Mechanical Performance Of Strain Hardening Cementitious Composites And Acoustic Emission Characteristics During Polyvinyl Alcohol Fiber Bridging

The development and innovation of high-performance concrete engineering structures require breakthroughs in material properties.The inherent characteristics of traditional concrete materials cause its brittle cracking mode at the serviceability and ultimate load states.The achievement of ductile failure is one of the common pursuit goals of related research.Since fiber toughening is an effective way,the study incorporated Polyvinyl Alcohol(PVA)fibers into cement-based materials for developing Strain Hardening Cementitious Composites(SHCC)with multiple micro-cracks based on optimizing the mix ratio scheme repeatedly,and systematically conducted research on mechanical performance according to engineering demand.On the basis of existing research,a total of 11 types of mix proportions were designed considering the factors of the fly ash/binder ratio,sand/binder ratio,water/binder ratio,silica fume/binder ratio,and sand type.Firstly,the direct tensile full curve and cubic compressive strength were measured.The compressive strength of series of mix proportions santisfies engineering applications such as repair and reinforcement of masonry structures and concrete structural members.The ultimate tensile strain of M8-S using sea sand reached 4%,and the compressive strength was 35 MPa,which provides a new idea for the development of ecological SHCC materials.Secondly,for the M8 ～ M10 with stable tensile properties,the laser displacement sensor was used to measure the full compressive curve of the cylinder,and the electromotive strain method was used to determine the compressive modulus and Poisson’s ratio.Different mixture proportions of ductility had different compressive failure modes.The SHCC of bilinear model under direct tension and three-stage model under axial compression were established.In addition,the mode Ⅰ,mode II and mixed mode Ⅰ-II fracture properties of SHCC were preliminary studied.The relationship curves of load and crack mouth opening displacement,load and crack tip sliding displacement,as well as the crack initiation angle for mode Ⅰ-II fracture were measured.Fiber bridging effect is the theoretical basis of SHCC material design,and the fibermatrix interaction also affects the above macro-mechanical performance.Considering the two variable parameters including relative notch length(0.2,0.4,0.6)and the matrix type(M8,M9,M10),the SHCC tensile behavior was analyzed by double-edge notched plate under direct tension.The stress-crack mouth opening displacement relationship of SHCC was measured,and the energy dissipated in linear elastic stage,strain hardening stage and softening failure stage were calculated.In addition,the acoustic signals generated during the test were monitored with the aid of acoustic emission technology.The development and distribution of internal defects in SHCC were tracked by sound source localization,and the possible damage mechanisms of fibers and matrix were explored from ring count and energy.The results showed that the ultimate tensile strain of three groups of SHCC reached more than 3%,of which M9 was up to 5.81%,and showed obvious multi-cracking.The cubic compressive strength ranged from 15 MPa to 65 MPa.Due to the large deformation of SHCC,the peak compressive strain was about 4 times higher than that of concrete.Because of the high ductility of SHCC,the fracture energy in the case of mode Ⅰ fracture was much larger than that of concrete and steel fiber reinforced concrete.There was a diagonal ‘shear zone’during mode II fracture.SHCC is a cement-based engineering material with excellent mechanical performance.Due to the fiber bridging effect,multiple micro-cracks appear in the SHCC under load,showing a ductile failure mode and dissipating a lot of energy.It is expected to be used in key structural parts of infrastructure construction.