Mode ? Crack Propagation Process Of Concrete Considering Specimen Thickness

The concrete structure abundantly exists in practical engineering in the form of threedimensional entity,but cracks inevitably occur in the pouring and severing process,of which the initiation,propagation and unstability cannot be studied just from the two-dimensional perspective.This paper studies the thickness effect of concrete fracture parameters by experiment and numerical simulation.The following three aspects of work is carried out:(1)Experimental study on the effects of specimen thickness on the model I crack propagation process of concreteTwenty-four concrete beams with prefabricated cracks,which the strength grade is C30,the initial crack-depth ratio is 0.3,the span is 600 mm,the height is 150 mm and thicknesses are 30 mm,60mm,90 mm,120mm,150 mm and 180 mm have been conducted using three-point bending test method.Respectively,many clip gauges are placed at the crack mouth to measure the crack mouth opening displacements at different thicknesses of the same specimen,four strain gauges are arranged near the prefabricated crack tip on the specimen surface to measure the initial cracking load at the specimen surface,and a clip gauge is set near the loading point to measure its displacement.The results show that within the scope of this experimental study,there is no obvious thickness effect of the unstable cracking toughness.The initial cracking toughness increases with the increase of the specimen thickness when the specimen thickness is less than 90 mm,the initial cracking toughness remains constant in other cases.The fracture energy increases with the increase of the specimen thickness,and the crack mouth opening displacement at the center is slightly smaller than it on the surface,but the difference is so small that can be ignored.(2)Effects of specimen thickness on the initial cracking load of model I crackANSYS is adopted to establish three dimensional models to analyze the concrete specimens above.Based on the initial cracking load of the specimen surface measured by the test,the stress intensity factors at the crack tips of difference thicknesses,the loading-point displacements and the crack mouth opening displacements are extracted.The external load is gradually reduced to the extent where the stress intensity factor at the crack tip in the central specimen equals to the stress intensity factor on the surface under the action of the initial cracking load,and this load is the initial cracking load corresponding to the center of the specimen.The results show that the closer it gets to the specimen center,the larger the stress intensity factor is,the smaller the loading-point displacement is and the smaller the crack mouth opening displacement is;with the increase of specimen thickness,the differences between the surface and the center on the stress intensity factor and the initial cracking load increase as well.(3)Effects of specimen thickness on the K_R curve and the FPZa curveFORTRAN is applied to calculate the K_R curve and the FPZa curve of the above concrete specimens.According to the measured elasticity modulus,the tensile strength,the Poisson's ratio,the initial cracking toughness,and the fracture energy of the concrete,the K_R curve is calculated by applying the crack propagation criterion.The results show that the K_R curve has the thickness effect,and the peak length of the fracture process zone increases with the increase of the specimen thickness.