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Asphalt mixture performance tests help agencies, contractors, and researchers
improve asphalt performance and prolong service life of flexible pavements.
This video is part of a series on asphalt performance testing
and demonstrates the Semi-Circular Bend Test, or SCB, at intermediate temperatures.
The data from this test can be used as a performance indicator to determine an asphalt mixtures’ cracking
resistance at climatic intermediate temperatures.
This test is based on "ASTM D8044-16: Standard Test Method for Evaluation of Asphalt Mixture Cracking Resistance
using the Semi-Circular Bend Test at Intermediate Temperatures."
All ASTM and AASHTO standards mentioned in this video are voluntary standards and are not required under Federal law.
The SCB test uses a half-disk test specimen with different notch depths cut parallel to the
loading and vertical axis. The notch depths are 25 millimeter, 32 millimeter, and 38 millimeter.
Load is applied along the SCB specimen’s vertical radius and the load and load line displacement,
or LLD, are measured during the entire duration of the test. The load is applied in a way to
maintain a constant LLD rate of 0.5 millimeter per minute for the duration of the test.
Equipment and supplies for this test include: Axial loading device;
Precise control and data acquisition system with a sampling rate of 10 Hertz or more;
LVDTs; Environmental conditioning chamber;
PTFE sheets; SCB Test fixture;
Four SCB test specimens at each notch depth of 25 millimeter, 32 millimeter, and 38 millimeter;
Dummy sample with temperature probe; Masonry saw.
Laboratory-prepared test specimens should be long-term oven conditioned as suggested in AASHTO R30.
Field cores do not need to be conditioned prior to testing.
Fabricate at least three SGC samples with a diameter of 150 millimeter by 120 millimeter thick
using a Superpave gyratory compactor. Target air voids to 7 percent plus or minus 0.5 percent
according to ASTM D6925. To prepare the semi-circular shaped
test specimens, cut a 150 millimeter by 120 millimeters SGC sample into
two identical 57 millimeter thick circular disks from the middle of the SGC sample.
Cut the disk into two identical semi-disks along its central axis. The height or radius of the two
samples should be within 1 millimeter of each other.
For the field cores, the dimensions of the cores should be 150 millimeter in diameter
and between 38 and 60 millimeter in thickness. If the thickness of the core
is greater than 60 millimeter, it should be trimmed to 57 millimeter and cut similar to
the laboratory prepared test specimen. Next, cut a straight vertical notch along
the axis of symmetry of each semi-circular specimen. A minimum of four semi-circular
specimens should be tested at each notch depth of 25, 32, and 38 millimeter.
The notch width should be less than 3.5 millimeter. Determine the test temperature based on
the climatic intermediate performance grade temperature as described in the standard.
Place a dummy specimen with a temperature probe in the chamber at the same time as the test specimen
to determine when the test specimen reaches the target testing temperature.
If a dummy specimen is not available, condition the test specimen for two hours plus or minus 30 minutes.
Inspect the fixture to ensure all contact surfaces are clean and free of debris.
Place PTFE sheets or pads on the bottom support roller to reduce friction between
the specimen and testing fixture. Mount the specimen in the testing device,
ensuring it is centered, level, and makes uniform contact on the support rollers.
To conduct the test, enter the information in the test controller.
Set the test temperature of the environmental conditioning chamber to the specified test temperature.
Allow the chamber to equilibrate.
Once the test temperature is reached,
apply a 45 Newton preload to the specimen for a maximum duration of 30 seconds to ensure
the sample is seated properly. Verify proper level and position, the release the load.
Now the test is ready to start. Apply load to the specimen in displacement
control at a rate of 0.5 millimeter per minute. Ensure that time, force, and displacement are
measured and recorded at a sampling rate of 10 Hertz or more.
The test can be terminated when fracture failure occurs
or when the applied load decreases to 25 percent of peak load.
When the test is complete, remove the specimen.
The process should be completed for each notch depth.
The load and vertical deformation relationship at each notch depth is recorded.
Analyze the test data and calculate the strain energy
for each test specimen at each notch depth.
Compute the critical strain energy rate as described in the standard.
This engineering property is a cracking performance indicator
of the asphalt mixture at intermediate temperature.
An SCB test data report should include the following parameters:
Asphalt mixture type; Test temperature, in Celsius;
Specimen air voids, in percentage;
Specimen thickness, in millimeters; Notch depth per sample, in millimeters;
Strain energy to failure per notch depth, in kilojoules;
Average strain energy per notch depth, in kilojoules;
Standard deviation of strain energy per notch depth, in kilojoules;
Coefficient of variation of strain energy per notch depth, percentage;
Coefficient of determination of standard deviation of U versus notch depth, percentage;
and J-integral value in kilojoules per square meter.
For more information about asphalt testing procedures or other pavement testing resources,
visit fhwa.dot.gov.
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