Plastic Cracking in Slabs Placed
on Vapor Barriers

Hotline Problem

A ready mix producer called to explain that one of his contractor customers had been having a series of problems with plastic cracking. The contractor insisted on pouring slabs on grade over a polyethylene vapor barrier. Would eliminating the polyethylene vapor barrier help the situation?

Troubleshooting Response

Yes, pouring without the polyethylene on the subgrade will very likely reduce plastic cracking. ACI 302 (Floor and Slab Construction) in comment on prevention of plastic cracking recommends not using vapor barriers if possible.

Remember, plastic cracking results from the shrinkage of the plastic concrete surface during rapid loss of surface moisture. It there is excess water in the mix when concrete is placed on a vapor barrier there will be immediate water gain on the surface. The more excess water there is at the surface, the greater the plastic shrinkage will be.

We can attest to some personal experience in witnessing the same concrete poured at the same time on a vapor barrier for a garage slab and directly on grade for an adjacent driveway apron, with plastic cracking occurring only on the garage slab.

In making this point we want to be sure we don't confuse the harmful effect of excess water in the surface concrete with the value of a fog spray applies to the surface. The fog spray is an effective means of preventing plastic cracking, not by adding water to the surface, but by preventing rapid loss of water from the concrete at the surface. When possible, the spray should be directed so that it drifts across the slab with the wind rather than being pointed directly onto the slab.

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