Formwork Stripping Pressure on a Cold Morning

Why tapping is not adequate:

• Surface hardness reflects only the outer few millimetres. The concrete that carries the load is the full section, and its strength depends on the maturity of the whole element, which is governed by temperature history, not surface appearance.
• At 4-12°C the hydration reaction is much slower than at 20°C. Concrete that looks and feels hard at 3 days in these conditions may have only a fraction of its 28-day strength - and possibly not enough to carry the props' load plus construction loads.
• The formwork was designed for 7 days at 20°C. That design assumption is invalidated by the cold. Stripping at 3 days in cold weather is a double reduction - shorter time and slower strength gain - and could cause deflection, cracking or collapse.

What actually controls the decision:

1. In-situ strength, not time. The accepted criterion is that the concrete must have reached a specified proportion of its design strength before props are removed - commonly 70% for soffits of slabs and beams, and higher where the span or loads demand it. The engineer's specification governs.
2. Maturity / temperature history. Use the actual temperature record of the element (thermocouples or a maturity meter) to estimate strength gain, or take companion cylinders cured alongside the element and test them.
3. Loads at stripping. Consider the props' load, any backpropping, construction loads and the sequence of striking. Striking the soffit while the slab above is loaded can overstress the young concrete.
4. Cold-weather precautions. Insulate or heat, use accelerators only if approved, protect from frost, and extend the curing and stripping period accordingly.

Conclusion: the 3-day stripping is not justified. The correct approach is to test or use maturity to confirm the required strength, and to extend the period to reflect the cold. 'Looks hard' is a visual impression, not a strength test.

Safe sequence for prop recovery:

1. Do not strip the whole soffit at once. Use a staged, partial striking sequence: remove props in a pattern that keeps the span supported while load is transferred gradually. Typically strike from the middle of the span outward, or as the engineer directs, leaving the props near supports longest.
2. Backprop before you strip. If the slab is to carry construction loads or the next level, install backprops under the span before removing the soffit props, so the load path is maintained.
3. Confirm strength first. Only begin when the in-situ strength criterion is met (tested cylinders or maturity), not on a calendar date.
4. Reuse props where the structure allows. Props can be recovered progressively from areas that have reached strength, while keeping the critical spans supported. Coordinate with the engineer's striking plan.
5. Monitor. Check for deflection, cracking and movement as props are removed; stop if anything changes.

Records required:

• Concrete temperature record for the element (and ambient), with the maturity calculation or cylinder test results showing the strength at stripping.
• The engineer's stripping/striking plan and approval for the staged sequence.
• The mix design and delivery dockets (to confirm what was actually placed).
• Inspection record of the props and formwork before and after striking.
• Any non-conformance report if the original 3-day plan was attempted.

Bottom line: props can be recovered, but only on a documented, staged plan backed by strength evidence - not on a fixed 3-day timetable carried over from a 20°C design assumption.