# Unit Weight and Yield of Fresh Concrete

The fresh concrete density test and what it tells you: yield, relative yield, actual cement content and gravimetric air, with worked equations.

- Canonical URL: https://pavementinteractive.org/reference-desk/testing/cement-tests/unit-weight-and-yield-of-fresh-concrete/
- Last updated: 2026-11-01
- Standards: AASHTO M 157, AASHTO T 121, ASTM C138, ASTM C94

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., 2019[^1])?

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, 2025[^2]).

$$D = \frac{M_c - M_m}{V_m}$$

where:

- $$D$$ = density (unit weight) of the concrete
- $$M_c$$ = mass of the measure filled with concrete
- $$M_m$$ = mass of the empty measure
- $$V_m$$ = volume of the measure

## Yield

[Yield](https://pavementinteractive.org/glossary/yield-concrete/) 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:

$$Y = \frac{W}{D}$$

where:

- $$Y$$ = yield, m³ (or ft³)
- $$W$$ = total mass of all materials batched: cement, supplementary materials, coarse and fine aggregate (as batched, including their moisture), water and admixtures
- $$D$$ = measured density

Dividing by the volume ordered gives the [relative yield](https://pavementinteractive.org/glossary/relative-yield/):

$$R_y = \frac{Y}{Y_d}$$

where:

- $$R_y$$ = relative yield
- $$Y_d$$ = volume the batch was designed or ordered to produce

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:

$$N = \frac{N_t}{Y}$$

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:

$$A = 100\,\frac{T - D}{T}$$

where:

- $$A$$ = air content, percent
- $$T$$ = theoretical air-free density: total batch mass divided by the sum of the absolute volumes of the ingredients
- $$D$$ = measured density

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., 2019[^1]).

## 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):

| Material | Mass, kg |
|---|---|
| Cement | 1,950 |
| Fly ash | 490 |
| Coarse aggregate | 8,250 |
| Fine aggregate | 5,850 |
| Water (batched plus free water on aggregate) | 1,150 |
| Total, W | 17,690 |

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 — https://pavementinteractive.org/tools/concrete-unit-weight-yield/

## Using Unit Weight for Control

Plotting density on a control chart for each mix through a job gives an early warning (see [Control Charts](https://pavementinteractive.org/reference-desk/qc-qa/statistical-acceptance/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

## Sources

[^1]: Taylor, P.; Van Dam, T.; Sutter, L. and Fick, G. (2019). Integrated Materials and Construction Practices for Concrete Pavement: A State-of-the-Practice Manual, Second Edition. National Concrete Pavement Technology Center, Iowa State University, for the Federal Highway Administration. Ames, IA. https://intrans.iastate.edu/app/uploads/2019/05/IMCP_manual.pdf
[^2]: Western Alliance for Quality Transportation Construction (WAQTC) (2025). Transportation Technician Qualification Program: Concrete Workbook. WAQTC/NAQTC. Hosted by the Alaska Department of Transportation and Public Facilities. Published October 2025. https://dot.alaska.gov/stwddes/desmaterials/mat_waqtc/assets/pdf/waqtcmanuals/workbook_concrete.pdf