🕑 Reading time: 1 minuteStrengthening of reinforced concrete beams with FRP reinforced systems (FRP plate or strips) have been utilize from around 1980s. FRP systems can be used for increasing shear strength of reinforced concrete beams by completely or partially wrapping FRP systems around reinforced concrete member. Since, most of reinforced concrete beams are constructed monolithically with other continuous members such as slabs or walls, therefore complete wrapping of FRP plates is not possible in most cases. Directing FRP fibers perpendicular to potential shear cracks is effective in providing extra shear strength. Moreover, enhancing shear strength might lead to flexural failure which is more ductile failure hence more desired compare with brittle shear failure. The additional shear strength achieved by applying FRP plates or strips depends on number of factors such as beam geometry, existing concrete strength, and applied wrapping scheme. There are three main types of FRP systems which includes Aramid, Glass, and Carbon FRPs. The Carbon Fiber Reinforced Polymer plate which is a high quality but expensive type of FRP plate, is shown in Figure-1. Externally bonding of FRP systems have been applied successfully for strengthening reinforced concrete beams in shear and for improving bridges especially in the United States of America. Shear design procedures provided by Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures (ACI 440.2R-08) is used in this article.
Figure-1: Carbon Fiber Reinforced Polymer Plate
Wrapping of FRP Plates or Strips to RCC BeamsThere are three major wrapping schemes that applied in the shear strengthening of reinforced concrete beams:
- Completer wrapping
- U- Shaped wrapping scheme
- Two side wrapping scheme
Complete Wrapping of FRP PlatesThis type is most efficient wrapping method. FRP system is wrapped around the concrete element completely as shown in Figure-2. As beams are monolithically poured it is hard to have all four sides available for wrapping. This method is suitable for strengthening of column.
Figure-2: Common Wrapping Scheme for Shear Strengthening by utilizing FRP Plates or Strips
U- Shaped Wrapping SchemeThis is used for cases where the beam is integrated with slabs and only three sides are available to use as shown in both Figure-2 and Figure-3.
Figure-3: U-Shaped FRP Wrapping for Shear Strengthening of RCC Beams
Two side wrapping scheme of FRP PlatesIn this case two sides of the member are bonded to the FRP system as shown in Figure-2 and Figure-4.
Figure-4: Two Sided FRP Systems for Strengthening of RCC BeamsMoreover, all three scheme types used for reinforced concrete elements to increase shear strength but the most efficient is full wrapped followed by three-sided bonding then two-sided bonding FRP systems. Furthermore, it is possible to either wrap all schemes continuously or discretely. In the latter case, center to center spacing between strips must be equal or smaller than (d/4+strip width). Finally, U-shaped and two-sided wrapping are subjected to de-bonding failure and therefore their strains are limited by shear bond reduction coefficient (kv). The shear bond reduction coefficient is the function of applied wrapped scheme type, concrete strength, and the stiffness of FRP strengthening system. ACI 440.2 R-08 provides an equation to compute (kv): Where: : design rupture strain of FRP system le: Active bonded length over which most of shear stress is transferred between concrete and FRP system. The active bond length is calculated as follow: k1, k2: Two modification factors which take concrete strength and wrapping scheme respectively into consideration. These modification factors are computed by the following equations: The formula used to compute (k2) for U-shaped scheme is: For two-sided scheme (k2) is expressed as follow: Where: df: Effective depth of the FRP shear strengthening systems. It is equal to the full height of the section in the case of U-shaped scheme whereas, in two-sided bond is the distance from main steel reinforcement to the top of FRP system.