Study On Failure Mechanism Of Concrete Ring Test Under Drying From Top And Bottom Surfaces

Early-age concrete is likely to experience shrinkage in response to hydration reaction,and moisture dissipation in the process of maintenance.When the shrinkage is restrained,cracking initials easily,which results in low structure durability and huge maintenance cost.Due to its simple operation and capacity of providing uniform restraint,the restrained ring test has been widely used to assess cracking propensity of concrete and recommended as standard test procedure by AASHTO and ASTM.To solve the deficiencies of large non-uniform shrinkage along radial direction in specimens suggested by AASHTO and small coarse aggregate in specimens suggested by ASTM,this paper proposes the large concrete ring specimens under top and bottom surfaces drying to study the failure mechanism under non-uniform shrinkage along height direction,aiming at providing the appropriate test procedure to assess the cracking resistance of large aggregate concrete.The main work of this paper was described as follows:(1)Experimental study on early-age material propertiesThe basic mechanical property tests and fracture performance tests were performed at the1 st,3rd,7th,14 th,21st,28 th days,respectively.By numerical fitting,material properties from1 to 28 days were obtained.In addition,free shrinkage specimens with three sectional dimensions(30 mm × 75 mm,50 mm × 75 mm,75 mm × 75 mm)were conducted to measure free shrinkage strains from 1 to 28 days.(2)Experimental study on cracking ages of ring specimens under top and bottom dryingAll ring specimens conducted in this paper were 75 mm-thick and drying from top and bottom surfaces.Specimen geometries were selected as circular and elliptical,respectively;specimen heights were 30 mm,50 mm,and 75 mm,respectively;and the thickness of steel ring were 12.5 mm and 19.5 mm,respectively(12 kind of ring specimens in total).It can be seen from experimental results that ring specimens with 19.5 mm-thick steel ring cracked earlier than that with 12.5 mm-thick steel ring;elliptical ring specimens cracked earlier than circular ring specimens,which demonstrated the advantage of elliptical geometry;30 mm height ring specimens cracked earlier than 50 mm height ring specimens and 75 mm height ring specimens did not crack until the 28 th day,which demonstrated that,under top and bottom drying,cracking propensity was inversely proportional to specimen height.(3)Numerical study on crack initiation and propagation process of ring specimens under top and bottom dryingFinite element software,ANSYS,was used to simulate crack initiation and propagation process under assumption of uniform and non-uniform shrinkage along height direction,respectively.The fictitious temperature field,derived from free shrinkage strain,was applied on numerical model to simulate its shrinkage effect.Based on fictitious crack model and crack propagation criterion,crack initiation and propagation process was analyzed.It can be seen from numerical results that the assumption of uniform shrinkage along height direction was only appropriate for 30 mm height ring specimen.Under non-uniform shrinkage along height direction,first cracking occurred at left top and left bottom corner of cross section and propagated into outer surface to form two horizontal cracks.The horizontal crack propagated into center until throughout the whole cross section.In addition,the power to drive crack propagation was mainly provided by the outer restraint from steel ring rather than the self-restraint caused by non-uniform shrinkage,which proved that the ring specimen proposed in this paper(i.e.,75 mm thick,50 mm height and drying from top and bottom surfaces)can assess the cracking resistance of large aggregate concrete.