Foundations constructed of poured concrete are the most common in American homes and there are several good reasons for their popularity:
- Concrete is a relatively low-cost building material and is widely available
- Construction is a fairly simple process compared to masonry
- The resulting structure is monolithic, adding to its strength and resistance to movement, pressure and water
Concrete is often wrongly called “cement,” but cement is only a component of the material. Making concrete requires combining Portland cement with an aggregate, usually gravel, and adding water to make the mixture plastic. The aggregate provides the concrete’s strength and the cement binds the whole thing together, resulting in a strong, solid structure.
Various additives to speed up or slow down the concrete’s curing process can be mixed in to make the product work better under certain conditions and hardware may be set into the wet mix to provide a strong anchor for other construction.
Even with all of these attributes, concrete foundations are not immune to damage. Pressure from oversaturated soil or other movement can cause cracks and shifting of a foundation and mistakes in mixing and pouring the concrete can lead to problems over time.
To better understand how a concrete foundation can suffer structural damage and for repair methods to make sense, it is helpful to know how a poured concrete foundation is built.
Construction of a residential foundation begins with an excavation. Heavy equipment digs a hole in the soil equal to the depth of the foundation and about ten feet wider all around that resembles a large bowl.
The bottom of the excavation is leveled and the undisturbed soil there serves as a stable platform for the foundation.
The first part of the foundation to be built is a system of footings, which are wide, shallow concrete slabs that describe the perimeter of the foundation. These footings serve as a flat, stable base for construction of the walls and to spread the weight of the foundation and the house it will support across a wider base.
When the footings have cured, the foundation contractor will set forms for the foundation walls, blocking out doors, windows and other necessary openings. Although various types of forms are used, including those that create insulation for the walls when finished, the most common style is a form of reinforced steel that is locked in place for stability.
After the forms are set, the contractor begins to pour the concrete, making use of a line of mixing trucks to ensure that the entire pour is completed quickly and efficiently. Workers use a heavy-duty vibrating cable to make sure that the mix settles into all parts of the forms and to eliminate air pockets that may cause problems later.
Once the forms are filled to the top, finishers level off the top edge of the walls and construction hardware is put into place to facilitate attachment of the wooden sill plate, the first stage of aboveground residential construction.
When the concrete has fully cured, the forms are removed, a basement floor is poured and leveled and the soil around the foundation is backfilled. The foundation is now ready for construction of the home to begin.
Even with its strength and solidity, a poured concrete foundation is vulnerable to structural damage of several kinds. In an earlier article, we described in detail the types of damage and how they occur but a brief recap would be helpful here.
Two kinds of structural damage are common to a concrete foundation, an inward-tipping or rotating wall and cracking and shifting caused by sinking, dropping or settlement.
When soil surrounding a foundation becomes oversaturated with water from rain or snowmelt (or improper grading or drainage) it begins to expand. The degree of expansion depends on the composition of the soil but all soil expands to some extent.
When the soil expands, it creates pressure against the foundation wall and, if the pressure becomes strong enough, it can actually cause the wall to move. In a concrete foundation the wall is so strongly anchored at the bottom that it will crack away at the top and sides and actually pivot or rotate inward with the base of the wall remaining fixed.
When this occurs, the wall separates from the structure above and destabilizes the entire structure.
Different damage can occur when soil below the foundation undergoes a significant reduction in its water content. The undisturbed soil left during the original excavation contains a certain amount of water, usually determined by the height of the water table. During normal weather patterns, this soil remains stable.
When drought occurs, trees and large bushes near the foundation will begin to expand their root systems deeper and wider in search of water they need to survive. When these roots extend under the foundation and withdraw water from the soil, the soil becomes compacted and the foundation resting on it can drop or sink into the void that is created. This causes damage ranging from cracking and sticking windows and doors to separation of building elements such as chimneys, decks and additions.
Either of these problems is dangerous to the stability and longevity of the home and must be repaired, but how?
Fixing Structural Damage in a Poured Concrete Foundation
There are two vastly different methods for fixing structural damage but one thing is true of both – they are both major repairs that shouldn’t be put off any longer than necessary when recommended by an engineer or structural foundation repair contractor.
Fixing a Tipped or Rotated Wall – The key to repairing a poured concrete foundation wall that has moved out of plane is to stabilize it rather than trying to restore it to its original position. Of course, if the damage to the wall is so significant that stabilization is not possible the wall can be replaced but this is a major undertaking done only as a last resort.
There are traditional methods of stabilizing a wall such as affixing large steel beams to the inside or installing plate anchors in the surrounding soil but the faults found in these methods have led to much simpler and more effective approaches.
If the wall movement is detected before it has reached two inches, the wall can be stabilized with carbon fiber strips applied to the wall in a number and placement determined by analyzing engineering data.
The installation of carbon fiber begins with the placement being marked on the wall and the area for each strip ground as smooth as possible to improve adhesion. The installer then uses industrial-strength epoxy to affix the 12-inch wide strips to the wall permanently. When the epoxy cures the strips will prevent further inward movement of the wall and will present only a slightly elevated profile that can be easily be painted over or covered by a 2 x 4 stud wall.
If the wall movement measures two inches or more, carbon fiber is insufficient for stabilization. Low-profile steel channels have replaced traditional I-beams for stabilizing a wall that has moved significantly.
To install channel steel, the technician cuts a small hole through the basement floor and bolts the steel strip to the foundation footings. The steel is then attached to a bracket between the floor joists above and tightened so that it conforms to the wall and creates enough pressure against the wall from the inside to prevent further inward movement. The steel is permanent and does not require adjustment or tightening; it can be covered by a standard-sized stud wall if the basement is to be finished.
Fixing a Dropped or Sunken Foundation – Mere stabilization is insufficient for repairing a dropped or sunken foundation; it is necessary to raise the foundation (and the house) back to its original level and then stabilize it. The best way to accomplish this is with a system of steel hydraulic push piers.
Installing push piers begins with digging down to the footings at each spot where an engineer has specified a pier is to be placed. The installer notches the footing and bolts on a steel bracket.
When the brackets are in place, a hydraulic ram is used to drive sections of steel column through each one until the column hits a load-bearing stratum in the earth. With the columns resting on this stratum, a hydraulic pump is attached to each bracket and used to raise the entire foundation at once back to its original level.
The hydraulic system keeps the house in place until each column has been bolted securely into its bracket so it can hold the structure in a permanent, stable position. When the holes have been backfilled the repair work is invisible.
Regardless of the type of damage or the recommended repair, a homeowner who needs structural repair on his or her home will require the advice and assistance of a qualified foundation repair contractor. At U.S. Waterproofing, our foundation repair exerts make use of engineering data and the latest repair technology to ensure each home is stable and secure at a minimum of cost and inconvenience. Why not ask for a free consultation if you suspect structural damage to your foundation?