What Is a CBR Test and When Is It Used?

What a CBR test is

A California Bearing Ratio (CBR) test measures how strongly a compacted soil or aggregate resists a standard piston pushed into it, compared with a standard crushed stone, usually limestone. The result is a percentage: a CBR of 10 means the material carried 10% of the reference load at the same penetration. It is used mainly to rate subgrades, subbases and bases for flexible pavement design.

The core question it answers

Before designing how thick a pavement, driveway or parking lot structure needs to be, engineers need to know how strong the underlying (subgrade) soil actually is. The CBR test provides a standardized, indirect measure of that strength.

The test came from California highway practice — its procedure and equipment were described by O. J. Porter of the California Highway Department — and a 1942 Highway Research Board paper reports that U.S. Army engineers had adopted it, on compacted and soaked specimens, for designing flexible runway pavements. CBR is not a fundamental material property, but it is relatively easy and inexpensive to run and has a long history in pavement design, so many highway agencies still use it.

When engineers use CBR

CBR is run where a pavement design method or specification calls for it — the agency and the design method decide, not the test itself. Typical uses:

• Roads and highways: rating the subgrade so a flexible pavement's thickness is designed for the actual soil rather than assumed.
• Airport pavements and parking lots, which are also commonly designed with CBR-based agency methods.
• Larger driveways and private roads, where the design is engineered rather than built to local practice.
• Fill, base and subbase acceptance: checking imported fill or granular base against a specification that sets a minimum CBR.
• Site investigation: field CBR or dynamic cone penetrometer tests give a quick read on in-place strength and its variability, with laboratory tests to confirm the correlations.

How the test works

A cylindrical piston with an end area of 3 in² is pushed into a compacted specimen at a steady 0.05 in (1.27 mm) per minute while the load is recorded. The loads at 0.1 in (2.54 mm) and 0.2 in (5.08 mm) of penetration are compared with the loads the standard crushed stone carries at the same penetrations.

• Specimens are compacted in a 6 in mold, at the moisture and density the engineer specifies — often those expected in the field.
• For design, specimens are commonly soaked for 96 hours (4 days) under surcharge weights that represent the pavement above, and swell is measured during soaking.
• Each set of conditions is typically tested on three identically compacted specimens, and series at different densities or moisture contents show how CBR changes with them.

Calculating the CBR

CBR = load on the specimen ÷ standard load at the same penetration × 100. ASTM D1883's standard loads are 3,000 lbf (about 13.345 kN) at 0.1 in and 4,500 lbf (about 20.017 kN) at 0.2 in. The 0.1 in value is normally reported; if the 0.2 in value is higher, the test is rerun, and the 0.2 in value is used if the check test confirms it.

The guide to calculating CBR covers the ASTM and IS 2720 standard loads, correcting the load-penetration curve and a worked example. The CBR Calculator does the arithmetic and applies the reporting rule.

Lab vs. field testing

Most CBR testing is done in the laboratory, typically on compacted, soaked specimens. ASTM's in-place method was D4429, which tests the soil at its field moisture content; ASTM withdrew it in 2018 with no replacement, so check which in-place procedure the project specification names. Field and laboratory values are not directly interchangeable and often need soil-specific correlations.

A faster field alternative is the dynamic cone penetrometer (ASTM D6951), whose penetration rate is converted to an estimated CBR with published correlations that differ by soil type.

What CBR values mean

CBR is a ratio, not a pass mark: whether a value is adequate depends on the traffic, the design method and the specification. For orientation, the Federal Highway Administration's pavement geotechnics manual reproduces the historical field CBR ranges below, from a 1953 U.S. Army Corps of Engineers source. They are reference values, not design criteria — project design values come from testing the actual material under the conditions the agency's design method specifies.

For comparison, the same manual gives a CBR of about 100 for the granular base and about 30 for the granular subbase at the AASHO Road Test.

How the result is used

A low CBR means a weak subgrade, which needs a thicker base or stabilization to spread traffic loads. Some design methods use CBR directly; others use resilient modulus (Mr), and the 1993 AASHTO Guide lists Mr (psi) ≈ 1,500 × CBR for subgrade soils with a CBR of 10 or less. Correlations like this are estimates, and the local agency's method should govern.

For how base material spreads load over a weak subgrade, see Road Base Materials Explained.

What affects the result

• Moisture — the same soil can test far lower when soaked; as-compacted values at optimum moisture can be significantly higher than soaked ones. See What Is Soil Moisture Content?.
• Density — better-compacted soil gives a higher CBR, so field compaction must reach the density assumed in the design. The In-Situ Dry Density Calculator works out percent compaction from a field sample.
• Specimen preparation — surcharge, soaking time and the flatness of the specimen surface all affect the curve.

Common mistakes

• Designing from unsoaked CBR values where the subgrade will get wet in service.
• Reporting the 0.2 in value without the confirming retest.
• Assuming field compaction will match the lab specimen without checking in-place density.
• Using a typical-values table as a design value instead of testing the actual soil.