Simulation And Experimental Study On Crack Resistance Of Early Age Concrete Under Different Temperatures And Humidities

Concrete is being one of the most important building material and the concrete engineering is widely used in the world,whose durability is very important for concrete structure.Early age concrete has microcracks due to shrinkage effect,which will reduce the strength and durability of concrete structure and bring hidden danger to the structure in the early stage of the project.Therefore,it is of great significance to carry out the research on the crack of early age concrete to evaluate the crack resistance.In view of this,the finite element program of early age concrete cracking under the coupling effect of temperature and humidity was developed,and the dynamic model based on the degree of concrete hydration was realized.The model is applied to early age cement mortar(C-0),Basalt Aggregate Concrete(BAC-40)and Shale Lightweight Aggregate Concrete(LAC700-40).Through the comparison of physical test and numerical simulation,the one-dimensional temperature and humidity variation rules and crack resistance of three kinds of early age concrete were analyzed,and the factors influencing the internal temperature and humidity fields and crack resistance of early age concrete were explored,the main conclusions are as follows.(1)The thermal insulation and moisture retention of LAC700-40 are obviously stronger than that of BAC-40 and C-0.The thermal conductivity of basalt is slightly higher than that of cement mortar.The change of inner temperature and humidity of BAC-40 is basically the same as that of C-0 at room temperature,but the inner temperature rise and inner humidity decrease vary faster than that of C-0 at higher temperature.(2)The drying shrinkage rate of C-0 is the highest and the crack resistance is the worst.The crack resistance of BAC-40 takes the second place.LAC700-40 has the best crack resistance.The drying shrinkage rates of three kinds of concrete can provide reference for shrinkage and cracking of early age concrete.(3)The numerical simulation shows that the changes of temperature and humidity,dry shrinkage rate and crack morphology are basically consistent with the experimental results,which can accurately simulate the internal temperature and humidity fields of early age concrete under different temperatures and humidities,as well as the strain change and crack propagation of early age concrete under the constraint of shrinkage strain.(4)The numerical simulation of temperature and humidity shows that the inner temperature and inner humidity of concrete are coupled.Wind speed,diffusion(drying)distance,internal and external humidity difference,internal and external temperature difference affect the internal temperature and humidity of early age concrete.With the increase of diffusion distance,the change rate slows down;Environmental wind speed and high temperature and low humidity environment will accelerate the loss of moisture in concrete,which is not conducive to concrete maintenance.(5)The numerical simulation of crack resistance shows that the increase of critical fracture energy release rate of concrete increases the difficulty of crack propagation,prolongs the cracking time,and effectively reduces the shrinkage strain.The increase of aggregate volume ratio does not affect the material damage and crack initiation,but makes the interface path between mortar and aggregate become more complex,which increases the crack development length,greatly consumes the potential energy increment generated by the shrinkage strain and reduces the shrinkage strain.The increase of the material strain threshold will delay the damage and crack initiation time,but it does not change the crack path and the energy consumed by the crack development,which has the effect of "strain lag".The research results of this thesis verify that the temperature and humidity coupled damage fracture numerical model is suitable for the shrinkage cracking of early age concrete,which has important reference value for improving the crack resistance of early age concrete in practical engineering,and proposes reasonable measures to improve the crack resistance of early age concrete.