Wall Pour With Missing Tie and Cold Overnight
A 9 m long by 4.2 m high wall is formed with modular steel forms. The pour is scheduled for 07:00 at 2.8 m per hour with 120 mm slump concrete at 12 degrees C. Ties are specified at 600 mm horizontally by 900 mm vertically. During the pre-pour walk, you find the bottom row missing one tie, one diagonal brace loose, and the formwork design drawings are not on site. The foreman says the mix is 'stiff enough' and the missing tie is 'near the corner where there is no pressure.' Overnight forecast is -3 degrees C with a 20 km/h wind, and the specified concrete has no accelerator. The pump is booked, 40 cubic metres is on order, and the client is pressing to pour. A previous pour at another site bulged with a similar mix and rate.
1. What hold points must be cleared before the pump starts, and can the pour proceed as scheduled?
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The pour cannot proceed as scheduled. The hold points are: approved formwork design drawings on site; all ties installed and tensioned to the specified values; braces and alignment checked; formwork inspected by a competent person; pour rate and concrete temperature/slump confirmed; cold-weather protection in place; and the pump and placing sequence agreed. The missing tie and loose brace are non-conformances, not minor defects. The foreman's argument that the corner has no pressure is wrong: formwork pressure acts on the whole face and is highest near the bottom, and the missing bottom tie transfers load to adjacent ties. The previous bulge is evidence that the mix and rate can overload the formwork. The 40 cubic metre order and the client's schedule do not change the hold point.
2. How do you deal with the missing tie and the loose brace? What information do you need to judge the formwork capacity?
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Replace the missing tie with the specified tie and torque it to the design value; do not substitute a shorter or lower-grade tie. Re-tension the loose brace and check the entire form for plumb and alignment. To judge capacity, obtain the engineer's formwork drawings and pressure calculation. The pressure depends on pour rate (2.8 m/h), concrete temperature (12 degrees C), slump (120 mm), vibration, cementitious content, and form type. At that rate and temperature, the pressure is not low. Verify tie spacing, tie capacity, wale and stud spacing, and the form manufacturer's ratings. If the drawings cannot be produced, the engineer must confirm the system before the pour. Document the missing tie, the correction, and the inspection sign-off.
3. What is required for the cold-weather placement, and who must authorize the change?
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Cold weather at -3 degrees C with wind requires a protection plan, not just 'no accelerator.' The concrete must be placed on a thawed, unfrozen base, protected from freezing, and maintained above the specified temperature until it reaches sufficient strength. That usually means heated enclosures, insulation blankets, temperature monitoring, and a contingency for the pump line. The mix may need to be adjusted by the supplier, but that is the supplier's and engineer's decision. The pour should be rescheduled or the protection plan approved by the site engineer or supervisor before the pump arrives. Record the concrete temperature at placement, the ambient temperature, and the protection measures. The client must be told the schedule change in writing.