Quantification of restrained shrinkage cracking of concrete with ring specimens

Restrained shrinkage cracking in concrete structures is a growing concern in the construction industry, particularly in concrete members such as floor slabs, walls, and overlays. Tensile stresses in concrete are developed from the restriction of volumetric change caused by differential drying, autogenous shrinkage, and thermal expansion/contraction. Once the tensile stresses exceed the tensile strength capacity of the concrete, cracking occurs. ASTM C1581, “Standard Test Method for Determining Age at Cracking and Induced Tensile Stress Characteristics of Mortar and Concrete under Restrained Shrinkage,” was developed to quantify the potential of concrete cracking under restrained shrinkage. In this study, the tensile stresses generated in concrete ring specimens were quantified and a limited comparison was made to the stresses generated in one large-scale slab specimen. In addition, analysis procedures were developed to characterize stress relaxation (creep) in concrete ring specimens subjected to restrained shrinkage. The determination of average stresses, maximum stresses, and the estimation of effective modulus of elasticity were conducted. A procedure is presented for estimating the stress in concrete under restraint and is believed to be applicable to any restrained system. From an equation that relates the measured or actual degree of restraint to effective modulus of elasticity (Moon et al. 2006), similar values of effective modulus, representing ultimate values, were obtained for initial degrees of restraint ranging from 50-85%. This suggests that an alternative approach to estimate the effective modulus of concrete should be considered, as one would expect a greater amount of relaxation to occur in concrete specimens exposed to greater restraint. When the average stresses across the section are considered, it was found that the effective modulus varies with both time and initial degree of restraint, with the thicker rings associated with a lower effective modulus value. This is believed to be caused by a higher degree of damage that occurs under a higher restraint. It was also observed that early-age tensile creep coefficient under restraint ranges from 1-2 and corresponds to an effective modulus that is 30-50% of the elastic modulus of the material for initial degrees of restraint ranging form 50-85%. The effectiveness and limitations of comparing the performance of restrained concrete ring specimens to that of large-scale restrained slabs are also discussed