What is the CBR? Everything You Need to Know

What is the CBR? Everything You Need to Know

Posted by Certified Material Testing Products on 22nd May 2023

First developed by the California State Highway Department in the 1930s, the California bearing ratio test is now a worldwide standard test method for assessing the strength of subgrade and subbase materials.

In this post, we'll take you through everything you need to know about the CBR test, including why it's so important and how CertifiedMTP can help with your CBR testing needs.

What is the California Bearing Ratio (CBR)?

The California Bearing Ratio is an all-important index used in all construction and civil engineering projects. The CBR ratio indicates the strength and load-bearing capacity of the ground, which is a vital factor to consider before you can begin any construction project.

As the name suggests, the California Bearing Ratio was first used in California in relation to its road construction program. Over time, it has become the international standard to measure load values.

You determine the CBR value by comparing the force required to penetrate a soil sample to the force needed to penetrate a standard crushed rock material through a penetration test. The resulting ratio from the CBR test, expressed as a percentage, is the CBR value.

Why is the California Bearing Ratio Important in Construction?

Even construction novices should appreciate the need to understand the CBR values for any given potential construction area. The CBR value generated by the CBR test provides valuable insights into the suitability of the ground for construction, particularly for roads and highways.

For example, if the CBR test procedure indicates a high CBR value, it means that the soil has a high resistance to penetration. This implies a strong, stable, firm ground suitable for construction.

It's not rocket science to follow that a CBR test indicating a low California Bearing Ratio may signal that the ground might not withstand heavy loads. This scenario tells project managers that the ground soil is of subgrade strength; it will need soil reinforcement or a different construction approach.

<23>How Do You Measure CBR?

We've mentioned it a couple of times already. Measuring the California Bearing Ratio requires a special penetration test called a CBR test.

You can carry out this procedure either on-site or in a laboratory setting. To do so, you need a specifically designed loading machine.

Brace yourself for the step-by-step process of the CBR test!

You start the process by preparing a soil sample, either collected directly from the proposed construction site or with laboratory compacted soils. Next, you use something called a penetration piston. You use this to achieve a specific and constant rate of penetration into the soil and record the force required to achieve the same penetration depths in the soil sample and in the standard crushed rock.

With these measurements, we create a load penetration curve. This curve effectively illustrates the force required at each depth of penetration. We can find the maximum ratio by comparing this curve to the standard values obtained from tests on well-graded crushed stone. This ratio is what we call the CBR value.

But what does it all really tell us?

Understanding CBR Tests Results

The results of your CBR test tell you exactly how suitable the ground is for your planned construction project. The percentage you get will determine your next course of action.

A score of 100% indicates incredibly hard, firm ground as hard as standard crushed rock. This is a rare occurrence. Most natural ground surfaces have a CBR value below 80%, while soils with a lot of clay or chalk will often have a value below 10%.

  • The percentage dictates what you must do next:
  • You may need to abandon the project if the ground is unworkable
  • You may need to employ ground improvement techniques to improve soil strength
  • You may need to opt for less-heavy construction materials
  • You may need to alter the budget for the project, based on changes mentioned above

This is why understanding and accurately determining the CBR test results to find a precise value is crucial for the success of your projects.

How CertifiedMTP Helps With CBR Testing

Here at CertifiedMTP, we understand the critical role of the California Bearing Ratio in your construction projects. That's why we provide high-quality equipment designed to deliver accurate and efficient CBR testing.

Our California Bearing Ratio (CBR) Equipment is built to last, offering long-term soil testing capabilities. We also try to prioritize equipment with compact designs that optimize your laboratory space if you do in-lab testing.

Critically, you'll find all our equipment meets the requirements of the rigorous ASTM test for the California Bearing Ratio, providing strength measurements for lab-compacted soil, in-situ soils, and base course materials.

Discover the Capabilities of our CBR Testing Equipment

Leveraging our California Bearing Ratio equipment, you'll have several options to help you conduct various tests to help you determine soil strength.

Leveraging our California Bearing Ratio equipment, you'll have several options to help you conduct various tests to help you determine soil strength.

Our flagship Manual CBR Press features distinct handles for platen positioning and soil testing operation. Also, the CBR Accessory Kit comes with all the essentials, including molds, surcharge weights, swell plates, and filter screens that meet ASTM D1883 and AASHTO T193 specifications.

If it's a specific product you need instead of the larger ticket items, we offer everything you could possibly need for your CBR test:

These offer just a selection of what's available. There are various options for each product type through CertifiedMTP.

Explore our extensive range of California Bearing Ratio Equipment available for purchase. Visit Certified MTP for an assortment of tools and equipment perfect for testing a broad spectrum of materials, including concrete strength testing and cement expansion testing.

Final Thoughts

The California Bearing Ratio (CBR) is an indispensable measurement for anybody involved in the construction industry. The information the CBR value gives you in relation to the load-bearing capacity of the ground is a vital consideration for any building project.

The results help you ensure the safety and structural integrity of a project.

Our equipment and tools meet the rigorous standards of the ASTM CBR test, meaning you can confidently assess the strength of your lab-compacted or in-place soils, as well as base course materials.

From the Manual CBR Press with its distinct operation handles to our comprehensive CBR Accessory Kit, CertifiedMTP delivers on the best tools for your testing needs.

Remember, understanding the California Bearing Ratio is one thing, but having the right tools to measure it accurately is what makes the real difference. Here at CertifiedMTP, we're committed to providing you with those precise tools and the knowledge to use them effectively.

Reach out to us today to learn more about our products and how they can support your construction testing needs.