By: Andrew Houston
As we all know, a building’s exterior envelope and roof system serve on the front lines of defense against wind, rain, and sun—the usual suspects that cause wear and tear on any structure over time. However, any building elements left directly exposed to nature will deteriorate much more quickly than those inside the building, even if they’re not explicitly part of the building’s exterior envelope or roof system.
Take these two examples: reinforced concrete framing above the roof slab atop a large-scale building that supports heavy mechanical equipment, and reinforced structural concrete in an enclosed yet unconditioned space below a building’s first floor. Both are bearing the full brunt of the climate at all times, even though neither are technically part of the exterior envelope or the roof system. In both of these examples, the concrete is fully integrated into the building structure and is a vital component to the building’s day-to-day functionality in the short term, and the overall health of the building in the long term.
After initial construction, the final surface treatment along this exposed concrete might have included any number of high performance anti-corrosive coatings, or the concrete may have been left fully exposed. Regardless of how good or bad the initial concrete treatment was at the time of installation, enough time and freeze/thaw cycles will eventually degrade and deteriorate exposed concrete. And for many building owners, this concrete exists out of sight and therefore out of mind.
So, when an owner discovers that exposed structural concrete on their building is severely deteriorated, what options do they have? The first step is to assess the level of deterioration and determine if the deteriorated concrete can be saved or must simply be replaced. The full replacement of a reinforced concrete column or beam is no small task; it takes considerable time and money. It also involves a sometimes-robust temporary shoring installation that usually requires its own design and is costly to install.
Until the early 1990s, the only options for strengthening existing reinforced concrete included adding more concrete and rebar around the outside of the structural member, external post-tensioning, or adding permanent structural steel shoring to remain in place. Although invasive, these treatments were often less expensive than the full replacement of structural concrete members. A new technology developed in the late 1980s and initially implemented in the early 1990s provided owners with another option to preserve and strengthen existing structural concrete members without having to fully replace them. Wrapping the deteriorated concrete member in what’s known as an FRP (fiber-reinforced polymer) composite wrap can add up to an additional 40% of structural strength without requiring any demolition, temporary shoring, new concrete, or post-tensioning. Sometimes referred to as simply “carbon fiber” wrap, FRP wraps are actually some combination of carbon fiber, glass, basalt, and aramid (commonly called Kevlar). Installing an FRP wrap is often much less expensive than any of the previous alternatives for strengthening existing concrete. It is less invasive and requires much less on-site equipment for installation.
Surface preparation is critical when it comes to FRP installation. The existing concrete must be fully cleaned to remove any chemicals or dirt. In order to promote the extremely tight adhesion that will allow the FRP wrap to best perform, the newly cleaned concrete is often shot blasted or water blasted in order to roughen the surface and create a bit of grip for the FRP wrap to be directly applied to the concrete. If for some reason this isn’t possible or is cost-prohibitive, another option is to fully bond the FRP to itself. In this case, the first layer of FRP wrap fully surrounds the concrete member and then continues to wrap around onto itself. This can allow less adhesion directly to the underlying concrete while still creating an impenetrable surface along the exterior. Several FRP manufacturers offer free 1- or 2-day installation classes for potential installers to ensure that correct installation practices are implemented.
Wrapping deteriorated reinforced concrete in FRP composites can extend its useful life by several decades. As previously mentioned, this technology was first introduced and implemented in the early 1990s, and there are many examples of 25-year-old treatments that are still completely intact and both strengthening and protecting the underlying concrete. Part of the reason FRP wraps can serve as such a long-term solution is that the FRP composites can be custom designed to not only strengthen the underlying structure, but to serve as an impenetrable waterproofing layer to protect against any further corrosion of the underlying concrete or rebar.
For building owners, discovering a highly deteriorated concrete column or beam in the dark corner of an underground garage or supporting heavy mechanical equipment on the roof may come as a shock. But owners should be aware of their options before making any knee-jerk reactions. Once a structural engineer assesses the situation, they might determine that FRP wrapping is a potential long-term solution that saves a lot of money in the process.