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Foundation Repair? Understanding How Austin TX Residential Slab Foundations Work

Two of the most frequent questions I get from my consumer clients is: Do I need to get my house slab repaired? And, if so, how?

As you might imagine, there are a myriad of variables that need to be considered before foundation repair is warranted. The fact is, knowing how slab-on-grade foundation systems work will help optimize the process of determining whether your foundation needs to be repaired. In any case, if you think you may need your foundation repaired, you will be in a better position to help the structural engineer in determining the health of your foundation, and which repair method is best for you. So, here are some items worth investigating before taking the significant economic step of getting your foundation repaired:

Get your house plumbing system statically tested. Assure your drain system (and domestic water supply, irrigation system, etc.) is leak free. Should plumbing leaks be present, get a flow test done to assess the leak discharge water volume.

Check your foundation drainage. Assure that the surface water drains away from the foundation along its perimeter, and no low areas allow water to pond for longer than a day or so after a heavy rain.

Understand how your foundation system works. The basic purpose of your house foundation system is to safely separate habitable areas from the exterior environment and limit damage or distress to interior/exterior brittle building materials.

One of the crucial underlying design principles of shallow bearing slab on grade foundation systems is its single unit behavior. To do their job, slab-on-grade foundation systems act as single structural elements with uniform stiffness. These foundation systems are designed to respond uniformly to resist upward or downward movement caused by soil pressure from below, and heavy structural loads from above. In doing so, these slab-on-grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting.

Know the limitations of your foundation system. The weight of your house walls, roof, appliances, furniture, or perhaps fireplace stone or brick are compressive loads that act downward on top of your foundation floor. These compressive loads average perhaps 300-500 pounds per square foot. Conversely, soil pressure acts upward from beneath your foundation system. Soil pressures average perhaps 3000-5000 pounds per square foot. And the winner i? You get the idea.

Understand your local soil conditions. Sand is non- plastic and quite stable. Clay is plastic and subject to volumetric changes with the addition or removal of moisture. When water is added to clay soil it expands, and when water is removed from the same soil, it shrinks.

As previously mentioned, expanding clay can produce tons per square foot of pressure on the underside of the foundation system, while the weight of a typical house produces less than 500 pounds per square foot average along its perimeter. With that said, excessive water input to clay soil poses the greatest threat to lightly loaded, shallow clay bearing foundations systems.

Get an independent engineer to check your foundation. A competent structural engineer can give an unbiased opinion and technical guidance based upon what the structural status of your foundation system. If your foundation system is structurally sound, then it is likely that underpinning will not be needed. If the engineer finds that your foundation is not structurally sound, then the proper repair will likely require underpinning.  

 Foundation Repair?  Understanding How Austin TX Residential Slab Foundations Work  Foundation Repair?  Understanding How Austin TX Residential Slab Foundations Work  Foundation Repair?  Understanding How Austin TX Residential Slab Foundations Work

Understanding Residential Slab Underpinning-Austin, TX

CRACKING EXTERIOR WALLS FOUNDED ON SLAB ON GRADE FOUNDATION SYSTEM

In Austin Texas, one of the crucial underlying design principles of shallow bearing slab on grade foundation systems is its “single unit” behavior. To do their job, slab-on-grade foundation systems act as single structural elements with uniform stiffness. These foundation systems are designed to respond uniformly to resist upward or downward movement caused by soil pressure from below, and heavy structural loads from above. In doing so, these slab-on-grade foundation systems are stiff enough to buffer potentially damaging soil and subgrade material movement, yet flexible enough to protect the supported structural elements from unsafe or excessive planar tilting.

Properly repairing a failed or structurally compromised slab on grade foundation system requires a design method that mirrors its original design principle. Since shallow bearing foundation systems are designed as single units, they likewise must be repaired as a single unit. At a minimum, the remedial structural design must uniformly stabilize the entire foundation so it can resist the soil, dead and live loads imposed upon it.

Austin, Texas-Case #1: Repairing Part of the Slab-on-Grade Foundation System-Partial Piering. Stabilizing only part of the system restrains and limits its movement in a localized area, allowing the remaining unrestrained portion of the foundation to continue to freely move. Like moving a lever up and down, the unrestrained part of the slab transfers and focuses its stress to the area between the restrained and movable portion slab. This transferred stress can and often does cause additional damage to the brittle concrete foundation structure, thus repeating the cycle of foundation system structural failure.

Austin, Texas Case #2: Repairing the Entire Slab-on-Grade System-Full Piering. Whether repaired or not, the foundation system is designed to behave as a single structural element. When an engineer designs this type of foundation system, shear, deflection, and strength is evaluated for the entire foundation, not just part of it. So why should an appropriate repair address only part of the slab?

The single unit slab-on-grade design principle allows the entire foundation system to react to soil movement uniformly. In doing so, the impact of excessive soil movement to the brittle house materials (brick, sheetrock, etc.) is minimized, thus protecting the slab and house from cosmetic or structural damage.

A reliable, durable, and time tested method for uniformly stabilizing (full Piering) slab-on-grade foundation systems includes underpinning, where a slab on grade foundation (interior and exterior grade beams) bears upon and span underpins (drilled and steel reinforced concrete piers, or DRCP’s). For residential foundation systems, these underpins are strategically placed beneath the expanse of the foundation system to depths exceeding eight feet to provide maximum support. Unlike the typical (and inexpensive) multiple segmented concrete pile components stacked atop one another like unstable kinder blocks, DRCP’s are poured in place, reinforced with vertical steel, and placed to bear on deep, yet stable soils, free from seasonal moisture variations. More importantly, and unlike segmented piles, DRCP’s provide superlative resistance to soil friction that tends to push the underpin up (float) or from side to side (lateral). As such, DRCP’s provide a stable bearing surface for the damaged foundation system by limiting its ability to move vertically or horizontally. For a residence of like kind, quality age and size, 30-40 reinforced concrete underpins drilled to a 10-15 foot depth and placed at the interior and perimeter of the foundation system may be expected.

So, if your unbiased engineer recommends foundation repair, DRCP’s, while expensive, must be strongly considered as the repair method of choice for slab on grade foundation systems. Other methods, while inexpensive, such as mudjacking, soil stabilization, using pilings, or partial piering may satisfy limited budgets in the near term, they will probably fail and cost you more money in the long run.

Understanding Residential Slab Underpinning – Austin, TX

Austin: Repair Foundation Cracks

By Aaron Kuertz

Concrete foundations can develop cracks over time. These cracks can then let in water and damage the interior contents of the basement. In severe cases, cracks in a concrete basement wall can signify that there is a structural defect with the wall. Whether it is to stop water or to structurally reinforce the concrete crack, epoxies can be injected into it.

Causes of Concrete Cracks

The most common is that the concrete actually shrinks as it cures. This curing process reduces the amount of water in the concrete and it shrinks in volume. This shrinkage causes stresses to occur and to relieve this stress, the concrete cracks. This is a normal process and it is not to be a cause of concern. However, they can still let in water and damage the interior contents of a basement. Shrinkage cracks usually are less than 1/16″ in width and remain a constant width throughout the life of a foundation.

A more serious concern is when the concrete has been structurally damaged. This could be occurring for a number of reasons. One of which is that the house is settling into the ground. Or the house may be sliding down a hillside. A very common reason, especially in older homes is the lateral force exerted by the soil onto the foundation. This will be evident by a bowing in of the concrete wall.

How to Determine if a Crack is Structural

There are a few easy ways to determine if a concrete crack is the result of a structural compromise in the foundation.

1. The crack runs on a severe diagonal
2. Horizontal cracks in a foundation wall indicate a serious problem
3. Crack widths that increase over time.
4. Cracks with widths in excess of 3/4″

If you are not sure or are concerned, you should contact a professional to determine if the crack is structural in nature and how to fix it.

Epoxy Injection

Whether it is a shrinkage crack or a structural crack, epoxies can be injected to make the repair. Epoxy injection is done from the interior of the basement, so no excavation on the outside needs to be done. The epoxies hardened in the crack and reinforce the concrete. They will fill the crack entirely from the bottom to the top and from front to back. In many cases, the epoxies are stronger than the concrete itself. The concrete crack will no longer be able to allow water to enter the basement.

This article is written by Aaron Kuertz with Applied Technologies. Aaron has been in the waterproofing industry since 1998. Applied Technologies is a manufacturer and supplier to professional waterproofing contractors and homeowners in the United States. To learn more about epoxy concrete crack repair visit Applied Technologies on the web.

Article Source: http://EzineArticles.com/?expert=Aaron_Kuertz

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