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Polymer Road Building Provides Solid Solution
Sep 3, 2010
Stabilization products ease maintenance, support the environment


Imagine building roads that are less expensive, easier to repair and environmentally friendly.

Lizano Construction of Costa Rica wanted to use cost-effective, environmentally friendly building options for a 1 kilometer road in a new subdivision under construction in Ciudad Quesada, Costa Rica, located about an hour north of the capital, San Jose. Like most of Latin America, Costa Rica has limited asphalt production and typically imports asphalt from Texas. Once on-site, the asphalt must be heated and combined with local aggregate, making the process expensive and time consuming.

"With our hot climate, we were looking for an aesthetically pleasing material that wouldn’t retain heat like asphalt," says Luis Lizano, CEO of Lizano Construction.

Lizano’s search for alternative road building methods led him to DirtGlue, a company that specializes in soil stabilization technology. The acrylic polymer used by DirtGlue is a water-based liquid plastic that bonds to the aggregate and cures to form a durable, water resistant, flexible road.

The DirtGlue Road Building System (DGRBS) offers advantages to the site developer. Environmentally friendly with no asphalt plant required, these roads are less expensive to build, less expensive to maintain and repair and can last up to 50 percent longer then traditional asphalt roads, according to DirtGlue officials. Polymer road building techniques can also help the contractor reduce the need for expensive excavation work and the importation of high-cost construction materials. Project costs are greatly reduced with DGRBS by getting roads back in service sooner and causing less disruption to the local infrastructure and population.


It Starts with Soil

Compaction of broken, angular stones held together by fines creates a strong load-bearing surface.
Chris Rider, technical director of DirtGlue, and Blake Kelso, a certified DirtGlue applicator and owner/operator of Texas DirtGlue, arrived on the site in Costa Rica in early May 2010. The first thing they did was perform a site inspection.

"Soil analysis is critical, because polymers react to the unique soil conditions encountered at each site," Rider says. "Variations in soils and climate affect what products are used and how they are applied."

After the initial site inspection and soil analysis, Rider and Kelso worked to develop an application strategy for the road. Due to the downtown location of the subdivision, they decided to install an optional "all in one" curbing and drainage system of poured concrete rather than typical country drainage of simple grass swales at the road’s edge.

Similar to traditional road building, it is important to start a polymer road with a strong stable subgrade. Many paved and unpaved roads fail because they are constructed using poor subbase materials. The problem with using inferior soils, such as clay and silt, for road construction is that they build up a structure of fine capillaries that absorb water, causing a reduction of stability and load-bearing capacity. The soil analysis indicated the subgrade material had high clay content, and Rider and Kelso needed to stabilize this material.

They considered using lime or Portland cement, but because of drainage issues and the high clay content, they decided to use TerraDry to stabilize the subgrade. TerraDry is a DirtGlue product that fills the capillary network with a silicone resin rendering the soil water-repellent. The benefits are enormous. TerraDry makes it possible to use "in-situ" material saving time, money and resources. With proper techniques and products, it is no longer necessary to remove inferior soil and haul it away to a landfill. Additionally, the cost of purchasing aggregate is also eliminated.

Using the Cat D5 track dozer, Lizano stripped off 14 inches of topsoil to start the earth-moving phase of the project. The blade of the track dozer was tilted to create the proper geometry for the eventual crown and slope of the finished road. With the subgrade exposed, Lizano stabilized it by mixing in TerraDry at a rate of 0.05 gallon/square foot to a depth of 6 inches. Diluting the TerraDry with water brought the material to the optimum moisture content. While still wet, the 7-ton, double drum Cat roller compacted the subgrade while maintaining the all-important geometry. The result was a strong stable subgrade ready for the base layer.


Adding Strength

The Land Pride Powered Rake applies crown and mixes the DirtGlue polymer into the wear surface.
After the subgrade was compacted, Mack 10-wheel dump trucks delivered aggregate for the base construction. DirtGlue Enterprises specs their aggregate based on the Macadam road building system, which relies on strong, angular interlocking stones held together by fines to create a strong load-bearing surface. The base was formed with 2-inch minus aggregate with at least 20 percent fines. The Cat D5 track dozer graded the base complete with the crown and slope, bringing the road up to within 2.5 inches from finished grade after compaction.

With the base layer completed, next came constructing the cap or the wear surface. Three-quarter inch minus aggregate with 20 percent fines was laid loose, not compacted, to bring the road up to finished grade after compaction. Typically this is when the DirtGlue polymer is applied, but because rain is not uncommon in Costa Rica during that time of year, Rider and Kelso decided to add PolyCure to the top layer, which will later become the wear surface. PolyCure is a rapid curing agent developed by DirtGlue Enterprises that increases wear surface strength and reduces the total drying time from weeks to days.

"PolyCure reduces the drying time exponentially," Kelso says. "Without it, this project would have been postponed until the dry season."


Final compaction reveals a smooth uniform surface with the flexibility of asphalt.
The curing agent is applied at a rate of two-thirds pound per square foot and mixed in with a Land Pride Powered Rake PR1690, taking care not to dig into the compacted subgrade material.

Once the PolyCure was properly mixed into the top layer of aggregate, the DirtGlue polymer was applied to the aggregate that was now very loose and fluffed from the powered rake. DirtGlue polymer is a "high solids" liquid polymer with unique hydrophobic characteristics, ideal for building strong, durable roads. The polymer was slightly diluted with water just enough so that the total liquid applied brought the aggregate to optimum moisture content.

The DirtGlue polymer was then mechanically mixed into the top 2.5 inches of aggregate using the powered rake. This required two passes with the liquid application system and two passes with the powered rake. The rake is adjustable both in depth and in angle to maintain or make a crown as it mixes. After final mix-in, the aggregate was thoroughly compacted with the 7-ton double drum roller. After a couple of sunny days, the road was ready for use.

According to Rider, the finished project cost about 65 percent less than a similar job using asphalt and the road should have a 50 percent longer life than the same road constructed of asphalt in Costa Rica. The finished road surface has the flexibility of asphalt but has strength much closer to concrete without the brittleness.

Testing has shown that with aggregate made from hard rock, it is possible to achieve unconfined compressive strength exceeding 2,000 psi, according to a study conducted at the University of Puerto Rico. This exceeds the strength of asphalt by 2.5 times.

Since new DirtGlue polymer easily bonds to old DirtGlue polymer, repairs are permanent and the seams are nonexistent as compared with the seaming quality of asphalt. According to DirtGlue officials, any repairs, if done with identical aggregate, will be nearly invisible, compared to typical asphalt repairs.