Experimental Study On Fire Resistance Of Rectangular Concrete Slabs With Bi-axial In-plane Constraint

At present,the in-plane constraint is not considered in the fire tests of most reinforced concrete slabs,and thus the in-plane free expansion occurs in the concrete slabs.However,the fire resistant performance of the concrete slabs in the building is affected by the constraints provided by the adjacent components.Hence,it is necessary to conduct the research on the fire behavior of the in-plane restrained concrete two-way slabs,and further investigate the effect of the in-plane restraint on the fire resistant performance of the concrete slabs.In this paper,the test furnace was designed and constructed in the original furnace,and the in-plane restraint apparatus was also designed.Four fires of the concrete two-way slabs were conducted,i.e.,one simply supported slab and three in-plane restrained slabs.In addition,the dimension of each slab is 3900mm×3300mm×100mm.The paper conducted the comparative analysis of the fire behavior of four tested slabs,including the cracks,failure mode,temperature,the out-plane(in-plane)deflections and corners' forces.Results show that the in-plane restraint has important effect on the deflection and deflection ratio of the concrete slabs,particularly the later stage.In other words,the deflection and deflection ratios tend to increase with the increasing in-plane restraint forces.The slab's corner should be reinforced and it early failure can be avoided.Otherwise,its early failure is not beneficial to the development of the tensile membrane action in the concrete slabs.VULCAN software was used to conduct the numerical analysis of the tested slabs,including the temperature of slabs,out-plane deflection(geometrical nonlinearity and geometrical linearity)and the membrane action.In addition,the parametrical analysis was conducted to investigate the effect of the aspect ratio,thickness,the restraint type,reinforcement ratio and reinforcement layers on the fire behavior of the restrained slabs.Results indicate that the concrete thermal strains and geometrical nonlinearity have important effect on the fire behavior of the restrained slabs.For any restraint condition,larger concrete thermal strains result in larger deflections of the restrained slabs.The geometrical linearity method tends to overestimate the deflection of the simply supported slab or underestimate that of the restrained slabs,and the results predicted by the geometrical nonlinearity were relatively reasonable.The in-plane restraint has important effect on the slabs' membrane action,and the concrete slabs with various in-plane restraint have different membrane action and failure mechanism.Fire behavior of the restrained slab,including the deflection,fire resistance,failure mode and membrane action,is dependent on the interaction of restraint types,restraint level and aspect ratio.Thus,the effect of the above factors should be considered in the failure criteria of the restrained slabs.Meanwhile,increasing slabs' thickness and reinforcement ratio will be beneficial to enhance the ductility of the constraint slabs under fire and prevent brittle failure.The steel strain failure criterion was proposed in this paper,and thus the corresponding model was established to calculate the limit bearing capacity of the restrained concrete slabs in fire,and the results predicted by the proposed method were compared to those obtained from the existing methods.Results indicate that the ultimate bearing capacity gradually decreases with increasing in-plane restraints.In addition,compared to that of the in-plane restrained slabs,the ultimate bearing capacity of bi-axial restrained slabs significantly decreased.