Galvanized steel: a) bond of concrete b) white rust

[ajk1]

We are designing a steel stair with concrete infill to the stair tread pans. It is open and exposed to the exterior and will have an electrical snow melting system in each tread. We are considering galvanized steel (we think it is superior to a zinc-rich paint or epoxy paint, in this type of situation). There are several flights of the stair; it goes up to the roof of a hospital and the helicopter pad on the roof.

Questions:

1. We would like the concrete to bond to the the galvanized steel, so that water cannot collect at the concrete to steel interface (and freeze and crack the concrete if the snow melting system fails). Many materials will not bond or bond poorly to galvanized steel. Will concrete bond to galvanized steel the same as to black steel?

2. Is white rust on galvanized steel only an issue of how the steel is stored before erection, or can white rust be formed on the galvanized steel after it is erected?

3. Is there any treatment that should be specified to prevent formation of white rust?

4. Does the galvanized steel routinely receive a passivating chromate treatment, or does that have to be specifically specified in addition to the galvanizing?

 

[BSVBD]

I don't believe you can permanently prevent moisture from seeping between the concrete and steel.

Why concrete-filled pans for exterior?

Why not open-grate treads?

 

[KootK]

I wouldn't expect to get a good bond with either galvanized or back steel. Given the choice, I'd prefer an all steel or all concrete stair for durability. I have seen some details where folks have welded rebar to the stair pans in order to improve bond.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

Concrete is absorptive and the potential for water to collect at the interface of either galvanized or black steel will always be there in an exterior environment. Further, concrete will react with the galvanizing to some degree, exacerbating its sacrificial loss at the interface. I agree with the grating tread idea. I would also hot dip galvanize all the steel. Galvanized treads are commonly available and can be bolted on.

White "rust" is the oxidation of the zinc in the galvanizing. It can occur at any time after galvanizing is done, whether in storage or in exposed applications.

 

[ajk1]

Thanks everyone for the responses.

This is a hospital stair up to a helicopter pad on the roof. Where I have seen open tread stairs, is seems to me to have been in industrial type buildings. I am not sure about its use in a hospital, and its acceptability as an emergency exit. It would solve the issue of water collecting in the bottom of the tread, but the open treads would be subject to icing up and I don't think that there is any way to install snow melting cables on such a stair.

If the open tread turns out not to be acceptable to our client, then we could perhaps use a bonding agent, except that may be difficult to apply with the electric heating cables there.

Ron - the literature that I see on the internet seem to say white rust is only an issue with respect to the "young" age of the zinc when the steel is stored prior to shipping, and that as the zinc coating ages it becomes resistant to formation of white rust. Is there any publication on which you rely in noting that the white rust also forms at later ages?

No one yet has responded to questions 3 and 4...

 

[Ron]

Questions 3 and 4....

3. Sometimes either chromate passivation or a light mineral oil coating is used to prevent the formation of "white rust" during storage. As I noted, the white rust is oxidation of the zinc. This can occur at any time before the zinc carbonate layer forms on the surface of the galvanizing. This layer is the dull appearance of galvanizing after weathering uniformly.

4. No. Chromate passivation is usually not done prior to galvanizing but can be done as a protection for storage conditions. It does not last long when done post-galvanizing.

If for any reason the zinc carbonate layer is removed or compromised during the life of the galvanizing, white rust can again form until the zinc carbonate is re-formed.

 

[Hokie93]

The advice of using an all-steel or all-concrete stair is spot on. Just two weeks ago I was asked to look at an exterior stair that was in poor condition. The construction was typical "interior" construction (metal pans with concrete fill and HSS stringers) but, in this case, it was an open staircase exposed to the weather. I would say it is a losing fight thinking you will keep moisture out of the concrete-to-metal pan interface for the next 50 years.

 

[XR250]

Concrete on pan usually last less than 20 years here. If they use de-icing salts, it goes downhill even faster. They are usually replaced with all steel.

 

[darthsoilsguy2]

you're on the right path. Galvanized or not, what you are talking about is an interior grade stair in an exterior condition. i've tried to convince institutional clients a few times to go with an exterior application whether precast slabs on angles or full metal, but many will just twist and moan about any deviation from the most standard stair. If you can enclose it, go for it. getting them to make promises to go easy on deicing salts will be false promises. the two i've seen were approx 10-15 years before complete replacement was necessary

 

[ajk1]

to Ron - thanks for the info.

Looks like everyone is advising against steel stair with concrete filled treads. Interesting.

 

[thaidavid40]

Consider possibly using solid cast aluminum treads and riser plates, along with aluminum channel stringers (or tubes) and handrails/guardrails. These make very strong, attractive, and long-lasting stairs, and can be made very slip-resistant.
Dave

Thaidavid

 

[ajk1]

To Thaidavid49 - an interesting idea. Thanks. Will aluminum resist the ravages of corrosive chloride deicing chemicals?
- Have you done a stair as you describe? In a deicing chemical environment like Canada or the north-east U.S.?
- How does the cost roughly compare to that of a galvanized steel stair with concrete fill in the treads?