1631Introduction

FRP reinforcing systems for strengthening structurally deficient concrete structural members and for repairing damaged or deteriorated concrete structures have been used since the mid-1980s. The first applications involved beams strengthened to increase their flexural capacity using high-strength, lightweight fiber reinforced epoxy laminates that were bonded to the soffits of the beams (e.g., Meier and Kaiser 1991; Saadatmanesh and Ehsani 1991; Meier 1995). The method is a modification of the one where epoxy bonded steel plates are used to strengthen concrete beams, which has been in use since the mid-1960s. The FRP systems were shown to provide significant benefits in constructibility and durability over the steel plates. Thereafter, significant work was conducted on strengthening of concrete columns to enhance their axial capacity, shear capacity, and ductility, primarily for seismic loadings (e.g., Seible et al. 1997). This method is a modification of the one using steel jackets to strengthen concrete columns. This was followed closely by work on shear strengthening of beams (e.g., Triantafillou 1998). A review of the state-of-the-art on the subject can be found in Teng et al. (2001) and Bakis et al. (2002). The method has also been used to strengthen masonry and timber structures; however, applications of this type are not discussed in this section.

Current FRP strengthening systems for concrete fall into two popular types. One type consists of factory manufactured (typically unidirectional pultruded) laminates (also known as strips or plates) of carbon- or glass-reinforced thermosetting polymers (epoxy or vinylester) that are bonded to the surface of the concrete using an epoxy adhesive. The manufactured laminates typically have a volume fraction of fibers in the range of 55 to 65% and are cured at high temperatures (typically >300°F), but are bonded in the field at ambient temperatures. The other type consists of layers (or plies) of unidirectional sheets or woven or stitched fabrics of dry fibers (glass, carbon, or aramid) that are saturated in the field with a thermosetting polymer (epoxy or vinylester) that simultaneously bonds the FRP laminate (thus formed) to the concrete. These ''formed-in-place'' or ''layed-up'' FRP systems typically have a fiber volume fraction of between 20 and 30% and are cured at ambient temperatures in the field.

A number of design guides and national standards are currently published that provide recommendations for the analysis, design, and construction of concrete structures strengthened with FRP materials (AC125 1997; TR55 2000; FIB 2001; JSCE 2001; ACI 440.2R-02 2002). This section will provide a brief review of the ACI guidelines.

Manufacturers of FRP strengthening systems for concrete typically provide their own design and installation guides for their proprietary systems. Since the performance of the FRP strengthening system is highly dependent on the adhesive or saturating polymer used, the preparation of the concrete surface prior to application of the FRP strengthening system, and the field installation and construction procedures, manufacturers typically certify''approved contractors'' to ensure that their systems are designed and installed correctly. In addition, code guidance is provided to ensure that FRP strengthening systems are appropriately installed (AC187 2001; ACI 440.2R-02 2002; TR57 2003).

Research in the use of FRP strengthening systems for concrete structures has been the focus of intense international research activity since the early 1990s. A biannual series of symposia entitled Fiber Reinforced Plastics in Reinforced Concrete Structures or ''FRPRCS'' has been the leading venue for reporting and disseminating research results. The most recent symposium, the sixth in the series dating back to 1993, was held in Singapore in July 2003 (Tan 2003).

0 0

Post a comment