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 HMA and OGFC sections. The difference between the noise measurements on the HMA and the OGFC sections was plotted against the age of the pavement. The black line drawn at 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 section was noisier than the HMA.
The graph shows that the OGFC-AR (green line) started out audibly quieter than the control section, but after only five months the difference was not audible. The OGFC-SBS (brown line) did not start out audibly quieter than the control section and was only quieter at the age of seven months and then again for a brief period between 11 and 14 months. In the short span of only 14 months both of the OGFC sections were not audibly quieter than the conventional HMA control section. The difference in noise levels between the OGFC sections and the control section continued to decrease over time until both OGFC’s become noisier than the control section.
The bar chart below shows the initial and final noise levels for the OGFCs and HMA control section.
How did the 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 section confirmed that the OGFC rutting was due to raveling, not due to plastic flow or secondary consolidation. The photo below is of the OGFC-AR section on I-5 near Lynnwood.
Where were the test sections on I-5?
The sections were installed on the four southbound lanes of I-5. Average daily traffic in this location is about 160,000 vehicles.
· OGFC - Polymer (MP 180.75 - MP 181.82) 1.09 miles
· OGFC - Rubber (MP 181.82 - MP 182.58) 0.76 miles
· Control - HMA (MP 182.58 - MP 183.50) 0.92 miles
When were the sections installed?
The I-5 Lynnwood test sections were installed at night during two weekend closures August 19-20 and 25-26, 2006.
What are the mix design characteristics of each pavement?
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: I-5 Lynnwood Test Section
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 indicates that it will need to be replaced at an age of about 13 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.
The chart below shows the life cycle cost comparison of the OGFC and HMA control sections based only on pavement performance. Current pavement performance data indicates the OGFC-AR would have lasted five years and the OGFC-SBS would have lasted nine years (both sections were removed as part of the project to construct the braided ramp at 196th).
The annual cost of the OGFC-AR was over twice that of the HMA control section but the annual cost of the OGFC-SBS was only about 15 percent higher than the HMA. Even if audible noise reduction were not an issue the OGFC-AR would be a very expensive pavement while the OGFC-SBS would be moderately more expensive than conventional HMA.