The Study On Fracture Toughness Of Multi-scale Polypropylene Fiber Concrete Based On The Double-K Criterion

The application of concrete is limited by its brittle fracture properties.Through the incorporation of polypropylene fibers in concrete,cracking,toughening and strengthening effect of fibers,significantly improve the mechanical properties of concrete.Polypropylene fiber reinforced concrete has been widely used because of its low price and excellent corrosion resistance.Many researchers have blended polypropylene fibers and steel fibers in concrete to obtain better performance concrete.But taking into account the steel fibers' defects of expensive price,poor corrosion resistance,and bearing capacity,mechanical properties of polypropylene coarse fiber is similar with steel fiber,multi-scale fiber concrete was obtained by mixing polypropylene fiber with different scale,and its fracture characteristics were studied by means of the combination of laboratory test and theoretical analysis.The main research contents and results are as follows:(1)Twelve groups of notched beam specimens was designed and manufactured By controlling the incorporation of three different sizes of polypropylene fibers,load-displacement curves and load-crack opening displacement curves were obtained by the three point bending tests of specimens.It can be seen that the incorporation of fibers improve the brittle fracture of concrete,make the concrete in the crack extended to a certain width also has a carrying capacity from the test phenomena and test results.(2)Initial fracture toughness and unstable fracture toughness are studied based on the double-K fracture criterion.It can be found that the incorporation of polypropylene fibers has little effect on the initial fracture toughness,but has significant effect on unstable fracture toughness.The unstable fracture toughness of specimens blended three kinds of fibers are improved most notably.This shows that the bridging effect of the polypropylene fibers in cracks limit the expansion of cracks and extend the path of cracks expansion,which make the unstable fracture toughness be greatly improved.Under the same precondition of total content,the mixing of different sizes of fibers can achieve better results.(3)The bridging effect in cracks is the essence of fiber enhancement effect.The bridging stress is related to the pull-out mechanical properties of single fiber and the distribution of fibers in concrete matrix.By means of reducing the pull-out load-displacement curve to a linear ascending segment and a descending segment,describing the peak load by a function of embedded angle and length of fiber,and combining with the distribution function of fiber in concrete matrix,the fiber bridging stress model is obtained.(4)The bridging stress of fiber in cracks can make the supporting structure work with crack and improve the deformation capacity of the structure.The stress distribution assumed by traditional material mechanics theory is different from the stress distribution of fiber reinforced concrete with cracks.Based on the traditional material mechanics theory,the design of supporting structure is not fit for the design of tunnel lining.According to the analysis of fracture mechanics,the stress distribution of the fiber reinforced concrete structure is obtained and the fracture mechanics method is proposed based on it.