How to Repair Cracks in a Concrete Foundation

November 21, 2012

Leaking cracks in concrete foundations are a common problem for many homeowners. While the cracks are caused by the natural process of concrete curing and shrinking, the cracks can be an annoyance for the homeowner. That is because many homeowners want their basements to act as an added living space with carpeting, finished walls and furniture.

Concrete Foundation Shrinkage Crack Causes

First, all residential foundations crack. Concrete has water as an ingredient in the mix. As the concrete cures, it goes through a chemical process that causes it to harden into a solid. As it hardens, the concrete actually loses volume; that is it shrinks. In fact a concrete wall 100' long can shrink by as much as ¾".

The shrinkage process causes stress forces to develop in the concrete and to relieve the pressure, it cracks. This is okay because a concrete foundation has steel reinforcing bars inside that maintain the strength of the foundation wall. It is common to see cracks develop in the middle of a long wall, under a window or where the foundation steps down.

You will notice that the cracks are usually wider at the top of the foundation and get narrower as you go down the wall. Again, this is normal. You should rarely see the reverse and if so, it may be signs of structural problems with the foundation.

Water Penetration into a Foundation Crack

Water can enter a foundation crack from two directions, from the top of the crack or horizontally from the sides. If a foundation crack occurs at the corner of a basement window, it is possible for rainwater to enter the crack from the top where the window sits back from the edge of the foundation. The water will run down the inside the foundation wall and be forced out to the interior surface somewhere below where the crack gets narrower.

Also, improper grading can cause water to enter from the top. Low spots in the yard or sloping towards the house will cause water to enter at the top. A home with improper grading can cause rainwater to pool against the foundation and enter the crack.

Another way for water to enter a crack is horizontally below the ground (called below grade.) As it rains, the rainwater saturates the soil around a foundation and water enters the foundation crack. Again, the water enters the crack and begins running down inside of it until the crack narrows and is forced out to the interior of the basement. The homeowner will then see water seeping down the wall and onto the basement floor.

Repairing the Foundation Crack Leak

There are several ways to stop the water from entering a foundation crack and leaking into the basement. The first and most expensive way is to excavate along the foundation wall and exposing it. A contractor can then apply one of several different waterproofing membranes to the foundation wall.

This is the most expensive repair process since a large machine must be employed to excavate the foundation wall. It is not a feasible solution for most homeowners because of the time and expense of the repair. A typical repair process may take 2-3 days, depending on how much must be excavated. The excavation requires a large area around the home to place the removed soil. And any landscaping around the home must be removed.

Interior repairs are much more common and feasible for a homeowner. They can be done quickly and for little expense. A traditional interior repair entails a contractor chiseling out a vee-notch approximately 6" wide on the interior surface of the foundation wall and 4" deep. This notch narrows as it is made to about 1" in width at the back of the notch. The vee-notch is made the entire height of the foundation wall.

Once the notch is made, the contractor will then fill the crack with hydraulic cement. This cement actually expands a tiny amount and bonds to the existing foundation walls. The disadvantage to a vee-notch repair is that water can still penetrate halfway into the crack; that is the other 4" of foundation wall width.

As the water evaporates out and into the basement, it leaves behind salts that were dissolved in the water. These salts build up along the bond between the hydraulic cement and the foundation wall. Eventually these salts will cause the bond between the cement and wall to fail and allow water to enter again.

Also groundwater exerts a horizontal force called hydrostatic pressure. This pressure can be transferred directly to the cement in the crack and cause it be forced out of the vee-notch.

Polyurethane Crack Injection Foundation Repair

A better solution is to perform a urethane foam injection. For over 15 years urethanes have been used to stop water from leaking into a basement. The repair is performed by a technician inside the basement. The urethane enters the crack as a two part liquid that is mixed by a static mixer as it enters the crack.

The liquid urethane fills the crack completely, from front to back and bottom to top. When the urethane encounters water that is already in the crack or placed there by the technician, it begins to foam. This foaming process greatly increases the volume of the urethane and it fills the crack.

Since the crack is completely filled, no water can enter the foundation crack. So there is no problem of the dissolved salts breaking the bond of the urethane to the concrete foundation. Also the hydrostatic pressure can't force the urethane out because its bonding strength is much higher than hydraulic cement.

A typical repair can be done in one hour per crack.

A Dry Basement

With the foundation crack filled, the basement area will be dry and free of water. The outside landscaping is not disturbed and for little expense the basement becomes a usable area. Now the homeowner can safely install drywall, carpeting and furniture.


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