To answer this question, it is first necessary to establish why wire mesh or fibers would be used in a concrete slab. One of the main reasons is for crack control. PCA, however, states that cracks in slabs on ground can be controlled with uniform subgrade support and proper joint spacing (as a function of slab thickness). See Table 6-3 from PCA’s Concrete Floors on Ground, EB075.

Reinforcement for Floors on Ground

Is reinforcement necessary?

 NO       With uniform support and short joint spacing.

YES     When long joint spacing is required or when joints are unacceptable in floor use.

Although short joint spacings alleviate the need for reinforcement, wire mesh will allow for increased distance between joints if correctly placed in the upper portion of the slab, at least two inches below the surface. In these slabs with longer joint spacings, the purpose of wire mesh is to hold random intermediate cracks tight.

Plastic fibers should not be expected to replace wire mesh in a slab on ground. However, although not affecting joint spacing, plastic fibers are used to reduce plastic shrinkage cracking.

Plastic shrinkage cracks are those that occur immediately following concrete placement, before the concrete has hardened. Plastic fibers are commonly dosed at 0.1 percent by volume for slabs on grade; this is equivalent to 1.5 pounds per cubic yard.  

Although polypropylene fibers are among the most common for controlling plastic shrinkage cracking, there are many types of fibers in addition to polypropylene, including other plastic materials, such as nylon.

Fibers serve different purposes based on the characteristics of the material from which they are made. For instance, steel fibers provide high flexural strengths and impact resistance and are found in heavy-duty industrial floors.