A slab-on-grade is 20 ft by 30 ft and 4 inches thick, with a thickened (turndown) edge around the full perimeter that adds a beam 12 in wide by 12 in deep below the slab. What is the APPROXIMATE total concrete volume?

Explanation

Slab field: 20 x 30 = 600 sq ft x (4 in = 0.333 ft) = 200 cubic feet. Turndown edge: perimeter = 2 x (20 + 30) = 100 ft, extra section 1 ft x 1 ft = 1 sq ft, so 100 x 1 = 100 cubic feet. Total = 300 cubic feet. Divide by 27: 300 / 27 = 11.1, about 11 cubic yards (c). Option (a) counts the slab only and omits the thickened edge. Option (b) roughly doubles the correct value. Option (d) leaves cubic feet without dividing by 27.