Equipment Store Decisions Before a Suspended Slab
Your small building firm is due to cast a 45 m² suspended slab, 150 mm thick, at 3.2 m above ground. The equipment store contains: a diesel concrete mixer with a cracked drum lip and a missing belt guard; an electric poker vibrator whose flexible hose is taped at one point; two wheelbarrows; a 5 m steel tape with the first 30 mm broken away; a dumpy level whose tripod clamp no longer locks firmly; and a 3 m aluminium straightedge. The site has single-phase power but no spare generator. The foreman plans to send everything to site and 'make do' because hiring is expensive. You must decide what is fit for the pour and what must be repaired, replaced or hired before work starts.
1. For the mixer, the 5 m tape and the dumpy level, state whether each is fit for the slab and why. Include the minimum action required.
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- Mixer: not fit as it stands. A cracked drum lip can leak slurry and change the mix, and a missing belt guard exposes the operator to a nip point. Minimum action: fit a guard and repair or replace the drum before use; if this cannot be done safely, hire a serviceable mixer.
- 5 m steel tape: not reliable for setting out or checking dimensions. The first 30 mm is missing, so every internal measurement taken from the broken end carries a 30 mm error unless a correction is applied. For a slab and formwork, that error can accumulate. Minimum action: replace it with a verified tape; a broken tape may be kept only for rough work and clearly marked.
- Dumpy level: not fit while the tripod clamp will not lock. A loose instrument can move between readings and give false levels, especially on a site with vibration from deliveries. Minimum action: repair or replace the tripod clamp, then carry out a two-peg check before use.
The straightedge and wheelbarrows are useful, but they do not compensate for unreliable measuring and mixing equipment.
2. Describe field checks you would use to confirm the tape and level are acceptable after repair.
Read the worked answer
For the tape, compare it against a known steel rule or a verified tape over at least 1 m and 5 m. Check that the zero end is sound and that the graduations are not stretched. If the tape has been shortened, measure from a sound graduation rather than the broken end, or reject it.
For the level, carry out a two-peg test on firm ground. Set two stable pegs about 30 m apart. Set up the level midway, take readings on both pegs and calculate the height difference. Move the level close to one peg, take readings again and compare the height difference. If the difference changes beyond a small tolerance, the instrument needs adjustment. Also check that the tripod head and clamp lock firmly, the bubble is centred after gentle tapping, and the cross-hairs are clear. Repeat the check before the slab is set out.
3. Prepare a short equipment plan for the pour, including the vibrator and any hire item.
Read the worked answer
The plan should be written and agreed before the pour:
- Mixer: repair the drum and guard, or hire a serviceable diesel mixer with the same or greater capacity. Confirm water-measuring equipment is available.
- Vibrator: the taped hose is a defect. Have the hose replaced and the electrical lead and plug inspected; use an RCD. If the vibrator cannot be repaired in time, hire one because a 150 mm slab needs proper compaction, especially around reinforcement and edges.
- Measuring: replace the broken tape with a verified 5 m or 8 m tape. Repair the level tripod and carry out the two-peg test.
- Hire items: a spare poker vibrator and, if the site power is unreliable, a small generator or a diesel vibrator.
- Ancillary: wheelbarrows, straightedge, floats, shovels, curing materials, and PPE.
Record the inspection and repair dates, and do not send condemned items to site 'to make do'.