ASTM C29: Rodded Unit Weight of Coarse Aggregates

Concrete, Tests
Rodded Unit Weight of Coarse Aggregates Test (ASTM C29)

In short, ASTM C29/C29M determines the bulk density, commonly called unit weight, and the void content of aggregate. At present, the active edition is ASTM C29/C29M-23.

Also, bulk density tells us the mass of dry aggregate that fills a known container under a specified placement condition. Because the volume contains particles and spaces, the value includes the solid particles and the voids between them. Therefore, it is not the same as the particle density or specific gravity measured by ASTM C127 or ASTM C128.

Therefore, the test result is used in concrete mixture proportioning, mass-volume conversions, and aggregate quality control.

Important: ASTM C29 bulk density is determined on a dry basis. However, a hauling unit or stockpile can have different packing and moisture, so the laboratory value should not be treated as its exact in-place density.

ASTM C29 Test Methods

Method Placement condition Typical application
Method A – Rodding Compacted in three layers with a tamping rod Aggregate with nominal maximum size of 37.5 mm or less
Method B – Jigging Compacted by lifting and dropping opposite sides of the measure Larger coarse aggregate within the method limits
Method C – Shoveling Filled without intentional compaction Loose bulk density

In general, ASTM C29/C29M covers fine, coarse, and combined aggregate not exceeding 125 mm nominal maximum size. More specifically, this article focuses on Method A, the rodding method.

Equipment for the Rodding Method

  • Balance or scale meeting the accuracy and capacity requirements of the current standard
  • Calibrated cylindrical metal measure of the required size for the aggregate
  • Straight steel tamping rod, 16 mm [5/8 in.] diameter and approximately 600 mm [24 in.] long, with rounded end
  • Shovel or scoop
  • Glass plate and water for calibrating the measure when its certified volume is not used
  • Dry representative aggregate sample, with enough material to overfill the measure
Equipment used for ASTM C29 rodded bulk density of coarse aggregate
Typical equipment for determining rodded bulk density.

Calibrating the Measure Volume

First, the measure volume must be known accurately. For example, one calibration method is to determine the net mass of water required to fill the measure at a known temperature.

V = (W – M) / D

In this equation:

  • V = volume of the measure
  • W = mass of measure, glass plate, and water
  • M = mass of dry measure and glass plate
  • D = density of water at the calibration temperature

For this reason, do not assume every bucket has its nominal volume. Instead, use the certified volume or complete the calibration required by the laboratory procedure.

ASTM C29 Rodding Procedure

  1. First, dry the representative aggregate sample as required by the method.
  2. Then, determine and record the mass of the clean, dry, empty measure. This is T.
  3. Next, fill the measure to approximately one-third of its volume.
  4. After that, rod the first layer 25 times, distributing the strokes uniformly. Therefore, do not strike the bottom hard.
  5. Then, fill the measure to approximately two-thirds and rod the second layer 25 times. Allow the rod to penetrate slightly into the first layer.
  6. Next, overfill the measure and rod the third layer 25 times. Allow the rod to penetrate slightly into the second layer.
  7. Finally, strike off the surface. For coarse aggregate, balance small projections above the rim with approximately equal voids below the rim.
  8. After strike-off, clean aggregate from the outside of the measure and determine the mass of the filled measure. This is G.
ASTM C29 rodding method showing three equal aggregate layers and 25 uniformly distributed strokes per layer
Fill the measure in three approximately equal layers and apply 25 uniformly distributed strokes to each layer.

Bulk Density and Void Content Formulas

Then, calculate the bulk density using the following equation:

Bulk density, M = (G – T) / V

Where:

  • M = bulk density of dry aggregate, kg/m3 or lb/ft3
  • G = mass of measure plus aggregate
  • T = mass of empty measure
  • V = volume of the measure

For example, use kilograms with cubic metres or pounds with cubic feet.

After that, when the dry bulk specific gravity is known, calculate the void content as:

Voids, % = 100 × [(S × D) – M] / (S × D)

  • S = bulk specific gravity of aggregate on a dry basis
  • D = density of water used with the selected unit system

As a result, the calculator below reports the bulk density in both SI and inch-pound units.

ASTM C29 Bulk Density Calculator






Dry aggregate mass: 15.850 kg
Bulk density: 1585 kg/m3 (98.9 lb/ft3)
Calculated void content: 40.1%

Worked Example

For example, assume the following measurements:

  • Filled measure mass, G = 24.85 kg
  • Empty measure mass, T = 9.00 kg
  • Measure volume, V = 0.010 m3
  • Dry bulk specific gravity, S = 2.650

First, subtract the empty measure mass:

Dry aggregate mass = 24.85 – 9.00 = 15.85 kg

Next, divide the dry aggregate mass by the measure volume:

Bulk density = 15.85 / 0.010 = 1585 kg/m3

Finally, calculate the void content:

Void content = 100 × [(2.650 × 998) – 1585] / (2.650 × 998) = 40.1%

Do Not Confuse Dry Bulk Density With SSD Mass

In summary, ASTM C29 reports the bulk density of dry aggregate. However, if a concrete mix calculation requires an SSD aggregate mass, make the moisture and absorption correction separately using the ASTM C127 or ASTM C128 results and the actual aggregate moisture condition.

For this reason, do not rename an absorption-corrected value as the ASTM C29 bulk density. Therefore, keep the measured dry result and the batch moisture correction as two separate steps in the mix-design record.

Common Test Errors

  • Using the wrong measure size for the aggregate grading
  • Assuming the nominal bucket volume instead of using the calibrated volume
  • Failing to dry the sample as required
  • Using unequal layers or fewer than 25 strokes per layer
  • Rodding only the center instead of distributing strokes over the surface
  • Forcing the rod through the entire previous layer or striking the measure bottom hard
  • Packing the surface during strike-off
  • Leaving aggregate on the outside of the measure during weighing
  • Mixing SI and inch-pound units in one calculation

How the Result Is Used

In practice, dry-rodded bulk density is commonly used when proportioning coarse aggregate by bulk volume. For example, it can help identify changes in grading, particle shape, or aggregate source when the same test procedure is repeated consistently.

For this reason, a result should be compared with the approved source data and previous test history. However, one number alone does not prove segregation or contamination because grading, shape, moisture preparation, and test execution can all affect the measured density.

References and Related Tests