The graph 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 HMA control pavement is the standard for judging the noise mitigation performance of the OGFC sections. If the noise level on the OGFC is more than 3 decibels lower than the control section it is considered to be an audible difference (a difference in noise level that is detectable by most human ears). If the difference is less than 3 decibels there is no audible difference and the noise levels between the OGFC and HMA would not be detectable. In the latter case the OGFC is failing to provide any noise mitigation over the conventional HMA control pavement. 

  

The graph below shows the history of the noise measurements on the I-405 Bellevue 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 both OGFC-AR and OGFC-SBS were audibly quieter than and control section after construction. The OGFC-AR (brown line) bounced around between being audibly quieter and not quieter for about three months. The OGFC-SBS (green line) continues to be in the not audible position through the current readings. In the short span of only three months both of the OGFC test pavements are not audibly quieter than the conventional HMA control pavement.

How are the I-405 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.

WSDOT uses three criteria to rate pavement performance: smoothness, structural condition and rutting. Rutting due to raveling has been the major downfall of the other two quieter pavement test sections. Currently there are no reports of any excessive raveling on this quieter pavement test section.

Where are the OGFC test sections on I-405?

All pavement test sections were installed on all lanes of the northbound direction of I-405. Average daily traffic in this location is about 165,500 vehicles.

• OGFC - Polymer (MP 10.60 – MP 10.93 and MP 12.06 – MP 12.40) 0.67 miles
• OGFC - Rubber (MP 10.26 - MP 10.60 and MP 11.76 - MP 12.06) 0.64 miles
• Control - HMA (MP 9.80 - MP 10.15) 0.35 miles

When were the test sections installed?

The I-405 Bellevue test sections were paved during the day over the weekend of August 15 and 16, 2009. 

Post-Construction Report and initial noise and pavement performance results:  I-405 Bellevue Test Section

What are the mix design characteristics of each pavement?

How did the cost of OGFC pavement compare to conventional asphalt (HMA) on the I-405 Bellevue test section?

The I-405 test project was part of a larger design build contract; therefore, line item costs are not available.