Study On Progressive Failure And Carrying Capacity Of Reinforced Concrete Beams And Plates Strengthened With FRP

The problem of inadequate carrying capacity of reinforced concrete(RC) often occurs in engineering.In many cases,in order to increase carrying capacity, the reinforcement for structures is more reasonable than the reconstruction. If adapting a simple and reliable method,the reinforcement will be a very effective approach.Therefore,the method to strengthen RC structures with fiber-reinforced plastic(FRP)has been applied extensively and in the same time many relative researches have been carried out.To increase carrying capacity of RC structures by FRP depends on the strength of adhesive interface and FRP strip,so it is necessary to study in detail adhesive failure process,its influence on carrying capacity and the acting mechanism of FRP, as well as the approach to enhance the reinforcement effects by using the appropriate modeling and numerical method.For the purpose,in this paper, a high-order shear model,correspondent finite element for FRP-strengthened RC beam/plate and FE program were presented and used to investigate the carrying capacity of RC beams and plates with FRP, the process of failure and the effect of fiber-reinforced RC and pre-stressed FRP, in addition the test for adhesive characteristics of FRP was carried out.The main researches are in the following:1.The adhesive layer between FRP and concrete is used for modeling the shear effects and failure behavior. It is assumed that the adhesive is subjected only to transverse shear deformation,in the FRP sheet there are membrance stresses and the classical assumption for RC beams and plates are valid still. Based on the above assumptions,referenced to Rabinovitch'etc research for FRP-strengthened beam,a high-order shear model for FRP-RC plates was proposed.According to the model the control equation in a virtual work form and its FE increment expression have been deduced.2.According to the model,a rectangular element of PRP-RC plate with four nodes and 28 d.o.f. is presented.In the element the Darwin-Pecknold nonlinear constitutive relations of concrete are used.When using the constitutive relations in uni-direction stress state,the element may also be applied to beams with rectangle section.The element along thickness is divided into a numeral sub-layers to simulate the distribution of damage.A FE program (by Fortran,with 4800 sentences)has been developed for nonlinear analysis of FRP-strengthened RC beams/plates and applied to determine the progressive failure process and carrying capacity of FRP-RC beams.The numerical analysis shows that,the tension failure of mid-span RC cover layer and the edge delamination lead to FRP peeling off. The link of edge and mid-span delamination generally means the beam will lose its carrying capacity. The calculation results of Saadmanesh's samples are good in agreement with the experimental results,and with higher precision than Rabinovitch'etc solution.Through numerical examples,the effects of adhesive toughness,the thickness of FRP and initial damage on carrying capacity of beams were discussed.3.For FRP-strengthened RC beams with fiber-reinforcement,the FE analysis of carrying capacity was carried out.The results show that in three cases of fiber-reinforcement, all the carrying capacity of FRP-beams with fiber-reinforcement is much higher than that without.In addition,the interaction between the fiber-reinforcement of concrete and FRP and the influence of fiber-filled position in beams were discussed.4.The analysis for pre-stressed FRP-strengthened RC beams indicates that adding pre-tension to FRP may be effective to delay the tension failure of concrete tension,to decrease the adhesive stresses and to enhance the effect of FRP-strengthening.5.Up to now the research on FRP-strengthened RC plates has been very few and the relative theoretical and numerical modeling have not been found. In this paper the high-order shear model and the developed FE program were applied to analyze the progressive failure and to determine the carrying capacity of FRP-strengthened RC plates.Darwin-Pecknold constitutive relation was refined to make it be applied to nearly mono-compressive stress states.The calculation results are good in agreement with Liman'etc experimental results.The numerical examples show that the carrying capacity of FRP-strengthened RC plates can be enhanced about 114% (depended on parameters of FRP and plates).6.Three kinds of concrete samples with FRP sheet were tested to investigate the shear failure mechanism of interface and effects of fiber reinforcement of concrete.The maximum average shear stresses in adhesive layers were estimated.It is found that the property of shear resistance may be increased about 15～27% due to fiber-reinforcement in concrete.