Experimental Study On Bending Property Of High Ductility Magnesium Phosphate Cement-based Composites Under Low Temperature Curing Condition

The failure of concrete often occurs without an obvious sign orcracksin tensile zone,and the form of structural failure is usually a large crack cracking through the whole section which leads to the failure of structure.Therefore,the repair and reinforcement of concrete structure require the repair materials possess a strong cracking resistance.In addition,if the reinforcement of concrete occurs in cold areas or in winter,this requires that the material used to reinforce and repair concrete has strong temperature adaptability to ensure its reinforcement and repair effect in low temperature environment.High ductile magnesium phosphatecement-based composite(MPC-ECC)is anideal material to solve the above problems which processes the advantages of high early strength,strong temperature adaptability,strong adhesion with ordinary concrete and low drying shrinkage.In this paper,by adding PVA fiber into the matrix of magnesium phosphate cement,a kind of composite material with high toughness and ductility can meet the requirement of strength at low temperaturewas obtained.By studying the compressive strength of magnesium phosphate cement mortar and the bending properties of high ductility magnesium phosphate cement-based composites under low temperature curing conditions,the mechanical properties and workability of this material under low temperature curing conditions are described,which provides the extensive experimental date and theoretical support for the reinforcement and repair of concrete in low temperature environment.The main research contents and conclusions are as follows:(1)The compressive strength of magnesium phosphate cement mortar under low temperature curing condition is studied by changing the curing temperature,the water temperature of cement casting,the substitution content of fly ash,sand-binder ratio,water-binder ratio and other factors.The results indicate that the lower the curing temperature is,the lower the compressive strength and the hydration rate is.The compressive strength can be improved by appropriately increasing the water temperature.But for curing temperature higher than 0?,thecompressive strength will decrease when the water temperature reaches 50?.Appropriate substitution content offly ash can improve the compressive strength of magnesium phosphate cement mortar,while the increase of water-binder ratio or sand-binder ratio will reduce the compressive strength.(2)The workability of MPC-ECCcomposite with different fly ash substitution content,sand-binder ratio,water-binder ratio and fiber volume fraction under 0?ambient temperature was studied.The results show that the workability of the composite can be improved by increasing the substitution content of fly ash because of the fly ash has morphological effect.Increasing sand-binder ratio will increase the friction between magnesium oxide particles and quartz sand,resulting in the decrease of workability.The workability of composite will be improved when the water-binder ratio is increased due to the lubrication of water.The increase of fiber volume fraction ratio will increase the friction between fibers and reduce the workability of composite.(3)The effects of different substitution content of fly ash,sand-binder ratio,water-binder ratio,fiber volume fraction ratio,curing temperature and curing age on the bending performance of MPC-ECCcomposite were studied.The results demonstrate that a moderate amount of fly ashsubstitution can enhance the bending performance of the composite,whereasexcessive fly ashsubstitution content will reduce the bending performance.The bending properties of the material vary with different values,and the ductility and toughness of the composite are best when the sand-binder ratio is 0.2.The increase of the water-binder ratio can improve the ductility and toughness of the composite.The fiber volume fraction ratio can improve the bending property of the composite,but with the increase of the fiber volume ratio,the strengthening effect of the fiber is gradually weakened,which will increase the risk of fiber agglomeration.The lower curing temperature,the worse the flexural toughness and ductility.The bending performance of specimen is the best at the 7days curing age,while there is no strain hardening behaviorandthe bending performance of testspecimenis poor at the 28 days curing age.