In a place known for pristine mountain views and limited sunlight, the Alaska DOT (ADOT) chose an alternative approach for the construction of a highway noise barrier that runs along 4000 feet of heavily traveled highway bordering Anchorage Airport: transparency.
Known as the C Street project, the development of a 10- to 17-foot high transparent noise barrier system proved to be the most viable solution. It allowed the ADOT to mitigate highway noise while also supporting a local residential community. Unlike a solid barrier, the transparent noise barrier ensured that the community would not be permanently cast in shadow and denied its view of the Chugach Mountain range.
Local authorities worked with ADOT in support of the project from April to October 2003. Transparency is highly uncommon among highway noise barriers in the United States, but Europeans have widely accepted transparent barriers to contain noise while preserving views of natural landscapes. For the implementation of this unique highway construction task, McLean, Va.-based Durisol USA Inc., a noise barrier manufacturer, constructed a distinctive barrier wall that combines the sound absorptive properties of a wood-concrete panel with the transparent and noise reductive qualities of Rockaway, N.J.-based CYRO Industries' new Paraglas Soundstop acrylic noise barrier.
The transparent barrier ensured that the community would not be permanently cast in shadow and denied its view of the Chugach Mountains.
Meeting Strict Standards
For more than 50 years, Durasol has developed and manufactured custom single- and double-sided noise barriers for various transportation systems throughout the United States and Canada. For the C Street project, ADOT provided Durisol with specifications that required the transparent noise barrier to be constructed as polycarbonate injection-molded blocks with a noise reduction coefficient (NRC) of 0.60 and light transmittance of at least 80%.
The difficulty in meeting both light transmission and noise absorption requirements with just the blocks, though, prompted Durisol to petition ADOT to consider broadening the specification to include alternative materials. “An entirely translucent wall could not provide the sound absorptive quality sought by the project engineers,” said Mike Edwards, vice president of Durisol. “We alerted ADOT and worked to open the spec to Durisol panels and transparent sheet panels and formulated a design proposal that would fit the spec.”
When ADOT agreed to open the specification to include materials other than polycarbonate. Durisol proposed combining panels made of its wood aggregate concrete mix with the noise barrier.
Wall Materials Stand Out
Durisol chose to work with these materials because the light diffusion, ultraviolet stability, and resistance to outdoor elements inherent in PARAGLAS SOUNDSTOP Sheet offer advantages over other translucent materials, such as polycarbonate sheet. “Durisol is not a plastics expert, but we've seen some polycarbonate walls in San Diego that had yellowed and been replaced by PARAGLAS SOUNDSTOP Sheet, which immediately directed us to CYRO when bidding this project,” said Edwards. “No other material was even considered.”
By combining its own wood aggregate panels with the acrylic noise barrier panels, Durisol achieved a unique construction solution that meets ADOT's stringent requirements for clarity and noise absorption. Durisol wood aggregate panels possess an NRC of 0.80 to 0.85, which easily exceeds the ADOT NRC specification of 0.60. The acrylic panels not only offer additional noise absorption, but also contribute 93% light transmission, surpassing the 80% light transmission minimum stipulated by ADOT.
The Selection Process
During the review process, the manufacturers provided ADOT's wall design committee with samples of both materials and demonstrated the ease of graffiti removal via hot pressure wash. To address concerns about reflected glare from the transparent noise barriers, CYRO presented ADOT samples of frosted acrylic noise barriers, with data from studies conducted in Europe showing that glare from the product is not a significant cause of driver distraction.
After reviewing the information and samples, the committee opted to construct the noise barrier using the wood aggregate panels along the bottom of the wall to absorb sound and the acrylic panels along the top to allow access to light and scenic views. The committee also decided that a frosted finish for the panels was unnecessary and instead specified the standard transparent product throughout the wall, except in areas where visibility and light transmission was unnecessary, where tinted panels would be used.
“The sun rides low in the winter, and the residents wanted to maintain their view of the Chugach Mountain range,” said Harold Henderson, ADOT project engineer. “This barrier is the best of both worlds, providing noise abatement and an attractive view.”
Durisol's partnership with CYRO also proved to be a bargain for the ADOT. Cost-effective construction processes allowed the total project cost for the transparent barrier system to be more economical than competing proposals.