Research And Application Of Concrete Fracture Toughnesses Under Steady And Unsteady Crack Growth

The fracture toughness expressed in terms of the stress intensity factor or the energy release rate has consistently been the research focus of concrete fracture mechanics.For concrete which shows the nonlinear characteristic during fracture,the contributions of cohesive forces on FPZ to the fracture toughnesses are not negligible.Thus,the cohesive force distribution along the FPZ is important,which in the past was assumed to be invariable as crack propagates,known as steady crack growth.However,many researches on FPZ demonstrated that the crack propagation in concrete is more like unsteady-state.Therefore,it is necessary to further study the concrete fracture toughnesses based on the steady and unsteady crack growth,especially considering their relation with the cohesive force,softening function and crack opening displacement.The researches on fracture toughnesses under steady and unsteady crack growth could bring an advance in concrete fracture mechanics.In this paper,the testing methods for determination of concrete fracture toughnesses,the theories and applications of concrete fracture toughnesses under steady and unsteady crack growth were studied.The detailed contents are listed below.1.Matters needing attention in concrete fracture tests were introduced.Load passing routes for fracture tests were analyzed,and the effects of loading device stiffness,loading support,steel platen form and loading control way on the test results were studied.2.Experimental and numerical test results were used to calculate the KR-resistance according to steady and unsteady crack growth.Then,the KR-resistance curves and the variation of FPZ length for different specimen sizes were investigated.The results showed that the FPZ length in concrete first increases and then decreases with crack growth,which is the unsteady crack growth.Unsteady crack growth in concrete was explained by a boundary effect.For large specimens,the crack tip is far from the back boundary when the FPZ length achieved its maximum value,thus the crack growth could be recognized as steady-state in a period of time after the FPZ length achieved its maximum value and before the crack tip approaches the specimen boundary as the boundary effect is little.3.The expression for GR-resistance in terms of the energy release rate was deduced based on linear elastic fracture mechanics considering the effect of cohesive forces on FPZ.The GR-resistance curve obtained by this expression could exhibit the same properties as the KR-resistance curve.Furthermore,the expression for GR-resistance under steady crack growth is validated by test results.4.The occurrence condition for steady crack growth was roughly determined based on crack opening displacement distribution,cohesive force distribution and GR-resistance curves by numerical tests.It is revealed that the occurrence condition for steady crack growth is related to specimen sizes,softening functions and specimen geometries.GR-resistance theories under the steady crack growth could be applied to calculate the initial fracture toughness by approximately choosing a softening function satisfying(?)0CTODc?(w)dw=(Klcun)2/E and to determine the parameters of the softening function.5.Wedge-splitting tests were performed on cylindrical specimens drilled from Danjiangkou dam.Then,double-K fracture parameters of dam concrete were determined by these tests.These double-K fracture parameters may be underestimated because the ratio of drilled specimen size to the maximum aggregate size is relatively small,and if they were directly used in the safety assessment in engineering,the result would be on the safe side.Besides,the stress intensity factors at the upstream vertical crack tip under water pressure for Danjiangkou dam were separately calculated by numerical and analytical method.It is found that the three-dimensional numerical method can be used to compute the stress intensity factors accurately,while the analytical method can roughly estimate the range of the stress intensity factors and is much simpler.Through comparing the relationship between the stress intensity factors at the crack tip and the double-K fracture parameters,the crack propagation state could be determined.