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Weathering Steel:
This material is a special, low alloy steel that forms a oxide matrix on the surface that prevents further rusting
under certain conditions. Officially called Atmospheric Corrosion Resistant Steel, the material has a narrow range
of application because it only resists normal rain, (see details below). For these bridges it is a fantastic
product, providing a nearly maintenance free and natural looking finish. Although the steel costs more than normal
structural steel, the weathering bridges are typically 20-30% less expensive because properly done coatings are costly.
Begins rusting like all steel.
Years later there is no deterioration.
Color: The color and variegated patina of weathering steel depends upon the pH of the local dust and rain.
It develops over years and varies from grayish to shades of brown, black, red and dark purple. Damage to the finish
will self heal over time. Notice the fine grain and tight adherence of the rust. This coat will typically stop at
about 6-8 mils thick.
Cannot be used everywhere: Weathering steel is NOT APPROPRIATE for the extremely dry desert or the always wet
conditions like the rain forests in the Pacific Northwest. It must go through many wet/dry cycles before the protective
layer develops.
Restrictions: Weathering steel must be freely exposed to the air on all sides; abutment and bridge member design
must keep this in mind to prevent areas that may entrap debris and moisture. If portions are buried in soil or covered
in vegetation or exposed to corrosive elements, most notably winter salting of walkways, it will deteriorate the same as
any other steel.
Rust runoff: Weathering steel will cause rust staining on concreted decks and sub structure. Sometimes concrete
dyes are used to mask the stains, or silicone treatments to minimize staining during the early years when oxide run off
will be the heaviest.
Called: Often specified as M270 by AASHTO. ASTM numbers for tubes plates and shapes are A847, A588, A242, A709
and A606. Bolts for splices are usually A325. Corrosion index is usually specified at 5.8. COR-TENŽ is the U.S. Steel
brand of weathering steel, due to it's brittleness COR-TENŽ is almost never rolled into tubes.
Web sites with technical data:
Painting:
For beauty or corrosion protection, epoxy coatings are commonly used on these structures. Nearly any coating system can be
provided by Excel, please call for discussion.
Painted Bridges:
Costing around 25% more than weathering steel, painted bridges also require periodic inspection, repair, and eventually blasting and
re-coating. The specifier should consider requiring outside inspection reports of the surface preparation and primer coat.
Paint manufacturers all agree that paint life is dependent upon these two operations, yet the owner cannot examine them and will
not see the performance of, for some years.
Weathering steel holds paint the same as regular steel. No formal research is known to exist regarding the benefit of using
weathering steel under paint, but field observations indicate that painted bridges
using weathering steel may last longer. Paint film injuries seem to self seal rather than bubble and expand. Another rationale
that has merit is when the specifier fears the owner may not maintain the paint, and the bridge is in a site suitable for
weathering steel, the weathering characteristic will protect the bridge whe the paint system breaks down.
Veneer:
Any steel bridge can be covered in a material to achieve a certain feel or look. Most commonly a stringer design is covered in
cedar or treated wood, and the railings made from timbers.
Wood look:
The strength and longevity of weathering steel girders with the warmth of wood. It is important that all veneers be stubbed
out away from the steel so air can circulate, or the steel should be painted.
Zinc coatings:
Sprayed on galvanizing, AKA metalizing, is an infrequent specification, but has been used on these bridges, especially by
the military. Hot dipped galvanizing (HDG) is typical for highly corrosive and low maintenance environments like ocean
side applications or industrial walkways. Due to the appearance and the 40-80% higher cost over weathering steel of
HDG, it is understandably unpopular but sometimes the best choice.
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