Disk Shaped Compact Tension Test (DCT) for Asphalt
The disk shaped compact tension test, or DCT, checks how well an asphalt mix resists cracking when it is very cold. A notched disk is pulled apart at a controlled rate, and the energy it takes to grow the crack is the result. It was built to predict the thermal cracks that open across northern roads every winter.
Asphalt tonnage calculator →The basics
- Standard: ASTM D7313.
- Specimen: a disk 150 mm in diameter and 50 mm thick, cut from a gyratory specimen or a field core, with two loading holes and a notch cut toward the center.
- Temperature: typically 10 °C above the project's low PG temperature.
- Control: the test is run at a constant crack mouth opening rate measured by a clip gauge across the notch, not at a constant load rate.
- Result: fracture energy in J/m², the area under the load versus opening curve divided by the ligament area (the length of uncut material times the thickness).
Why test at a cold temperature
Thermal cracks form when the pavement shrinks faster than the binder can relax. That happens on cold nights after a sharp drop. The low-temperature PG grade tells you the binder's limit; the DCT checks whether the whole mix, with its stone skeleton, can hold together near that limit. For background on the binder side see the low-temperature number on a PG grade and the bending beam rheometer.
Step by step
- Compact or core the sample and cut the disk to thickness.
- Drill the loading holes and saw the notch.
- Condition the disk at the test temperature for several hours.
- Mount it in the grips, attach the clip gauge and pull until the load falls to a small fraction of the peak.
- Calculate fracture energy from the recorded curve.
Example of reading results (made-up numbers)
Two surface mixes are tested at −24 °C for a PG 58-34 project. Mix A records 520 J/m² and Mix B 380 J/m². Mix A absorbed about 37 percent more energy before breaking, which suggests better resistance to thermal cracking. Whether either passes depends on the agency's minimum, which commonly rises with traffic level.
What improves DCT fracture energy
- A softer low-temperature binder grade or a polymer-modified binder.
- Adequate effective binder content; dry mixes tend to score low.
- Limiting or offsetting high recycled binder contents.
- Good field density, because air voids weaken the ligament.
Practical limits
The DCT needs a saw, a core drill, a cold chamber and a closed-loop test frame, so it is slower than quick-QC tests such as the IDEAL-CT. It targets cold cracking specifically; fatigue and rutting need other tests. For a broader view of the lab work behind a mix, see asphalt testing basics.
Why homeowners might care
Thermal cracking is the long, straight crack that crosses a driveway or road every 20 to 50 feet in cold country. A mix designed with cold-weather testing in mind will usually crack less. Your tonnage still depends on area and thickness, which the asphalt tonnage calculator works out.
Questions people ask
What does the DCT test measure?
Fracture energy: the work needed to grow a crack through a notched asphalt disk at a cold temperature, reported in joules per square meter.
What standard covers the DCT test?
ASTM D7313, the standard test method for determining fracture energy of asphalt mixtures using the disk-shaped compact tension geometry.
What temperature is the DCT run at?
Usually 10 °C warmer than the low-temperature PG grade for the project. A PG 58-28 climate would be tested at about −18 °C.
Is a higher DCT fracture energy better?
Yes. More energy to break means the mix can tolerate more thermal stress before a crack forms and travels.
Which states use the DCT?
It has been used most in cold-climate states, with Minnesota among the early adopters, and in research on low-temperature cracking nationally.
How is DCT different from the bending beam rheometer?
The BBR tests the binder alone for stiffness and relaxation. The DCT tests the whole mix, aggregate and binder together, for fracture resistance.
Related guides
- Transverse cracking
- Bending beam rheometer
- The low-temperature number on a PG grade
- Asphalt testing basics
- Superpave mix design
As an Amazon Associate, AsphaltTons.com earns from qualifying purchases. Product links below are sponsored.