Concrete
Why does new concrete crack?
Concrete shrinks as it cures and it has no give, so it cracks somewhere, and the job of the saw cuts in a new slab is to decide where.
The numbers
- Concrete shrinks as it dries. Shrinkage that cannot move becomes a crack.
- Control joints are cut in early so the slab cracks along a straight line underneath them.
- A hairline crack in a slab that has not moved is cosmetic.
- A crack with one side higher than the other, or one that keeps widening, is ground movement.
What the saw cuts are for
Those straight lines cut into a new driveway or sidewalk are control joints. They make the slab thinner along that line, so when the concrete shrinks it breaks there rather than wandering across the middle.
The crack still happens. It happens under the joint where nobody sees it, which is the whole trick. Joints cut too late, too shallow or too far apart is the usual reason a new slab cracks somewhere ugly.
Which cracks matter
A tight hairline running from joint to joint, with both sides level and no movement year to year, is shrinkage and it is cosmetic. Keeping water out of it with a joint sealant is all it wants.
A crack where one side has lifted or dropped is different. That is the ground under the slab moving, from frost, from a washout, or from fill that was never compacted, and the slab is following it. Those get looked at, because the concrete is reporting a problem that is underneath it.
What keeps it from spreading
Water, again. An open crack or an open joint lets water into the base under the slab, and in northwestern Wisconsin that water freezes and lifts. Sealed joints and a slab that drains away from the building are most of the maintenance a concrete surface ever asks for.