Experimental Study On Bonding Properties Of High Ductility Magnesium Phosphate Cement Based Fiber Composites To Existing Concrete

Magnesium phosphate cement（MPC）has many advantages,such as early strength,fast hardening,strong temperature adaptability,strong bonding,low dry shrinkage,good wear resistance,and good volume compatibility with old concrete in engineering.When MPC was used as a building material,it is especially suitable for the rapid construction,emergency repair and reinforcement of all kinds of buildings and structures,which can meet the strict requirements of timeliness and seasonality in repair and reinforcement projects.Magnesium phosphate cement based fiber composite（HDMPCC）prepared by adding PVA fiber to MPC can give full play to the superior performance of magnesium phosphate cement in repairing and strengthening.Compared with ordinary Portland cement ECC,magnesium phosphate cement-based fiber composite material has better mechanical properties and durability,and works better in coordination with old concrete.It has a wide prospect to apply it to the field of concrete structure reinforcement.In this paper,on the basis of the previous research group,the age,W/C,M/P,W/C,FA replacement amount,B content,fiber content were studied,and the drying shrinkage strain was tested.The effect of material ratio on drying shrinkage property was studied.Then,the bonding tensile and shear properties were tested through the bonding positive tensile and bonding shear tests,and the influencing reasons were analyzed.The main conclusions were as follows:（1）With the increase of W/C,the shrinkage strain of the specimen increases,and the bonding tensile strength and shear strength decrease.When W/C was 0.10,the shrinkage strain of the specimen was the minimum,and the bonding tensile strength and shear strength were the highest.Decrease of W/C could reduce the evaporation of free water and not only reduce shrinkage but also the generation of shrinkage cracks.（2）With the increase of S/C,the shrinkage strain of the specimen decreases,and the bond tensile strength and bond shear strength had little change.When S/C was 0.2,the shrinkage strain was the minimum and the bond strength is the highest.（3）With the increase of M/P,the shrinkage strain of the specimen increases significantly,the bonding tensile strength decreases,and the bonding shear strength increases.When M/P is 3,the shrinkage strain was the minimum,the bond tensile strength was the highest,and the bond shear strength was the highest when M/P was 5.When M/P was low,there was more phosphate,the hydration reaction can be fully carried out,the free water evaporation was less,and the shrinkage strain was lower.（4）With the increase of FA substitution rate,the shrinkage strain of the specimen decreased significantly,and the shrinkage strain at 40%FA substitution rate was the lowest.The tensile strength and shear strength of the specimen are the highest when the substitution rate is 30%FA.On the whole,when the substitution rate is between 20%and 30%FA,the bonding effect is the best.The main reasons are as follows:fly ash has filling function,which can reduce shrinkage;It can also react with Mg²⁺in the matrix to form cementitious material,which improves the bonding performance and strength.However,excessive FA substitution reduces the cement matrix material,weakens the hydration reaction,and the hydration products are also reduced accordingly,so the bond strength is reduced.（5）With the increase of the borax content,the shrinkage strain of the specimens increased significantly.The 3%borax content had the smallest shrinkage strain,which was only-40×10^-6 and slightly expanded.When the borax content was 6%,the bond tensile strength and bond shear strength of the specimens are the highest.Too high or too low borax content was not conducive to the development of bond strength.（6）With the increase of fiber volume ratio,the shrinkage strain of the specimen decreases and the bonding strength increases.When the fiber volume ratio was 2.0%,the shrinkage strain was the minimum,and the bond tensile strength and bond shear strength were also the highest.Increasing the fiber content had positive effects on the drying shrinkage and bond strength.（7）After three days of standard curing,the tensile strength of the specimen could reach about 40%of that of the ordinary concrete control group,the shear strength of the single shear bond can reach 82.8%,and the shear strength of the double shear bond can reach 66.67%.After curing for 7 days,the bond strength of the positive tension specimen can reach 80%of the bond tensile strength of the normal concrete control group,while the bond shear strength can reach 100%of the ordinary concrete control group.Different interface modes had significant effects on the bond tensile strength and bond shear strength.The specimens treated with high-pressure water injection had the highest bond strength.