The graph below shows the initial and current noise measurements for the three pavement types:
- new hot mix asphalt (HMA), or control pavement
- new rubber asphalt open graded friction course pavement (OGFC-AR)
- new polymer-modified asphalt open graded friction course pavement (OGFC-SBS)
The graph below shows the history of the noise measurements on the SR 520 Medina test section. The numeric difference between the noise measurement on the HMA and OGFC sections is plotted against the age of the pavement. A black line is drawn at the 3 decibel level to differentiate between noise level differences that are audible and not audible.
The graph shows that the OGFC-AR (brown line) is audibly quieter than the control section before it slips below the 3 decibel difference line at six months. The OGFC-SBS (green line) is never 3 decibels quieter than the control section, even right after construction. The OGFC-AR noise levels follow the pattern of the I-5 quieter pavement test section by becoming noisier than the HMA control section as the pavement ages. The area below the 0.0 decibel line on the chart is where the OGFC pavements are noisier than the HMA control section. In the short span of only six months the OGFC test pavements are not audibly quieter than the conventional HMA control pavement.
How are the SR 520 test sections evaluated?
For noise, the On-Board Sound Intensity (OBSI) measurement method is used and measurements are taken approximately every month. OBSI is the best method for measuring the acoustic performance of pavements because the microphones are mounted very close to the tire pavement interface.
How are the test sections performing as pavements?
WSDOT uses three criteria to rate pavement performance: smoothness, structural condition and rutting. The OGFC-AR section began to show excessive raveling after the winter of 2008-09. Raveling is the loss of aggregate from the pavement. Loose aggregate on the shoulders of the test sections confirms that the OGFC rutting on SR 520 Medina is due to raveling, not due to plastic flow or secondary consolidation. Raveling this early in the pavement life predicts a much shorter pavement life compared to the standard HMA life of 16 years in western Washington. Photo below shows pavement wear from raveling exposing the underlying asphalt pavement in the OGFC-AR section of SR 520.
Where are the OGFC test sections on SR 520?
OGFC pavement test sections were installed on the two general purpose lanes both the eastbound and westbound directions and in the westbound HOV lane. Average daily traffic in this location is about 90,000 trips.
- OGFC - Polymer
- Eastbound (MP 5.26 – MP 5.82) 0.56 miles
- Westbound (MP 5.26 – MP 5.82) 0.56 miles
- OGFC – Rubber
- Eastbound (MP 4.18 – MP 4.68) 0.50 miles
- Westbound (MP 4.18 – MP 4.68) 0.50 miles
- Control - HMA
- Eastbound (MP 4.68 - MP 5.26) 0.58 miles
- Westbound (MP 4.68 - MP 5.26) 0.58 miles
When were the test sections installed?
The SR 520 around Medina test sections were installed during the day on August 15 and 16, 2007.
What are the mix design characteristics of each pavement?
| OGFC-AR | OGFC-SBS |
|---|
| 0.75 inches thick |
0.75 inches thick |
| PG 64-22 |
PG 58/64-22 modified to PG 70-22 |
| 3/8 inch maximum aggregate size |
3/8 inch maximum aggregate size |
| 9.2 percent binder content |
8.3 percent binder content Fibers added for draindown |
Post-Construction Report and initial noise and pavement performance results: SR 520 Medina Test Section
How does the cost of quieter OGFC pavement compare to the HMA control pavement?
WSDOT uses life cycle cost analysis (LCCA) to compare the cost of different pavement types. LCCA is a method of economic analysis that takes into account the initial as well as discounted future costs. In the case of the OGFC and the HMA control section, the future cost is the cost of repaving the roadway at the end of the pavements life. The life cycle cost then becomes a function of how much it cost to pave the road and the time between each repaving of the road.
The chart below compares the OGFC-AR and OGFC-SBS using LCCA if they were replaced as soon as they were no longer audibly quieter than the HMA control section. The life cycle cost is expressed as uniform annual cost in order to directly compare the different pavement types. Although the audible noise reduction capability of the OGFC is six months or less, one year was used in the LCCA calculations as the audible pavement life for the OGFC for simplicity. The life cycle cost for the HMA control section is also included for comparison.
The short duration of audible noise reduction for the OGFC leads to a high life cycle cost. Current performance data for the HMA control section indicates that it will need to be replaced at an age of about 11 years. The short time period that the OGFC is audibly quieter than the HMA results in a life cycle cost that is much higher than conventional HMA in order to achieve the noise reduction.
But, is there an application for the OGFC where noise reduction is not an issue? Current pavement performance data indicates the OGFC-AR would have lasted 5 years and the OGFC-SBS would have lasted 14 years (noise testing was suspended as a result of construction on this section of SR 520). The chart below shows the life cycle cost comparison of the OGFC and HMA control sections.
The annual cost of OGFC-AR is over twice that of the HMA control section but the annual cost of the OGFC-SBS is lower than the HMA. The data appears to indicate that the OGFC-SBS may be an alternative to dense graded HMA. However this may not be the case. The HMA control section on SR-520 is under performing WSDOT standard HMA which lasts about 17 years west of the Cascades. If the HMA control section lasted 17 years, its life cycle cost would be much lower than the OGFC-SBS. On the I-5 Lynnwood project the OGFC-SBS test section has a annualized life cycle cost that is higher than the HMA control section. The I-5 data indicates that the OGFC-SBS may have a higher life cycle cost than the HMA. WSDOT continues to evaluate alternate pavement types, including OGFC-SBS, for their potential to reduce life cycle cost.