Q.: Your Problem Clinic item on expanded metal bulkheads (November 1986) is interesting but I'd still like to have more information. Are there jobs that have been in place for 15 or more years so the long-term performance can be evaluated? Can zinc in the galvanized expanded metal product form a corrosion cell with the rebar?

A.: We talked with two manufacturers of expanded metal products. Neither had long-term performance data for their product when it's used in bulkheads. We then contacted several concrete industry sources. John Richardson, a concrete specialist from the United Kingdom, reports that expanded metal is used there for both stay-in-place forms and bulkheads. He says it was used by the acre on the Thames barrier and that there were no reports of corrosion problems. He cautions that there must be adequate cover over any metal left embedded in concrete. The drawing below shows a detail for a construction joint with waterstop. Note that the expanded metal has the same depth of cover as the top rebars.

Both design and construction people in the United States have told us of successful use of expanded metal for form bulkheads in nuclear power plants and water retaining structures. Some of this work has been in place for more than 15 years with no problems reported.

Expanded metal has been used in ferrocement boats for many years. Martin Iorns, a ferrocement consultant from Sacramento, California, told us that corrosion isn't a problem in boats. Even when hull abrasion scrapes off the mortar cover and exposes expanded metal to salt water, rusting doesn't develop enough pressure to cause further spalling of surface mortar. This is because there isn't enough steel mass to generate a significant volume of corrosion products.