The HMA control pavement was the standard for judging the performance of the OGFC sections. If the noise level on the OGFC section was more than 3 decibels lower than the control section it was considered to be an audible difference (a difference that is detectable by most human ears). If the difference was less than 3 decibels an audible difference would not exist and the OGFC would not be providing any noise reduction over the conventional HMA control pavement.
The graph below shows the history of the noise measurements on the SR 520 Medina section. The numeric difference between the noise measurement on the HMA and OGFC sections was plotted against the age of the pavement. A black line drawn at the 3 decibels differentiates between noise level differences that are audible and not audible. The red line at 0.0 decibels marks the level below which the OGFC is noisier than the HMA.
The graph shows that the OGFC-AR (green line) was audibly quieter than the control section before it slipped below the 3 decibel difference line at six months. The OGFC-SBS (brown line) was 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 section by becoming noisier than the HMA control section as the pavement aged. In the short span of only six months the OGFC sections were not audibly quieter than the conventional HMA control section.
The bar chart below shows the initial and final noise levels for the OGFCs and HMA control section.
How did the test sections perform 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 is due to raveling, not due to plastic flow or secondary consolidation. Excessive raveling early in the life of a pavement results in a much shorter pavement life. The photo below shows pavement wear from raveling exposing the underlying asphalt pavement in the OGFC-AR section of SR 520.
Where were the sections on SR 520?
The sections were installed on the two general purpose lanes in 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 (MP 5.26 – MP 5.82) 0.56 miles
· OGFC – Rubber (MP 4.18 – MP 4.68) 0.50 miles
· Control - HMA (MP 4.68 - MP 5.26) 0.58 miles
When were the sections installed?
The OGFC-AR was installed during the day of August 15. The OFGC-SBS was installed during the night and early morning of August 16, 2007.
What were the mix design characteristics of each pavement?
|OGFC-AR ||OGFC-SBS |
|0.75 inches thick
||0.75 inches thick |
||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 to prevent draindown |
Final Report with complete noise and pavement performance results: Final Report SR 520
How did 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 was 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 resulted in a high life cycle cost. Current performance data for the HMA control section indicated that it will need to be replaced at an age of about 11 years. The short time period that the OGFC was 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, was there an application for the OGFC where noise reduction was 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 was over twice that of the HMA control section but the annual cost of the OGFC-SBS was 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 was 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 had an annualized life cycle cost that was 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.