Study On Evaluation Method Of Cracking Resistance For Dry-mixed Plastering Mortar

Because of the strict quality control, stable performance, energy conservation, environmental protection, Dry-mixed mortar has been more and more widely used. But cracking often occur in the Dry-mixed plastering mortar(Hereinafter referred to as plastering mortar), therefore research and evaluation of the crack resistance become an important subject in application of Dry-mixed plastering mortar.In this paper, the research object is the plastering mortar with strength grade M5, M10, M15 and M20. Based on the experimental research and comprehensive analysis of basic mechanical properties, drying shrinkage performance under the no constraint condition, cracking characteristics under the constraint conditions, the ring constraint test method was proposed to evaluate cracking resistance for plastering mortar. Within the scope of this study, the following conclusions can draw:(1) In the case of same range of the consistency, with the increase in the amount of cement in the plastering mortar, compressive strength and flexural strength increase accordingly; in the case of same strength grade and the different consistency, strength decreases when the water are increase. For the same mortar, strength under the dry condition is lower than that under standard curing condition.(2) With the increase of cement content, the drying shrinkage of mortar under the no constraint and constraint conditions increase. The shrinkage cracking index under constraint condition shows that the possibility of cracking of mortar increases with the increase of cement content.(3) In the case of same curing time, the water loss rate in mortar decreased with strength grade increasing. in the case of same strength grade, the water loss rate decreased with curing time increasing. During the drying period, the shrinkage rate is roughly proportional to the water loss rate.(4) The results obtained in ring constraint shrinkage test showed that, the longer the curing time, the faster development rate of the steel ring strain is, and the bigger the maximum strain of steel ring at the cracking time is, but the cracking time is almost same. In the case of same curing time, with the increase of strength grade the steel ring strain rate become faster and the cracking time become shorter, but there is no obvious difference between the maximum strain of steel ring when mortar cracking.(5) The stress inside of the steel ring is larger than outside according to ring constraint test. It means that the cracking in ring mortar specimen may be developed from the inside to the outside. During drying of the mortar, the stress development rate decreased when the thickness of the steel ring increase. When the thickness of the steel ring are same, the longer the curing time, the faster the development rate of internal stress is, and the bigger the maximum stress at the cracking time is. When the curing time is same, the greater the strength grade, the faster development rate of the stress is, and the shorter cracking time is.