When we added a room onto the house my ex brought the concrete pad to the level of the original house with the exception of a 5′ x 7′ area that was an outdoor shed and already had a concrete pad. In order to bring that area up to the level (about 6″)he needed, he built it up with 2’x 4′ beams and laid plywood on top of it. Now I am having trouble with carpenter ants (again) and would like to rip that part out and replace all the wood with concrete over the original pad. It is enclosed so there would be no need for a form. I am at my wits end as this is the second time – I replaced the flooring about 4 years ago and also had an exterminator. Is this feasible? If not, what are the other options?

I would suggest you drill holes about 1 foot apart, and set some rebar into the old slab with concrete tampen into the holes with the bar. Then, coat with concrete glue (it makes new concrete adhere to old). This should give you a good bond.

If you pour concrete on top of concrete you will need to put visqueen over the existing concrete. If you don’t it will crack in the same places as before.

I am not familiar with using this product in this situation. I looked it up on the net but wonder if the product is like a net or something. If it were sheeting, how would the top lay stick to the bottom layer of concrete?

The idea behind the polyethylene sheeting–Visqueen is a brand name of it–is to prevent the two layers from sticking together, so they can expand and contract of their own volition.

When the existing concrete has cracks in it, and you pour new concrete on top of old concrete it will crack in the same spot. The plastic will keep the new concrete from cracking. I have poured concrete over old concrete.

Thanks for the reply but my question is not about redistributing the stresses but rather adhesion of the old concrete to the underlying layer if a plastic sheet intervenes. How thick of a layer are we talking about here?

If you’re just trying to lock the new slab to the old slab, drilling some holes in the old slab and bending some rebar to fit into the drilled holes, then just let the bent rebar float in the new slab. Once the concrete sets the rebar will be locked into position and the new slab will be ‘pinned’ to old slab. There are also glues that are a lot like carpenters glue that bond concrete. I’ve used carpenters glue on some mortar on a block wall, it worked. Dusting that area with Boric acid might help too. It should do the ants in.