Every concrete mix design assumes a certain weight per cubic metre — usually around 2400 kg/m³ for normal-weight concrete. The unit weight test checks whether the concrete actually delivered matches that assumption. It is one of the oldest and most honest quality-control tests we have: weigh a known volume of fresh concrete, divide, and you immediately know if the batching, the air content, or the compaction is off.
If you have ever wondered why a “1 m³” batch seemed to produce noticeably more or less concrete than expected, this test gives you the answer.
For how these properties change as the mix sets and hardens, see the properties of fresh and hardened concrete.
What the Test Measures
The unit weight test determines the density (unit weight) of freshly mixed concrete — its mass per unit volume, in kg/m³ (or lb/ft³). From that one measurement you can also calculate:
- Yield — the actual volume of concrete produced by the batch (are you getting the cubic metres you paid for?)
- Relative yield — actual volume ÷ designed volume (over-yield or shortfall)
- Air content (gravimetric method) — by comparing the measured density with the theoretical air-free density
Practical note: Unit weight is a fresh-concrete property. It tells you about batching accuracy and air content, not directly about strength — but a mix that is 100 kg/m³ light almost always has a story worth investigating.
Governing Standard
ASTM C138/C138M — Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete.
The procedure below follows ASTM C138. Note that unlike SCC workability tests, this test does use rodding to consolidate the concrete in the measure — the aim is to remove entrapped air the same way every time, so the result reflects the mix, not the filling technique.
Apparatus
- Density measure — a rigid cylindrical container of known volume, typically 7 L (0.25 ft³) or 14 L (0.5 ft³), with a machined flat rim. Its exact volume must be calibrated (usually by weighing it full of water).
- Balance or scale accurate enough for the measure size (ASTM requires accuracy consistent with reporting density to 1 kg/m³)
- Tamping rod — 16 mm diameter, 600 mm long, with a rounded end
- Rubber or rawhide mallet
- Flat strike-off plate
- Scoop
Practical note: Recalibrate the measure’s volume periodically — dents and wear change it, and every litre of error goes straight into your density. A quick water-weighing check takes two minutes.
Test Procedure (ASTM C138)
The whole test takes about five minutes once you have a fresh sample. Work steadily and in exactly the same way every time — the result only means something if the filling and rodding are consistent from one test to the next.
- Dampen the inside of the measure and drain off standing water. Weigh the empty measure and record its mass (Mm).
- Using the scoop, place fresh concrete in the measure in three layers of approximately equal volume, moving the scoop around the perimeter so the concrete distributes evenly.
- Rod each layer 25 times (for the 14 L / 0.5 ft³ measure; 50 times per layer for the 1 ft³ measure) with the tamping rod, distributing the strokes uniformly over the cross-section. On the second and third layers, let the rod penetrate about 25 mm into the layer below. Take care not to damage the bottom of the measure on the first layer.
- After rodding each layer, tap the outside of the measure 10–15 times with the mallet to close the voids left by the rod.
- On the final layer, avoid overfilling — about 3 mm of excess concrete above the rim is ideal. Add or remove material as needed before strike-off.
- Strike off the top surface flush with the rim using the flat strike-off plate, with a sawing motion, leaving the measure just level-full.
- Clean all concrete from the outside of the measure, then weigh it full and record the mass (Mc).
Why three layers, and why rod each one?
Filling the measure in one go traps air at the bottom and leaves the top loose, so the test fills it in three roughly equal layers instead. Rodding each layer 25 times spreads the compaction evenly through the whole measure, and letting the rod sink about 25 mm into the layer below knits the layers together, so there is no weak plane between them. Tapping the sides with the mallet closes the holes left by the rod, and striking off flush with the rim fixes the volume exactly — a heaped or hollow top would change the volume and spoil the result.
Common mistakes
- Rodding only the top layer, or counting the strokes unevenly across the surface.
- Skipping the mallet taps, which leaves rod holes — empty space — inside the measure.
- Weighing the measure with concrete stuck to the outside, which adds mass that is not inside the known volume.
- Using a dented measure without recalibrating its volume.
- Testing a sample that has been left standing and started to stiffen, instead of a fresh sample.
If a result looks wrong, check these five things before blaming the mix — most bad results come from the test itself, not the concrete.
Calculation
Density (unit weight)
D = (Mc − Mm) / Vm
where:
– D = density (unit weight), kg/m³
– Mc = mass of measure filled with concrete, kg
– Mm = mass of empty measure, kg
– Vm = calibrated volume of the measure, m³
Report to the nearest 1 kg/m³ (0.1 lb/ft³).
Yield
Y = M / D
where M is the total mass of all materials batched (kg) and D is the measured density (kg/m³). Y is the actual volume produced, in m³.
Relative yield
Ry = Y / Yd
where Yd is the volume the batch was designed to produce. Ry > 1.00 means over-yield (excess concrete); Ry < 1.00 means the batch is short.
Air content (gravimetric)
A = [(T − D) / T] × 100
where T is the theoretical density of the mix computed on an air-free basis (total batch mass ÷ sum of absolute volumes of ingredients), and D is the measured density. A is the air content in percent.
Worked Example
A 14 L measure is used on a site batch:
- Mass of empty measure, Mm = 5.40 kg
- Mass of measure + concrete, Mc = 39.10 kg
- Calibrated volume, Vm = 14.0 L = 0.0140 m³
Step 1 — net mass of concrete:
39.10 − 5.40 = 33.70 kg
Step 2 — density (unit weight):
D = 33.70 / 0.0140 = 2407 kg/m³
This is right in the normal range for non-air-entrained concrete — no alarm.
Step 3 — yield check. The batch ticket shows total batched mass M = 2421 kg for a designed 1.000 m³:
Y = 2421 / 2407 = 1.006 m³ → Ry = 1.006 / 1.000 = 1.01
The batch is yielding 1% over design — within normal tolerance, and the producer is not short-changing the volume.
Step 4 — air content. The mix design’s theoretical air-free density T = 2485 kg/m³:
A = [(2485 − 2407) / 2485] × 100 = (78 / 2485) × 100 = 3.1%
A plausible entrapped-air value for non-air-entrained concrete. If this had come out at 6–7%, you would be looking at unintended air entrainment (or a faulty air-entraining admixture dose) — and a strength investigation.
Practical note: Do this arithmetic on site, not back at the office. A density 80–100 kg/m³ below design is your cue to check the batch ticket, retest, and hold the pour before the concrete is in the formwork.
Second worked example — a quick site check
A smaller 10 L measure is used for a quick check on a delivery:
- Calibrated volume of the container, Vm = 0.01 m³
- Mass of container + concrete = 27.5 kg
- Mass of empty container = 3.5 kg
Step 1 — mass of concrete: 27.5 − 3.5 = 24 kg
Step 2 — unit weight: 24 / 0.01 = 2,400 kg/m³
That is exactly the usual design value for normal-weight concrete, so this delivery is on target. The same arithmetic with a lighter result — say 23 kg of concrete in the same container, giving 2,300 kg/m³ — would be a 100 kg/m³ shortfall, and worth investigating before the pour goes on.
Typical Unit Weight Ranges
| Concrete type | Typical unit weight |
|---|---|
| Normal-weight concrete | 2200–2500 kg/m³ (design commonly taken as 2400 kg/m³) |
| Lightweight concrete | 1600–2000 kg/m³ |
| Heavyweight concrete (radiation shielding etc.) | 3000–3800 kg/m³ |
For the unit weights of the individual materials that go into the mix — cement, aggregates, steel — see the unit weight of construction materials guide.
Factors Affecting the Unit Weight
The unit weight is not a property of the cement alone. It is the combined result of everything in the mix and how it was placed in the measure, so when a result drifts, the cause can be in the materials, the batching, or the test itself. The four factors that matter most on site are the air content, the water–cement ratio, the aggregate type, and the compaction.
- Aggregate type and density. The aggregate is ~70% of the volume, so it dominates. Normal crushed stone gives ~2400 kg/m³ concrete; lightweight expanded-clay aggregate drops it toward 1800 kg/m³.
- Air content. Every 1% of additional air reduces density by roughly 20–25 kg/m³. This is exactly why the test doubles as an air-content check.
- Water content. Extra water beyond the design increases the measured fresh density slightly but weakens the hardened concrete — a high unit weight is not automatically good news.
- Water–cement ratio. A higher water–cement ratio means more water for the same cement, which shifts the fresh density away from the design value and weakens the hardened concrete.
- Compaction in the measure. Under-rodding leaves entrapped air and a falsely low reading; over-enthusiastic rodding can drive out intended entrained air. Follow the 25 strokes + mallet taps consistently.
- Moisture in aggregates. Wet aggregates weigh more, and if the batch plant doesn’t correct for free moisture, both the density and the water-cement ratio shift.
- Cement content. Richer mixes (more cement, denser than water) run slightly heavier; lean mixes run lighter.
Practical note: When a delivery’s unit weight drifts from the trial-mix value, check in this order: (1) was the measure filled and rodded correctly — rerun the test; (2) compare with the batch ticket weights; (3) check air content with a pressure meter. Nine times out of ten, the answer is in one of those three.
Measured vs Theoretical Unit Weight
The measured unit weight only tells the full story when you compare it with the theoretical unit weight from the mix design. The theoretical value is the total mass of all the materials in the batch divided by the sum of their absolute volumes, worked out on an air-free basis — in other words, what the concrete would weigh per cubic metre if it contained no air at all.
Work out the difference in kg/m³, and as a percentage of the theoretical value. A small gap is normal: even non-air-entrained concrete traps 1–3% air, so the measured value sits a little below the theoretical one. In the worked example above, the theoretical density was 2,485 kg/m³ and the measured density 2,407 kg/m³ — a gap of 78 kg/m³, or about 3.1% air, which is exactly what you would expect.
A big gap is a warning. If the measured value is much lower than the theoretical value, the mix probably carries too much air, or the batching is short on cement or aggregate — or the measure was under-compacted, so rerun the test first. If the measured value comes out higher than the theoretical value, the mix probably has less air than designed. Either way, do not just file the number away: record the measured and theoretical values side by side on the test sheet and check the batch ticket. A drift that grows over several deliveries points at the batching plant, while a one-off low result points at that load — or at the test itself.
Unit weight hints at strength but does not measure it – the compressive strength of concrete is tested on cured cylinders or cubes after 7 or 28 days.
Frequently Asked Questions
What is the unit weight of normal concrete?
About 2400 kg/m³ (150 lb/ft³) is the standard design value. Measured fresh values typically fall between 2200 and 2500 kg/m³ depending on the mix and air content.
What is the difference between density and unit weight?
Strictly, density is mass per volume (kg/m³) and unit weight is weight per volume (kN/m³). In construction practice the terms are used interchangeably, with kg/m³ the common site unit.
How accurate is the unit weight test?
ASTM C138 reports density to the nearest 1 kg/m³, and the single-operator standard deviation is about 10 kg/m³ — so two properly run tests on the same sample should agree within roughly 30 kg/m³.
Can the unit weight test detect extra water in the mix?
Indirectly. Added water changes the density only slightly, but it changes the yield and the slump noticeably. Use the unit weight test together with the slump test — a high slump and a low density points at water or air problems.
Why is my concrete’s unit weight lower than the design value?
The usual suspects: higher-than-designed air content, under-compaction in the measure (rerun the test first), lighter aggregate than the design assumed, or a shortfall in cement content. Work through them in that order.
How often should I test the unit weight on site?
Test every trial mix and the first loads of a pour, then again whenever the mix, the materials or the supplier changes, or a delivery looks or feels wrong. If the project specification sets a testing frequency, follow it. One test takes about five minutes, so test when in doubt rather than guessing.
What should I do if the result is much lower than 2400 kg/m³?
Do not just carry on pouring. Rerun the test on a fresh sample first — poor rodding is the commonest cause of a low reading. If the low result stands, check the batch ticket weights and measure the air content with a pressure meter, then tell the engineer and hold the pour until the cause is found.
Does the unit weight test tell me the strength of the concrete?
Not directly. It is a test on fresh concrete, while strength is measured on cured cylinders or cubes. But a low unit weight caused by high air content or poor compaction usually goes with lower strength, so treat it as an early warning, not as a strength result.
Can the test be used for lightweight concrete?
Yes — the procedure is the same. Only the expected result changes: lightweight concrete typically measures 1600–2000 kg/m³, so judge it against its own mix design, not against 2400 kg/m³.
The unit weight test is five minutes of weighing that protects everything downstream — yield, air content, and batching accuracy. Run it on trial mixes to lock in your baseline, then on site deliveries whenever something looks or feels off. The scale rarely lies.
Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.
Mohsin Raza Adil
BSc Civil Engineering Technologist (NTC-registered), Pakistan. I write practical, field-tested guides on construction materials, soil testing and site engineering — the way it is actually done on site.
