Carbon Fiber and Concrete Repair – An Unlikely But Very Compatible Duo – Part 2

In part two of this three part series, we look at how carbon fiber can be used in conjunction with concrete on Structural Foundation Repairs.

Structural Foundation Repairs  

Wrapping bridges, foundations, sea walls

Carbon Fiber and structural epoxy is often used for wrapping structures such as bridge pylons, foundations, and even sea walls. If a crack is present or the support structure has become weakened, the crack itself may be filled by a resin alone or with an added CF mixture. Then the structure itself is laid over by wrapping unidirectional or woven carbon fiber and epoxy matrix over the existing surfaces as designed. This effectively strengthens the overall construction and reduces stress on that existing structure. When this type of repair is carried out, it is commonly practiced using an epoxy resin formulated for higher chemical resistance and even water-proof epoxies for prolonged exposure to elements. These types of repairs can vary greatly in their design and practice. While it may seem like a cure all, there is a lot at stake when structures such as bridges or structurally loaded buildings are repaired. There are years behind the experience and materials knowledge it takes to make the critical decisions and designs on these repairs.

Bowed Interior walls

Interior walls of basements have a history of being troublesome in regard to bowing due to the pressure placed by the soil. This inward pressure will in time cause the concrete foundation walls to bow inward. To combat additional deterioration, concrete walls can be reinforced by carbon fiber straps. These straps are applied vertically along the wall. Carbon Fiber and Epoxy can be used effectively because it will conform to the slightly bowed walls and stabilize the interior wall from bowing more. This type of repair is done by placing the straps perpendicular to the bowing surface. The bow being inward, the repair will go up and down the wall. The repair may apply differently if the wall had a vertical crack in it from another cause. In this case, the carbon straps may be applied from side to side, combating future movement of the wall. The concept being, always applying the reinforcement perpendicular to the discrepant area. In these types of repairs, unidirectional CFs are used more effectively.

General Practices for Repair with CF and epoxy systems on construction applications with concrete follow the same guidelines as that of bonding to other composite substrates. The area in which is being bonded must be clean and free of any FOD (Foreign object debris). With most concrete repairs in construction, a clean room like atmosphere is nearly impossible. This can be a challenge as there is usually an amass of dirt and grime accumulated on most construction surfaces whether they are indoors or out. One of the best methods of cleaning these construction substrates involves use of different tools and cleaning methods.

Achieve an even bonding substrate. With concrete, it will need to be as even as possible, removing any high spots and sharp edges. Any sharp edges of walls should be sanded down to get a smooth enough surface to lay the CF fabric over smoothly. On sharp edges, the radius should be more than 20-30 mm. This may be achieved by grinding, grit blasting, sand blasting, sanding and / or pressure washing.

Surface Prep for Bonding

An etching chemical such as Muriatic Acid may help even the dirtiest surfaces of concrete become clean enough for composite for bonding. It is highly important to note that special PPE (Personal Protective Equipment) is required for use. Chemical resistant gloves, sleeves, googles, and a respirator would be required for safe effective use of this product. It is also important that the acid does not reach any metals as it can deteriorate and etch metal surfaces. It can be mixed 50/50 or less and poured onto the concrete surfaces, it will begin to bubble and work through everything it contacts. After it sets for (~5) minutes, it can be removed by pouring water over it to neutralize the acid. It is important to read the Safety Data Sheet associated with any chemical before using it, especially with Muriatic Acid.

Soap and water may also be used to clean concrete or other construction substrates for bonding. This may be the cheapest and safest option, requiring the lowest amount of PPE. Whether chemicals are used or not, a vacuum cleaner or shop vac should be used to rid any remaining debris from the proposed bonding area. When an effective prep is complete, and the concrete surface(s) is FULLY DRY the real fun can begin.

Mixing

Most concrete specific epoxies have special additives that make them more suitable for various applications. Depending on the repair strength needed, temperature conditions, or type of repair, or differing chemical exposures, special types or formulated construction epoxies are made. Regardless, most epoxy construction adhesives come as a (2) part system that will need to be mixed according to the adhesive’s technical data sheet (TDS). If the adhesive was not shipped with one, a TDS can usually be found by a quick online search. Some formulations may require a low-speed mixer to be most effective, but some may be completed by hand depending on the amount. Construction epoxies vary heavily in their physical, working, and cure characteristics; so be sure to study the adhesive well before buying a specific construction adhesive. When mixing is complete, be sure to clean all mixing tools for future use.