Cold-Weather Concreting of a Foundation Slab at -30°C
An apartment block in Darkhan is behind schedule. The 850 m² foundation slab, 400 mm thick, C25/30, is scheduled for placement in late December. Forecast: -20°C by day, -30°C at night. The ready-mix plant is 50 minutes away and can deliver 18°C concrete at 60 m³/h; the pour will take about seven hours. The contractor proposes to use 60°C mixing water, add 2% calcium chloride by cement weight as an accelerator, cover the slab with tarps after finishing, and strip formwork after three days because the mix is rich. There is no heated enclosure, no insulation schedule and no temperature monitoring plan. The client wants the slab poured before year-end to secure next-year funding. The site engineer must approve or reject the method statement.
1. Evaluate the contractor's cold-weather method statement. Which parts are unacceptable and why?
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Reject the method statement. Three parts are unacceptable. First, 2% calcium chloride by cement weight in reinforced concrete is far above normal chloride limits and creates a serious corrosion risk; chlorides should not be used as an accelerator in reinforced concrete unless the mix design and code explicitly permit a controlled non-chloride system. Second, 60°C mixing water can cause flash set and uncontrolled concrete temperature; hot water must be combined with aggregates and controlled so that the fresh concrete temperature stays within the mix-design limit. Third, tarps alone at -30°C cannot prevent freezing. Concrete must not freeze until it has reached at least about 5 MPa, and the slab edges and top surface are the most vulnerable. Stripping formwork after three days because the mix is rich is not a strength criterion. The funding deadline does not justify a non-compliant pour.
2. Specify the measures you would require before allowing the pour.
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Acceptable measures: heat the mixing water and aggregates so that concrete at discharge is typically 10-20°C, within the mix design; use a non-chloride accelerator or an approved antifreeze admixture; place concrete in a heated enclosure or under insulated covers with supplementary heat; maintain the slab above 5°C for at least 48 hours and until the specified early strength is reached; insulate formwork and edges; avoid thermal shock when removing covers; and install thermocouples or maturity sensors at representative locations. Provide a curing and protection plan, a temperature record, and standby heating. The contractor should also demonstrate that the plant can maintain supply without cold joints in the 400 mm slab.
3. What evidence should be required before the slab is accepted and formwork is removed?
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Acceptance evidence: site-cured cylinders stored with the slab, temperature and maturity records, strength results at the critical locations, and a visual inspection for freezing, cracking and honeycombing. Formwork and insulation should be removed only when the engineer accepts the strength and thermal gradient. Payment for the slab should be linked to that acceptance and to a completed cold-weather record.