Experimental Research And Analysis On Durability Of AFRP Strengthened Concrete Under Chloride And Freeze-Thaw Conditions

In the past three decades,fiber reinforced polymer(FRP)composites are being increasingly used worldwide as a rehabilitation option for concrete structures,which makes the issue concerning durability of FRP strengthened concrete structures attract more and more attentions.So far,there has been plenty of researches which are about the durablity of carbon fiber reinforced polymer(CFRP)or glass fiber reinforced polymer(GFRP)strengthened concrete structures,however,similar studies involving aramid fiber reinforced polymer(AFRP)are very limited.In order to promote the application of AFRP,it is necessary to reveal the durability of AFRP strengthened concrete structures scientifically and quantitatively.Therefore,a systematic research is presented in this dissertation concerning durability of AFRP strengthened concrete under chloride and freeze-thaw conditions.The main work and conclusions are as follows:1.Effects of chloride condtion(35 g/L NaCl solution)on mechanical behaviors of AFRP sheets,epoxy adhesive and concrete were studied through testing 120 specimens.It can be seen that:(1)the change in the tensile strength of the AFRP sheet is not obvious after immersion in the salt solution for 360 days,while the modulus of elasticity of the sheet decreases by 11.1% comparing with the control sheet;(2)the shear strength of the steel-epoxy adhesive joint related to an immersion time of 360 day is 36% lower than that of the control joint,indicating that the bond between steel and adhesive experiences great degradation after being immersed in the salt solution;(3)the compressive strengths of high-strength concrete(HSC)and normal-strength concrete(NSC)are improved after immersion in the salt solution,the reasons for which are hydration of concrete,formation of Friedel's salt and secondary hydration caused by fly ash.2.Mechanical behaviors of the bond between AFRP and HSC/NSC under chloride condtion(35 g/L NaCl solution)were investigated through testing 60 specimens.The findings show that:(1)after immersion in the salt solution,the load carrying capacity of the AFRP-HSC/NSC bond increases by 20% at most when comparing with that of the control joint;(2)the maximum local bond stress of the AFRP-HSC/NSC bond generally rises first and then decreases with the increasing of immersion time;(3)Overall,effects of the salt solution on the deformability of AFRP-HSC/NSC bond are limited,while the initial shear stiffness of the bond is not affected after being immersion in the salt solution within 360 days.3.Effects of chloride condtion(35 g/L NaCl solution)on mechanical behaviors of AFRP strengthened HSC/NSC beams are studied through testing 22 specimens.Based on the experimental results,the validity of the existed models,which were estabilished for predicting mechanical behaviors of FRP strengthened beams in non-corrosive environments,on predicting the behaviors of AFRP strengthened beams under chloride condtions was evaluated.It can be seen that:(1)cracking load of the AFRP strengthened HSC/NSC beam generally increase after immersion in the salt solution,but this condtion causes limited influences on load carrying capacity and load-deflection curve of the strengthened beams;(2)the performances of Teng et al.model,Said and Wu model and Lu model seem good on predicting debonding moment capacity and debonding FRP strain of AFRP strengthened concrete beam,when the modulus of elasticity of AFRP sheet and the compressive strength of concrete are known.4.Mechanical behaviors of aramid fiber bundles,epoxy adhesive,AFRP sheets and concrete,which were placed in air,immersed in water or subjected to freeze-thaw cycling respetively,were evaluated by testing 529 specimens.The experimental results show that:(1)effects of these three condtions on the load carrying capacity of the aramid fiber bundle are all very limited;(2)water uptake by the epoxy adhesive subjected to freeze-thaw cycling is always between that immersed in water and that placed in air;(3)variations of tensile strength and elastic modulus of the epoxy adhesive,which is immersed in water or subjected to freeze-thaw cycling,with conditioning time are similar to that placed in air,and these two values could recovered to their initial levels after 1200 hous' conditioning;(4)bond strength of the aluminum-epoxy adhesive joint,which is immersed in water or subjected to freeze-thaw cycling,experiences an initial increase followed by a decrease within 1200 hours,and the bond strength of the joint immersed in water is lower than that subjected to freeze-thaw cycling in the later stage during condtioning;(5)the tensile strength and elastic modulus of the AFRP sheet exposed to freeze-thaw cycling decrease more significantly than that placed in air or immersed in water;(6)non-air-entrained concrete(NAC)degrades severly after 100 freeze-thaw cycles,while the reduction in the compressive strength of air-entrained concrete(AC)after 300 freeze-thaw cycles is only 2.1% comparing with its initial value.5.Mechanical behaviors of the bond between AFRP and NAC/AC,which were placed in air,immersed in water or subjected to freeze-thaw cycling respetively,were investigated through testing 96 specimens.The findings show that:(1)siganificant deterioration of the AFRP-NAC bond occurs after being exposed to the freeze-thaw condtion,with its load carrying capacity,maximum bond stress,maximum slip at loaded end and initial shear stiffness decreasing greatly after 100 freeze-thaw cycles,meanwhile the strain distribution of the AFRP and load-slip curve experience an apparent change;(2)generally,placing in air has no adverse effects on the mechanical behaviors of the AFRP-NAC bond,however,the maximum bond stress of the bond decreases after being immersed in water and the maximum slip at loaded end in the case of immersion in water is lower than that in the case of placing in air;(3)the load carrying capacity and the initial shear stiffness of the AFRP-AC bond could remain stable in these three conditions,which have slight impacts on the strain distribution of the AFRP and the load-slip curve;(4)comparing with the case of placing in air,immersion in water and freeze-thaw cycling have relatively larger effects on the maximum bond stress of the AFRP-AC bond,which decreases by 20% and 6%,after 300 freeze-thaw cycles and immersion in water for 1200 hours,respectively.