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.

    Another alterntive for reducing concrete slab cracks, used only for larger slabs, is described in our article Why is there a "WARNING! POST-TENSION SLAB" sticker in my house?

• • • • • • • • • • • • • • • • • • • • • • • • • • • 

Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• What are the building code requirements for notching and boring holes in a wall stud? 

• What causes dark or light "ghost" lines on ceilings and walls?

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code? 

• Why is my garage ceiling sagging? 

• How can I identify what kind of wood flooring I am looking at?

• Why does my concrete floor slab sweat and get slippery?

• What is the minimum ceiling height for rooms in a house? 

• Why are there score line grooves in the concrete floor of the garage?

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

   Visit our STRUCTURE AND ROOMS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.