Laminated Model And Damage Mechanism Of Fiber Reinforced Cementitious Composites In Severe Environments

Traditional concrete is unable to satisfy the mechanical and durability requirements in severe environments due to the easy cracking,tensile softening and high-brittleness characteristics.Polyvinyl alcohol(PVA)fiber reinforced cementitious composites(FRCC)exhibits significant strain-hardening characteristics,excellent cracking-control ability and ultra-high toughness,showing great potential to solve this problem.Therefore,focusing on three severe environments including 1)the freezing thaw cycles(FTCs),2)the combined condition of FTCs,chloride erosion and flexural strength(FTCs-Cl~--σ),3)the combined condition of dry wet cycles(DWCs),chloride erosion and flexural strength(DWCs-Cl~--σ),and taking fiber volume fraction(V_f),the number of FTCs or WDC_S and the flexural stress levels(FBLs)as testing variables,and from the perspective of degradation of macro and microstructures,the following researches were carried out:(1)The evolution characteristics of macro performance of FRCC under the three kinds of severe environments were investigated through uniaxial tension,compression,four points flexural and fast FTCs tests.The results shown that the Biphasic-Hill model could well describe the evolution characteristics of freezing thaw damage of FRCC under compression,and the calculation error is reasonable and acceptable.(2)The compressive damage evolution equation of FRCC under FTCs was established based on the Biphasic-Hill model and the basic theory of damage mechanics.The results shown that the Biphasic-Hill mode could well describe the compressive damage evolution characteristics of FRCC under FTCs,and the calculation errors were reasonable and acceptable(3)The laminated beam model of FRCC under FTCs,FTCs-Cl~--σand DWCs-Cl--σwere established based on the classical laminated beam theory and the basic theory of damage mechanics,respectively.The results shown that the theoretical calculation values were in good agreement with the experimental results.(4)The degradation mechanism of FRCC materials in the three environments were revealed through multiple micro-structure methods including XRD,FTIR,XPS,MIP,SEM-EDS,and X-CT,the results shown that the degree of hydration and carbonization of FRCC increased slightly with the increasing of FTCs or DWCS under the three environments.The degree of polymerization of C-S-H in FRCC decreased with the increase of the number of FTCs under FTCs or FTCs-Cl~--σ.However,it was increased under the enviroment of DWCs-Cl~--σwith the increase of DWCs,and in this eviroment,the FRCCs were more compact.These results could well explain the evolution characteristics of the macro properties of FRCC under the corresponding environment.(5)The concept of"artificial quantization pores"was proposed.The complete freezing thaw deformation curves of FRCC with different quantitive pore structure parameters were obtained by methods of resistance strain gauge(RSG)and linear displacement sensor(LVDT).The results shown that the frost heaving damage of inner-closed pores were more serious than those of surface-opening pores in FRCC under FTCs environment,while the wall of inner-closed pores was different from that of natural-surface pores.The freezing thaw damage degree of the wall of surface-opening pore were basically the same.These results provided strong data supports for freezing pressure theory and osmotic pressure theory of freezing thaw damage of FRCC.