An Experimental Study On Cracking Resistance Of Dry-mixed Plastering Mortar Under Different Drying Conditions

It is a quality defect of dry-mixed plastering mortar(DMPM)that is susceptible to cracking in practical applications.Therefore,experimental investigations and evaluations on cracking resistance of DMPM have become an important premise on expanding its applications in civil engineering.In order to reasonably evaluate cracking resistance of DMPM,it is need to develop test methods which can fully consider its real service conditions.This paper presents an experimental study on evaluation of cracking resistance of a commercially available DMPM with a strength class of M15.The experimental parameters mainly include initial drying age(1 d,2 d,3 d,5 d,and 7 d)and wind speed(0 m/s,4 m/s and 8 m/s).For the purpose of comprehensively evaluating the cracking resistance of DMPM,a series of experiments are performed on basic mechanical properties,free(drying)shrinkage,water loss ratio,and external restriction-induced cracking behaviors based on ring and plate specimens of DMPM.In the range of this thesis,the following conclusions could be drawn:(1)Both initial drying age and wind speed have little effects on the strength of DMPM at early stages(before 5 d).The earlier the initial drying age is and the greater the wind speed is,the more the loss of strength at later stages(at 28 th d)is.(2)The earlier the initial drying age is and the greater the wind speed is,the faster both the rates of water loss and drying shrinkage of DMPM develop.The correlation curve between drying shrinkage and water loss can be divided into two basic stages in terms of the slope.The slope is small at early stage and is large at later stage.(3)In the ring-restrained test of DMPM,the earlier the initial drying age is and the greater the wind speed is,the faster the restrained stress of the DMPM ring specimen develops,and the slower the restrained stress of inner surface of the DMPM ring specimen develops with an increase of free shrinkage strain.When the DMPM ring specimen fractures,the corresponding free shrinkage does not converges.The later the initial drying age is and the smaller the wind speed is,the larger the unit equivalent stress of the DMPM ring specimen at the cracking instant is.When the initial drying age keeps constant,as an increase of wind speed,restrained stress at the cracking instant of the DMPM ring specimen and the corresponding splitting tensile strength reduce.(4)In the plate-restrained test of DMPM,the earlier the plate specimens are exposed to drying environment(wind + strong light)and the greater the wind speed is,the greater the cracking index is.Compared with different wind speeds(4 m/s and 8 m/s),the influence whether the wind is performed or not on the cracking index is more obvious.