Colorado Public Works Journal, Spring 2021

10 | THE ROAD AHEAD WWW.CO-ASPHALT.COM INDUSTRY NEWS The National Center for Asphalt Technology (NCAT) Pavement Test Track is a world- renowned accelerated pavement testing facility that combines full-scale pavement construction with live, heavy truck trafficking for rapid testing and analysis of asphalt pavements. Located on a 309-acre site, the Test Track is a 1.7 -mile oval comprised of 46 200-foot test sections sponsored on three-year research cycles. Since its original construction in 2000, findings from this unique facility have helped improve specifi- cations for aggregate, binder and mix design as well as more cost-effective asphalt pave- ment design methods. The research will continue to pay dividends for years to come. NCAT has recently released a summary re- port of key findings thru the end of the sixth research cycle. The focus of their practical NCAT Releases Report Of Key Findings From Test Track Research research is on application of findings that lead to specification improvements agencies can put into contracts, including: • MIX DESIGN • AGGREGATE PROPERTIES • BINDER CHARACTERISTICS • STRUCTURAL DESIGN • TACK COAT • FIELD/LAB RELATIONSHIPS • INTERLAYERS • FOUNDATION SUPPORT • TIRE-PAVEMENT INTERACTION If you would like a copy of the summary report, contact Mike Skinner, CAPA’s Director of Engineering at: RESEARCH FINDING: DESIGN GYRATIONS The Test Track, along with data from NCHRP proj- ect 9-29, showed that the gyratory compaction effort specified in AASHTO standards was too high. The lab compaction effort was not representative of what actually occurs in pavements since high Nde- sign numbers tend to grind aggregate particles and break them down much more than what occurs during construction or under traffic. Mix designers were typically using coarse-graded mixes to meet the volumetric mix design criteria, but those mixes are more challenging to compact in the field and tend to be more permeable, making pavements less durable. Numerous mixes on the Test Track designed with 50 to 70 gyrations in the Superpave gyratory compactor held up to the heavy loading with great performance. As a result, many states have significantly reduced their Ndesign levels. RESEARCH FINDING: ELIMINATION OF THE RESTRICTED ZONE Part of the original Superpave mix design procedure included a restricted zone within the gra- dation band for each nominal aggregate size. In the first cycle of the Test Track, sections with a variety of aggregate size. In the first cycle of the Test Track, sections with a variety of aggregate types proved that mixtures with gradations through the restricted zone were not necessarily susceptible to rutting. The restricted zone was subsequently removed.