Fiber mesh refers to steel, glass, or synthetic (polypropylene) fibers added to a concrete mix which provides the concrete with increased resistance to curing stresses (plastic shrinkage & settlement cracking), impact resistance (shock absorber), and lowered permeability. As far as fiber mesh, it’s benefits really happen mostly in the first several hours after concrete placement, reducing the chances of shrinkage cracking occurring. As the bleed water makes its way to the top of the concrete, if drying out or evaporation of the surface happen too quickly, the surface may begin to generate very fine surface cracks. When added to the mix, plastic fiber mesh can provide the subtle boost of flexural strength needed in the upper portion of the slab to counteract the shrinkage cracking during the initial curing stage. In addition to polypropylene fibers, there are also nylon and polyolefin mesh. Polypropylene fibers come in 2 different types: fibrillated and monofilament.
Fibrillated fibers come deformed and shredded to create a net-like mesh that grips more tightly with cement paste than monofilament fibers. The main disadvantage with fibrillated fibers is their tendency to protrude from a slab. Monofilament fibers are less tenacious and are slightly less likely to stick up than fibrillated polypropylene. Another possibility is to use nylon fibers, which also lay down better than fibrillated polypropylene. The dosage rate of polypropylene fibers is commonly +/- 1.5 lbs. This is the dosage that was formulated to maximize efficiency in relation to the cost of wire mesh.
Many think the use of polypropylene fiber mesh is an equal substitute to wire mesh in a slab, but the two actually perform separate functions. Wire mesh is most commonly installed in the center of the slab section to provide additional flexural (bending) strength to the slab member if the subgrade becomes unstable and settles. Also, wire mesh helps keep the slab intact when typical uncontrolled cracking eventually occurs, often several years down the line. Fiber mesh really does not provide these two advantages.