International Concrete Abstracts Portal

In this paper, the crack propagation along FRP-concrete interface of FRP-strengthened concrete structures is analyzed by using nonlinear fracture mechanics, in which the concept of mode II fracture is applied to describe the interfacial fracturing behavior by means of a cohesive crack model with a local shear stress-slip relationship. Two types of the shear stress-slip relationship were proposed, and have been implemented with the mixed finite element methods to perform numerical simulations. A simulation for a simple shear test is carried out to verify the interface crack model. It is found that the interfacial fracture energy is the most important parameter for the bond behavior and the ultimate load can be expressed in terms of the fracture energy. The finite element numerical results agree with the theoretical derivation. Choosing different bond strength and shear stress-slip relationship may influence the effective bond length between FRP sheets and concrete. In addition, an example of a FRP-strengthened concrete beam is also analyzed, in which the composite behavior is significantly dependent on the bond strength of strengthened beam, and the debonding propagation and the failure load due to debonding may also be expressed with fracture energy. The fact that cracks are localized or distributed, for plain concrete beams without reinforcing steel bars, is regarded to be affected by bond strength, interfacial fracture energy, concrete tensile strength and mode I fracture energy of concrete.

Large-scale (80%) tests were conducted on one "as-built" and four composite jacketed rectangular flexural bridge spandrel columns to assess the effectiveness of different retrofit schemes using fiber reinforced polymer composite jackets. Retrofit challenges were in (1) the unknown response of the inclined interface between spandrel column and the arch rib and (2) the behavior of the column reinforcement lap splice located at the top of the spandrel column pedestal. Three of the four FRP retrofit systems only addressed the lap splice region, where as the fourth system connected the column jacket to the arch rib to improve the column/arch rib interface response. Final damage patterns and failure modes showed that only the latter scheme improved the seismic response whereas the other systems resulted in a sliding failure mode without improving the displacement capacity which for the prototype bridge response is less desirable than the original “as-built” lap splice debonding failure. All retrofit schemes successfully clamped the column reinforcement lap splice above the column pedestal construction joint. The tests showed that fiber reinforced polymer composite jacketing systems clearly can be installed without affecting the overall geometry or appearance of the structure, and emphasizes the importance of designing retrofit strategies to control the mode of failure. Retrofitting of one weakness without considering the next mode of failure can lead to ineffective and poor designs.

A series of R.C. beams were strengthened with carbon-fiber-reinforced plastic (CFRP) laminates and tested in an experimental program to study the influence of the cross-sectional area of the CFRP laminates on the shear capacity of the strengthened beam. The used technique enhances the flexural capacity of the original beam but at the same time may decrease the shear capacity. The strengthened beams are noticed to behave and fail through various modes. Also a general modified equation is proposed to predict the load carrying capacity of the strengthened beams taking into account all the existing parameters. The results obtained using the modified equation are discussed and evaluated according to the obtained experimental results.

Increasingly, existing concrete structures are being assessed as having insufficient capacity in shear; the development of an efficient and durable means of upgrading such structures is becoming of utmost importance. An exciting solution is the use of tensioned non-laminated carbon fiber reinforced plastic (CFRP) straps as active (stressed) external shear reinforcement for concrete. The use of an active system has several advantages over a passive (unstressed) reinforcement system. In particular, the prestressed CFRP straps provide confinement and enhance the performance of the concrete. Details of a series of tests carried out on a concrete beam strengthened using these novel CFRP shear reinforcing elements are presented. The strain in each of the straps was measured during testing and valuable insight into the shear behaviour of the concrete beam was gained. It was found that the strengthened beam had a much higher shear capacity than the predicted resistance of an equivalent unstrengthened beam.

This paper presents the results of an experimental investigation on the response of continuous reinforced concrete (RC) beams with shear deficiencies, strengthened with externally bonded carbon fiber reinforced polymer (CFRP) sheets. The experimental program consisted of nine full-scale, two-span, continuous beams with rectangular cross section. The tested beams were grouped into three series. Three beams, one from each series, were not strengthened and taken as reference beams, whereas, six beams were strengthened using different schemes. The variables investigated in this study included the amount of steel shear reinforcement, amount of CFRP, wrapping schemes, and 900/00 ply combination. The experimental results indicated that the contribution of externally bonded CFRP to the shear capacity of continuous RC beams is significant and is dependent on the tested variables. In addition, the test results were used to validate shear design algorithms. The proposed algorithms show good correlation with the test results and provided conservative estimates