Reinforcement Cover on a Curved Slab Edge
You are fixing N16 bars to a curved edge beam on a suspended slab. The drawings specify 40 mm cover, and the formworker has set the edge form 50 mm outside the outermost bar line, giving a nominal 50 mm gap. The concrete supplier confirms a 20 mm maximum aggregate. Your bar chairs are 45 mm plastic, but the site has run short and a labourer offers 25 mm chairs 'to get the pour away'. The pour is scheduled in three hours and the inspector is due in 45 minutes. The engineer's specification states exposure classification B1 with a minimum cover of 40 mm and a tolerance of minus 5 mm. The edge form has already been braced and cannot easily be moved.
1. Decide whether to proceed with the 25 mm chairs, the 45 mm chairs, or to stop and escalate. Justify your decision against the specified cover and tolerance.
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Do not proceed with 25 mm chairs. The specification requires 40 mm minimum cover with a minus 5 mm tolerance, so the absolute minimum acceptable as-built cover is 35 mm. A 25 mm chair places the bar at roughly 25 mm cover before any construction tolerance, which is 10 mm below the absolute floor and 15 mm below nominal. That is a non-conformance, not a judgement call.
The 45 mm chairs are also marginal. They give 45 mm nominal, which sits inside the 40 mm +5 mm tolerance band, but only if the bar does not roll or settle during the pour. On a curved edge the bars tend to drift outward under vibration, so you should tie the chairs down and check cover after tying.
The correct action is to stop, record the shortfall, and escalate to the supervisor and engineer before the pour. Options include sourcing more 45 mm chairs, using bar spacers tied to the form, or asking the engineer for a documented concession. Never let a schedule pressure override a cover non-conformance, because cover governs durability and fire resistance.
2. The labourer argues the 50 mm gap to the form means cover is fine regardless of chair height. Explain why this reasoning is wrong and what actually controls cover.
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The 50 mm gap is the distance from the form face to the bar line only if the bar is held exactly at that position. Cover is controlled by the bar's actual position, which is set by the chairs and ties, not by the form. If the bar sits on a 25 mm chair, the bar is 25 mm from the form face regardless of how far the form was set out.
The 50 mm set-out is a nominal allowance for form deflection, surface preparation, and construction tolerance. It does not add to cover. In fact, if the form bows outward during the pour, the effective cover reduces further.
Cover is verified by measuring from the finished concrete surface to the nearest bar, using a cover meter or physical check before the pour. The set-out dimension is a design input, not a compliance measurement. The labourer is confusing the form position with the bar position, which is a common and expensive error.