If there is a vapor barrier such as a plastic sheet under the slab, this method will not be allowed as it would puncture the vapor barrier. The concrete in contact with the box and the rebar will probably provide a better ground path than a couple of spikes in the subsoil. (Google "Ufer" grounds").
In addition the box may be fed by a galvanized, grounded conduit.
I agree with David in regards to the best method, however this method may depend on cooperation from the concrete contractor and such cooperation is not always forthcoming.
As for corrosion, I don't see the straps causing a problem.
I have seen corrosion problems but almost always as a result of poor quality porous concrete. With good quality concrete the straps may corrode below the concrete but the corrosion will not travel into the concrete more than a small part of an inch, at least in our lifetimes.
However if you desire rat slabs as David suggests, I would suggest negotiation and compromise on this job and for future work, be sure to specify the method of mounting floor boxes.
A contractor I was with years ago would drive short pieces of 1/2" EMT into the sub soil and wire them to the sides of the floor boxes. We never had a problem or a complaint.
I see a wide range of quality in jobsite methods. When the customer is willing to pay for quality, David's suggestion would prevail. With a building built "On the cheap" by the lowest bidder, as long as there is a box in the floor that is almost at the correct level and close to the correct location, the installation may be acceptable.
Bill
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"Why not the best?"
Jimmy Carter
