Assessing residual stress development and stress relaxation in restrained concrete ring specimens

Early-age cracking can be a significant problem in concrete pavements, floors, and bridge decks. Early age cracking occurs when the volumetric changes associated with drying, hydration, and temperature reduction are prevented. While free shrinkage tests can quantify length change, they may not always be sufficient for detecting materials that are prone to cracking since the potential for cracking is influenced by complex interactions of strength gain, stiffness development, creep, shrinkage, the degree of restraint, and toughness. One test which can be used to assess the cracking potential of a material is the restrained ring test; where a concrete annulus is restrained by an inner steel ring. The simplicity of the ring test enables it to be used as a comparative test to screen potential mixture designs. Based on the use of this test, AASHTO developed a provisional standard ring test which establishes specimen geometry, however the provisional standard does not provide an approach for quantifying stress development or indicating how close a specimen may be to failure. The main objective of this research is to provide quantitative information about stress development in the ring test that may be used to assess the potential for cracking in concrete. An analytical stress formulation is presented to enable the actual residual stress level in the concrete to be computed using only the measured strain in the steel ring. The theoretical elastic stress is computed using the free shrinkage, ring deformation, and elastic properties of the concrete. Comparison of the elastic stress and actual stress provides a measure of the stress relaxation (i.e., creep) in a material. A series of ring tests has been performed to investigate the influence of the degree of restraint and specimen boundary conditions on the stress development, microcracking, and visible crack development in the restrained ring specimens. Specimen geometry was found to have a significant effect on the stress development and cracking behavior of the restrained ring specimens. Specimens that shrink uniformly along the radius show the greatest variation in the age of cracking with thicker specimens cracking at a later age. Acoustic emission testing has illustrated that specimens with a higher level of restraint exhibited more microcracking as a part of the stress relaxation process. It has been also found that specimen boundary condition substantially influence crack development and propagation. Rings that dry from the top and bottom show a crack that propagates from the inside edge while specimens that dry from the outer circumference show cracking from the outside radius