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What Homeowners Need to Know About Concrete Slab Foundations

Generally, there are two common types of concrete slab foundations being built today, T-Shaped foundations and Slab-on-grade foundations. Below is a brief discussion of the major features of each type of foundation.

T-shaped foundations are used in areas where the ground freezes. The footing is placed first, the walls are then constructed and poured, and finally the slab is poured. Slab-on-grade foundation is used in areas where ground doesnt freeze to any significant depth. The slab-on-grade is monolithic and the edges of the slab, the perimeter beams, are often thicker than the interior of the slab. Slab-on-grade foundations are poured in one operation whereas T-shaped foundations usually require three separate pours.

Theoretically, concrete should not have defects if the concrete is properly mixed, the ground under the concrete properly prepared and compacted, concrete properly mixed, and allowed to properly cure. But in practice, concrete often has some minor defects.

Cracking is the most common defect and occurs when the concrete separates. Though cracks can be vertical, horizontal or both vertical and horizontal, crack width is perhaps the most important concern. If the width is more than one-eighth of an inch then professionals should be contacted for an inspection.

Other defects include spalling, dusting, and crazing. Spalling is when small areas of the finished surface pop off leaving a small depression. When the surface produces a fine powder that behaves like dust, it is termed dusting. When the concrete surface shows several very small cracks looking like a dry lake bed in a random pattern, then this is crazing. These are non-structural cosmetic defects caused by improper placing, finishing, or mixing of the concrete and are considered minor defects.

Occasionally, a defect can look worse than it really is, so it is advisable to always consult a company offering concrete repair service. A foundation crack that widens over time can result in loss of structural integrity and perhaps water seepage. There are a number of methods to repair foundation cracks in basement areas. All repair methods have their pros and cons so listen carefully to a contractor's opinion. Sometimes, minor foundation cracks can be sealed by using low-pressure injection of polyurethane or epoxy foam material.

The more serious foundation problems are called settlement and upheaval, with settlement being much more obvious to the home owner. After settlement occurs, all or a portion of the home's concrete foundation can be lifted by either slab-jacking or piering. Slab-jacking can be used with smaller areas of settlement and the space under the slab is filled with a grout mixture that forces the foundation to float back to its original position.

However most foundation problems are larger and more serious and require a stronger structural approach or solution. Piering is a process where holes are dug under the problem areas of the foundation. These holes are they filled with poured concrete and steel rebar and they are designed to provide support to those areas of the concrete foundation that need it.

There are other forms of underpinning a concrete slab foundation but the drilled pier method offers the greatest strength and reliability. Other repair methods are cheaper but they also offer less strength, reliability, and usually there is no method to verify the proper installation.


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