Considering that we made the decision early on to have a concrete house we have to take into account the extra weight of the whole structure. The “soil” (sandy marine clay and rock fragment) we will be building on is probably fill the developer threw on top of a marsh to create “dry” land and from the surrounding canals.
Considering the average elevation of the lot is +3.5 feet above sea level, you can bet the foundation will be sitting in salt water.
The current method is to drive precast concrete piers into the existing soil “driven to refusal”. That means a pile driver will hit the pier into the soil until there is enough compacted material to stop the the pier from going farther.
Across the tops of the driven piers, lie the grade beams. Theses are 16″ x 16″ horizontal concrete beams that carry and distribute the weight of the house structure on to the piers.
Above that is a 6″ concrete slab with a waterproof plastic mat under it.
One of the issues I am researching is “can the concrete for the piers be made with fly ash or Type V concrete”? This concrete mix does not react to salt water as regular concrete does. The fly ash is created from coal fired electric-producing plants as a byproduct. The ancient Romans used the fly ash from volcanos for their concrete mix.
A beneficial result of fly ash is helps prevent spalling. Spalling is the reaction to salts in concrete that causes a chemical reaction. The chemical reaction causes expansion of the material and corrosion for the rebar. The result is weakened concrete.
Another way to prevent spalling is to use coated steel rebar. The rebar can be galvanized with zinc to coat it against corrosion. The zinc will corrode before the steel. There is rebar coated with a plastic but I am unsure if any of these are cost effective for a small residence. There is composite rebar which is made from glass fiber reinforced polymer. It is rust proof, which would be great in a marine environment. It’s limitations preclude it from being used in a small project. It cannot be bent in the field or shaped with heat. In residential construction, where bending of rebar is the norm, composite rebar won’t be used. For precast concrete products, with a specialized need, this product, in a marine environment sounds great.
In a foundation corrosion and spalling can be costly to fix, if even possible.