As an installer of two different underpinning systems, I have firsthand knowledge of both thermal set plastic coated and galvanized piers. I have been a part of the debate between the two protective coating systems for many years. I have seen firsthand how the products react over time in the soil.
First let’s discuss the threaded rods that hold the brackets in place. These threaded rods hold the building up and are put under a tensile load to do so. These threaded rods are very close to the surface and therefore are exposed to high amounts of both water and oxygen making them more susceptible to corrosion at that location. If these threaded rods fail in tension, then the building settles. Below is a picture of threaded rods and the corrosion that takes place in open air conditions. We can all agree that embedded in soils that are wetted on a regular basis the corrosion will be much more accelerated than in open air conditions.
The alternative is to use galvanized coil rods that will provide some protection from oxidization such as these pictured.
Below is pictured are typical foundation brackets with Thermal Set epoxy coating after it has been installed in the ground for about a year and a half. Notice all of the peeling that is taking place. Again this bracket is located very near the surface and therefore exposed to high amounts of water and oxygen.
The argument that was regularly made for epoxy coating was that if the tip of it was scratched off during installation, that this tip was located below the active zone and therefore not exposed to too much water or oxygen. And that the weakness of the galvanized system is that it would dissipate over time.
The first assumption that only the tip of the epoxy coated system would be exposed is not true. Much of the entire shaft epoxy coating is removed during the installation process. I have withdrawn many powder coated piles that I installed, and most of the shaft was bare. Furthermore when small amounts of epoxy coating are removed it works very similar to epoxy coating on rebar in that it focuses the electrochemical oxidization process entirely on a very small area exacerbating the oxidization there.
With galvanized products, a small scratch does not have this effect as the galvanization is nobler than the steel, protecting even the small scratch area from oxidization. The added advantage is that zinc anodes can be added to the pile and replaced on a regular basis as needed to complement a galvanized product. This also extends to providing protection to the coil rods as anything in connection with the galvanized products is protected.
One bit of misinformation propagated by the users of Thermal Set epoxy coated products is that the galvanized products leach deadly zinc products into the soil. This is not true, as the actual product that is leaked into the soil is zinc oxide. This is the same zinc oxide that we take as a vitamin supplement. Hardly deadly.
One last bit of advantage is that the hot dipped galvanized products actually gain strength from this process and provide more overall strength because of it.
Feel free to weigh in with your opinion in the comments portion.