Hardscaping with Caltrans
California tests new vegetation-control methods for those
hard-to-manage areas along roadsides.

by Kerry L. Clines, Associate Editor

The California Department of Transportation manages and maintains vegetation along approximately 15,000 miles of highway statewide. Until 1992, the use of herbicides was their main choice for controlling problem vegetation, but that changed with the Environmental Impact Report regarding vegetation control practices. In response to the report, Caltrans adopted an integrated vegetation management program and set goals for the reduction of herbicide use — 50% reduction by 2000 and 80% reduction by 2012.

This left the state with a very difficult problem that every roadside maintenance crew faces — controlling unsightly and sometimes hazardous vegetation in difficult, hard-to-manage areas where vegetation is not feasible because mowing is impossible or risky. Clas-sic examples include that strip of ground along and under guardrails, and the road that runs along a steep cliff where it’s difficult to put workers without risking their safety — either from falling or from traffic.

Caltrans has been studying this problem for some time and has been testing possible solutions in selected areas across the state. These methods, known as hardscaping, not only help control vegetation, but can also be pleasing to the eye.

What is hardscaping?

“Basically, hardscaping is the use of hard inert material surfaces such as stamped asphalt, patterned concrete, and rock cobble — in comparison to living soft material surfaces such as organic mulches and fully landscaped areas,” says Senior Landscape Architect Jennifer Malcolm, Maintenance Division, Caltrans. “Where hardscaping tends to work best is in small areas that are hard to maintain. Caltrans has lots of small areas; such as under guardrails, around signs and delineators, and in narrow areas that the traveling public tends to drive over, such as gores.”

The use of hardscaping isn’t without challenges, however. “Generally, structural methods involve higher costs during initial installation, and sometimes can be quite pricey,” says Malcolm, “but they lead towards reduced maintenance needs, resources, and costs in the long run. Structural elements are the best way our designers can help maintenance in achieving our herbicide reduction goals.”

According to Caltrans, 10 different hardscaping treatments have been approved for use along their roadsides. Some of these methods have been used for quite some time, while others are newer and are just being tested in a few areas to see how well they perform.

Rocks

The use of rocks is one of the oldest hardscaping treatments in California. Rock cobble, rock blanket, and rock slope protection are used throughout the state in gore areas, around guardrails and sign posts, on road edges, and on side slopes.

Rock cobble is loose, round river rocks — usually 6 to 12 inches in size — that are used to discourage plant growth and to beautify areas. Cobbles are readily available and have natural color tones that are easy to match if repairs are needed. Commonly used along local streets and roadways, this method is aesthetically pleasing, but does not work well for all areas. Rock cobble can not be placed directly adjacent to a roadway, because loose cobble should not be driven on.

Rock blanket, or mortared cobble, uses the same round river rocks but sets them in place with mortar, so they are no longer loose. The blanket then forms a continuous and uniform hard surface that can be driven on ocassionally, providing a load-bearing surface for maintenance operations and errant vehicles. However, repairing cracked mortar and loose cobbles is difficult and labor intensive.

Rock slope protection uses much larger rocks, 1- to 2-ton boulders, which are set on steep slopes along roadsides to hold the slope back and protect it from water and wind erosion. When used with a weed fabric, effective vegetation control can be achieved. This method is easy to install and repair, and is recommended for slopes that are too steep for mowers and maintenance equipment.

Paving

There are three types of paving on Caltrans’ hardscaping list — asphalt concrete pavement, stamped asphalt concrete pavement, and patterned concrete pavement.

Asphalt concrete pavement is recommended for roadside management in medians, gores, and in narrow strips adjacent to sound walls and retaining walls. This commonly used road construction material is readily available, and most contractors and Caltrans maintenance crews have the necessary equipment and knowledge for installation and repair.

Stamped asphalt concrete pavement involves the imprinting and/or coloring of standard asphalt concrete. Woven wire templates are pressed into the asphalt surface by standard compaction equipment while the asphalt is still warm and pliable. When the templates are removed, color can be applied as a finishing coat. A variety of colors and patterns are available. Use is limited to areas where the asphalt can be compacted, which means it can not be used on steep slopes. Matching colors and patterns can be difficult during repairs.

Patterned concrete pavement is standard concrete pavement imprinted or stamped with a pattern that may be colored or stained prior to curing. This works well in high-visibility urban/suburban locations such as gores, road edges, median strips, and slope paving. This method has a low life-cycle cost when designed and constructed correctly and is better suited for slopes than stamped asphalt. However, construction is more labor intensive than stamped asphalt. As with stamped asphalt concrete pavement, matching colors and patterns can be difficult whenever repairs are needed.

Fiber weed control mats

Fiber weed control mats are made of synthetic polyester fibers spun together to create a mat that prevents weed growth but allows water and air to filter through.

These mats are less expensive than most other hard surface treatments, require limited training to install, and are simple for Caltrans maintenance crews to replace and repair. The product comes in rolls with  holes pre-punched at intervals, making it great for new sign post installations or long straight runs of new guardrail.

However, the mats are not easily applied to curves or to existing guardrails without becoming labor intensive, thereby raising installation costs. They work best on level, compacted ground that is free of vegetation. The product is not recommended for high-wind areas as the wind can lift the edges of the mat. The edges must be secured according to the manufacturer’s recommendations to ensure that this does not happen.

Many of the fiber mats being installed by Caltrans are called WeedEnder, manufactured by U-Teck of Fayetteville, North Carolina. So far, testing has gone fairly well for this product. “If the mat is placed properly, as per the manufacturer’s instructions, it will last,” says District Landscape Specialist Bob Melendez in Caltrans District 1. “I don’t know if it’s the greatest thing, but when you can’t spray and don’t have other options, it seems to work well.”

Polyureas

Polyureas, or elastomers, are made by combining isocyanate and resin-blend components to form a hard, yet semi-flexible product that cures in less than a minute. Polyureas are usually applied as a spray with the two components combined at the nozzle.  Very thin coats are applied over geotextile fabric stapled to the ground surface, which must be well prepared, compacted, and smooth.

Polyureas can be colored to match or blend with the surrounding area. The product is UV stable and bonds well to wood. Because it is sprayed on, it can be applied around existing features such as curving guardrails and sign posts.

However, polyureas require specialized equipment, a high level of expertise, and the use of safety gear for installation. It does not bond well to asphalt or concrete, and wind and water can get under the edges of the material. Snow removal operations can destroy the product during the winter months, so this must be taken into consideration when deciding where to use it.

One polyurea being tested extensively by Caltrans, especially around existing guardrails, is Bio-Guard. “We haven’t had any real issues with the Bio-Guard yet,” says Caltrans Landscape Specialist John Dobson in Caltrans District 2, “but by the time we got done with some of the test plots, it cost too much, and we’re not sure that it won’t burn with a real intense fire.”

Rubber weed control mats

Rubber weed control mats are made from recycled tire rubber pieces that are bonded together with a resin and pressed into a mat. The mat lies directly on the ground, preventing sunlight and air from reaching the ground surface, so weeds don’t grow. Overlapping mats are sealed with asphalt crack filler or resin adhesive to ensure that the edges stay fastened together. The weight of the mat tiles keeps them in place, so staking is not required.

During the mat-making process, size and color options are available, allowing the mats to be customized for each job. The product can be used under new and existing guardrails, sign posts, and fences. It is flexible and easily repaired by one person after a hit.

However, long continuous runs of rubber mats with multiple joints are labor intensive, slow to install, and may be unsightly. Rubber mats do not work well on curves because of their square/rectangular shapes. Tiles can separate if they are not sealed correctly.

“There’s quite a bit of labor in installing rubber mats,” says Dobson, “you have to caulk each joint and make sure there’s a real good fit around each post.”

The effects of UV light and long-term degradation are not known due to the newness of the product. Oil content in the mats presents potential stormwater concerns, as well as a potential to burn. Weight may be beneficial in keeping the product in place, but it also increases shipping costs.

CRMCrete

CRMCrete, short for Crumb Rubber Material Concrete, is a concrete-based product that blends recycled scrap tire crumb rubber material with homopolymer polypropylene high-performance reinforcing fibers into a slurry. CRMCrete, also known as minor concrete, is poured into place, tamped and leveled, and then finished like regular concrete, with a wide variety of finishes.

The product can be made to match soil or vegetation colors by using concrete stain or pigment. Standard equipment and concrete mixes are used for the installation, which can be performed by contractors or Caltrans maintenance crews. Unlike concrete, CRMCrete doesn’t always require formwork, so it can be applied as Shotcrete. It can be placed much faster than concrete due to the fact that it does not need rebar or welded wire mesh, thus saving on labor costs.

The consistency of the mix may limit its use on slopes, however, and it may be difficult to match the original color when repairs are made. CRMCrete’s rigidity requires a 150-millimeter gap between the product and nearby guardrails or sign posts for expansion and contraction. In addition, a leave-out section is filled with expanded polystyrene foam covered with a thin layer of two-sack mix so the guardrail can flex and move slightly when impacted by an errant vehicle, leaving the CRMCrete undamaged. There is a limited history of maintainability and life-cycle costs due to the newness of the product.

Overall, Caltrans thinks CRMCrete looks very promising. “You can pour it in any shape,” says Dobson. “We have different configurations of guardrail and you can just form it and pour it to the configuration. Once it’s down, you can drive equipment over it and mow right up over the top of it. Even in snow country, it’s not an issue with the plow trucks hooking the edge of it since it is placed at the existing grade.”

Caltrans maintenance personnel are so impressed with CRMCrete that Jack Broadbent, Supervising Landscape Architect, Roadside Management, is looking to initiate a new design standard statewide that if new guardrail is going in, some sort of CRMCrete weed treatment must go in around it. This should help meet their goal of 80% reduction in herbicide usage by 2012.

New possibilities

There are several new products being tested by Caltrans that are not on their list of approved hardscaping methods. Many of these involve the recycling of waste products that might otherwise end up in a landfill. This would not only help control vegetation along the roadsides, but help the environment as well.

Cullet. Cullet is simply recycled glass that has been processed in a glass recycling plant — tumbled to round off the rough, sharp edges. The glass pieces range in size from 0.25 to 1 inch and come in a wide variety of colors that can be mixed to blend in with the surrounding area. Cullet is spread over a barrier fabric at a depth of 4 to 6 inches and works best with a border to keep it from spilling into other areas. One main concern is the glass may cause glare for the traveling public, so this has to be considered when choosing a site.

Turboscape. Turboscape is composed of recycled tire crumb rubber that has been chopped into small pieces and mixed with resin. The product is then sprayed around guardrails and sign posts along the roadside. Resin holds the rubber pieces together and seals them into a fairly solid piece. The California Integrated Waste Management Board provided funding for this research project because it recycles unwanted tires.

However, turboscaping is still in the initial test phase. The fact that turboscape is made from tires means the product is flammable. If the product caught fire, it would create black smoke, resulting in air pollution and possible traffic problems. Early test results indicate that vegetation will grow through turboscape if it isn’t sprayed on thick enough or mixed with enough resin to seal the crumb rubber pieces tightly together.

Recycled Plastic. Universal Weed Cover is a panel made of 100% recycled plastic. Panels interlock around guardrail posts to form a weed barrier. This product is being tested around existing guardrail at one site near Shasta Lake, north of Redding.

“The plastic panels are fairly easy to install because they are formed to fit between the guardrail posts, so you just snap each panel together as you go,” says Dobson. “That’s one of the reasons it seems to have promise — because it’s so basic that just about anyone can install it.”

There have been some concerns about the fit around the post, but the panels are currently being redesigned. A new collar is being developed by the contractor/manufacturer to ensure a better fit around the post.

“He has a good concept because it's a recycled product," says Dobson. "The mil is thick enough that, if the panels are hooked together with an overlap, they'll keep all weeds from growing through.”

EnviroWeld. EnviroWeld, also known as WeatherWeld, is made of general roofing tar with fiberglass strands added to the mix. This unique fiberglass-strand extrusion process has been used in the roofing industry and for lining ponds and ditches. The binder matrix is made of pure asphalt suspended in a blend of inorganic bentonite and water. As the materials  dry, the bentonite forms a UV and oil barrier that protects the material below the surface from deterioration.

Ground preparation is relatively simple — requiring only grubbing to remove vegetation, an application of pre-emergent, and compaction — before applying a tack coat and then the emulsion. The process is relatively low in cost at about $21 per linear meter and sprays on.

In the Spring of 2005, the material was applied around test sections of guardrail along Highway 166 and is currently in the evaluation process. If tests show that EnviroWeld does the job of controlling unwanted vegetation, it could prove to be the spray-on solution Caltrans has been looking for to use around existing guardrail.

“It’s too early to tell how well the product will work,” says Landscape Specialist Roy Freer, Caltrans District 5. “It does have promise, but more testing needs to be done.”

Preferred method

So, which of the hardscaping methods is preferred by Caltrans? “None,” says Malcolm. “No single product is perfect! We haven’t found our silver bullet yet.”

While this may be the case, each treatment has its own advantages and disadvantages, and each fills its own niche. Meanwhile, Caltrans will continue testing along the California roadsides to improve these treatments and develop new treatments.

Will Caltrans ever find their silver bullet? Only time, and more testing, will tell.

The Roadside Management Toolbox

Keeping up with the latest roadside vegetation management methods can be challenging, so a couple years ago, the California Department of Transportation put a Roadside Management Toolbox on their Web site to keep  their employees informed. The toolbox offers design techniques and treatments for roadsides that can improve traveler and worker safety, protect highway infrastructure, and reduce the need for repetitive maintenance activities.

”As any other DOT, we develop highways,” says Supervising Landscape Architect Jack Broadbent, Roadside Management, “and we need to do our plan specifications and estimates in an efficient, productive manner so we don’t wind up reinventing the wheel every time. The toolbox is a place where a civil engineer or landscape architect — the designers of the highway environment — can find the most current information on roadside design options for permanent vegetation control. They can pick from the specifications and details, and if need be, modify that specification and detail for their specific project. This allows better efficiency.”

Information about Caltrans’ hardscaping methods is available, as well as information about other roadside treatments, such as the use of vegetation and mulch. The site is easy to navigate and packed with information that can help you find the method/methods best suited to tackle problem areas such as gores, guardrails and signs, medians, road edges, and side slopes. The toolbox offers a list of recommended treatments for each problem area, as well as pictures of test areas where that treatment is being used.

Since the methods used are undergoing constant evaluation and testing, the toolbox is open to constant change. Updating will occur as existing methods are improved upon and new treatments are found.

The Roadside Management Toolbox can be found on the Caltrans Landscape Architecture Programs Web site at www.dot.ca.gov/hq/LandArch/roadside .

Reprinted from Better Roads Magazine
April 2006

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