Investigating Crack Initiation and Propagation of Concrete in Restrained Shrinkage Circular/Elliptical Ring Test

The restrained ring test, which is recommended by AASHTO and ASTM, has been used for assessing the potential of early-age cracking of concrete and other cement-based materials. Recently, a novel elliptical ring test method has been proposed to replace the circular ring test method for the purpose of shortening ring test duration and observing crack initiation and propagation more conveniently. In order to explore the mechanism of this novel test method, a numerical model is developed to analyze crack initiation and propagation process in restrained concrete rings, in which the effect of concrete shrinkage is simulated by a fictitious temperature drop applied on concrete causing the same strain as that induced by shrinkage. First, an elastic analysis is conducted to obtain the circumferential stress contour of a concrete ring subject to restrained shrinkage. Combined with the fictitious crack model, a fracture mechanics method is introduced to determine crack initiation and propagation, in which crack resistance caused by cohesive force acting on fracture process zone is considered. Finite element analysis is carried out to simulate the evolution of stress intensity factor in restrained concrete rings subject to circumferential drying. Cracking age and position of a series of circular/elliptical concrete rings are obtained from numerical analyses which agree reasonably well with experimental results. It is found that the sudden drop of steel strain observed in the restrained ring test represents the onset of unstable crack propagation rather than crack initiation. The results given by the AASHTO/ASTM restrained ring test actually reflects the response of a concrete ring as a structure to external stimulation, in this case restrained concrete shrinkage.The financial support from the National Natural Science Foundation of China under the grants of NSFC 51478083 & 51421064, Engineering and Physical Sciences Research Council under the grant of EP/I031952/1, and the National Basic Research Program of China (973 Program, Grant No. 2015CB057703) is gratefully acknowledged

Shrinkage Behaviour of Cement-Treated Crushed Rock Base in Western Australia

Shrinkage cracking is a significant problem when using cement stabilised materials in the construction of road pavements. Reflective (upward) cracks travel from the cement stabilised base layer to the top of the asphalt surface can cause water ingress through the underlying pavement layers. This paper examines the shrinkage behaviour of cement-treated crushed rock base as applied to pavement conditions in Western Australia. The testing protocol to examine the shrinkage behaviour of the material was adapted from the cement shrinkage test in Australian Standards, AS 1012.13. The test results showed that the amount of shrinkage in the cement-treated material did not increase with additional amounts of cement. The highest shrinkage values were found for the 2% and 6% cement specimens, where shrinkage was approximately 17% greater than the lowest shrinkage value found for the 4% cement sample. Based on the results of this study, it seems that shrinkage in the samples with relatively higher cement content of 5% and 6% mainly results from loss of water during the hydration reaction process between cement and water. Shrinkage in the low cement content samples of 2% and 3% is governed by the evaporation of excess water after the hydration reaction. The 4% cement content sample demonstrated the optimum cement content when considering the lowest shrinkage values among other cement content samples and an appropriate unconfined compressive strength value