Cracking Mechanism Research Of Early-age High Performance Cement-based Materials Based On Experiment

As concrete is more and more widely used in engineering field, high performance concrete has some excellent performances like high workability and high durability. On the other hand, abundant cement and low water-cement ratio also cause volume instability, which makes the early cracking problem of structure more and more serious. The cracking problem of early age concrete has become the important reason leading to performance degradation of concrete structure and affecting its durability.Concrete is mainly composed of cementitious materials such as cement cementing aggregate and sand together, and its early cracking mainly result from the shrinkage deformation of cement gel is restrained by aggregate, steel bar and gel itself. Cracks are produced primarily in the cement mortar matrix interior,as well the interface between it and aggregate. Therefore, this article chooses cement mortar as the main research object to effectively study the early mechanical properties and cracking mechanism of cement based materials.This paper has tested the flexural strength, compressive strength, splitting tensile strength and elastic modulus of the early-age mortar specimen, and studied the influence of the water-cement ratio and expansive agent on the basic mechanical performance. Shrinkage tests, with different water-cement ratio, content of expansive agent, and different moisture dry direction, were carried on to explore the shrinkage deformation law of mortar, and then the creep properties of cement mortar specimens were studied. The conclusion showed that:(1) the strength, elastic modulus, and shrinkage deformation of mortar will increase when the water-cement ratio decreases, while adding expansive agents can effectively reduce the shrinkage of mortar, and not affect the development of the later strength, then the shrinkage deformation under limiting moisture drying condition will decrease;(2) lower water-cement ratio reduces the early extensibility of mortar and adding suitable expansive agents can increase its creep relaxation ability, which is beneficial to control cracking sensitivity.This paper also used circle cracking device to test the crack resistance of early-age mortar and analysis the influence of water-cement ratio, expansive agent and different constraint degree on the cracking sensitivity of cement based materials. Temperature sensors were used to monitor the temperature development of ring specimen, and the development trend of ring specimen internal cracks were observed by using industrial CT. The research found that:(1) reducing water-cement ratio and increasing the thickness of the steel ring would lead to the early cracking of ring specimens, while adding expansion agent could significantly prolong the cracking time.(2) CT scan was carried out on the mortar ring, which found cracks gradually widened from inside to outside after the shrinkage crack developed through the circle surface.During the ring cracking test, strain measurement tests were carried out on the mortar ring specimens with given water-cement ratio. In addition to traditional strain monitoring method, a new method was also put forward to measure the internal strain of the constrained mortar ring, which used embedding the combination of carbon fiber and strain gauge. The experiment results show that:(1) the means of using burying the combination of carbon fiber and strain gauge to measure the internal strain of mortar ring is feasible, and the cracking of mortar specimens can be monitored by observing the mutation or the inflection point of the strain curve;(2) The cracking time detected by using buried strain gauge was earlier than that with artificial direct observation, so it was confirmed that the internal cracking of mortar is earlier than the external. It also showed that burying the combination of carbon fiber and strain gauge in cement-based materials is a good method for accurately warning material’s cracking, since it could monitor the cracking phenomenon which couldn’t be observed by artificial macroscopic observation.