Messages and Online Forum Q&A

Re: Cracked Concrete - Aggregate

From: info@builderswebsource.com
Category: Concrete
Remote Name: 63.198.181.43
Date: 31 Jul 2001
Time: 11:04 PM

Comments

We're sure you paid a considerable sum to install 3,000 square feet of aggregate concrete around your pool. Now, just a few months later, you have hairline cracks as well as one large section which is settling.

Without seeing the site and soil conditions first hand, it's hard to know what the cause of the failure is. While hairline cracks are relatively common, complete settlement (1/2" - 3/4") is unacceptable.

First, the type of soil has a lot to do with performance of concrete. For example, clay soils expand when wet and shrink when dry, leading to premature failure or cracking.

Regardless of your soil type, it is the responsibility of your contractor and/or designer to ensure that the concrete has been engineered for the conditions.

Keys to a successful pour include:

1) Sufficient excavation of the native soil, including proper ground compaction and base preparation (which may include base rock and/or sand). The subsoil should be compacted with a mechanical compactor to at least 90% compaction prior to pouring. Failure to do this could result in disturbed soil that later settles.

2) Sufficient rebar in the concrete should be used to hold the entire system together. Some contractors use wire mesh, which often ends up at the bottom of the slab, providing virtually no structural reinforcement. For this reason, we almost ALWAYS prefer rebar placed on intermediate supports. This helps to keep adjacent sections from settling at different rates.

3) Finally, moisture conditions and the rain could have weakened some of the concrete or undermined the base, resulting in premature failure.

So, while hairline cracks can be expected from time to time, large differential settlements should be fixed. The concern is whether the hairline cracks are precursors to further settlement if the ground was not properly compacted and prepped prior to concrete placement.

Our position is clear on this one: have your contractor fix it. Hopefully your contractor was licensed (and you verified the license). If not, you have little leverage. The key here is to determine why the problem is happening in the first place and address the source of the issue, whether it's a soil preparation issue...or a workmanship issue.

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