NPCA - Precast Products - Noise Barrier FAQ - Frequently Asked Questions

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Frequently Asked Questions about Precast Concrete Noise Barriers

1. What is a noise barrier?
A solid wall or structure built between a roadway and a community that mitigates traffic noise before reaching the community.

2. What is the difference between a noise barrier and a screen wall?
Noise barriers are typically larger walls or structures installed along a roadway or highway. Noise barriers are required to be designed, manufactured and installed in accordance with the requirements set forth by the authority having jurisdiction (AHJ) over the highway or roadway. Screen walls are typically smaller and designed to block the line of sight between a commercial or residential area and a structure, business and/or utility that is perceived to be visually unappealing. Screen walls may also be constructed to reduce the noise associated with a local business and/or utility structure.

3. How does a noise barrier work?
A noise barrier intercepts sound waves and either deflects the noise, absorbs the noise or a combination of both. An effective noise barrier typically reduces noise levels by 5 to 10 decibels. A 5-decibel reduction is noticeable, while a 10-decibel reduction typically means reducing the perceived noise level by half.

4. What is the difference between a reflective and an absorptive noise barrier?
A reflective noise barrier, like a mirror, reflects the sound. An absorptive noise barrier, like a black surface, absorbs some of the sound to reduce the reflected sound.

5. Who decides if a noise barrier will be used on a highway construction project?
In the United States, a state’s Department of Transportation must perform a traffic noise impact study on any projects in which federal funding participation is a factor. If a project results in a significant increase in noise level in a community or interferes with a community’s activities, then the state is required to take measures to abate the noise pollution to meet federal guidelines.

6. What is the difference between a Type I and Type II highway project as defined by FHWA?
Type I refers to projects in which noise abatement is considered a feature on a new or expanded highway. Type II refers to projects in which noise abatement is considered a retrofit on an existing highway.

7. What are the advantages of precast concrete noise barriers over other materials and construction methods?
Research has shown that concrete provides the highest sound transmission loss value when compared with common noise wall materials. In addition, precast concrete noise barriers require a considerably smaller footprint compared with earth berms, which can make only a barely perceptible decrease in the noise.

Precast concrete noise barriers can be manufactured and inspected prior to installation, which reduces a contractor’s construction timeline. Precast concrete noise barriers are manufactured with a wide array of textures and finishes using various form liners and finishing techniques. Because precast concrete products are produced in a controlled environment, they exhibit high quality and uniformity. In addition, precast concrete noise barriers are virtually maintenance free.

8. What aesthetic options are available for precast concrete noise barriers?
The aesthetic options for precast concrete noise barriers are virtually limitless. Concrete may be colored by using integral color pigments or by staining after the casting process. Textures can be created easily on the traffic side and/or the residential side of the posts and panels. Many community planning groups utilize the versatility of precast concrete to capture and preserve a community theme or identity within the precast concrete noise barrier.

9. What industry standards and specifications address precast concrete noise barriers?
The Federal Highway Administration (FHWA) has developed the “Highway Noise Barrier Handbook” which provides guidelines on how to design a highway noise barrier that fits with its surroundings and performs its intended acoustical and structural functions at a reasonable life-cycle cost. A majority of state Departments of Transportation have also developed state-specific guidelines for noise barriers constructed within their jurisdiction.

The FHWA Highway Noise Barrier Handbook may be accessed at:
http://www.fhwa.dot.gov/environment/noise/manual.htm

Additional information from FHWA may be accessed at:
http://www.fhwa.dot.gov/ENVIRONMENT/noise/index.htm
http://www.fhwa.dot.gov/Environment/probresp.htm#intro

10. What engineering constraints must be taken into consideration when designing a noise barrier?
The most important aspects that drive design are the height of the barrier to be constructed, soil conditions, wind loads, snow loads (when applicable), nearby utilities and proximity to the roadway.

11. How do precast concrete noise barriers perform when exposed to corrosive agents or repeated freeze-thaw cycles?
Precast concrete mix designs are formulated to provide an economical and durable finished concrete product. Mix designs can easily be adjusted to account for various levels of anticipated exposure to corrosive agents such as salts. Precast concrete noise barriers are manufactured with a low water-to-cement ratio, which typically contains an air-entraining admixture. By utilizing a low water-to-cement ratio, higher strengths and a lower permeability can be achieved. Purposely entrained air voids provide additional space for freezing water to expand, minimizing the potential for scaling and deterioration of the concrete.

12. How does the height of the noise barrier affect its noise reduction capabilities?
Typically a 5-decibel reduction in noise can be expected for a noise barrier that blocks the line of sight to the roadway. An additional 1.5-decibel reduction typically can be expected for each additional 40 inches (1 m) of barrier height above the line of site.

Affects of height on noise reduction capabilities

13. How does the length of the noise barrier affect its noise reduction capabilities?
To minimize the potential for a reduction in the noise barrier’s performance, the barrier should extend a minimum of four times the perpendicular distance from the receiver to the barrier along a line drawn between the receiver and the roadway, according to the FHWA. Another way of looking at this is that the angle subtended from the receiver to a barrier end should be at least 80 degrees as measured from the perpendicular line from the receiver to the roadway.

Affect of length on noise reduction capabilities

14. Can access openings for maintenance and/or fire rescue be provided for precast concrete noise barriers?
Yes, secured doors and frames or access hatches can easily be incorporated into the design of a precast concrete noise barrier. Blockouts for access to fire hydrants may also be cast in or can be cored in the field. Barriers may also be designed with an overlap, providing a larger opening for maintenance crews and/or emergency vehicles to access the roadway. A properly designed overlap will ensure the wall’s performance is not compromised.

15. Do precast concrete manufactures have their own internal quality control program?
Departments of transportation often require each manufacturer to maintain an active plant-specific Quality Control plan. Manufacturers may also be required to maintain a Quality Control certification through an independent third party such as the NPCA Plant Certification Program. In addition, departments of transportation inspect finished product at the plant and job site on a regular basis.

16. What types of footings or foundations are normally used for precast noise barriers?
Precast concrete noise barriers typically utilize a drilled shaft foundation, an auger cast foundation or a spread footing located at each column. Columns can extend into the drilled shaft and be encased in the concrete footing or can be attach to anchor bolts that are cast into the concrete footing.

17. Are soil borings important to the design of a precast noise barrier?
Yes, soil properties are probably the single most important design aspect of a precast concrete noise barrier. Soil borings should be performed at least every 100 feet along the length of a noise barrier, especially when soil conditions are expected to vary. Borings should be taken along the proposed location of the noise barrier, not in the center of the proposed roadway or at bridge abutments located on the same project.

18. Do all precast concrete noise barriers utilize precast concrete posts?
No, noise barriers can be built using concrete posts or steel H beam posts, or they can be freestanding using a serpentine design along the centerline of the noise barrier that offsets wind loads. Noise barriers constructed in a serpentine pattern also provide additional space for landscaping along the roadway. Noise barrier panels have also been mounted on specially designed concrete highway barriers when right-of-way is limited, while lightweight precast concrete noise barriers may be mounted to bridge structures.

19. Are precast concrete noise barriers designed for impact loads or earth retaining applications?
Typically they are not, however they can be designed for impact loads and earth retaining applications. It is important that these design requirements be specified prior to the bidding process so the precast concrete manufacturer can take such design requirements into consideration.

20. What is the typical construction sequence when using precast concrete noise barriers?
After shop drawings have been approved, surveyors lay out the alignment of the wall and place stakes at each column location. Offset stakes are also provided in case of the accidental removal of an alignment stake. The contractor then begins the process of drilling and pouring concrete footings; the posts are then placed on or in the footings, depending upon which style is used. Once the posts are in place, the panels are then installed between the posts to the desired height.

21. What can be done to prevent graffiti?
There are many types of permanent and sacrificial graffiti protection products that can be applied on the wall after construction that will prevent graffiti. In addition, graffiti artists prefer a smooth, flat canvas. You can reduce the likelihood of graffiti by specifying a varying texture such as stone. Wax-based coatings also may be applied. Once an artist leaves his mark, the material can be removed with a steam cleaner, and a new wax coating can be applied immediately. This eliminates issues with matching paints when covering graffiti. There are also products available to remove graffiti from untreated walls. Essentially, these compounds or solvents breakdown the paint itself.

22. What happens if a precast concrete noise barrier panel is damaged?
If the precast concrete noise barriers are constructed from standard products readily available, then replacement is typically not an issue. However, when utilizing a custom design involving detailed form liners and integral color or stains, the issue becomes more critical. To address this concern, some agencies have instituted stockpiling policies where the contractor or precast concrete manufacturer, at the time of construction, supplies additional components to the organization responsible for maintenance. Typically, an additional 1 percent or 2 panels per 100,000 square feet may be supplied to the responsible organization for stockpiling purposes. The disadvantage to this practice is that the responsible organization may end up with a large amount of stock on hand.