Axial Compressive Mechanical Behavior Study Of FRP Intermitted Helical Strips Confined Concrete Columns

Carbon Fiber Reinforced Polymer(CFRP)is widely used in the field of civil structural reinforcement due to its outstanding advantages of high tensile strength,light weight,good corrosion resistance,convenient construction,and no increase in structural size.Existing researches show that the failure form of FRP-confined concrete columns is mainly that the FRP sheets is broken.And the FRP limit hoop strain at break is significantly lower than the rupture strain measured by the flat coupon tests.The reasons for this phenomenon: first,due to the impact of the FRP on-site wrapping quality,the stress concentration of the FRP sheets is caused,which leads to premature failure;second,for FRP fully-wrapped confined concrete and FRP hoop-wrapped concrete,the existence of geometrical discontinuous in the FRP overlapping area is prone to result in stress concentration and cause premature failure.Based on the above points,this paper proposes the use of concrete confined with spiral FRP strips,which can eliminate the impact of the geometrical discontinuous and increase the ultimate strain at columns failure.Besides,the new reinforcement method also has the following advantages:(1)suitable for the integrated construction of mechanical operations to improve the wrapping quality and reinforcement efficiency;(2)with the coordinated optimization of the bearing capacity and the deformation capacity;(3)with the superior seismic capacity.The main contents of this thesis include the following aspects:(1)Monotonic axial pressure performance study of the spiral FRP strips wrapped concrete column with spacing.Through the monotonous axial pressure test of 28 pcs of FRP strip spiral partly confined specimens with a diameter of 200 mm and a height of 400 mm,the impact of strips' width,number of layers,spiral angle and effective refinement rate on monotonic axial pressure performance has been researched.The experimental data show: the failure mode of the confined concrete is that the fiber fracture At a half height position;the FRP strip width,number of layers and effective restraint rate increase;the pressure strength and axial strain limit of the constrained concrete are improved;the spiral angle is closer to the hoop direction;the effect of FRP straps confinement is more obvious.(2)Cyclic axial pressure performance study of the spiral FRP strips wrapped concrete column with spacing.Through the cyclic axial pressure test of 18 pcs of wrapped specimens,considering the impact of the FRP effective confinement rate and loading and unloading rules on the cyclic axial pressure performance of confined-concrete,the test results show that the stress-strain curve of the concrete under monotonic axial pressure basically coincides with the stress-strain curve of the confined concrete under cyclic axial pressure,and that the failure mode is similar to that of monotonic axial pressure.CFRP fiber rupture occurs in the middle of height of the specimen.Under the impact of cyclic axial pressure load,the loading history will have a cumulative effect on plastic residual strain and stress damage: as the number of cyclic loading / unloading increases,the cumulative degree of plastic residual strain continues to decrease until it remains unchanged;as the number of cycle loading / unloading increases,the degree of stress damage continues to decrease until it remains unchanged;(3)Establish a monotonic axial pressure constitutive model of FRP strip spiral partly wrapped concrete column.The definition standards of sufficient and insufficient confinement of FRP strips spiral partly confined concrete column are put forward.The calculation formula of ultimate axial stress and axial strain is obtained based on regression analysis of test data.A monotonic axial pressure stress-strain design-model and analysis-model of FRP strip spiral partly confined concrete column is established.There is coherence between the results predicted based on the model and the test results and thus,a scientific reference is offered for the axial pressure performance design of this type of reinforced structure.(4)Establish the cyclic axial pressure model of the spiral FRP strips wrapped concrete column with spacing.By analyzing the loading and unloading curve law under cyclic axial loading,combined with the monotonous axial pressure constitutive model,hysteresis model of the spiral FRP strips partly wrapped concrete column is established and compared with existing model and test data.The results show that the model proposed in this paper can better predict the test results.