Numerical Simulation On The Cracking Mechanism Of Early-age High Performance Restraint Mortar Ring

High Performance concrete is a new kind of high technology concrete. It’s widely applied in the high-rise buildings, large span buildings or bridge engineering. But as construction materials, its tensile ability is poor, which has not be solved with the improvement of the performance of the concrete. On the contrary, due to the rapid development of high performance concrete early strength, its chemical reactions are quick and more complicated. On account of the high dosage of cement and low water-binder ratio, the instability of the volume is easy to make the concrete crack at an early age. And the crack will reduce the durability of concrete, which makes the concrete structure degrade prematurely. Based on the early-age high performance restraint mortar ring cracking test, This paper has developed a numerical model for calculating the temperature field, humidity field and restraint stresses development of the restraint mortar ring. finite element analysis of the influence factors, stress and strain development in the early-age high performance mortar is used.Based on the heat conduction theory, this paper has developed a finite element model for calculating the early-age restraint stresses development of the restraint mortar ring by considering the synthetic effects of hydration heat, autogenous shrinkage, drying shrinkage, creeping and restraint by the steel ring. The development of thermal stress caused by cement hydration heat, drying shrinkage stress produced by moisture diffusion and autogenous shrinkage stress produced by hydration reaction in the early-age restraint mortar ring is analyzed. The rationality and practicability of the present numerical model are proved by comparing with the restraint ring test, and the soundly good agreements are observed.Based on Fick’s second law, this paper has developed a finite element model for calculating the humidity field and drying shrinkage stress development of the mortar ring by considering moisture diffusion. It has also been found that drying depth and ambient humidity significantly influence on mortar internal humidity distribution. The hyperbolic model is more applicable for low water-binder ratio cement-based materials, and it’s more accurate than the exponential model, which are proved by comparing the numerical model and the Parrot moisture prediction model. The drying shrinkage proportion of high performance mortar total shrinkage is smaller than that of autogenous shrinkage, which is proved by comparing with the shrinkage test. Moreover, In the process of cement hydration, temperature and humidity are interacting. The coupling of temperature and humidity is one of the reasons that the numerical model predictions and experimental data differ large. Ambient humidity change, the relative stiffness of the steel ring to the mortar ring and inner diameter to thickness ratio of the mortar ring significantly influencing on mortar ring drying shrinkage stress are discussed. The analysis shows that the restraint shrinkage stress of the mortar ring decreases with the constraint degree decreasing; The drying shrinkage stress increases with the inner diameter to thickness ratio of the mortar ring decreasing. The internal moisture gradient and drying shrinkage stress of the mortar ring are larger with ambient humidity lower, The mortar ring cracking earlier with ambient humidity lower is proved by comparing the drying shrinkage stress with mortar tensile strength, and the soundly good agreements are observed.