Why does concrete crack, and is it covered?

Fresh concrete holds more water than the cement needs to cure. As the extra water leaves, the slab shrinks. The usual figure is about 1/16 of an inch for every 10 feet of slab, which sounds like nothing until you remember a 40-foot driveway is trying to get a quarter inch shorter while it is glued to the ground. Friction with the subgrade holds it back, the concrete pulls against itself, and somewhere it lets go. That is a shrinkage crack.

Shrinkage cracks are thin, usually narrower than 1/16 of an inch, and both sides of the crack sit at the same height. They show up in the first weeks after the pour, sometimes within days in Texas summer heat when the surface dries faster than the middle. They do not get wider over time, they do not hurt the strength of the slab, and they are not a sign the concrete was mixed wrong. Industry guidance, including the American Concrete Institute's, starts from the same premise: concrete cracks, and the job of the contractor is to control where.

Curing matters here. A slab that is kept damp or sprayed with curing compound and left alone for the first days shrinks slower and cracks less. A slab that bakes dry in August afternoon sun with no cure shrinks fast and cracks more. That is one of the reasons we talk about curing time with every customer before we pour.

Since the concrete is going to crack somewhere, we give it a place. A control joint is a straight groove, tooled in while the concrete is wet or saw-cut the next morning, about a quarter of the slab thickness deep. That groove is the weakest line in the slab, so when the concrete shrinks, it cracks underneath the groove where nobody sees it instead of wandering across the middle of the driveway.

The rule of thumb from ACI and the ready-mix industry is to space joints at 2 to 3 times the slab thickness in inches, read as feet. A 4-inch slab gets joints every 8 to 12 feet. We cut at the tighter end, 8 to 10 feet, because DFW heat and low humidity push shrinkage toward the high side of normal. Panels should be close to square. A long skinny panel, say 4 feet by 12, cracks across its middle no matter how good the mix is.

Joints also have to line up with the corners. Anywhere the slab turns a corner, like where a walkway meets a driveway or where a patio wraps a house, the concrete concentrates stress at the inside corner and cracks from that point. A joint run to every re-entrant corner catches it. When you see a diagonal crack shooting out from a corner on an older slab, that is what was missing.

Most of Arlington and the Metroplex sits on expansive clay. It swells when it is wet and shrinks when it dries, and it does both every year. The slab does not move on its own; the ground under it does, and the concrete goes along for the ride. A patio edge that lifts in a wet spring and drops in a dry August is the clay breathing under it, not the concrete failing.

That is why subgrade is most of the job. We strip the topsoil and organics, bring the grade up with compacted base where the clay is bad, and compact it in lifts so the slab rests on something that does not settle. We keep water moving away from the slab with slope, because the worst movement comes from one side of the slab staying wet while the other dries. A downspout dumping at the corner of a driveway is the most common cause of a cracked and sunken corner we see.

Steel helps too. Rebar does not stop concrete from cracking, but it holds the two sides of a crack tight together so the crack stays hairline and the panels stay level with each other. On driveways and anything that carries a vehicle we use rebar on chairs, in the middle of the slab where it does the work. Wire mesh lying on the dirt at the bottom of the pour does nothing. Thickness and steel are covered in more detail on our page about driveway thickness.

Width is the first check. A crack you cannot fit a credit card into, under about 1/16 of an inch, is cosmetic. A crack wide enough to fit a coin edge, 1/8 inch or more, is moving, and the question becomes why. Put a pencil mark across it with the date, and look again in a few months. A shrinkage crack stops. A settlement crack keeps opening.

Height is the second check. Run your hand across the crack. If one side is higher than the other, the ground under one panel has moved. That is a subgrade or drainage problem, not a concrete problem, and it will keep moving until the water that caused it is dealt with. Raised edges are also a trip hazard, which matters on a walkway or a pool deck.

Pattern is the third. A single straight or gently curved crack between two joints is shrinkage. A crack that follows a gutter downspout, a tree root, or the edge of a planting bed is water or roots. A web of fine cracks across the whole surface, called crazing, comes from the top drying too fast and is purely cosmetic. Spalling, where the top layer flakes off in patches, is a finishing or freeze problem and is a different conversation, usually about concrete repair.

Few concrete contractors in North Texas warrant against cracking. The shrinkage is physics and the clay is geology, and neither one reads the contract. What a contractor can stand behind is workmanship: the slab is the thickness you paid for, the steel is in it and in the right place, the base was prepared and compacted, the joints were cut on time and at the right spacing, the surface drains the way it was supposed to, and the finish is what was agreed. If any of those are wrong and the slab cracks because of it, that is on the contractor.

What is usually not covered: hairline shrinkage cracks, crazing, color variation in the mix, cracks that come from movement of the ground under the slab, cracks caused by something heavier than the slab was built for, and damage from water that was directed at the slab after we left, like a new downspout or a sprinkler head soaking one corner every morning. A patio built for furniture that gets a parked truck on it has been overloaded, and the crack that follows is not a defect.

Before you sign with anyone, ask what is covered in writing, for how long, and what the fix is: sealed, patched, a panel replaced, or the whole slab. Get it in the contract, not in a conversation. Ask how they cut joints, what steel they use and where it sits, and what they do about the subgrade. The answers tell you more about how the slab will age than the warranty term does. We pour pads and slabs the same way our sister shop, CJM Metals, builds the steel that goes on top of them: to a standard, so we are not back fixing it. Call (214) 603-9142 and we will walk the site with you.