My project calls for phosphatized and painted white bottom composite deck. What is it and why haven’t I seen it before?

What it is: Bare steel that is phosphatized (chemically treated to help paint bond to it) and has a primer coat of paint on the underside.

Phosphatized and painted composite steel deck exists because the unit price of cold rolled bare steel with a primer is less than galvanized steel. However, painting the top of composite floor deck would inhibit the composite action it was designed for because the paint does not form a chemical bond with the concrete like galvanized or lightly rusted bare steels do.

This means two things:

1. The top cannot be painted and
2. The top needs to develop light tight rust for the deck to work.

If you happened to read our last installment of ASK D-MAC: “Why is the white-bottom deck on my job covered with red streaks across the bottom?” you should be asking yourself:

“How do I prevent unsightly red rust stains across the bottom of my phosphatized and painted white bottom composite deck when the deck is designed to rust?”

In this case, we recommend you do two things:

1. Notify the owner and architect in writing that the white primer should be expected to exhibit red rust stains and will need to be cleaned and painted with a finish coat of paint if it is meant to be exposed AND,

2. Recommend to the owner and architect that a galvanized and painted white bottom deck be used instead.

Why you do not see it often: As the saying goes, “the lowest price option is usually not the lowest cost option.” In order for phosphatized and painted composite deck to be low price and low cost, the following conditions need to be met:

1. The project requires at least one truckload of material. Since the deck is not common, you may face a setup charge and longer lead times if you need less than a truckload of material, negating the lower price you hoped for.

2. The deck will be used in a dry interior and non-corrosive environment. Since composite deck is the positive bending reinforcement in the slab, it must be designed to last the life of the structure.

3. The material will not be stored outside for extended periods of time. While you need some light rusting to bond with the concrete, excessive rusting means that you are losing material thickness. Since you are using thin metal to begin with, you will quickly lose capacity as the material thickness is reduced.

As always, please contact us for more information about this, or any other questions you would like to ASK D-MAC!