The Illinois Center for Transportation recently completed a project sponsored by the Illinois Department of Transportation to address concerns about the long-term performance of asphalt concrete mixtures that use higher amounts of asphalt binder replacement such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS). The use of RAP and RAS in asphalt concrete mixes has environmental and economic benefits because those materials help reduce demand for virgin aggregates and asphalt binder; however, the impact on pavement performance must be verified.
In the project, Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS, ICT researchers developed a simple yet effective performance index to predict the asphalt concrete mixture’s resistance to cracking obtained from a semi-circular bending (SCB) fracture test also known as the Illinois Flexibility Index Test, or I-FIT. The measure is called the flexibility index, and it ensures that asphalt concrete mixtures have the flexibility necessary to resist premature cracking.
I-FIT is performed on a 2-inch-thick puck cut from 6-inch gyratory-compacted asphalt concrete or on 6-inch cores taken from in-place pavement. The pucks are cut in half to form two semi-circular samples. The samples are notched, and a semi-circular bending test is performed on a three-point loading apparatus. The test calculates two parameters, fracture energy and the flexibility index (FI). Fracture energy represents the overall energy required to fracture the specimen. The FI indicates the flexibility of the material and its ability to delay cracking. In general, the larger the FI number, the more resistant the mixture is to premature overall cracking-related damage.
The different stages of the SCB test.
ICT director Imad Al-Qadi says of I-FIT: “We finally have a simple, rapid, affordable, robust, and reliable test that allows engineers and contractors to have better control of the development of durable, crack- and rutting-resistant asphalt mixes that provide long-term performance. Our project’s findings show that I-FIT is very successful in predicting cracking resistance.”
IDOT is planning to implement I-FIT by constructing 11 pilot projects using its new I-FIT specification. That specification requires I-FIT testing and compliance for mix design verification and plant-produced mixtures.
In addition, a new AASHTO provisional standard has been introduced for I-FIT: “Determining the Fracture Potential of Asphalt Mixtures Using Semicircular Bend (SCB) Geometry at Intermediate Temperature” (TP-124). The project in which I-FIT was developed and validated was selected as an AASHTO Sweet 16 High Value Research project for 2016.
The I-FIT software can be downloaded from ICT’s website, as shown in the screenshot below. There is also a video available online.