Short answer
Wire mesh and rebar do not prevent cracking. NRMCA says properly placed reinforcement keeps cracks from opening wide. Synthetic fibers, typically 1 to 2 pounds per cubic yard, reduce plastic shrinkage and settlement cracks and add toughness and impact resistance. Joints still decide where a slab cracks.
Key facts
- Wire mesh and reinforcement cannot prevent cracking; properly placed, they reduce crack width (NRMCA CIP 4)
- Cut alternate wires or stop the mesh across contraction joints (NRMCA CIP 6)
- Fibers at 1 to 2 lb per cubic yard need no mix changes (NRMCA CIP 24)
- Fibers reduce plastic shrinkage and settlement cracks and add impact resistance (NRMCA CIP 24)
Mesh and rebar
NRMCA’s cracking guide says wire mesh and reinforcement in slabs cannot prevent cracking, but when placed at the proper location they can reduce crack width. It recommends at least 2 inches of concrete cover over steel to keep salt and moisture away from it.
Its joints guide adds that mesh should be cut, alternate wires or preferably discontinued, across contraction joints, so the joint can do its job.
Synthetic fibers
NRMCA describes synthetic fibers as polypropylene, nylon or polyethylene added during mixing. At typical rates of 1 to 2 pounds per cubic yard the mix does not need to change. Benefits it lists: fewer plastic shrinkage and plastic settlement cracks, more toughness and impact resistance. Macro-synthetic fibers at higher doses can serve for crack control in hardened concrete in some uses.
What to ask for
Ask what reinforcement, if any, the estimate includes and where the joints go. Joints, subgrade and curing do more for cracking than any reinforcement.
From Tim’s jobs
