The eighth research cycle continued NCAT’s mission of advancing asphalt pavement technology through accelerated performance testing, featuring 17 new test sections and 17 carryover sections. This cycle also introduced the Test Track’s first off-ramp experiments and marked a major milestone—11 million miles driven by the fleet of five heavily loaded trucks.
Balanced mix design (BMD) remained a central focus, with new sections evaluating rutting and cracking resistance compared to traditional Superpave designs. Data from NCAT and companion MnROAD sections are helping agencies refine and implement performance-based mix specifications.
A new Additive Group experiment assessed asphalt mixtures modified with synthetic fibers, recycled plastics, and recycled tire rubber. NCAT sections emphasized fatigue performance under heavy truck traffic, while MnROAD companion cells focused on reflective and thermal cracking in colder climates.
CCPR was explored extensively. A legacy section from 2012 exceeded 40 million ESALs and was slated for 2024 removal and deep forensic analysis. The eighth cycle also piloted re-recycled CCPR and off-ramp studies using CCPR with stabilizing agents and rejuvenators; all performed strongly under accelerated loading.
New sections evaluated reflection cracking treatments and the performance impacts of tack coat to guide construction practices that improve long-term pavement life.
Surface friction research examined mixtures and treatments to improve roadway safety and durability. Carryover sections continued to provide insights into preservation treatments, rejuvenators, and long-term BMD performance.
The seventh cycle of accelerated performance testing began in 2018, providing unique opportunities to determine the field performance of breakthrough materials and concepts without the risk of failure that local and state agencies are unwilling to accept. Research during this cycle primarily focuses on pavement preservation, balanced mix design, cracking tests, and rejuvenators.
The pavement preservation group study is quantifying the life-extending and condition-improving benefits of different pavement preservation treatments on both low-volume and high-volume roadways across northern and southern climates.
By determining the field performance of treatments applied at various pavement life stages, agencies will gain data-driven insights for selecting preservation treatments. A secondary focus of the study is developing specifications and quality assurance guidelines for pavement preservation.
Find out more about our pavement preservation study here.
Many of the same treatments used in Alabama are also being tested in Minnesota using MnDOT materials and methods. The northern pavement preservation experiment includes sections on U.S. Route 169 (high-volume) and County State Aid Highway 8 (low-volume).
Four test sections on the Test Track focus on balanced mix design, sponsored by the Oklahoma and Texas Departments of Transportation. Oklahoma’s approach uses the Hamburg wheel tracking test for rutting resistance and the Illinois flexibility index for cracking resistance.
Texas’ balanced mix design specification requires the Hamburg wheel tracking test for rutting resistance and the overlay test for cracking resistance. Their goal is to compare the performance of balanced mix design asphalt mixes against those designed using the Superpave volumetric approach under accelerated loading.
The cracking group experiment aims to develop and implement performance tests that predict pavement cracking across various distress types found in North America. While several lab tests claim to assess cracking, most lack validation beyond the state where they were developed.
Both MnROAD and NCAT have established test sections to validate laboratory cracking tests by correlating results with real-world performance. This research helps establish specification criteria for cracking susceptibility.
Regular performance monitoring includes assessments of ride quality, distresses, pavement strength, and response.
Cargill is sponsoring research at both the NCAT Test Track and MnROAD to evaluate balanced mix design procedures in asphalt mixes containing rejuvenators and high levels of reclaimed asphalt pavement.
Test sections made with Cargill’s Anova rejuvenator and a 45% reclaimed asphalt mix will be compared to control sections with lower recycled content. Researchers will assess factors such as pavement ride quality, cracking, and rutting under different climate conditions.
Mississippi and Tennessee DOTs have also sponsored rejuvenating seal experiments. NCAT conducted preliminary screening of seven rejuvenating seals, ranking them based on rheological properties and friction test results.
The Mississippi DOT selected ReGenX™ (Blacklidge Emulsions) and Delta Mist™ (Collaborative Aggregates LLC) for evaluation on its S3 Test Track section. The Tennessee DOT selected Reclamite® (Ergon) and e-Fog S (Ergon) for its S4 section. These four rejuvenating fog seals will be evaluated over the two-year Test Track cycle.
Additionally, Section N7 with Delta S®, sponsored by Collaborative Aggregates LLC, continues to be monitored following completion of the 2015-2017 research cycle.
NCAT launched a landmark collaboration with MnROAD to validate asphalt mixture cracking tests for use in balanced mix design and acceptance testing.
Seven new sections at NCAT and eight rebuilt sections at MnROAD studied top-down and low-temperature cracking. After 10 million ESALs, Section N8 (20% RAP + 5% RAS) showed significant surface cracking, while most other sections exhibited little to none. I-FIT, IDEAL-CT, and Overlay Test (OT) aligned best with field performance; Energy Ratio and SCB showed limitations. Continued trafficking was planned to expand the dataset.
ALDOT evaluated three modified OGFC mixtures. After 20 million ESALs, they showed minimal rutting, no raveling, and excellent durability. ALDOT extended evaluation through 2021.
Section N7 (35% RAP + Delta S) was redesigned after early slippage issues; the revised mix (no RAS, adjusted dosage, silo hold) delivered good ride and rutting performance, with cracking comparable to the control.
Asphalt-bound surfaces with calcined bauxite were tested as lower-cost alternatives to resin-bound HFST. Micro-surfacing and thin overlays provided strong friction but did not outperform the “gold standard” bauxite–resin HFST.
FDOT evaluated mixes up to 30% RAP with softer binders; after 10 million ESALs, all four sections performed well with only low-severity reflective cracking.
GDOT’s double chip seal with sand seal significantly outperformed an open-graded interlayer for limiting reflective cracking.
KYTC’s fine-graded mix achieved tighter, more durable joints without sacrificing rutting resistance compared to a coarse-graded control.
MDOT’s redesigned Thinlay with local aggregates and RAP endured 20 million ESALs with no distress. TDOT’s thicker 4.75-mm mix section showed low rutting and no cracking under heavy traffic.
ODOT’s sandstone OGFC mixes with higher tack rates improved interface bond strength and maintained strong friction after 10 million ESALs.
VDOT’s CCPR and FDR studies performed strongly after 20 million ESALs. Stabilized bases reduced strain and are expected to extend pavement life.
2009 Group Experiment: Controls, WMA, and 50% RAP mixes exceeded design expectations after 17+ million ESALs. RAP sections had the least rutting and cracking and showed promise as high-modulus base layers for perpetual pavement design.
Green Group Experiment: Partnering with five DOTs, NCAT evaluated high RAP, RAP/RAS, and GTR-modified binders. Several sections reached cracking thresholds, underscoring the importance of adequate layer bonding. SMA mixes with recycled materials demonstrated strong rutting resistance.
CCPR & Stabilized Base: VDOT sections performed well with no cracking and minimal rutting after 10 million ESALs, validating CCPR for high-volume roadways.
Eight new and one existing PFC sections were evaluated for durability and raveling resistance. Permeability dropped early but stabilized; ALDOT’s three PFC mixes had no raveling or major rutting across two years of trafficking.
On a 45% RAP section, MDOT’s rout/fill and blow/band crack sealing slowed crack progression and extended service life.
ODOT’s perpetual vs. non-perpetual comparison showed the perpetual design had lower life-cycle cost despite higher initial cost. A HiMA section (Kraton) demonstrated exceptional fatigue resistance with a thinner cross-section.
FHWA’s HFST study confirmed calcined bauxite as the top friction performer, while many domestic aggregates still outperformed conventional asphalt. GDOT’s interlayer comparison favored the open-graded interlayer for rutting control.
Two 50% RAP sections showed excellent rutting resistance and no cracking after 10 million ESALs. Softer virgin binders improved durability; high RAP with unmodified binders offered cost savings.
Multiple WMA sections matched HMA performance, with lab testing indicating improved fatigue resistance at lower production temperatures.
Results showed SMA can use a broader range of aggregates without sacrificing performance, helping reduce costs.
Sulfur asphalt, Trinidad Lake Asphalt, highly polymer-modified mixes, and GTR were evaluated. All performed strongly; GTR offered a sustainable, SBS-comparable option.
Sections expected to fail at 10 million ESALs survived 30 million+ with minimal rutting and no fatigue cracking, supporting thinner perpetual designs.
Research recommended increasing the asphalt concrete structural coefficient from 0.44 to 0.54, enabling up to ~20% thickness reduction.
OGFC contributed to structural strength, cracking resistance, skid resistance, and noise reduction. Hamburg, APA, and Flow Number tests were validated as rutting predictors; MEPDG rutting predictions improved with new calibration.
Original 2000 sections and new mill-and-inlay sections with high RAP were assessed for rutting potential. Six RAP sections (20–45%) performed favorably vs. virgin mixes in rutting and cracking resistance.
FDOT validated its energy ratio methodology; the lower-ER section cracked first as predicted. Forensics confirmed mechanistic properties and bond strength were not the cause of early cracking.
GDOT compared aggregate sources and paving methods; dual-layer paving delivered the most effective drainage and noise reduction.
APA and Flow Number tests correlated strongly with field rutting, enabling proposed rut criteria for future use.
Instrumentation in 11 sections supported calibration of mechanistic–empirical design. Field data showed MEPDG over-predicted load duration by ~80% and informed field-based strain thresholds for perpetual design.
Granular material characterization favored the MEPDG/universal models. For dynamic modulus, the Hirsch model was recommended over Witczak models (with caution at low temperatures/high frequencies).
23 sections continued from Phase I. Despite 20 million ESALs, maximum rutting was ~7 mm; most rutting and alligator cracking occurred in the outside wheel path.
22 new sections were added; three structural sections failed due to alligator cracking. Non-failing new sections had ≤ ~9 mm rutting.
Cracking (mostly top-down) appeared in only four non-structural sections and was minor at reporting.
SMA sections showed early densification rutting then stabilized; none cracked. Lower gyration SMA designs (50/75) performed well, suggesting improved durability with higher optimum binder. Modified binders reduced rutting by 50%+ vs. unmodified; fine-graded mixes were less permeable, quieter, and easier to compact while matching rut resistance of coarse-graded mixes.
APA correlated well with rutting. Structural performance matched 1993 AASHTO predictions; instrumentation reliability was high; measured responses compared well to theory; truck wander matched typical highways. Fatigue progression and model calibration were documented.
Moisture and temperature gauges proved highly reliable (>80% accurate after 9 million ESALs). Automated belt/mix sampling improved safety and representation.
Short sections were challenging, but quality was achievable with attention to detail. Transverse joints required diamond grinding to improve smoothness; unsealed ground joints performed well. About 10 million ESALs (≈1.6M miles) were applied, offering valuable logistics insights.
Seven-day max temperature at 20 mm depth averaged 61.4°C, aligning with Superpave predictions. Surface temps peaked near 2:30 p.m.; 10-inch depth around 10 p.m. Rutting followed three stages (initial seating, summer 1, summer 2) and tapered when average max air temps fell below 28°C. Rutting was lower in year two despite higher temperatures.
PG-67 mixes densified more than PG-76 under traffic; PG-76 mixes showed >60% less rutting. Adding 0.5% binder increased rutting in PG-67 mixes (~50%) but had negligible effect in PG-76. Coarse- and fine-graded mixes performed similarly in rutting.
Calculated densification-based rutting exceeded measured rutting, indicating stable mixes. IRI rose modestly (mid-60s to mid-70s in/mile over two years). Skid numbers declined into the 30s; one polishing aggregate section dropped below 20 and was overlaid. Dynamic modulus did not correlate well with rutting; confined repeated load and wheel-tracking did. Many sections continued beyond two years to strengthen lab–field correlations.