Concrete block was used extensively for foundation construction in the mid 1900’s. The primary problem we see with block foundation walls is inward bowing of the wall and cracking of the mortar joints.
Concrete block foundation walls which are tall enough to form basements, and are constructed in clay soils (very common in northern Illinois), generally do not have the strength to resist the forces of lateral soil pressure unless they are grouted solid and reinforced with steel bars. Unfortunately, most homes built in this area with block foundation walls in the mid 1900’s were not built with any type of grouting or reinforcement. Over time the pressure of the soil acting against the foundation wall forces it inward causing the walls to crack and bow. The integrity of the block foundation wall continues to degrade the further it is pushed in as more and more cracks develop between the block joints. And as the wall is forced inward, the weight of the home above the foundations accelerates the inward bowing process. If the situation is not stabilized early on in this process, the walls will continue to move inward and the mortar joints will continue to crack and deteriorate.
The lateral earth pressure increases significantly as the depth of the basement increases or as the soil becomes saturated with water. With older concrete block foundation walls the problem is made worse as perimeter drain tiles were rarely installed. This is usually evident as the basement area may be wet.
If the integrity of a concrete block foundation wall has not been overly compromised by cracking within the mortar joints, the service life can usually be extended by stabilizing them. The most economical and effective way to stabilize a block foundation wall is done with vertical braces in the form of vertical steel beams placed against the wall. These beams are connected to the floor framing at the top and anchor bolted into the concrete floor slab at the bottom. If the floor slab is less than 3″ thick then we may need to break out the concrete floor and embed the base of the braces into the concrete. The beams must have adequate strength to resist the lateral forces of ground pressure and are designed based on the height of the foundation wall and the height of the exterior grade.
If the integrity of a concrete block foundation wall has been overly compromised by inward bowing and cracking within the mortar joints then bracing with vertical beams may not be effective. Vertical braces rely on the concrete block foundation walls having enough integrity to resist the soil pressure and span from bracing beam to bracing beam. If the foundation wall is bowed inward more than approximately 1″, and/or a significant number of the mortar joints are cracked and deteriorated, then bracing the foundation wall with vertical steel beams may not be very effective as the wall will continue to bow and crack between the vertical braces. In these cases the foundation wall may require replaced or reinforcement with a new concrete wall in front of it.
Tie backs with a single plate to the interior are generally not very effective at bracing a block foundation wall as the mortar joints at the perimeter of the plate will crack as the wall tries to distribute the tie back restraining force to the adjacent areas of the foundation wall. An example of this is shown in a photo below. Tie backs are effective when they incorporate a vertical steel beam to the inside of the foundation which engages and restrains the entire height of the foundation wall.
Carbon fiber was developed to reinforce areas of excessive tension stresses within concrete structures, like bridges and buildings, typically in seismic zones. It was not intended to provide bracing for foundation walls. To properly brace a concrete block foundation wall the bracing element must be able to transfer end reactions (shear forces) at the ends. Steel beams can be bolted to the floor framing and to the floor slab to transfer those forces. Carbon fiber cannot be connected in anyway. Below are a couple of photographs showing failure of carbon fiber straps. Unfortunately the foundation wall continued to move inward to the point where the wall was so compromised that it required the installation of another concrete wall in front of it.