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Aluminum and Concrete Don’t Mix!

A puzzling problem surfaced on the Eads bridge project. Within an hour and a half of the first concrete place- ment on this cast-in-place Exodermic deck project, small blisters began to appear on the surface of the still plas- tic concrete. When cut open, small gas bubbles could be seen rising to the surfaces through the paste.

The Mississippi River’s 1874 Eads Bridge is one of the world's most important historic bridges.

Eads Bridge Work Nears Completion

There were many theories about what was going on: something was wrong with the concrete mix; the con- crete was reacting with the hot dip gal- vanizing coating; or stray currents from the light rail catenary on the bridge were enabling some reaction to take place. After a number of experi- ments, the culprit was found. In the course of manufacturing the grid pan- els, it was necessary to touch up the

The Eads Bridge over the Mississippi River in St. Louis is one of the landmark structures in the history of bridges. Opened to traffic in 1874, the Eads Bridge pioneered the use of cast steel, removable bridge parts, caisson con- struction, and cantilever construction techniques. Configured with rail traffic on its lower level and vehicular traffic on its upper level, the bridge was essential to the economic growth of St. Louis. The three main spans of the bridge are 500 foot (153 meter) arch trusses, giving St. Louis the arch form as its trademark.

Before closing to traffic in 1991, the roadway of the Eads Bridge had seen steadily declining load postings as cer- tain elements of the structure deteriorat- ed. It was rated only at 5 tons (45 kN) when closed. The current reconstruc- tion project is returning the roadway to service with an HS-20 live load rating. In order to achieve this rating, a number of key changes were required by the designers.

In order to accommodate four lanes of the HS-20 live load in combination with the Light Rail Vehicle loading, consultant

Reaction between wet concrete and aluminum in touch up spray paint caused concrete “blisters.”

  • galvanizing with a “cold galvanizing” paint. Two products labeled for galva- nizing touch-up were available from a well-known manufacturer of specialty paints. One contained 93% zinc and 7% inert materials. The other con- tained 80% zinc, 8% inert materials, and 12% aluminum. Unfortunately, the latter product was chosen. Further experiments showed that the touch-up spray paint was the culprit, the alu- minum in the paint reacting with the cement paste to form hydrogen gas. As a number of grid panels had been touched up with this product, the contractor developed a procedure for eliminating the problem. The painted areas were power washed with a basic solution and then repainted with a zinc-rich paint free from aluminum. Problem solved.

Light weight, 36 foot long grid panels were delivered out on the bridge using a simple trailer

EXODERMIC BRIDGE DECK, INC. • 60 LONG POND RD, LAKEVILLE, CT 06039 • TEL: 860.435.0300 • FAX: 860.435.4868 • info@exodermic.com


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