5.1 Purpose, Equipment, and the Calibrated Measure

What ASTM C138 Does and Why It Matters

ASTM C138/C138M, "Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete," is one of the seven performance tests an ACI Concrete Field Testing Technician Grade I must master. A single field measurement — the density (also called unit weight) of freshly mixed concrete — feeds four separate calculations:

• Yield — the volume of concrete actually produced from a known batch of materials.
• Relative yield — whether the batch delivered exactly the volume the mix was designed for.
• Cement content — the actual mass of cementitious material per unit volume of concrete produced.
• Gravimetric (theoretical) air content — the air percentage derived by comparing measured density to a theoretical air-free density.

Because every one of these answers flows from density, an error of even a few pounds per cubic foot ripples through all four results. That is why C138 procedure precision — filling, consolidating, striking off, and weighing — is tested so heavily on the exam.

The Density Equation

The core relationship the exam expects you to recite is:

D = (Mc − Mm) / Vm

where D is density (lb/ft³ or kg/m³), Mc is the mass of the measure filled with concrete, Mm is the mass of the empty measure, and Vm is the calibrated volume of the measure. The difference (Mc − Mm) is the net mass of concrete alone — the empty measure mass is always subtracted out. Forgetting to subtract Mm is one of the most common conceptual traps.

Worked density example

An empty 0.25 ft³ measure weighs 9.50 lb. Filled and struck off, it weighs 45.50 lb. Net concrete mass = 45.50 − 9.50 = 36.00 lb. Density = 36.00 / 0.25 = 144.0 lb/ft³. A normal-weight concrete density of 140–150 lb/ft³ is a sanity-check range; a result far outside it signals a procedural or weighing error before you ever report it.

Required Equipment

C138 prescribes specific apparatus, and the exam asks you to match tools to the right measure size:

The measure should be the same one used for the air-pressure (C231) test in many field setups, but its volume must still be independently calibrated for C138.

Calibrating the Measure (Water-Fill Method)

The volume Vm is not assumed — it is calibrated by water. You fill the measure with water at a known temperature, slide a glass plate across the top to seal it with no air bubbles trapped underneath, dry the exterior, and weigh. The net water mass divided by the density of water at that temperature gives the volume:

Vm = (mass of water) / (density of water)

The density of water is approximately 62.4 lb/ft³ (about 997–998 kg/m³ at typical 16–29 °C field temperatures; 1000 kg/m³ only near 4 °C). Using 62.4 lb/ft³ is the standard inch-pound shortcut.

Calibration example

A measure holds 15.60 lb of water (net). Vm = 15.60 / 62.4 = 0.250 ft³. Calibration is repeated at least annually and whenever the measure is dented, because a deformed measure changes Vm and silently corrupts every density and yield result that follows.

Note the temperature sensitivity: water is densest near 4 °C (about 1000 kg/m³, 62.43 lb/ft³) and slightly less dense at field temperatures. At 60–80 °F (16–27 °C) the density is roughly 997–998 kg/m³, which is why the metric value is given as a range rather than a flat 1000. For inch-pound calibration, the 62.4 lb/ft³ value is the accepted working figure unless an unusually hot or cold day demands the temperature-corrected number.

Why Density Drives Everything Else

It is worth pausing on how central a single number is to this method. Once D is in hand, yield, relative yield, cement content, and gravimetric air content all follow by arithmetic — no further field measurement is required. This makes C138 efficient but unforgiving: a 2% error in density (about 3 lb/ft³ on a normal mix) becomes a roughly 2% error in yield and shifts the computed air content by close to a full percentage point.

That sensitivity is why the standard is so prescriptive about consolidation, strike-off, cleaning, and calibration. Every one of those steps exists to make (Mc − Mm)/Vm represent the true concrete as placed. A technician who treats the procedure casually — heaping the measure, leaving slurry on the outside, or trusting an uncalibrated stamped volume — is not just risking the density number; they are corrupting four downstream quality-control values that the producer and owner rely on. Keep that chain in mind: clean apparatus and disciplined procedure are the whole point of the method.