Assessment of restrained shrinkage cracking of concrete through elliptical rings

Abstract

An elliptical ring test method is presented to replace the circular ring test method for assessing cracking potential of concrete and other cement-based materials under restrained condition. The latter is recommended by both ASTM (American Society of Testing Materials (C1581/C 1581M-09a) and AASHTO (American Association of State Highway and Transportation Officials (PP34–39, 2004) as a standard test method for such purpose. However, there is no research published so far on comparing circular and elliptical ring test for assessing the relative likelihood of cracking of concrete and other cement-based materials as proposed in this study. Besides, ASTM proposes to use thin concrete rings with the wall thickness of 1.5 inches while AASHTO to use thick concrete rings with the wall thickness of 3 inches. The implication of these two ring wall thicknesses has not been well studied. The elliptical ring geometry employed here was used to facilitate unique ways of analysing cracking sensitivity of concrete. In line with this, the test program and numerical model developed was focused on investigating the mechanism of the elliptical ring test, irrespective of the concrete materials used which is also novel in this research as efforts towards this assessment are still yet to be published. The new experimental method investigated the use of elliptical rings for assessing the potential of concrete cracking under restrained condition to enable a faster and more reliable assessment of cracking tendency of concrete and other cement-based materials. A series of thin and thick elliptical concrete rings were tested alongside circular ones until cracking. Cracking age, position and propagation in various rings were carefully examined. It was found that thin elliptical rings with appropriate geometry can initiate crack quicker than circular ones which is desirable for accelerating the ring test. There were multiple visible cracks that occurred in elliptical rings and some cracks initiated but did not propagate through the ring wall, an interesting finding reported as the first time by this study. The features of multiple cracks in restrained elliptical rings were examined and their impact on interpreting elliptical ring test results was discussed. In addition, in restrained thin concrete rings, cracks initiated at their inner circumference and propagated towards their outer one while cracks initiated at the outer circumference and propagated towards their inner one for thick rings. To explore the mechanism of this new test method, a numerical model was developed to simulate stress development and crack initiation in concrete ring specimens under restrained shrinkage in which the effect of concrete shrinkage was simulated by an artificial temperature field externally applied on concrete causing the same strain as shrinkage does. A uniform artificial temperature field across the concrete ring wall generated good results and works well for simulating shrinkage cracking of thin concrete rings while an artificial temperature field with linear gradient across the concrete ring wall should be employed in order to predict cracking behaviour of thick concrete rings under restrained shrinkage reasonably well. Stress developed in concrete rings in the restrained shrinkage test was thus obtained through a combined thermal and structural analysis. Based on the maximum tensile stress cracking criterion, cracking age and position of a series of circular and elliptical, thin and thick rings were obtained from numerical analyses. It was found that numerical results match the experimental results in terms of initial cracking ages and position for a number of circular and elliptical concrete rings subject to restrained shrinkage. The effects of ring geometry on cracking in concrete were equally investigated by comparing the behaviour of the elliptical and the circular rings under restrained shrinkage. Both experimental and numerical results indicated that the ratio between the major and the minor semi-axes of an elliptical ring emerges as the main factor which affects the maximum circumferential tensile stress in concrete when subjected to restrained shrinkage. Thin elliptical rings with appropriate geometry can enable crack initiating earlier than circular rings, which is able to accelerate the ring test for assessing the potential of cracking of concrete mixtures. On the other hand, thick elliptical concrete rings do not to possess a favourable geometry effect over circular ones in accelerating ring test due to a relatively weaker restraining effect provided by the central steel ring. The results on crack position and stress development in concrete rings based on numerical analysis were examined to further explore the mechanism of the proposed elliptical ring test for assessing cracking potential of concrete. It was also found that the drying direction of the concrete ring has a substantial influence on the cracking age when thin concrete rings are considered.