Density (Unit Weight), Yield, and Gravimetric Air Content
Measurements
Density, unit weight, yield, and gravimetric air content come from one set of measurements. Determine the volume of the measuring bowl by filling it with water and weighing; use that calibrated volume, not a nominal size. Fill the bowl with concrete in layers, consolidate by rodding or vibration as required for the bowl size, strike off, and determine the mass of concrete in the bowl. Density (unit weight) = mass of concrete ÷ volume of bowl.
Calculations
Theoretical density is the total mass of all batch materials divided by their total absolute volume, with no air. Gravimetric air content = [(theoretical density − measured density) ÷ theoretical density] × 100. Yield = total batch mass ÷ measured density. Relative yield = yield ÷ design yield; cement content = cement mass ÷ yield. Keep units consistent, and subtract the empty bowl mass.
Traps and worked example
Traps include using an uncalibrated bowl, guessing the bowl volume, failing to consolidate, and mixing pounds with kilograms. Another trap is reporting air content as a fraction; it is a percentage.
Worked example: bowl volume = 0.25 ft³; concrete mass = 36.0 lb; density = 36.0 ÷ 0.25 = 144 lb/ft³. Theoretical density = 150 lb/ft³. Air = (150 − 144) ÷ 150 × 100 = 4.0%. For a 1,800 lb batch, yield = 1,800 ÷ 144 = 12.5 ft³.