It has been found that the -strength of concrete regularly diminishes as the proportion of cement becomes less. Approximately the results follow the formula F = 150 - 10 B, where F = crushing force in tons per square foot, and B = quantity of ballast to 1 of cement. (See vol. iii., "Notes in Building Construction," pp. 208-9.) Sutcliffe's" Concrete" quotes three tests by Kirkaldy for strength of concrete beams as follows. (1) Beam of 1 Portland cement and 1 coke breeze, seven days old, 3 in. broad, 5 in. deep, 72 in. clear span. Breaking weight loaded in centre average 1 3.85 cwt., or allowing half - weight of beam between supports a gross central load of 4.07cwt. (2) Beam of 1 Portland cement and 2 crushed bricks, two or three months old, 12 in. broad, 8 in. deep, (30in. span. Breaking weight loaded in centre averaged 13.25 cwt., or a gross central load of 15.08cwt. (3) Beam of 1 Portland cement to 6 gravel, ninety days old, 12in. by 12 in. by 36in. span. Average breaking weight on central 6 in. = 16.67 cwt. But it must be mentioned that the strength is subject to so many contingencies that experiments cannot be relied upon very closely.

A reasonable practice is to let the thickness of concrete in inches equal the span in feet between main joists, and to put cross joists of about half-depth at half the distance apart.