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Slab Cracks

Slab cracks in slab foundations can cause great anxiety to homeowners. However, many slab cracks do not pose a structural problem to the foundation. For example, many times, slab cracks appear soon after the concrete is installed and are usually caused by poor quality control by the concrete contractor. Most of the time, these types of cracks are hairline in width and are only superficial (do not penetrate deep into the slab) – however, there are exceptions. Generally speaking, these types of cracks are related to the excessive loss of moisture in the concrete and are called plastic shrinkage cracking. Plastic cracking is related to many factors during the slab installation process, including: wind, low humidity, excessive water content in the concrete mix, exposure to the sun, etc. These types of cracks will not usually have a negative impact on the structural ability of the slab.

The cracks that concern us the most are the cracks caused by foundation movement. Most of these types of cracks are hairline to 1/16” wide or so, however, I have seen slab cracks that are ¼” and wider. Many times, an extensive network of slab cracks is an indication of poor rebar placement and/or significant foundation movement.

Unless a slab crack is covered with floor tile (which will also crack) or the slab crack allows excessive moisture penetration from the subgrade soils, most slab cracks cannot be easily detected under carpet or wood flooring.

The usual remedy for a slab crack caused by foundation movement is twofold:

  1. Stabilize the slab so it cannot move. This is typically done by installing piers.
  2. Epoxy inject cracks that are 1/32” wide or wider.  This type of work needs to be done by an experienced technician.

There are other reasons concrete slabs crack but that is another topic for another day.

Jim McNeme, P.E.

Foundation Engineer

www.GeoDFW.com

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Posted by on January 2, 2011 in foundation repair

 

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Foundation Movement

It can be disturbing for a homeowner to realize that his residence has experienced foundation movement. Many homeowners have heard some of the horror stories about super expensive foundation repairs.

Foundation movement, almost all the time, is related to soil movement. Slab foundation stability depends on the support of the soil, so when the soil moves, most foundations (unfortunately) move differentially.  “Differential Foundation Movement” means that the foundation moves more in one area than in another. The principle reason for this kind of movement is that the foundation is not rigid enough to overcome the movement of the soils. This type of movement creates sheetrock cracks, slab cracks, brick cracks, out of level doors and floors, etc. Unfortunately, this is the most common type of foundation movement in expansive clay soils.

The other kind of foundation movement (as opposed to differential movement) is the type where the foundation tilts (like when you pick up the edge of a card table). Tilting usually indicates that the foundation is well designed and constructed (it is rigid) but since the supporting soils below moved, the foundation moves as a unit. In these instances, a foundation that experiences tilt many times does not have a lot of cosmetic distress (sheetrock cracks, brick mortar cracks, etc.) or structural distress (slab cracks).

Neither type of foundation movement is pleasant to remedy. Both types typically require underpinning with piers. There are two reasons to install post construction piers:

1.   To stabilize that area of the foundation from experiencing additional settlement, and/or

2.   To attempt to make the foundation more level

Both of the above goals require using an experienced engineer, the right pier for the soil conditions and a reputable foundation repair contractor.

For more information on foundation movement, go to www.GeoDFW.com

 
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Posted by on January 2, 2011 in foundation repair

 

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