Slab Thickness Conflict Between Drawing and Specification
On a clinic store in Swakopmund, the structural drawing shows a ground-bearing slab 150 mm thick with A142 mesh at 200 mm centres, while the specification clause says 125 mm thick with A98 mesh. The slab area is 8.4 m x 6.2 m. The contractor has already ordered 22 m³ of ready-mix concrete based on the drawing and wants to pour tomorrow. The pour is scheduled for 07:00, and 14 m³ of the order is for the slab; the rest is for two ground beams. The architect is on leave, the engineer is reachable by phone, and the client has asked for the store to be finished early. The site diary has no entry about the discrepancy. The concrete supplier requires changes 24 hours before delivery. A site instruction book is available but has not been used.
1. Set out the correct procedure for resolving the discrepancy before the pour, naming the documents and people involved.
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Stop the pour and do not let the supplier deliver until the discrepancy is resolved. The drawing and specification are both contract documents, but they conflict; the contractor must not pick the cheaper option. I would raise a formal request for information (RFI) to the engineer and architect, copied to the client, stating both clauses, the slab area and the pour date. The engineer's written response is the instruction. I would record it in the site instruction book, update the drawing register, and confirm to the supplier before the 24-hour deadline. The site diary should record the date, time, people contacted and the decision. If the engineer is unreachable, the safer default is the more onerous requirement (150 mm and A142), but this must still be confirmed in writing. A verbal instruction should be confirmed the same day. The reason is that the slab's thickness and mesh affect strength and cover; choosing 125 mm could be a design change, not a contractor's choice.
2. Calculate the concrete volume for the slab at 150 mm and at 125 mm, and comment on the ordered 22 m³. Show your working.
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Area = 8.4 × 6.2 = 52.08 m². At 150 mm: 52.08 × 0.150 = 7.81 m³. At 125 mm: 52.08 × 0.125 = 6.51 m³. The difference is 1.30 m³, about 17% less concrete. The ordered 22 m³ is 14.19 m³ more than the 150 mm slab, so it cannot be justified by the slab alone. The setup says 14 m³ is for the slab; that is still nearly double the calculated 7.81 m³. Either the order includes ground beams, waste and a different area, or it is a serious over-order. I would check the order breakdown, allow 5–10% waste (about 8.2–8.6 m³ at 150 mm), and reconcile the difference before delivery. Over-ordering wastes money and can lead to unauthorized thick slabs or disposal problems. The calculation shows why a measured take-off, not a guessed order, is needed.
3. The engineer confirms 150 mm and A142 mesh. Explain how you would instruct the site team and record the change, and what quality checks you would make at the pour.
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Once the engineer confirms 150 mm and A142, I would issue a written site instruction and revise the inspection and test plan. The site team briefing would cover: slab dimensions, 150 mm thickness, A142 mesh at the specified spacing, lap and cover, and the pour sequence. I would check the sub-base level and compaction, the setting-out of edges, the damp-proof membrane, and the mesh position on spacers so it is not on the ground. At the pour I would check concrete grade and slump against the specification, measure thickness with a depth pin at several points, check mesh laps and cover, and record the time and weather. I would also take cubes if specified and cure the slab. The records would include the RFI, engineer's reply, site instruction, delivery notes, photos and a signed pour card. This matters because the change affects structural performance and payment; a clear paper trail prevents disputes.