5. NOISE BARRIER MATERIALS AND SURFACE TREATMENTS

Features - Almost half of the noise walls constructed in North America to date are made of concrete. The proliferation of the use of concrete is not without reason. Concrete, if formulated, cast (precast or cast-in-place), and cured properly, is considered to be one of the most durable materials currently used for many highway products, including noise barriers. It is rugged and able to withstand severe temperatures, intense sunlight, moisture, ice, and salt. It is a versatile material capable of being shaped, molded, and textured to take on the appearance of anything from weathered wooden boards to rock face to stone blocks to virtually any sculpted mural topic imaginable. Its mass, even at a thickness of only 12 mm (0.5 in.), is well within any Sound Transmission Class requirement (see Section 3.4.2).

Concrete products lend themselves well to coloring or tinting by either incorporating pigments into the concrete mix before pouring or afterwards by applying a stain onto the surface of the cured products. For more details, see Section 5.9.2.1

Typical Use - The versatility of concrete also extends to the shape and the size with which the panels can be produced (e.g., precast stacked panels, cast-in-place and precast full height panels, and precast concrete block). In addition, concrete allows for a complete range of installation techniques including post and panel, post integral with the panel, free standing, direct buried, and on top of spread footings, continuous footings, traffic barriers, and retaining walls. Cast-in-place concrete walls have been typically used on bridges and retaining walls because of their flexibility of design, high structural strength, and resistance to vehicle impact damage.

5.1.1 Special Considerations.

• Mix Design - There are 2 basic types of concrete mix produced, dry cast and wet cast. Both are generally composed of aggregate, Portland cement, and water. The major difference between the two is the amount of water used.

  - Wet Cast Concrete - Mix contains enough moisture to allow proper chemical reaction between all the ingredients in the mix to form a sufficiently strong bond between each other. The concrete mix must be allowed to set in the mold before it can be removed, typically about 8 hours. Wet cast concrete flows better in a mold than the dry cast mix allowing the use of finely detailed form liners.

  This type of mix is suitable for both cast-in-place and plant production products.

  - Dry Cast Concrete - Mix contains significantly less water and only enough to allow the mixture to retain its shape after being compacted into a mold. This allows the product to be removed immediately from the mold after pouring. The product is then hydrated by the introduction of steam during the curing process. Since it is not as fluid as the wet cast concrete, it is not suitable for panels requiring fine surface details. This type of mix typically has superior strength capabilities over the wet cast mix. It is much more suitable for mass production processes under controlled plant conditions.

• Size Limitations

  - Precast - Precast panel sizes are typically confined in one direction to approximately 4.5 m (15 ft) due to limitations in shipping, with no limit on length other than by size and weight for handling. The minimum thickness is usually directly related to the amount of concrete cover required over the reinforcing bars or mesh, but is typically about 100 mm (4 in) plus an additional 25 mm (1 in) in total to allow for the reinforcing and any surface texturing.

  - Cast-in-Place - The minimum wall thickness is approximately 150 to 200 mm (6 to 8 in) due to the space requirements needed to place and vibrate the concrete around the reinforcing steel.

• Precast vs Cast-in-Place - Precast panels can be erected quickly if crane and truck access are readily available. Traffic hold-ups can be minimized with off-site panel fabrication and landscape damage can be avoided by the use of proper sized cranes which can span over the landscaping when setting the panels. On the other hand, the presence of a crane and truck haulage unit together, which are necessary during erection of the panels, can become a traffic problem, sometimes necessitating a lane closure.

• Reuse of Precast Panels - Precast concrete walls have the potential to be relocatable and have been used for temporary walls as well as permanent installations.

• Off-Site Casting Yard vs Off-Site Casting Plant - Casting yards are typically established by the manufacturer as a temporary manufacturing facility for a specific project, usually to reduce shipping costs when the permanent casting plant is too far from the site. Casting plants inherently have a higher level of quality control in the manufacturing, handling, and hauling than can be achieved at a casting yard. As a result, a greater number of imperfections will appear in products supplied from a temporary casting yard and will have to be dealt with on site through a more active quality assurance process.

• Regional Differences - Concrete products are suitable for any climate condition imaginable. However, the emphasis on specific characteristics of concrete may vary from region to region. For example, in the northern regions, freeze/thaw and salt scaling resistance is critical. Where as, in the southern or warmer climates, expansion coefficients and proper curing practices are more important. In coastal regions, the emphasis would be on density which helps to repel the penetration of salt laden moisture.

5.1.2 Verification of Quality.
For all testing, it is important to select samples which are a true representation of the finished products or of the material(s) being used in the casting of the noise barrier components.

he following are standard tests, normally conducted at the casting plant, on concrete to verify overall quality and to confirm desired properties of the products. The tests discussed within this section are described in detail in Section 10. which discusses product evaluation of all types of barrier materials. Although most of these tests are suitable for both wet and dry cast concrete, some are more suitable for one type as compared with the other. Suitability of the test is noted where applicable.

• Slump Test (Suitable for Wet Cast Concrete Only) - This test determines the stiffness and consistency of freshly mixed concrete and, in general, is a good indicator of the amount of water in the mix.

• Air Content (Suitable for Wet Cast Concrete Only) - This test determines the amount of air in cured concrete. It is, primarily, a good indicator of durability of concrete which may be frequently exposed to freezing and thawing conditions.

• Compressive Strength - This test determines the maximum compressive strength of cured concrete samples.

• Air Void Analysis (Suitable for dry-cast concrete products) - This test determines the shape and size of air voids in cured concrete samples.

• Freeze-Thaw/Salt Scaling - This test is a combination of two tests, which determines, to some degree, the cured concrete's resistance to salt scaling and also to frequent freezing and thawing cycles. It is a very good indicator of the quality of the curing process.

• Density - Determining the density of the concrete material provided information related to the degree of compaction the concrete mix was subjected to in the mold before curing. The denser the product, the better the quality of concrete, assuming that a suitable mix design was used and the product was cured properly.

• Water Absorption - This test determines the amount of water the sample can absorb over a given time period. Generally, the more water absorbed, the poorer the quality.

• Minimum Cover Over Reinforcing - Panels and posts should be checked to ensure that the minimum concrete cover over the reinforcing is maintained during the casting operation. Adequate cover is critical in preventing or slowing down the penetration rate of salt laden moisture from reaching the reinforcing bars. This results in the corrosion of the bars and subsequent spalling of the concrete surface along with the drastic deterioration of the structural properties of the components.

• Dimensions - All precast or cast-in-place concrete products should be checked for proper dimensioning of key features (see Section 11.5.1).

• Visual Inspection - All precast and cast-in-place concrete products should be visually examined to identify any unusual and unwanted features which will affect the structure, durability, and performance of the noise barrier wall, such as honey combing, knuckling, cracks, and voids.

• Color Consistency - A consistent color from panel to panel may be difficult to achieve; however, it is an important aesthetic factor in achieving a successful barrier system. To ensure a panel-to-panel color consistency, a surface-applied stain may be more effective than the use of integral colors or pigments in the concrete mix.

5.2 Brick and Masonry Block
Brick (see Figure 72) is typically manufactured using a clay and sand mix which is fired in a kiln to increase the brick's strength and durability. Bricks can be produced in varying sizes but most commonly in 70 x 95 x 200 mm (2 x 3¾ x 8 in).
	Figure 72. Brick noise barrier photo #6511

Masonry block (see Figure 73) is manufactured using a dry-cast concrete mix. These blocks can be produced in any size but with the most common in the range of 200 to 300 mm (8 to 12 in) thick by 200 to 250 mm (8 to 10 in) high and 355 to 460 mm (14 to 18 in) long.
	Figure 73. Masonry block noise barrier photo #2456

Features - Both brick and masonry block walls can be either hand-laid or preassembled by machine. Hand-laid walls have greater versatility in their ability to conform to the variety of ground contours encountered in the roadway environment and in their layout than do the preassembled panels with their fixed panel sizes and heavy equipment requirements. Preassembled panels have an advantage in speed of erection, provided that the site environment allows for easy maneuvering of the necessary cranes and transport vehicles. In addition, brick and masonry block walls can be constructed satisfactorily with no special leveling courses on grades of up to 6 percent. In some cases, brick is used as a facing or veneer on masonry block or cast-in-place walls.

5.2.1 Special Considerations.
All brick and masonry walls, whether they are hand or machine laid, require a continuous concrete foundation (see Figure 74). The wall must be anchored to the foundation with reinforcing bars. Vertical and horizontal reinforcing bars are also needed in the wall itself to provide structural strength. Preassembled panels usually must be braced while the supporting concrete gains its strength.
	Figure 74. Barrier concrete foundation photo #568

Scaffolding for Installation - In most cases, scaffolding is needed to install brick and masonry block noise barriers (see Figure 75). Cranes may be used to install prefabricated or preassembled panels, but crews are still needed on scaffolding to fasten the panels to the posts and framework. Scaffolding needs room, a good solid foundation, and a considerable amount of effort and time to install. All of these factors should be considered before this type of material is selected for a specific site.

Figure 75. Scaffolding for barrier installation photo #2455

• Regional Differences - Masonry blocks are less suited for the more northern regions where the blocks would come into frequent contact with substantial amounts of deicing salts, which tend to deteriorate the blocks and the bonding mortar between them.

5.2.2 Verification of Quality.
The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials.

• Compressive Strength - The compressive strength of the brick or masonry block, the concrete used to fill the voids inside the wall, and the concrete used in the foundation should be tested, since these are the structural components of the wall system.

• Dimensions - Since concrete blocks and bricks are shipped on pallets, it may only be necessary to check the dimensions of one unit per pallet (see Section 11.5.1).

• Mortar - The mortar used in most concrete block noise barriers is an integral part of the structural strength of the wall enabling, it to withstand lateral forces against the wall. Therefore, the quality of the mortar becomes quite critical and should be checked to assure adherence to project specifications.

5.3 Metals

Three type of metals are most commonly used: (1) steel; (2) aluminum; and (3) stainless steel.

Steel - Steel is the least expensive and most common of all metals used in construction (see Figure 76). It is composed of a mixture, in varying proportions, of iron ore, carbon, and small amounts of other metals depending on the physical characteristics desired. Most steel panels, posts, and girts are either: (1) coated with plastisols, bonded powders, enamel paints, or galvanizing material; or (2) manufactured as a self protecting weathering steel.

Figure 76. Metal noise barrier photo #158

Aluminum - Aluminum is a lightweight alloy commonly made from bauxite and is typically coated with a bonded powder, enamel paints or anodized (see Figure 77). It is not compatible with galvanized coatings.
	Figure 77. Metal noise barrier photo #5692

Stainless Steel - Stainless steel is a highly durable and corrosion resistant metal alloy. It is a mixture of steel carbon, nickel, and chrome (in varying proportions). Since this material is virtually corrosion resistant, the surface does not need to be coated.

Features - Metal panels have a weight advantage which makes them particularly useful for vertical extensions of existing sound walls, for mounting on existing retaining walls which have limited residual strength, or on bridge structures, because of their light weight.

Typical Use - This type of material can be used anywhere. However, bridges and retaining walls are ideal locations for the use of these light-weight type of panels.

5.3.1 Special Considerations.

• Weathering Steel - Such unpainted, rusting panels are found to stain adjacent concrete.

• Adjacent Vegetation - Plantings do not grow well next to sun-heated metal panels. In addition, vines have difficulty in gaining a foothold on coated panels.

• Non-compatibility of Various Metal Combinations - Care should be taken to ensure that differing metal which come in contact with each other do not have an adverse effect on one another. This is particularly true for aluminum coming in contact with steel. The aluminum acts similarly to the zinc in the galvanizing material where it is the sacrificial element and will eventually disintegrate over a short period of time.

• Sound Transmission Class (STC) - Most metal sheeting materials do not meet the typical minimum panel weight and/or sound transmission class required in typical noise barrier specifications (see Section 3.4.2). However, adding corrugations or ribs to the profile of the panel material tends to improve the sound transmission class of the panel.

• Appearance - Occasionally, metal walls impart an industrial appearance which is considered undesirable by some residential neighborhoods. This is particularly true for the back side where the girts and posts are exposed to the view of the residents. To overcome this perception, a double faced wall may be used. This system incorporates the use of steel panels on both sides of the post and framework.

  Climbability - The girts, in combination with horizontally mounted corrugated or ribbed metal panels, provide an opportunity for the wall to be climbed from the side containing the girts or ribs (see Figure 78). This can be prevented by the addition of a sheet of metal facing to cover the girts and posts on this side of the wall
  	Figure 78. Metal barrier: climbability photo #157

• Glare - All metal barriers are susceptible to glare from opposing light sources. This issue is addressed in more detail in Section 9.6.

• Conductivity - Since all metals are electrically conductive, the installation of metal noise barrier walls should be avoided near power lines unless all metal components can be properly grounded.

• Scaffolding for Installation - In most cases, scaffolding is needed to install metal noise barriers. Cranes may be used to install prefabricated or preassembled panels, but crews are still needed on scaffolding to fasten the panels to the posts and framework. Scaffolding needs room, a good solid foundation, and a considerable amount of effort and time to install. All of these elements should be considered before this type of material is selected for a specific site.

• Panel Thickness - Metal noise barrier panels are typically in the range of 18 to 22 gauge thickness making them quite susceptible to damage from vandalism, debris, errant vehicles, snow plow operations, and other maintenance equipment. Therefore, consideration should be given to the thickness of the panel, the structural strength that can be achieved through corrugations, and the distance from the roadway that they are installed.

5.3.2 Verification of Quality.
The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials.

• Dimensions - The panel, profiles, size, and gauge should be verified since any deviation from that specified in the design plans will effect the structural strength, durability, and performance of the noise barrier system (see Section 11.5.1).

• Coating Thickness - Coating thickness, whether it is a galvanized, painted, sprayed, or dipped, must be verified to ensure durability.

• Accelerated Weathering - This test provides information on how well coatings withstand extreme moisture, high temperatures and harsh light conditions. The test method normally involves exposing the samples to prolonged water spray and high doses of ultraviolet light, all carried out under high temperatures (approximately 63 degrees Celsius or 145 degrees Fahrenheit). Although accelerated test methods are not true representations of actual conditions, it does provide a reasonable tool to compare product performances and to be able to reasonably predict the results of long term exposure to harsh climatic conditions.

• Corrosion Resistance - This is another procedure used to test coatings, but is used mostly for testing their effectiveness in preventing corrosion of metal surfaces to which they are applied. The typical test method subjects a coated metal sample to constant exposure of salt and moisture.

• Structural Strength - Single, thin, flat sheets of metal usually do not have the structural strength to resist the wind loads to which noise barrier panels are normally subjected. Therefore, panels made of this type of material may require some form of stiffening before they are able to meet local structural design and testing procedure requirements.

• Metal Properties - Brittleness, hardness, and tensile strength should be verified by appropriate standard test method. Mill classification certificates should be available for all metal components.

5.4 Wood
Most wood noise barrier walls are constructed of pressure preservative treated lumber, plywoods (see Figure 79), and glue laminated products (see Figure 80).

Figure 79. Plywood noise barrier photo #657	Figure 80. Glue laminated post and plank noise barrier photo #736

A number of different species of wood have the potential for being used as a noise barrier product, but this does not mean that all perform equally. Some species, such as the pines, are well suited for pressure treatment. Whereas, it may be difficult to obtain a deep, uniform penetration of the preservative in spruces. Some of the more common species used are as follows:

Pacific Coast Douglas Fir Interior Douglas Fir White Fir Western and Eastern Hemlock Western Larch Jack Pine Red Pine Ponderosa Pine

Eastern White Pine Lodgepole Pine Western White Pine Southern Yellow Pine Red Spruce White Spruce Poplar Red Alder

Features - Panels can be either installed piece by piece in the field or partially assembled in a plant or on the ground prior to attachment to the post. Power nailers, which are commonly used in the plant as well as in the field, make quick work of assembly. Some wood barriers can also be easily dismantled if future highway changes are needed. This material blends well with natural or residential background and does not conduct electricity.

5.4.1 Special Considerations.

• Safety - Consideration to safety issues, such as shatter resistance, should be given when mounting wood noise barrier walls on traffic barriers.

• Burning Characteristics - Wood noise barrier walls will burn under the same conditions as any other wooden fence. The smoke and emissions that are generated from burning treated wood are considered toxic. The ash left from the burning of this type of wood is also toxic and can leach into the surrounding soil and water supply.

• Warping and Shrinkage - Wood products are not dimensionally stable and tend to warp/shrink leaving open cracks between joints, particularly if they have not been properly seasoned or kiln dried. The thicker the wood products are, the more problematic warping can become.

Tongue and Groove Planking (see Figure 81) - To prevent the occurrences of gaps between planks as the walls weather and the planks shrink or warp, specifications for wood plank walls should include deeper tongues and grooves than the industry standards.
	Figure 81. Wood barrier:tongue and groove planking photo #411

• Pressure Treating - Most wood will decay rapidly when in contact with moisture. To combat this, the common practice is to pressure treat the wood with a chemical preservative. There are several acceptable chemical solutions used, all with relatively equal performance. Some of the more common ones are ACC, ACA, CCA, Penta, and Creosote.

  ACC	Acid Copper Chromate.
  ACA	(Ammoniacal Copper Arsenate) is a bright greenish colored waterborne preservative.
  CCA	(Chromated Copper Arsenate) is a mild green colored waterborne preservative. This cannot be used on fir wood.
  Penta	(Pentachlorophenol) comes in either a gas or oil borne formula. Colors vary, with the gas borne generally being lighter in color. This product has a distinct odor for a season and tends to draw out natural pitch leaving deposits and streaks on the wall facing.
  Creosote	is a Coal Tar based oil borne preservative, usually dark brown in color.

• Glue laminated posts and planks - Due to the chemical composition of the glues commonly used to fabricate glue-laminated post and planks, these products are not suitable for treatment with water based preservatives, only the oil based Penta is recommended for this product.

• Cutting - Cutting pressure treated wood will expose untreated interior portions to the elements. These areas should be retreated with a compatible brush-on preserving solution.

• Fasteners - Fasteners used to assemble a wood noise barrier can either be staples, nails, lag bolts, carriage bolts, nuts and bolts, or screws. Ideally, they should be made of a non-corroding metal such as stainless steel or aluminum. However, aluminum and steel have been known to react unfavorably with some types of pressure treating chemicals; steel nails are also susceptible to this type of chemical reaction.

• Scaffolding for Installation - In most cases, scaffolding is needed to install wood noise barriers. Cranes may be used to install prefabricated or preassembled panels, but crews are still needed on scaffolding to fasten the panels to the posts. Scaffolding needs room, a good solid foundation, and a considerable amount of effort and time to install. All of these issues should be considered before this type of material is selected for a specific site.

• Wood Posts - Large posts have a tendency to check or split open, exposing untreated surfaces to decay and attack by insects. Checking can be minimized by allowing the posts to season properly before being pressure-treated. In addition, the butt end of the post should be kerfed to allow deeper penetration of the preservative solution.

• Color - Initial color is a problem with timber walls since it is usually governed by the type of preservative chosen. These barriers will eventually fade to a weathered brown or gray color. The waterborne preservatives are initially green with some fading rapidly and more uniformly than others. Repairs to damaged sections will be conspicuous unless repair planks are acquired at the time of construction and allowed to weather in the yard.

  Some saw mills stain their products with a preservative having an identifying hue or "mill bright." This practice creates visual chaos when different-sourced products are mixed in one installation.

5.4.2 Verification of Quality.
The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials.

• Structural Grade - Specifying a good structural grade of lumber does not guarantee that all pieces of wood will be straight enough to permit the tight fit normally required for wood barriers. Therefore it is essential to visually confirm the grade of the wood used and removing any pieces that are warped, checked, split, or have excessive knots.

• Dimensional Stability - Structural graded lumber does not ensure that the product will never shrink, particularly if the wood has not been properly seasoned or kiln dried before pressure treating. Therefore, all wood components, particularly large members, should be checked for dimensional tolerances (see Section 11.5.1).

• Determination of Penetration - This test determines the depth of penetration of the preservative into the wood. The penetration rate may vary between species.

• Moisture Content - This test determines the amount of moisture in the wood. It is a nondestructive test and should be conducted on all larger pieces and those where warping, checking, and splitting may be un-preventable or may have a serious impact on the overall performance of the wall.

5.5 Transparent Panels
The typical transparent noise barrier (see Figure 82) may use panel material made of either glass or a clear plastic product such as Plexiglas, Butacite, Surlyn, Lexan, or Acrylic. Glass panels are commonly made of single tempered or laminated tempered glass sheets. Both plastics and glass can be tinted and can also be etched or given a frosty appearance. Tempering of the glass is a heat treating process which strengthens the glass, producing a much more shatter resistant product. When it does shatter, the shards are small and granular in appearance, with pieces typically not larger than 12 mm (½ in). These are much safer than the long knife-like shards produced from shattering common non-heat treated glass. In addition to the tempering, the glass panels can also be laminated. This type of glass panel is produced by adhering two sheets of tempered glass sheets to both sides of a clear rubbery type flexible sheeting. When this type of glass panel is shattered, the glass will break into small granular-like pieces, where the pieces will remain adhered to the sheeting.
	Figure 82. Transparent panel noise barrier photo #1981

Features - The transparent panel materials are an ideal way of reducing or virtually eliminating the visual impact of a noise barrier.

Typical Use - Transparent barriers are normally only built for 3 reasons:

- To prevent hindering the scenic view for the driving public;
- To prevent hindering the scenic view for the residents adjacent to the roadway; or
- To prevent hindering the view of retail establishments for the driving public.

Since transparent noise barriers costs can be as much as 20 times that of common concrete or steel panels, the decision to use transparent noise barriers should not be made lightly. Possibly, the only other reasons for their use would be to improve safety, where opaque noise barrier walls may have an adverse affect on stopping sight distance, visibility in merge areas, lighting, and shading.

5.5.1 Special Considerations.
Transparent noise barriers come with their own unique set of engineering, safety, and environmental considerations which are significantly different than most other types of material normally used for noise barrier panels. Vandalism - Plastic panels are particularly susceptible to vandalism (see Figure 83), not only from the typical paint can, but also from knives, lighters, or matches.
	Figure 83. Transparent panel barrier: vandalism photo #1947

• Mounting - Depending on the type of material used for the panels, (glass, acrylic, etc.) and the size, the method of mounting can vary significantly. The general principle for mounting this type of thin, flat sheeting is that the method used must sufficiently reduce or remove the stresses on the material to eliminate the possibility of the panels breaking or falling out from between the supports. It is recommended that the manufacturer be contacted to obtain the best mounting method to be used for their specific product on every site.

• Dimensions - In order to enhance transparency, it would be preferable to use large pieces of material and to limit the number of supporting brackets. While transparent plastic sheeting materials are available in lengths exceeding 5 m (16 ft), the same is not true for tempered or laminated glass. Manufacturing constraints limit the maximum dimensions of glass plates. In addition, as the area of the panel increases, the thickness must also be increased to maintain structural integrity, thus increasing weight and cost (see Section 11.5.1). To reduce the need for thicker material, it is common to use smaller post spacing and/or framed panels that can be stacked between posts. Size of panels is also limited by handling capabilities.

• Edge Conditioning - To avoid thermal and stress cracking, all edges must be smooth and without defects. This is extraordinarily critical if the edges are not cut in a straight line.

• Ultraviolet Light Stabilizers - Some of transparent plastic sheeting materials are sensitive to ultraviolet light. If exposed to sunlight for extended periods of time without being protected by UV stabilizer additives or coatings, the sheeting will haze and discolor, leaving them translucent or even opaque in some cases. Even with the stabilizers, the sheeting will eventually be affected by the light. Acrylic based sheetings are much less sensitive to sunlight and tend to stay transparent for a longer period.

  Note that glass, by itself, is not affected by sunlight. However, if laminating material is used, this material may be sensitive to ultraviolet light.

• Shatter Resistance - Although most commonly used plastic products are relatively shatter resistant, glass is not; even when the glass is tempered and/or laminated, the panel will shatter

• Glare - All transparent barriers are susceptible to glare from opposing light sources. This issue is addressed in Section 9.6.

• Road Debris Damage - These types of panels are more susceptible to damage from flying debris than most other types of barrier materials. They are also very susceptible to the abrasive damage caused by the sand blasting action from stirred up road dirt.

• Cost - Depending on the size and type of material selected, the cost of transparent barrier sheeting can be anywhere from 10 to 20 times more than that of other barrier panel materials. However, in some areas of the country where the use of transparent barriers are prevalent, the costs may be much lower, even comparable to that of other barrier panel materials.

• Cleaning - To maintain their transparency, these types of panels need to be washed on a regular basis. This is of particular concern if the wall or the individual panels are tilted, which tends to hinder the natural cleaning process provided by rain on the underside of the panel. Access for cleaning of the panels is normally not a problem on the traffic side, which is usually the dirtier side of the wall. However, the opposite side may not be as accessible, and, in some cases, cleaning may not be feasible at all. This limitation should be considered when selecting barrier material. Cleanliness is particularly critical if the transparent noise barrier was constructed for safety reasons such as to improve visibility for stopping sight distance or merging.

• Breakage - Damaged panels cannot be repaired by patching. The only option is to replace the damaged sections.

• Regional Differences - There are generally no climatic restrictions for the use of any of the transparent sheeting materials.

5.5.2 Verification of Quality.
The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials.

• Dimensions - The panel, profiles, size, and thicknesses should be verified since any deviation from that specified in the design plans will effect the structural strength, durability, and performance of the noise barrier system (see Section 11.5.1). Of particular importance with the transparent sheeting material, whether it is glass or one of the plastics, is the strict adherence to the tolerances for the mounting hardware and the sealants to avoid uneven stress points which can result in material fracturing or warping. Such consequences can occur if a panel is placed between two posts with insufficient room for expansions or inappropriate expansion or caulking material. If too much room is provided at the panel-to-post connection points, the panel can actually become separated from the posts under certain conditions which will result in substantial contraction of the barrier material and/or excessive panel movement caused by vibration or wind.

5.6 Plastics
There are several types of plastic materials available for use as a barrier material, including Polyethylene, PVC, and fiberglass (see Figure 84). Features - The most unique features of plastic products are their versatility and moldability. This material can be produced to perform and appear the same as almost any construction material on the market today. Its light weight nature improves ease of handling both in the plant and in the field. In addition, most of these products are recyclable.
	Figure 84. Plastic noise barrier photo #782

Typical Use - Plastic noise barrier panels can be installed in almost any situation. However, due to there light weight characteristics, they are particularly suitable for structure mounted applications.

5.6.1 Special Considerations.

• Burning Characteristics - Plastic noise barrier walls tend to be more flammable than barriers made of other materials. The smoke and emissions that may be generated from burning plastics should be considered toxic. The ash left from any burnt material should also be considered as toxic and can leach into the surrounding soil and water supply.

• Shrinkage - Some plastic products are not dimensionally stable and may tend to shrink leaving open cracks between joints or may be susceptible to accelerated creep and deformation.

• Ultraviolet Protection - Some plastic products are very sensitive to ultraviolet light and tend to cause rapid deterioration of pigments, surface appearance, and material strength. To avoid this, it is possible to slow down the deterioration process by adding ultraviolet protection into the composition of the plastic at the time of molding.

• Creep - Creep, which is evident in most plastics to varying degrees, should be considered during the design of the barrier system by reducing the amount of strain to which the plastic components may be subjected.

• Vandalism - Plastic panels are particularly susceptible to vandalism from paint, knives, and lighters.

• Shatter Resistance - Although most commonly used plastic products are relatively shatter resistant, this characteristic tends to deteriorate over time, and the product becomes more brittle and may shatter on impact by flying objects or vehicles. Damaged panels can usually not be repaired by patching. The only option is to replace the damaged sections, thus increasing the cost of repairs and possibly jeopardizing the appearance of the barrier if similarly molded panels are no longer available or are difficult to reproduce at a reasonable cost.

• Glare - Depending on the surface texturing applied to the plastic surfaces, the barrier panels may be susceptible to glare from opposing light sources. This issue is addressed further in Section 9.6.

5.6.2 Verification of Quality.
There are countless standard test methods published to assist in the verification of plastic products. Each of these test methods are normally only relevant to very specific plastic formulations. The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials. This handbook will not attempt to list or describe these and suggests current testing information be obtained from other sources.

• Dimensions - The panel's profile, size, and thickness should be verified since any deviation from design specifications will affect the structural strength, durability, and performance of the noise barrier system. Of particular importance is the strict adherence to the tight tolerances for the mounting hardware and the sealants to avoid uneven stress points (see Section 11.5.1).

5.7 Recycled Rubber
The issue of using recycled rubber from tires in products used for roadway construction has been under investigation for many years by numerous government agencies, world wide. The results of their efforts indicate widely varying success in trying to adapt this type of material into a usable product (see Figure 85). Recycled rubber can refer to a wide range of products, made from an equally wide range of rubber compounds. In practice, however, the rubber waste stream is dominated by scrap tires. There are two other significant sources: (1) tire trim and off-spec tires from tire production and (2) buffings from rubber product manufacturers.
	Figure 85. Recycled rubber noise barrier photo #3124

5.7.1 Special Considerations.

• Flammability and Smoke - Rubber is notorious for its high flammability and the dense smoke which is produced when it burns. If a noise barrier made from this material should ignite as a result of such incidents as grass fires, accidents, or vandalism, the accelerated flame spread and the dense smoke produced could result in safety and legal issues. To reduce rubber's susceptibility to these concerns, flame and smoke retardants are available that can be added to the mixture during the manufacturing process.

• Toxicity - Recycled rubber tire material has been found to be nontoxic under leachate testing. However, additives, such as binders, retardants, coatings, and coloring, included in the mix to form and enhance the material, can create potential toxicity problems. These additives are, in some cases, proprietary with the specific formulations kept in confidence by the manufacturer.

• Structural Strength - Rubber material, on its own, does not have sufficient rigidity to be considered as a structural component of a noise barrier panel. Therefore, bonding agents must provide adequate stiffness to enable the panels to be considered strong enough to withstand wind loading, or the rubber material must be firmly attached to a suitable stiffener, such as channel backings, cores, or casings.

• Binders - Rubber and some binders tend to oxidize over time when exposed to the elements. They may also be susceptible to certain chemical or petroleum products. This increases the potential of premature disintegration of the panels. If concrete is used as a binder, concrete modifiers and special treatment of the crumb rubber are required before they will bond properly to each other. This is particularly important when these panels are exposed to salt, cold weather, and flexing for a long period of time.

• Coatings - Some coatings suitable for rubber have a questionable life expectancy. They have a tendency to oxidize prematurely, particularly when used in conjunction with certain pigments. If the surfaces of the noise barrier panels are being manufactured to be sound absorptive, the coatings may clog the surface openings thereby reducing the Noise Reduction Coefficient (NRC).

• Sound Transmission Class (STC) - Although the weight of the panels may be sufficient to meet general requirements for minimum STC ratings, it may not be sufficient when produced as a porous panel. Even when stiff backers or cores are used, the nature of this material may require the cores or backers to be extensively perforated to promote bonding.

• Recyclability - The recyclability of the final product may have been reduced drastically by the type of additives needed to alter the physical properties of the panel so that it can meet the various fundamental requirements for an effective, safe, and durable noise barrier product.

5.7.2 Verification of Quality.
The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials.

• Flame Retardants - To ensure that the retardants are adequate, the minimum allowable rate of flame spread and smoke generated should not be greater than the rate for a typical fence material, such as pine.

• Toxicity - Concerns for environmental damage and health hazards should be addressed by requesting leachate testing or other methods to determine the toxicity of the final noise barrier panel material.

• Structural Strength - The structural strength of the panel must be verified through load testing on a production panel.

• Bonding - To optimize the bond between the rubber crumb particles, it is necessary to ensure that the rubber crumb is new or has been protected from the elements. The binders used should be stable under prolonged exposure to ultraviolet light. The manufacturing process should ensure that each rubber particle is completely encapsulated by the binder. If cement is used, the rubber surface should be treated or impregnated with a bonding agent compatible with both the rubber and the concrete. Or, the concrete should contain modifiers that will allow it to firmly bond to the rubber and be able to stand the test of time.

• Coatings - The coated panels should be subjected to weatherometer testing to determine the longevity of the coating.

• Noise Reduction Coefficient (NRC) - If the panels are to be coated, the NRC rating should be verified after the panels have been coated.

• Sound Transmission Class (STC) - The assembled noise barrier system should be tested to verify the STC rating. Even though the mass requirement for a suitable STC has been theoretically met, the finished panel may be two porous to actually achieve the desired STC.

5.8 Composites
Composite noise barrier materials, in general terms, can be defined as any product composed of two or more primary materials, such as plywood with a fiberglass skin (see Figure 86), or wood mixed with concrete and then layered onto concrete (see Figure 87). Since the possibilities are almost endless, this section will mainly focus on the special considerations which should be used in evaluating their safety, durability, and performance.

Figure 86. Composite noise barrier photo #132	Figure 87. Composite noise barrier photo #707

5.8.1 Special Considerations.
The combining of basic materials has a tendency to change the performance, durability, and, in some cases, the safety characteristics of the final product. These changes should be investigated thoroughly before the composite materials are used in an actual installation.

• Burning Characteristics - Some composite materials may have a tendency to burn, or be severely damaged under certain conditions. The smoke and emissions that may be generated from burning materials might also be toxic. The ash left from any burnt material may also be considered as toxic and will most likely leach into the surrounding soil.

• Shrinkage - The shrinkage rate of the primary materials differ significantly and may cause dimensional instability and leave open cracks between joints or promote accelerated creep, warping, or delamination.

• Ultraviolet Protection - Some products are very sensitive to ultraviolet light and tend to cause rapid deterioration of pigments, surface appearance, and material strength. To avoid this, it is possible to slow down the deterioration process by adding ultraviolet protection into the composition of the material at the time of molding.

• Creep - If plastic is part of the composite material, creep should be considered during the design of the barrier system by reducing the amount of strain which the plastic components may be subjected to.

• Vandalism - Some materials are particularly susceptible to vandalism from paint, knives, and lighters.

• Shatter Resistance - Although most commonly used products are relatively shatter resistant, this characteristic tends to deteriorate over time and the product becomes more brittle and may shatter on impact by flying objects or vehicles. Damaged panels can usually not be repaired by patching. The only option is to replace the damaged sections, thus increasing the cost of repairs and possibly jeopardizing the appearance of the barrier if similarly molded panels are no longer available or are difficult to reproduce at a reasonable cost.

• Structural Strength - Some primary used in composite panels do not have sufficient rigidity to be considered as a structural component of a noise barrier panel. Therefore bonding agents must provide adequate stiffness to enable the panels to be considered strong enough to withstand wind loading, or the material must be firmly attached to a suitably stiff backing, core, or casing.

• Binders - Some binders tend to oxidize over time when exposed to the elements. They may also be susceptible to certain chemical or petroleum products. This increases the potential of premature disintegration of the panels. If concrete is used as a binder, concrete modifiers, and special treatment of the crumb rubber are required before they will bond properly to each other. This is particularly important when these panels are exposed to salt, cold weather, and flexing for a long period of time.

• Coatings - Many coatings have a questionable life expectancy. They have a tendency to oxidize prematurely, particularly when used in conjunction with certain pigments. If the surface of the noise barrier panels are being manufactured to be sound absorptive, the coatings may clog the surface openings thereby reducing the Noise Reduction Coefficient (NRC).

• Sound Transmission Class (STC) - Although the weight of the panels may be sufficient to meet general requirements for minimum STC ratings, it may not be sufficient when produced as a porous panel. Even when stiff backers or cores are used, the nature of some materials may require the cores or backers to be extensively perforated to promote bonding.

• Recyclability - The recyclability of the final product may have been reduced drastically by the type of additives needed to alter the physical properties of the panel so that it can meet the various fundamental requirements for an effective, safe, and durable noise barrier product.

• Future Disposal - The nature of the primary materials or when combined with other materials may render the final product unsuitable for future disposal in land fill sites.

• Safety - Consideration to safety issues, such as shatter resistance, should be given when panels are mounted on traffic barriers, such as Jersey barriers.

5.8.2 Verification of Quality.
Some combinations may not have sufficient in-field performance history to be able to determine the long term durability, safety, and performance of specific composites. Therefore testing is much more critical for these types of materials than most others used for noise barrier panels. There are countless standard test methods published to assist in the verification of various materials. Each of which are normally only relevant to very specific material formulations. This handbook will not attempt to list or describe these and suggests current testing information be obtained from other sources. The tests discussed within this section are described in detail in Section 10 which discusses product evaluation of all types of barrier materials. However, the following fundamental tests should be considered for all of these types of materials.

• Dimensions - The panel's profile, size, and thickness should be verified since any deviation from design specifications will effect the structural strength, durability, and performance of the noise barrier system (see Section 11.5.1).

• Flame Retardants - To ensure that the inherent, or added, flame retardants are adequate, the minimum allowable rate of flame spread and smoke generated should not be greater than the rate for a typical fence material, such as pine.

• Toxicity - Concerns for environmental damage and health hazards should be addressed by requesting leachate testing or other methods to determine the toxicity of the final noise barrier panel material.

• Structural Strength - The structural strength of the panel must be verified through load testing on a production panel.

• Bonding - To optimize the bond between the composites, it is necessary to ensure that the primary materials and binders used are stable under prolonged exposure to ultraviolet light and that the proper binders are used for the specific materials.

• Coatings - The coated panels should be subjected to weatherometer testing to determine the longevity of the coating.

• Noise Reduction Coefficient (NRC) - If the panels are to be coated, the NRC rating should be verified after the panels have been coated.

• Sound Transmission Class (STC) - The assembled noise barrier system should be tested to verify the STC rating.

• Freeze-Thaw/Salt Scaling - This test is a combination of two tests, which determines the material's resistance salt scaling and also to frequent freezing and thawing cycles. It provides a good indicator as to how the final material combination(s) will perform under harsh weather conditions.

5.9 Barrier Surface Treatment
This section describes various surface treatments, including textures, colors, and coatings which may be applied to a noise wall.

5.9.1 Textures.
A vast majority of surface textures are available to the noise barrier designer (see Figure 88). In many cases, such treatments can be applied to several elements of the barrier systems (e.g., posts, panels, caps, etc.). Different barrier surface treatments can be obtained by having different combinations of treatments on these separate barrier elements. The intent of the following discussion is not to address all possible combinations of treatment, but rather, to discuss the more common surface texture types available. The following discussion of texture types is categorized by material type.
	Figure 88. barrier surface treatment textures photo #512

5.9.1.1 Concrete.
Smooth Surface - Such a surface is produced by traditional concrete finishing techniques (see Figure 89). The top side of precast panels cast in a horizontal precast bed are typically "floated" to a smooth texture. Obtaining a smooth surface on the bottom side of such a panel or on either side of a vertically formed panel requires finishing after the initial concrete cure, including filling of voids and final "rubbing" of the concrete with a thin cement mixture.
	Figure 89. Concret:smooth surface photo #996

• Exposed Aggregate - A stone aggregate surface is typically obtained using precast concrete barrier elements (see Figure 90). The surface is obtained using the selected type, color, and gradation of aggregate in the barrier's concrete mix itself. Care must be taken to assure that the aggregate selected for its aesthetic appearance also meets the required structural requirements related to strength, angular size, shape, etc. Also, the aggregate should be sufficiently screened, graded, and inspected to assure the removal of any iron ore aggregate which could give the appearance of rust bleeding from the panel. Prior to the panel being cast, a retarding chemical is placed on the form work of the surface which will ultimately have the exposed aggregate finish. The retarder extends the curing time of the concrete on the surface to which it is applied. Different grades of retarder are available which provide varying degrees of penetration into the concrete surface and thus will provide varying degrees of exposure of the aggregate. Following the initial cure of the panel to a degree of strength which enables it to be lifted out of the horizontal form to a vertical position, the surface treated with retarder is power washed with a high pressure (2,000 psi) water wash. This process removes the "retarded" (soft) concrete and exposes the aggregate. An acceptable exposed aggregate surface is most easily obtained on the bottom (down side of a precast panel). Obtaining an acceptable exposed aggregate surface on the top side of a panel is significantly more difficult and may require seeding of the top surface with additional aggregate in addition to the application of a retarder directly to the top panel surface immediately after its pour. Producing consistently acceptable exposed aggregate panels is an art requiring experience on the part of the precaster as well as an emphasis on quality control.

Figure 90. Concrete: exposed aggregate photo #1180

Form Liners - A wide variety of surface treatments can be obtained by the use of form liners (see Figure 91 to 93). Form liners are essentially sheets of material (usually a flexible material such as rubber, but also may be made of wood, metal, or other material) which have been fabricated or molded with one flat side and one side containing the "mold" of the desired surface treatment. These sheets are made to be re-useable for multiple applications. While form liner treatments (related to noise barriers) are most often applied on the down side of panels cast in a precast plant, they can be applied to one or both sides of a panel if cast in a vertical position. Recent innovations have produced machinery capable of pressing a form liner onto the top side of a panel, thus allowing form liner finishes on both sides.

Figure 91. Concrete: form liner photo #1180

Figure 92. Concrete: form liner photo #498	Figure 93. Concrete: form liner photo #698

Care must be taken when such form liner sheets are butted together such as in long precast bays where bulkheads may be moved to produce the required lengths of individual noise barrier panels. In such an application, the joints between form liners will occur mid-panel on many of the noise panels (see Figure 94). To avoid unsightly joint lines which can interrupt the desired surface texture, care must be taken to assure that the joints are tight, adequately secured to the form work, and "lined up" in terms of the form liner pattern. The potential for such joint lines is greatest with form liners having slight relief (depth of pattern). A deep relief pattern such as a deep rib texture can more easily hide such joints, assuming that the rib patterns are carefully lined up at the joints. Since such form liners by nature create non-smooth surfaces, care must be taken to avoid jagged edges at the ends and sides of the panels. This is particularly critical if the concrete will be painted or stained since any jagged edges can more easily be chipped or broken off, exposing areas of unpainted or unstained concrete.

Figure 94. Concrete: form liner panel joints photo #7029

Raked, Broomed or Other Applied Finishes - These types of treatments are applicable to the top side of precast panels (see Figure 95). They can result in various degrees of patterns created by hand-applied techniques, with less or equal effort as compared to "floating" a smooth concrete surface.
	Figure 95. concrete: raked finish photo #508

• Stamped Finish - It is possible, with specialized techniques, to press or stamp a design into the top surface of a horizontally poured panel (see Figures 96 and 97). Such techniques have been used (in conjunction with the use of a pigmented surface layer of cement) to create a brick-type surface. Such a treatment is somewhat more labor-intensive than other treatments, particularly if a grout-type look is desired. Any stamping process also requires that the aggregate in the panel be sufficiently deep to allow such stamping.

Figure 96. Concrete: stamped finish photo #6512	Figure 97. Concrete: stamped finish photo #6514

• Inserts - Special and unique aesthetic treatments can be obtained by the use of panel inserts (see Figures 98 and 99). These inserts are typically precast or manufactured separately from the concrete panel and either imbedded in the panel during the pouring process or set into an equal size and shape indentation within the precast panel. In either case, care must be taken to assure an adequate bond/attachment between the insert and the noise barrier panel. Such bonds are best accomplished by mechanical attachments (studs or anchors) although chemical bonding techniques may also be employed.

Figure 98. Concrete: inserts photo #8014	Figure 99. Concrete: inserts photo #8015

• Veneers - In the context of this discussion, veneers are meant to represent a separately manufactured material applied to the surface of a concrete noise barrier. Such materials are usually applied for aesthetic reasons although certain veneers may be applied to make the surface sound absorptive. Examples of such veneers include full width and thin brick, ceramic tile, and porous composite sound absorptive materials. The primary concern of the use of veneers is related to assuring an adequate attachment bond as discussed above.

Stucco - Stucco is a finish coat of cementitious material which can be textured in a variety of ways. It bonds directly to the concrete noise barrier without any additional attachments. As such, the cleanliness and roughness of the concrete noise barrier surface is critical in order to assure an adequate bonding surface for the stucco (see Figure 100).
	Figure 100. Masonry block: stucco photo #1066

5.9.1.2 Masonry Block.

• Exposed Aggregate - Compared to the variety of exposed aggregate textures available in a concrete panel, such treatments with masonry blocks are significantly limited. This is due to the limited range of aggregate used in the mass production of concrete blocks.

• Form Liners - While some molds may be applied to the production of concrete blocks, their use is rare and limited.

• Veneers - Veneers can be applied to concrete block in a manner similar to that of concrete panels.

• Fractured Fin - A common means of achieving a rough, textured surface in concrete block is through the use of fractured fin surfaced block (see Figure 101). This surface is achieved by mechanically sheering the block to create a rough surface.

• Stucco - Stucco can be applied to concrete block in a manner similar to that of concrete panels (see Figure 102).

Figure 101. Masonry block: fractured fin photo #948	Figure 102. Masonry block: stucco photo #1061

5.9.1.3 Brick.

• Type of Brick - Literally hundreds of types of brick are available for use in the construction of noise barriers (see Figures 103 and 104). These include more common types of standard brick, including pavers, plus the slump stone and adobe style of brick. Bricks may be laid up with mortar in multiple courses or used to face concrete or concrete block walls as discussed above. Whether bonded to other brick courses or to concrete or block walls, bonding straps of some form are used to secure the system components to each other.

Figure 103. Brick: surface texture photo #560	Figure 104. Brick: surface texture photo #6518

• Type of Mortar - Various colors and types of mortar are available for bonding bricks.

• Type of Bond - Similarly, various styles or patterns (known as bonds) are used to construct brick or brick-faced barriers. The most common are stacked bond and running bond. Bricks of different widths, styles, and colors may also be employed to create unique and interesting patterns and/or to create designs representative of area landmarks, themes, etc.

5.9.1.4 Metal.
Mechanically Formed Shapes - Most textures created in metal noise barriers are via the use of roll-formed mechanical brakes, benders, and similar factory operated devices (see Figure 105). A variety of such formed shapes are available for both preassembled components as well as for components assembled in the field. Although different styles and shapes are possible on each side of the noise barrier (through the use of double-faced panels), surface treatments are most often similar on both sides.
	Figure 105. Metal: surface texture photo #1708

• Pressed (dimpled) surfaces - Smaller relief impressions in metal panels may also be obtained by plant-applied processes.

5.9.1.5 Wood.

• Plank Orientation - Different visual appearances can be obtained via the orientation of wood planks used in noise barrier construction. Horizontal, vertical, and diagonal configurations have been employed (see Figures 106 and 107). Planks of different widths and depth can also create interesting visual effects.

Figure 106. Wood: horizontal plank photo #745	Figure 107. Wood: vertical plank photo #730

Battens - As well as providing a mechanism for preassembling panels on the ground (prior to their attachment to posts) battens can also be employed to create patterns in the noise wall design (see Figure 108). Grain - Selection of wood grain and roughness can also create the desired surface texture treatment.
	Figure 108. Wood: patterns on battens attached to panels photo #471

Lamination - Various design patterns are possible with laminated panels through the particular orientation of the laminated component elements (see Figure 109). Designs with such a system will most often be similar on both sides of the barrier.
	Figure 109. Wood: pattern on laminated panels photo #663

• Post Type and Orientation - Wood posts of various shapes and sizes have been employed in the construction of noise barriers. Circular, square, and rectangular post sections are common (see Figures 110 and 111). The exposure or concealment of the post can create different textural and shading patterns.

Figure 110. Wood: circular post type photo #744	Figure 111. Wood: square post type photo #435

5.9.1.6 Transparent Materials.
Surface texture treatments are limited to aesthetic type designs (stencils) applied to such barriers (see Figure 112).
	Figure 112. Transparent: surface stencil design photo #1954

5.9.1.7 Plastics.
Surface textures of plastic panels are generally limited to shapes and textures which can be accomplished via the panel component molding process (see Figure 113).
	Figure 113. Plastic: surface texture photo #792

5.9.1.8 Rubber.
Similarly, barriers constructed of recycled rubber materials are limited to shapes obtainable through molding of their components. The surface texture of such panels is also influenced to some degree by the density and porosity of the rubber (see Figures 114 and 115).

Figure 114. Rubber: surface texture photo #2948	Figure 115. Rubber: surface texture photo #2949

5.9.1.9 Composites
. Composite panel texture treatment opportunities reflect the availabilities and constraints of the particular components used to form the outside barrier face (see Figures 116 and 117).

Figure 116. Composite: surface texture photo #136	Figure 117. Composite: surface texture photo #708

5.9.1.10 Other Applications

• Planted Walls - Textures of planted walls are limited to what can be accomplished via the planted vegetation within the wall system.

Gunite on Reinforced Chain Link Fence - A unique texture treatment has been successfully used in a noise barrier application for over twenty-five years. This system consists of a chain link fence which was reinforced with expanded metal mesh and then sprayed with gunite to provide the appearance of a stucco wall (see Figure 118). While such a treatment is probably not advisable close to travel lanes, where it is more prone to damage from airborne debris or vehicular hits, it appears to be a viable and economical technique for use in areas somewhat removed from active travel lanes. The particular wall shown has also withstood several major and many minor earthquakes with little or no damage.

Figure 118. Gunite: surface texture photo #2243

5.9.1.11 Special Considerations.
As discussed briefly in the introduction of Section 5, the selection of a particular surface treatment texture depends on a number of factors including aesthetic requirements of both sides of the barrier, constructability issues, maintenance concerns, and the type of barrier material. The selection of a form liner finish on both sides of a barrier requires specialized equipment, could negate the ability to use horizontally cast precast barrier elements and could require the use of either vertically cast precast elements or cast-in-place barriers. The inability to use full height precast panels (in a situation where placing such panels is restricted due to overhead wires or other factors) could limit the barrier type, material, and therefore the surface texture options. Texture treatments used with stacked panels should be coordinated so that the joints are either concealed by the pattern or become a part of the pattern (see Figures 119 and 120).

Figure 119. photo #902	Figure 120. photo #533

5.9.2 Color.
The desired color of noise barriers is provided by one of the following two general techniques or a combination thereof:

• by the natural color of the noise barrier material being used (possibly enhanced by a clear coating)

• by application of a paint, stain, pigmented coating, or integral pigment added to the noise barrier material.

Options related to colors of noise barriers are discussed below for various types of barrier materials.

5.9.2.1 Concrete and Masonry Block
These types of materials are quite versatile in their ability to be colored (see Figures 121 to 123). The color of their natural elements (sand, stone, and cement) can be varied to obtain different earth tone colors. Addition of a pigment to such a coating can provide a uniform color to a plain concrete surface wall. Pigments can also be added to the mix prior to the forming of barrier components. Quality control and consistency are critical in the production of pigmented concrete since unevenness and blotchiness can become apparent, particularly on smooth surfaces. A consistent color from panel to panel is an important aesthetic factor in achieving a successful barrier system. To ensure a panel-to-panel color consistency, a surface-applied stain may be more effective than the use of integral colors or pigments in the oncrete mix. Natural and pigmented barriers have the advantage of not showing damaged areas (from chips, scrapes, etc.) as much as painted or stained surfaces. The maximum protection against the visual effects of such damage can be provided by use of a pigmented panel with a surface stain of a matching color. This reduces any unevenness in the pigmenting process while providing for consistent color throughout the panel. The added cost of such a dual treatment may, however, not be warranted in many cases.
	Figure 121. Concrete: color photo #2337

Figure 122. Concrete: color photo #1218	Figure 123. Masonry block: color photo #2373

5.9.2.2 Brick.
While many variations in color are available with these types of materials, the color is limited to the that of the material itself (see Figure 124).
	Figure 124. Brick color photo #2655

5.9.2.3 Metal.
Except in cases where a natural (e.g., aluminum, stainless steel, etc.) or rusted look (e.g., weathering steel) appearance is desired, color on metal panels is usually obtained by painting or application of some bonded type of surface coating (see Figure 125). In many cases, barrier elements (particularly posts) are designed with their protective galvanized finish as the ultimate color. This process protects the post while negating any maintenance requirements associated with paint. When colors are desired on metal barriers they may be either applied at the origin of manufacture or in the field. If applied where the barriers are manufactured, such coatings may be applied using a variety of techniques such as a baked enamel finish, a sprayed-on finish, by using a plastisol (a poured-on PVC emulsion), or via a bonded powder coating applied via an electrostatic process. In any coating application process, surface preparation is critical. Such preparation needs to consider the initial, intermediate, and final coats in terms of their undercoating and surface preparation requirements. Manufacturers' requirements (related to both coating and material coated) such as extent of sand blasting (white blast, grey blast, etc.), cleaning materials, temperature and moisture controls, etc. need to be adhered to closely. Certain coatings may also allow for easier removal of graffiti without any detrimental effects to panel aesthetics.
	Figure 125. Metal: color photo #1720

5.9.2.4 Wood.
A wide variety of natural colors is available in wood products (see Figure 126). The selection of a particular wood species for its color attributes must also consider other factors of the wood. While a particular wood species may have the exact natural color desired, its qualities related to other areas of concern (durability, warping, rot resistence, etc.) may be unacceptable. Most woods can be stained or painted to obtain a desired color. Clear and pigmented preservative treatments are also available. The type of treatment, stain, or paint must be compatible with the type of wood, nails, and other fasteners used in the barrier assembly, and with the environment in which the barrier will be placed. Chemical reactions between steel nails and certain wood preservative treatments have caused these nails to corrode, requiring their replacement with stainless steel fasteners. Moisture content at the time of paint or stain application is also critical. It is essential that a wood barrier be properly seasoned before application of any paint or stain, and that no such protective materials are applied when the barrier is damp. Specific requirements will vary for each coating material, necessitating strict compliance with the specifications of the coating manufacturer.
	Figure 126. Wood: color photo #464

5.9.2.5 Plastics, Fiberglass, and Acrylics.
Color of these materials is most often obtained through the pigmentation of the emulsifiers used in the molding process of barrier elements (see Figure 127). A scratch coat may also be added for protective purposes.
	Figure 127. Plastic: color photo #1728

5.9.2.6 Rubber.
Recycled rubber material generally cannot be pigmented. Rubber can be coated (usually with a more expensive polyurethane coating) to obtain a desired color.

5.9.2.7 Composites.
Composite panel color treatment opportunities reflect the availabilities and constraints of the particular components used to form the outside barrier face (see Figure 128). Particular concern should be paid to ensure compatibility between the barrier materials (including any glues, attaching devices, etc.) and the applied coating to negate any potential for damaging chemical reactions.
	Figure 128. Composites: color photo #2533

5.9.2.8 Planted Walls.
Color in such walls can be obtained by the selection of the appropriate plants. Different colors and patterns can be obtained with changes in season.

5.9.3 Coatings.
The discussion in this section is limited to coatings on concrete and masonry barriers, since coatings on other types of barriers are inherently addressed within the discussions of color found in Section 5.9.2.

Coatings are typically applied to concrete or masonry barriers for protective and/or aesthetic reasons. Protection against the elements (wind, rain, salt spray, ultraviolet light, etc.) and potential vandalism (anti-graffiti) are common reasons for application of such coatings. While coatings can significantly enhance the appearance of a barrier or be its primary aesthetic element, care should be taken in application to assure positive results. Addition of a clear protective coating can have positive benefits in terms of enhancing the color and brilliance of a concrete exposed aggregate surface. Conversely, application of the same clear protective coating on a plain concrete barrier can have a negative impact by making the barrier surface look wet and enhancing any unevenness or blotchiness.

5.9.3.1 Anti-Graffiti Coatings.
A number of products are on the market which can provide varying degrees of protection against graffiti. Few, if any, barriers can be made graffiti-proof. Quick removal (within 24 hours) of the graffiti may discourage "artists" who try to return to embellish upon the previous night's work. Rougher surfaced and darker colored barriers and barriers covered with vegetation or vines may also provide more resistence to being "hit" by graffiti artists as compared to light colored, non-planted, and/or smooth-surfaced barrier surfaces. However, there is no guarantee that a barrier will not be susceptible to graffiti. Once hit with graffiti, an unprotected concrete or masonry surface can be marred for life due to staining and penetration of the graffiti paint into the concrete substrate, even if the graffiti is "removed." Anti-graffiti coatings are meant to prevent such penetration and to make the removal of the graffiti somewhat easier. Anti graffiti coatings may be clear or pigmented. Clear coatings are usually applied to architectural surfaces such as exposed aggregate or brick (see Figure 129), where the source of the barrier's color is in the natural barrier material itself. In some instances, such coatings applied to exposed-aggregate surfaces tended to enhance its appearance by deepening the aggregate colors. Pigmented coatings are typically applied to natural (unpainted, unstained, un-pigmented) concrete surfaces. Application of coatings is usually done by spraying, although rolling and brushing may occasionally be performed, especially in areas where over-spray is a concern. Anti-graffiti coatings may be of either the permanent or sacrificial variety as described below:
	Figure 129. Anti-graffiti coating photo #5332

• Permanent Type - This type of treatment is aimed at providing a coating which will enable multiple removals of graffiti by high pressure water wash and/or chemical washing techniques without having to replace the coating. Such coatings may be either urethane or water based and usually require a two- or three-coat application process.

• Sacrificial Type - This treatment type provides a coating which itself is wholly or partially removed along with the graffiti during the cleaning process. Certain sacrificial coatings may be capable of several cleanings before they must be reapplied. These types of coatings are typically one-coat, water-based or wax-type systems. They are generally less expensive, but do require re-application following a certain degree of graffiti removal.

5.9.3.2 Stains.
In addition to providing the desired color in a noise barrier, a concrete stain can provide a degree of protection against the elements as well as be reapplied to areas "tagged" with graffiti. Stains can be either oil-based or water-based, with the latter being more widely used for concrete applications.

5.9.3.3 Application Process.
Coating of precast barrier elements may be performed at the point of their manufacture (normally a precast plant) prior to their erection or in the field after their erection. Only on rare occasions would coating of precast barrier elements be performed in the field prior to the erection of the barrier. Coating of cast-in-place noise barriers must occur in the field. If coating is done at the plant, conditions may be better controlled, but barrier elements must be stored during drying or between coats at a location free of dust and potential damage from plant operations. Additional care must also be taken during transport of coated panels to the job site and during their erection process to protect them against possible damage. It is for these reasons that the majority of coating is performed in the field following the erection of the barrier. Even in this situation, coating must be done when conditions (rain, temperature, wind) are acceptable.

5.9.3.4 Relationship of Coating Type to Maintenance Philosophy.
Anti-graffiti coatings should only be applied if the responsible organization has a policy which dictates the removal of graffiti from barriers. If the standard operating procedure related to the treatment of graffiti is to paint over it, then anti-graffiti coating is not only a waste of money, but also creates a surface to which the "paint over" paint will not easily adhere. With this maintenance philosophy, the use of stains appears to be more consistent and cost-effective.

5.9.3.5 Relationship of Coating to Barrier's Acoustical Performance.
In addition to the factors listed above, it is essential that the coating be compatible with the intended acoustical performance of the barrier. If the barrier is designed to be sound reflective (see Section 3.5.4), then any type of coating may be applied without affecting its acoustical erformance. However, if the barrier is to perform a sound absorptive function, any coating used must not interfere with the barrier's sound absorptive characteristics. For instance, coating a sound absorptive, porous surface with a paint or a urethane-based anti-graffiti coating could seal up the voids which provide the barrier's sound absorptive characteristics. A water-based penetrating stain may be an appropriate coating in this instance. Before applying any coating to a sound-absorptive surface, research and coordination with both the barrier manufacturer and the coating supplier is strongly recommended.

5.9.3.6 Health and Environmental Issues.
While significant benefits in several areas can be realized by the use of coatings, there are health and environmental issues which need to be addressed in association with their use. As a general rule, products should be stored, applied, and disposed of in strict compliance with the recommendations of the manufacturer and in accordance with all applicable federal, state, and local regulations. Urethane-based coatings require particular attention due to their higher content of volatile organic compounds (VOC) and their more potent odors. Water-based and other more recently developed coatings will have little or no VOCs and less of an odor problem. They also have less toxicity. Over spray is still a factor requiring careful consideration where barriers are in close proximity to either residences or vehicles or where coating application is being performed near water courses or swales leading to such sources of water. Adequate covering of the ground with tarpaulins can help to minimize ground and water contamination.

Section Summary

Noise Wall Materials.
Item#	Main Topic	Sub-Topic	Consideration	See Also Section
5-1	Concrete	Aesthetic	For cast-in-place: form liners and architectural inserts must be placed on vertical surfaces of the form work which can increase the chance of imperfections in the wall surface.	5.1
			For cast-in-place: application of concrete retarding chemicals to the vertical form work surfaces for the purposes of obtaining an exposed aggregate finish is difficult.	5.1
			For cast-in-place: other surface textures obtained through raking, brushing, or stamping of concrete are not possible.	5.1
			Surface textures: Smooth Surface - Ensure a smooth surface by performing a final "rubbing" of the concrete with a thin cement mixture. Exposed Aggregate - Provide sufficient screening, grading, and inspection to ensure the removal of any iron ore aggregate which could give the appearance of rust "bleeding" from the panel. Form Liners - The joints must be tight, adequately secured to the form work, and "lined up" in terms of the form liner pattern. Care must be taken to avoid jagged edges at the ends and sides of the panels. Inserts - Care must be taken to ensure an adequate bond/attachment between the insert and the noise barrier panel. Veneers - The primary consideration of the use of veneers is related to assuring an adequate attachment bond. Stucco - The cleanliness and roughness of the concrete noise barrier surface is critical in order to ensure an adequate bonding surface for the stucco.	5.9.1.1
			Quality control and consistency is critical in the production of pigmented concrete since unevenness and blotchiness can become apparent, particularly on smooth surfaces. The maximum protection against the visual effects of damage can be provided by use of a pigmented panel with a surface stain of a matching color. The added cost of such a dual treatment may, however, not be warranted in many cases.	5.9.2.1 5.9.3
		Drainage and Utility	Application of concrete retarding chemicals to the vertical form work surfaces for the purposes of obtaining an exposed aggregate finish is difficult.	5.1
		Safety	Consider on-site material testing and inspection during construction.	5.1
		Installation	For precast , consider size limitations, shipping requirements, traffic implications, reusability of precast panels, quality assurance process	4.1.2.3.1 5.1
			For cast-in-place, consider on-site material testing and inspection procedures during construction, and weather concerns for on-site casting and curing.	4.1.2.6 5.1
		Maintenance	For free-standing noise walls, consider access for landscaping.	4.1.2.6
5-2	Brick and Masonry Block	Aesthetic	Surface textures: Exposed Aggregate - Consider limited use due to mass production constraints. Form Liners - While some molds may be applied to the production of concrete blocks, their use is rare and limited. Veneers - Ensure an adequate attachment bond. Stucco - The cleanliness and roughness of the noise barrier surface is critical in order to ensure an adequate bonding surface for the stucco.	5.2 5.9.1.2
			Quality control and consistency is critical in the production of pigmented blocks since unevenness and blotchiness can become apparent, particularly on smooth surfaces.	5.2 5.9.2.1 5.9.3
		Structural	Consider the need for a continuous concrete foundation.	5.2 8.4
			Consider the compressive strength of the concrete materials.	5.2
		Installation	Hand-laid versus preassembled panels: Consider each type's versatility to conform to ground contours. Consider each type's speed of erection. Consider special leveling courses on grades of up to 6 percent. Consider the scaffolding requirements for hand-laid panels.	5.2 11.1
5-3	Metal	Acoustical	Consider the Sound Transmission Class requirements.	5.3
		Aesthetic	Consider the possible "industrial" appearance of metal walls.	5.3
			Consider weathering steel concerns because unpainted rusting panels can stain adjacent concrete.	5.3
			Manufacturers' requirements (related to both coating and material coated) such as extent of sand blasting, cleaning materials, temperature and moisture controls, etc. need to be adhered to closely.	5.9.2.3 5.9.3
		Structural	Consider the non-compatibility of various metal combinations.	5.3
			Ensure corrosion resistance.	5.3
			Consider the metal's structural strength.	5.3
		Safety	Consider the possible glare due to on-coming vehicles.	5.3
			Consider implementing a deterrent for climbing on barrier girts.	5.3
		Maintenance	Debris and errant vehicles easily causes noticeable damage.	5.3
5-4	Wood	Acoustical	Consider possible shrinkage and warping causing noise leakage through gaps.	5.4
			Ensure a tight fit for tongue and groove planking to avoid noise leakage.	5.4
		Aesthetic	Selection of a wood species for its color attributes must also consider durability, warping, rot resistence, etc. The type of treatment, stain, or paint must be compatible with the type of wood, nails, etc. used in the barrier assembly, and with the environment in which the barrier will be placed. Chemical reactions between steel nails and wood preservative treatments may corrode nails. Moisture content at the time of paint or stain application is also critical.	5.9.2.4 5.9.3
		Safety	Consider the wood's burning characteristics when choosing a wood.	5.4
			Consideration should be given to a wood's shatter resistance.	5.4
5-5	Transparent Panels	Structural	Consider the various methods of mounting.	5.5
			Consider edge conditioning the panels.	5.5
		Safety	Consideration should be given to shatter resistance.	5.5
			Consider the possible glare due to on-coming vehicles.	5.5
		Maintenance	Consider the methods of cleaning the panels.	5.5
			Consider the maintenance concerns related to vandalism and scratches.	5.5
			Consider the need for ultraviolet light protection.	5.5
			Debris and errant vehicles easily causes noticeable damage.	5.5
		Cost	Transparent noise barriers costs can be more costly than common concrete or steel panels.	5.5
5-6	Plastics	Acoustical	Consider the possible shrinkage in plastic materials.	5.6
		Safety	Consideration should be given to the material's shatter resistance.	5.6
			Consider the possible glare due to on-coming vehicles.	5.6
			Consider the burning characteristics of the materials.	5.6
		Maintenance	Consider the need for ultraviolet light protection.	5.6
			Consider the maintenance concerns related to vandalism and scratches.	5.6
5-7	Recycled Rubber	Acoustical	Consider possible coating interference with the material's Noise Reduction Coefficient.	5.7
			Ensure an adequate Sound Transmission Class.	5.7
			Recycled rubber material generally cannot be pigmented. Rubber can be coated (usually with a more expensive polyurethane coating) to obtain a desired color.	5.9.2.6 5.9.3
		Structural	Consider the panel's structural strength requirements.	5.7
			Consider the material's bonding requirements.	5.7
		Safety	Consider the need for flame retardants.	5.7
			Consider the material's possible toxicity concerns.	5.7
5-8	Composites	N/A	(Refer also to concerns for individual materials within composites) Particular concern must be paid to ensure compatibility between the barrier materials (including any glues, attaching devices, etc.) and the applied coating to negate any potential for damaging chemical reactions.	5.8 5.9.3