Research On Mechanical Properties Of New Hybrid Fiber Cement Mortar After High Temperature

The high temperature of the fire will cause concrete bursts in the building structure and deterioration of mechanical properties,significantly reduce the bearing capacity,safety and durability of the structure,and even cause serious casualties and economic losses.With the support of the National Natural Science Foundation of China(51779095)and the Natural Science Basic Research Program of Shanxi Province(2021JQ-174),this thesis systematically studied the effect of a new hybrid fiber system composed of steel fibers(S),millimeter-scale polyvinyl alcohol fibers(PVA)and micron-scale calcium carbonate whiskers(CW)on the physical and mechanical properties of cement mortar specimens(cylinders,cubes and reinforced beams)after high temperature,including burst resistance,mass loss,ultrasonic wave velocity loss,compressive strength,splitting tensile strength,flexural performance of reinforced beams,etc.The main research contents and conclusions of the full text are summarized as follows:(1)The macroscopic morphology,mass loss and ultrasonic wave velocity loss of hybrid fiber cement mortar with different mix proportions after high temperature were studied.The results show that under the slow heating system,hybrid fibers can effectively inhibit the cracking of cement mortar,reduce the number of cracks on the surface of mortar and reduce the mass loss.The filling effect of CW in mortar can increase the density of mortar,optimize the pore structure of mortar and reduce the loss of ultrasonic wave velocity.Under the rapid heating system of ISO 834 curve(simulating real fire),CW and steel-PVA hybrid fibers can significantly improve the anti-explosion performance of hybrid fiber cement mortar,inhibit the generation and expansion of cracks in specimens after high temperature,and maintain good integrity.(2)The effects of hybrid fibers with different mixing ratios on the mechanical properties of cement mortar at room temperature were studied.The results show that the introduction of hybrid fibers into the plain mortar can change its axial compression failure mode,transform the non-symptom brittle failure into the symptom ductile failure mode,and play a micro-reinforcement role to improve the compressive strength and splitting tensile strength of mortar.The toughness of hybrid fiber cement mortar can be further improved by adding appropriate amount of CW,and the filling effect of micro-aggregate and crack resistance and strengthening effect can be exerted.However,excessive addition of CW will produce aggregation and agglomeration,which will deteriorate the mechanical properties of mortar.(3)The effects of hybrid fibers with different mixing ratios on the mechanical properties of cement mortar after high temperature were studied.The results show that CW can play a microscopic role in hybrid fiber cement mortar,and optimize the compressive and splitting tensile properties of hybrid fiber cement mortar.The addition of hybrid fibers can improve the ductility and axial compressive strength of mortar.Compared with the mortar specimens without fibers,the incorporation of appropriate amount of steel-PVA hybrid fibers into the mortar can significantly improve the splitting tensile strength after high temperature,and weaken the degradation effect of high temperature on the splitting tensile strength of mortar.(4)The bending properties of hybrid fiber cement mortar reinforced beams before and after high temperature were studied.The results show that the addition of hybrid fibers before high temperature can significantly improve the initial cracking load,yielding load and ultimate load of the reinforced beam,improve the initial cracking strain of the reinforced beam,delay the cracking of the reinforced beam,reduce the crack width of the reinforced beam under normal use,and improve its durability.Hybrid fiber system can not only effectively avoid reinforcement beam burst and limit the expansion of thermal cracks under high temperature.Moreover,it can still play a role of crack resistance after high temperature,and improve the loading capacity of the reinforced beam under bending.