huangyhg

paint in lieu of galvanizing
folks,
i am not a structural guy, but i have a structural problem of sorts. at an ongoing project, i received a call first thing this morning from the gc that "the steel arrived, but it isn't galvanized." by steel, he means about 250 feet of w21x73 to be used as a pipe bridge across a creek. the steel arrived with the red (zinc?) primer only. the specifications and drawings (prepared by others) clearly show the steel to be galvanized, but evidently the steel contractor "missed that part".
i'd love to tell him to send it back and get it right, but that won't fly with a two week turnaround time, so i need some alternative coatings that can be field applied. the steel will be exposed to the elements (not marine).
any ideas?
thanks.

if you are going to paint it, i would use tnemic paint. it bonds to the steel much better than regular paint. do you have any dissimilar metal issues, where you need a break between them, to consider?
what is your long term time frame? this bridge is supposed to stay in place for how long without requiring maintenance?
there are zinc rich steel coatings that are available.
i am unsure of their performance in the long term or compared to hot dip galvanizing.
get the steel galvanized.
the only metal to metal interface will be with the anchor bolts, but this is steel to steel, so i don't think that will matter. the long-term time frame could be as short as three years or as long as 30, but the current thinking is closer to the three year mark. i would anticipate zero maintenance will be conducted on this once it is built.
epoxy enamel paint will last 20 years with no traffic if correctly applied.
if it has been spec'd as galvanized, then almost any coating will be inferior and the long term corrosion of the material will be greater.
they should supply the correct specified material. alternative coatings, approved by the eor, can be considered, but these are high end coating systems.
dik
require the contractor to either hot dip galvanize the material or provide a interest bearing note that will fund recoating after 10 years. i suspect that he'll figure out a way to get the galvanizing done.
even though your schedule is tight, you're going to have to live with the structure for a long time. don't let him (or her) use the schedule as a club to make a bad decision.
a paint system including a zinc rich primer can give good performance but is dependant on (i) good surface treatment and (ii) good painting conditions and procedure.
(i) probably means removing the existing primer by blasting
(ii) may be difficult to achieve in the field.
galvanising may well be just as easy to obtain as a good paint system, and is the more robust solution.
if you get people who know what they're doing (say, bridge painters), you should be able to get a zinc-rich coating applied well. in theory. with good inspection and a good specification. and yes, you do need to remove the existing primer, or the zinc in the coating won't accomplish anything. you need close to a white-metal blast. and you want something that will provide a dried film with at least 94% zinc dust by weight. look for coatings recommended for bridges.
if you want something resembling galvanizing, try metallizing. again, you'll still need to blast off the shop primer, but spray metallizing is comparable to galvanizing in its ability to protect from corrosion over the long term.
hg
i would also say metallizing in the field is the way to go. metallizing may not be as good as hot-dip galvanizing, but it’s better than painting (even with a zinc rich paint). this is a somewhat common coating used for highway and railroad bridge girders which are too large to be hot-dipped.
the cost will be higher than painting, so expect resistance from the contractor if you choose to go that way.
in my mind, it is not your call to make on weather to accept a painted system. it is the owner's. they have to live with the system for many, many years, and will be responsible for maintaining the coating.
if the structure is above a creek, it will be a little difficult to maintain a paint coating. and so, more than likely, it won't get done.
painting is inferior to hot-dipped galvanizing. that is why you spec'd it in the first place.
ask the owner if he is willing to accept painting, and if so, how much of a credit he want back for accepting an inferior product than the one specified in the contract documments.
if the owner does decide to accept a painted system, you will need to strip the primer paint off. painting over the primer is a very bad system that likely will not hold up very long.

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