Experimental Study On The Influence Of Different Factors And Conditions On Concrete Fracture Parameters

As a widely used multi-phase quasi-brittle composite material in construction engineering,concrete has an extremely important role in studying its mechanical properties and fracture properties in practical engineering.In the process of dam construction and operation,there is already a saying "no dam and no crack",so the problem of concrete cracking has become a very important problem to be solved Determining the real fracture parameters of concrete is an important criterion for the risk analysis of concrete structure cracking.In order to obtain the actual fracture parameters of a dam concrete in the southwest region more accurately,the wedge splitting method was used to study the fracture process of concrete.The so-called different factor condition refers to the wedge-splitting test of concrete with different specimen size,different initial crack-height ratio and different maximum aggregate size under different cement-based conditions(low heat Portland cement or medium heat Portland cement),different boundary effect models are used to analyze the test results,and at the same time,the fracture toughness and tensile strength of concrete materials are obtained to verify the accuracy and applicability of the model.The main research contents and conclusions are as follows:(1)First of all,by carrying out the fracture performance test of different types of concrete,the test parameters of concrete in a specific age are measured.Combined with the calculation method of double-K fracture parameters provided by 'Hydraulic Concrete Fracture Test Regulations'(DL/T5332-2005),the fracture parameters of different sizes,different initial crack-height ratio,different maximum aggregate particle sizes and different cement-based concrete are calculated.The effects of specimen size,initial crack-height ratio,coarse aggregate size and cement base on the fracture performance of concrete were studied systematically.The results show that the fracture process of wet-sieved low-heated concrete specimens of different sizes is different;when the ratio of the ligament height of the wet-sieved low-heated concrete specimen to the maximum particle size of the aggregate is greater than or equal to 7.5,the fracture parameter value of the concrete material tends to be stable;Under the same specimen size,the initial fracture toughness of low-heated concrete with different initial crack-height ratio is an inherent parameter of the material,and has nothing to do with the initial crack height ratio;however,coarse aggregate has resistance to the crack propagation of concrete,the larger the particle size of the aggregate is,the greater the double-K fracture toughness and fracture energy of concrete are;Under the same other conditions,the fracture parameters of long-term low-heated cement concrete are significantly higher than that of medium-heated cement concrete(2)The rationality and accuracy of the three existing boundary effect models are verified.In order to obtain more real tensile strength and fracture toughness,Hu-Guan boundary effect model introduces the parameters related to the maximum aggregate size on the basis of the original Hu-Duan boundary effect theory,defines the failure mode of different forms of concrete,and explores the impact of the maximum aggregate size on the failure mode of concrete.The Koval-Gao boundary effect model combines the energy release rate with LEFM to predict the true fracture parameters of the material.The results show that the tensile strength and tensile strength fitted by Hu-Duan boundary effect model are quite different from the experimental values.Hu-Guan boundary effect model and Koval-Gao boundary effect model are fitted with tensile strength values that differ from the test average by 4.85%and 1.45%,and the error does not exceed 10%.The obtained tensile strength can be used as the actual fracture of concrete.The parameters solve the difference between the test values obtained by the direct method and the indirect method,and meet the actual requirements of the project.The difference between the fracture toughness obtained by the fitting and the average value of the unstable fracture toughness calculated by the double-K fracture criterion is 4.76%and-11.90%,and the error is not more than 15%.It is verified that these two boundary effect theories are suitable for determining concrete The material stabilizes the true fracture toughness.Through analysis and comparison,the Hu-Guan boundary effect model uses concrete aggregate maximum particle size and discrete coefficient to describe the virtual crack propagation,but the only discrete coefficient may be difficult to obtain accurate fracture toughness and tensile strength,while Koval-Gao boundary effect model can accurately predict the fracture toughness and tensile strength of the material simply by substituting the test results into the model.