Q.: We delivered concrete to a poured wall contractor building residential foundations in a subdivision. He says we shorted him about 1/4 yard per 9-yard load.

I ran a unit weight test on the fresh concrete and divided the total batch weight by the unit weight. The yield came out to be exactly 27 cubic feet per cubic yard delivered, but the contractor still thinks we shorted him.

Is there a standard waste allowance that contractors use when estimating the amount of concrete they need? If so, is the 1/4 yard per 9 yards within that allowance?

A.: Wasting 1/4 yard per 9 yards calculates out to 2.8% waste. The book Concrete Construction & Estimating says to include 3% waste for concrete placed on the ground but no waste for concrete placed in forms. However, the book also says: "If an item calls for 31 1/4 cubic feet of concrete, call it 32, picking up part of a cubic foot on each item. This is how to include waste on the job. But you should still add 3% additional waste for all concrete placed on earth or fill." In the example quoted, the estimated waste then is 0.75/31.25 = 2.4%.

In the book Estimating & Project Management for Small Construction Firms, the normal waste for foundations is given as 5% and for superstructures, 2%. The book also suggests ignoring any openings in walls and slabs less than 15 square feet but deducting from quantity take-off for larger openings.

Some contractors figure their concrete requirements closer than others and might have no more than a couple wheelbarrow loads left over after pouring a 40-cubic-yard foundation. Even at 4 cubic feet per wheelbarrow, that's a waste factor of less than 1%.References 1. Concrete Construction & Estimating, Craftsman Book Co., P.O. Box 6500, Carlsbad, CA 92008. 2. Estimating & Project Management for Small Construction Firms, Van Nostrand Reinhold Co., 450 W. 33rd St., New York, NY 10001.