FDOT Research - NCAT

PHASE VIII (2021 – 2023) FUTURE NCAT TEST TRACK

For the new cycle, FDOT is sponsoring traffic continuation on E5 and E6 as well as a slot in the Additive experiment with a focus on aramid fibers.

Research in E5 and E6 is intended to relate surface mix performance to density that varies between a low of 87.8 percent to a high of 93.2 percent as seen below:

E5A = 93.2

E5B = 92.0

E6A = 87.8

E6B = 89.7

Results from the start of this study in Phase VII (2018 – 2020) showed:

• Laboratory cracking test results did not present expected trends in terms of density or air voids. There was no consensus among parameters to define a top performer out of the four mixtures.
• None of the mixtures resulted in considerable amounts of rutting, and all were considered non susceptible to rutting in the laboratory. This agrees with the minimal rutting and no moisture damage observed at the test track.
• At the end of trafficking, field performance was good with cracking at less than 5% of the lane in each section, no changes in roughness, little permanent deformation, and no significant changes in texture.
• Observed cracking in the test sections was classified as low severity.

Multistate Group Experiment to Measure the Performance Benefit(s) of Sustainable and Resilient
Pavement Additive Technologies

Agencies are routinely approached by vendors who promote the use of premium mix additives.  In many cases, it is claimed that the cost increase resulting from specifying a premium mix additive is more than offset by the ability to achieve equivalent performance from a thinner pavement structure.  Independent, peer reviewed research is needed to validate these claims and build an objective framework to evaluate future premium mix additives.  The Sustainable and Resilient Pavement Additive Technologies experiment (referred to in shorthand as the “Additive Group” (AG) study) was developed for the 2021 NCAT Pavement Test Track to meet this need.  Relatively thin pavement structures will be built using an array of popular premium mix additives that can include fibers, plastics, and rubbers.  The measured service life of each structure under accelerated traffic using legally loaded axles will be compared to a conventional control structure.  Any treatment section that statistically outlasts the control section will be a demonstration of improved structural performance.  Additional traffic supported as a result of the premium mix additive can be used to assess return on investment.  High-speed response instrumentation will be used to establish an ideal but practical laboratory characterization methodology and develop an objective and reliable framework to evaluate future premium mix additives.  Support for implementation of the additives evaluated on the NCAT Pavement Test Track as well as the framework for the evaluation of future premium mix additives will be provided.

• The group experiment will consist of relatively thin sections that should fail within one 10 million ESAL research cycle with conventional mix.
• Sections will likely be placed in a single lift to prevent bond strength failures from confounding outcomes.
• Implementable products will be layer coefficients and ME calibrations for pavements containing additives selected for the study by sponsor states including Florida.

PHASE VII (2018 – 2020) NCAT TEST TRACK FINDINGS

Florida Department of Transportation Cracking Study (Phase II)

As many state agencies have successfully mitigated rutting as a primary cause of pavement deterioration, more emphasis has been placed on identifying properties of mixtures that may influence the overall durability of a pavement structure. One such distress that affects durability is top-down cracking, which has been documented worldwide. The objective of the Florida Department of Transportations study was to evaluate different amounts of RAP on cracking performance within three test track subsections.

Sections E7 and E8 were divided into four 100-foot subsections with varying amounts of RAP.

Mixes:

Conclusions:

• Field cracking did not follow the expected trend with regards to RAP contents. Sections E7A and E7B showed the highest amounts of cracking.
• The use of the softer 58-28 modified binder did not show significant difference in field performance.
• At the end of trafficking, cracking was classified as low severity for sections E8A and E8B with a cracking percentage below 10%, and a medium severity for sections E7A and E7B with a cracking percentage above 10%.
• A large number of cracks mapped from these sections can be characterized as reflective for E7B. As for the other three subsections, there is not enough evidence to conclude if the cracks are reflective or not. However, visual inspection of extracted samples indicates that the most likely cause of these cracks was bottom-up and reflective cracking.
• After 20 million Equivalent Single Axle Loads of traffic, field performance for these sections was good overall in terms of roughness (constant and below 125 in./mi.) and permanent deformation (below 5 mm).

Florida Department of Transportation Density Study

The objective of the Florida Department of Transportation’s (FDOT) study in two Test Track sections was to evaluate the effect of density on pavement performance. Recent national efforts have increased attention on raising in-place densities to improve durability.

Sections E5 and E6 were divided into four 100-foot subsections having almost identical mixtures with target density being the key distinction between them. They are all fine-graded mixes with 20% RAP and a PG 76-22 asphalt binder modified with styrene-butadiene-styrene (SBS). E5A has a targeted density of 94%, E5B has a target of 92%, E6A has a target of 90%, and E6B has a target of 88%. The laboratory portion of this study included cracking and moisture susceptibility performance testing along with rheological and chemical tests of the extracted binders from each subsection. The field evaluation was typical for the Pavement Test Track and included cracking, rutting, texture, and roughness (IRI).

Conclusions:

• Laboratory cracking test results did not present expected trends in terms of density or air voids. There was no consensus among parameters to define a top performer out of the four mixtures.
• None of the mixtures resulted in considerable amounts of rutting, and all were considered non susceptible to rutting in the laboratory. This agrees with the minimal rutting and no moisture damage observed at the test track.
• At the end of trafficking, field performance was good with cracking at less than 5% of the lane in each section, no changes in roughness, little permanent deformation, and no significant changes in texture.
• Observed cracking in the test sections was classified as low severity.

PHASE VI (2015 – 2017) NCAT TEST TRACK FINDINGS

Florida Department of Transportation Cracking Study

With the identification of top-down cracking as a major factor in durability of pavements, FDOT has sponsored test section E7 and E8 during the reconstruction of the 2015 test cycle. The first objective was to determine which asphalt mixture were more prone to surface cracking, and the second was to characterize the mixtures’ properties in the laboratory to determine which cracking tests might most successfully predict cracking resistance.

Four mixtures were placed in 100-foot test strips, and varied in terms of binder grade and recycled material content.

Mixes:

• All mixtures utilized styrene-butadiene-styrene (SBS) modified binders.
• Main difference between the four mixtures is the percent of RAP, which varies from 20-30%.

Conclusions:

• The 25% RAP, PG 76-22 mixture was ranked as the poor performed based on laboratory test results, and was the first to crack in the field; however, the test sections with this mixture did not result I the highest amount of field cracking.
• All FDOT mixtures were not susceptible to rutting, as confirmed by the field and laboratory testing.
• Laboratory cracking results didn’t exhibit expected trends in terms of cracking potential.
• Field aging compared to the laboratory aging level may potentially influence the lack of correlation between the measured field cracking and laboratory parameters.

PHASE V (2012 – 2014) NCAT TEST TRACK FINDINGS

Florida Department of Transportation Tack Coat Effect on OGFC Performance Study

FDOT’s test sections N1A, N1B, and N2 were used in 2012 to study the effectiveness of various tack methods, following tack research from Phase IV, that could potentially provide the same life extension for OGFC surfaces as current (2012) thick polymer-modified tack coats applied with a spray paver.

The three tacks included cationic non-tracking tack (CRS-1HBC), a non-tracking hot applied polymer tack (NT-HAP), and an anionic non-tracking tack (NTSS-1HM).  These sections were tracked to 10 million ESALs over the course of two years.

Conclusions based on field performance results:

• IRI started higher in section N1, but increased as a lower rate than in the other sections. IRI at the end of the evaluation period was similar for all test sections except Section N1A which had a large patch.
• The heavier tack rate would reduce field permeability or drainage of the OGFC surface during and after heavy rains.
• It is recommended that a thicker tack coat be used to improve the performance of OGFC surfaces.

Florida Department of Transportation Cracking Study

With the identification of top-down cracking as a major factor in durability of pavements, FDOT has sponsored test section E7 and E8 during the reconstruction of the 2015 test cycle. The first objective was to determine which asphalt mixture were more prone to surface cracking, and the second was to characterize the mixtures’ properties in the laboratory to determine which cracking tests might most successfully predict cracking resistance.

Four mixtures were placed in 100-foot test strips, and varied in terms of binder grade and recycled material content.

Mixes:

• E7A – Virgin asphalt mixture using an asphalt binder modified by styrene-butadiene-styrene (SBS)
• E7B – Same aggregate skeleton as E7A, binder modified using FDOT experimental specification for ground tire rubber (GTR).
• E8A – SBS modified binder with the addition of RAP in the mix.
• E8B – SBS modified binder with the addition of RAS in the mix.

Conclusions based on field performance results:

• After 10 million ESALs of traffic, all but the SBS RAS had less than 22.5% cracking in their respective sections.
  • SBS RAS section had the highest cracking area despite being the last section to crack.
• Majority of the cracking in these sections was low severity.
• Correlations between laboratory performance and field cracking area are different when comparisons are made at 5 million and 10 million ESALs. This trend reversal is likely driven by rapid crack development in the RAP/RAS mixture in the second half of the research cycle.
• Results showed that polymer-modified binders should not be used in conjunction with RAP/RAS mixtures. These can become too stiff and this, are susceptible to cracking.

PHASE IV (2009 – 2011) NCAT PAVEMENT TEST TRACK FINDINGS

The main objective of this study was to evaluate the influence of a heavier tack coat on the field performance of OGFC by comaring the performance of the same OGFC mixture placed in Sections N1 and N2. In section N1, a heavier polymer-modified tack coat was applied using a spray paver right before the OGFC layer was placed. In section N2, a trackless tack was applied at a regular application rate using a distributor truck. A secondary objective of this study was to evaluate the use of RAP in an OGFC mixture.

A polymer-modified tack coat (CRS-2P modified with SBS) was applied at a spray rate of 0.21 gal/yd² in section N1 using a spray paver and a trackless tack was applied at a regular spray rate of 0.05 gal/yd² in Section N2. The spray rate for the trackless tack is between the specified application rates of 0.04 and 0.08 gal/yd² .

Conclusions:

• Section N1, with a heavier tack coat, had better performance than Section N2 with a conventional tack coat used.
• The use of 15% RAP in the OGFC did not have any negative effects on the mix characteristics or field performance.
• Both sections had cracking, but the level of severity and the area of severe cracks were greater in Section N2 than in Section N1.
• It is recommended that a heavier tack be used to improve the performance of OGFC surfaces.