Study On Mechanical Properties Of CFRP Reinforced Self-Stressing Concrete Columns Under Axial Compression

Fiber Reinforced Polymer(FRP)has advantages of high specific strength,fatigue resistance,corrosion resistance,and convenient construction.It has been gradually applied in civil engineering.FRP confined concrete column utilizes the tensile properties of FRP and the compressive properties of the concrete.And the confinement of FRP make the concrete in a state of three-dimensional restraint,the strength and deformation performance of concrete are improved effectively.In recent years,FRP confined concrete column has received extensive research by scholars at home and abroad.But the constraint effects of FRP can only be played out when the concrete have a great lateral deformation,it has a certain hysteresis.But when using self-stressing concrete,due to the expansion of the concrete confined by FRP,prestress can be built in FRP and concrete,the hysteresis of FRP stress was alleviated.In addition,this method is also convenient for construction and low cost compared with tensioning FRP.The axial mechanical properties and theoretical study of FRP reinforced self-stressing concrete columns were conducted in this article.The main research work is as follows:(1)mechanical properties test of FRP confined self-stressing concrete column under axial compressionA total of 18 FRP reinforced concrete columns were constructed to investigate the axial mechanical properties,including 9 FRP confined self-stressing concrete column and 9 FRP confined concrete columns.The variables of the test inculed confinement stiffness(FRP layers)and self prestress level(with or without using expansive agent).The static strain monitoring test and axial compression test were conducted on the composite columns.The experimental results showed that,prestress in concrete caused by expansion of concrete is in the range of 1.85 to 2.13 MPa which is quite significant.O n one hand,The product of the expansion reaction tends to fill the inner holes and interfacial transition zone(ITZ).It make the interior structure of concrete denser and the compressive strength of self-stressing concrete was improved.On the other hand,FRP is tensioned,and prestress was formed in FRP and concrete.And the inflection stress is improved nearly 17%.The ultimate load capacity of composite column has also been improved slightly.(2)mechanical properties test of FRP-steel confined concrete tubular columns made of expansive self-consolidating concrete under axial compressionA total of 32 FRP confined self-consolidating concrete columns were manufactured in this test including 8 FRP confined self-consolidating columns(FCSCs),4 FRP confined self-consolidating concrete filled steel tube columns(CCFTs),and 20 hybrid FRPconcrete-steel double-skin tubular columns(DSTCs).Key parameters included confinement stiffness(FRP layers),self prestress level(with or without using expansive agent),diameter of steel tube and solid and hollow core(concrete filling steel tube or not).The experimental results showed that,due to the effect of expansive agent,prestress was detected in all specimens with expansive self-consolidating concrete(ESCC).For FCSCs and CCFTs,the intercept,inflection and ultimate load for ESCC-specimens were improved due to the effect ofself prestress.For hollow DSTCs,however,self prestressdid not improve the axial stress-strain behavior.But when the inner steel tube was filled with concrete,it was observed that the ultimate load and the stiffness of linear strengthening section can be improved by comparing ESCC-with SCC-specimens.(3)A stress-strain model for FRP confined self-stressing concrete columnBy fitting the relations between the increment of intercept stress and FRP expansion strain,the self-stressing concrete compressive strength was calculated with revised intercept stress.According to the calculated compressive strength of self-stressing concrete,the ultimate stress and ultimate strain of FRP confined self-stressing concrete column were calculated by using an existing of FRP confined concrete stress-strain model.With the effect of prestress taken into account,a calculation method of the inflection stress with the self-stressing factors was put forward.The calculation results show that,the theoretical results are in good agreement with the experimental data,providing a good reference for the further research of FRP confined self-stressing concrete column and engineering application.