Research On The Fracture-strength Of Concrete And Its Application

To prevent concrete cracking is a significant issue in the construction of momentous hydraulic concrete structures.The determination of the true fracture parameters of concrete is a key basis for cracking risk evaluation and crack stability analysis of concrete structures.Thus,in this dissertation,the method for calculating the fracture-strength of concrete was proposed,the fracture experiments of concrete exposed to different environment were carried out and the model for predicting the fracture parameters of concrete under arbitrary conditions was established.Finally,the true fracture-strength of concrete was obtained.The main research content and achievement of this study are as follows:(1)The concept of the fracture-strength of concrete was introduced and its calculation method was proposed.In order to characterize the effect of initial defects in concrete on its macroscopic crack resistance,a calculation method for determining the fracture strength of concrete based on toughness was proposed.The accuracy and rationality of the proposed method for calculating fracture-strength were verified using exiting experiment results.The results show that the ratio of fracture-strength to the axial tensile strength is equal to 0.8,which is independent of the gradation,mix ratio,and strength of the concrete.Thus,the fracture-strength of concrete can be used as an intrinsic material property of cracking analysis.(2)The fracture parameters of the concrete under variable temperature and humidity conditions were obtained.The influence of curing temperature,ambient humidity and age on the fracture properties of concrete were systematically studied by carrying out the fracture tests on wedge splitting specimens exposed to different curing conditions.The fracture of concrete under different curing condition were measured at typical ages.Then,the applicability of the proposed fracture-strength calculation method in the variable temperature and humidity conditions was verified by comparing experimental data and predicted values of fracture-strength.The results show that a high temperature inversion phenomenon was observed in the test for fracture energy,unstable fracture toughness,initial fracture toughness and fracture-strength of concrete,with larger later-age fracture parameters corresponding to lower temperature and vice-versa.The optimum curing temperature was found to be approximately 40°C for development of concrete fracture properties.As time elapsed,the influence of curing humidity on the concrete fracture properties became increasingly pronounced,and larger values of fracture parameters were measured by specimens kept at high humidity.The concrete fracture mode also gradually changed from ductile to brittle with curing humidity decreasing.Furthermore,the ratio of the fracture-strength to the axial tensile strength of concrete is independent of its curing temperature and ambient humidity.(3)A new model was proposed to estimate the fracture properties of concrete cured at variable temperatures and humidity levels.In order to characterize the influence of environmental humidity on the development of concrete fracture properties,an innovative humidity rate function was introduced and its expression was given by theoretical derivation.On this basis,the fracture parameter prediction model that considers the coupling effect of the curing temperature,relative humidity and age was established.The physical meaning and the calibration methods of the key parameters in the proposed model were discussed.Then,the values of effective fracture toughness,initial fracture toughness and fracture energy were predicted using the proposed model.A comparison between experimental data and theoretically predicted results shows that the new model could be used to predict the fracture parameters of concrete exposed to arbitrarily changed temperatures and relative humidity levels with a relative error of less than 6%.