Three-day stripping on a suspended slab
You are the formwork supervisor for a suspended slab 100 mm thick over an area 4.5 m x 6.0 m, designed at 21 MPa at 28 days. The deck is 12 mm plywood on 50 x 100 bearers at 400 mm centres, carried on props at 1.2 m spacing both ways. The pour was Monday. The project specification states that soffit formwork to slabs may be removed once the concrete has reached 70 per cent of design strength. Cylinders tested at three days return 13.5 MPa, and the crew wants to strip on Thursday morning so the props can be moved to the next bay. In addition, the props under the first-floor slab below were struck last week, and the current props bear directly on that slab. The form oil ran out on Tuesday and the crew has been wiping the plywood with diesel from the site bowser.
1. Using the numbers given, decide whether to strip on Thursday and show your working.
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Do the arithmetic before the argument:
- Required strength = 70 per cent of 21 MPa = 14.7 MPa.
- Achieved at three days = 13.5 MPa = 64 per cent of 21 MPa.
- 13.5 < 14.7, so the stripping criterion is not met. Do not strip the soffit on Thursday.
Then sanity-check the evidence, because one number carries the decision. A single cylinder set is weak evidence: confirm the cylinders were cured and stored under the same conditions as the slab, that the break was a proper test, and that the result is representative of that pour. Curing in the slab in the first three days is what actually drives strength gain, so check that curing started on time and has continued.
Give the crew a constructive path: keep the soffit props in place, book a seven-day break, keep all construction loads off the slab until the criterion is met, and notify the engineer of the result. Side forms and beam sides, which carry no slab load, can generally be removed earlier - so some of the crew can be productive without touching the soffit.
Never strip to a date because it suits the programme. Stripping early risks deflection, cracking, and a repair that costs more than the whole formwork cycle. If an earlier strip is genuinely needed, that is an engineering decision, not a supervisor's call.
2. Explain the issue with the props bearing on a slab whose own props were already removed. What should be done?
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Formwork loads travel down through the building. The fresh concrete, the formwork, the props, and the impact and stacking loads of the crew all pass into the slab below. That slab is itself young, and last week its props were struck, so it is carrying load in a condition it was not designed for at this age - it may deflect, crack, or take a permanent set. The props also apply concentrated point loads that can punch locally.
The standard answer is reshoring: maintain a load path through the levels below. Leave the original props under the lower slab, or re-install reshores there before any stripping above, sized and spaced so the load is shared, not dumped on a few points. Never strike two levels at once, and never remove the props below a slab that is still carrying the props above. Props should bear on sole plates or bearers to spread the load, and the layout should be set by the formwork design or the engineer.
Record the reshore layout, the dates, and who inspected it. The failure mode here is a progressive collapse during a pour, which is exactly the kind of event a supervisor is expected to prevent without being told twice.
3. The crew used diesel as a form release agent. What are the consequences and what do you do now?
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Diesel is not a release agent. It is a hazardous, unapproved product, and the damage is in three directions:
- Product: it soaks into the plywood and contaminates the concrete surface, causing dusting, staining, and poor bond for plaster, tile adhesive, or screed. The defect often appears months later, after the finish is on.
- Health: skin contact causes dermatitis, and the vapour is a fire and inhalation risk in an enclosed space.
- Environment: spills and wash water must not reach drains, soil, or waterways; diesel is a controlled waste.
Action now: stop the practice, place a drip tray and absorbent under the bowser, contain any spill and dispose of the absorbent as hazardous waste (never hose it into the drain), and report the incident to the safety officer and the engineer in writing. Obtain the approved release agent specified for the job and check its safety data sheet. If diesel was applied to forms that have not yet been poured, clean and re-coat with the correct product. If concrete has already been cast against diesel-coated forms, flag those areas and get the engineer to decide on testing - an adhesion or pull-off test is far cheaper than a failed tile job. Continue with a short toolbox talk so the whole crew understands why the shortcut was refused.