Q.: I want to build a concrete floor to be covered with ceramic tile. I'd like to be sure that no shrinkage cracks appear in this floor later and crack the tile. Would polypropylene fibers do the job? What difference would the fibers make in the batching, mixing and finishing operations?

A.: Polypropylene fibers have been used as crack arresters and using them is one of several ways you might consider. (Others are using steel fibers, using welded wire fabric, or using a mix made of shrinkage-compensating cement.) The fibers are said to be easy to mix; usually they can be obtained in premeasured bags to be added at the rate of one bag per cubic yard of concrete. The manufacturer can recommend the length of fibers to choose for this application. The fibers don't affect the workability much but they tend to slow down the rate of bleeding, so this may delay the time when the slab can be troweled.