Concrete repair projects vary widely in scope and cost, from patching cracks and holes to applying decorative coatings or overlays to replacing major structural elements. Performing each kind of repair efficiently and effectively calls for a particular set of skills, materials, and equipment, but all repair projects share certain requirements. The finished repair has to be structurally reliable, visually acceptable, and durable over the long term.
Several steps are critical to achieving these goals. The first is determining the cause and extent of deterioration in the existing concrete. The next is removing damaged materials so that only sound concrete and rebar remain. Another key is to choose repair materials that are both compatible with the existing substrate and capable of performing as needed. Finally, the substrate must be prepared and the repair material applied in ways that ensure they bond strongly together.
Evaluate corrosion and deterioration
Concrete delamination and spalling are problems that often necessitate repairs, and the build-up of corrosion on embedded reinforcement is most commonly the cause. Corrosion byproducts (primarily iron oxides, or rust) have greater volume than the original rebar, effectively increasing the rebar’s diameter. This puts pressure on the surrounding concrete and leads to cracks, delamination, and spalls.
Corrosion is an electrochemical process that requires an anode, a cathode, an electrical path between the two, and an electrolyte. The anode and cathode are points where ions and electrons flow in and out of a metal object. An electrolyte is a substance through which ions and electrons can pass. The corrosion process consumes metal at the anode; metal electrons move along the electrical path from the anode to the cathode, and metal ions move from the anode into the electrolyte. The ions combine with oxygen in the electrolyte to form rust deposits (corrosion) around the anode.
Moisture in the concrete around embedded steel is enough of an electrolyte to initiate corrosion, but plain water is a weak electrolyte and creates a slow corrosion process. Water with dissolved chloride (salt) is a much stronger electrolyte, however, and can greatly increase the corrosion rate. Concrete that is exposed to deicing salt solutions or airborne chloride particles in marine environments is especially prone to rebar corrosion.
The location and extent of rebar corrosion are not always evident from the surface. The area of corroded rebar often extends beyond visible cracks and delamination detected by sounding. In order for a repair to be successful over the long term, rebar corrosion has to be completely removed, even where it has not yet expanded the rebar diameter enough to cause cracking. When initiating repairs, you must remove concrete beyond any signs of corrosion.
It’s also important to inspect for and remove any loose or delaminated concrete. Avoid using large, heavy chipping hammers or pavement breakers to remove concrete for partial-depth repairs, however. These larger machines can damage rebar and create micro-fractures, ortiny cracks below the surface that weaken concrete.