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How to Fix Shrinkage Cracking in Concrete Overlays

By Chris Sullivan – ChemSystems, Inc.

Question:

Why did this integrally colored stampable overlay crack so drastically? It was applied at 8 a.m. on a hot July day. The air temperature was 85 F, but the surface temperature was over 100 F.

Answer:

Even though stampable overlays are polymer-modified, they are still concrete mixtures and exhibit all the characteristics of concrete, including shrinkage cracking if care is not taken during installation. The dry, hot weather and high surface temperature, in conjunction with the thin cross section of the overlay, created a situation where the little moisture present in the overlay evaporated too quickly, resulting in shrinkage cracks.

Here are the key factors to remember when working with overlays in hot conditions:

The temperature of the surface to receive the overlay is as important as the air temperature. Only apply overlays when [air and surface?] temperatures range from 50 F to 90 F. If in doubt, hold off until temperatures moderate.
Avoid application in direct sunlight or excessive wind.
To slow hydration, add approved set-retarding agents to the overlay mix or apply evaporative-control agents to the surface.
Cure the overlay with plastic or an approved curing compound.
Not all overlays are the same. Some products contain water reducers or hydration control admixtures to help control shrinkage cracking. Check with the manufacturer of the product you're using.

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