The ability to safely slow and stop a vehicle is not only dependent upon its tires, but also the surface characteristics of the pavement.
Pavement surfaces can be constructed with a more open surface texture to provide greater macrotexture for better high-speed performance during wet-pavement surface conditions. This has been shown to reduce crash rates on high-speed facilities. However, one issue with more open-textured surfaces is that there is less fine aggregate material in the mix to prevent the coarse aggregates from moving under traffic loading. This lack of fine aggregate material tends to make the pavement structure weaker at the surface when using the same asphalt binder grade that would be used for dense-graded mixes. The asphalt used in open-textured mixes must play the important role of holding the mix together and adding strength and structural integrity to the pavement.
In critical locations, the Connecticut Department of Transportation (ConnDOT) often specifies an ultra-thin bonded hot-mix asphalt (UTBHMA) wearing surface as part of its pavement preservation program. UTBHMA also can be specified for increasing frictional characteristics of the pavement in critical areas. The UTBHMA mix, however, requires the use of specialized paving equipment for construction, which limits the number of contractors able to place this material. It was desired to investigate a high-friction mix for use in Connecticut that could be placed with readily available conventional paving equipment in order to increase contractor bidding pools.
In 2012, ConnDOT contracted the Connecticut Advanced Pavement Laboratory (CAP Lab) at the University of Connecticut to design a paver-placed high-friction thin lift (PPHFTL) wearing course for placement on S.R. 12 in Preston/Ledyard, Conn. The mix was specified for placement at no more than 1 in. thick. The asphalt binder was required to meet the performance grade (PG) criteria for PG 76-22 and be modified with styrene-butadiene-styrene (SBS) polymer. No recycled products were permitted for use with this mix.
The CAP Lab selected a 3/8-in. nominal maximum aggregate size for the PPHFTL mix, with no more than 50% aggregate passing the No. 4 sieve. The produced mix was sampled at the plant and tested at the CAP Lab. Laboratory performance tests included moisture susceptibility via tensile strength ratio and Hamburg wheel-track testing, and rutting susceptibility via the asphalt pavement analyzer. Performance testing of the mix indicated no potential for premature failure of the wearing surface. This is consistent with the results of nearly all asphalt pavement mixtures containing SBS polymer that have been tested for performance by the CAP Lab. The addition of polymer to the asphalt binder significantly enhances the performance of the mixture when considering both dynamic loading and environmental loading. This expected performance was substantiated during several field visits to the site, as visual inspections indicate that the PPHFTL is performing well regarding its durability.
To evaluate the PPHFTL surface properties, locked-wheel pavement skid resistance was measured with full-scale ASTM E-501 standard ribbed and ASTM E-524 standard smooth tires to give an indication of the microtexture and the overall macrotexture, respectively. Pavement texture depth, expressed as Mean Profile Depth (MPD), was measured with a Circular Track Meter (CTMeter). Both field-testing apparatuses are shown in Figure 1.
A comparison of skid numbers measured at 40 mph with the smooth (SN40S) and ribbed (SN40R) tires for the PPHFTL and an adjacent dense-graded control section are shown in Figures 2 and 3, respectively. Immediately following construction, the SN40S values were slightly lower for the PPHFTL than for the control section, but the PPHFTL values quickly increased and were comparable to the control after just three months.
After 12 months of service, it is clear that the PPHFTL SN40S values were significantly higher than the dense-graded control section values. This trend continued, and after two years, the average PPHFTL SN40S value was 52, versus an average control SN40S value of 38.
Regarding the ribbed-tire values presented in Figure 3, it can be seen that, immediately following construction, the PPHFTL SN40R values were significantly lower than for the control section; however, average PPHFTL SN40R values increased over time and currently align with the control-section values.
The CTMeter test results showed that the PPHFTL-wearing course had an average MPD of 0.78 mm, while the adjacent dense-graded control section had an average MPD of 0.35 mm. The difference in the MPD values gives an overall indication of the open nature of the PPHFTL surface as compared with the traditional dense-graded surface. The surface texture of the PPHFTL is shown above with a U.S. quarter dollar coin for size reference.
Based upon the stated skid test results and texture measurements, it is evident that in time the PPHFTL surface provides greater macrotexture than the control, which greatly aids in preventing skidding during wet weather. After two years, the PPHFTL texture depth was more than twice that of the control, and the SN40S values were 37% greater than the control values. Similar to tire threads, the open pavement texture allows water to escape from between the tire and surface. This provides the potential to reduce wet-weather crashes.
The initially lower ribbed-tire values for the PPHFTL surface suggest that some caution should be exercised regarding the treatment’s ability to provide superior friction characteristics during the first year after placement. This is likely a result of a thin asphalt binder film on the surface aggregate, which has worn off over time. When viewing both of these figures, the friction trend over time is positive. This gives confidence that this mixture will eventually perform as intended. This pavement surface section will continue to be monitored over time.
There are currently plans to place this PPHFTL mix (or a slight variation thereof) on a high-volume/high-speed section of I-84 in Connecticut during the 2015 construction season. Surface properties will be monitored to see if the initial skid numbers following construction are initially lower, as they were for the S.R. 12 project, or if adjustments to the mix resolve this issue. If not, close monitoring to see how quickly the skid numbers improve under higher volumes will be conducted.
ConnDOT pavement engineers are still hopeful that the PPHFTL will provide a possible alternative to UTBHMA; however, because of its initially lower skid resistance following construction, they indicated that further research is needed before applying PPHFTL as a high-friction surface treatment. At this time, suggestions that would be better suited for the following applications include:
1. To arrest continued loss of friction, as it would first stabilize and within a year improve the friction characteristics. This would make it ideal to proactively treat segments where existing conditions are reducing friction over time. The treatment would provide acceptable friction numbers initially and then for a sustained period of time (even improving friction after the initial installation period); and
2. To provide superior splash/spray/macro-texture performance for high-volume/high-speed roadways in a preservation application.