Research On Mechanical And Pore Structure Characteristics Of SAP Internal Curing Concrete

With the continuous advancement of China’s infrastructure construction,the requirements for concrete materials are becoming increasingly strict.As a new type of internal curing material,super absorbent polymer（SAP）possesses the advantages of good water absorption and water retention,light weight and hard to dehydrate after compression.The hydration process of concrete will be improved by SAP continuous water release and shrinkage process after it is added into concrete.Many scholars have studied and carried out a series of tests and explorations on the early cracking and shrinkage resistance of internally cured concrete,and they have reached a basically consistent conclusion:internally cured concrete had a strong ability to resist early cracking and shrinkage.However,in terms of internal curing concrete strength,due to the different types of SAP materials and mixing methods,the existing research has not reached a consistent and universal conclusion,and there is also a lack of enough research on durability.To explore the mechanical properties and durability of SAP internal curing concrete,several groups of concrete with different SAP content and particle size were set up,and the internal micro pore structure of SAP concrete was observed and analyzed by means of nuclear magnetic resonance（NMR）and scanning electron microscope（SEM）.The main research contents and achievements are as follows:（1）Strength and elastic module tests of SAP internal curing concrete.Adding SAP material could significantly reduce the early strength of concrete.The maximum reduction of cube compressive strength at 3 d curing age could reach 25%.The cylinder compressive strength and splitting tensile strength also had different degrees of reduction.However,as the age increased,the internal curing effect of SAP on concrete gradually worked,there was little difference in strength between SAP internal curing concrete and ordinary concrete.The change in compressive elastic modulus of concrete was consistent with the strength.（2）Micro pore structure characteristic tests of SAP internal curing concrete.The T₂ NMR spectrum of each group of concrete showed three obvious peaks,the first peak representing micropores accounts for the largest proportion,the second and third peaks representing mesopores and macropores account for a relatively small proportion.The effect of SAP material on the internal pore structure of concrete was obvious.With the growth of curing age,the internal curing effect of SAP was gradually brought into play,and more dense hydrates were generated.The number of pores in concrete was reduced,and the pore structure was also significantly optimized.The mixing method of dry mixing would still lead to the aggregation of SAP water sacs.After the SAP water sacs released water inside the concrete,they would leave the hole of the size of water sacs,which would become the weak point of strength.（3）Durability analysis of SAP internal curing concrete.The chloride migration coefficient of concrete gradually decreased with the increase of age,and the resistance to chloride erosion was gradually strengthened.The addition of SAP material could improve the resistance to chloride penetration of concrete,and the anti-permeability property of 56 d would be increased by 38%at most.（4）Based on the neural network fitting in MATLAB software,the chloride migration coefficient of SAP concrete was predicted,and the regression equation with mean square error of 0.40 and correlation coefficient of 0.975 was obtained.The average error of the prediction of chloride migration coefficient of SAP concrete by this neural network was 5.29%,indicating excellent prediction accuracy.