Structural strengthening with externally bonded FRP has grown in popularity in recent years and provides a viable option for strengthening reinforced concrete for seismic applications, remediations or retrofit of existing structures.  Many products are available on the market, and design guidance has been included in building codes for a number of years.  Are you aware of the benefits and limitations of this type of external reinforcement?

In the July 2021 SE University session, Mark Jarvinen, PE, from Simpson Strong-Tie, presented Design Methodology and Calculations using Fiber-Reinforced Polymer Systems.  Mark covered the design philosophy presented in ACI 440.2R-17 and ACI 318 and explained the strengthening limitations of FRP.  Mark walked through the design methodology for FRP strengthening of a reinforced concrete beam for shear and flexure, as well as improving reinforced concrete column strength with FRP confinement.

Mark reviewed important considerations when using FRP for shear strengthening of reinforced concrete beams.  FRP has been shown to increase the shear strength of existing reinforced concrete beams by wrapping or partially wrapping the beam.  The FRP fiber is oriented transverse to the longitudinal axis or perpendicular to shear cracks in the member.  Mark noted that shear cracks must be epoxy injected before the addition of FRP in order to reestablish any aggregate interlock that has been lost due to the cracks.  This is imperative in order to consider the contribution of the concrete when determining the combined shear strength of the member.  However, depending on the quality of the workmanship for the crack repair, the engineer may want to consider the implication on the shear strength provided by the concrete during design.  Mark noted these cracks should be repaired using a product that meets ASTM C881, Type IV criteria for such a load bearing application.

Three types of FRP wrapping schemes can be used for shear strengthening of RC beams: completely wrapped, 3-sided or U-wrapped, or 2-sided wraps.  These can be continuous or discrete strips.  Mark advised that discreet strips allow for moisture migration which can be essential when working with beams exposed to the elements.  FRP as well as the primer used before its application are vapor barriers and will contain moisture within the element if used continuously.  

Follow along, as Mark walks through a design example using discrete FRP U-wraps to increase the shear strength of a reinforced concrete beam according to the guidelines in ACI 440.2R-17 and ACI 318 by watching this short 4 minute video: 

After Mark determines that the reinforced shear strength is greater than the shear demand on the member, he checks the shear strengthening limit to ensure the total shear strength provided by the steel plus the FRP is not greater than the criteria provided in ACI 318 for steel alone according to ACI 440.2R-17, Sec. 11.4.3.  Lastly, he determines how far into the span the discrete strips are needed by plotting the shear diagram of the member to determine where the strips are no longer necessary.  For further guidance, click here to access the full design example shared by Mark to learn more about the design calculations and limit checks needed to use FRP for shear strengthening of reinforced concrete beams.