Cast in Place Concrete Foundation Walls

Our experience is that cast in place concrete has been used for foundation walls in this area of the country since around the 1920's. It is generally a very durable material which can be formed and cast into almost any configuration. It’s ability to support or resist compressive loads far exceeds what it can support or resist in tension. If concrete is used in construction where tension stresses will develop beyond what the concrete alone can safely resist, steel reinforcing bars are installed to take up those tension stresses.

Generally speaking the tension stresses which develop in an 8' tall concrete foundation wall, 8" thick, do not exceed what the concrete alone can safely resist. The tension stresses being referred to here are the stresses at the interior face of the foundation wall which result as the foundation wall resists the lateral soil pressure from the outside. This is assuming that the foundation wall is properly secured by the floor framing above and the floor slab along the bottom.

Some of the most common problems we find with cast in place foundation walls includes a failure of the connection between the top of the foundation wall and the floor framing, a failure of the beam line to properly restrain the foundation walls, instability caused by the construction of a framed knee wall on top of the concrete wall, and general deterioration of the concrete which is typically found in older foundation walls.

Foundation Walls Not Properly Secured To The Floor Framing

One of the most common problems we see with concrete foundation walls is that they are not properly secured or connected to the floor framing. This is usually caused by a lack of toe nails connecting the floor joists to the sill plate, an inadequate number of anchor bolts connecting the sill plate to the foundation wall, or improperly placed anchor bolts which cause the sill plate to split. As this connection to the floor framing along the top of the foundation wall fails, the foundation wall tips or leans inward under the lateral soil pressure and develops significant tension cracks. The cracks that form are generally diagonal cracks at each end of the foundation wall and a vertical crack in the center. These cracks are much more pronounced and obvious than your typical shrinkage cracks (shrinkage cracks are usually 1/8" or less in width and are generally vertical). This problem is made worse as the foundation wall height increases and as the soils behind the wall become more saturated with ground water. Proper grading down and away must be maintained to direct water away from the foundations.

Stabilizing concrete foundation walls which are shifting inward because of a failed connection between the floor framing and the top of the foundation wall can be accomplished in a number of ways. Bracing the foundation walls with vertical steel beams is effective but may be more than is required except in areas where the strength and integrity of the concrete foundation wall has been compromised by numerous cracks, this typically occurs at the ends of the foundation wall where diagonal cracking develops. Other ways to stabilize the foundation wall involve re-establishing a connection between the top of the foundation wall and the floor framing. This can be accomplished by installing properly designed angle braces, or by installing continuous 2x material to the underside of the floor joists and tight against the foundation wall. This may not be feasible if there is interference with utilities. In some cases, if access allows, the floor joists can be toe nailed into the sill plate.

Foundation Walls Not Properly Secured By Beam Lines

Foundation walls which run parallel to the floor framing typically rely on the beams which support the floor framing being pocketed into the foundation wall to provide lateral resistance against soil pressure. Another problem we see with concrete foundation wall stability, that is not so common, is where the concrete around the beam pocket fails as the foundation wall is forced inward by the lateral soil pressure, or the beam line may not be continuous across the basement so it is free to shift as the foundation wall moves inward under the lateral earth pressure. The foundation wall cracks that develop are the same as those that develop when the floor framing is not properly connected to the foundation wall as discussed above.

Instability Caused By Framed Knee Walls

Foundation instability can also result from a framed knee wall being constructed on top of a concrete wall. The problem with this is that the connection between the concrete foundation wall and the framed knee wall acts as a hinge and the concrete foundation wall is free to rotate inward as the lateral soil pressure pushes against the concrete foundation wall. This is generally not a problem where the exterior grade is 3'-6" or less up along the concrete wall, and if the wall length is not relatively long. This situation can be stabilized using tie backs or vertical beam braces. If the foundation wall needs to be pushed back to a plumb position the exterior will have to be excavated to relieve the soil pressure.

Older Deteriorated Concrete

Some early concrete mixes were made by hand on the construction site. Very often these concrete mixes did not contain an adequate amount of cement to prevent premature weathering and deterioration. This was most likely because the cement was, and continues to be, the most expensive part of a concrete mix.

The weathering and deterioration appear as the sand and gravel in the concrete mix become loose on the surface of the foundation wall. Sometimes it is very easy to remove the loose material and dig through the surface of the wall. We have seen cases where it is easy to remove the material all the way through the foundation wall.

Often times we will find “seams” within an older concrete foundation wall which are weaker and more weathered or deteriorated than other parts of the foundation wall.

If the level of deterioration is not too severe, it may be possible to clean the concrete surface and patch it with specific materials designed for patching concrete. This may not be a permanent repair for the foundation wall but will extend the service life. If the deterioration is significant, the walls may need to be removed and replaced.