Experimental Study On Flexural Performance Of Cracked RC Beams Retrofitted With CFRP Sheets

Compared with the traditional strengthening techniques, structural retrofit and rehabilitation with FRP is a very hot one which can reduce the restrictions, and expedite its popularization. Because FRP has great advantages in strength and efficiency, convenience in construction, good durability and corrosion resistance, extensive fields of application, low deadweight, and hardly changing the structural dimensions, so it's broadly used in construction engineering.The rehabilitation of flexural members such as beams and slabs bonded with FRP is the most common forms. And it's manifested by tests and engineering applications that the bearing capacity of flexural members will be greatly enhanced by bonded with FRP. In most cases, attention should be paid both to bearing capacity and deflection of structural members in order to satisfy the applicability, durability and security. Therefore it's essential to study the stiffness and bearing capacity of structural members strengthened with FRP under sustaining load.There is very few studies on deflection of RC members strengthened with FRP all over the world. The deflection formula both used in classic RC theory and in RC code is correspond to service state. However after RC members strengthened with FRP, the main rebar is under the post-yielding stage. Therefore the normal deflection formula is not suitable for calculating the deflection of RC members strengthened with FRP.On the other hand, though there are a great deal experimental studies on flexural bearing capacity of RC members strengthened with FRP, whereas most of the test members are normal ones, so there is a great difference with the actual structural members. Therefore it has a practical significance to study the flexural performance of RC members strengthened with FRP which simulate the practical loading state.In order to investigate the effects of CFRP sheets on flexural bearing capacity and stiffness of the beam, test of 18 RC beams retrofitted with CFRP sheets was carried out. 16 beams were retrofitted while they were bearing loads and there were bending cracks on their surfaces. The parameters of the test include the ratio of the rebar, quantities of CFRP and loading procedure. Through the experimental and theoretical analyzing results, the conclusion can be summarized as follows.1) The yielding load and ultimate bearing capacity of the beam are not affected by the value of the load under that load the CFRP sheets were retrofitted, but the value will affect the stiffness of the beam, especially the average tangent stiffness after rebar yielding. 2) The stiffness of cracked beams retrofitted with CFRP sheets is larger than that of the normal RC beams. 3) The average-strain of cross-section still can be assumed to be keep-in-plane within the whole loading procedure; the theoretical moment-curvatures obtained by the cross-section keep-in-plane is consistent with the experimental ones. 4) The analytical flexural bearing capacity and CFRP-strain are more accurate by assuming the concrete ultimate strain to be 0.07 instead of 0.0033; 5) Limiting the CFRP-strain under 0.01 is reasonable to evaluate the flexural bearing capacity of RCbeams retrofitted with CFRP; 6) The initial load is hardly influence the flexural bearing capacity of CFRP-strengthened RC beams. 7) The tri-linear hypothesis between moment and curvature is fit to actual state, and the deflection formula which deduced by the hypothesis is agreed with the test results. Therefore it can be used as the formula to calculate the deflection of RC members strengthened with FRP under sustaining load. 8) The experimental and analytical results of this paper are valuable for authorizing codes and practical engineering.