Question: In our part of Florida, concrete slab floors are used for residential and commercial buildings, whether the walls are concrete block, wood frame or metal. Typically cracks develop at the locations shown (figure below)

How can these be prevented?

Answer: Wire mesh and a plastic vapor barrier are used, but no other bars are implanted. Will the new polypropylene fibers in the concrete be of help?

I am not a concrete contractor but a builder of residences and small commercial buildings. Other builders here would be happy to know how to prevent this cracking.

This kind of cracking in concrete commonly occurs at re-entrant corners, that is, corners that are directed inward rather than outward. We do not think using polypropylene fibers in the concrete would be likely to prevent the cracking because of the high stress concentrations at these corners. The preferred method of handling this problem in floors is to install joints that break up the slab into panels so that there are no re-entrant corners within panels. In the example you provided this could be done in a way such as that shown in Figure 2 (below), but there also could be other ways of dividing the slab.

If possible, however, it would help if the architect could redesign the slab layout on some modular system that would allow the floor to be divided into approximately square panels no more than 15 feet on a side and without re-entrant corners. The existing layout makes it impossible to do this without creating a number of panels whose ratios of length to width are more than the recommended maximum limit of 1.5. Figure 2 shows six such areas (in color) in the floor itself plus two in the porches. There is no way to avoid overly long panels with the existing slab outline.

Another way to solve the problem is to insert reinforcing bars in the positions shown in Figure 3. Bars should be Number 4 bars or larger and they should be supported on chairs so that they will be located at mid-height of the slab. This should replace the large cracks with numerous unobjectionable tiny cracks too small to notice. Typical slab showing probable crack locations. Install joints to eliminate re-entrant corners. However, this leaves the problem of many panels (crosshatched) with lengths more than 1.5 time their widths. Install reinforcing bars, Number 4 or larger, in positions shown.