Parametric Assessment of Stress Development and Cracking in Internally Cured Restrained Mortars Experiencing Autogenous Deformations and Thermal Loading
A finite element model is used to examine how the properties of cementitious mortar are related to the stress development in the dual ring test. The results of this investigation are used to explain the thermal cracking behavior of mixtures containing prewetted lightweight aggregates (LWA) by quantifying the contribution of several material properties individually. In addition to the beneficial effects of using the LWA as an internal curing agent to reduce the autogenous shrinkage of concrete, the LWA also helps to reduce the potential for thermal cracking due to a lower elastic modulus and increased stress relaxation. The rate of stress development, age of cracking, and magnitude of the temperature drop necessary to induce cracking in a dual ring specimen are dependent on a variety of factors, including the coefficient of thermal expansion of both the cementitious mortar and the restraining rings, elastic modulus of the mortar, creep effect of the mortar, and rate of thermal loading. Depending on the rate of cooling, cracking may or may not occur. The slowest rate of cooling ( C/h) minimizes the effects of creep while cooling rates faster than C/h can produce a thermal gradient through the mortar cross-section that needs to be considered. 1. Introduction When volume changes caused by the heating or cooling of concrete are restrained, residual tensile stresses can develop [1, 2]. These residual stresses can result in cracking if they reach the tensile strength of the concrete. The potential for cracking depends on a combination of a variety of factors including the degree of restraint in the concrete [3], the early-age mechanical property development of concrete [4], the shrinkage of concrete [5], the thermal properties of the concrete (e.g., coefficient of thermal expansion and heat capacity) [6], the heat of hydration [6], the rate of temperature change in concrete [7], and the environmental conditions such as ambient temperature and wind speed [8]. The dual ring test has previously been used to study the stress development and cracking behavior of concrete materials that show shrinkage or expansion at early ages [9–11]. The dual ring test consists of a mortar or concrete specimen that is cast between two concentric restraining rings. If materials for restraining rings are not selected properly in the dual ring test, temperature changes can substantially move the restraint boundaries and alter the degree of restraint. As such, the restraining rings in tests considered in this paper are constructed from Invar 36, a metal alloy having a minimal
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