ABSTRACT
Shear failure is a common problem in concrete structures. Natural disasters, such as hurricanes and earthquakes, may also cause shear failure of structures before full flexural capacity is achieved (MCEER 2005). Reinforced concrete (RC) and prestressed concrete (PC) structures, such as buildings and bridges that were designed several decades ago, also exhibit shear cracks because of regular and unintended or unforeseen loads, unaccounted loads in the earlier designs, inferior material behavior, and loss of concrete strength due to aging (Bousselham & Chaallal 2004). Efficient and cost-effective method of strengthening concrete members in shear is of utmost importance to encounter shear-deficiency problem in RC and PC structures. Fiber reinforced polymer (FRP) systems have been used in the United States for almost 20 years and are becoming a widely accepted method for strengthening concrete structures in flexure and shear. American Concrete Institute (ACI) Committee 440 (ACI 2002) has developed guidelines for the design and construction of externally bonded FRP systems for strengthening concrete members. In this paper, the term ‘externally bonded FRP’ refers to FRP composites attached on the external surface of concrete members by means of epoxy or cement-based adhesive. In case of externally bonded FRP systems, FRP sheets, strips or laminates are externally attached on the concrete surface. The use of externally bonded FRP laminates has been derived from the practice of steel plate bonding. On the other hand, near surface mounted (NSM) FRP system is not a widely used method yet for strengthening deficient concrete members in flexure and shear (Alkhrdaji et al. 1999; De Lorenzis et al. 2000), as well as strengthening
unreinforced masonry walls (Tumialan et al. 2001). The NSM technique was developed in Europe for strengthening reinforced concrete structures in 1950s. In 1948, an RC bridge deck in Sweden needed to be upgraded in its negative moment region (Asplund 1949) due to an excessive settlement of a steel cage during construction. This was accomplished by inserting steel reinforcement bars in grooves made in the concrete surface and filling them with cement mortar. In the NSM technique, grooves are cut into the surface of the concrete members, and FRP bars, sheets or strips are inserted into the grooves and attached using epoxy or cement-based adhesive. The grooves are normally cut 1.5 to 2 times the diameter of the FRP bars to ensure adequate bonding between FRP composites and surrounding concrete. Unlike externally bonded FRP strengthening, NSM technique does not require additional precaution in surface preparation. In fact, the technique has been proved to be very practical, efficient, and economical in strengthening negative moment regions of beams and slabs. Different methods of strengthening concrete members in shear have been developed and are being used. The lighter weight and greater tensile strength of CFRP laminate greatly reduce the overall installation and maintenance costs as compared to steel plate bonding techniques. Moreover, FRP materials have gained popularity due to their non-corrosive nature and low maintenance cost. Shear steel reinforcement is internal; however, FRP composites are bonded externally, which makes the assessment of the FRP performance and bond mechanism more complex. RC and PC members strengthened in shear with externally bonded FRP composites show a number of failure modes that are largely related to the bond behavior at the concrete/ composite interface. Debonding, delamination, and
fracture of FRP composites at the ultimate load or thereafter are examples of such failure modes (Triantafillou and Antonopoulos 2000). Geometry of the member, type of applied loading, ratios of internal longitudinal and shear reinforcement, and shear span ratio (defined as the ratio of the shear distance to the effective depth of the member) are some of those parameters, some of which have been investigated in this research. The experimental and analytical study conducted in this research measured the corresponding strains in the FRP bars and shear steel at various shear critical locations along the beam, and developed methods to calculate the nominal shear strength provided by the NSM CFRP bars. The outcomes of this research are expected to provide useful guidance in developing design criteria for shear strengthening of concrete members with FRP bars attached using NSM technique.
