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Early-age cracking has been found to occur in some concrete bridge decks, slabs, and pavements 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. The simplicity of the ring test enables it to be used as a comparative test to screen potential mixture designs. From the use of this test, AASHTO developed a provisional standard ring test that 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. Described is a simple stress solution for quantifying the results of the ring test. Issues related to ring and free-shrinkage specimen geometries are discussed to improve the fundamental understanding of the information provided by the ring test. Also described is how elastic stress and actual stress can be compared to measure the stress relaxation in a material. To better illustrate the microcracking and visible-crack development process, acoustic-emission testing was performed. These experiments indicated that specimens with a higher level of restraint exhibited more microcracking as a part of the stress relaxation process.

This paper describes an experimental study that investigated the influence of ultrafine fly ash (UFFA), a relatively new pozzolanic admixture, on the early age stress development, stress relaxation, and cracking in restrained low water-tobinder ratio (w/b) mortars due to shrinkage. The restrained ring test was used to assess the early age residual stress development in mortar ring specimens. In addition, free shrinkage strain, elastic modulus, and splitting tensile strength measurements were performed to assess the stress relaxation (creep effect) and cracking potential. It was found that the restrained mortar mixtures containing the typical fly ash (FA) (ASTM Type-F) or the ultrafine fly ash demonstrated lower levels of residual tensile stress than the control mixture. Also, both typical fly ash mortar and ultrafine fly ash mortars demonstrated a reduction in free shrinkage. In addition, an increase in the age of cracking and a decrease in stress relaxation (creep effect) were observed in the restrained mortar ring specimens containing FA or UFFA. However, at equal replacement of cement with either FA or UFFA, the delay in the age of cracking and the reduction in stress relaxation were higher for the UFFA modified mortar.