| Crumb rubber concrete is applied to the road and bridge engineering,which can fully exert its excellent toughness and impact resistance.With the continuous expansion of the construction scale of the road and bridge,reducing the water-cement ratio is generally adopted to make the mechanical properties of CRC meet the design requirements.However,the reduction of water-cement ratio seriously affects the volume stability of the pavement material,increases the risk of shrinkage and cracking of the structure,and limits the development and application of CRC in engineering.Super Absorbent Polymer(SAP)can be introduced into concrete as an internal curing agent.Through its slow release behavior in concrete during curing,the autogenous shrinkage caused by the reduction of internal humidity can be effectively decreased.Therefore,SAP with three different molecular weights were used as internal curing agents to study their effects on auto-shrinkage and compressive strength of low water-cement ratio CRC.The hydration process and microscopic pore structure of the internal curing CRC were analyzed by isothermal calorimetry,X-ray diffraction test,mercury intrusion test and SEM electron microscopy,meanwhile,the mechanism of different molecular-weight SAPs were analyzed.Considering the rubber content and the influence of SAP on the internal humidity of CRC,the rubber increase coefficient and humidity parameters were introduced,and the traditional concrete auto-drying shrinkage model was simplified and compared with the test results.(1)Pre-soaked SAP with different molecular weights has different water release rates and particle volumes,thus affecting the hydration process and pore structure characteristics of low water-cement ratio CRC.Isothermal calorimetry test shows that rubber addition prolongs the hydration induction period of CRC cement and delays the final setting time,and the addition of SAP significantly promotes the accelerated reaction phase and the post-accelerated reaction phase of cement hydration.The law is also obtained that the promotion effect is more significant with the higher molecular weight and the faster water release rate.Mercury intrusion test results illustrates that the rubber addition increases the porosity of micropores in the range of 10?100nm,while SAP increases the pores of which size are 100?1000nm and above 1000nm.Besides that,the higher the molecular weight,the higher the porosity of the macropores.(2)Internal curing CRC hydration process and differences in pore structure distribution characteristics affect the reduction effect and compressive strength.The auto-shrinkage test shows that the blending of SAP has a good shrinkage effect on low water-cement ratio CRC.It can be concluded that the shrinkage effect gets better as the molecular weight of SAP is higher.When the specimens of 5%rubber content are maintained after 35 days,the shrinkage of the specimens of internal curing by three different kinds of SAP decreased by 16.7%,26.3%and 34.0%,respectively.The compressive strength test indicates that the internal curing is beneficial to the development of CRC compressive strength.From low to high of the molecular weight of the three kinds of SAP,the compressive strength at 28d increased by 8.55%,11.45%and 4.78%,respectively.The internal curing method effectively compensates for the shortcomings of low water-cement ratio CRC in bridge engineering.(3)Simplify the expression of self-drying shrinkage model of the low water-cement ratio CRC by introducing two parameters,and the influence factors are elastic modulus E,small capillary porosity ?mr,and internal relative humidity RH of CRC The calculated values of the model are well fitted to the test results,and it can be applied to the pavement design in predicting the shrinkage rate of the internal curing CRC. |
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