Based On Mesoscopic Mechanics Of Cement Base Material With Expansion Agent Stress Analysis Research

The early shrinkage and cracking of cement-based materials is one of important factors, which decreases the durability of concrete structures and shortens the in-service lifetime. Adding the expansion agents into the cement-based materials can effectively restrain the shrinkage deformation and improve the durability of the concrete structure. Surrounding the expansion mechanics problems of cement-based materials mixing with expansion agents, this paper investigates the deformation and stress of the cement-based materials with expansion agents by the experiments, modeling and numerical analysis. The main contents and conclusions are as follows:(1) The cement paste and mortar cylinder specimens with the different water cement ratios and expansion agent contents were designed and manufactured. The expansion ratios in the radial direction of the cylinder were measured during the different curing period. The influences of the curing period, water cement ratio and expansion agent content on the radial expansion rate of the cylinder specimen were analyzed. The results show that the radial expansion rate of the cement-based material cylinder increases with the curing period before14day, but it decreases slightly and tends to be stable after14day. The radial expansion rate of every cylinder increases with the expansion agent content, but it decreases with the increasing of water cement ratio. In the same condition, the radial expansion rate of cement paste cylinder is higher than that of mortar cylinder.(2) Based on the mesoscopic mechanics theory of the composite materials, some models of the composite representative volume element associated with ettringite expansion, which are two-phase cement paste and three-phase mortar materials, are established. According to the interaction between each phase of materials, the representative volume elements of cement-based materials are suggested to calculate the microscopic displacement, microscopic strain and microscopic stress. Using the homogenization approach, the macroscopic stress in the infinite cement paste and mortar specimen are calculated. Finally, the expansion stress of cement paste with the different expansion agent contents are analyzed. The results show that the moderate expansion agent can compensate for the shrinkage of cement paste. But less expansionve agent can not compensate. However, more expansion agent can produce the great expansion stress in the cement paste, which will cause the cracking of cement paste. (3) Using Powers hydration model and Moil-Tanaka method of mesoscopic mechanics, some calculation models for the elastic modulus and poisson’s ratio of cement-based materials with cement hydration degree, such as cement paste, mortar and concrete, are given, and they are used to analyze the influences of cement hydration degree and water-cement ratio on the elastic modulus and poisson’s ratio of cement-based materials. The results show that, with the increasing of the cement hydration degree, the elastic modulus increases, but the poisson’s ratio decreases. At the early cement hydration process, both the elastic modulus and poisson’s ratio increases quickly, but at the later, they tend to be stable. The higher water-cement ratio, the smaller elastic modulus.(4) Using MATLAB language to complete the programme, the influences of curing age, porosity and water-cement ratio on the deformation and stress of the representative volume element are analyzed. Taking the calculation of the radial expansion rate of cylinder as an example, the calculated results are analyzed and compared with the experimental results. The results show that, the expansion stress of cylinder specimen increases with the curing age, but decreases with the increase of water-cement ratio and porosity, the calculated results are basically agreed with the experimental results.