Moisture–Density Relationship (Proctor Test)

When a soil is compacted, its density depends on how much water it holds. Dry soil resists compaction because friction between the particles holds them apart. As water is added it lubricates the particles, and the same compactive effort packs them more tightly. Past a certain point, though, the water itself starts to fill space that soil particles could occupy, and the dry density falls again. The water content that gives the highest dry density for a given effort is the optimum moisture content, and that density is the maximum dry density.

R.R. Proctor described this relationship in 1933 while building earth dams in California, and the laboratory tests that measure it still carry his name. The results are the reference that field density tests are compared against: a specification that asks for "95 percent compaction" means 95 percent of the maximum dry density from a named Proctor test (Christopher et al., 2006¹).

Standard and Modified Tests

Two levels of compactive effort are in common use:

• Standard Proctor (AASHTO T 99, ASTM D698) drops a 2.5 kg (5.5 lb) rammer 305 mm (12 inches), compacting the soil in three layers.
• Modified Proctor (AASHTO T 180, ASTM D1557) drops a 4.54 kg (10 lb) rammer 457 mm (18 inches), in five layers.

Each layer gets 25 blows in the 101.6 mm (4 inch) mold or 56 blows in the 152.4 mm (6 inch) mold. The modified test applies roughly four and a half times the energy of the standard test. It was developed after World War II for airfields carrying heavy aircraft and is now common for base courses and heavily loaded subgrades. The modified test gives a higher maximum dry density at a lower optimum moisture content than the standard test on the same soil (Samtani and Nowatzki, 2006²).

Each standard has several methods (A to D) that pair a mold size with the sieve the material is screened on, the 4.75 mm (No. 4) or the 19.0 mm (¾ inch). The coarser the soil, the larger the mold. Material retained on the screen is left out of the test, which is why field results on coarse soils need an oversize correction (see Field Density of Soils).

Running the Test

A sample is dried enough to break up, screened, and split into several portions. Each portion is mixed with a different amount of water, typically in steps of 1 to 2 percent, chosen to bracket the expected optimum with at least two points on each side. For each portion the technician:

1. compacts the soil into the mold in layers, with the specified number of rammer blows spread evenly over each layer;
2. trims the top flush with the mold and weighs the mold and soil to get the wet density; and
3. takes a sample from the middle of the specimen for an oven-dry water content.

The dry density of each point follows from the wet density and the water content:

The dry densities are plotted against water content and a smooth curve is drawn through them. The peak of the curve gives the maximum dry density and the optimum moisture content (Texas DOT, 2011³).

Some clays and soils with aggregations of fines change when they are dried and rewetted, so many procedures allow or require preparing each point from moist soil rather than reusing one dried portion. Soils that break down under the rammer should not be reused from point to point.

The Zero Air Voids Curve

A useful check is the zero air voids (ZAV) curve: the dry density the soil would have if every void were filled with water. No compacted point can lie above it.

The wet side of a good Proctor curve runs roughly parallel to the ZAV curve, a little below it; at optimum, most soils still have a few percent air voids. A point plotting on or above the ZAV curve points to an error in a weight, a water content, or the specific gravity used (Samtani and Nowatzki, 2006²).

Calculator

Moisture-density (Proctor) curve

AASHTO T 99 · T 180

Fits a curve to your compaction points for maximum dry density and optimum moisture, with the zero-air-voids line.

Units

US customary SI (metric)

TestAASHTO T 99 (standard effort, 5.5 lb rammer, 12 in. drop)AASHTO T 180 (modified effort, 10 lb rammer, 18 in. drop)

Enter each point asMass of mold and wet soil, with moistureDry density, with moisture

Mass of mold and base plate, lb

Volume of mold, ft³

From standardizing the mold with water. A 4 in. mold is about 0.0333 ft³ (944 cm³); a 6 in. mold about 0.075 ft³ (2,124 cm³).

Compaction points

Point	Mold and wet soil, lb	Moisture, %
1
2
3
4
5
6

Specific gravity of the soil, G_s

For the zero-air-voids line. Measured by AASHTO T 100, or an assumed value such as 2.70.

CalculateExample values. Aim for three points dry of optimum and two wet of it.

One-Point Checks and the Family of Curves

A full Proctor test takes most of a day, and the soil on a project changes from place to place. Agencies handle this in two ways.

The first is to check a field sample against a family of curves: a set of Proctor curves developed in advance for the soils of one source or project, plotted on one chart (AASHTO R 75). In the field the technician compacts a single point of the sample (AASHTO T 272), plots its wet density and water content on the chart, and reads which curve it falls on. That curve's maximum density becomes the reference for that test. For the result to be valid, the point has to be compacted somewhat dry of optimum, close enough to the peak that it identifies one curve clearly.

The second, where no family exists, is a one-point test that is compared with a full test on the same material, run in the laboratory. If the two disagree by more than the agency allows, a new full test is run.

Things That Go Wrong

• Too few points, or none past the peak. A curve with all points on one side of optimum doesn't locate the peak. Add a point.
• Wrong method for the gradation. Testing a coarse soil in the small mold, or screening out too much, gives a reference density that doesn't represent the field material.
• Water content samples not representative. Taking the moisture sample from the top of the specimen, or letting it dry before weighing, shifts every point.
• Specific gravity guessed. An assumed specific gravity can put the ZAV check in the wrong place. Use a measured value where it matters.

Standard Test Methods

• AASHTO T 99, ASTM D698: Moisture–density relations of soils using a 2.5 kg (5.5 lb) rammer and a 305 mm (12 in.) drop
• AASHTO T 180, ASTM D1557: Moisture–density relations of soils using a 4.54 kg (10 lb) rammer and a 457 mm (18 in.) drop
• AASHTO T 272: One-point method for determining maximum dry density and optimum moisture
• AASHTO R 75: Developing a family of curves

@misc{pavint-moisture-density-relationship-proctor-test,
  author = {{Pavement Interactive}},
  title = {Moisture–Density Relationship (Proctor Test)},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/soil-tests/moisture-density-relationship-proctor-test/}},
  note = {Accessed [date]}
}