Study On Mechanical Properties And Mechanism Of UHP-ECC

In order to solve the problems of structural safety and environmental pollution and unsustainability caused by the lack of mechanical properties and durability of traditional RC structures,there is an urgent need to design a high-performance engineering construction material with durability,high mechanical properties,high energy consumption and sustainability.At present,Ultra-high performance engineered cementitious composite（UHP-ECC）is expected to be an ideal construction material due to its excellent mechanical properties.In particular,its compressive strength is nearly three times higher than that of ordinary concrete,which facilitates the diversification of building components and also ensures a higher safety threshold.In addition,the excellent bridging effect of polyethylene（PE）fibers facilitates the high deformation capacity of UHP-ECC.Its excellent tensile ductility（tensile strain above 3%）and saturation cracking behavior（crack width less than 200μm）improve the stability and sustainability of the structure under extreme conditions.However,the extremely low water-cement ratio and the use of large amounts of supplied cementitious materials in the mix ratio led to extremely high internal shrinkage of the UHP-ECC matrix,which can reach more than1900μm/m within 7 days,which,if not controlled,can easily lead to shrinkage cracking of the material and seriously affect the construction safety of the structure.And the high cost of PE fibers in UHP-ECC is also a limitation.The high cost of PE fiber in UHP-ECC is also one of the bottlenecks limiting the large-scale engineering application of UHP-ECC.In this study,the dense packing principle is used to optimize the particle gradation of the matrix and design a cement-based matrix with a high degree of particle packing.The micromechanics are used to ensure the excellent deformation capacity of UHP-ECC.The effects of different water-binder ratios,fly ash content and PE fiber content on the compressive and flexural properties of UHP-ECC were investigated respectively,and finally the UHP-ECC with high strength and high ductility was obtained.To solve the problem of high shrinkage of UHP-ECC,different shrinkage-reducing admixtures such as superabsorbent polymer（SAP）,expansion agent（EA）and shrinkage-reducing agent（SRA）were used in this study.The autogenous shrinkage of UHP-ECC was controlled by combining various shrinkage-reducing admixtures.When each shrinkage-reducing admixture is incorporated separately,the shrinkage-control effect of UHP-ECC is low and leads to the deterioration of properties of UHP-ECC.For example,the incorporation of SAP leads to the decrease of UHP-ECC matrix strength due to the increase of total water-binder ratio;the incorporation of EA leads to the deterioration of UHP-ECC flowability due to the consumption of internal free water.In contrast,the autougenous shrinkage of UHP-ECC was more significantly controlled by the combination of multiple shrinkage-reducing admixtures,and the negative effect of single admixture on the performance of UHP-ECC was mitigated or even reversed.In particular,when SAP,EA and SRA admixtures were incorporated simultaneously,the autougenous shrinkage value of UHP-ECC was reduced to less than 600μm/m,and the mechanical properties and fluidity of UHP-ECC were better than those of the control group.Secondly,through the determination of internal relative humidity,pore distribution,hydration degree inside UHP-ECC and micromechanics tests,the effects of different shrinkage-reducing admixtures on the microstructure of UHP-ECC are investigated,which provides some theoretical basis for the performance regulation of UHP-ECC.In order to solve the problem of high cost of UHP-ECC,this study focuses on the cost of PE fibers.By introducing cheap,high-strength aragonite-type micron-sized fiber-calcium carbonate whisker（Ca CO₃ whisker,CW for short）to partially replace the PE fibers in the fit ratio.The price of CW is only 1/100 of that of PE fiber when the volume fraction is constant,and the material cost of UHP-ECC can be significantly reduced by replacing PE fiber with CW.The experimental results show that CW can fill the microscopic pores in the matrix and thus improve the compressive strength of UHP-ECC to a certain extent;CW in the form of needles can play a reinforcing and toughening role at the microscopic level through the mechanisms of microcrack bridging,deflection and whisker-pullout,and can play a synergistic limiting effect on microcracks with PE fibers as multi-scale fibers,thus improving the strain hardening and multiple cracking behavior of UHP-ECC.According to the result of the technique for order preference by similarity to an ideal solution,the multi-scale fiber combinations of PE fibers and CW can effectively improve the performance and cost economy of UHP-ECC.Finally,a semi-empirical prediction formula for the tensile performance of multi-scale fibers reinforced UHP-ECC was obtained based on the hybrid fiber reinforcing index.