Almost every concrete building shows cracks at some point. Most are harmless hairline cracks in plaster or skim coat, but a few are structural cracks, a sign that a member is working beyond its capacity or that the foundation is moving. This article helps you tell the two apart, monitor cracks in simple ways and know when to call an engineer.
Why concrete cracks
Concrete is strong in compression but weak in tension. In reinforced concrete the steel carries the tension, and fine cracks in tension zones are expected in design. The ACI 224R guide on cracking in concrete structures also notes that some cracking in a structure should be expected. The question is not "are there cracks?" but "what kind, what caused them, and are they still moving?"
Common causes include:
- Plastic shrinkage, when the surface of fresh concrete loses water too fast.
- Drying shrinkage, as concrete or plaster shrinks over time.
- Temperature changes, for example on roof slabs exposed to heat.
- Loads, which cause flexural and shear cracks.
- Foundation settlement, which makes parts of the building move unevenly.
- Corrosion of reinforcement, which pushes the concrete cover until it cracks or spalls.
Hairline cracks: usually harmless
Hairline cracks are usually fine, shallow and found on the surface of plaster, skim coat or slabs. Typical signs:
- A random, map-like pattern, or short lines with no clear direction.
- Only in the finishes, not running through structural members.
- Not getting wider or longer over time.
These are usually caused by shrinkage or temperature. Repair is often cosmetic, but cracks in wet or outdoor areas should still be sealed, because they let in water and air that can speed up corrosion of the reinforcement.
Structural cracks: know the patterns
Some crack patterns deserve closer attention:
| Crack pattern | Typical location | Possible cause |
|---|---|---|
| Vertical cracks at the bottom of a beam or slab near midspan | Positive moment zone | Bending from overload or large deflection |
| Diagonal cracks at about 45 degrees near beam supports | Near columns or walls | Shear |
| Diagonal cracks in walls, often from door or window corners | Masonry or concrete walls | Uneven foundation settlement |
| Cracks along reinforcement with rust stains or spalling | Columns, beams, slabs in damp areas | Corrosion of reinforcement |
| Cracks in columns, especially at the top or bottom | Columns | Overload or earthquake damage |
This table is a starting point, not a diagnostic tool. The same pattern can have different causes, and the final judgement needs an engineer who sees the building in person.
Warning signs that need prompt action
Call a structural engineer promptly if you see:
- Cracks that keep getting wider or longer over days or weeks.
- Cracks in columns, or diagonal cracks in beams near supports.
- Spalled concrete with exposed, rusting reinforcement.
- Doors or windows that suddenly stick, sloping floors or a slab with visible sag.
- New cracks after an earthquake, a fire, or added loads such as water tanks or heavy equipment.
If there is immediate danger, such as falling concrete, cracking sounds or sag that keeps increasing, keep people away from the area and call an engineer at once. Do not wait for monitoring results.
Tip: Do not cover cracks with paint or skim coat before they are inspected. Covering them removes the evidence an engineer needs to find the cause and see whether the crack is still moving.
How to monitor a crack
Many cracks need no immediate action but should be monitored to see whether they are active or have stabilised. Simple steps:
- Photograph with a scale. Photograph the crack with a ruler or tape beside it, from the same position each time.
- Mark the crack ends with a date. Draw a short line at each end with a permanent marker and write the date. If the crack grows past the mark, you will see it straight away.
- Fit a crack gauge. A simple crack monitor fixed across the crack lets you read changes in width at regular intervals.
- Note what is happening around the building. Wet or dry season, construction next door, or a change in how rooms are used can help explain movement.
Monitoring may run for several months. The results help the engineer decide whether a cosmetic repair is enough, further monitoring is needed, or structural repair and strengthening are required.
What an engineer does
When evaluating cracks, an engineer usually reviews the drawings and the building's history, maps the crack pattern, measures width and depth, and where needed carries out tests such as core drilling or non-destructive testing. If settlement is suspected, the soil and foundations are reviewed too, including the foundation type, as discussed in Bored Piles vs Driven Piles. Repair methods such as epoxy injection, patching or strengthening are chosen after the cause is known, not before.
Conclusion
Hairline cracks in plaster are usually cosmetic. Cracks in columns, diagonal cracks near supports, settlement cracks and cracks that keep moving are signals to call an engineer. Document them, monitor them and do not cover them up before they are inspected. You can see examples of buildings whose structures Stadin designed on our projects page.
Worried about cracks in your building? Stadin's structural audit team can help assess the cause and how urgent it is.
This article is general information. Every project still needs to be analysed and calculated by a competent structural or geotechnical engineer, in line with the applicable SNI codes and regulations.
