Concrete has a reputation for being tough. And it is. But it’s also one of those materials that almost always cracks at some point, and when it does, a lot of homeowners are caught off guard. You’re not dealing with poor-quality material. You’re dealing with the nature of concrete itself. Understanding what causes concrete to crack is the first step toward protecting your driveway, patio, pool deck, or walkway before minor issues turn into major ones.
The good news is that most concrete cracking is predictable, and a lot of it is preventable when you know what to look for and act early. Whether you’re dealing with hairline cracks that have been there for years or something that appeared suddenly after a heavy monsoon season, this breakdown will give you the real picture.
Shrinkage During The Curing Process
One of the most fundamental answers to what causes concrete to crack is shrinkage. Fresh concrete contains a significant amount of water. As it cures, that water evaporates, and the concrete shrinks slightly as it dries. If the mix proportions weren’t right, or if the curing process wasn’t properly managed, that shrinkage creates tension in the slab that results in cracking. This is why freshly poured concrete in the right conditions includes control joints, which are intentional weak points designed to guide where the cracking happens so it doesn’t run randomly through the surface.
Thermal Expansion And Contraction
Concrete expands when it gets hot and contracts when it cools. In Arizona, where summer temperatures regularly exceed 110 degrees and winter nights can drop significantly, that thermal cycling happens in extremes. Over time, the repeated stress of expansion and contraction weakens the surface and the joints between sections. This is a major factor in what causes concrete to crack on pool decks and driveways in the Valley, where the temperature differential throughout the year is significant.
UV exposure compounds this. Concrete that sits in direct sunlight for years without proper sealing loses surface strength faster, making it more susceptible to cracking from thermal movement.
Ground Movement And Settlement
The ground beneath a concrete slab is rarely perfectly stable. Desert soils expand when they get wet and contract as they dry out. Tree roots grow. Erosion occurs around edges. When the soil beneath a slab moves unevenly, one section of concrete is supported, and another isn’t, which creates tension and leads to cracking. This is called settlement cracking, and it’s one of the more serious types because it often signals an underlying issue that needs to be addressed before the surface is repaired.
If your concrete is cracking in patterns that follow a diagonal or that seem to mirror movement in specific sections of the slab, settlement is likely a contributing factor.
Overloading The Surface
Every concrete slab is designed for a certain load. Exceeding that load, whether from heavy equipment, vehicles that weren’t accounted for in the original design, or structural changes to the building above, creates stress that the concrete wasn’t built to handle. This is less common in residential applications but worth knowing. Driveways that suddenly need to accommodate heavier vehicles or equipment may show stress cracking sooner than expected.
Water Infiltration
Water is one of concrete’s most consistent adversaries. Concrete cracks when moisture infiltrates the surface, either through existing small cracks or directly through a porous unsealed surface. In Arizona’s monsoon season, water enters cracks, saturates the base, and then evaporates rapidly when the heat returns. That cycle of wetting and drying puts stress on the concrete from within. Over time, it widens existing cracks and creates new ones.
Proper sealing is the most effective preventative measure against water-driven cracking. A UV-stable sealer applied to a clean surface protects against moisture entry and significantly extends the useful life of any concrete installation.
Poor Mix Design Or Installation
Sometimes the answer to what causes concrete to crack comes back to how it was mixed and poured in the first place. Too much water added to the mix reduces the concrete’s strength. Inadequate curing time, pouring in extreme heat without proper precautions, and wrong aggregate ratios all create conditions where cracking becomes likely well before it should be. This is why the quality of the contractor and the installation process matters so much.
What To Do When You See Cracks
Small, stable cracks that haven’t changed in size are often manageable with proper sealing and monitoring. Cracks that are widening, that show vertical displacement between sections, or that appear suddenly after a significant weather event need professional assessment. Catching concrete cracking early is almost always less expensive than addressing it after it’s progressed.
At Toros Pool Decking, we handle concrete repair, resurfacing, and new installations throughout the Mesa area. If you’ve got concrete that’s showing its age or cracking in ways that concern you, we’ll assess it honestly and tell you what it actually needs. Now that you know what causes concrete to crack, take action before it gets worse. Contact us for a free concrete assessment and honest advice on the best path forward.