Experimental Study On Bond Behavior Between Geopolymer Concrete And FRP Bars After High Temperature

Geopolymer is a new type of environmentally friendly inorganic cementitious material.It is the most potential substitute for ordinary cement due to its excellent mechanical properties,high temperature resistance,acid and alkali corrosion resistance and low energy consumption in the production process.Fiber reinforced polymer(FRP)bar shows remarkable superiority in weight,tensile strength,corrosion resistance and fatigue resistance.The combination of FRP bars and geopolymer concrete can effectively solve the problems of high energy consumption and durability caused by traditional reinforced concrete structures.However,FRP bars will occur glass transition and thermal degradation after exposed to high temperature,which will lead to a significant change of the bond property between FRP bars and concrete.Considering the fire-protection of FRP bars reinforced concrete structure,it is very important to study the bond performance between FRP bars and geopolymer concrete after high temperature exposure.Therefore,in this paper experimental study was conducted on mechanical properties of geopolymer concrete and the bond performance between FRP bars and geopolymer concrete after exposure to elevated temperature.The main research contents of this paper include the following four aspects:(1)Geopolymer concrete,mortar and paste specimens with three different grades(C30,C40 and C50)were prepared.and OPC concrete,mortar and paste specimens with grade of C40 were also prepared as a reference.Compressive strength,splitting tensile strength and flexural strength tests were conducted on large sets of geopolymer and ordinary concrete,mortar and paste after exposure to elevated temperatures.TGA,XRD,SEM tests were also conducted on geopolymer and ordinary paste.From these test results,geopolymer exhibits excellent resistance to elevated temperature.Compressive strength of geopolymer concrete,mortar and paste displayed an initial increase and then decreases gradually with the increase of exposure temperature.The higher content of slag of geopolymer causes the lower residual strength.Resistance in elevated temperature of C40 geopolymer concrete,mortar and paste demonstrated a better performance than that of ordinary concrete,mortar and paste at the same grade.XRD,TGA and SEM results revealed that the high temperature decomposition of calcium silicate hydrate(C-S-H)produced by slag is the main reason for the degradation of C30,C40 and C50 geopolymer in turn after exposure to elevated temperatures.(2)Through the central pull-out test of FRP bar geopolymer concrete bond specimen at room temperature,the influence of surface form,diameter,bond length of FRP bars,strength grade and cover thickness of geopolymer concrete and other factors on the interfacial bond performance between FRP bars and geopolymer concrete were analyzed.The results show that the surface form of FRP bar has a great influence on its bond performance.With the improvement of the surface roughness of FRP bars,the bond strength between three kinds of FRP bars(shallow ribbed,sand coated and deep ribbed)and geopolymer concrete increases in turn.The interfacial bond strengths between FRP bars and geopolymer concrete are similar to those between FRP bars and ordinary concrete.Meanwhile,the bond strength between FRP reinforcement and geopolymer concrete decreases with the increase of FRP bar diameter and bond length,while increases with the increase of concrete strength and cover.(3)Through the central pull-out test of FRP bar geopolymer concrete bond specimen after high temperatures exposure,the influence of surface form,concrete type,strength grade of geopolymer concrete and FRP bar type on the interfacial bond performance between FRP bars and geopolymer concrete were analyzed.The results show that the bond strength between FRP bars and geopolymer concrete first increases and then decreases with the increase of exposure temperature,and the bond strength of all specimens reaches the highest at 100℃ or 200℃.When the temperature is between100℃ and 350℃,the bond strength retention rate of C30 specimens is significantly higher than that of C40 and C50 specimens,which has better high temperature resistance.After exposure at 400℃,the retention rate of bond strength of all specimens is less than 21%.After high temperature exposure,the bond strength retention rate of bonded specimens of sand coated FRP bars and geopolymer concrete is significantly lower than that of shallow or deep ribbed specimen,which is mainly due to the peeling off of sand coated layer on the surface of FRP bar at high temperature.In addition,the types of FRP bars have no significant effect on the interfacial bonding properties after high temperature exposure.(4)CMR and mBPE models were used to fit the rising section of bond-slip curve of FRP bars and geopolymer concrete after high temperature exposure.The results show that CMR model has better fitting effect than m BPE model and the regression coefficient is greater than 0.98,which can be used to simulate the bond behavior between FRP bars and geopolymer concrete after high temperature exposure.Finally,based on the test results and the existing achievements,a formula of the basic anchorage length of FRP bars and geopolymer concrete structure at room temperature and after high temperature was proposed.