Evaluating Shrinkage and Cracking Behavior of Concrete Using Restrained Ring and Free Shrinkage Tests

Abstract

Free shrinkage and restrained ring tests are used to evaluate concrete mixes designed for use in bridge decks. The study consists of a series of preliminary tests and three test programs. In each program, the concrete is exposed to drying conditions of about 21C (70F) and 50% relative humidity. The concrete mixes include a typical concrete bridge deck mix from both the Kansas (KDOT) and Missouri (MoDOT) Departments of Transportation, plus seven laboratory mixes, including a basic mix used as a control, a mix similar to the control but made with Type II coarse-ground cement, the control mix cured for 7 and 14 days, a mix with a shrinkage-reducing admixture, a mix with a reduced cement content compared to that of the control, and a mix with quartzite in place of the limestone coarse aggregate used for the other mixes. The free shrinkage specimens were 76 x 76 x 286 mm (3 x 3 x 11_ in.). The concrete ring specimens were 76 mm (3 in.) or 57 mm (2_ in.) thick and 76 mm (3 in.) tall and were cast around a 13 mm (_ in.) thick steel ring with an outside diameter of 324 mm (12_ in.). The results show that as the paste content of the concrete increases, the ultimate free shrinkage also increases. Replacing Type I/II Portland cement with Type II coarse-ground cement lowers the free shrinkage and shrinkage rate, and adding a shrinkage-reducing admixture significantly reduces these values. Extending the curing time lowers free shrinkage at early ages due to delayed drying and expansion during curing, but does not affect the restrained shrinkage rate at the start of drying. The free shrinkage and restrained shrinkage decrease as the surface to volume ratio of the concrete decreases. One out of 39 restrained rings cracked during testing, and the mix that did crack, MoDOT, had the highest paste content and highest shrinkage rate of all the mixes.