The bar chart below represents the following 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 pavement (OGFC-SBS) 

The HMA control pavement is the standard for judging the performance of the OGFC sections. If the noise level on the OGFC section is more than 3 decibels lower than the control section it is considered to be an audible difference (a difference that is detectable by most human ears). If the difference is less than 3 decibels there is no audible difference, and the difference is not detectable. In the latter case the OGFC is failed to provide any noise mitigation 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 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) started out audibly quieter than the control section, but after only five months the difference was not audible. The OGFC-SBS (green line) did not start out audibly quieter than the control section and was only quieter for one measurement at the age of seven months and then again for a brief period between 11 and 14 months. The difference in noise levels between the OGFC sections and the control section continues to decrease over time until both OGFC’s becomes noisier than the control section, especially the OGFC-AR In the short span of only 5 to 14 months both of the OGFC test pavements are not audibly quieter than the conventional HMA control pavement. 

How are the I-5 Lynnwood test sections evaluated?

For noise, the On-Board Sound Intensity (OBSI) measurement method was used and measurements were 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 and pavement interface.

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 section confirmed that the OGFC rutting was due to raveling, not due to plastic flow or secondary consolidation. Photo below is of the OGFC-AR section on I-5 near Lynnwood.



Where are the OGFC test sections on I-5?

All pavement test sections were installed on all lanes of southbound I-5. Average daily traffic in this location is about 160,000 vehicles.

• OGFC - Polymer (MP 180.75 - MP 181.82) 0.76 miles
• OGFC - Rubber (MP 181.82 - MP 182.58) 1.07 miles 
• Control - HMA (MP 182.58 - MP 183.50) 0.92 miles 

When were the test sections installed?

The I-5 Lynnwood test sections were installed at night on two weekend closures August 19-20 and 25-26, 2006.

What are the mix design characteristics of each pavement?

Post-Construction Report with initial noise and pavement performance results:   I-5 Lynnwood 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 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 of 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 13 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 five years and the OGFC-SBS would have lasted nine years (both test sections were removed as part of the project to construct the braided ramp at 196th). 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 only about 15 percent higher than the HMA. Even if audible noise reduction were not an issue OGFC-AR would be very expensive pavement while the OGFC-SBS would be moderately more expensive than conventional HMA.