Concrete Plastic Shrinkage – What It Is and How Prevent It

Posted by Rima on 3/12/2014
The thin, long parallel cracks noticeable on freshly placed horizontal concrete surfaces are what are called plastic shrinkage cracks. They have nothing to do with plastic materials – the plasticity in question refers to irreversible changes that happen to concrete while it’s drying and setting. Plastic shrinkage cracks are more of an aesthetic problem than anything else, as they don’t affect the structural soundness of concrete.

Plastic shrinkage cracks in concrete usually appear when the concrete is exposed to strong winds, low humidity and high temperatures. All of these conditions contribute to concrete losing water much faster than it should, which creates a surface tension. If the concrete has not developed enough tensile strength yet, plastic shrinkage cracks will appear on the surface of concrete. Even if the surface of the concrete dries rapidly without any visible cracks, it doesn't mean the cracks won’t appear before the concrete is fully set.

To prevent plastic shrinkage cracks, it’s more important to understand the conditions that lead to them appearing than tampering with the cement mixture. Pretty much any cement mixture can have plastic shrinkage cracks appear, although adding some synthetic fiber reinforcement into the mixture may help it resist the formation of the cracks better.

To deal with the strong wind, installing windbreaks will help protect the cement surface from the strong wind. Windbreaks can also be used to prevent damage from wind gusts that may appear even if there’s no strong wind in the forecast. Protecting the concrete from direct exposure to the sun is also a good idea, and it can be achieved by mounting sun shades over the surface.

To help concrete retain moisture longer in those really dry and hot periods, using fog sprays to create a layer of moisture above the concrete surface will help. It’s also important to moisten any dry, absorbent formation levels before the concrete is placed over them in the hot periods. There shouldn't be any puddles left after it’s done – it should by damp, not wet. Other than that, the concrete should be placed as fast as possible, cured as soon as possible and the setting time should be accelerated as much as possible.