EngiMetric Arena / Spot the Error
Review this concrete order: wastage counted twice
Competency Quantitative reasoning primary
What you should take away: Recognise that a percentage allowance is applied once to the measured quantity, not compounded onto an already adjusted total.
The situation
What is happening
A ground slab is 12 m long, 8 m wide and 250 mm thick. The quantity surveyor calculated 24 m3, added 5 % wastage, then added a second 5 % for pump losses and issued an order for 26.46 m3. Review the order before it is placed.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Slab length | 12 | m | Measured plan dimension. |
| Slab width | 8 | m | Measured plan dimension. |
| Slab thickness | 250 | mm | Design thickness. |
| Wastage allowance | 5 | % | The project allowance for spillage and loss. |
| Issued order | 26.46 | m3 | What the review has to check. |
Assumptions this challenge makes
- The slab is a simple rectangle with no openings or thickened edges.
- Wastage is a single project allowance, not a separate allowance per trade.
Your call
Your answer
Hints
- Start from the measured geometry. Multiply the three dimensions and keep the units consistent.
- Apply the project wastage allowance once to that measured volume. Ask what the second allowance is being applied to.
- Compare the issued quantity with a single allowance. A repeated percentage is not a second physical loss.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- The measured volume is 12 x 8 x 0.25 = 24 m3.
- A single 5 % wastage allowance adds 1.2 m3, so the ready-mix order should be 25.2 m3.
- The issued 26.46 m3 is 24 x 1.05 x 1.05. The second 5 % was applied to the already adjusted total, so the same allowance is counted twice.
- The practical effect is a small but real over-order. On a single slab it looks harmless; repeated across a project it distorts both cost and the concrete-delivery programme.
- The review finding is therefore about the method, not about the arithmetic of the slab dimensions.
What this challenge does not establish
- The slab is treated as a plain rectangle; openings, chamfers and thickened edges would change the measured volume.
- The 5 % allowance is taken as the project allowance. A different measured loss would change the correct order.
- No allowance is made for pump priming, washout or rejected loads beyond the stated percentage.