Corrosion of Embedded Reinforcing Steel
Horizontal cracks in concrete block, or cast in place concrete foundation walls, are rarely an indication of foundation settlement. In concrete block foundation walls horizontal cracks are typically caused by the foundation wall being forced and bowed inward under the lateral or inward pressure of the soil. We rarely see cast in place concrete foundation walls which have developed a horizontal crack because of lateral or inward soil pressure. These rare instances are typically the result of excessive lateral soil pressure caused by super saturated soils or very deep basements with tall foundation walls.
The more likely cause of horizontal crack development in concrete foundation walls is corrosion of embedded reinforcing steel. As the steel corrodes it expands which causes the concrete to split apart and spall off on the surface.
In very rare instances have we seen vertical crack development caused by corrosion of embedded reinforcing steel as vertical reinforcing steel is rarely used in concrete foundation walls. The only situations which would require the use of vertical reinforcing steel would be in relatively tall foundation walls. Vertical cracks in concrete foundation walls are generally the result of concrete shrinkage which is normal and expected.
Corrosion of reinforcing steel is typically the result of either the steel not having enough concrete cover to protect it or by excessive chlorides being present in the concrete mix at the time it was placed. Common sources of chloride in concrete used for residential foundations include admixtures which are used to accelerate curing at the time the concrete is placed and contaminated aggregates. Chlorides may also be added to the concrete in an attempt to lower the freezing point, although this not very effective as it may lower the freezing point by only a couple of degrees. The level of chloride required to start the corrosion process of embedded reinforcing steel is extremely low at approximately .026%.
There are two problems associated with horizontal crack development. The first and most obvious, is the crack itself which is unsightly and may leak. Unfortunately epoxy or urethane injection may only be a short term fix as the steel continues to corrode and expand opening the crack up even further. The second and more significant problem is that the integrity of the foundation wall has been compromised as it is not longer a continuous section from top to bottom. If you can imagine the lateral soil pressure pushing against the foundation wall from the exterior, with the floor framing on top and the concrete floor slab below holding the wall in place, it will want to fold inward along the crack location.
In situations where corrosion of embedded reinforcing steel is effecting only a small area of the foundation, usually along the top of the wall, the most common repair is to first dig the reinforcing steel out of the wall. This may require removing the wall from the crack location up. If the crack development is along the bottom of the wall it is possible that the entire foundation wall may have to be removed and replaced. After the steel is removed the foundation wall is re-capped or patched and then possibly braced to resist the lateral or inward soil pressure.
In cases where there is reinforcing steel around the entire perimeter of the foundation which is corroding and expanding, the installation of a impressed cathodic protection system may be a viable option compared with removal and replacement of part or all of the foundations. An impressed cathodic protection system utilizes a sacrificial anode which is embedded into the surface of the concrete and electrically charged with a very low level DC current. The current flows out of the sacrificial anode and into the reinforcing steel which stops the corrosion process. The foundation walls can then be stabilized against inward lateral movement with the use of carbon fiber straps and steel bracing.
Examples of Horizontal Cracking Caused by Corrosion of Embedded Reinforcing Steel
Repair of Foundation Walls With Corroded Embedded Reinforcing Steel
Removal and Replacement