Mechanical Behavior And Calculation Methods Of Fiber&Nanosized Materials Reinforced Concrete At High Temperature

This thesis is one of the main research contents of the project “Fiber and Nanosized Materials Reinforced Concrete(FNMRC)at and after High Temperature”(No.51178434)supported by Natural Science Foundation of China.In this thesis,the tests of mixture proportion,basic mechanical properties of FNMRC at ambient temperature and high temperature,microstructure and phase analysis of FNMRC were carried out.The compressive properties,splitting tensile properties,shear properties,flexural toughness,strengthening mechanism of nanosized materials and degradation mechanism of FNMRC at high temperature were studied.Then,the relevant calculation models were established.The main contents in this thesis are presented as below:(1)The mixture proportion test of 22 groups was conducted to investigate the effects of concrete strength grade,amount of nanosized materials and volume fraction of steel fiber on the extra coefficient of cement paste and mortar.Based on the workability of FNMRC,the method of mixture proportion design was proposed.Compared to traditional method,the proposed method with a smaller amount of tests has a wider applicability.Meanwhile,the results of the proposed method can be converted to the effects of various factors on the water requirement and sand ratio,which is compatible to the traditional method.(2)The tests compressive properties,splitting tensile properties,shear properties and flexural toughness of 156 FNMRC specimens with 13 mixture proportions were conducted to investigate the volume fraction of steel fiber,amount of nanosized materials and concrete strength grade on the compressive strength,splitting tensile properties,shear properties and flexural toughness.With consideration of the effects of fibers and nanosized materials,the calculation models of the compressive strength,splitting tensile strength,shear strength and flexural strength were put forward.Finally,the evaluation method of flexural toughness,which is suitable for the FNMRC characters,is proposed.(3)The axial compression test at high temperature of 300 FNMRC prism specimens of 150mm×150mm×300mm with 10 mixture proportions was conducted to investigate the effects of temperature,amount of nanosized materials and volume fraction of steel fiber on the compressive strength and constitutive relation of axial compression.With consideration of the effects of temperature,fibers and nanosized materials,the calculation formulas for compressive strength of cube specimens,peak stress and corresponding strain as well as initial elastic modulus were proposed.Meanwhile,the constitutive relation of axial compression of FNMRC at high temperature was modeled.(4)The axial compression test at high temperature of 150 FNMRC prism specimens of 150mm×150mm×150mm with 10 mixture proportions was conducted to investigate the effects of temperature,amount of nanosized materials and volume fraction of steel fiber on the splitting tensile strength,deformation at peak load and area under splitting tensile load-deformation curve.With consideration of the effects of temperature,fibers and nanosized materials,the calculation formula for splitting tensile strength and the evaluation method for splitting tensile toughness of FNMRC at high temperature were proposed.(5)The four-point flexural test of FNMRC beams of 100mm×100mm×400mm with 10 mixture proportions was conducted to investigate the effects of temperature,amount of nanosized materials and volume fraction of steel fiber on the flexural strength,deflection at peak load and area under flexural load-deformation curve.With consideration of the effects of temperature,fibers and nanosized materials,the calculation formula for flexural strength and the evaluation method for flexural toughness of FNMRC at high temperature were proposed.(6)The SEM microstructure and XRD phase of FNMRC were analyzed to investigate the effects of temperature and nanosized materials on the microstructure and phase composition of FNMRC.The influence mechanism of nanosized materials on the micro properties and the degradation mechanism of FNMRC at high temperature were discussed.