Quality with FransThe five you will see most
| Crack type | When it shows | What it looks like | Usual cause |
|---|---|---|---|
| Plastic settlement | While the concrete is still plastic, usually in the first few hours | Straight lines that follow the top bars, or form at changes in depth | Concrete settles and is held up by the bars or formwork; heavy bleeding, low cover over the top steel, poor compaction |
| Plastic shrinkage | Within hours of placing, before the concrete hardens | Short, often diagonal or random cracks across the surface, sometimes in parallel; can be wide at the top and narrow fast | The surface dries faster than bleed water can replace it: sun, wind, low humidity, curing started too late |
| Crazing | Within days; easiest to see when the surface is wet and drying | A fine "map" of small hexagons on the surface only | Finishing too early or overworking the surface, bleed water worked back in, poor curing |
| Early thermal | In the first days to weeks, as the element cools | Cracks through thick sections or walls cast on an older base, often at regular spacing | Heat from the cement; the concrete shrinks as it cools and is held back by the base, older concrete or a hot core |
| Drying shrinkage | Weeks to months later | Cracks at re-entrant corners, openings, and across long bays with no joints | The hardened concrete loses moisture and shrinks while something restrains it; joints too far apart or cut too late |
Timings above are typical ranges from industry guidance, not limits. Sources give slightly different windows, so use them as a guide to the likely cause, not as proof.
The weather number worth knowing
Plastic shrinkage is driven by the evaporation rate from the fresh surface, which depends on air and concrete temperature, humidity and wind. Guidance does not agree on one trigger value. US highway guidance (FHWA) uses 0.5 kg/m²/h as the limit for normal bridge-deck concrete, and half that for low water-cement concrete. An older Virginia field study found that cracking may start above roughly 0.5 kg/m²/h and that precautions become almost essential around 1.0 to 1.5 kg/m²/h. On a hot, windy Gulf afternoon, you can pass those figures easily.
New research also shows that when you cure matters, not only whether you cure. A 2023 University of Pretoria study found that applying curing water at about the start of the shrinkage period (around an hour after casting in their tests) prevented the cracking, while applying it earlier or later was less effective. The exact time will change with your mix and weather, but the message is clear: curing that starts after the tea break is often too late.
Why this matters for QA/QC
When a crack is found, the first question on most sites is "who pays?". The better first question is "which crack is it?", because that tells you which stage failed. A crack that follows the top bars points to cover, compaction and bleeding, which is a placing and inspection issue. Random surface cracks on a windy afternoon point to curing and protection. Regular cracks through a wall cast on an old base point to thermal control and pour sequence, which is a planning and design issue.
That is why the crack record matters. "Cracks observed" in an NCR tells nobody anything. Date, location, length, width, pattern, and the weather and curing on the day of the pour let the engineer judge whether it needs repair and let you fix the process for the next pour. Any crack that is getting wider, is offset across its faces, runs diagonally near supports, or shows rust staining is a different story: report it straight away for the structural engineer to assess.
Check on your site tomorrow
- Before the next slab pour, check the weather forecast for wind, temperature and humidity. Agree in the pre-pour meeting who brings the wind breaks, sunshades, evaporation retarder or fog spray, and when curing starts.
- At rebar inspection, measure cover over the top bars, not only the bottom. Low top cover is a classic cause of settlement cracks along the bars.
- Watch the finishing gang. Is anyone sprinkling water on the surface or troweling while bleed water is still present? That is how crazing starts.
- For thick sections or walls cast onto an old base, ask if there is a thermal control plan (pour size, joints, insulation, temperature monitoring).
- For any crack you find, record date, grid location, length, width (crack gauge or card), pattern and photo, plus the pour date and weather. Then classify it using the table above.
Bar sizes, spacing, cover (top and bottom) and laps, checked against the approved drawing before the concrete arrives. The 31-point checklist I use before every pour.
Download the checklist
Sources: Concrete New Zealand: early-age cracking · FHWA: evaporation rate limits · TRB: Virginia bridge-deck cracking study (Newlon) · University of Pretoria (2023): effect of surface curing timing on plastic shrinkage cracking · FHWA: early-age behaviour of concrete pavements · UltraTech: shrinkage cracks, causes and prevention. Timings are typical ranges; the responsible engineer decides whether any crack needs repair.