Yang

Investigation of Microstructural Damage in Ultrahigh-Performance Concrete under Freezing-Thawing Action

This work aims to investigate the damage in ultrahigh-performance concrete (UHPC) caused by freezing-thawing action. Freezing-thawing tests were carried out on UHPCs with and without steel fibers. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and X-ray computed tomography (X-ray CT) were applied to detect the microstructure of the UHPC matrix before and after the freezing-thawing tests. The results showed that UHPC possessed very excellent freezing-thawing resistance due to its dense microstructure. After the freezing-thawing action, cracks occurred and were prone to initiate at the sand-paste interface in the UHPC matrix. MIP results also indicated that cracks appeared in the UHPC matrix after the freezing-thawing action. The number of defects that can be seen by X-ray CT increased in UHPC after the freezing-thawing action as well. The mismatch of the thermal expansion coefficients of the aggregate and the paste is considered to be the reason for the cracking at the sand-paste interface. The steel fibers in UHPC inhibited the propagation of cracks in the matrix and improved the freezing-thawing performance of UHPC.

Influences of Environmental Conditions on the Cracking Tendency of Dry-Mixed Plastering Mortar

Cracking tendency is one of the important performances of dry-mixed plastering mortar (DMPM). Environmental condition is a key factor to affect the cracking tendency of DMPM. For the purpose of evaluating the cracking resistance of DMPM and revealing the influence of environmental conditions on the cracking tendency of DMPM, a series of experiments were performed on restriction-induced cracking behaviors as well as free shrinkage, water loss, and mechanical properties of DMPM. The restricted shrinkage tests were based on ring tests and plate experiments. The results showed that the initial drying age exhibits significant influence on the cracking tendency of DMPM, and there was a stress balance period when the initial drying age was 2 days. But, the phenomena cannot be observed when the initial age was 3u2009d, 5u2009d, and 7u2009d. In order to eliminate the cracking tendency of DMPM, it should avoid water loss from the plaster layer during construction in practical engineering, especially, before initial drying ages.