to a week). Four methods for converting creep compliance to relaxation modulus were discussed in detail and used for the one‐dimensional (1D) and three‐dimensional (3D) simulation of stress evolution in the Mini‐TSTM test. Furthermore, virtual creep and relaxation tests were conducted using an exponential algorithm with either the Kelvin or Maxwell chains to show their applicability in simulating the viscoelastic behavior of early‐age cementitious materials. The results showed that the exponential algorithm with the Maxwell chain using an exponential conversion function from creep to relaxation obtains good prediction accuracy of EAS in 3D analysis. The numerical solutions of the Volterra integral of creep compliance can lead to a negative relaxation modulus, thus introducing stress calculation errors in both 1D and 3D analysis.

Research on the Early-Age Cracking of Concrete Added with Magnesium Oxide under a Temperature Stress Test Machine.

Concrete cracking is a significant issue in the global construction industry, and the restraint stress of concrete is a crucial contributing factor to early concrete cracking. The addition of magnesium oxide additive (MEA) to concrete is a method to enhance its crack resistance. In this paper, concrete specimens with four different contents of MEA were tested with a temperature stress testing machine. The deformation characteristics and mechanical properties of concrete with varying contents of MEA were investigated using both free deformation tests and fully constrained deformation tests. The prediction model for the early restrained stress of concrete was developed by integrating the stress relaxation phenomenon of concrete with models for autogenous shrinkage, temperature deformation, and elastic modulus. According to the results, (1) the thermal expansion coefficient exhibits a pattern of initially increasing and subsequently decreasing with the increasing ratio of MEA; (2) the addition of 3% and 8% MEA can offset 23% and 35.1% of the concrete's self-shrinkage, respectively. Nevertheless, when the added MEA content is 5%, the self-shrinkage of concrete increases by 6%; (3) the addition of 3-8% MEA can result in a 0.5-1.67 times increase in the maximum expansion stress of concrete, as well as a 0.5-0.95 times increase in cracking stress; (4) as the MEA content continues to increase, the stress relaxation level of concrete also increases. In comparison to concrete mixed without MEA, the maximum increase in the stress relaxation level of concrete is 65.5%, thereby enhancing the concrete's anti-cracking ability. However, when the MEA dosage reaches a certain threshold, the stress relaxation enhancement brought about by the addition of MEA will no longer be significant; (5) when compared to the experimental data, the established model of early-age constraint stress accurately predicts the tensile constraint stress of concrete.

Effects of temperature on autogenous deformation and early-age stress evolution in cement pastes with low water to cement ratio

This paper investigates the influence of temperature on autogenous deformation and early-age stress (EAS) evolution in ordinary Portland cement paste using a recently developed Mini Temperature Stress Testing Machine (Mini-TSTM) and Mini Autogenous Deformation Testing Machine (Mini-ADTM). In the Mini-TSTM/ ADTM, CEM I 42.5 N paste with a water-cement ratio of 0.30 was tested under a curing temperature of 10, 15, 20, 25, 30, and 40 °C. X-Ray diffraction (XRD) tests were conducted to measure the amount of ettringite and calcium hydroxide, which reveals the micro-scale mechanisms of autogenous expansion. The applicability of the Maturity Concept (MC) for the prediction of autogenous deformation and relaxation modulus under different temperatures was also examined by the experimental data and the viscoelastic model. This paper leads to the following findings: 1) The autogenous deformation of ordinary Portland cement paste is a four-stage process comprising the initial shrinkage, autogenous expansion, plateau, and autogenous shrinkage; 2) Higher temperature leads to higher early-age cracking (EAC) risk because it accelerates the transitions through the first three stages and causes the autogenous shrinkage stage to start earlier. Moreover, higher temperatures also result in increased rates of autogenous shrinkage and EAS in the autogenous shrinkage stage; 3) Autogenous expansion and plateau are attributed to the crystallization pressure induced by CH. Temperature-dependent CH formation rates determine the duration of the plateau stage; 4) Low-temperature curing can delay but not completely prevent the EAC induced by autogenous deformation; 5) The MC cannot predict the autogenous deformation at different temperatures but can be used to calculate the relaxation modulus, which in turn aids in EAS prediction based on autogenous deformation data.

Autogenous shrinkage and tensile creep of supersulfated cement concrete at early age

Supersulfated cement (SSC) is an environmentally friendly cementitious material known for its low hydration heat, high flexural strength, as well as resistance to sulfate attack. While existing research on SSC concrete mainly focuses on mechanical properties and hydration mechanisms, there exists a gap concerning the optimization of SSC concrete and the exploration of its early-age behavior. Early-age tensile creep of concrete has the potential to alleviate tensile stresses induced by restrained shrinkage. The current research aimed to fill this research gap by investigating the autogenous shrinkage and tensile creep of SSC concrete under different granulated blast furnace slag (GBFS) substitution rates (0%, 15%, 30%, and 50%) utilizing the Temperature Stress Test Machine. Experimental findings demonstrated that with a higher GBFS substitution rate, the mechanical properties and autogenous shrinkage decreased, while the tensile creep behavior of SSC concrete increased at early age. Additionally, models for predicting the autogenous shrinkage and tensile creep compliance of SSC concrete were proposed considering the GBFS substitution rate at early age.

Assessment and mitigation of early-age cracking for on-site concrete structures by a combined scheme using Temperature Stress Testing Machine (TSTM) and long-span restraining frame

From the field survey, thermal cracking frequently occurs in substructural concrete walls, although a high dosage of Calcium Sulfoaluminate (CSA) expansive additive is widely utilized. Conventional assessment methods fail to characterize the failure of CSA expansion under coupled realistic conditions. Therefore, an assessment scheme is proposed in this study: Firstly, mix proportions, temperature, restraint and curing conditions are used as input for a developed Temperature Stress Testing Machine (TSTM). By testing, limited expansions of CSA additive under low water-to-binder ratio, high temperature or drying condition, is unfolded. Corresponding countermeasures are also proposed. Secondly, long-span restraining frames are constructed to validate, and upscale findings from TSTM. Eventually, normal and optimized mixtures are applied and compared in real substructural walls. This combined scheme newly reveals that the lack of free water is the mechanism for CSA's stagnant expansion and a combination of CSA and lightweight aggregates shows promising potential as a countermeasure.

Influence of seawater and sea sand on early-age performance and cracking sensitivity of concrete

Due to resource constraints, there has been a growing demand for alternative sources of water and sand in construction engineering. Recent advancements in technology, such as the use of fiber-reinforced polymers (FRP) reinforcement or impressed current cathodic protection-structural strengthening (ICCP-SS), have enabled the utilization of seawater and sea sand, garnering significant research attention. The study investigated various aspects of four types of early-age concrete based on the temperature stress testing machine (TSTM) method, including fresh properties, basic mechanical properties, hydration temperature history, restraint stress, autogenous shrinkage, creep, and cracking sensitivity. The results showed that seawater significantly increased the early tensile strength of concrete, while sea sand had the opposite effect. Compared with ordinary concrete, seawater and sea sand increased the early autogenous shrinkage of concrete, and the impact of seawater was more significant. For the early creep of concrete, seawater and sea sand reduced the maximum compressive creep of concrete, and the effect of sea sand was more remarkable. Seawater increased the tensile creep of concrete, while sea sand decreased the tensile creep of concrete. The comprehensive evaluation index indicates that seawater and sea sand have no significant effect on concrete's cracking sensitivity. This research may serve as a valuable reference for the use of seawater and sea sand in concrete.

Early-Age Autogenous Shrinkage and Cracking Risk of 5D Hooked-End Steel Fiber–Reinforced High-Strength Concrete under Uniaxial Restrained Condition

The incorporation of steel fiber exhibits great benefits to reduce cracking risk and autogenous shrinkage (AS) of high-strength concrete (HSC). 5D hooked-end steel fiber (5DSF) is a kind of novel hooked-end steel fiber. Previous investigations mainly focused on the mechanical properties and shrinkage deformation of 5DSF reinforced HSC (5DSFRC). However, the early-age cracking risk of 5DSFRC under uniaxial restrained conditions considering temperature evolution, AS, and restrained stress simultaneously has not been thoroughly investigated. A temperature stress test machine, which is capable of measuring these factors simultaneously, was used to investigate the effect of 5DSF volume fraction (0%, 0.12%, 0.24%, and 0.36%) on the early-age cracking risk of HSC in the present study. Experimental results and analysis showed that the cracking age, cracking stress, ratio of cracking stress to axial tensile strength, and reserve strength of HSC increased with an increase in 5DSF volume fraction. The AS of HSC decreased with an increase in 5DSF volume fraction, and an AS formula was proposed to calculate the early-age AS of HSC reinforced with steel fiber considering the effect of the volume fraction and shape of the fiber. The cracking risk of HSC decreased with an increase in 5DSF volume fraction. The early-age cracking risk and AS of HSC reinforced with 5DSF were lower than that of HSC reinforced with 3D hooked-end steel fiber with a similar steel fiber volume fraction.

Early-Age Cracking Potential of Fly Ash High Performance Concrete Internally Cured with Super Absorbent Polymers

Abstract Early-age cracking is problematic for high performance concrete (HPC) made with a low water/binder ratio. The application of fly ash (FA) as a cement replacement can contribute to reduced CO2 emission and clinker factor, as well as improved durability of HPC. Super absorbent polymers (SAPs) were also applied for better curing and a higher rate of hydration to optimize the FA HPC. The current study utilized a temperature stress test machine to investigate the early-age cracking potential of FA HPC with internal curing by SAPs. Test results and the related analyses suggested that the introduction of internal curing alleviated the autogenous shrinkage, restrained stress, as well as tensile creep for HPC with 0 % and 20 % FA replacement level. The application of SAPs reduced the cracking potential of HPC with a low FA replacement level, whereas it aggravated the cracking when the FA replacement level was high.

Development and Preliminary Application of Temperature Stress Test Machine for Cast-in-Place Inner Shaft Lining.

Over the past 20 years, as the depth and diameter of shaft lines increased in China, the cracking and water leakage of the inner walls of frozen shafts have become increasingly severe, resulting in significant safety threats and economic losses. Understanding the stress variation patterns of cast-in-place inner walls under the combined effects of temperature and constraint during construction is a prerequisite for evaluating the crack resistance performance of inner walls and preventing water leakage in frozen shafts. The temperature stress testing machine is an important instrument for studying the early-age crack resistance performance of concrete materials under the combined effects of temperature and constraint. However, existing testing machines have shortcomings in terms of applicable specimen cross-sectional shapes, temperature control methods for concrete structures, and axial loading capacity. In this paper, a novel temperature stress testing machine suitable for the inner wall structure shape, capable of simulating the hydration heat of the inner walls, was developed. Then, a reduced-scale model of the inner wall according to similarity criteria was manufactured indoors. Finally, preliminary investigations of the temperature, strain, and stress variations of the inner wall under 100% end constraint conditions were conducted by simulating the actual hydration heating and cooling process of the inner walls. Results show that the hydration heating and cooling process of the inner wall can be accurately simulated. After approximately 69 h of concrete casting, the accumulated relative displacement and strain of the end-constrained inner wall model were -244.2 mm and 187.8 με, respectively. The end constraint force of the model increased to a maximum value of 1.7 MPa and then rapidly unloaded, causing the model concrete to crack in tension. The temperature stress testing method presented in this paper provides a reference for scientifically formulating technical approaches to prevent cracking in cast-in-place concrete inner walls.

Early-age properties and shrinkage induced stress of ultra-high-performance concrete under variable temperature and uniaxial restrained condition

Researches have been continuously carried out on the mechanical properties and control measures on autogenous shrinkage of ultra-high-performance concrete (UHPC), while studies on the stress evolution and cracking behavior of UHPC under variable temperature and uniaxial restrained condition are insufficient. The development of restrained stress is not only closely associated with the elastic modulus and shrinkage, but also affected by creep. In this research, the volume deformation, stress evolution, and cracking behavior of UHPC incorporating steel fibres under variable temperature and uniaxial restrained condition were investigated by using temperature stress test machine, and the autogenous shrinkage, tensile creep, as well as stress relaxation were analyzed. The development of stress was aggravated by the shrinkage caused by the temperature drop and self-desiccation, which eventually resulted in the premature cracking of UHPC without fibres. The autogenous shrinkage of UHPC obtained from the free strain was reduced gradually when the volume proportion of fibres was raised. Incorporating steel fibres significantly reduced the early-age tensile creep of UHPC, and the tensile creep of UHPC changed little when the volume proportion of steel fibres was raised from 1.67% to 2.50%. The combined effect of creep and damage reduced the restrained stress by 77.3%, 76.1%, 76.1%, and 75.1% for UHPC incorporating 0%, 0.83%, 1.67%, and 2.50% volume proportion of steel fibres under the restrained condition, respectively.

Cracking failure behavior of high strength concrete containing nano-CaCO3 at early age

Nano-calcium carbonate (CaCO3) has been added to high strength concrete (HSC) to reduce cement usage, control greenhouse gas emissions, increase durability, and control excessive shrinkage of HSC. Many investigations on the mechanical properties and shrinkage of HSC containing nano-CaCO3 have conducted. However, the early-age cracking failure behavior of that considering temperature, shrinkage, restrained stress, and tensile creep simultaneously was rarely investigated. Temperature stress test machine, which could measure these factors simultaneously, was used to investigate the influence of nano-CaCO3 contents (0%, 1%, 2%, and 3% by weight of cement powder) on the early-age cracking failure behavior of HSC in the present investigation. Mechanical properties of HSC containing nano-CaCO3 were tested. The addition of nano-CaCO3 to HSC improved the mechanical properties, and reduced the tensile creep as well as autogenous shrinkage of that at early age. A simplified stress-strain failure criterion was proposed.

New insights into restrained stress and deformation mechanisms of concretes blended with calcium sulfoaluminate and MgO expansive additives using multi-scale techniques

The early-age cracking performance of expansive concrete relies heavily on the temperature, restraint and curing conditions. Empirical methods to use expansive additives are not always reliable in diverse engineering conditions. By using a novel approach with the self-developed temperature stress testing machine (TSTM), this study evaluates the early-age cracking sensitivity of concretes blended with Calcium Sulfoaluminate and MgO expansive additives under different engineering conditions. Their performances under wetting–drying cycles are also compared. To interpret the deformation mechanism in a microscopic scale, pore structures are characterized by mercury intrusion porosimetry and nitrogen adsorption/desorption tests. It is found that MgO concrete with proper reaction activity can offer more effective shrinkage-compensation effect than CSA concretes under high temperature or drying condition, due to its less demand for hydration water, delayed expansion and finer pore structure. The findings in this study offer us new insights in modelling and application of expansive concretes on site.

Autogenous deformation induced- stress evolution in high-volume GGBFS concrete: Macro-scale behavior and micro-scale origin

This study aims to experimentally investigate the autogenous deformation and the stress evolution in restrained high-volume ground granulated blast furnace slag (GGBFS) concrete. The Temperature Stress Testing Machine (TSTM) and Autogenous Deformation Testing Machine (ADTM) were used to study the macro-scale autogenous deformation and stress evolution of high-volume GGBFS concrete with w/b ratios of 0.35, 0.42, and 0.50. The early-age cracking (EAC) risk (quantified by stress-strength ratio) and stress relaxation were analyzed extensively based on ADTM and TSTM results. Furthermore, Environmental Scanning Electron Microscopy (ESEM), X-ray Diffraction (XRD), and Mercury Intrusion Porosimetry (MIP) were conducted to explore the micro-scale origin of the autogenous deformation of high-volume GGBFS concrete, which supports the observations on the macroscale measurement of TSTM/ ADTM tests. This study finds that the ettringite formation in the first two days results in autogenous expansion, which can delay the appearance of tensile stress. The magnitude of autogenous expansion depends on the compatibility of ettringite content and pore size. The w/b ratio of 0.42 turns out to be optimal because it produces the highest amount of ettringite and results in the highest autogenous expansion. In comparison, the w/b ratio of 0.35 introduces significant autogenous shrinkage after the expansion peak and therefore corresponds to a high early-age cracking risk.

Early-age autogenous shrinkage, tensile creep, and restrained cracking behavior of ultra-high-performance concrete incorporating polypropylene fibers

Ultra-high-performance concrete (UHPC) as a kind of progressive cement-based material has attracted wide attention in the field of concrete engineering due to its superior performance. Nonetheless, the high early-age cracking potential of UHPC under restrained conditions due to the considerable autogenous shrinkage (AS) becomes one major impediment. Polypropylene fibers have been added to UHPC for increasing the tensile strength. However, the influence of polypropylene fibers on the cracking potential of UHPC under restrained conditions is not well studied. The restrained cracking behavior of UHPC incorporating polypropylene fibers was examined by the temperature stress test machine tests in the present study. To study the influencing mechanism of polypropylene fibers on the cracking potential of UHPC, workability, mechanical properties, shrinkage-induced stress, AS, and tensile creep were investigated. Results indicated that the incorporation of polypropylene fibers improved the tensile strength and flexural toughness, decreased the elastic modulus and AS, and increased the tensile creep of UHPC. The shrinkage-induced tensile stress obtained from test was reduced by 64.9%, 65.9%, 67.8%, and 67.5% attributed to creep when compared to the calculated theoretical stress for UHPC incorporating 0%, 1%, 2%, and 3% volume proportion of polypropylene fibers, respectively. As a result, the cracking potential of UHPC under uniaxial restrained condition estimated by the integrated criterion decreased by 21.7%, 60.7%, and 44.1% with the increasing volume proportion of polypropylene fibers from 0% to 1%, 2%, and 3%, respectively.

The practical application of a self-developed temperature stress testing machine in development of expansive concrete blended with calcium sulfoaluminate additives

Early-age cracking of concrete is a complex problem related to temperature, restraint and curing conditions. Cracking sensitivity is hard to evaluate quantitatively and the use of expansive additives to mitigate cracking remains empirical in-situ. This study developed a Temperature Stress Testing Machine (TSTM) to evaluate early-age properties of concretes under varied engineering conditions. The general design of TSTM and the mathematical derivation of its working principle are provided. With the developed TSTM, early-age properties (free deformation, restrained stress, elastic modulus and early-age creep effect) of Calcium Sulfoaluminate (CSA) expansive concrete are tested comprehensively. Experimental results indicate that under high temperature condition, CSA concrete is not effective to compensate shrinkage, while the combination of CSA and lightweight aggregate resolves the problem. It offers us valuable insights on potential application of TSTM into mix design of crack-resistant concretes under complex engineering conditions.