Why Concrete Wants to Crack

Concrete is practically a builder’s best friend, with its universally appreciated characteristics of strength, durability and versatility. Its affordability and thermal mass make it an eco-friendly material too, so it’s no wonder concrete is among the most-used building materials in the world.

But the darn stuff has a propensity to crack, and in many cases there’s simply nothing that can be done about it. Why is that? Here, we seek to answer that question, and provide some pointers on how to diminish the extent of cracking in your next project.

A concrete destiny. Given enough time, your concrete will look like the driveway in this photo. Not exactly a pretty sight. There are many forces that lead to cracking, some of which can begin to appear only hours into the drying process.

There’s an old saying that there are two kinds of concrete: concrete that will crack and concrete that is cracked.


But why does it crack? How is it possible that the concrete that looks so stable and gorgeous in this backyard will certainly crack to some extent as the years go by? Forces such as soil upheaval, expansion and freeze-and-thaw cycles all guarantee that your perfect concrete patio won’t stay perfect forever.

But more immediate forces are most commonly responsible for the early, hairline cracks that can blight concrete slabs almost as soon as they’re poured.


Shrinkage cracks. Hairline cracks are common and can’t be entirely avoided. The most common type is called a shrinkage crack, which materializes as the water evaporates from the concrete mix during hardening.

Moist concrete fills a defined space, but as it dries, water evaporates, and the resulting reduction in volume literally pulls the concrete apart.

A hairline crack typically is not merely a surface crack, but runs through the slab. That said, it’s not usually a sign of a major problem. Hairline cracks are inevitable, unfortunately, but good contractors carefully monitor the water and mix ratio (too much water will increase the likelihood of shrinkage cracks), use synthetic-fiber additives to minimize cracks, and provide crucial control joints to accommodate unavoidable cracks.


Concrete floors and countertops. Interior concrete slabs have a propensity to crack just as exterior slabs do, and this is often listed as a disadvantage of exposed interior concrete surfaces.

The good news is, interior floors are typically large and structurally designed, with more substantial reinforcement than an exterior patio. Also, for concrete surfaces poured over an existing slab, the interior climate is often manageable, allowing installers to create perfect installation conditions.

I’m sorry to say, even with these advantages, interior concrete still cracks.


Control joints. In the slab shown here, the contractor has attempted to patch extensive hairline cracks of recently poured concrete. Not pretty, is it?Properly spaced control joints, grooved 1 inch deep into this section, would have alleviated the extent of visible cracking.

Control joints are separations in the outer layer of the concrete (see the next photo for an example of well-spaced control joints), providing a weakened location across the visible plane where hairline shrinkage cracks occur but are not visible. From the surface, you see the intentionally placed line (or joint), but not the crack itself.

Concrete installations in hot weather often develop more shrinkage cracks than those poured under more moderate weather conditions. The reason is the quick drying caused by the heat, which requires more water mixed into the concrete during the pour. The poured concrete fills a defined space, but as evaporation and hardening occur, the water disappears. If there was too much water in a soupy mix, the remaining matter is forced to fill excess volume, pulling the concrete apart. In short, too much water results in more shrinkage, which results in more hairline cracks. Proper control joints (the lines you can see on the surface of concrete slabs) and the use of synthetic fibers and sufficient rebar all help control cracking.

It’s difficult to provide hard and fast rules on slab thickness or the amount of metal support needed. In some cases, as with structural slabs for home foundation systems and some driveways, all details for sub-grade rebar installation and slab thickness will be carefully determined by a structural engineer. Driveways that bear heavy loads require thicker slabs and more reinforcement than, say, a backyard dining patio, where a 4-inch slab with minimal rebar (24-inch spacing or wire mesh) may be sufficient.


Expansion cracks. The new slab shown here was made with extensive control joints (the lines dividing the surface of the slab into sections, typically cut 1 inch deep). They provide a natural place for the concrete to crack beneath the surface, keeping the top from looking unsightly.

The slab also uses an expansion joint between the concrete slab and bricks on the left, providing a space (the line separating the brick and concrete) where the materials can expand during hot weather. The expansion joint runs the entire depth of the slab, providing total separation between the two materials.

Hot summers cause slab expansion even in mature slabs, and without a proper expansion joint where the forces of two static objects (like bricks and a concrete slab) meet, the force of the expansion can cause substantial cracking over time. This expansion joint on the left is filled with a flexible, compressible material such as an expansive foam, fiber or asphalt mixture.


Heaving and subsidence. Tree roots, freeze-and-thaw cycles and other forces of differential soil movement are unavoidable causes of concrete cracks.

Substantial steel reinforcement and proper sub-grade preparation can alleviate major cracks early in a slab’s life. Making sure not to install a slab too near mature trees (20 feet is a good rule of thumb, but the fact is, some root types can extend as much as 40 feet from a tree) and not planting new trees closer than 20 feet from concrete can help reduce the effect of thrusting roots.

There’s a reason concrete always cracks eventually, and it’s not that subcontractors don’t know what they’re doing. Unfortunately the forces of time are insuppressible, and eventually they win.


Uncertainty. In some cases, there’s no precisely verifiable cause of a crack. The cracking seen here was likely caused by upheaval, but it might also be related to expansion and freeze-and-thaw cycles.

If you live in Florida, your cracked concrete probably wasn’t caused by freeze-thaw cycles. But Florida has unstable soil, causing subsidence issues, so there are differences in slab requirements resulting from different geographical climates. An experienced professional in your area will know the proper techniques to use to minimize cracks.


Benefits. Concrete isn’t going anywhere. Its durable, adaptive nature and attractive, chameleon-like qualities will keep trucks pouring. But the surface cracks will also keep coming. They are unavoidable. An experienced professional who understands the forces described above and takes the necessary steps to minimize cracking will go a long way toward ensuring you’re satisfied with your finished product.


More
How to Tear Down That Concrete Patio
5 Benefits to Concrete Floors for Everyday Living
See a Concrete House With a $0 Energy Bill

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