The Effect Of Initial Defect And Moisture On The Concrete Strength Research

The complex physical and mechanical properties of concrete are closely linked to its microstructure and moisture state.On the mesoscopic scale,the concrete is regarded as a three-phase heterogeneous composite material consisting of aggregate,mortar,and interface transition zone(ITZ),and initial defect and moisture are distributed in the mortar and ITZ phases.In this paper,the concrete three-phase mesomechanics strength predictive model was first established based on mesomechanics theory.And considering the effect of initial defect and moisture on mortar and ITZ phases respectively,the predictive models of the effective strength of concrete with initial defects and under different moisture were obtained.In addition,on the macroscopic scale,the additional water pressure of fissure water in the concrete structure caused by the disturbance load has an impact on the structural fracture strength.Analytical formula of the fracture strength of the concrete structure considering the fissure additional water pressure was deduced based on the inclusion theory and fracture mechanics theory.The main contents and results of this paper include the following aspects:(1)Concrete is regarded as a three-phase composite material consisting of coarse aggregate,cement mortar and ITZ.The coarse aggregates and their peripheral ITZ were equated with equivalent particles based on the IDD method,and then considering the interaction between equivalent particles,the second equivalent of equivalent particles and mortar was deduced based on the Mori-Tanaka method,by which the three-phase mesomechanics strength predictive model of concrete was established.Selecting the tensile and compressive failure criterions of the mesoscopic phases,the cracking strength and effective strength of the concrete were calculated.The model predictive results are compared with test data to verify the effectiveness of the model.The influence of the aggregate elastic modulus,the aggregate content and the elastic modulus of ITZ on the cracking strength and effective strength are discussed.(2)Initial defect in the mortar and ITZ phases are taken into account on the basis of the concrete three-phase model.Based on the Mori-Tanaka method,the equivalent deformation parameters of the mortar and ITZ phases with initial defect were obtained.The relationships between the effective tensile and compressive strengths of meso-phases and porosity were expressed respectively from the proposed simplified center pore model,and the damage values of tensile and compressive strength caused by porosity were defined.The total influence coefficient of pore size,obtained by combining the pore size distribution function and influence function,was multiplied by the damage values,and the effective strength of meso-phases with initial defect were deduced.The equivalent mechanical parameters of the mortar and ITZ phases with initial defect were substituted into the concrete three-phase mesomechanics model,and the effective strength predictive model of concrete considering initial defect was established.The effects of porosity and pore size on the effective strength of meso-phase are discussed.The effect of porosity on compressive strength exceeds that of tensile strength,and the effective strength of meso-phase with the same porosity can be improved by decreasing the pore size.(3)The effects of moisture on the mortar and ITZ phases are considered on the basis of the concrete three-phase model,and the pores distributed in the mortar and ITZ phases are divided into active and inactive pores according to the moisture state.Based on the multiphase inclusion formula,the equivalent deformation parameters of the mortar and ITZ phases under different moisture were obtained.Considering the effects of moisture on the meso-phase strength based on the Griffith theory and the effective stress principle from the two aspects of surface energy and pore water pressure,the effective strength of mortar and ITZ phases under different moisture was obtained.The equivalent mechanical parameters of the mortar and ITZ phases were substituted into the three-phase mesomechanics predictive model,and the effective strength predictive model of concrete under different moisture was established.The effect of moisture on the effective strength of mortar phase is discussed.The mortar strength decreases with the increase of moisture,and the change rules of strength are related to the influence function of moisture.The proportion of pore water pressure to the mortar strength increases with the increase of moisture when the mortar fails.(4)On the macroscopic scale,taking the representative volume element containing the single fracture as the research object,the relationship between the fissure additional water pressure and the disturbance load was established based on the inclusion theory.With the application of the compress-shear fracture criterion,analytical formula of the fracture strength of the concrete structure with considering the fissure additional water pressure was deduced.The analysis of an example shows that fissure additional water pressure reduces the fracture strength of the concrete structure and increases the inclination angle range of fracture failure,which makes concrete structure easier to lose stability and fail along the fracture surface.The fracture strength increases with the increase of concrete elastic modulus and fracture shape coefficient,and its increasing trend is more noticeable with the increase of fracture inclination angle.