DEBONDING OF EXTERNALLY BONDED POLYPARA PHENYLENE BENZOBISOXAZOLE (PBO) MESHES FOR FLEXURAL STRENGTHENING OF REINFORCED CONCRETE BEAMS

Abstract

Nowadays, there are a lot of existing concrete structures that do not satisfy design and lifetime requirements due to suffering from many adverse conditions such as aging, overload and corrosion. Maintaining, repairing, strengthening and retrofitting for these structures are necessary to extend their lifetime. Several techniques based on practical experiences and scientific researches have been proposed during recent decades. Among these techniques, fiber reinforced polymer (FRP) strengthening systems made of fiber sheets and epoxy resin have been widely accepted to increase the load-carrying capacity of reinforced concrete (RC) structural members because of their favorable properties, such as high strength-to-weight ratio and corrosion resistance. However, there are some drawbacks of FRP systems that are unavoidable due to the usage of epoxy resin. In fact the epoxy bond agent has low permeability, poor fire resistance, is impossible to apply on humid surfaces and is susceptible to UV radiation. To overcome some of these obstacles, fiber reinforced cementitious mortar (FRCM) systems made of fiber meshes embedded in a cementitious matrix have been proposed. These materials of the FRCM systems have good mechanical performance, high resistance to temperature and fire, and have good vapor permeability. They can be applied on wet surfaces. Therefore, the FRCM systems have become an alternative option to the FRP systems for strengthening and repairing RC structures. The innovative strengthening system made of polypara phenylene benzobisoxazole (PBO) fiber mesh embedded in cementitious matrix and concrete recently for external strengthening of RC structures has emerged as one of the most exciting and promising technologies in material and structural engineering. Debonding phenomenon is an important characteristic to evaluate the effectiveness of any strengthening systems and it strongly depends on the transfer load mechanism at the FRCM strengthening material and concrete substrate interface. Until now, very few studies have investigated on the debonding phenomena in RC beam strengthened with PBO-FRCM system. So that, we continue to investigate on the debonding behavior of PBO-FRCM strengthening RC beams under four-point flexure tests in this study. My research included both experimental work and analytical work on the use of PBO-FRCM for strengthening RC beams. The main objectives of my research are: (1) the effective bond length of PBO mesh for PBO-FRCM system, (2) the bond slip law between PBO mesh and concrete, (3) the intermediate crack induced debonding (IC debonding) behavior of PBO-FRCM strengthened RC beams under bending load, and (4) proposed model for predicting IC debonding for beams strengthened with PBO-FRCM under flexural condition. To achieve these objectives, this study was divided into two parts. The first part showed the experimental work while the second part presented the analytical work. There were two phases in first part. The first phase included the shear test of 12 specimens for determining effective bond length. And the second phase included 9 specimens for investigating bond slip law. There were also two phases in second part. The first phase included developing an analytical model to obtain bond slip law between PBO materials and concrete, and the second phase included analyzing and predicting the behavior of RC beams strengthened with PBO-FRCM systems in flexure load. The efficiency and accuracy of these models were verified by comparing their results to the experimental results. The experimental work was also used to investigate the effects of different parameters. The tested results are showed in terms of deflections, strains in materials and failure modes. Based on the experimental and analytical work, useful conclusions and recommendations for beams strengthened with PBO-FRCM system were provided.