The fracture mechanism of circular/elliptical concrete rings under restrained shrinkage and drying from top and bottom surfaces

Due to uniform shrinkage along the radial direction, drying from both top and bottom surfaces has been recommended to replace drying from outer circumference surface in the restrained circular ring test to assess cracking tendency of concrete. However, non-uniform shrinkage along the height direction under drying conditions is significant, and its effect on crack initiation and propagation in a concrete ring is not clearly understood. To investigate the fracture mechanism of the restrained ring test under drying from top and bottom surfaces, three series of circular and elliptical ring specimens with heights of 30 mm, 50 mm and 75 mm are tested to measure the cracking ages. A fracture mechanics based numerical method is proposed by introducing fictitious crack model to simulate the fracture process and predict the cracking age of a concrete ring under restraint. The effects of ring geometric profile, specimen height and moisture gradient on crack development are discussed. The results indicate that, under drying from both top and bottom surfaces, crack initiates partly along the height direction at the inner circumference of a concrete ring, and propagates along the radial direction, one by one, until the crack propagated throughout the whole cross-section. The moisture gradient along the height direction has significant effect on the crack driving force, which is dominated by the moisture gradient and steel ring restraint near the exposed surface, whose proportion increases with the increase in distance from the exposed surface.