Eighth Test Track Cycle Focuses On Innovative Materials

Aggregate traveling to the drum at East Alabama Paving in Opelika, Alabama. 

Operations at the NCAT Test Track began in 2000 to improve materials, tests, specifications and design policies by providing sponsors confidence to move innovations into practice. The only high-speed, full-scale accelerated pavement testing facility of its kind, the 1.7-mile oval consists of test sections sponsored by U.S. highway agencies and the asphalt pavement construction industry. Now in its eighth three-year cycle of accelerated pavement research, the track will include new and continuing experiments to evaluate innovative additives, balanced mix design (BMD) trials for several states and pavement preservation research.

Surface Performance Sections

Surface mix performance sections are built on a robust cross-section that limits distresses to the experimental surface layers. Seven test sections on the track have been milled and inlaid with new mixtures to evaluate how they can stand up to the severe loads from the track’s fleet of five trucks, each weighing in at 156,000 lbs.

Alabama DOT (E9)

Section E9 is a high-performance open-graded friction course (OGFC). The OGFC mixture was designed with a new method requiring gyratory compacted specimens to have air void contents between 15 and 20%, and less than 20% loss in the Cantabro test after four hours of aging at 275°F. To test the new design methodology, the OGFC mixtures incorporated aggregates with a history of marginal performance in Alabama.

Kentucky Transportation Cabinet (S7)

The experiment on S7 aims to establish a BMD+ approach for Kentucky surface mixes that will include popular BMD test criteria as well as provide good long-term friction performance with skid resistance values in the mid 30s and 40s. The experiment began with a laboratory evaluation of several mixes with Kentucky aggregates using NCAT’s Three-Wheel Polishing device followed by periodic friction and surface texture measurements using the Dynamic Friction Tester and Circular Track Meter.

The crew paves Kentucky's balanced mix design Section S7.

North Carolina DOT (W4)

The main goals of Section W4 are to investigate the effects of tack coat type and application rate on bond strength over time to assess the influence of aging and traffic loading. The experiment will also evaluate NCDOT’s specifications regarding the shear stability of high recycled content mixes under high stress conditions. The research will help compare NCAT shear bond strength results to those performed in North Carolina and determine if the test is useful as a forensic tool for investigating projects with early distress.

Tack coat is applied to North Carolina's Section W4 in the Test Track's west curve.

Oklahoma DOT (N8, N9)

Section N8 is a mill and fill section with two layers providing a total thickness of 5 inches. The lower layer is the same mix used in the 2018 cycle as the intermediate layer in Section S1. The surface layer contains 30% reclaimed asphalt pavement (RAP) compared to a 15% RAP content in the surface layer of S1. Both layers were designed as BMD and used rejuvenators to treat the RAP. Strain gauges were installed at the bottom of N8-2 and pressure plates previously used in that section are being reused. The overall purpose of the experiment is to evaluate a recently developed rejuvenator with a substantially higher RAP content in the surface layer.

Section N9 was built as a 1.5-inch mill/inlay using BMD and incorporated 12% recycled tire rubber (by mass of binder) added via the dry process during mix production. The aggregate structure is similar to the previous N9 surface mix that was BMD-optimized and placed at the same 1.5-inch thickness in 2018. In both cases, the perpetual pavement cross-section had a small amount of top-down cracking that extended below the depth of micro-milling. The objective of this research is to assess the performance of the rubber modified mix and its potential to resist reflection cracking from the underlying layer. Top-down and/or reflection cracking with the new rubber modified BMD mix will be compared to the previous conventional BMD mix to assess the impact of the recycled tire rubber on performance.

Oklahoma's Section N9 is compacted by a vibratory smooth drum roller.

Tennessee DOT (S4)

Tennessee DOT has embarked on an important study to define the limits of their proposed BMD specification thresholds. Starting with a mix design that far surpassed TDOT’s proposed BMD thresholds, the mix was “economized” by reducing virgin asphalt binder content, increasing RAP percentage and adjusting the gradation to lower the cracking test index to barely pass the threshold. Considering that TDOT is still a Marshall mix design state, the research team conducted the IDEAL-CT test on specimens prepared using the Marshall hammer to assess the viability of using the compactor for BMD testing. This mix design has been placed on Section S4 and will be monitored for performance.

Texas DOT (N6)

TxDOT is further expanding its efforts to validate their BMD criteria with a new test section to complement its other research on S10 and S11, described below under “Continuing Experiments”. The underlying pavement structure for N6 is similar to those in S10 and S11. The goal of the BMD mix for this new section was to use the same constituent materials that were used in S10 and S11 with a gradation that is between S10 and S11. Although the same aggregate sources and asphalt binder source and grade were available, another RAP material had to be located from the same area as the RAP used in S10 and S11. TxDOT also wanted to target a Texas Overlay test crack progression rate (aka β value) of 0.45 for the mix in this test section to help define the threshold for their BMD specifications. NCAT successfully designed the mix meeting all of the goals, and the new BMD mix was placed on N6 for field validation.

Assistant Research Engineer Grant Julian verifies mixture temperature for Section N6.

Structural Sections

Structural sections have designed pavement thicknesses that closely resemble real-world pavements with embedded strain and pressure sensors that measure pavement responses to loads for validation of mechanistic-empirical pavement design procedures.

Rachel Cousins (left) works with Matthew Sasser (right) to sieve mix for embedding the instrumentation in Section S5 of the Additive Group experiment.

Additive Group (N1, N2, N5, N7, S5, S6)

An experiment to study the impact of various mix additives on pavement life is the core of the structural research effort within the new group experiment. Group experiments are projects that address a national need where the costs of construction, operations and research are shared in a cooperative manner. The Additive Group is sponsored by Alabama DOT, Florida DOT, Mississippi DOT, New York State DOT, Tennessee DOT, Texas DOT and the Federal Highway Administration. Recycled plastic additives, recycled tire rubber additives and aramid fiber additives will be evaluated using balanced mix design methods. The objectives are to comprehensively evaluate the field performance impact of multiple mix additives at the same time, develop a laboratory testing framework to provide a good indication of field performance and establish a framework to evaluate future mix additives with a validated pavement model.

Section S6 of the Additive Group is paved on the south side of the NCAT Test Track.

BASF (S13)

Section S13 is sponsored by BASF Corporation to evaluate their new reactive isocyanate based modifier (B2Last®). This section has the same pavement structure as the Additive Group test sections and is considered to be complementary to that experiment. The only difference between this section and the Additive Group control section is the binder used to produce the mixture contains B2Last® and a linear styrene-butadiene-styrene (SBS). The modified binder is graded as a PG 82-22.

The mix for BASF's Section S13 is transferred from the truck to the paver by a shuttle buggy.

Virginia (S12)

Virginia DOT has continued their sponsorship of cold central plant recycling (CCPR), but this time with a new spin. After the success of Section S12 undergoing 30 million equivalent single axle loads (ESALS) with no deterioration, it was decided to investigate what happens when the CCPR is re-recycled. The surface and CCPR layers were separately milled, the base re-worked to consist of track subgrade with a 6-inch aggregate base, and then the CCPR was re-recycled using a mobile CCPR plant and surfaced with a 2-inch Virginia SMA. The asphalt in the CCPR is now living its third life in an innovative experiment to further explore the circular economy of asphalt pavements.

Rejuvenated CCPR (Off-ramp)

In a first-of-its kind experiment, a rejuvenated CCPR experiment has also been constructed on the Test Track off-ramp. Produced using either a typical mobile CCPR plant or a mobile pugmill, five sections were constructed: a foamed asphalt, an engineered emulsion, two rejuvenated emulsions, and a rejuvenator. Because the rejuvenated mixtures are expected to perform somewhere between a typical CCPR and a hot-mix asphalt (HMA) base mixture, an HMA mixture was placed as the base of a sixth section for comparison. All sections were topped with a 4.75 mm NMAS thin overlay. The sections will be monitored closely and laboratory characterization of all of the mixes has commenced.

Continuing Experiments

Several sections on the Test Track will continue to be evaluated with additional traffic and environmental exposure during this cycle. Detailed information for these test sections is available in NCAT Report 21-03, Phase VII (2018-2021) NCAT Test Track Findings.

Alabama DOT (N10, N11)

This study is evaluating the performance of two thinlay test sections placed in 2018 as a pavement preservation treatment for high volume roads. Section N10 used a 4.75 mm NMAS stone matrix asphalt (SMA) mix, and Section N11 used a 4.75 mm NMAS dense-graded Superpave mix. I-FIT results showed statistically similar FI results for both mixes, however, they were lower than the criterion established for Illinois DOT surface mixes. On the other hand, IDEAL-CT results were significantly higher for the SMA in N10 compared to the Superpave mix in N11. Despite these results, there has been no field cracking; therefore, monitoring of these sections will continue to assess their long-term cracking performance.

Cargill (N3)

This experiment is being continued to evaluate the effectiveness of Anova asphalt rejuvenator with high RAP mixtures within the balanced mix design framework. The experiment compares two surface mixtures placed in 2018. Section N3A was milled and inlaid with a control mixture containing 30% RAP and a PG 64-22 binder. Section N3B was inlaid with a mixture containing 45% RAP, a PG 64-22 binder and Anova asphalt rejuvenator. The BMD test results indicated that mix with Anova asphalt rejuvenator improved cracking resistance without affecting the mixture’s resistance to rutting. So far, the two subsections have shown comparable field performance.

Florida DOT (E5, E6)

The objective of this experiment is to evaluate the effect of density level on pavement performance. The testing plan will also conduct BMD tests on the mixture at the same density levels achieved in the test sections. For this experiment, one asphalt mixture containing 20% RAP and a polymer modified binder was placed and compacted in four 100-foot test strips in Sections E5 and E6 in 2018. Average in-place densities were 87.8, 89.7, 92.0 and 93.5% of G_mm. So far, the lowest density section is the only one to exhibit cracking. An additional round of traffic is needed to better determine the effects of density on performance.

Georgia DOT (N12, N13)

This study aims to evaluate six different reflective cracking treatments including two geosynthetic interlayers, two chip seal systems, an open-graded interlayer mix and a rubber modified asphalt interlayer. After 10 million ESALs of trafficking, no reflection cracking was evident, the surface roughness of all subsections remained about the same, and all sub sections exhibited comparable surface macrotextures. The only meaningful difference in the subsections has been the RAP chip seal subsection (N13A) with a greater rut depth. Trafficking and field performance of all subsections will continue to be monitored in this research cycle.

Mississippi DOT (S2, S3)

Section S2 was constructed in 2018 to evaluate a flexible pavement with a stabilized subgrade and base foundation. The performance of the pavement through one cycle has been excellent. Total rutting was less than 0.15”, no cracking was observed, and smoothness did not appreciably change. Structural health monitoring from imbedded instrumentation also showed no significant changes over time. Measured stresses in the pavement structure followed expected trends, with stresses decreasing with depth and increasing exponentially with temperature of the asphalt concrete. Linkages between performance measurements and structural characterization will provide needed data sets for M-E analysis and design of stabilized foundation sections. This section will remain in place with continued surface performance monitoring (i.e., rutting, cracking and ride quality) and structural response measurements (i.e., stress, strain measurement and FWD testing). To further investigate and validate the findings from S2, MDOT is also constructing an instrumented test section on State Route 76 near Tupelo, Mississippi.

Mississippi DOT sponsored an experiment on Section S3 to evaluate the field performance of two spray-on rejuvenator products over time. The study is examining the short- and long-term effectiveness of the rejuvenators to renewing asphalt surfaces as well as their impact on  friction. The existing eight year old dense-graded surface with no distresses was divided into two subsections: one treated with a plant-based topical rejuvenating seal and the other treated with a proprietary age-regenerating surface treatment. The rejuvenating capability of each product is being assessed by examining rheological parameters and surface friction measurements. After reviewing the results from the first cycle, Mississippi DOT feels that the two rejuvenator products show promising effects and will continue to sponsor S3 for the next research cycle.

Oklahoma DOT (S1)

The main objective of this experiment is to help ODOT with implementation of mixture performance testing and criteria for BMD. Two sections were built in 2018 as mill-and-inlays using asphalt mixtures designed with a BMD approach. Section N9 was placed as a 1.5-inch layer while S1 was placed in two layers with a total thickness of 5.0 inches. ODOT sought to determine if the proposed criteria in their BMD provisional specification were sufficient, or if they needed to be adjusted to achieve good rutting and cracking field performance. In the previous cycle, both sections performed very well with minimal rutting and cracking and exhibited steady smoothness and texture results. Skid numbers declined slightly but remained well above the general safety threshold. Trafficking and monitoring of Section S1 will continue during this research cycle.

South Carolina (S9)

Section S9 was constructed in 2018 as an 8.05” single thick-lift pavement on a prepared crushed granite aggregate base to evaluate the constructability, performance, and structural characteristics of this rapid reconstruction technique. The section has shown excellent performance similar to conventional multi-lift sections with less than 0.25” rutting, very little cracking, and no change in smoothness. Structurally, the temperature-corrected backcalculated AC moduli and measured pavement responses were remarkably consistent over time, indicating good structural health despite the small amount of cracking observed in the section. Laboratory determination of AC dynamic modulus produced consistent data, and either data set could be used for future mechanistic modeling. Cracking tests using the bending beam and cyclic fatigue tests produced vastly different transfer functions and additional trafficking and cracking development will be required for calibration. Since only minor cracking has been observed through 10 million ESALs, further monitoring will help determine an acceptable threshold.

Texas DOT (S10, S11)

In the 2018 research cycle, TxDOT sponsored a BMD surface experiment to compare the field performance of asphalt mixes designed using a BMD approach (S10) versus the traditional volumetric approach (S11). Both sections were built as 2.5-inch mill-and-inlays over an existing asphalt pavement with approximately 15 to 20% cracked lane area to challenge the surface mixes. Both sections performed well after 10 million ESALs. The BMD mix exhibited a little over 0.3” of rutting and the volumetric mix design had a little over 0.2” of rutting, both well below the threshold of 0.5 inches. The BMD mix had considerably less reflection cracking than the volumetric mix design, which was consistent with the Texas Overlay test results. The two sections had similar smoothness and friction characteristics. Both sections will remain in place for additional trafficking and long-term performance evaluation.

United Soybean Board (W10)

A bio-derived polymer produced from the transesterification of glycerol from the triglycerides in epoxidized soybean oil was tested as part of the 2018 research cycle. This bio-polymer, which includes epoxidized benzyl soyate (EBS), is thought to also improve the resistance of asphalt binders to oxidative aging as the epoxide rings within the EBS react to form crosslinks and block nucleophilic sites where asphalt oxidation occurs. In this experiment, the EBS-modified surface mix is compared against a control section (E5A) containing a traditional SBS polymer for texture, rutting and cracking performance. Both sections have carried 10 million ESALs of trafficking with comparable field performance thus far. No cracking was observed for the EBS-modified mixture while low severity cracks were recorded for 0.4% of lane area with the SBS-modified mixture. These sections will be kept in place for continued trafficking, which will be important to evaluate the long-term performance of the bio-polymer modified asphalt against the conventional SBS modified asphalt.

Virginia DOT (N4)

In 2012, Virginia DOT initiated an experiment to evaluate how a cold central plant recycled (CCPR) pavement layer would perform under heavy traffic loads. The experiment also sought to understand the stress and strain distribution in the pavement, as well as identify the failure mechanism and the rate of deterioration to help plan future maintenance and rehabilitation strategies. Three test sections using CCPR performed exceptionally well through two test cycles. In 2018, VDOT decided to continue trafficking on the two that had 4” of HMA over the CCPR layers. After the third test cycle, both of those sections were still performing very well. VDOT has decided to continue trafficking only on Section N4 so that the deterioration rate can be observed. This, along with data collection and forensic analysis at the conclusion of the test, will provide VDOT with insight on how CCPR materials fail, the rate at which they may fail, and validation data that can be used for mechanistic-empirical pavement design.

Pavement Preservation Group

NCAT launched a productive partnership with the Minnesota Department of Transportation’s Minnesota Road Research Project (MnROAD) in 2015 aimed at producing findings that can be directly implemented by a larger geographic base by incorporating a cold-weather climate. NCAT is continuing this collaboration for the Additive Group experiment and continues the research partnership on the pavement preservation group study. This highly controlled experiment includes monitoring numerous preservation treatments on public roadways to quantify the condition-improving benefit curves for each treatment or treatment combination under different levels of traffic and very different climates.

Examining Performance

Trafficking begins this fall and will continue over a period of two years. Each section on the track is subjected to 10 million equivalent single axle loads of heavy truck traffic and performance is closely monitored on a weekly basis. Drivers currently operate NCAT vehicles in order to best induce representative vehicle wander, but autonomous systems are expected to be implemented in the future.

NCAT added two new trucks to its fleet for the track's eighth cycle.

An automated pavement distress data collection vehicle is used to quantify roughness, macrotexture, rutting and cracking in the same manner used by most state highway departments for their pavement management systems. Other tests such as surface friction, falling weight deflectometer, tire-pavement noise and permeability data are also conducted. Similar performance data is conducted for off-track sections on a less frequent basis due to the open traffic on these roadways.

The final part of the three-year cycle will involve forensic analyses of damaged sections in order to determine the contributing factors to pavement distresses. Investigations conducted during this stage include destructive testing such as trenching and coring, as well as additional laboratory testing. Test sections will either be replaced or remain in place for additional evaluation during the ninth research cycle in 2024.