The Study On Fracture Toughness Of Hybrid Fiber Reinforced Concrete Based On Acoustic Emission

As a brittle material,concrete often works with cracks in engineering,which is easy to produce brittle fracture.How to improve its deformation performance and enhance its fracture toughness has become an important problem to be solved.Fiber reinforced concrete technology can not only significantly improve the toughness of concrete,but also play a role of reinforcement and crack resistance.It is one of the breakthrough achievements in the research of concrete modification and improvement.In particular,hybrid fiber concrete can give full play to the advantages of different fibers and produce synergistic effect,which can improve the performance of concrete more effectively.The basalt fiber with small size and large elastic modulus can inhibit the development of early micro cracks;the plastic steel fiber with large size and high tensile strength can inhibit the development of later cracks.Therefore,this paper has made 13 groups of 39 specimens of fiber-reinforced concrete notched beams by using two kinds of fibers,plastic steel fiber and basalt fiber,for single mixing and mixing and make 13 groups of 39 specimens of fiber reinforced concrete notched beams to study on the influence of fiber on the fracture toughness of concrete by the research method of combining laboratory test,numerical simulation and theoretical analysis.The main research contents and achievements are as follows:①Through the three-point bending test of the notched beam,the load displacement curve(P-δ curve)and load crack opening displacement curve(P-CMOD curve)of the concrete with single mixed plastic steel fiber,single mixed basalt fiber and mixed fiber are obtained.It is found that adding fiber can improve the instability load and critical displacement of concrete,and improve the brittleness of concrete.Hybrid fiber can produce positive effect and play the above role more effectively.②Fiber can prolong the crack initiation and instability time of concrete and reduce the average energy of specimens.Basalt fiber has longer crack initiation time and smaller reduction of average energy,and the amplitude of acoustic emission signal generated when it is pulled or pulled out is mainly between 60-70db;plastic steel fiber has longer instability time and larger reduction of average energy,and the amplitude of acoustic emission signal generated when it is pulled or pulled out is mainly between 70-80db.The initiation time,fracture time and initiation load of hybrid fiber A5 are the largest among the four groups of specimens,and the peak energy and average energy are the smallest,which shows that the hybrid of the two fibers can play a positive effect.In addition,the crack initiation load of all specimens is determined by using the characteristic of obvious crack initiation point of cumulative ring count time curve.③ Fiber can improve the fracture toughness parameters of concrete,but different fibers can improve different fracture parameters.Basaltic fiber can improve the initial fracture toughness KIcini greatly,up to 18.9%;plastic steel fiber can improve the unstable fracture toughness KIcun greatly,up to 26.9%.When the mixing ratio of steel fiber and basalt fiber is 2:1,the initiation toughness,instability toughness and fracture energy of concrete are improved greatly,up to 32.2%,38.7%and 330.2%,which means that the mixing of steel fiber and basalt fiber can work together to produce positive effect,and can improve the fracture toughness of concrete more effectively.Based on the two-line softening constitutive curves of three kinds of ordinary concrete,the two-line softening constitutive curves of hybrid fiber concrete are obtained.④ The reliability and rationality of the two-line softening constitutive curve of hybrid fiber reinforced concrete are verified by applying the obtained two-line softening constitutive curve to the numerical simulation test.By fitting the test data,the fitting equation of the mixed fiber concrete’s content with the initial fracture toughness KIcini and the unstable fracture toughness KIcun is calculated under the condition that the total content is not more than 9 kg/m3:When the content of plastic steel fiber is 3.28 kg/m3 and basalt fiber is 5.72 kg/m3,the maximum value of the initial fracture toughness KIcini of hybrid fiber concrete is 1.534 MPa*m-0.5;when the content of plastic steel fiber is 4.77 kg/m3 and basalt fiber is 4.23 kg/m3,the maximum value of the unstable fracture toughness KIcun of hybrid fiber concrete is 2.225 MPa*m-0.5.