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The density (unit weight) of concrete (ASTM C 138) is measured using a Type B pressure meter (see photo) to verify agreement with the approved project mix design. The information obtained through this test can also be used to determine yield and relative yield, which helps you verify that you are getting the volume of concrete you ordered and paid for. You can also use this data to calculate the air content of the mix.

The unit weight is determined by the formula below. Subtract the weight of the measuring base from the combined weight of the measuring base and the concrete it contains. Next, divide this weight (in pounds) by the volume of the measuring base (cubic feet) to obtain the density expressed as lb/ft3:

D = (Mc – Mm) / Vm
D = Density of the concrete, lb/ft3
Mc = Weight of the measure holding the concrete
Mm = Weight of the empty concrete measure (base of air meter)
Vm = Volume of the measure (usually about 0.25 ft3 for a pressure meter base) (Fig. 3)

Testing tip: Having the unit weight data gives you "a third point to check a straight line." For example, when slump increases, the air content will generally increase. If significant, look for the unit weight to decrease measurably. If that is not reflected in the test results, keep an eye on the testing and examine the data, procedures, or reporting accuracy.