Q. We are going to do a 2-inch overlay on an existing concrete floor. The floor is in different sections and has had plumbing and electrical strips cut in and repoured. The floor varies from a flat plane by 1/2 inch to 3/4 inch. The client wants the overlay to be the finished product with 4-foot squares cut in the floor for a pattern. He also wants the roof deck (with a plywood base) to have the same overlay.

We propose to use 3000-psi concrete with 3/8-inch aggregate, a water reducer, a superplasticizer, and polypropylene fibers. The general contractor is going to prepare the floor by sandblasting. I am worried about cracking, which would be a problem for a finished exposed concrete floor. Should I put a bonding agent down for both the concrete-to-concrete pour and the concrete-to-plywood pour? Is it feasible to pour on a plywood deck? This project is going to be an art gallery and I don’t want any problems after the fact.

A. First, let’s consider the 2-inch topping. You will need a very rough profile on the existing floor. Shotblasting would be the minimum. Create a deeper profile. Do not use a sandblasted surface; it’s not aggressive enough. Given that the floor is interior, the cracks between the existing concrete and the trenches for plumbing and electrical probably won’t move.

The topping mix that you mentioned is OK. The day before you place concrete, wet down the surface and keep it wet until you place concrete but don’t have any standing water on the slab during placement. Use a 50% mix of portland cement and sand and make a slurry of it. You can use a 50% mix of water and latex bonding agent (like Acryl 60) to make the slurry. Squeegee or sweep with a stiff broom the slurry into the existing floor just ahead of concrete placement. The slurry must not dry before concrete is placed over it.

After the concrete is finished, it should be wet-cured for 14 days. A product like burlene is good for this, but keep it wet. The concrete won’t shrink when it is wet, so it shouldn’t crack during this period. When you remove the burlene, saw cut the slab immediately. You can use a 10x10-foot grid, but 8x8 is better. This should provide a slab without cracks, but it’s not a guarantee.

Don’t try to use a regular concrete mix for the 1/2-inch-thick area or over the plywood, rather use a polymer overlay cement rated for that thickness. My recommendation is to staple down diamond mesh over the plywood before placing the overlay. Overlay cements are sticky and require experience in terms of troweling a finish.

The only problem with this approach is that the floors are likely to be different colors due to the two types of cement, and there will very likely be some efflorescence on the floor that is wet cured. They will take chemical stains differently, too, if that’s what you intend to use.

Another approach is to use a polymer-based overlay cement for the entire project. Most products come packaged for different thickness applications. So, one product is good for applications in the 0- to 1/8-inch category, another for up to 1/2 inch, and another for 2-inch-thick applications. I suggest that you bring the entire floor up to within 1/2 inch of the top elevation with one product, and then use a product rated for 1/2 inch over everything. Polymer cements have good flexural and tensile strength ratings and are much more crack-resistant. They also have much higher compressive strengths than the concrete mix you are thinking of using. Remember that in topping work, handling the details makes all the difference.