Mechanism And Control For Early-Age Shrinkage Crack Of High Performance Concrete Used For Bridge Engineering

The concrete will produce shrinkage deformation on account of the hydration and the unsaturated environment in the early age. And the shrinkage strain accumulates by the age of the concrete. If the shrinkage deformation is restrained, the risk of cracking will increase in the early age, especially for the high performance concrete. During the construction period of concrete bridge, the cracking of high performance concrete is often observed, and the shrinkage deformation is treated as the main cause. So how to avoid the early-age cracking of concrete is becoming the difficulty and hot point of research now.The macroscopic shrinkage deformation of the high performance concrete is closely related to its mesostructure. So the study of the macroscopic shrinkage deformation must base on the mesostructure of concrete, and the influence of the mesostructure on the macro appearance must be take into consideration. Only in this way we can find the underlying cause of the shrinkage deformation of concrete, and then put forward measures to control the cracking of concrete in the early age.Based on the research status and field research of the early-age cracking of high performance concrete, it is realized the time, form and characteristic of the early-age cracking of concrete in the bridge engineering. On this basis, it is illuminated the influence of water to cementitious materials ratio, fly ash content, air content and aggregate to cementitious materials ratio on the early-age shrinkage strain by laboratory test. And it is proposed a predicted model of early-age shrinkage of concrete, which is able to reflect the influence of mix proportion parameter. Meanwhile, it is discussed the influence of reinforcement ratio, reinforcement way, curing way, curing time on the early-age shrinkage deformation of concrete, and get the corresponding conclusions.By the combination of macro and mesoscopic method, the scan imagery of industrial CT and image reconstruction technique, it is established the mesoscopic mechanics model of concrete. At mesoscopic level, the concrete is treated as a composite material of aggregate and mortar. It is simulated the development process of the early-age shrinkage deformation of high performance concrete by the the explicit algorithm of FEM. And considered the differences of the mechanical and shrinkage characters between the aggregate and mortar, it is analyzed the influences of the distribution and size of aggregate and the shrinkage of mortar on the early-age shrinkage deformance of concrete at mesoscopic level. And the analysis result is verified by the laboratory test. It is acquired by simulation the stress distribution of the concrete and its relationship with the size and distribution of aggregate which under the freedom of shrinkage.Unite the elastic mechanics, microscopic damage mechanics and fracture mechanics, it is analyzed the process of shrinkage deformation, damage and destory of concrete at the mesoscopic level. Based on the dispersion crack model, it is established a mesoscopic mechanical system of concrete which can stimulate the early-age cracking process of concrete and reveal its inner mechanism.From the stimulated result, it can be found that the position of cracking is mostly at on the interface of aggregate and mortar and the interstice. This is due to the most significant constraint effect of mortar on the aggregate surface, and the stress concentration of interface resulted in the first cracking of the interface. And the initial crack on the interface will extend to the other weak areas with age. The bigger the interface area is, the stronger the inner stress caused by the constraint effect is. And the interface becomes crack initiation. With the extending, expanding and coming together of the micro crack, the macro crack occurs on the concrete. The meso cracks from the crack point location are extension, extension, and gradually converge with age, and finlly form the macroscopic crack of concrete.The formation mechanism of early-age shrinkage cracking for high performance concrete is analyzed both in meso and macro methods. The prophylaxis and treatment approach of the early-age cracking is put forward in this paper. And the method is verified in theoretical analysis, laboratory test and practical engineering. The prophylaxis and treatment approach will play an important role in avoiding early-age crack, enhancing durability and extending the service life of the concrete structure.