Concrete Cracks vs Structural Cracks: How to Tell the Difference
A crack in a concrete slab, path or foundation wall is one of the most common things that worries homeowners, buyers and builders. The good news is that most cracks are harmless shrinkage cracks. The trick is knowing which ones are not, and when it is time to get an engineer to look.
Why concrete cracks in the first place
BRANZ (the Building Research Association of New Zealand) sets out three main causes of cracking in concrete:
- Shrinkage — as concrete dries it shrinks slightly. A mix that is too wet or weak, poor curing, badly placed or timed control joints, too few joints, or restraint from other parts of the building can all make shrinkage cracking worse. "Plastic cracking" can even happen before the concrete has fully set, if it dries out too quickly.
- Ground and loading issues — poor compaction or an incorrectly assessed subgrade under a slab, a structure not designed for the loads actually placed on it, or damage during handling of precast elements like tilt-up panels.
- Corrosion — water getting into the concrete corrodes the reinforcing steel inside it, especially where the concrete cover is thin or the concrete quality is poor. As rust forms it expands and cracks the surrounding concrete from the inside out.
Cosmetic or structural? What to look at
There is no single test that settles it on the spot, but BRANZ guidance points to several things worth checking:
- Hairline cracks are common and normally need no action at all.
- Width matters. As a simple guide, BRANZ notes maximum acceptable crack widths of around 0.3 mm for exterior elements in the ground, and 0.4 mm for a protected ground floor slab or other elements fully enclosed within a building (NZS 3101 sets out the fuller limits). Wider than that is worth getting checked.
- Is it still moving? A stable crack is a different story from one that keeps widening. Simple gauges, tape or glass "tell-tales" can monitor movement over weeks or months.
- What caused it? A crack after an earthquake, a heavy impact, or alongside ground movement deserves more attention than one that appeared gradually during normal drying.
- Where it sits relative to the reinforcing steel. Narrow cracks crossing bars at right angles are generally lower risk. Cracks running parallel to, and directly above, a bar can mean corrosion has already started.
- Does it let water in? A crack that lets water through, threatens durability, or damages internal linings is a problem even if it is not structurally serious.
Repairing a crack
BRANZ notes that cracks are generally easier to repair while still new, though shrinkage cracks in new concrete floors can keep widening for up to two years, so waiting a while before filling can make sense. The right method depends on the crack: plain cement fill suits cosmetic shrinkage cracks with no further movement expected; polymer-modified cement suits narrow cracks needing waterproofing but no movement; epoxy resin is for structural repairs and waterproofing under water pressure (small jobs can be a builder's job, larger ones need a specialist); specialist injection systems should only be done by specialist firms; and flexible sealants suit cracks that keep moving, though they do not restore structural strength.
When to call an engineer
BRANZ is clear that if there is any doubt about the cause or seriousness of a crack, a structural engineer should assess it before any repair is carried out — guessing wrong can turn a minor issue into a bigger one. That applies especially to irregular, wide or still-moving cracks, cracks following an earthquake or other event, and any crack where reinforcing steel may have fractured or corroded. An engineer can confirm the cause, check the element is still performing as designed, and recommend the right fix before a builder or specialist carries it out.
If you have noticed a crack on your property and are not sure whether it is cosmetic or something more, get in touch with Fil Engineers — we are happy to take a look.
Sources
This article is general information only and is not a substitute for site-specific engineering advice. Every building and every crack is different — talk to a qualified structural engineer about your own property.
