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ASTM C138 · gravimetric measure test
Fresh concrete unit weight
The density of the concrete in front of you, from a measure of known volume and two scale readings — the field test everything else is checked against.
Verified engine
What the engine returns
The net mass is what the concrete itself weighs; the unit weight is that mass spread over the measure’s volume. Compare it against the density the mix design implies. A result below it points at air or at consolidation before it points at the materials, and the difference is worth converting: a small density shortfall across a full load is a surprisingly large volume of concrete you paid for and did not receive.
Inputs the result is built from
Measure volume
Mass of the empty measure
Mass of the filled measure
How the result is derived
MethodGravimetric measure test: net concrete mass divided by the calibrated measure volume
StandardASTM C138/C138M — Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete
GuardA filled mass that does not exceed the empty mass is refused as a bad input rather than computed. It would produce a zero or negative density — arithmetically fine and physically impossible — and it almost always means the two readings were entered in the wrong order.
What the gravimetric measure test tells you
Unit weight is the density of fresh, consolidated concrete — mass per unit volume, as placed. It is measured rather than derived: a rigid measure of known volume is filled and consolidated to a standard procedure, struck off level, and weighed.
The net concrete mass is the filled mass less the mass of the empty measure. Dividing it by the measure’s volume gives the unit weight. Both the subtraction and the division are trivial; everything that makes the test reliable happens before the scale reading.
The number is a check on the whole batching operation at once. Concrete lighter than expected usually means more air than intended — entrained, entrapped, or from poor consolidation. Heavier usually means less air or a denser aggregate than the design assumed. Either way it is the first evidence that the mix leaving the plant is not the mix that was designed.
It is also the input to a measured yield: the batch’s total mass divided by this density is the volume it actually produced, which is the same question a mix design answers by calculation and this test answers by weighing.
Gravimetric measure test: net concrete mass divided by the calibrated measure volume
When this calculation is used
- Routine field QA on a delivered load, alongside slump and temperature.
- Investigating a short pour where the delivered volume fell below the ordered volume.
- Confirming that air entrainment is doing what the specification asked for.
- Establishing the density a measured yield calculation needs.
Worked example
A standard measure of known volume is filled and consolidated on site, and weighed empty and full on the same scale.
The net mass is what the concrete itself weighs; the unit weight is that mass spread over the measure’s volume. Compare it against the density the mix design implies. A result below it points at air or at consolidation before it points at the materials, and the difference is worth converting: a small density shortfall across a full load is a surprisingly large volume of concrete you paid for and did not receive.
The figures are produced by the certified engine when the calculator loads, from this pack’s own declared example. Nothing on this page is a stored answer.
What each input represents
Measure volume
The internal volume of the container, from its calibration — not from its nominal size. Measures are calibrated by weighing them full of water at a known temperature, and a measure that has been dropped is a measure whose calibration has expired.
Mass of the empty measure
The tare. Taken clean and dry: residue from the previous test rides along into every reading afterwards, and it is always in the direction that makes the concrete look lighter.
Mass of the filled measure
The measure filled, consolidated and struck off level, with the outside wiped clean. Consolidation is where this test is won or lost — under-rodded concrete carries entrapped air that the scale faithfully records as missing mass.
Assumptions and limits
- The measure’s volume is its calibrated volume. Nominal capacity is a label; the calibration is the measurement, and the whole result scales directly with it.
- Consolidation followed the standard procedure. Rodding and tapping are not a formality — they are the part of the test that decides how much entrapped air is in the sample.
- Both masses come from the same scale, in the same session. A tare from a different scale introduces an offset that no part of this calculation can see.
- The sample is representative of the load. One measure from the first discharge is not, and the standard has something to say about when to take it.
What the guards protect against
- A filled mass that does not exceed the empty mass is refused as a bad input rather than computed. It would produce a zero or negative density — arithmetically fine and physically impossible — and it almost always means the two readings were entered in the wrong order.
- That guard is on the readings rather than on a domain range, which is the difference between this page and the design calculators. There is no invalid density here: whatever the concrete weighs is what it weighs, and refusing an unusual value would be refusing the measurement.
- An unexpectedly low or high result is therefore reported, not blocked. It is evidence about the batch, and evidence is exactly what this test exists to produce.
Provenance
ASTM C138/C138M — Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete
Gravimetric measure test: net concrete mass divided by the calibrated measure volume
The citation travels inside the signed pack and is displayed from the verified leaf, not transcribed here. What you see rendered with the result is what was signed.