From Tech Transfer Newsletter, Summer 2006 » printer-friendly

Permanent Vegetation-Control Methods for those Hard-to-Manage Roadsides

by Kerry L. Clines, Associate Editor, Better Roads

Along California's roadsides, there are countless spots where vegetation creeps up. You've probably seen these spots, marveled at the strength and persistence of the growth, and perhaps have even been tasked with the challenge of destroying the growth. This vegetation is unsightly and sometimes even hazardous, and it grows in those hard-to-manage areas where landscaping is not feasible because mowing is impossible or risky. The need for roadside vegetation control is fueled not only by aesthetic considerations, but also must be managed with limited use of pesticides out of concern for safety, environmental mitigation, storm water pollution prevention, and erosion control. Caltrans has studied the problem and has tested possible solutions across the state. These methods not only help control vegetation, but can also make your roadsides more attractive.

Rocks

The use of rocks is one of the oldest permanent vegetation control methods 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 comprised of 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 occasionally, 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

Asphalt concrete pavement, stamped asphalt concrete pavement, and patterned concrete pavement are all methods that can be used for permanent vegetation control.

Asphalt concrete pavement is recommended for roadside management in medians, gores, and in narrow strips adjacent to sound walls and retaining walls.

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 when 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 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.

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. They do not bond well to asphalt or concrete, wind and water can get under the edges of the material, and snow removal operations can destroy the product.

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. Mat size and color can be customized. 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, installing long, continuous runs of rubber mats with multiple joints is labor intensive, slow, 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. 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 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.

Caltrans maintenance personnel are so impressed with CRMCrete that Jack Broadbent, Supervising Landscape Architect of the Roadside Management division, is looking to initiate a new Caltrans design standard statewide that says some sort of CRMCrete weed treatment must go in around any new guardrail.

Reprinted with permission from Better Roads, April 2006. For a subscription to Better Roads, please visit www.BetterRoads.com.

Special thanks to Jack Broadbent at Caltrans for providing photos of Permanent Vegetation Control methods.

New Possibilities

Caltrans is currently testing and evaluating several new products. Many of these involve the recycling of waste products that might otherwise end up in landfills. If these products are found to be effective permanent vegetation control methods, they would not only help control vegetation along the roadsides, but help improve 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. The glass may cause glare, a concern that must 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 thickly 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.

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.

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.

Virtual Toolbox
Caltrans maintains an Internet toolbox offering design techniques and treatments for roadsides that can be used by local agencies statewide to improve traveler and worker safety, protect highway infrastructure, and reduce the need for repetitive maintenance activities. The Roadside Management Toolbox is located at www.dot.ca.gov/hq/LandArch/ roadside.

© 2008 Regents of the University of California