Chris' Corner: Here a crack, there a crack (part 3)

Here a crack, there a crack (part 3)


Russel Ray is a very professional, knowledgeable and experienced home inspector from San Diego CA and fellow Active Rain blogger.  Over the last while, he has written a series of posts on the subject of cracks in a property.  If you have a crack and are unsure of the impact on your property, consult with a professional, such as a structural engineer, however, as a primer, and to gain general knowledge, I have found Russel's information very helpful.  This is Part Three of his series and discusses a fairly common occurence, namely cracks on the concrete slab floor of a garage ...


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Here a crack, there a crack....
(part 3)

DISCLAIMER: This discussion is based on several decades of experience in real estate, including as a home inspector and general contractor. I am not now a concrete professional or licensed structural engineer, so if you are not comfortable with common concrete cracks and our discussion here, hire one of those two professionals to help you. This discussion, however, should help you be a little more knowledgeable about cracks so that you don't just automatically assume the worst. Cracks happen, kind of like ActiveRain blogs.

I'll presume that you have read previous lessons.

Lesson Three

Let's move on to cracks in the concrete slab in the garage. Generally, cracks in the garage floor are not of a structural concern to the building because the garage floor concrete is poured separately from the house foundation. It's what we call a "floating slab." The reason why that is done is because you're going to be parking heavy automobiles on the garage floor and you don't want that heavy weight causing the concrete floor to pull against the foundation of the house.

However, cracks in the garage floor can indicate other conditions, such as expansive soils, that might also be affecting the house. So if you see lots of cracks in the garage floor, it's worthwhile to look for cracks and settling problems in other areas of the house.

There are several different types of cracks that can appear in the garage floor slab, all resulting from different conditions.

  1. A crack in the corner of a concrete slab is typically the result of inadequate rebar or poor rebar placement. Corners are difficult to work in, so people choose not to.
  2. A crack at the midpoint of the edge of the slab, shown in Figure 1, is very similar to the crack at the midpoint of the foundation stem wall discussed in Lesson One and Lesson Two. Generally not indicative of structural concerns.

    Figure 1

  3. A lot of small cracks interconnected with each other, looking very similar to alligator skin, is called "alligator cracking." It usually is caused by an improper concrete mixture but can also be caused by lack of a vapor barrier between the concrete and the soil, thus allowing moisture from the ground to move up through the concrete slab.
  4. Large cracks such as those shown in Figure 2 and Figure 3, while uglier, are not necessarily any more problematic. They usually appear in very old concrete, so if you see them in new construction, yes, you've probably got problems. Figure 3 shows cracks that have been "repaired," i.e., filled with goop by the homeowner, not the best repair job here but better than nothing.

    Figure 2

    Figure 3

  5. Cracks with a lot of white stuff (efflorescence) around them, like Figure 4, indicates that there is a lot of water moving up through the concrete and evaporating, leving all those mineral salts behind. This condition is most often associated with lack of a vapor barrier between the concrete and the soil, as well as overwatering the yard.

    Figure 4


All of the cracks discussed above can usually be prevented by:

  1. Cutting down on watering, especially if you live in a desert to begin with. Our desert soils are just a wee bit different from tropical soils and clays.
  2. Installing a vapor barrier between the soil and the concrete, rarely done in dry environments such as Southern California and some areas of Arizona, New Mexico, and west Texas. Sometimes the garage slab is so ugly with cracks that it's time to remove it and start again. Use a vapor barrier this time, even if the concrete professionals don't want to. Make 'em!
  3. Making sure that there is an adequate amount of rebar in the concrete.
  4. Making sure that the rebar is positioned properly within the concrete.
  5. Making sure that the concrete mixture is correct for the job and the environment.
  6. Installing control joints in the concrete such as that shown in Figure 5. Control joints will focus stress along the joint so that the slab itself doesn't crack from the stress.

    Figure 5


Unfortunately, doing items 1-5 above typically requires more time, and time is money someone once said. Readers familiar with San Diego County homes might know that many of the homes in Del Mar Heights, 4S Ranch, and similar new subdivisions along Highway 56 between I-15 and I-5 have vapor barriers, lots of rebar, and expansion joints in the garage slab. Of course, homes out there are about $100,000 higher than similar homes without such quality construction methods.

As always, if you are not comfortable discussing common concrete cracks, hire a professional to guide you.

Join me soon for Lesson Four.


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Comment balloon 0 commentsChris Smith • April 20 2011 02:42PM