Study On Mechanics Performance Of Mode Ⅰ Fracture Of C40 Self-compacting Concrete Beams

Self-compacting concrete,also known as self-leveling concrete,belongs to a large branch of high-performance concrete.With the rapid development of railway transportation and the improvement of infrastructure in China,the self-compacting concrete will be used more and more widely in civil engineering construction.The fracture failure of concrete has always been the main reason for the decrease of durability and deterioration of concrete structures（components）.The study on fracture mechanics of concrete is also an important part of the study of fracture mechanics.Up to now,there have been a lot of researches on the fracture properties of ordinary concrete with different strengths,but few aimed at the fracture properties of self-compacting concrete.In order to study the influence of volume fraction of coarse aggregate and crack depth on the fracture characteristics of self-compacting concrete,45 three-point bending test beams with prefabricated cracks in mid-span were studied by electrical measuring method and digital image correlation method,which included 15 sets of specimens with three kinds of volume fraction of coarse aggregate and five kinds of crack depths.In this paper,initial cracking load and peak load of the test beams is measured by electric measurement method,and the calculated value of crack opening mouth displacement was obtained;the measured values of fracture process zone length and crack opening mouth displacement were obtained by digital image correlation method;based on the theory of critical distance,the relationship between the generalized critical distance and the length of fracture process zone is studied;the fracture toughness and tensile strength of the beams were calculated by double-K fracture criterion and boundary effect theory.The main work is as follows:（1）The working performance and basic mechanical performance of the three kinds of self-compacting concretes with different proportion of aggregate volume are studied by tests.The results show that the three kinds of self-compacting concretes are up to the mustard;the compressive strength and split strength increase slightly with the increase of the coarse aggregate volume fraction.（2）The initial cracking load and peak load of the three-point bending self-compacting concrete beams is measured by using electric measurement method.The results show that under the principle of single variable,both the crack initiation load and the peak load decrease with the increase of crack depth,and they increase with the coarse aggregate volume fraction increases.（3）The length of fracture process zone and opening displacement of crack mouth in the three-point bending test beams are measured by using digital image correlation method.The results show that in the process of specimen fracture,the length of fracture process zone is lowering as the crack depth increases,but it is not obvious with the change of coarse aggregate volume fraction.The crack mouth opening displacement is positively correlated with the load,and it’s growing as the crack depth or the coarse aggregate volume fraction increases under the same load.（4）Based on the double-K fracture model,the double-K fracture toughness,including K_IC^{in i} andK^u_IC ⁿ,of three kinds of self-compacting concrete beam specimens with different coarse aggregate volume fraction was studied.The comparison between crack depths varies from 0.2 to 0.6 indicate that the calculated double-K fracture toughness is independent of crack length size.For beam specimens with different coarse aggregate volume fraction,the double-K fracture toughness increased with the increase of coarse aggregate volume fraction.It can be considered that the double-K fracture toughness of self-compacting concrete is greatly affected by the change of coarse aggregate volume fraction.（5）The point method and line method in theory of critical distance is taken to analyze the length of fracture process zone.Although the generalized critical distance values obtained by point method and line method are close,the predicted results of line method criterion are more conservative than that of point method criterion,and the length of the process zone predicted by the two methods decreases with the increase of the initial crack-depth ratio,which is consistent with the experimental results from digital image correlation method.The comparison between the theoretical results and test results by digital image correlation method indicate that the experimental value of FPZ length tends to 2L as the initial crack-depth ratio decreases when the initial crack-depth ratio is small（?₀=0.2 0.4）;the experimental value of fracture process zone length tends to the experimental value L with the increase of the initial crack-depth ratio when the initial crack-depth ratio is relatively large（?₀=0.4 0.6）.（6）The boundary effect theory is used to calculate the fracture toughness and tensile strength of three kinds of self-compacting concretes.The equivalent mean particle size d_ave,including the bulk ratio of aggregate and the maximum particle size of coarse aggregate,is added to the model.By comparing with the calculation results of the double-K fracture criterion,the fracture toughness obtained by the boundary effect theory fitting can be established to be close to theK^u_IC ⁿ in the double K fracture criterion,and the tensile strength obtained by the theory of boundary effect is in good agreement with the splitting tensile strength obtained by the experiment.