Highway agencies have traditionally used open-graded friction course (OGFC) as a road’s final riding surface to provide safety and environment benefits. Those benefits include improved friction, minimized hydroplaning, reduced splash and spray, and reduced noise level.
With improvements in design and construction practices and the use of better materials, especially polymer-modified asphalt (PMA) binders, the performance of OGFC has improved through the years. However, compared to a conventional hot mix asphalt (HMA) mix, OGFC mixes are often more prone to pavement distresses such as cracking and raveling, which results in a shorter service life.
The performance of OGFC as a surface layer depends on three things:
• durability of the mix
• integrity of the underlying layer
• interface bond
One way to potentially improve the performance of OGFC is to enhance the interface bond between the OGFC and underlying layers by applying a heavier tack coat. The Florida Department of Transportation (FDOT) sponsored a study in the fourth NCAT Pavement Test Track research cycle to evaluate the effectiveness of a heavier tack coat application on the field performance of OGFC.
Crews milled test sections N1 and N2 on the NCAT test track and inlaid them with three asphalt layers for the study. The buildup, which consisted of four asphalt layers, was the same for the two sections. The only difference in the two sections was the tack coat applied at the interface of the OGFC and the underlying layer.
For section N1: the crew used a spray paver to apply a polymer-modified tack coat (CRS-2P modified with SBS) at a heavy spray rate of 0.21 gal/yd2.
For section N2: the crew used a conventional tack coat distribution truck to apply a trackless tack (NTSS-1HM) at a spray rate of 0.05 gal/yd2, as required in the FDOT Standard Specification.
The OGFC mix was designed with a PG76-22 asphalt binder modified with styrene-butadiene-styrene (SBS) according to the Florida DOT Construction Specification for an FC-5 mix. The aggregate mix was a blend of virgin granite aggregate, hydrated lime and 15 percent reclaimed asphalt pavement (RAP).
The RAP consisted of two fractionations from East Alabama Paving in Opelika, Ala. The first RAP stockpile was crushed and screened on a 1-inch screen. The second stockpile was fractionated on the #4 (4.75-mm) sieve. In addition, the OGFC mix design was evaluated and passed the requirements for drain-down susceptibility, moisture susceptibility and abrasion resistance.
After construction was completed in August 2009, these test sections and other test track sections were trafficked with a fleet of five triple-trailer trucks operating two shifts a day, five days a week. By the end of the 2009 research cycle, the test sections were loaded with approximately 10 million equivalent single-axle loads (ESALs).
Researchers conducted weekly evaluation and monitoring of each section on Mondays. Sections were inspected for signs of cracking, and multiple measurements of rutting and surface texture were made. Falling-weight deflectometer (FWD) testing was conducted several times a month, and strain at the bottom of asphalt structure, vertical pressure in the aggregate base and subgrade pressure measurements from 15 truck passes were obtained weekly in each section.
Measurements of asphalt concrete (AC) strain, base pressure and subgrade pressure were remarkably stable over time for Section N1. This observation is supported by the relatively stable moduli for N1 obtained from the FWD back-calculation. These observations (i.e., AC modulus and measured responses) indicate a structure in reasonably good health.
Conversely, the pavement response measurements for Section N2 increased between mid-November 2010 and early February 2011. This time period corresponded with the general decline in back-calculated AC modulus experienced in Section N2 from October 2010 through February 2011. Clearly, the drop in back-calculated AC modulus had an impact on the measured pavement responses.
Section N1 had about half the total rutting compared to Section N2. The International Roughness Index (IRI) for Section N2 started lower than that for N1 but increased more quickly starting in October 2010. This increase corresponded to the time period when the AC strain increased and the AC modulus declined. Cracks can be seen throughout Section N2, and the level of severity and the area of severe cracks were greater in Section N2 than in Section N1.
Permeability test results showed that permeability was not significantly affected by the tack coat application rate or method. However, during heavy rains, Section N2 with the lower tack coat rate appeared to provide better drainage than Section N1. The OGFC layer in Section N1, where the heavier tack coat was applied, performed better than that of Section N2, where a conventional tack coat was used. Therefore, it is recommended that a heavier tack coat be used to improve the performance of OGFC surfaces.