Measure the Strength of Subgrade Soil with CBR Test Machines

1. Measure the Strength of Subgrade Soil with CBR Test Machines California bearing ratio (CBR) is a test to measure the strength of highway subgrade and sub base, as well as subgrade soils through a penetration test. It is primarily designed to determine the cohesive materials with particle sizes that are smaller than 19mm. CBR test machines are used to make testing faster and more reliable in yielding accurate results. The California bearing ratio test was developed in the 1930s by the California Division of Highways, and it has since been adopted by other states and countries around the world. A standard CBR test requires load application to a small penetration piston at 1.3mm per minute. The total range to be recorded is from 0.64mm to 7.62mm. CBR test machines are used following standard test methods like ASTM D 1883 (Bearing Ratio of Laboratory Compacted Soils) and AASHTO T 193 (California bearing ratio). CBR testing was initially designed and developed to measure soil’s load-bearing capacity for road construction, but it can be used to measure the load- bearing capacity of soils under paved airstrips and unimproved airstrips. A high CBR rating corresponds to a hard surface. For instance, a tilled farmland may have a CBR of three, moist clay or turf can get 4.75, and moist sand may get a 10. High-quality crushed rocks typically get a CBR that is greater than 80. CBR machines come in different types to serve laboratory and field testing requirements. Laboratory machines typically come with a dial gauge and load ring or digital gauge to measure the piston travel and force. They should have a 50 kN capacity and they must be able to meet two travel speeds, which are 1mm/min and 12.27mm/min. Automatic CBR test machines are computerised with varying ram speeds from 0.5 mm/min to 5 mm/min. They are compatible with testing and data acquisition software, and the built-in system can record corrected CBR values along with percentage of CBR. Automatic testing machines offer load control or displacement and a test graph display in real-time. Output can be saved as an Excel document for later reference. CBR test machines for field testing are best used for sites that have been undisturbed by any construction activity. That way, you can saturate the test sites to acquire conservative data.