Concrete is appreciated by both builders and homeowners for its strength, versatility, and affordability. But if you’ve ever had a concrete driveway or patio, you know the frustration of discovering cracks where the surface was once smooth and pristine.
Despite its many benefits, given enough time, all concrete will develop cracks. But there are methods for reducing the extent of cracking.
So why does concrete crack and what can you do about it?
Too Much Water in the Concrete Mix
While water is necessary to create a pourable mix, sometimes too much water is added. This becomes problematic because the more water added, the more shrinkage occurs as the water evaporates. Shrinkage causes the concrete to pull apart, creating fissures.
To reduce the amount of cracking, make sure you know the advised water to cement ratio for the grade being poured. A knowledgeable contractor will recognize that while it may take more effort to pour a stiffer mix, a lower water to cement ratio results in less cracking and greater durability. If you’d like to calculate the proper water to cement ratio, the Concrete Network provides helpful information.
While you don’t want a mix that is too wet, you also don’t want to allow the concrete to dry too quickly. The shrinkage caused by rapid drying results in greater cracking, so it’s important that the cement is cured. Ask your contractor how he/she intends to cure the cement, as there are a few methods. The most common is to flood or mist spray the concrete. When this is done for seven days, the resulting slab will be about 50% stronger than an uncured slab.
Lack of Control Joints
Control joints are planned cracks that permit concrete to expand and shrink as temperatures change. According to builder Tim Carter, properly placed control joints allow you to influence where and how your concrete cracks rather than leaving it to chance. Ask your contractor about the joints he/she intends to cut to ensure they are deep enough, spaced properly, and cut at the right time (typically within 6 to 12 hours of pouring).
Insufficient Subbase and Subgrade Support
Many homeowners assume that concrete is strong enough to support itself but slabs need foundations much like houses do. According to Matt Clawson, contributor at Houzz.com, driveways that bear heavy loads will require thicker slabs and more reinforcement than backyard patios. There are a few levels of support that can be used beneath concrete slabs. The subgrade is compacted soil; the subbase is a layer of gravel that sits on top of the subgrade; and the base course is the layer of material directly underneath the slab or vapor barrier.
According to the American Concrete Institute, slabs should, at minimum, rest on a uniform and well-compacted subgrade. However, soil quality must be considered. If the soil is too wet, doesn’t drain well, or is not easily compacted, then additional support may be needed. A subbase and base course can provide a more even foundation as well as reduce the amount of groundwater that seeps up into the slab. Also, depending on how much weight will likely sit on top of the slab, steel rebar or fiber-reinforced concrete may be recommended.
While you cannot prevent all cracks, having a better understanding of the mechanics of concrete will allow you to ask your contractor the right questions. Experienced professionals should be able to answer your questions and articulate a plan to reduce cracking.
Stay tuned for our next post about laying concrete in zones vulnerable to flooding and high winds!
Blog by Jessica T. Smith for the Coastal Cottage Company