Research On The Fracture Characteristics And Size Effect Of High-strength Concret

The construction industry is rapidly developing towards high-rise and large-span buildings due to the growth of the social economy and science and technology.This trend has led to higher demands for the strength,durability,impermeability,and other properties of concrete materials,resulting in the emergence of high-strength concrete（HSC）.Compared to ordinary concrete,HSC has higher strength and greater brittleness,making its fracture mechanism more complex.In addition,studies in the field of concrete have found that concrete structure failure is often related to their fracture properties and the fracture properties of HSC change as the size of specimens increases.This leads to differences between the fracture properties of large-scale components in actual engineering and those of scaled-down models in the laboratory.Therefore,exploring the size effect of HSC is necessary to improve the applicability and reliability of models in actual engineering.The study of the fracture properties and size effect of HSC has essential theoretical and practical significance.Firstly,this paper conducted compressive tests on cylindrical and cubic specimens,as well as splitting tensile tests on cylindrical specimens to determine the basic mechanical properties of HSC,and the results showed that the elastic modulus of HSC was 28.3 GPa,while its compressive strength and tensile strength were 58.4 MPa and 4.3 MPa,respectively.Secondly,to study the fracture properties and size effect of HSC,three-point bendingtests were performed on HSC beam specimens with prefabricated cracks,and electronic speckle pattern interferometry（ESPI）was used to observe the surface deformation of the HSC beam specimens.The study obtained the load-displacement curve’s through three-point bending tests and investigated the impact of geometrically similar specimens on fracture mechanics parameters.These parameters included fracture energy（G_F）of the fictitious crack model,equivalent elastic crack length（a_e）,crack tip opening displacement（CTOD_c）,and fracture toughness(K_IC^s)of the double-parameter model,as well as critical crack length（a_c）and fracture toughness(K_IC^{u n})of the double-K fracture model,etc.The results show that when the beam span S is less than 600 mm,the fracture parameters such as G_F,a_e,CTOD_c,K_IC^s,a_c,K_IC^{u n}are related to the size of the specimen;When S is more than 600 mm,the size effect of the fracture parameter is not significant.Based on the result of ESPI,the full-field horizontal displacement and strain fields on the surface of HSC beams were analyzed,and the fracture process zone’s length,width,and shape were determined using the strain threshold and critical crack opening displacement（w_c）.Additionally,the study examined the influence of size variation of geometrically similar specimens on the fracture process zone.The results indicated that during the peak load stage of the HSC specimen and the fully developed stage of the fracture process zone,the length(l_FPZ),width（b_m）,and shape(l_FPZ/b_m)of the fracture process zone were all related to the specimen size.Thirdly,based on the experiment,the numerical simulation software Abaqus was used to establish an HSC beam model with a single-size variation（initial seam height ratio,span,and thickness）using the extended finite element method（XFEM）and the impacts of single size variation of HSC specimens on the P-CMOD curve,the initial cracking load,peak load,the initial cracking fracture toughness and unstable fracture toughness were analyzed.By comparing the numerical calculation results with the experimental results,the accuracy of the numerical model was verified.The results showed that as the initial beam height ratio（a₀/D）and span（S）increased,P_iniand P_maxalso increased,and P_iniand P_maxwere inversely proportional to the B.K_IC^{u n}was inversely proportional to a₀/D and proportional to S,however,it was not affected by B.K_Iⁱ _C^{n i}was an inherent property of concrete materials and remained stable throughout.Finally,a summary of the research content and main conclusions of this paper was provided,and suggestions for further research were proposed.