Study Of Fracture Parameters Of Concrete Using Modified Round Compact Tension Method

The modified round compact tension method has small specimens and its operation is simple.It is suitable for drilling core samples,and is convenient for detecting and evaluating the fracture performance of concrete of existing building.Therefore,the tests and research on the fracture properties of the materials are carried out by the modified round compact tension method.The double-K and double-G fracture model proposed by Xu Shilang et al.is adopted as the calculation theory.This article is supported by the National Natural Science Foundation(51378461)and the Zhejiang Outstanding Youth Fund(LR16E080001).First,the stability of the new test method is verified.Then the size effect of double-K and double-G fracture toughness and its intermediate parameters measured by this test method is studied.Secondly,under the guidance of double-K and double-G fracture model,the effect of steel fiber and carbon nanotube on the fracture properties of RPC was studied.Modified round compact tension specimens were poured with concrete.The diameter of the specimens are 150mm,200mm,250mm and 300mm.A total of 4 groups and 16 specimens were poured.By analyzing the variance and variation coefficient of double-K and double-G fracture parameters measured by specimens with different size,the discreteness of the data was compared.The stability of the modified round compact tension method is verified.Then the size effects of the fracture toughness and the intermediate parameters of double-K and double-G fracture model were analyzed.It can be found that cracking load Pini,peak load Pmax,critical crack opening displacement CMODc,critical crack length ac have obvious size effect,increases with the increase of specimen diameter.When the specimen diameter is greater than 200mm,in this test,the initial fracture toughness KICini and initial fracture energy release rate GICini have no obvious size effect;unstable fracture toughness KICun and the unstable energy release rate of GICun increases with the increase of specimen diameter,but the rate of growth is small.A total of 3 groups of 15 specimens were poured by RPC,steel fiber reinforced RPC,steel fiber and multi-walled carbon nanotubes reinforced RPC.The modified round compact tension method was used to carry out this test.By analyzing the change of cracking load Pini,crack opening displacement CMODini,initial fracture toughness KICini and initial fracture energy release rate GICini,it can be found that adding fiber materials can effectively delay the cracking of cracks.By analyzing the change of peak load Pmax,critical crack opening displacement CMODc,critical crack length ac,unstable fracture toughness KICun and unstable energy release rate of GICun,it can be found that the addition of fiber material can increase the crack propagation resistance and postpone the crack growth process.By comparing the fracture toughness of the three materials,it can be found that the fracture properties of the material were greatly enhanced by the addition of steel fiber and multi-wall carbon nanotube.After adding steel fiber,the initial fracture toughness KICini of the material increased by 743.5%,and the initial fracture energy release rate GICini increased by 24 times.The unstable fracture toughness KICun increased by 1947.1%,and the unstable energy release rate of GICun increased by 153 times.After adding multi-walled carbon nanotubes,the mechanical properties of the materials are further improved.Compared with steel fiber reinforced reactive powder concrete,the initial fracture toughness KICini increased by 14.6%,the initial fracture energy release rate GICini increased by 43.1%,the unstable fracture toughness KICun increased by 26.7%,and the unstable energy release rate GICun increased by 70.7%.