Unit Weight and Yield of Fresh Concrete

The unit weight test measures the density of fresh concrete: the mass of a known volume. It takes about ten minutes and uses a container most technicians already carry, often the base of the pressure air meter. On its own a density is just a number, but combined with the batch ticket it answers two important questions: is the customer getting the volume of concrete they paid for, and has anything changed in the mix (Taylor et al., 2019¹)?

Normal-weight paving concrete usually has a density of roughly 2,200 to 2,400 kg/m³ (140 to 150 lb/ft³).

The Test

The test (AASHTO T 121, ASTM C138) uses a rigid, watertight measure of known volume, calibrated by filling it with water of known temperature. The technician fills the measure in layers, consolidates each layer by rodding (or by internal vibration for stiff concrete), taps the sides to close the voids left by the rod, strikes off the surface flush with a flat plate, cleans the outside, and weighs it (Western Alliance for Quality Transportation Construction, 2025²).

Yield

Yield is the volume of concrete actually produced by a batch. It comes from the total mass of everything in the batch, read from the batch ticket, divided by the measured density:

Dividing by the volume ordered gives the relative yield:

A relative yield above 1.00 means the batch produced more concrete than ordered, usually because it has more air or water than designed. A relative yield below 1.00 means underyield: less concrete than ordered, so the customer runs short and each cubic meter holds more cement than intended. Ready-mixed concrete specifications (ASTM C94, AASHTO M 157) treat the volume delivered as the batch mass divided by the measured density, so this test is how a volume dispute is settled.

Actual Cement Content

The cement actually in each unit volume is the mass of cement batched divided by the yield:

where $N_t$ is the mass of cement (or total cementitious material) in the batch. If a batch underyields, $N$ is higher than the mix design; if it overyields, lower.

Gravimetric Air Content

If the absolute volume of each ingredient is known (from the mix design's masses and specific gravities), the theoretical density of the concrete with no air can be calculated. The difference between that and the measured density is air:

Because it depends on accurate specific gravities and batch weights, the gravimetric air content is best used as a check on the pressure or volumetric test, not instead of it (Taylor et al., 2019¹).

A Worked Example

This example uses made-up numbers. A truck ticket shows a 7.65 m³ load with these batched masses (aggregates as weighed, wet):

The unit weight test gives D = 2,290 kg/m³. Then:

• Yield, Y = 17,690 ÷ 2,290 = 7.72 m³
• Relative yield, Ry = 7.72 ÷ 7.65 = 1.01
• Cementitious content = (1,950 + 490) ÷ 7.72 = 316 kg/m³

The load slightly overyields. A relative yield of 1.01 is within normal testing variability; a value of 1.03 or more on several loads would be worth checking against the air content.

Calculator

Concrete unit weight, yield and gravimetric air

AASHTO T 121 · ASTM C138

Weigh a filled measure of known volume, then take the batch masses from the ticket. A low density usually means more air or water than designed, and more volume than was paid for per sack of cement.

Units

US customary SI (metric)

Mass of the measure, M_m, lb

Mass of measure and concrete, M_c, lb

Volume of the measure, V_m, ft³

From the measure’s standardization: the mass of water that fills it ÷ the density of water at that temperature.

From the batch ticket

Total mass of the batch, W, lb

Cement and other cementitious materials, aggregates as batched, and all the water: at the plant, in transit and on site.

Cementitious materials in the batch, N_t, lb (optional)

Volume ordered (design yield), yd³ (optional)

From the mix design

Air-free (theoretical) density, T, pcf (optional)

The batch masses ÷ their absolute volumes, with no air. Leave blank to skip the gravimetric air content.

Specification

Relative yield minimum (optional)

T 121 sets no limit; use your agency’s, if it has one. Leave blank for no check.

Relative yield maximum (optional)

Air content minimum, % (optional)

From your specification’s exposure class.

Air content maximum, % (optional)

CalculateExample values: one truckload. Your agency sets any relative yield or air limits.

Using Unit Weight for Control

Plotting density on a control chart for each mix through a job gives an early warning (see Control Charts). A drop in density usually means more air or more water; a rise, less air. Either change should prompt an air test and a look at the batch ticket.

Standard Test Methods

• AASHTO T 121, ASTM C138: Density (unit weight), yield and air content (gravimetric) of concrete
• AASHTO M 157, ASTM C94: Ready-mixed concrete

@misc{pavint-unit-weight-and-yield-of-fresh-concrete,
  author = {{Pavement Interactive}},
  title = {Unit Weight and Yield of Fresh Concrete},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/cement-tests/unit-weight-and-yield-of-fresh-concrete/}},
  note = {Accessed [date]}
}