Cold-Weather Basement Wall Pour With Reused Forms
An 18 m long by 2.7 m high basement wall is being formed with 250 mm thick walls. The forms are modular aluminum, rated by the manufacturer for a maximum fresh concrete pressure of 60 kPa. The forecast is 3°C with wind, and the supplier proposes a 180 mm slump mix with a retarder, placed in one continuous pour at 1.8 m/h using a 50 mm vibrator. Ties are installed at 600 mm horizontally and 600 mm vertically, but the layout is from a previous job with a 300 mm wall. The formwork has been reused four times, and several tie washers show deformation. The pump truck is on site, and the crew wants to start before the rebar inspection is signed off. A junior carpenter asks whether the pour can proceed.
1. Should the pour proceed? Identify the controlling issues and the information needed before a decision.
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The pour should not proceed as described. Four issues control the decision. First, the form pressure is unverified: with a 2.7 m wall, full hydrostatic pressure at the base is roughly 65 kPa, already above the 60 kPa form rating, and a cold, retarded, high-slump mix placed at 1.8 m/h with vibration can keep pressure high. Second, the tie layout was designed for a 300 mm wall, not this 250 mm wall, so the 600 mm by 600 mm spacing cannot be assumed adequate. Third, deformed tie washers indicate the forms may have been overloaded and should be rejected or inspected. Fourth, rebar inspection is a hold point and has not been signed off. Before deciding, the supervisor needs the form manufacturer’s pressure tables for the actual pour rate, temperature, slump, and vibration, plus a tie layout for this wall and a competent inspection of the form hardware.
2. If the pour is allowed later, what pour procedure and monitoring would you require?
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The pour should be controlled by a written formwork and pouring plan, not by the pump operator’s preferred speed. The plan should state the maximum pour rate and lift height that keep calculated pressure below the form rating, the approved tie pattern and hardware, and the vibration procedure. Cold-weather measures should protect the concrete from freezing and keep it above the specified temperature, but they do not remove the need to verify form pressure. During the pour, a competent person should watch for form deflection, tie slippage, washer deformation, leaking joints, and unusual movement; the pump should be stopped if any of these appear. Concrete should be placed in controlled lifts and not dumped in one point. The crew should know the stop-work trigger and the evacuation area in case of a blowout. The rebar inspection must be signed off before concrete is placed.
3. What records and inspections would you require before, during, and after the pour?
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Before the pour, the records should include the approved formwork design or manufacturer’s pressure calculation, tie layout, inspection of reused forms and washers, rebar and embedment inspection sign-off, concrete mix approval, cold-weather protection plan, and a pre-pour checklist signed by a competent person. During the pour, the supervisor should record the actual pour rate, lift heights, concrete temperature and slump, vibration use, and any observed movement or corrective action. After the pour, the formwork should not be stripped until the concrete has reached adequate strength, and the wall should be inspected for alignment, honeycombing, cold joints, and tie-hole condition. Any form distress should be documented and the forms should not be reused until repaired or approved. These records matter because the next pour may use the same forms and the same crew.