Q.: The contractor on one of our jobs is placing concrete underwater through 10- to 12-inch-diameter tremie pipes 90 to 120 feet long. He has proposed using aluminum instead of steel pipes. I know that alkalies in concrete can attack aluminum. Can the concrete passing through an aluminum pipe also be affected?
A.: If concrete abrades the aluminum as it falls through the tremie pipe, you could have a problem. Aluminum shavings react with calcium hydroxide in the concrete, producing hydrogen gas. The voids created by this gas generation can significantly reduce concrete strength. This was dramatically demonstrated on jobs in which concrete pumped through aluminum pipeline experienced strength losses of 20% to 50% (Refs. 1, 2) Because of this, aluminum pipeline shouldn't be used for concrete pumping (Ref. 3).
Concrete falling through a tremie pipe isn't as likely to abrade the surface because the concrete isn't under pressure and the pipe diameter is larger than the typical pump pipeline. However, concrete velocities of 4 to 6 feet per second might cause abrasion (Ref. 4)., and concrete falling 100 feet could easily reach these velocities. Aggregate hardness, particle shape and texture can also affect the degree to which aluminum tremie pipe is likely to be abraded. Rough-textured, angular particles are more abrasive than smooth, rounded ones.
To determine if abrasion could be severe enough to cause a concrete strength-loss problem on your job, make a test placement of a nonstructural caisson seal, then core the concrete to determine in-place strength. For more useful results, compare concretes placed through steel and aluminum tremie pipes. Considering the cost of testing, however, it might be less expensive to simply use steel tremie pipe for the job.
1. Howard Newlon Jr. and Michael A. Ozol, "Delayed Expansion of Concrete Delivered by Pumping Through Aluminum Pipeline," Concrete Case Study No. 20, Virginia Highway Research Council, Charlottesville, Va. 1969.
2. E.L. Fowler and E.F. Holmgren, "Expansion of Concrete Pumped Through Aluminum Pipeline," ACI Journal, American Concrete Institute, Farmington Hills, Mich., December 1971, pp. 950-953.
3. ACI 304.2R-91, Placing Concrete by Pumping Methods, American Concrete Institute, 1991.
4. Howard Newlon Jr. and Michael A. Ozol, discussion of "Expansion of Concrete Pumped Through Aluminum Pipeline," ACI Journal, June 1972, pp. 357-359.