Only three things are certain in life: death, taxes and cracks in concrete. We can’t help you with the first two, but, fortunately, there are some steps you can take to postpone the inevitable cracks in concrete.
First, make sure to properly prepare the subgrade before pouring concrete. While it’s drying, concrete contracts and shrinks—which typically leads to its cracking. A smooth subgrade reduces the tension with the ground as the concrete dries, shrinks and hardens. Also, a subgrade of gravel or similar material allows for drainage, keeping those crack-causing, freeze-thaw cycles at bay.
Second, add steel reinforcement to your concrete. By its very nature, concrete has high compression strength, but low tensile strength. In order to offset its tensile-strength shortcomings, steel reinforcement should be added. Steel mesh or rebar (depending on the size of the slab) is the peanut butter to the concrete’s jelly—making a delicious, and strong, combination.
Third, and arguably the most effective way to reduce cracking in residential applications, is adding joints to concrete. Joints take the stress off of the slab, create a weakened area to help control where the concrete cracks and separates one cementitious surface from another—such as a home’s foundation from its patio. There are two types of concrete joints: control and expansion.
Since concrete is bound to crack, control joints are simply “planned” cracks. These cracks are added by tooling them into the still-wet concrete with a hand tool, or sawing them into the slab once it has cured enough to prevent chipping during the cutting process. These straight grooves create a place for the slab to crack during expansion and contraction. Sometimes these cracks occur on the underside of the concrete, completely out of sight.
Control joints should be cut into the concrete no less than two or three times (in feet) the thickness (in inches) of the slab. So, for a 4-inch slab (fairly standard for residential applications), control joints should be 8 to 12 feet apart. What about the depth? Well, they should be no less than one-quarter of the thickness of the slab. For that same 4-inch slab, control joints should be 1 inch deep.
Expansion joints, which are usually a pliable material such as cork or plastic, are installed in an adjacent surface before the concrete is poured. These joints create a type of “shock absorber” next to the poured concrete, to allow for the concrete’s natural expansions and contractions as it dries. Expansion joints should be inserted wherever a new slab abuts a building or an existing slab.
The material used for expansion joints should extend the depth of the slab and stretch its full width. Also, it’s a good idea to seal any exterior expansion joints with high-quality, flexible urethane caulk to keep water out.
Not much in life is guaranteed—except for cracks in concrete. Taking the steps to properly prepare the subgrade, use steel reinforcement material and strategically place control and expansion joints will go a long way in delaying the inevitable.
What else is guaranteed? Intermountain Concrete Specialties is here to answer your concrete questions and help with any of your DIY project needs. We rent specialty equipment and tools. We have years of experience, expertise and know-how. And with seven locations from St. George to Idaho Falls—all with friendly and knowledgeable staff members—help is never far away.