The use of Carbon fiber reinforced polymer (CFRP) strips for shear strengthening of reinforced concrete beam has gain great success. Structural beams may loss shear capacity due to environmental influences or design and construction errors and hence rehabilitation of the such beam is a must. Sometimes, the exertion of extra load may enforce the improvement of beams in order to increase its ability to contain the extra loads.  

The use NSM and EB for shear strengthening of reinforced concrete beams using Carbone fiber reinforced polymer strips (CFRP) can effectively rehabilitate and improve shear capacity of reinforced concrete beams. The CFRP strips do not corrode and its weight is low which make them easy to install.

The anti-corrosion capability enables the placement of CFRP strips without the need for thick concrete. Both epoxies and systems using high quality cement mortar can be used. For all strengthening methods it is of utmost importance to understand how the strengthening would affect the final structure.

Shear Strengthening Technique of Concrete Beams

The CFRP strip strengthening is based on fixing, epoxy adhesive, Carbon Fiber Reinforced Polymers. Strengthening procedure for both EB and NSM using CFRP strips are presented below:

Procedure for EB CFRP Strengthening

CFRP strips can be externally bonded onto reinforced concrete beams in three forms namely full sided wrapped, three sided wrapped, and two sides wrapped as shown in Fig. 1. Added to that, the CFRP strips can be installed vertically or horizontal as shown in Fig. 2. The following procedure can be considered for externally bonded Carbone fiber reinforced polymer.

  1. Prepare the surface of the beam especially its sides.
  2. Apply epoxy to the prepared surface
  3. Install the CFRP laminates
  4. Allow the epoxy to cure
Fig. 1: Various Wrapping Forms of Externally Bonded CFRP Strips
Fig. 1: Various Wrapping Forms of Externally Bonded CFRP Strips
a) Vertical CFRP Strips, b) Inclined CFRP Strips
Fig. 2: a) Vertical CFRP Strips, b) Inclined CFRP Strips
Full Wrapping EB CFRP Strips
Fig. 3: Full Wrapping EB CFRP Strips

Procedure for NSM CFRP Strengthening  

Detailing of the near surface mounted reinforcement is an important issue; we need to select the most suitable FRP cross-section and adhesive. In design there should be considered the minimum distance between adjacent reinforcement to avoid horizontal propagation of the splitting cracks, and the minimum distance from the edge of the member to avoid edge splitting effect. The following procedure can be used for NSM CFRP shear strengthening of concrete beams:

  1. Sawing slots in the concrete cover of lateral surfaces of beams, with the depth depending upon product used and the depth of concrete cover.
  2. Cleaning of the slots after sawing using high-pressurized water, approximately 100-150 bars is recommended.
  3. If an epoxy system is used, the slot must be dry before bonding. If a cement system is used, it is recommended that the existing surfaces are wet at the time of concrete mortar casting.
  4. Adhesive is applied in the slot, or with a cement system, cement mortar is applied in the slot.
Procedure for Strengthening with NSM
Fig. 4: Procedure for Strengthening with NSM

Groove Dimensions for NSM Technique

  • The minimum dimension of the grooves should be taken at least 1.5 times the diameter of the FRP bar.
  • When a rectangular bar (strip) with large aspect ratio is used, the minimum dimensions must be 3 times the bar width and 1.5 times the bar height.
  • In other instances, the minimum groove dimension could be the result of installation requirements rather than engineering. For example, the groove width may be limited by the minimum blade size and the depth by the concrete cover.We should always avoid cutting of the existing steel reinforcement.
  • Optimal dimensions of the groove may depend on characteristics of the adhesive, surface treatment of FRP, and concrete tensile strength, surface aggregates
Groove Dimensions
Fig. 5: Groove Dimensions

Spacing of NSM CFRP strips

Spacing of FRP shear reinforcement should not exceed l net /2 or 600 mm

Spacing of CFRP Strips in NSM Strengthening Technique
Fig. 6: Spacing of CFRP Strips in NSM Strengthening Technique

Advantages of CFRP Strips

  1. Low weight of the fiber makes it easy to handle without lifting equipment at the site.
  2. Negligible changes of cross-section, self-weight and free height of a structure.
  3. Quick to apply

Disadvantages of CFRP Strips

  1. Without protection the reinforcement is fire and impact sensitive.
  2. Design consultants, contractors and clients have limited experience. 

Read More:

Failure Modes of RCC Beams Strengthened with NSM FRP Techniques

Design of FRP Axial Strengthening of RCC Columns -ACI 440.2R-08

Strengthening Techniques of Reinforced Concrete Beams-Bond Characteristics