The foundation of a building is the part that sits directly on the supporting soil. The purpose of a foundation is to support and evenly distribute all loads to the soil it rests on. The most durable and common material used for foundations is concrete. Concrete is a mixture of cement, aggregate, and water. The cement alone consists of a combination of lime plus silica, alumina and iron oxide.
The nature of concrete causes it to harden, and when it does, small surface cracks may develop. This is a natural process and is nothing to be concerned about. Bear in mind, concrete will often develop cracks, which does not necessarily mean it needs repair.
Today, the most common type of foundation found in the Houston area is the concrete slab-on-grade (usually referred as a “slab”). Concrete has great compressive strength, but little tensile strength (the inability to resist tension or stretching), which is why reinforcing steel (usually called rebar) is necessary. Post-tension cables are another type of reinforcement that is used in residential foundations. These foundations use steel cables that are laid through the form boards where the concrete slab is to be poured. Once the concrete has set, a truck with a hydraulic jack stretches the cable to provide tension. Because tension is applied after the concrete has set, the foundation is said to be post-tensioned. To better understand this, imagine lifting several books together by pushing in on the sides. The post-tensioned system uses the same principal to hold the slab together.
The second type of foundation, pier and beam, consists of piers spaced evenly under the house to support the floors and building load. One major difference between the pier and beam and concrete slab foundations is that pier and beam foundations have a crawl space between the floor of the house and the soil the house is built over. There are vents in the outer foundation wall and it is important that these not be covered. If proper ventilation is not maintained, the floor framing and sub-floor can begin to rot.
In most areas of Houston you will find houses with some foundation movement. Generally, the reason for this is not poor construction, but poor maintenance. “How do I maintain a concrete slab?” you might ask. Good question! Let us explain why maintenance is necessary and then how you can maintain your foundation.
Factors Affecting Your Foundation
- Soils In most areas around Houston the soils are heavy clay. The heavy clay that causes the most foundation problems contains montmorillonite. Soil with a high percentage of montmorillonite can change in volume (increase or decrease) ten to fourteen times, with the addition or subtraction of water. What this means in plain language is, if the soil your home is built on is expansive clay it can expand and contract tremendously if moisture content is allowed to fluctuate. When the clay soil is increasing in volume it can exert a force of three to fifteen tons per square foot. Most houses only weigh 350 to 450 pounds per square foot; consequently, the volume change can move a house up and down easily. Some people ask us, “Is this the black clay that does this?” Yes and no, color is not a good indication of whether soil is expansive. Expansive soils can be red, black, gray or even orange.
Moisture Content Now let’s get back to the subject of maintaining your foundation. Since the moisture content under and around the perimeter of your foundation determines foundation movement, it follows that you should maintain the moisture content as evenly as possible to minimize movement. “Why can’t nature take care of this?” you may ask. Let me explain it this way. “If you lay a large board on the ground and leave it for a week or so, moisture will build up underneath it, because the moisture cannot evaporate into the air with the board on it” The same thing happens with a concrete slab foundation, only on a larger scale.
Uneven Watering: When watering the foundation or yard, always remember the important thing is to maintain an even amount of moisture under and around your foundation. Now, we don’t mean you should float your foundation; too much water is just as bad as not enough. The easiest way to maintain a constant amount of moisture under and around your foundation is with an automatic sprinkler system.
Soaker Hoses: If you do not have an automatic sprinkler system, you can maintain your foundation with soaker hoses. These hoses are either flat with small holes in them, or are a black rubber “leaky hose”, If you use a soaker hose you should be aware that timers are available to use with them and will greatly simplify their use. Pier and beam foundations should be treated the same as slab foundations, when concerning watering programs.
Plants: Another problem you might experience concerns the vegetation and trees around or close to the foundation. Some trees and shrubs require large amounts of water, and they are not particular where they obtain it. An ideal moisture supply for trees and shrubs is under concrete slab foundations. If there is not an ample supply of moisture, the tree and shrub roots will pull moisture out from under your foundation, causing the soils to dry and shrink, allowing possible foundation problems.
When hot, dry weather comes along, some of the moisture will evaporate from the soil around the perimeter of your foundation, causing the soil around the perimeter to dry and shrink away from the foundation. This allows more moisture to evaporate from under the edge of the foundation. At this point the dry soil under the edge of the foundation has lost its load-bearing capacity in comparison to the soil under the middle of the foundation and allows the edge of the foundation to sag. If the foundation slab is cracked, it may or may not warrant repairs. This judgment should be left in the hands of someone that is experienced in inspecting foundations, a structural engineer.
Hot, dry weather is one of the biggest reasons for changes in moisture content under and around your foundation, but it is not the only factor in our area. Two others are uneven watering around the perimeter of the foundation or yard, and removal of moisture from under your foundation by trees and shrubs.
“What can I do about such items?” you might ask. Lets look at each item: