| Reactive powder concrete(RPC),a type of cement-based ultrahighperformance concrete,possess ultrahigh compressive strength,good toughness and excellent durability.Since its introduction in the early 1990 s,RPC is f inding increasing applications in civil engineering.Nevertheless,owing to its densely packed microstructure and lower permeability,RPC is more susceptible to explosive spalling at high temperature,which not only threatens the fire safety of RPC structures,but also restricts the application and popularization of RPC material.At present,the pore pressure calculation for the RPC at high temperature is not accurate,the spalling criterion of RPC is still under the uniaxial stress state,and there is a lack of the design method to prevent RPC beams from “similar to less reinforcement” failure under fire and the simplified calculation method to improve the fire resistance of RPC beams based on the fire-retardant coatings.Against this background,this thesis intends to investigate the spalling property and fire resistance design method of RPC beams:(1)In order to solve the problem that the existing methods for calculating the fire resistance of reinforced concrete(RC)beams are not applicable to hybrid fibre-reinforced(2% steel fibers + 0.2% PP fibers)RPC beams,based on the fiber element model and introducing the thermal and mechanical properties of hybrid fibre-reinforced RPC at high temperature,a computer program for calculating the sectional moment-curvature relations and deflection of RPC simply supported beams was compiled and verified.Through the flexural capacity and deflection analysis of fire-exposed RPC beams under 825 working conditions,a formula was proposed for the fire resistance calculation of hybrid fibre-reinforced RPC beams under ISO834 standard fire exposure.The results showed that as compared to the compressive strength of RPC,span-depth ratio and reinforcement ratio,RPC cover thickness,load ratio and beam width have a more significant effect on the fire resistance.Additionally,in order to prevent the “similar to less reinforcement”failure of hybrid fibre-reinforced RPC beams under fire,a formula was proposed to calculate the minimum reinforcement ratio of RPC beams corresponding to different fire-resistance ratings.(2)In order to study the beneficial effect of fire-retardant coatings on the fire performance of RPC beams,a three-dimensional sequentially coupled thermalstress finite element(FE)model was developed.Comparison between the FE simulations and the existing fire test results was conducted to demonstrate the accuracy of the developed FE model.Then,the effect of the fire insulation parameters such as thermal properties,thickness,painting position,setting height and local damage on the fire performance of RPC beams was investigated.It was found that the fire resistance of RPC beams can be reduced by 15% if 5% fire insulation along their span is peeled off.Additionally,the influence degree of the structural parameters such as beam width,RPC cover thickness,load ratio,spandepth ratio,RPC compressive strength and reinforcement ratio on the fire resistance of insulated RPC beams was also quantified.The results showed that the effect law of the structural parameters on the fire resistance of insulated RPC beams is the same as that on the fire resistance of uninsulated RPC beams.(3)In view of the problem that there is a lack of the simplified method to calculate the fire resistance of the RPC beams with fire-retardant coatings,the fire resistance performance FE analysis of the prestressed RPC double tees with fire insulation was conducted.It was found that the fire resistance of insulated RPC beams with section width less than 200 mm calculated based on the coating thickness equivalence method is unsafe.To solve the above problem,the corner rebar temperature equivalence method was proposed based on the principle that the flexural capacity of RPC beams and insulated RPC beams is same when their fire resistance are reached respectively.Then,the improvement factor that quantifying the beneficial effect of fire insulation on the fire resistance of RPC beams was defined,the mathematical expression of fire resistance between RPC beams and insulated RPC beams was established,and the simplified formula for the fire resistance calculation of insulated reinforced RPC beams was proposed.Additionally,the recommended thickness of fire-retardant coatings required for reinforced RPC beams to achieve the specified fire resistance was given.(4)In view of the problem that the calculation of the RPC pore pressure at high temperature is not accurate,the calculation model for the intrinsic permeability of RPC at high temperature was proposed with considering the effect of temperature and PP fiber melting.Introducing the sorption isotherms model proposed by Davie,the calculation method for the mass of liquid water inside RPC at elevated temperatures was proposed with considering the effect of both RPC microstructural characteristics(porosity and intrinsic permeability)and liquid water properties(density and surface tension).The computer program for the calculation of RPC pore pressure based on the ideal gas law was compiled,and the accuracy of the program was demonstrated through the comparison with the existing fire test results.Then,the effect of sorption isotherms and intrinsic permeability on the RPC pore pressure was investigated.It was found that the calculation accuracy of RPC pore pressure can be improved by 10%-20% when using the intrinsic permeability model and liquid water mass calculation method of RPC proposed in this thesis.(5)In view of the problem that RPC is under the complex stress state at high temperature,the method to reasonably consider the superposition effect of both pore pressure and external load was proposed,and the spalling criterion of RPC based on the biaxial strength criterion was also constructed.Through the comparison with the existing fire test results,the accuracy of the proposed spalling judgement method was demonstrated.Then,the computer program for calculating the fire resistance of RPC beams with considering the effect of spalling was compiled and verified.The results showed that as compared to the uniaxial strength spalling criterion,the calculation accuracy of the fire resistance of RPC beams based on the biaxial strength spalling criterion can be improved.The fire resistance of RPC beams calculated without considering spalling may be overestimated by 26.4%-65.3%.The above research can improve the fire resistance of RPC beams and provide the scientific basis for achieving the fire resistance design objectives of not collapse and controllable spalling of RPC beams during fire. |
