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Community and Q&A

Concrete Cracks when using sub-slab insulation

Ron Flax | Posted in Green Building Techniques on

On a number of projects we seem to encounter problems when pouring concrete slabs over thick rigid insulation. The insulation seems to interfere with the curing process and we get surface cracks that are problematic when using the concrete as the final finish surface. The concrete that is being poured in unconditioned spaces (like a garage) at the some time do show any cracking even though they were the same mix and poured at the same time. Has anyone found a strategy that might lead to better results?

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  1. wjrobinson | | #1

    Spray sealer on right away? Use flow additives instead of added water.

  2. Ron Flax | | #2

    I have not been able to find a spray sealer that will not interfere with the desired look of the polished concrete finish.

  3. user-946029 | | #3

    Curing (or bleedwater evaporation) needs to take place. Wouldn't a sealer inherently prevent that from happening?

  4. user-946029 | | #4

    AJ - Sounds more like a curing compound than a sealer. By definition, a sealer shouldn't have permeability.

    Plasticizers and super-plasticizers are the way to go when needing high slump without adding detrimental amounts of water. Check out some videos of self-consolidating concrete for some incredible stuff.

  5. DWBuilder | | #5

    So the first thing to figure out is why is it cracking when same mix/same time does not in the garage.

    Are you pouring the garage over poly or right on grade? My first thought is that the cracks are shrink cracks caused by the top of the slab drying out and thus shrinking at a faster rate than the bottom of the slab. This creates an internal tension and the cracks appear as the top surface tries to shrink faster than the rest of the slab will allow. If the garage is poured directly on grade the soil absorbs moisture and the top and bottom of the slab hydrate at a similar rate.

    Have you tried moist curing and covering with burlap or plastic to even out the cure rate from top to bottom?


  6. wjrobinson | | #6

    Mike, my contractor uses a spray on (product) right upon finishing power troweling. I believe it has a permeability to it, anyway, it is not inexpensive. Not sure today of product name. The other part of my post mentions keeping the water content to a minimum. Those that do not do commercial work cheat often by over watering. There are lots of neat additives, for flow, speeding and slowing cures, cold, hot weather and more. Even rebar or wire mesh substitutes.

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