When a homeowner discovers cracks in their floor slab just a few months after moving into their brand-new home, the builder gets an irate phone call. Usually, the cracks are small and the result of the concrete shrinkage as it sets up. It’s normal and not a structural problem. But complaints about cracks in concrete floor slabs—especially at the garage, where the slab is most visible—have bedeviled homebuilders for as long as floors have been made of concrete.
Because concrete has tremendous strength in compression, but minimal tensile (bending) strength, the standard way to add tensile strength has been to use a heavy steel wire mesh with a 6” grid, placed slightly above the center of the slab depth when it is poured. The wire mesh provides additional stiffness but is not always that effective controlling cracking.
The addition of small fibers of glass, steel, or a synthetic material such as polypropylene, to concrete, became popular in the 1960s as a way to make concrete more crack-resistant; although asbestos was used previously (before it became recognized as a health hazard), and even ancient builders added horse hair or straw to mortar for reinforcement. Research continues today in using other materials, such as cellulose and even recycled carpet fibers.
When you see bagged pre-mix concrete at your local home improvement warehouse store marked “crack resistant,” that means it has a fiber additive. Also, some homebuilders have used a higher ratio of the fiber admixture as an alternative to steel mesh reinforcement for their concrete floor slabs.
Van Hibberts, CMI
Certified Residential Building Code Inspector ICC-5319905
IBHS Fortified Certified Inspector #FEV32561020109
ARA Certified Inspector #20302 (Applied Research Associates)
Florida-State Certified Master Inspector Lic. #HI 89
Certified Owens-Corning Roof Data Inspector
Florida-Certified Wind Mitigation Inspector
WDO Certificate #JE190791
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