| Ultra-high Performance Concrete(UHPC)suffers from rapid development of early autogenous shrinkage,increasing the risk of structural cracking significantly.Internal curing can efficiently inhibit internal self-desiccation of UHPC,which is regarded as the most direct and effective approach to control the development of autogenous shrinkage.However,several aspects remain unclear concerning about the internal curing effect of porous aggregate on performance of UHPC,including: The unreached agreement on potential driving mechanism of autogenous shrinkage and drying shrinkage evolution of UHPC after incorporation of porous internal curing medium;Inadequate investigation on regulatory mechanisms of pore structure,particle size,water absorption state and total amount of entrained water on internal curing efficiency;The insufficient understanding towards effective modification approaches to stimulate the internal curing potential of porous materials with fine pore structure.Herein,the targeted research related with topic stated above is carried out on UHPC matrix.The main results are as following:(1)Clarifying multiple evolutionary behaviors potential driving mechanism of UHPC autogenous shrinkage by characterization and monitoring on hydration products,hydration process and internal humidity/temperature field after incorporation of porous aggregate with different prewetting degree.The results reveal that the multistage evolution of autogenous shrinkage of UHPC matrix is comprehensively controlled by compensation in liquid phase volume,hydration dynamics,thermal deformation and capillary negative pressure.During the stage dominated by hydration dynamics and capillary negative pressure,the development of UHPC autogenous shrinkage is binomial-functionally correlated with hydration degree of cementitious system and decline degree of internal relative humidity.Meanwhile,there are limitations concerning about the application of traditional Powers model of water demand for internal maintenance based on traditional to design the parameters for UHPC system with porous internal curing medium.The best strategy to prepare low-shrinkage UHPC requests employment of prewetting aggregate as internal curing medium with low water absorption and relatively high content.Under optimized condition,the mechanical properties maintain the same level,but total autogenous shrinkage deformation at 7 day reduced by 60% compared with reference group.(2)The modified shrinkage testing method is applied to separate autogenous and drying shrinkage of UHPC composites with prewetting porous aggregate.The structural characteristics of pores and C-S-H gels are systematically evaluated.The adverse effects and mechanisms of absorbed water induced by internal curing medium on long-term drying shrinkage of UHPC matrix are clarified.The results indicate that the proportion of drying shrinkage in total shrinkage reaches 35%-67%,and long-term drying shrinkage is positively correlated with practical total w/b ratio of matrix.The development of drying shrinkage of UHPC includes two stages,in which the first stage is dominated by characteristics of interconnected pores in matrix and the second stage is driven by dehydration of C-S-H gels.(3)Natural lightweight pumice is introduced as internal curing medium for UHPC matrix.After systematic evaluation on pore structure,pore parameters and water absorption-release process of pumice aggregate,the impact of particle size,state of water absorption and total amount of entrained water on mechanical properties,volume stability,pore structure,hydration characteristics and microstructure of matrix are investigated.The results demonstrate that it is feasible to prepare UHPC matrix with low-strength pumice aggregate instead of river sand.Incorporation of prewetting pumice reduces the early autogenous shrinkage of UHPC matrix by 30%-50%,and pumice aggregate with larger size possesses higher potential of internal curing.The drying shrinkage behaviors of UHPC matrix after introduction of pumice exhibit higher sensitivity towards aggregate size.Excessive entrained water by internal curing medium may result in deterioration of long-term volume stability of matrix.The impact of entrained water by internal curing medium on hydration of UHPC cementitious system consists of inhibition during early age and acceleration during later age.Prewetting aggregate with larger size can refine pore structure of UHPC matrix,leading to higher fraction of connected pores.However,the aggregate with exceedingly small size brings detrimental effect to structure development of matrix.The ITZ between prewetting aggregate and paste is remarkably narrowed and densified,improving micromechanical properties notably.(4)The calcination and acid erosion processes are applied to modify the pore structures of zeolite.The structure and properties of modified zeolite,and the comprehensive performances of zeolite-based UHPC matrix are evaluated,from which the positive effect of modified pore structure of zeolite on stimulating the potential of internal curing is proven.The results indicate that the water-releasing rate increases with promotion on the water saturation rate of modified zeolite.Incorporation of prewetting zeolite aggregate after 400 ℃ calcination and acid erosion can improve the mechanical properties and shrinkage-reduction efficiency of internal curing of UHPC matrix significantly.The optimum modification process is acid erosion optimization on zeolite structure.Compared with original prewetting zeolite,introduction of acid-modified zeolite to UHPC matrix induces 15%improvement on compressive strength at 28 day,25% reduction on total deformation of autogenous shrinkage at 7 day and 55% decreases on diffusion coefficient of rapid chloride at 7 day.Based on results presented above,the potential driving mechanism of UHPC matrix after incorporation of internal curing strategy during evolution process of early autogenous shrinkage and long-term drying shrinkage is clarified,laying the foundation for regulating and controlling of volume stability of UHPC.The feasibility and superiority of internal curing technology with lightweight and modified aggregate are demonstrated,providing the technical support for material design of UHPC with light weight,low shrinkage and specific functions. |
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