Q: We're a wet-mix shotcrete repair contractor and the engineer on an upcoming job doesn't want to use steel fibers as a replacement for 4x4-W2.0xW2.0 (4x4-8x8) welded wire mesh. Can polypropylene fibers be used instead?

A: According to D. R. Morgan of AGRA Earth & Environmental Limited, you would have to add about 1% by volume (15.3 pounds per cubic yard) of collated, fibrillated polypropylene fibers to achieve the same residual load-carrying capacity as 4x4-W2.0xW2.0 welded wire mesh. The practical limit for most polypropylene fibers in shotcrete, however, is about 0.5% by volume (7.7 pounds per cubic yard). If you add more than this, the shotcrete becomes paste-starved and unworkable because too much cement paste is used to coat the fine fibers.Morgan has, however, recently used synthetic polyolefin fibers to achieve residual load-carrying capacity similar to 4x4-W2.0xW2.0 welded wire mesh. The polyolefin fibers for shotcrete applications are 21_2 times thicker than typical fibrillated polypropylene fibers. They can be added at a dosage rate of 15 to 20 pounds per cubic yard without adversely affecting the workability of the mix.