Crack Width In Concrete And When It Matters

Every reinforced concrete structure cracks. It is designed to.

Concrete is weak in tension, so once a member bends, the tension face cracks and the reinforcement takes over. That is the mechanism working as intended.

Which means finding cracks in concrete is not a finding. Assessing them is.

What the width is actually about

Crack width limits exist mainly for durability, not strength.

A wide crack lets water, oxygen and chlorides reach the reinforcement. The steel corrodes, it expands, and it forces the concrete off. That is the process behind [concrete cancer](/blog/concrete-cancer-in-apartment-buildings), and it starts at a crack.

So the allowable width depends on the exposure. A slab inside a dry building tolerates more than a balcony soffit in a coastal suburb, which is the substance of [coastal exposure and concrete durability](/blog/coastal-exposure-and-concrete-durability).

Appearance and watertightness are the other two reasons a limit gets applied. A water retaining structure has tighter limits than anything else, because leaking is the failure.

Reading the pattern

The pattern says more than the width.

Fine cracks across the tension face of a beam or slab, roughly evenly spaced. Flexural cracking, expected, and evidence the reinforcement is doing its job.

Diagonal cracks near a support. Shear. These deserve attention, because shear failure is not gradual the way flexural failure is.

Cracks following the line of the reinforcement, with rust staining. Corrosion driven. The steel is expanding. This is the serious one and it does not improve.

Map cracking, fine and random over a surface. Usually shrinkage in the surface layer. Generally cosmetic, and it does admit water.

A single straight crack across a slab in one line. Frequently restraint: the element wanted to shrink and something held it. Related to the reasoning behind [expansion and control joints](/blog/expansion-and-control-joints-in-concrete).

Cracks radiating from a re-entrant corner or a penetration. Stress concentration, very common, usually addressed with trimmer bars that were not there.

Active or dormant

The question that decides everything.

Shrinkage cracking largely stops once the element has dried and stabilised. Cracking from ongoing movement, settlement or corrosion does not.

Monitoring over months is the only way to tell, and it is worth the wait. Repairing an active crack without addressing the cause produces a repaired crack that reopens next to the repair.

What to measure and record

Width, with a crack width gauge rather than an estimate. Length and location, mapped. Whether there is any displacement across the crack. Whether there is staining, damp or efflorescence. And whether the concrete sounds solid or drummy when tapped alongside it.

A cover meter reading tells you how much concrete is protecting the steel, which is frequently the underlying explanation. Low cover is the most common reason a structure corroded early, and it is a construction issue rather than a design one.

The repairs

Sealing, for dormant cracks where the aim is keeping water out.

Resin injection, where structural continuity needs restoring across a dormant crack.

Cut out and patch repair, where the steel has corroded. The concrete comes out past the affected area, the steel is cleaned and treated, and the section is rebuilt. Patching over corroded bar fails.

Cathodic protection, on larger structures where chloride contamination is widespread.

Strengthening, where the assessment finds the element genuinely lacks capacity.

Choosing between them requires knowing the cause, which is why assessment comes first.

When to get it looked at

Any crack with rust staining. Any crack in a structural element that is growing. Any diagonal cracking near a support. Any drummy concrete. Anything on a balcony or an exposed soffit, which is where the consequences are highest and covered in [balcony remediation](/blog/balcony-remediation-structural-assessment).

Fine cracking on an internal slab, stable for years, is generally exactly what the design anticipated.

ACSES provides structural engineering and condition assessment for concrete structures across Sydney. Get in touch.