Almost all concrete cracks, and most cracks are harmless. The ones to worry about are cracks that are wide or getting wider, that show displacement from one side to the other, that follow reinforcing steel with rust staining, or that come with other signs of movement such as sticking doors or bowing walls. The pattern, size and change over time tell you far more than the crack's existence.
Why concrete cracks
Concrete is strong in compression and weak in tension. As it cures it shrinks, and when that shrinkage is restrained by the ground, the forms or the reinforcing, tension builds and the concrete cracks. Control joints are cut or formed to make these cracks happen in planned, straight lines. Cracks also come from temperature changes, settlement, loads, corrosion of reinforcing steel and freeze-thaw damage.
Normal cracks and warning cracks
Cracks that are usually not serious
- Hairline shrinkage cracks: fine, often random or map-like on slabs, or vertical in foundation walls.
- Cracks at control joints: that is what the joints are for.
- Plastic shrinkage cracks: short, shallow cracks that formed while the concrete was fresh, often in hot or windy conditions.
- Crazing: a fine network of surface cracks, mostly cosmetic.
Even harmless cracks can let water through. A dry crack in a basement wall can be left and watched; a wet one needs sealing and a look at drainage.
Cracks that need attention
| What you see | What it can mean |
|---|---|
| Horizontal crack in a basement wall, often with inward bowing | Soil or frost pressure on the wall |
| Stair-step crack in block or masonry | Differential settlement or lateral pressure |
| Crack wider at one end than the other | Rotation or settlement of part of the structure |
| One side of the crack higher than the other | Vertical movement or heave |
| Cracks along the line of rebar, with rust stains | Corrosion of reinforcing steel |
| Diagonal cracks near supports in beams or slabs | Possible shear or overload; needs an engineer |
| Cracks that keep growing | Active movement |
Call an engineer promptly for structural cracks. Diagonal cracks in beams or columns, cracks with visible displacement, bowing walls, or cracks with spalling around reinforcing steel can indicate structural problems. Don't load the area further, and have it assessed by a qualified engineer.
How to measure and monitor a crack
A crack's behaviour over time is often more telling than its size on one day.
- Photograph the crack with a ruler or tape in the frame
- Measure the width at several marked points with a crack-width card
- Mark the ends of the crack with a pencil line and the date
- Check whether one side is offset from the other
- For important cracks, install a simple crack monitor (a tell-tale) across it
- Re-check at regular intervals and across seasons
- Note any water, staining or efflorescence
Cracks often open and close slightly with temperature and moisture. A crack that steadily widens, lengthens or offsets is the one to act on.
Cracks in specific places
Basement and foundation walls
Vertical shrinkage cracks are common in poured walls. Horizontal cracks, bowing and stair-step cracks in block walls are more serious. Many basement leaks come through cracks combined with poor grading and drainage. See foundation problems: the warning signs.
Slabs on grade
Garage and basement slabs often crack from shrinkage and settlement of the soil below. Wide cracks with height differences can be a trip hazard and may indicate soil problems.
Parking structures and suspended slabs
Here cracks matter more, because they let water and de-icing salts reach the reinforcing steel. Corrosion then causes more cracking and spalling. Regular inspection and sealing are part of looking after a parkade. See parking garage inspection basics.
Repair approaches
The right repair depends on the cause:
- Dormant, dry cracks: often left alone or sealed for appearance.
- Leaking cracks: injected with a suitable material, combined with fixing drainage outside.
- Structural cracks: repaired to an engineer's design, which may include injection, reinforcement or other strengthening.
- Corrosion cracking: needs removal of damaged concrete, treatment or replacement of steel and patching, plus protection against further water and salt.
Sealing an active crack without dealing with the cause usually just moves the crack.
How ehZee can help
We map and measure cracks, look for the related signs that point to the cause, and tell you whether a crack is cosmetic, needs monitoring or needs a structural engineer. See our concrete inspections and X-ray, or call 1-833-888-0091.
Common questions
Is there a crack width that is always a problem?
Guidance varies with the type of structure and exposure, so there is no single number that applies everywhere. Width matters, but so do location, pattern, offset and whether the crack is growing.
Should a new slab have cracks?
Some shrinkage cracking in a new slab is normal, especially between control joints. Wide cracks, cracks with offsets, or extensive cracking soon after placement are worth having looked at.
Can I fill cracks myself?
Sealing small, dry, non-structural cracks in a slab is often a reasonable DIY job. Leaking cracks, cracks in structural members and anything showing movement should be assessed first.