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Concrete Cracking — Four- vs. Five-Inch Slabs

Dear Builder’s Engineer,

Will a 5-inch slab have less chance of cracking than a 4-inch slab? Also, what is the best way to eliminate surface cracks on porches and garages without pouring at a different time than the main house? — Mike DuPont, DuPont Construction, Ponchatoula, La.

Mike, thanks for your question. Unfortunately the answer is not so easy. A little background:

In general, concrete is at its largest volume the moment it oozes into the forms. As it loses water from hydration and evaporation, it shrinks. As it shrinks it cracks. Period. It cracks. Regardless of thickness or anything else, it’s going to crack.

The trick is to control the cracking. Here’s how:

1. Use as little water as possible in the mix. I hate beating a dead horse (referring to previous columns), but this is the single most important variable to ensure strong, wear-resistant, minimally-shrinking concrete. Use plasticizers (aka water-reducers), not more water, if you want thinner mud.
2. Use reinforcement. Rebar or welded wire fabric won’t stop cracks but they help distribute shrinkage stresses evenly through the slab, thereby keeping cracks small. You are better off using more, smaller reinforcement than a few large bars. Welded wire fabric is tough to beat for slabs on grade because the mesh pattern is tightly spaced (usually 6-inches square). The problem, though, is ensuring that the fabric winds up at mid-height and not smashed down into the dirt subgrade.
3. Use control joints. Since there will be cracks no matter what, why not tell the concrete where to do its cracking? This is precisely the reason for tooled, sawcut or proprietary (e.g., “zip-strip”) joints. Left to its own, concrete tends to crack in chaotic, zany directions that look like something from a Frankenstein movie. Use control joints in nice, neat, parallel and perpendicular directions as closely spaced as practical for best results. My own stained concrete floor has diagonal sawcuts at 3-foot centers in a diamond pattern. Every sawcut did its job concealing a crack. Even at this close spacing, however, I wound up with a few random (small) cracks, mostly at wall corners and doorways.
4. Make sure subgrade is rock-solid. The fate of any slab is forever tied to the performance of the dirt under it. Pour your slab on oatmeal and all the control joints and reinforcement in the world won’t help. This is especially true if the slab supports vehicle or other heavy loads. The only way to achieve proper subgrade is to not use any fill (assuming native soils are competent); or if you must fill, compact in thin lifts (not exceeding 6-inches) at proper moisture content, using a large enough vibratory compactor (a hand-operated plate unit won’t cut it).

Now, to answer the questions. Regarding 4-inch or 5-inch: 4-inch-thick slabs work just as well as five in residential applications IF all of the above items are adhered to. If subgrade is suspect and vehicle loads will be present you are better off with a 5-inch slab. For slabs that will only see foot traffic, certainly 4-inch is fine, but again, proper preparation is important.

I once saw plans for a new fire station. The slab was designed 10 or 12 inches thick with two mats of rebar. If the subgrade was really weak, i.e., an old peat bog or bay mud, this structural section might have been warranted. However, this fire station was to be located on a stable, mildly sloping hillside with soil bearing capacities in excess of 2,000 pounds per square foot. I think the structural engineer got a little carried away; I would have designed a slab half that thick with half the rebar.

The best way to eliminate surface cracking is by doing what I recommend in 1-4 above. Also, it is a good idea to place a couple 3-foot long sticks of #4 rebar diagonally at every reentrant corner because you know a crack will emanate from that location. Place the first stick about 3 inches from the corner and the next about 9 inches from the corner, both at mid-height of slab. You’ll still get a crack, but it should be hairline at most.

Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to buildersengineer@constructioncalc.com. Tim reads every one.

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