Stress Development In Concrete Research Articles

Numerical evaluation of built-in temperature distribution effects on stress development in concrete pavements

Built-in temperature distribution (BITD) is one of the critical factors affecting the stress development and performance of a concrete pavement. However, evaluation of the BITD has not been conducted and its influences on early-age and long-term concrete stress development have not been well understood. In this study, the effects of BITD on the stress development of concrete pavement slabs subjected to environmental loads were numerically investigated. A series of numerical models were developed to predict the early-age and long-term stress and temperature in the concrete slab. The effect of creep on the concrete stress development was then incorporated. Different concrete placement times on a day were simulated to evaluate the BITD and the concrete stress development. The results of the numerical model were validated with field measurements of the concrete temperature and stress developments of a full-scale concrete pavement test section under real climatic conditions. It was found that results generated by the numerical model for concrete temperature and stress developments were reasonable. The modelling results indicated that the effect of creep or stress relaxation could reduce the concrete stress by up to 34% at a pavement age of 20 d. The BITD obtained from the different concrete placement times has a significant effect on the early-age concrete stress, but it slightly affects long-term concrete stress development.

Finite-Element Modeling and Analysis of Early-Age Cracking Risk of Cast-In-Place Concrete Culverts

Extensive cracking was found in several cast-in-place concrete culverts in Alabama. This condition can decrease the long-term durability of the culverts. Early-age stress development in concrete is influenced by temperature changes, modulus of elasticity, stress relaxation, shrinkage, thermal coefficient of expansion, and the degree of restraint. The objective of this study is to determine means to mitigate early-age cracking in culverts by evaluating the cracking risk. Finite-element analysis was used to model the early-age stress by accounting for the following factors: construction sequencing, support restraint, concrete constituents, temperature effects, and the time-dependent development of mechanical properties, creep/relaxation, and drying shrinkage. Experimental results from restraint to volume change tests with rigid cracking frames were used to verify the accuracy of the finite-element analysis. A parametric study was performed to quantify the effect of changing joint spacing, joint type, construction sequence, concrete coefficient of thermal expansion, placement season, and concrete type on the risk of early-age cracking. The finite-element model results revealed that the use of the following measures will reduce the risk of early-age cracking in cast-in-place concrete culverts: concrete with lower coefficient of thermal expansion, contraction joints, sand-lightweight concrete or all-lightweight concrete, and scheduling the casting of the culvert wall to minimize the difference in its placement time relative to its previously cast base. Alternatively, to minimize the contribution of thermal effects on risk of cracking, the construction schedule should be developed to avoid concrete placement during hot weather conditions.

Stress prediction in very early-age concrete subject to restraint under varying temperature histories

Assessing the stress development in concrete requires an appropriate tensile creep model which is capable of incorporating the effect of the field environment conditions. This study quantifies the effect of temperature variation on the very early-age stress developments in restrained concrete by adopting a modified microprestress-solidification (MPS) theory-based creep model. The MPS creep model is first calibrated and verified based on the measured direct tensile creep data under normal and high temperature histories, it is then used to predict the very early-age stress development of the fully restrained concrete specimens under variable temperature history since casting. The predicted results are in good agreement with the experimental results. The tensile stress in restrained specimens can be relaxed by 80 %–87% within the first three days since casting. The predicted stress exhibits an obvious deviation from the measured one if temperature effect is not considered. Therefore, it is of importance to consider the temperature effect on concrete creep when the temperature variation in concrete is significant, and MPS creep model is valid for tensile stress prediction in concrete at very early ages.

Microwave decontamination of concrete

Microwave heating has been recently considered as an effective method to remove the contaminated surface layer of concrete elements. In such applications, an accurate simulation of the microwave–concrete interaction involves the solution of Maxwell's equations. However, owing to the inherent complexity of Maxwell's equations, some studies have used approximate methods such as Lambert's law to estimate the microwave power dissipation in concrete. Invariably, such approximations are prone to introduce errors in the results and verification using the more accurate methods is thus necessary. In this paper, Maxwell's equations are numerically solved to simulate accurately the microwave–concrete interaction. The electric field and energy dissipation as well as the resulting temperature rise and thermal stress development in concrete are computed. The results are used to verify the results obtained using a modified Lambert's law approximate method. Moreover, the effect of steel reinforcing bars embedded in a concrete specimen on the microwave decontamination process is investigated. The results show that Lambert's law can be satisfactorily used as an easy-to-use method to predict the temperature rise and stress development within a concrete specimen subjected to microwave heating.

Analysis of Shrinkage and Thermal Stresses in Concrete Slabs Reinforced with GFRP Rebars

The corrosion resistance of glass-fiber-reinforced polymer (GFRP) rebars makes them a promising substitute for conventional steel reinforcing rebars in continuously reinforced concrete pavements (CRCPs). Studies are conducted concerning the effect of using GFRP rebars as reinforcement in CRCP on the concrete stress development, which is directly related to the concrete crack formation that is inevitable in CRCP. In this study, an analytical model of a freely supported reinforced concrete slab is first developed to simulate the shrinkage and thermal stress distributions in concrete owing to the restraint provided by GFRP rebars in comparison with the stresses induced by steel rebars. The results show that the stress level in concrete is reduced with GFRP rebars owing to a low Young’s modulus of GFRP. In addition, the analytical model is utilized to estimate the concrete strain variation in the reinforced concrete slabs resulting from changes in the concrete volume, and the results are compared with the experimental observation. Finite-element (FE) analyses were also conducted to calculate the stress distribution and crack width of a GFRP-reinforced CRCP section subjected to both the concrete shrinkage and thermal change. By using the FE method, the crack spacing and crack width of a CRCP reinforced with GFRP rebars were predicted and compared with those of a steel-reinforced CRCP. The result shows that the crack spacing and the crack width of the GFRP-CRCP are larger than those of the steel-CRCP.

Prestress Loss Measurements in Missouri's First Fully Instrumented High-Performance Concrete Bridge

Prestress losses have a direct impact on concrete stress development and deflection behavior of highway bridge members. A poor estimate of prestress losses can result in a structure in which allowable stresses are exceeded or camber and deflection behavior is poorly predicted, such that the serviceability of a structure may be adversely affected. This paper reports the prestress losses observed throughout fabrication, shipment, erection, and the first 2 years of service for the first high-performance superstructure concrete bridge in Missouri. The prestress losses investigated included prerelease losses, elastic shortening losses, relaxation losses, creep losses, and shrinkage losses. Results from the study were compared with eight commonly used loss estimate models for total prestress losses, including AASHTO and Prestressed Concrete Institute methods. Recommendations were proposed by the authors for the most appropriate methodology to use to predict prestress losses in high-strength concrete girders accurately.

Prestress Loss Measurements in Missouri's First Fully Instrumented High-Performance Concrete Bridge

Prestress losses have a direct impact on concrete stress development and deflection behavior of highway bridge members. A poor estimate of prestress losses can result in a structure in which allowable stresses are exceeded or camber and deflection behavior is poorly predicted, such that the serviceability of a structure may be adversely affected. This paper reports the prestress losses observed throughout fabrication, shipment, erection, and the first 2 years of service for the first high-performance superstructure concrete bridge in Missouri. The prestress losses investigated included prerelease losses, elastic shortening losses, relaxation losses, creep losses, and shrinkage losses. Results from the study were compared with eight commonly used loss estimate models for total prestress losses, including AASHTO and Prestressed Concrete Institute methods. Recommendations were proposed by the authors for the most appropriate methodology to use to predict prestress losses in high-strength concrete girders accurately.