Rebound, Waste, and Yield Reconciliation
A crew is shooting a 1.5-inch dry-mix lining on a vertical wall. The specification calls for a 1.5-inch nominal thickness with a minimum of 1.25 inches. The crew is placing from a working platform at a 6-ft standoff with the nozzle angled downward about 30°. At the end of the shift, the crew has used 40 bags of material and placed approximately 220 square feet. The manufacturer's yield is 0.45 cubic feet per bag. The foreman reports 'a lot of rebound' on the floor and wants to make up the shortfall by shooting a thin flash coat over the finished wall.
1. Calculate the expected material use and assess whether the crew's consumption indicates a problem.
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Volume required: 220 sq ft × 1.5 in = 220 × 0.125 ft = 27.5 cu ft at nominal thickness. At 0.45 cu ft per bag, that is 27.5 / 0.45 ≈ 61 bags at nominal thickness with zero waste. The crew used 40 bags, which is about 18 cu ft - roughly 0.98 inches average thickness, below the 1.25-inch minimum.
So the crew is not overusing material; it is under-placing. The 'rebound' the foreman sees is partly real waste from the 6-ft standoff and downward angle, but the dominant issue is that the wall is too thin. A flash coat will not fix it.
2. Evaluate the flash-coat proposal and specify the correct corrective action.
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A thin flash coat over a finished wall is not an acceptable repair. It creates a cold joint, poor bond, and a layer that will delaminate; it also does not restore thickness or encapsulation. The correct action is to measure the placed thickness (probe or core) across the wall, identify areas below 1.25 inches, and remove and replace those areas to full thickness with proper technique.
To prevent recurrence: reduce standoff to 2-3 ft, hold the nozzle closer to perpendicular, and place in controlled lifts. Reconcile yield at the end of each shift against area placed so thin areas are caught before cure. Document the shortfall and the repair.