Abstract
An elliptical ring test method is proposed to replace the circular ring test recommended by ASTM and AASHTO for faster and more reliable assessment of cracking tendency of concrete. Numerical models are also established to simulate stress development and crack initiation/propagation in restrained concrete rings. Cracking age, position and propagation in various rings are obtained from numerical analyses that agree well with experimental results. Elliptical thin rings of certain geometry can shorten the ring test duration as desirable. In thin rings, crack initiation is caused by external restraint effect so that a crack occurs at the inner circumference and propagates towards the outer one. In thick rings, crack initiation is mainly due to the self-restraint effect so that a crack occurs at the outer circumference and propagates towards their inner one. Therefore, thick elliptical concrete rings do not necessarily crack earlier than circular ones as observed from experiment.
Highlights
Shrinkage cracking of concrete occurs when the tensile stress generated due to restrained volume contraction of concrete exceeds its tensile strength
It is desirable if the position of cracking can be predicted in advance so that the resources required for detecting crack initiation and tracking crack propagation can be minimised during the experiment
For elliptical rings with a  b = 150  60 mm2 (see Fig. 13(a)) and a  b = 150  75 mm2 (see Fig. 13(b)), cracks initiated close to the vertices on the major axis of the inner elliptical circumference; for rings with a  b = 150  125 mm2 (see Fig. 13(c)), crack occurred close to the minor axis. It can be seen from the circumferential stress contour (see Fig. 14(a) and (b)) that the stress concentration is significant in elliptical concrete rings with a  b = 150  60 mm2 and 150  75 mm2
Summary
Shrinkage cracking of concrete occurs when the tensile stress generated due to restrained volume contraction of concrete exceeds its tensile strength. As a crack propagates close to the inner circumferential surface of a concrete ring, the combined restraining effects from the self-restraint and external restraint dominate These results, which are totally new and reveal the mechanism of the elliptical ring test, did not appear in the conference paper. The driving force for crack initiation and propagation in a thick concrete ring is the selfrestraining effect due to nonlinear circumferential drying rather than the external restraint effect from the central steel ring This is the reason why elliptical thick rings do not crack earlier than the circular ones. If any sealing condition allows the moisture distribution, and shrinkage, across the ring wall to become uniform, the selfrestraint effect from concrete itself will be largely reduced In this case, the restraining effect to a thick concrete ring will mainly come from the central steel ring, i.e. external restraint and is revealed for the first time in this paper. The elliptical ring test can be adopted for evaluating factors that may affect concrete cracking such as curing time, curing method, and evaporation rate
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