Semi Circular Bend Test (SCB) and the Flexibility Index
The semi circular bend test (SCB) rates how well asphalt mix resists cracking by bending a notched half disk until it breaks. Several versions exist. The best known in the United States is the Illinois Flexibility Index Test, or I-FIT, which turns the load curve into a single flexibility index.
Asphalt tonnage calculator →The specimen
A gyratory cylinder 150 mm across is sliced into disks about 50 mm thick. Each disk is cut in half, giving two semicircles, and a notch about 15 mm deep is sawn in the center of the flat edge. The half disk then sits on two rollers with the notch underneath, and the load is applied at the top of the curve. As it loads, the bottom of the specimen goes into tension and a crack grows up from the notch.
Main versions
| Version | Temperature | Main output |
|---|---|---|
| --- | --- | --- |
| I-FIT (Illinois, first published as AASHTO TP 124) | 25 °C | Flexibility index (FI) |
| Louisiana SCB | Intermediate, about 25 °C | Critical strain energy release rate (Jc) |
| Low-temperature SCB | Near the PG low temperature | Fracture energy and toughness |
How the flexibility index works
The I-FIT loads at 50 mm per minute and records load against displacement. Software finds the fracture energy and the slope of the curve at the inflection point after the peak. The flexibility index is the fracture energy divided by the absolute value of that slope, times a scaling factor. A tough mix that loses load slowly gets a high FI; a brittle mix that snaps gets a low one. Illinois uses the index in mix approval, with different targets for different mix types and aging conditions.
What changes the result
- Binder content and grade: more effective binder or a softer grade tends to raise FI.
- Recycled binder: high RAP or shingle content often lowers it unless compensated.
- Aging: long-term aged specimens score lower and better reflect a road several years old.
- Air voids and notch accuracy: both need tight control or results scatter.
SCB vs other cracking tests
The SCB and the IDEAL-CT both run at 25 °C and both reward toughness plus gradual failure. The IDEAL-CT skips the cutting, which makes it faster for quality control. The disk shaped compact tension test is the cold-weather cousin, aimed at thermal cracking. Most balanced mix design programs pair one cracking test with a rutting test such as the Hamburg.
Example (made-up numbers)
A surface mix records fracture energy of 2,400 J/m² and a post-peak slope of −2.1 kN/mm. With the I-FIT scaling factor of 0.01, FI is about 0.01 × 2400 ÷ 2.1 ≈ 11.4. A second mix with the same energy but a steeper slope of −4.0 kN/mm scores about 6.0, telling the designer that mix fails more abruptly.
Why it matters outside the lab
Mixes that rate well in bending tests usually hold up better over cracked concrete or old asphalt, where reflective cracking is the main threat. For your own paving quantities, use the asphalt tonnage calculator.
Questions people ask
What is the semi circular bend test?
A lab test where a half-disk of asphalt with a notch at its flat edge is loaded at the top of the curve until it cracks. The load curve is used to rate cracking resistance.
What is the flexibility index?
The output of the Illinois version (I-FIT). It divides fracture energy by the post-peak slope and scales the result, so tough mixes that fail gradually score higher.
What temperature is the I-FIT run at?
25 °C, with a loading rate of 50 mm per minute.
Are there other SCB versions?
Yes. A Louisiana version measures critical strain energy release rate at an intermediate temperature using several notch depths, and low-temperature SCB versions are run near the PG low temperature.
How is the SCB specimen made?
A 150 mm gyratory cylinder is sliced into disks about 50 mm thick, each disk is cut in half, and a notch about 15 mm deep is sawn at the middle of the flat side.
Is the SCB better than the IDEAL-CT?
Neither is universally better. The SCB needs more cutting but has a long research record; the IDEAL-CT is faster. Agencies choose based on their own validation work.
Related guides
- IDEAL-CT test
- Disk shaped compact tension test
- Hamburg wheel tracking test
- Asphalt testing basics
- Reflective cracking through an overlay
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