Q: We will be using steel-fiber-reinforced concrete (SFRC) in a pavement we'll be building. The specified dosage is 50 pounds of fibers per cubic yard and the fibers will be added at the ready mixed concrete plant. How can we tell if the delivered concrete contains the specified amount of steel fibers?

A.: Visual inspection isn't enough. Even an experienced eye couldn't detect the difference between concrete with 35 pounds of steel fiber per cubic yard and concrete with 50 pounds per cubic yard. MB Engineering Ltd. in Canada developed a simple test, however, for measuring the concentration of steel fibers in concrete. The device, first built by Intex Construction Ltd., a contractor for an SFRC slab on grade, is a plywood box with three screens.

The screens have openings of decreasing size (1 1/4 inches, 3/4 inch, and 1/4 inch).When the SFRC arrives on site, a technician fills an air meter bowl exactly full, then dumps the concrete on the top screen. He wet sieves it and uses a magnet to remove the steel fibers from each of the three screens. The fibers are washed, dried, and weighed.

Several samples were tested on Intex's slab job; the table shows the results.Each test took about 15 minutes and proved to the consultants and owners that the specified concentration of steel fibers was present in the concrete. Domecrete Canada Ltd., a steel fiber manufacturer, now includes the use of a fiber screening device as part of its design proposals.

For more information about the test, contact Ivan Kawulka, Domecrete Canada Ltd., 493 Sioux Road, Sherwood Park, Alberta, T8A 4H2, Canada. Our thanks to Assir Melamed of Domecrete for bringing this test to our attention.

Testing device for measuring steel fiber content consists of three screens with successively smaller openings. A concrete sample of known volume is washed over the screens and a magnet is then used to remove steel fibers for weighing.

Test no. Fiber weight (grams) Concrete volume(cubic meters) Measured dosage (kg/cu. meter) Specified dosage (kg/cu. meter)
1 162.5 0.00707 23.0 20.0
2 146.8 0.00707 20.7 20.0
3 137.9 0.00707 19.5 20.0
Average 21.1