Concrete cracks: what each type means and how to fix it

Why concrete cracks at all

Concrete shrinks as it dries and expands and contracts with temperature. It is strong when squeezed but weak when pulled. When a slab tries to shrink and something holds it back (the ground, a wall, a step), it cracks. NRMCA puts it directly: all concrete tends to crack, and completely crack-free concrete is not possible^[1]. Joints are planned cracks, cut or tooled so the cracking happens in a straight line^[4].

That is why NAHB guidance says shrinkage cracks in slabs are expected, very common, and do not compromise structural integrity^[7]. The skill is telling those apart from the few cracks that point to a real problem.

The crack types, one by one

Crazing: a fine spider web

A network of very fine cracks across the surface, forming small irregular shapes. NRMCA says craze cracks are rarely more than 1/8 inch deep, the shapes between them are usually no more than 1 1/2 inches across, and they do not affect structural integrity^[3]. They show up best when the surface is drying after rain. Cause: a paste-rich top layer from extra water, overworking, finishing with bleed water on the surface, or late curing^[3].

Fix: none needed. If the look bothers you, a resurfacing overlay can cover it.

Plastic shrinkage cracks: short parallel lines

These form while the concrete is still fresh, when the surface dries faster than water can rise to replace it. NRMCA describes them as usually parallel, 1 to 3 feet apart, shallow, and not reaching the slab edge. They rarely impair strength, but they can let in water and chemicals and become starting points for later cracks^[2]. Hot, dry, windy days raise the risk; NRMCA flags wind over 5 mph, low humidity and high temperatures^[2].

Fix: seal them in freezing climates to keep water out. Otherwise, cosmetic.

Drying shrinkage and random cracks

Cracks that wander across a panel, often from a corner or midway between joints. Most come from joints that were too far apart, too shallow or cut too late. NRMCA's rules^[4]:

Joint rule	NRMCA guidance
Spacing	24 to 36 times the slab thickness; about 10 ft for a 4-inch slab; max 15 ft
Panel shape	Square or nearly; length no more than 1.5 times width; avoid L shapes
Depth	At least 1/4 of slab thickness, never less than 1 inch
Timing (saw cut)	Within 4 to 12 hours after finishing

If your 4-inch driveway has joints 20 feet apart, a crack in the middle of each panel is what you would expect.

Fix: rout and seal. ACI describes routing and sealing as the common, simple method for non-structural cracks on floors and pavements, and recommends a flexible sealant over cement grout^[5].

Restraint cracks at walls, steps and corners

Cracks that start where a slab touches something fixed: the house, a column, a step, a drain. NRMCA calls for isolation joints between slabs and walls, footings, columns, steps and other fixed points so each part can move on its own^[4]. Without them, the slab cracks at the point of restraint^[1].

Fix: seal the crack and, where possible, cut or add a proper joint at the fixed point.

Settlement cracks: one side lower

A crack where one side has dropped below the other. NRMCA lists settlement cracks among common forms^[1]. In flatwork, the cause is lost support underneath: poor fill, washout or shrinking clay. See why concrete sinks.

Fix: stop the water, then lift the low side if the slab is sound. Sealing alone will not hold. ACI notes that new cracks will probably form near a repaired crack unless the cause is corrected^[5].

D-cracks: cracks that start at joints in cold climates

In freezing climates, cracks that show up near joints and edges of exterior slabs years after the pour. NRMCA explains that D-cracks come from freeze-thaw damage to certain porous aggregates. They start at joints near the bottom of exterior slabs and usually appear years after placement^[1]. Unlike most early cracks, they do shorten the life of the concrete^[1].

Fix: sealing slows water entry. Badly D-cracked slabs are usually replaced.

Cracks over rusting steel

Cracks that follow a straight line over reinforcing steel, often with rust stains, then spalling. Rust takes up two to four times the volume of the steel, which cracks and breaks off the concrete cover^[6]. Deicing salt is a common source of the chlorides that start corrosion^[6].

Fix: a qualified repair contractor or engineer. Patching over active corrosion fails.

How to tell if a crack is moving

You cannot judge a crack from one look. Track it:

1. Measure the width with a crack-width card or feeler gauges. FPA notes that separations in concrete become noticeable at about 0.04 inch (1 mm)^[8].
2. Check for offset. Lay a straightedge across the crack. Any step means vertical movement. NAHB guidance says cracks with significant vertical displacement may indicate a structural concern^[7].
3. Mark the ends with a pencil line and the date.
4. Photograph with a ruler in the frame, from the same spot, every month or two through a full year of seasons.

Our crack assessment checklist lays this out in a printable form.

Myths to ignore

• "Any crack over 1/8 inch is structural." No standard says that. For house slabs, see the crack width guide.
• "Mesh or rebar prevents cracks." NRMCA states that wire mesh will not prevent cracking; it tends to keep cracks from opening up^[4].
• "Fill it with mortar." ACI says to avoid cement grouts for routing and sealing because they are likely to crack^[5].

Quick reference: crack > cause > fix

Crack	Likely cause	Fix
Fine web	Crazing[3]	None, or resurface
Short parallel lines	Plastic shrinkage[2]	Seal in cold climates
Mid-panel random	Joints too far apart or cut late[4]	Rout and seal[5]
At wall, step or corner	Missing isolation joint[4]	Seal; add joint
One side lower	Lost soil support[1]	Fix water, lift, seal
Starting at joints, cold climate, years later	D-cracking[1]	Seal; replace if severe
Straight line with rust	Steel corrosion[6]	Qualified repair

When to call a pro

• One side of the crack is higher than the other and it is getting worse.
• A crack keeps widening over several months of tracking.
• Cracks with rust stains or spalling over steel.
• A crack in concrete attached to the house appears at the same time as cracks or sticking doors inside: see signs of slab foundation problems.