Reinforcing Mechanism Of Highly Active Admixtures And Textile Reinforcement System Of Lightweight Engineered Cementitious Composites

The brittle characteristics of cement-based materials make them face problems such as easy cracking,low toughness and poor durability.Engineeried cementitious composites(ECC)have good ductility and crack control capabilities,which provides new ideas for solving the above problems.Lightweight engineeried cementitious composites(LECC)are formed by introducing pores or bubbles on the basis of ECC,which combine the strain hardening of ECC,low thermal conductivity and low density,and has great application prospects in self-weight sensitive buildings such as high-rise,long-span and offshore platforms.At present,fibers,lightweight aggregates and external admixtures are used in the development of LECCs,but the characteristics of various materials are not considered enough,resulting in the advantages of component materials not being fully utilized,and the cost-effective and excellent performance LECCs cannot be effectively prepared.Therefore,it is of great significance to systematically analyze the characteristics of each component of LECC and optimize its proportion,which is of great significance to improve the performance of LECC and enhance its engineering application potential.In this paper,the characteristics of Fly ash cenospheres and high-strength fibers were analyzed in detail,and the composition and ratio of LECC were optimized by combining with highly active admixtures,aiming to improve the performance of LECC.On this basis,the combination of LECC and textile grid to develop a composite reinforcement system,and its mechanical properties and constraint application are studied.(1)Research on simplified design method of ECC based on single-crack tensile test.The fiber bridging stress-crack opening width(σ(δ))curve of ECC was tested by single-crack tensile test.Based on the obtained σ(δ)curve,the strain hardening index were calculated.The feasibility of simplifying the ECC design method by replacing the single-fiber pull-out test σ(δ)curve with the single-crack tensile test σ(δ)curve is analyzed.The results show that the hardening index calculated based on the σ(δ)curve of the single-crack tensile test can better reflect the multiple cracking ability of ECC,which has great reference value for simplifying the design method of ECC.The difficult single-fiber pull-out test is expected to be avoided in the ECC design.(2)The effects of fly ash hollow microspheres and polyethylene fibers on the performance of LECC were studied.Through the tests of LECC density,compressive properties and tensile properties,the effects of fiber content and aggregate content on the mechanical properties and crack development of LECC were discussed.The results show that fly ash hollow microspheres were effective in reducing the density of LECC,but its addition also weakened the mechanical properties of LECC.The tensile properties of LECC are directly related to the fiber bridging properties,and reducing the amount of fibers leads to weakened fiber bridging properties,resulting in lower tensile properties.During the multple cracking process of LECC,the crack width was positively correlated with the fly ash hollow microspheres content and negatively correlated with the fiber content.Combining the high strength properties of PE fibers and the low toughness properties of LECC can provide more space for the balance between the amount of ECC fibers and the cost and mechanical properties.(3)The mechanical properties and modification mechanism of LECC modified by highly active additives were studied.The mechanical properties of LECC were analyzed by compressive,tensile,and single-crack tensile tests,and the hydration products and microstructure of LECC were analyzed combined with XRD,TG and SEM methods.The results show that the nucleation effect of nano-silica promotes the hydration of cement,and the pozzolanic effect promotes the conversion of more calcium hydroxide into hydrated calcium silicate gel.The compressive strength and tensile strength of LECC can be increased by 30% and 24% with the addition of 3% nano-silica.The composite use of nano-silica and silica fume can further improve the mechanical properties of LECC.(4)Analysis of tensile behavior and enhancement mechanism of textile reinforced LECC composite system(TR-LECC).The effects of textile type,reinforcement ratio and LECC mix proportion on tensile properties,reinforcement efficiency,and stress fluctuation of TR-LECC were investigated.Based on the concrete crack width theory,the tensile constitutive model of TR-LECC was established by considering the bearing capacity contribution of LECC in the hardening stage.The results show that the TRLECC tensile strain capacity depends on the textile and the less deformable term in the LECC.Using textile with large fracture strain,TR-LECC with ductility equivalent to LECC can be designed,so that LECC deformability can be fully utilized.The established constitutive model can better describe the relationship between tensile stress/strain in TR-LECC,which provides theoretical support for the calculation and analysis in the application of TR-LECC.(5)Study on compression behavior of TR-LECC constrained concrete.Concrete columns were restrained using textile-reinforced mortar,LECC,and several TR-LECCs.Through the characterization of compressive strength,deformation capacity and restraint efficiency,the influence mechanism of TR-LECC characteristics on restraint effect was analyzed.Referring to the existing stress/strain models of FRP-confined concrete,and considering the bearing capacity contribution of the confinement layer,a design-oriented model suitable for TR-LECC-confined concrete is established.The research shows that using the LECC matrix as the transition layer of the confinement system can significantly improve the confinement efficiency.Compared with the restraint system with mortar as the transition layer,the strength of the restrained concrete column is increased by 42% and the ductility is increased by 50%.The established model is in good agreement with the experimental results,and can effectively predict the stress/strain relationship,compressive strength and deformation capacity of confined concrete.In this paper,the mechanism of modification of LECC high-activity admixture and the textile reinforcement system provide experimental and theoretical references for the improvement of the mechanical properties of LECC and the construction of highstrength and high-ductile TR-LECC reinforcement systems.