The California bearing ratio test (CBR) measures a given material’s strength. That’s a big deal because we need strong materials to make the roads, runways, and other infrastructure we depend on every day. In this article, we’ll cover:
Let’s begin!
CBR is short for California bearing ratio. This test measures how much a given material—such as soil or asphalt—resists penetration. This resistance is the bearing capacity. Engineers compare the material’s bearing capacity to that of a standard crushed aggregate sample to get the bearing ratio.
But how did California get into the mix? In the late 1920s, the California Division of Highways (now known as the California Department of Transportation or “Caltrans” for short) developed the CBR test to help create better quality roads that could withstand the state’s increasing vehicle traffic.
Today, the CBR is most common for road construction, airport runways, parking lots, and other paving projects. Many state transportation departments, the Federal Highway Administration, and the Federal Aviation Administration use it.
The California Division of Highways invented the r-value test in the late 1940s with the intention of improving the CBR. The r-value test is more extensive, including three different pressure testing components, while the CBR includes only one. However, the CBR remains the more widely used of the two tests in most parts of the U.S.
CBR testing gives us a number called the CBR value, which represents how strong soil, aggregate, and paving materials are. The higher the CBR value, the stronger the material. The more we know about a material’s strength, the better we can predict how it will perform when traffic drives over it or when environmental conditions impact the roadway.
Engineers use the CBR value to determine design parameters, like the thickness of a road’s subgrade and pavement. With that information, they can help contractors decide how to strengthen the material they have on-hand or whether to replace it with something stronger.
For example, imagine that a roadbuilder performs a CBR test on the soil for an unpaved road, and the soil has a low CBR value. That weak soil is likely to erode and shift, which could cause structural failures. Now, the roadbuilders know they need to stabilize the soil so it can withstand traffic.
The U.S. has two standardized CBR tests: the ASTM D-1883 and the AASHTO T-193. To keep things simple, we’ll provide a high-level overview of a laboratory CBR test for soil. The basic steps of this test are:
First, engineers collect three to five soil samples. CBR testing works best on particles less than 0.75 inches wide, so loose soil is great for this test. Its finer particles deliver more consistent results because the testing equipment contacts more of the soil’s surface area.
Engineers generally prepare CBR soil samples the same way they would prepare samples for a laboratory proctor test.
First, they weigh the soil to meet CBR testing standards. Next, they place each soil sample into a mold and compact it, usually in five layers called lifts. Using multiple lifts ensures thorough compaction throughout the sample. This process is crucial because it simulates compacting soil on the jobsite to create a road’s subgrade.
Engineers soak the compacted samples in water for 96 hours to account for real-world conditions, like heavy rainfall and flooding, that could lower soil’s strength. (Labs sometimes skip this step for CBR testing in extremely dry regions since the soil will rarely get wet.)
After 96 hours, they’ll measure how much the soil swelled and how much water it absorbed. That information is important because wet soil can swell under roadways and cause structural damage. By accounting for soil’s reaction to moisture during construction, roadbuilders can prevent structural failures due to changing moisture.
Engineers put the soil samples into a CBR testing machine known as a load frame. They put a ring-shaped weight called a surcharge load over the samples to simulate overburden pressure, which is the pressure that soil subgrade will experience under the layers of aggregate and pavement in a road.
The load frame has a standard-sized penetration piston, usually 50 millimeters in diameter, that applies pressure to the soil. Sensors measure the piston’s applied pressure and the soil’s vertical displacement—in other words, how much the soil moves down under a given amount of pressure.
A screen attached to the load frame displays these values for the engineer to see during the test. They log the soil’s penetration load ratings at 0.5, 1, 1.5, 2, 2.5, 4, 5, 7.5, 10, and 12.5 millimeters.
Then, engineers use that data to calculate the soil’s CBR value as a percentage of the actual load, where 100% represents the standard sample. They display the CBR value in a graph. Interestingly, each type of soil tends to deliver a consistent range of results on the CBR test. Check out the graph to see for yourself.
After completing the CBR test, engineers remove each soil sample from its mold to test its moisture content. They break up the sample, weigh it, dry it in an oven, and then re-weigh it to calculate both the current and optimum moisture content.
Optimum moisture content is the amount of water soil must contain to compact properly. Proper compaction is essential for road construction because it maximizes the road’s strength and keeps the soil subgrade from shifting under the pavement.
Sometimes, geotechnical engineers perform CBR tests in the field instead of the lab. That may happen because of quick timelines or tight budgets, since lab testing often takes longer and costs more, or because the contractor has run into a problem onsite that needs an immediate solution.
The field CBR test runs on essentially the same principles as the lab test, with a few adjustments. For instance, the engineer won’t soak the soil for 96 hours. And instead of a load frame, they put the soil samples into a gear-driven load jack. They’ll brace the jack against a piece of heavy equipment—like an excavator or loaded dump truck—instead of using surcharge loads. CBR field test results also tend to rely on a mechanical dial, instead of a digital screen.
With all these adjustments, CBR tests can be convenient and useful in the field. However, they may not be as accurate as lab CBR results. Generally, field CBR tests tend to show slightly higher CBR values than lab tests, so soil may appear stronger on the field test than it really is. It’s important to keep these differences in mind and stabilize your soil accordingly.
