# Intermediate-Temperature Cracking Tests

IDEAL-CT and I-FIT: quick room-temperature cracking tests on asphalt mixture, used in balanced mix design and production, and what their indexes mean.

- Canonical URL: https://pavementinteractive.org/reference-desk/testing/asphalt-tests/intermediate-temperature-cracking-tests/
- Last updated: 2026-11-01
- Standards: AASHTO T 393, ASTM D8225

Volumetric mix design controls air voids, VMA and binder content, but it doesn't measure how a mixture will crack. As mixtures took on more recycled binder, softer and stiffer additives, and new binders, agencies wanted a direct check. Two simple tests have become the most widely used for intermediate-temperature cracking, the cracking that comes from repeated traffic and aging rather than from extreme cold:

- the **Indirect Tensile Asphalt Cracking Test (IDEAL-CT)**, ASTM D8225 and Texas Tex-250-F; and
- the **Illinois Flexibility Index Test (I-FIT)**, AASHTO T 393 (formerly the provisional TP 124).

Both run at room temperature, about 25°C, on specimens made in a Superpave gyratory compactor or cut from cores, and both finish in minutes. They are central to **balanced mix design**, in which a mixture must pass both a rutting test such as the Hamburg wheel-tracking test and a cracking test. Older performance tests are described in [HMA Performance Tests](https://pavementinteractive.org/reference-desk/testing/asphalt-tests/hma-performance-tests/) and [Laboratory Wheel Tracking Devices](https://pavementinteractive.org/reference-desk/testing/asphalt-tests/laboratory-wheel-tracking-devices/).

## IDEAL-CT

The IDEAL-CT uses an ordinary cylindrical gyratory specimen, 150 mm in diameter and 62 mm tall, with no cutting, notching or gluing. It is loaded across its diameter between two curved strips, like an indirect tensile strength test, at a constant rate of 50 mm per minute until it fractures. The load and displacement are recorded throughout (Texas DOT, 2026[^1]).

From the load–displacement curve:

- the **failure energy**, $$G_f$$, is the work done to break the specimen (the area under the curve) divided by its cross-sectional area (diameter × thickness);
- the **post-peak slope**, $$m_{75}$$, is the slope of the curve after the peak, taken around the point where the load has fallen to 75 percent of the peak; and
- the **deformation tolerance**, $$l_{75}$$, is the displacement at that 75-percent-of-peak point.

These combine into the [cracking tolerance index](https://pavementinteractive.org/glossary/ct-index/):

$$CT_{index} = \frac{t}{62}\cdot\frac{l_{75}}{D}\cdot\frac{G_f}{|m_{75}|}\times 10^{6}$$

where:

- $$t$$ = specimen thickness, mm
- $$D$$ = specimen diameter, mm
- $$l_{75}$$ = displacement at 75 percent of peak load after the peak, mm
- $$G_f$$ = failure energy, J/m²
- $$m_{75}$$ = post-peak slope at 75 percent of peak load, N/m

A higher CT index means the mixture tolerates more deformation and spreads the fracture over a larger, slower failure, which indicates better cracking resistance (Texas DOT, 2026[^1]).

## I-FIT

The I-FIT uses a **semicircular bend** geometry. A gyratory specimen is sliced into discs 50 mm thick and each disc is cut in half. A notch about 15 mm deep is cut in the middle of the flat edge, to force the crack to start there. The half-disc is set flat edge down on two rollers, and loaded at the top of the curve at 50 mm per minute at 25°C. Specimens are prepared to about 7 percent air voids and conditioned at the test temperature for at least two hours (Federal Highway Administration, 2020[^2]).

The result is the [flexibility index](https://pavementinteractive.org/glossary/flexibility-index/), which, like the CT index, divides the fracture energy by the post-peak slope:

$$FI = A\,\frac{G_f}{|m|}$$

where:

- $$FI$$ = flexibility index
- $$G_f$$ = fracture energy, J/m²
- $$m$$ = slope of the load–displacement curve at the post-peak inflection point, kN/mm
- $$A$$ = unit conversion and scaling factor, 0.01

The larger the FI, the better the cracking resistance. Mixtures with little recycled binder often have FI values above 10, while some high-recycled mixtures with a history of cracking test below 4. Agencies set their own minimums (Federal Highway Administration, 2020[^2]).

## Comparing the Two

| | IDEAL-CT | I-FIT |
|---|---|---|
| Specimen | Whole gyratory cylinder, 150 × 62 mm | Half-disc, 50 mm thick, notched |
| Preparation | No cutting | Slicing, halving and notching |
| Loading | Indirect tension across the diameter | Three-point bending |
| Result | CT index | Flexibility index |
| Typical time per specimen | A few minutes | A few minutes, plus cutting |

Both indexes are sensitive to binder content, binder stiffness and aging, recycled binder and air voids, in the directions engineers expect, which is why agencies adopted them. Neither predicts cracking life on its own. Results depend on how the mixture was aged before testing, so specifications state the aging protocol along with the threshold.

## Using the Tests

- **Mix design.** A cracking test result is added to the volumetric requirements, so the designer may adjust binder content, binder grade or recycled content to pass.
- **Production.** Because the tests are fast and inexpensive, some agencies run them on plant-produced mix as a quality control or acceptance check.
- **Aging.** To represent the mixture after some years of service, specimens can be made from loose mix aged in an oven for a set time and temperature before compaction.

## Standard Test Methods

- ASTM D8225: Cracking tolerance index of asphalt mixture using the indirect tensile cracking test at intermediate temperature (IDEAL-CT)
- AASHTO T 393: Fracture potential of asphalt mixtures using the Illinois Flexibility Index Test (I-FIT)
- TxDOT Tex-250-F: Ideal cracking test

## Sources

[^1]: Texas Department of Transportation (TxDOT) (2026). Ideal Cracking Test (Tex-250-F). Materials and Tests Division, Texas Department of Transportation. Austin, TX. https://ftp.dot.state.tx.us/pub/txdot-info/cst/TMS/200-F_series/pdfs/bit250.pdf
[^2]: Federal Highway Administration (FHWA) (2020). Illinois Flexibility Index Test (I-FIT). Mobile Asphalt Technology Center fact sheet FHWA-HIF-21-019. Federal Highway Administration. Washington, D.C. https://www.fhwa.dot.gov/pavement/asphalt/MATC/pdf/illinois_flexibility_index_testing.pdf