I recently moved to Louisville from a suburb of Los Angeles where I was born and raised—a city that was incorporated in 1989. Louisville, on the other hand, was established in 1778 when there was no President and, like Lexington, Cincinnati, Indianapolis, Nashville, and many other 18th century cities, contains snapshots of history through its architectural magnificence…The perspective of what “history” is, completely transforms when you drive past the Mississippi river!
Preserving these historic buildings is not an easy feat and certainly comes with a price. With this in mind, it’s worth introducing FRP strengthening as a means of repairing and/or restoring existing concrete building elements like walls, beams, columns, trusses, even pipes.
What is FRP?
If you need a visual of this, imagine a sheet of twist-ties you get in the box of plastic trash bags then glued to the face of a wall (don’t try this at home, kids)—very similar.
Fiber-reinforced polymer (FRP) is a composite material consisting of high tensile-strength fiber strands, uni-directionally woven together, and bonded to the concrete surface using an epoxy resin. If you need a visual of this, imagine a sheet of twist-ties you get in the box of plastic trash bags then glued to the face of a wall (don’t try this at home, kids)—very similar. The concept is this: the FRP fabric is oriented longitudinally along the span of the concrete member, to the face that is expected to be in tension (primarily due to flexural bending), such as the underside of a simply-supported beam. Once bonded and anchored to the concrete member, the FRP can then be painted with a protective coating and would appear and function in the same manner that conventional rebar would with tensile demands being resisted by the high-strength FRP fabric. The net effect is a concrete strengthening solution without an invasive impact to the use, function, aesthetic, etc. of the space.
We all know the nature of the building industry is such that ‘change’ does not come quickly or easily: Engineers tend to lean towards ‘convention’ and Owners don’t want to be the playground for ‘science experiments’. The reality is this: FRP concrete is NOT NEW. Fiber reinforcement in concrete started being researched when a gallon of gas cost $0.13 (1930’s) and was utilized in the U.S. to retrofit building structures from the 1980’s. Research, interest, and use of FRP concrete accelerated in the last decade and the advent of documents such as AC 125 and ACI 440 gave structural engineers a leg to stand on when recommending, designing, and implementing FRP.
What are Common FRP Applications?
FRP is not only used in building alterations, building additions, or change of occupancy projects, but it can be used to repair concrete structures damaged by corrosion or overstressing. Parking structures, for example, have consistently been found to be great case studies given their exposure to the elements and propensity to be damaged as a result of the wear/tear/normal use.
In addition to increasing the tensile capacity of flexural members, FRP has also been shown to produce great results for strengthening the shear capacity of concrete members. It may not be readily apparent unless you’ve taken a mechanics of materials course, but the shearing action within a beam, column, or wall actually results in truss-like effects that produce tensile stresses within the cross-section; thus, an opportunity for FRP strengthening to come into play.
Working in Los Angeles meant that understanding seismic design concepts, load path, and detailing was the only way to be a successful structural engineer. In my experience there, I’ve seen FRP come to be an effective means of seismically retrofitting concrete buildings—here’s a few reasons why:
- The overall lateral force resisting system strength can be increased using FRP without significantly impacting the building’s overall stiffness, like you would if you thickened or added concrete walls. Changing the stiffness of a building changes the force distribution and response of the building during a seismic (or wind for that matter) event—an undesirable effect that warrants modeling the entire building and could require alterations to surrounding structural building components;
- You can greatly increase the ductility of a concrete moment frame building by confining/wrapping/encasing the columns with FRP. ‘Ductility’ is a quality that you WANT in a building in order to dissipate energy without losing load-bearing capacity—it is the opposite of being brittle. We want the failure of a member to occur through yielding of the steel in tension rather than from shear loading or concrete crushing in compression. Encasing the longitudinal reinforcement with FRP limits the ability of the concrete to crush and spall, restricts the vertical bars from buckling in compression, and all but eliminate shear as the governing failure mechanism;
- Instances where inadequate lap splices in columns have been provided can be remedied by encasing them in FRP.
FRP has also successfully been utilized on masonry (CMU) buildings, utilizing the similar concepts as we do with concrete. A former colleague of mine was recently preparing testing criteria for a historic renovation project with wood trusses, where FRP to rehabilitate these members was a viable candidate—hopefully more published results on these applications to come!
How Much Does FRP Cost?
I will say this: I have yet to see a project where FRP was eliminated as an option due to cost.
An entirely separate discussion on the cost analysis between FRP vs. Alternatives could be done, perhaps for FRP Concrete Strengthening 201; there are far too many project-specific parameters that come into play. It goes without saying though, that the cost of utilizing FRP should be carefully considered against traditional alternatives such as enlarging, jacketing, or even replacing the elements. I will say this: I have yet to see a project where FRP was eliminated as an option due to cost. Moreover, if you are dealing with a building that is experiencing noticeable deterioration, you should probably be asking yourself how soon do you spend the money to repair (FRP or otherwise), not IF.
What are Some Limitations with FRP?
As with most materials, there are indeed limitations to using FRP---Here is a brief list of some:
- FRP can only serve to supplement existing steel reinforcement (i.e. rebar, PT strands, etc.)—at least for now. Currently, the existing concrete element must be capable of supporting 110% of calculated dead loads and 75% of prescribed live loads without the aid/benefit of FRP strengthening.
- The FRP must extend a sufficient distance beyond the point where the additional tensile capacity is required, so that it can be properly anchored. To understand this, consider the extreme case: applying 4” of FRP in the middle of a beam would have zero impact on the beam capacity...This aspect makes it rather complex (though not impossible) to strengthen existing columns or the top and bottom of two-way slabs for instance;
- The fire rating of the FRP system and it’s compatibility with the fire rating of the structure needs to be considered;
- FRP design and equations in ACI 440 are predicated on a proper installation: the substrate needs to be clean and free of laitance material, sharp edges, and the proper procedures/methods for bonding the FRP is critical. This is where the selection of a reputable, experienced concrete strengthening contactor is tantamount. A structural engineer should provide a building evaluation per ACI 364 and ACI 437 where/when it is applicable.
- Durability concerns could limit the use of FRP if subject to harsh/abrasive environments. Testing may be required to demonstrate that bond strengths aren’t compromised.
Like buying a new Honda Accord, there are numerous options available when specifying FRP: the type of epoxy resin, type of coating, strands or bars, internal or external, carbon or glass, wet or dry application, fibers or resins, cloth or leather upholstery? …You get my point. Selecting the appropriate FRP system can be done with the help of a knowledgeable structural engineer, concrete repair/restoration contractor, and FRP manufacturer.
Commitment to restoring and maintaining today’s infrastructure will not only ensure that its beauty and history is preserved for posterity, but will provide the many who arrive from those brick and vinyl communities to experience the awe-inspiring architecture, just as I did and do.
Many thanks to Sean Gallagher at Sika Corporation for the use of these photos.