Study On Prestress Loss Of RC Beams Strenthened With Pre-tensioned Prestressed CFL

Carbon Fiber Reinforced Polymer(CFRP) has been widely used currently in the reinforcement of engineering structures or structural members at home and abroad. However, a relative limited effect has been witnessed in the traditional non-prestressed strengthening method due to the fact that CFRP will not play a significant role before the cracking or even yielding of engineering structures. Instead, high tensile strength, light quality and other properties of CFRP can be fully played in the technology strengthening reinforced concrete with prestressed CFRP which can also effectively control the formation and expansion of cracks, enhancing sectional flexural stiffness as well as improving the structure stress distribution. So the technology is developed dramatically. But the theories of CFRP prestressing loss in strengthening reinforced concrete is still not enough for engineering application. And there is almost no related research on the influence of end anchorage to maximum control stress for prestressing. Thus there is a great significance and value in the theoretical research of CFRP prestressing loss in strengthening reinforced concrete and maximum control stress for prestressing.Pretensioning method is used in this paper to prestress a new kind of Carbon Fiber Laminate(CFL) invented by our research group, and experimental study and theoretical analysis are carried on prestressing loss taking the whole process of prestressed CFL strengthening reinforced concrete beam as the object of the study. Furthermore, the finite element model of CFL strengthening reinforced concrete beam is established to study maximum control stress for prestressing. The research contents and conclusions can be listed as follows:1) With the application of anchor clamp and tension device developed by our research group, pretensioning method is used in this paper to prestress Carbon Fiber Laminate(CFL) with prestress levels of 8%、15% and 22%, and experimental investigation is conducted for CFL prestressing loss in strengthening reinforced concrete. The prestressing loss corresponding to the reinforcement method is divided into five stages according to the experiment. The first prestressing loss stage is in the stretching process followed by the stages prestress growth in CFL pasting process, curing process prestressing loss, pretension process prestressing loss, and long-term prestressing loss after the pretension process.2) In order to grasp the general rule of effective prestress distribution of prestressed CFL beam reinforcement of RC, the prestressing loss of prestressed CFL reinforced RC beams are analyzed. Through the research, this paper argues that the main factors of prestress change in the five stages are: it is caused by CFL slippage in waveform tooth plate and mouthpiece pressing as well as deformation of anchor joint in unloading lifting jack in the first stage. While in the second stage, the change is mainly brought by the height difference between the end of the RC beam and the CFL adhesive surface. In the third stage, the prestressing loss mainly lies in the stress relaxation of CFL while the concrete rebound contributes to the fourth stage prestressing loss and concrete shrinkage and creep lead to the fifth. The paper also deduced the theoretical calculation formula of effective prestress of prestressed CFL reinforced RC beam and through the comparison of experimental data and calculation results, it is known that the formula can accurately predict the effective prestress of RC beam strengthened with prestressed CFL under pretensioning method as the results of theoretical calculation agree well with experimental data.3) In the interest of fully taking advantage of the high tensile strength property of CFL, theoretical analysis and calculation on the maximum control stress for prestressing is presented in the paper from three aspects of crack of beam tension side, CFL tension fracture as well as insufficient anchoring strength and the theoretical calculation formula of the maximum control stress for prestressing is obtained. Depth analysis of the anchoring effect and its influence on the maximum control stress for prestressing by fully swathing FRP around the beam end is also actualized in the article. In addition, the finite element model of the RC beam in ANSYS is set up to present numerical simulation on the prestress of the beam after the pretension process. Through this the relationship between the FRP band width of the end anchorage and the maximum control stress for prestressing is obtained, then the effectiveness of the theoretical calculation formula is validated through the comparison with numerical analysis results.