Experimental Study On Size Effect Of Large-Shear-Span-Ratio RC Beams Shear-Strengthen With CFRP U-Strips

Fiber Reinforced Polymer（FRP）has been increasingly applied in the strengthening of reinforced concrete structures due to its advantages such as light weight,high strength and good corrosion resistance.Among various strengthening schemes,shear strengthening using FRP U-strips is widely used because of easy installation and flexibility of application.However,the majority of current research on FRP shear-strengthened beams has been focused on small-to-medium-sized beam specimens,as the design methods/formulae in existing guidelines have been validated only against the test database of mall-to-medium-sized beams.As the beams needing strengthening in practice are full-scale members,ignoring the size effect may overestimate the strengthening effect of FRP and lead to unsafe design.Against the above background,this MSc thesis presents an experimental carried out on the size effect of RC beams with large shear-span-ratio shear-strengthened with CFRP U-strips.The main contents covered in this thesis are as follows:（1）The behavior of RC beams shear-strengthened with CFRP U-strips was experimentally studied by testing strengthened beams with different sizes（300mm,600 mm,900mm,respectively）and different steel shear reinforcements scenarios（with and without steel stirrups）.The results show that the strengthening effect of CFRP U-strips on the beams without stirrup is limited,but the CFRP U-strips can significantly improve the ductility of the strengthened beam.For the specimen beams with stirrup,only the small beams has obvious strengthening effect;for the medium-sized beams and the large-sized beams,the strengthening effect is not obvious,and the ductility is also reduced.（2）The interaction between FRP,stirrup and concrete was explored using the measured data of FRP strains,stirrup strains and recorded loads and displacements.The results show that there is an unfavorable interaction between FRP,stirrup and concrete: the shear contribution of FRP U-strips first increases and then progressively decreases associated with the successive debonding of the U-strips,during the process,the shear contribution of concrete may be well maintained but some of the steel stirrup may not reach their yield strengths due to the quite small crack width upon the debonding failure.As a result,it is unreasonable to use the formula based on simple superposition(V = V_c + V_n + V_frp)to estimate the shear capacity of the strengthened beam.（3）The size effect of the RC beams shear-strengthened with FRP U-strips was analyzed.The analyses results show that there is obvious size effect in the shear strength of strengthened beams regardless of whether the beams are reinforced with stirrup or not,By comparing the test results and design formulae in the existing guideline it can be shown these formulae are not capable of considering the size effect and generally overestimate the FRP shear contribution.