Study On Early Plastic Tensile Strength Of Polypropylene Fiber Reinforced Concrete

If the fresh concrete is not cured effectively in the early plastic stage（within 2-10 hours after pouring）,it is prone to produce plastic cracks,which will affect the normal use of the structure or components,and will bring a series of durability problems as time goes by.In practical engineering,adding short-cut polypropylene fiber can effectively inhibit the early plastic cracking of concrete.In fact,the early plastic cracking strength of concrete is directly related to the early tensile strength.However,there are few studies on the early plastic tensile strength of concrete,and also the study on the early tensile strength of polypropylene fiber concrete is a blank.Furthermore,at present,the calculation formula of the tensile strength of fiber reinforced concrete is based on macro mechanics,but for the tensile strength of fiber reinforced concrete in the early plastic stage,it is more reasonable from the perspective of micro mechanics.Therefore,based on the capillary negative pressure theory of concrete,through theoretical derivation and experimental research,and combined with simulation method,this paper suggests the calculation formula of early plastic tensile strength of polypropylene fiber concrete.The main research work is as follows:（1）The early plastic tensile strength of plain concrete under different capillary negative pressure was tested,and the theoretical formula of early plastic tensile strength of concrete was derived based on the theoretical formula of shear strength and matrix suction（i.e.capillary negative pressure）proposed by Shen Zhujiang in soil mechanics and Griffith and Griffith brace failure criteria.According to the test results,the calculation formula of early plastic tensile strength of plain concrete based on capillary negative pressure was established.（2）It is not only capillary negative pressure,but also various fiber parameters,such as fiber volume ratioρ_f,length l_f and diameter d_f,that affect the early plastic tensile strength of polypropylene fiber reinforced concrete.In this paper,the early plastic tensile tests of polypropylene fiber reinforced concrete with different fiber parameters and different capillary negative pressure were carried out to study the influence of fiber parameters on the early plastic tensile strength of concrete with different capillary negative pressure.（3）Due to the random distribution of fibers on the tensile failure surface,MATLAB software was used to simulate the random generation and placement of polypropylene fibers to determine the relationship between fiber parameters and the number N_fof fibers at the failure section;The influence of fiber parameters on the relationship between fiber residual embedment length L and tensile displacement W was analyzed,and the relationship between fiber parameters and fiber initial embedment length L ₀ was studied.（4）Combining the force-displacement（F-W）relationship of tensile test of early polypropylene fiber concrete with the relationship between the residual embedded length of fiber and the tensile displacement（L-W）obtained by MATLAB simulation,the functional relationship between the early bond strengthτ_f of polypropylene fiber and concrete and the capillary negative pressure u_s was calculated and deduced.（5）The calculation model of fiber early plastic tensile strength consists of two items,one is the tensile strength of matrix concrete considering the influence of fiber parameters,the other is the strength enhancement item provided by fiber participating in tensile action.Among them,the fiber additive expression is derived from the bond strengthτ_fobtained above.Finally,combined with the experimental data of different capillary negative pressure and different parameters of polypropylene fiber early plastic tensile strength,the calculation formula of polypropylene fiber concrete early plastic tensile strength including various fiber parameters and capillary negative pressure was proposed.