The Experimental And Numerical Research On The Failure Mechanism Of Early-Age Concrete Under Restrained Shrinkage

Early-age concrete is likely to crack because its restrained shrinkage, these cracks often exit in concrete structures with large surface exposed to air, such as concrete pavement surface layer and industrial floors, which is one of the major diseases of concrete. The restrained ring tests are widely used for evaluating crack resistance of early-age concrete, but the performance of concrete, specimen geometry and drying direction all influence the experimental result and fracture mechanism. Thus, the researches on the fracture mechanism in the restrained ring test are significant to accurately assess crack resistance of early-age concrete. Based on the above mentioned, the relative researches are carried out in the dissertation as follows:(1) Experimental investigation on the early-age concrete characteristicsEarly-age concrete characteristics, such as axial compressive strength, splitting tensile strength, modulus of elasticity, fracture energy, fracture toughness and free shrinkage, were measured at the age of 1,3,5,7,14,21 and 28. And the variation of early-age concrete characteristics with age were gained.(2) The restrained circular and elliptical ring tests on concrete under restrained conditionThe restrained circular and elliptical ring tests with 37.5mm concrete wall thickness are carried out, and the influence of specimen geometry on concrete cracking time is discussed under out circumferential drying. The results show that, elliptical ring specimen can accelerate shrinkage cracking and shorten test time at the same condition.(3) Numerical investigation on the shrinkage fracture mechanism of concrete under restrained conditionConcrete shrinkage is simulated through applying the fictitious temperature filed on concrete, the crack location of concrete ring is determined through the circumferential stress contour from an elastic analysis. Combining with the fictitious crack model and propagation criterion, the crack initiation and propagation of ring specimens with circumference exposed is simulated. At the same time, the proposed method is proved to be feasible because that concrete cracking time from numerical results and experimental results agree well. The results show that, concrete directly go into unstable propagation stage after crack initiation, and the sudden drop of steel compressive strain represents crack initiation and unstable propagation.(4) Theoretical research and experimental application on the degree of restraint in the restrained shrinkage elliptical ring testA numerical analysis method is adopted to simulate concrete shrinkage under restrained condition through applying fictitious temperature field on concrete. The effects of concrete ring thickness, steel ring thickness and concrete elasticity modulus on degree of restraint are discussed, and the formula of degree of restraint for elliptical ring specimen is fitted based on the numerical results. Finally, a design approach of restrained elliptical ring test is introduced to assess crack resistance of concrete structural under high degree of restraint. The results show that, meeting demand of degree of restraint, the elliptical specimens require thinner steel ring and facilitate the operation than circular specimens.