In construction materials laboratories, the California Bearing Ratio (CBR) test is used to assess the strength of soil subgrades and base course materials. CBR test values are used by those who build and engineer highways, airport runways and taxiways, parking lots, and other pavements to determine pavement and foundation thicknesses.
CBR Test Methods
The ASTM D1883 and AASHTO T 193 laboratory test technique for the CBR test is a simple empirical approach that compares the test specimen's resistance to penetration to that of a "standard" sample of well-graded crushed stone material using a standard-sized piston. Aside from penetration resistance, the test does not characterize any other soil qualities. The FM 5-515 Limerock Bearing Ratio (LBR) Test was created by the Florida Department of Transportation as a modified version of this method.
CBR testing on soil subgrades can also be done in-place with an equipment that uses the same size penetration piston or a dynamic cone penetrometer (DCP). ASTM D4429 is frequently used as a preliminary test to establish in-place soil or foundation conditions or to corroborate laboratory results.
Why CBR Test Is Important
Economic expansion fueled the desire for more miles of better-quality roads in the early 1900s, resulting in greater traffic. The CBR test was devised by engineers at the California Division of Highways (now Caltrans) in 1928 and 1929 to confirm that pavements could be built affordably while still carrying the predicted axle loads. They used the penetration resistance of optimum crushed-rock foundation material as a benchmark against which all other soil and base course materials were measured. The CBR test was first used in California in 1935, and it is now a standard method in ASTM, AASHTO, the US Army Corps of Engineers (USACE), British Standards, and other organizations. State Departments of Transportation and the Federal Highway Administration all recognize the importance of CBR values in pavement and runway design.
CBR Test Procedure
CBR Laboratory Test Preparation
The majority of the time spent in the geotechnical laboratory testing CBR samples is spent on sample preparation. The materials that will be used in the study are sampled and processed in the lab to create a series of test specimens.
The same or equivalent steps used in ASTM D698 or D1557 (AASHTO T 99 and T 180) moisture/density relationship (Proctor) tests are used in the preparation. The CBR test specifier may request that the dry density of the specimens be changed. The dry density can be adjusted by changing the needed number of blow counts. Spacer discs, surcharge weights, and other expansion measuring equipment are also required.
The test is most commonly used to determine the strength of cohesive materials with maximum particle sizes of less than 19 mm (0.75 in), although it can also be used to determine the strength of granular soils and aggregates. Before the penetration test, three to five samples are prepared and immersed for a length of time. The prepared test specimens are placed in a load frame and exposed to regulated penetration stresses from a piston with a 3in2 (1,935mm2) surface area. Loads are measured and graphically shown at predetermined penetration intervals.
In the CBR test, there are several sample preparation test procedures that can be used. A single sample can be tested at optimum water content (as defined by ASTM D698 or D1557), or a series of three test specimens can be tested at optimum water content (as determined by ASTM D698 or D1557).
Soaked CBR Test Specimens
Most CBR testing employ soaking to account for adverse moisture conditions from predicted rainfall or flooding. In addition to the compaction procedure, each specimen is normally soaked in water for 96 hours before to the penetration test.
A surcharge load of 10lbf (4.54kg) or more must be applied on the sample during the soaking time using surcharge weights to approximate the weight of the pavement and other loads required. Before penetration testing, the soil swell caused by soaking must be measured using expansion measurement apparatus and swell plates placed on the sample. Preparing several compacted specimens for a single test requires a significant amount of time. To maximize testing, complete CBR testing sets featuring numerous CBR molds and components are provided.
CBR Penetration Test
The sample is kept in the mold for penetration testing. The piston is seated against the test sample with the perforated plate and the necessary surcharge weights in place on the soil, and loading begins at a rate of 0.05in (12.7mm) per minute. Test loads are recorded at eleven predefined depths up to 0.500in as the piston enters the soil (13mm).
Penetrations vs. load readings are shown on a graph and rectified according to the test standard's method. Standard stresses of 1,000psi (6.9mPa) and 1,500psi (10.3mPa) are divided by corrected stress values from penetration depths of 0.100 and 0.200in (2.54 and 5.08mm), then multiplied by 100. CBR values often drop as penetration increases. The CBR value is the stress at a penetration depth of 0.100in (2.54mm). The ratio at the 0.200in (5.08mm) penetration depth is occasionally higher than the 0.100in depth, necessitating a repetition of the test. If the check test yields similar findings, the reported CBR is the stress from the 0.200in penetration depth.
Field CBR Test
Field CBR testing is done on soil subgrades in order to determine in-place soil and base conditions or to corroborate laboratory test results. For loading, the ASTM D4429 test technique necessitates the use of specialized CBR field equipment. Because soil saturation can be controlled in the lab but not in the field, a direct correlation between the two types of tests is challenging.
Note: This is an ASTM standard that has been withdrawn but will be reintroduced as a new or improved test method soon.
DCP Test
Another field test that is frequently used to assess in-place CBR values is the Dynamic Cone Penetrometer (DCP). The conditions for using this test method for pavement design applications are outlined in ASTM D6951, and the measured values are widely accepted.
DCP test equipment is more portable than field CBR apparatus and is often easier to set up in remote areas. A cone-shaped point is forced down through soil layers using a sliding hammer, measuring penetration resistance. The DCP can also be used to define soil strata and relative strengths to a depth of at least 39 inches (1 meter).
CBR Values
California bearing ratios from field tests show the material's in-situ strengths under current field conditions and might not always correspond to laboratory CBR values for the same material. For soaking laboratory samples, saturation levels are near 100%. Because field materials are rarely more than 80% saturated, they are more resistant to penetration. The standard test methods' significance and use sections provide thorough interpretation instructions. The graph below gives an idea of what to expect in terms of laboratory and field CBR values for various soil types.
CBR Testing Equipment
CBR Lab Equipment (ASTM D1883):
- CBR penetration testing requires a top quality laboratory load frame with a capacity of 10,000lbf (44.5kN) or more and the ability to operate at a strain rate of 0.05in (1.3mm) per minute.
- With CBR Testing Components for load and displacement measurement, the load frame will be ready to execute precise, repeatable testing.
- A Load Ring, Penetration Piston, and Dial Indicator Set will suffice for occasional CBR testing.
- Gilson offers a Digital CBR Component Set for efficient testing of many samples since it optimizes data collecting, calculation, and reporting. A 10,000lbf Load Cell, Penetration Piston, 2in Linear Variable Displacement Transducer, and a Two-Channel Data Readout are all included in the kit.
- CBR Data Acquisition Software uses real-time data collection to expedite the calculating, graphing, and reporting process, and plots results to ASTM/AASHTO specifications.
- The CBR Test Set contains all of the components needed to execute the CBR lab test.
- Molded by CBR
- Annular Surcharge Weight of 5 lbs
- Slotted Surcharge Weight of 5 lbs
- Plates for Swelling
- Filter Paper CBR
- CBR Disc Spacer
- CBR Swell Tripod
- Mechanical Dial Indicator 1x0.001in
Compaction of the soil For compacting soil samples for testing, hammers are used.
Equipment for CBR Field Testing (ASTM D4429):
- The CBR Penetration Piston is the same as the one used in the laboratory, but different equipment is needed to generate enough resistance to load the piston.
- Field CBR Test Apparatus consists of a number of components that are positioned against a response load, such as a big truck or a weighted framework, and which apply force to the penetration piston via extension rods that are spaced properly. Surcharge plates imitate loads from above materials, while a support bridge isolates the dial gauge from the test area's loads.
- 2-speed or 3-speed transmission Jack with a Rotary
- Extension Rods and Connectors
- Field Surcharge Plates, 10 lb and 20 lb
- Bridge of Support
- Mechanical Dial Indicator 1x0.001in
ASTM D6951 DCP Test Equipment:
Drive rods and cone-shaped tips truncated to a point at a 60° angle make up the Dynamic Cone Penetrometer Set (with a single or dual-mass hammer). The cones are forced to penetrate the soil layers by a sliding drop hammer installed on the rods, and the number of hammer blows per increment of penetration indicates penetration resistance. The DCP is the greatest option for testing in remote places or doing several tests in a short amount of time.
Resources:
- The most often used standard for this test is ASTM D1883 Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils.
- ASTM D1883 is closely mirrored by AASHTO T 193.
- ASTM D4429 Standard Test Method for CBR (California Bearing Ratio) of Soils in Place (Withdrawn 2018) ASTM is currently considering revising or replacing this standard.
- Standard Test Method for Using the Dynamic Cone Penetrometer in Shallow Pavement Applications (ASTM D6951). The DCP is extensively used to estimate field CBR values.
- CRD-C654-95, Standard Test Method for Determining the California Bearing Ratio of Soils, is a standard test method for determining the California Bearing Ratio of soils developed by the United States Army Corps of Engineers (USACE). In December 1995, MIL-STD-621A was replaced.
- FM 5-515, Florida Department of Transportation The Florida Method of Test for LIMEROCK BEARING RATIO (LBR) is a test for soil materials unique to the Southeastern United States that uses methods and equipment comparable to CBR tests.
- The British Standard BS 1377-1:2016 Methods of Test for Soils for Civil Engineering Purposes, which contains nine parts, was amended in July 2016. A method for determining the CBR value of a compacted or undisturbed soil is included in Part 4, Compaction Related Tests.
- The CBR Test is described in Chapter 5, Section 5.4, of the Federal Highway Administration's (FHWA) Geotechnical Aspects of Pavements Reference Manual.
- Airport Pavement Design and Evaluation is a Federal Aviation Administration (FAA) guideline. For CBR testing in the lab or on the ground while designing flexible pavements for airports or airfields. See 2.5.6 for more information.
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