Cold-Weather Slab and Upstand Pour
On a five-storey car park project, the ground-floor slab-on-grade was placed on 12 November: 24 MPa design strength, 180 mm thick, 620 m², ready-mix without accelerator, delivered at 12 °C and finished at 15:40. The forecast gives 3 °C at 18:00 falling to -4 °C overnight with 20 km/h wind. The site has one curing-compound sprayer and no insulating blankets. Formwork for the 900 mm upstand beams is due to be struck at 36 hours to give the following trade access. On the second morning, plastic shrinkage cracks appear along the upstands and the slab edges are friable and dusty. Seven-day cylinder results come back at 11 MPa. The project manager wants to strike the beam formwork on schedule and asks whether the edges are just a finishing problem.
1. Assess whether the concrete was adequately protected, and explain what the friable edges and the cracking indicate.
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The protection was inadequate. Concrete placed at 12 °C with ambient falling to -4 °C, 20 km/h wind, no accelerator and no insulation is exposed well beyond accepted cold-weather limits. Fresh concrete should be at least 10 °C at placing (often 13 °C for thin sections) and protected from freezing until it has gained about 3.5 MPa, which is far beyond the first night at those temperatures. When free water in the paste freezes it expands by about 9 % and disrupts the paste irreversibly; strength lost this way is not recovered by later warm curing.
The symptoms match that mechanism. Friable, dusty slab edges and surface scaling are classic freeze damage to the surface that cooled and dried fastest, and wind accelerated both cooling and evaporation. The cracks along the upstands are a second mechanism: plastic shrinkage, driven by a high evaporation rate before the concrete had strength to resist, on an exposed vertical surface with no curing cover.
Seven-day cylinders at 11 MPa against 24 MPa design are consistent with slow, cold strength gain plus damage - but they are not conclusive alone, since cylinders stored in the cold would also read low.
2. Decide on striking the upstand formwork at 36 hours and state the criteria you would apply.
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Do not strike at 36 hours.
Formwork and props may be removed only when the in-situ concrete has reached the strength the structural designer specified for that operation - typically around 70 % of design for soffits, or an explicit value for a 900 mm upstand, where the formwork also resists hydrostatic pressure and thermal restraint. Nothing in the record shows the upstand is close to that. Eleven megapascals at seven days is roughly 46 % of 24 MPa, and those cylinders were probably not cured under the same conditions as the member.
There is also a thermal-shock risk. Striking formwork on a thick section while the core is warmer than the surface can crack the surface; a 900 mm upstand hydrating warm and then exposed to a cold night can readily exceed a 20 °C differential.
What to do: keep the formwork in place as insulation, determine in-situ strength by maturity monitoring or by companion cylinders stored beside the member, and require the designer's stated strength before striking. Re-sequence the following trade rather than compressing curing - the schedule problem is real, but a cracked upstand costs more than a delayed trade.
3. List the measures you would require for the next pour on this site, and say what the failure reveals about planning.
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Requirements for the next pour:
- Agree a cold-weather mix with the supplier in advance - accelerator or increased cementitious content - and specify a delivered temperature of at least 10 °C, checked at the truck, not at the pump.
- Never place on frozen ground or against frozen formwork; clear ice and snow and pre-warm formwork and subgrade.
- Have insulation on site before the pour: blankets for slab and upstand tops, insulated formwork or heated enclosures for thin edges.
- Do not rely on curing compound alone below about 5 °C; use blankets and, if needed, temporary enclosures.
- Record concrete temperature at placing and through the first 72 hours. Keep the surface above 5 °C and hold the core-surface differential within about 20 °C.
- Monitor evaporation rate; use windbreaks, fog spray or an evaporation retarder before finishing when it is high.
- Keep companion cylinders beside the member for striking decisions.
What the failure reveals: this was a planning and procurement failure as much as a placing failure. One sprayer and no blankets were known before the pour, and the 36-hour striking date was fixed before anyone checked whether the weather allowed it. Cold-weather provisions must be a hold point in the pour permit, not an improvisation on the morning.