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.
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)
- 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).
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.
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
- 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.
- 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.
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.
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.
