Multiple Systems

I’m going to interrupt the defunct-airport sideshow to return to the regularly-scheduled post. The picture above shows an almost-completed repair on a facade project. If you do this kind of work, this is an almost boring picture. It shows three waterproofing systems and implies a fourth, any one of which is theoretically capable of protecting the steel.

What we’re looking at: the rear facade of a steel-frame building constructed in the 1920s. The green fabric is new waterproofing that has been installed over a spandrel beam – it’s adhered directly to the steel. The dark brick above is the original; the lighter brick below was maybe a week old when I took the picture. All of the brick outboard of the beam was stripped to expose the steel for rust-removal, painting, and waterproofing. The four bolts you see are new, replacing damaged rivets at the connection to an interior floor beam behind the spandrel.

As originally built, all of the steel was coated with red-lead paint. It wasn’t a health hazard embedded in the brick and it generally does a good job of protecting steel from rust. The solid brick wall (no cavity as in a modern masonry curtain wall) was built in direct contact with the steel. This was meant to provide waterproofing via the alkaline cement in the mortar. Solid walls also only allow water entry via ordinary absorption through the pores in the masonry, unlike cavity walls, where the differential pressure between the cavity and the outside air tends to suck rainwater into the pores. The new bolts are galvanized, which explains their pretty bronze-like tint. And our work specifically includes installing the new (green) waterproofing. So we have four possible systems of protecting the steel: the (unseen) lead paint, the solid masonry wall, galvanizing, and adhesive fabric waterproofing.

Empirically, lead paint works. It worked on old bridges and it worked on all but the most exposed parts of old steel-frame buildings. I’m not recommending we go back to using lead, but as a steel-waterproofing system, it was pretty effective. We’re looking at 100-year-old buildings and the steel is intact.

Given the gaps we see between the old masonry and the steel, the idea of the cement providing protection was wrong. But 12-inch-thick solid brick walls generally do a pretty good job as a water barrier. Unfortunately, as seen in the photo, there’s usually only one wythe of brick – 4 inches – between the steel and the outside world.

Galvanizing works fine as protection, but proper galvanizing can’t be done in the field to a piece of steel in place, so it’s not a realistic method except for new pieces of steel. There’s also a limit on how big pieces of steel to be galvanized can be. You can go bigger than the limit, but the price increases dramatically. So, in general, this kind of work does not depend on galvanizing.

Finally, the adhesive waterproofing fabric is our modern cure-all. It can cover irregular geometry and it bonds well to the steel. It’s not forever – nothing is – but it will last long enough for the next generation or two of facade inspections.

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