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Preventing and Repairing Cracks in Slabs

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Cracks are not inevitable if you understand why they form. In Part 1 of this three part article, we'll review cracks that form in plastic concrete.

Despite our best efforts, concrete cracks frequently enough that anyone working with it should understand why and how to repair the cracks when necessary.

Cracks form in a slab when there is a tensile force in the concrete that’s greater than the concrete’s strength in tension. Concrete’s direct tensile strength is much less than its compressive strength—about 10%--so when it is pulled too hard it cracks. And cracks can continue to grow, at even lower stresses. The tension can have of a lot of different causes that are either external (overloads, restraint, poor subbase) or internal, inside the concrete itself, such as pressure caused by freezing water or expanding aggregates.

In many cases, cracks in a concrete slab do not detract from its performance. Plastic shrinkage cracks in a slab’s surface often have no negative effect on performance and may only be objectionable because the owner doesn’t like the way they look. Shrinkage cracks that aren’t offset vertically or unstable under heavy loads are really nothing more than natural contraction joints.

In this three-part article we are going to look at cracking in slabs only, not structural cracks or concrete disintegration such as from freeze-thaw action or alkali-silica reaction (ASR). Those situations certainly do crack the concrete, but that’s a different article.

Cracking of Plastic Concrete

Plastic concrete is concrete that has been mixed but has not yet gotten completely hard. The most common cracks in plastic concrete are plastic shrinkage cracks and crazing. Many people confuse these two types of cracks but there is a difference.

Both these types of cracking are caused by the surface drying out faster than the underlying concrete—when the evaporation rate on the surface, due to dry or windy conditions, is greater than the rate of bleeding that can keep the surface wet. Shallow plastic shrinkage cracks form as parallel lines on the slab surface, typically between strike-off and finishing. Crazing cracks that create a fine map or alligator pattern are similar but start after the finishing operation. Plastic shrinkage cracks and crazing don’t typically have a negative impact on slab performance but may be objectionable due to the aesthetics. On the other hand, I’ve heard of some owners asking for a crazed surface on their decorative concrete—tell them, no extra charge!

Plastic shrinkage cracks and crazing can usually be avoided by preventing the surface from drying out before the concrete gains enough strength. These cracks often form between strike-off and finishing. Use moisture retaining covers, plastic sheeting or burlap, windbreaks, shading, fog sprays, or monomolecular evaporation retarders to keep the surface from drying out. Similar precautions can prevent crazing or you can just start curing as soon as possible to prevent surface dry-out. Don’t overwork the surface and never “bless” the concrete with water during troweling. That may make it easier to finish but it adds water to the surface thereby weakening the wearing surface that needs to be the strongest.

Settlement cracks are another common type of crack in plastic concrete. These form directly above reinforcing bars in the slab and are due to the concrete continuing to settle below the bar. Vibrating the concrete is one cause of these cracks and they get worse for larger bars. Settlement cracks, plastic shrinkage cracks, and crazing can be reduced or eliminated by using synthetic microfibers in the mix.

So how do you repair these surface cracks? You don’t. As mentioned earlier, they don’t often detract from slab performance so unless an owner insists, just leave them. Under duress you could grind/polish the surface deeply enough to remove them or place an overlay to cover them up. But the best solution is to convince the owner that the crazing is a special decorative treatment you gave him for free!

In next week’s Part 2, we’ll review drying shrinkage cracks—completely different than plastic shrinkage cracks.

This article is adapted from an article in Concrete Construction, February 2019.

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