Study On The Influence Of Aggregate Gradation On Concrete Fracture Properties Based On Boundary Effect Theory

Concrete is an artificial building material made of water,cement,aggregate and admixtures in a certain proportion and usually through mixing,pouring,vibrating and curing processes.Each component of concrete material is the main factor that affects the performance of concrete.At present,although the research on the effects of aggregate gradation on concrete fracture properties has obtained abundant results,which mainly focus on the selection of the maximum aggregate size d_max.However,the study on the fracture behavior of concrete with the same maximum aggregate size d_max and different large aggregate content is scarce.In this paper,the influence of aggregate gradation on the performance of concrete is studied by designing five mixture ratios(C_D-15,C_D-30,C_D-45,C_D-60 and C_D-100)whose large aggregate content C of 8¹0mm are 15%,30%,45%,60%and 100%.In this paper,the uneven coefficient C_U is used to describe the quality of aggregate gradation.The C_U of C_D-100,whose aggregate distribution range is the narrowest belong to poor gradation,is the smallest.The C_U of C_D-60,which is not the best gradation because of lack of intermediate aggregate size,is the largest.Therefore,with the increase of the content of large aggregate,the change tendency of cubic compressive strength f_cu,axial compressive strength f_cp,splitting tensile strength f_ts and fracture peak load P_max of each aggregate grade is first to increase and then to decrease.It reaches its maximum at 45%.Analying the experimental results based on double-K fracture model and Yang shutong analytical theory,it is found that the equivalent crack extended length?a_c of each aggregate gradation has a positive correlation with the height of concrete ligament.The fracture toughness is not sensitive to the type of aggregate gradation and initial crack-depth ratios.Analying the experimental results by use of size effect theory,it shows that the critical fracture process zone equivalent length c_f of C_D-100-100 is the smallest,which means it is more brittle and emerges brittle failure before its fracture process zone may be fully developed.When 1.33?C_U?1.51,with the increase of the content of large aggregate,the change tendency of the critical energy release rate G_f of each aggregate grade is first to increase and then to decrease.It reaches its maximum at 45%.Analying the obtained fracture toughness by using boundary effect theory,it is found that BEM 1 established by direct application of boundary effect theory is not suitable for significant data dispersion caused by the small number of specimens or loss of data validity caused by the small difference of equivalent crack a_e.BEM 2-1 constructed by using maximum equivalent average aggregate size d_aveve to calculate the average aggregate size G is only suitable for concrete with less large aggregate content?about 15%?.BEM 2-2 constructed by using the nominal average aggregate size d_avg presented in this paper to calculate the average aggregate size G is reasonable and can fit reasonable fracture toughness K_Ic and tensile strength f_t in the range of?=1.0².0.For concrete with d_max=10mm and C_U?1.48,BEM 2-2 can use?=1.18+0.03467C to fit unstable fracture toughness in the double-K fracture model,and its corresponding tensile strength is closer to flexural strength and the error is less than 23%and can use?=0.68+0.021C to fit equivalent fracture toughness in Yang shutong analytical model,and its corresponding tensile strength is flexural strength and the error is less than 11%.