Plastic Cracking on a Windy Slab Pour
A large external slab (approximately 400 m²) is being placed on a warm, breezy day: air temperature 28 °C, wind speed around 25 km/h, relative humidity 35%. The concrete is a C25/30 mix with a slump of 120 mm and no fibres. Curing compound is specified but is not applied until finishing is complete, which takes about 90 minutes after each bay is struck off. Within two hours of placing the first bay, the finisher reports short, roughly parallel cracks near the surface, mostly perpendicular to the wind direction, appearing before the concrete has set. The contractor asks whether the cracks are structural and whether the bay can be accepted.
1. Identify the likely cause of the cracking and explain why it occurs under these conditions.
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The cracks are almost certainly plastic shrinkage cracks, not structural cracks.
Reasoning:
- They appear before the concrete has set, which rules out most structural or thermal cracking mechanisms.
- They are short, roughly parallel, and near the surface, and they are oriented perpendicular to the wind - a classic signature of plastic shrinkage, where rapid surface moisture loss causes the surface to shrink while the interior has not yet gained strength.
- The conditions strongly favour it: high temperature (28 °C), high wind (25 km/h) and low humidity (35%) combine to give a high evaporation rate from the fresh surface.
- The 90-minute delay before curing is the critical contributing factor: the surface is left exposed to evaporation during the most vulnerable period.
Plastic shrinkage cracking is a durability and appearance issue rather than a structural one, but it should not simply be ignored.
2. What should the engineer recommend to address the cracks in the affected bay and to prevent recurrence?
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For the affected bay:
- Assess the cracks: if they are shallow and narrow (typical plastic shrinkage), they are mainly a durability concern. The engineer should decide whether they need sealing or whether they are acceptable, based on the specification's crack-width limits and the exposure conditions.
- Document the cracks and the conditions at the time of placing.
To prevent recurrence on the remaining bays:
- Reduce evaporation. Use windbreaks, shade, or fog/mist spraying to raise humidity at the surface.
- Apply curing earlier. Do not wait for full finishing; use an evaporative retarder or a spray-applied curing compound as soon as the surface will accept it, or cover with plastic sheeting between finishing stages.
- Adjust the mix where possible - e.g. a mix less prone to rapid moisture loss - but the primary fix is construction practice, not mix design.
- Sequence the work so bays are placed and finished in smaller areas that can be protected quickly.
Key point: the root cause is the delay in protecting the fresh surface under high-evaporation conditions, so the fix is procedural.