Reading the cracks in a concrete driveway or patio

Concrete cracks. That is not a defect in itself, it is a property of the material: it shrinks as it cures and it moves with temperature, and unless it is allowed to do that somewhere controlled, it will choose its own place. The useful question is never whether a slab has cracked, but what kind of crack it is.

Fine surface crazing

A network of very fine cracks across the surface, looking a little like a dried out riverbed, is called crazing. It sits only in the top skin of the concrete, it does not affect strength, and in most cases it is cosmetic. It usually comes from the surface being worked while there was still bleed water on top, or from the surface drying much faster than the body of the slab.

It is worth knowing what it is mainly so you do not panic about it, and so nobody sells you a replacement slab on the strength of it.

Straight cracks between corners

A single long crack running in a fairly straight line, often between two inside corners or across a narrow section, is a shrinkage crack that the slab placed itself. Concrete has to relieve shrinkage stress somewhere. Control joints exist to give it a weakened line to crack along, out of sight and in a straight tooled groove.

When those joints are cut too late, too shallow, or at spacings that do not suit the shape of the slab, the concrete relieves itself wherever it likes instead. That is the crack you are looking at. Structurally it is often not serious, but it is an entry point for water, and water is what turns a cosmetic problem into a real one over a few winters.

Flaking and pitting at the surface

Where the top few millimetres flake away and leave the aggregate exposed, that is spalling. In this climate it is very often a freeze and thaw problem, made considerably worse by de-icing salts. Water gets into the surface, freezes, expands, and pushes the skin off. Salt increases the number of freeze cycles the surface goes through and draws more water in.

Newly placed concrete is most vulnerable to this in its first winter. If you have a new driveway or path, that is the season to use sand for grip rather than salt. It is far less convenient and it will save the surface.

One corner sitting lower than the rest

This is the one that actually matters. A slab that has dropped at an edge or a corner has lost support beneath it, either because the fill under it was never properly compacted or because water has been running underneath and washing the fines out. The concrete is not failing. The ground is.

The reason this one is serious is that it will continue. Grinding the resulting trip hazard off the top removes the symptom for a while and does nothing about the cause. Where the ground beneath has genuinely gone, the honest options are to address what is under the slab or to replace the affected area, and any assessment that skips straight to a surface treatment is not really an assessment.

A quick way to sort what you are seeing

• Fine web across the surface only, no lip you can feel: crazing, usually cosmetic
• Single straight crack, edges level with each other: shrinkage, worth sealing to keep water out
• Crack with one side higher than the other: movement underneath, needs looking at
• Surface flaking with aggregate showing through: freeze and thaw, often salt related
• Whole area visibly lower than it was, especially at a corner: loss of support below

The lip test is the most useful single check. Run a foot or a hand across the crack. If the two sides are still level with each other, the slab has cracked but has not moved. If one side is proud of the other, something underneath has shifted, and that is a different conversation.

What is worth doing, and when

Sealing cracks while they are still hairline is the cheapest and most valuable maintenance there is, because it keeps water out of the base. Keeping drainage clear matters for the same reason. Beyond that, most cosmetic cracking is best left alone rather than repaired in a way that draws attention to it.

If you have a slab with a lip, a corner that has dropped, or cracking that keeps opening year on year, it is worth having someone look at it properly. Call (201) 945-0064 with the address and we will tell you honestly whether it needs replacing, an overlay, or nothing yet.