Experimental Study On Flexural Properties Of Seawater Sea Sand Concrete Beams Reinforced With Basalt Fiber Reinforced Polymer Bars

Fiber reinforced polymer(FRP)has received more and more attention and application in the field of civil engineering due to its high strength,high corrosion resistance and fatigue resistance.In particular,the outstanding corrosion resistance of FRP enables its replacement of the easily corroded steel rebar in concrete members in the corrosive environment.Among the FRP materials,basalt fiber reinforced polymer(BFRP)is cost effective considering its low production cost and excellent mechanical properties,which can give it a promising wider application in the engineering field.The combination of fiber reinforced polymer bars(FRP bars)and seawater and sea sand concrete(SWSSC)can effectively solve the problem of chloride ion induced corrosion,alleviate the shortage of river sand resources and bring considerable economic and social benefits.However,the strong alkalinity of the concrete pore solution can hydrolyze the resin matrix and corrode fiber,eventually resulting in the mechanical performance deterioration of the FRP bars.Therefore,the research on the durability of FRP bars reinforced SWSSC members are oriented to the demand of the application of SWSSC,which has obvious practical significance.Given the sensitivity of FRP bars to concrete alkalinity,a new low alkalinity SWSSC is developed to prepare BFRP bars reinforced SWSSC beams.Through the four-point bending test and digital image correlation(DIC)test,the evolution laws of failure mode,flexural capacity,and ductility of BFRP bar reinforced SWSSC beams in the process of accelerated deterioration are obtained.The influence of exposure time,immersion temperature,internal alkalinity of concrete and the surface type of reinforcement on the long-term bending performance of SWSSC beams is studied.A calculation formula with less error is proposed by comparing the theoretical calculation formulas of flexural capacity in Chinese and American codes.The main research contents and conclusions are as follows:(1)Accelerated aging will change the failure mode of normal SWSSC beams with BFRP bar reinforcement,and cause a significant decrease in the bearing capacity.The failure mode of low alkalinity SWSSC beams remain unchanged and the bearing capacity increases after the aging process.After aging,the failure mode of normal SWSSC beams changes from concrete compression failure in the compression area to tensile failure of BFRP reinforcement.With the increase of aging time and temperature,the ultimate bearing capacity decreases significantly.After soaking at 55℃ for 4months,the ultimate bearing capacity decreases by 70.0%.In contrast,the failure mode of aged low alkalinity SWSSC beams after immersion aging remains unchanged and the ultimate bearing capacity increases due to the increase of concrete strength,and the maximum increase is 27.8% after 55℃ immersion for 4 months.(2)Alkaline environment corrosion will significantly degrade the interface performance between normal SWSSC and FRP bars,which increases the crack width of the beams and reduce their ductility.Due to the severe deterioration of the reinforcement itself and the interfacial bond strength with the concrete in normal SWSSC beams,the failure mode changed after immersion.The number of cracks in the beam is significantly reduced and the crack distribution is sparser,resulting in a sharp increase of the crack width.The crack-induced load fluctuations of the experimental beams are more significant than that of the control beams.The average load fluctuation of the experimental beams under tensile failure is 111.3% higher than that of the control beams in the N-H beams series.After immersion,the concrete strain of normal SWSSC beams at failure decreases significantly(the maximum decrease is 64%).However,the ductility and the crack development of low alkalinity SWSSC beams are improved.(3)The interface treatment of the reinforcement will change the long-term durability of BFRP bar reinforced SWSSC beams.The performance of sand-coated BFRP bar beams in bearing capacity retention,crack development,and ductility retention after aging is better than that of helically-wrapped bar reinforced beams,indicating that sand-coated treatment is helpful to improve the durability of BFRP bar reinforced SWSSC structures.(4)The theoretical calculation values of Chinese and American codes are conservative regardless of the failure mode of the beams.The theoretical value calculated in the Chinese code is more consistent with the test results in this paper.By comparing with the current codes,the calculation formula of flexural capacity of SWSSC beams strengthened with BFRP bar is proposed.Compared with the codes of the two countries,the accuracy of Chinese code is better,and the safety redundancy of American code is more sufficient.The new calculation formula of flexural capacity of SWSSC beams strengthened with BFRP bar proposed in this paper has excellent accuracy for compression damaged beams.Compared with the experimental values,the error is very small and the average error is 4.3%.