Cracks in concrete make people nervous, but most of them are cosmetic. Concrete shrinks as it cures, and virtually every slab develops some cracking in the first few years. The skill is reading a crack correctly: understanding whether it's dormant or active, cosmetic or structural, and what the right response is. This guide covers the main crack types you'll find in Kamloops residential and commercial slabs, what they mean, and what actually fixes them.
Why concrete cracks in the first place
Fresh concrete contains a significant amount of water. As that water evaporates and the concrete cures, the slab shrinks — by roughly 1/16th of an inch per 10 feet of length. That shrinkage creates tensile stress, and concrete is weak in tension. Control joints are saw-cut into slabs to give cracks a place to happen in a controlled, straight line that can be managed. But concrete cracks even with good joint placement, especially if the subgrade settles unevenly, the slab is restrained at its edges, or temperature changes cause thermal movement.
The main crack types and what they mean
Hairline cracks
Hairline cracks are very narrow — often less than 0.5mm — and are typically the result of normal shrinkage during curing. They run fairly randomly across the slab. If a crack has been there for years without widening or displacing, it's essentially dormant. These can be filled and finished over without concern, and in a polished or coated floor they become part of the natural character of the slab.
Shrinkage cracks at control joints
If you see cracking at or near a saw-cut control joint, the joint did its job — the crack happened where it was supposed to. The joint may have faulted (meaning one side is slightly higher than the other) if there's a gap under the slab, but if the displacement is less than a few millimetres and it's not growing, it's typically a cosmetic issue. We fill the joint and feather it smooth before polishing.
Diagonal corner cracks
Diagonal cracks from the corners of slabs are common in garages and indicate either thermal movement or sub-slab settlement at the corners. Corner areas of slabs are the first to settle if the fill under them wasn't compacted properly. These are worth monitoring — if the crack is growing or the corner is dropping relative to the rest of the slab, the sub-base needs attention. If it's been stable for years, fill and finish.
Cracks with vertical displacement (faulted cracks)
A faulted crack is one where one side of the crack is higher than the other. This is a different situation from a hairline crack — it means the slab has moved differentially, with one section dropping or rising relative to the adjacent one. Small faults of a couple of millimetres that have been stable for a long time can often be ground down and filled. Larger faults, or faults that are still changing, indicate ongoing movement that may need to be addressed at the foundation or sub-base level before cosmetic repair makes sense.
Pattern cracking (map cracking)
Map cracking looks like the cracks on dried mud — a network of interconnected cracks covering the surface. This is usually alkali-silica reaction (ASR) in the concrete itself, a chemical reaction between the cement and certain aggregates that causes the concrete to expand internally and crack. ASR is rare but serious — it indicates the concrete itself is deteriorating, and in advanced cases may affect the structural integrity of the slab. This warrants a professional assessment before any surface treatment.
Active vs dormant: how to tell
The key question with any crack is whether it's still moving. Here's a simple way to monitor it at home: mark both sides of the crack with a pencil line or a piece of tape across the crack, then photograph it and measure the crack width. Check it again in three months. If the crack has widened, if the tape or pencil lines have shifted relative to each other, or if new sections have appeared, the crack is active. If nothing has changed, it's dormant.
The repair process for different crack types
- Hairline dormant cracks: chase out with a grinder or crack chaser blade to widen slightly (makes filling easier and more durable), clean with compressed air, and fill with a semi-rigid polyurea or polyurethane crack filler. Allow to cure, then grind flush.
- Wider dormant cracks: same process but may need a sand-filled epoxy mortar for larger voids. The semi-rigid qualifier still applies — a rigid fill in a concrete slab will re-crack eventually.
- Faulted cracks with small displacement: grind down the high side, fill the crack, allow to cure, sand flush.
- Active cracks: identify and address the movement source first. This may mean compacting the sub-base via mudjacking or foam injection, addressing drainage that's undermining the slab, or removing and replacing a badly settled section.
When to call a structural engineer vs when to call us
Call a structural engineer if you have: cracks with significant vertical displacement (more than 10mm), cracks that are actively growing week over week, multiple interconnected cracks covering a large area, signs of heaving or tilting, or cracks in a concrete wall that is also carrying load. These situations go beyond surface repair and need a proper assessment of what's happening under the slab.
Call us if you have: cosmetic cracking you want filled before polishing or coating, salt-damaged surfaces that need grinding and repair, spalling and pitting from freeze-thaw, or cracks that have been stable for a long time and you want assessed and treated. We look at slabs free of charge and tell you honestly what category the damage is in. We serve all of Kamloops including Valleyview, Batchelor Heights, and nearby areas.