Mechanical Behavior And Calculated Methods Of RC Beams Strengthened In Shear With FRP Grid And Cementitious Matrix

Since the 21 th century,fiber reinforced polymer(referred to as FRP)has been attracted great interest in the strengthening field of the existing concrete structures,due to its high strength-weight ratio,corrosion and fatigue resistance as well as ease to assemble,etc.Usually,the strengthening method of FRP laminate takes epoxy-based organics as the adhesive to externally bond FRP to the concrete surface,so the existence of the organic adhesive can ensure the effective stress transfer mechanism and synergistic work between the FRP and concrete structures.However,the epoxy has many drawbacks including low glass transition temperature,easy degradation under the conditions of over high or low-temperature,and poor durability in harsh environment like moisture and so forth.Recently,some scholars at home and abroad consider that using both FRP grid and cementitious matrix to strengthen existing concrete structures may be a better choice,because this method can not only avoid aforementioned shortages of FRP laminate used,but further upgrade the strengthening effects and the utilization ratio of FRP.As a result,for the purpose of meeting the demand of FRP grid-cementitious matrix used in strengthening field in the future,and realizing the feasibility of this method in practice applications,this thesis conducts four-point bending tests and theoretical analysis regarding concrete beams strengthened with FRP grid and cementitious matrix,based on different types of concrete beam’s cross section.A novel calculated method of FRP grid strengthening for concrete beams strengthened in shear is finally developed.The main conclusions are as follows:(1)With increasing load level,specimens successively appear fine flexural cracks,inclined cracks and critical shear cracks,and all of them finally occur shear-compression failure.By comparing the mechanical characteristics of the reference beam and strengthened beams at the same load level,it can be seen that FRP grid can inhibit the development of inclined cracks,and delay the appearance of critical shear cracks,so that the shear capacity of strengthened beams is significantly improved.Showing that the strengthening effect of FRP grid-cementitious matrix for concrete beams is obvious.(2)The shear contribution of FRP grid is closely related to concrete strength,shear-toeffective depth ratio and FRP reinforcement amount.In detail,the shear contribution of FRP grid increases when the concrete strength or shear-to-effective depth ratio are reduced,while the increment of FRP reinforcement amount would promote the shear contribution.Additionally,for the FRP strengthening layer,the possibility of debonding would increase with the concrete strength and shear-to-effective depth ratio.(3)After critical shear cracks occurred,compared to concrete beams strengthened in the web only,better performance in terms of flexural stiffness and shear capacity is presented when concrete beams are retrofitted in both the web and the haunch regions;moreover,the strain distribution of FRP grid is more uniform at the same load level,and the integrated strengthening effect is more obvious,both of which can avoid premature debonding caused by the stress concentration and thus improve the bond behavior at the interface to some extent.Deducing that FRP grid-cementitious matrix used for the full sectional reinforcement of concrete beams with the I-shaped profile is more effective.(4)According to the non-linear finite element analysis conducted by Abaqus/CAE 6.14-3 for several concrete beams with the I-shaped profile,the analytical results and corresponding test results fit well regarding shear capacity.Conclusion can be drawn that the modeling method based on the concrete damaged plastic model and interface spring element in software Abaqus can be used to effectively predict the shear capacity of concrete beams strengthened with FRP grid.(5)Through the series tests of regression analysis,an improved calculated method for shear capacity,which is based on FRP grid’s effective strain,is developed finally in this thesis.The comparison results between the predicted and actual values show good fitness that covers high prediction accuracy,low data discrete and so forth.Therefore,this kind of calculated method used for predicting the shear capacity of concrete beams strengthened with FRP grid is effective and feasible,which can be considered as a reference for the design of concrete structures.