A+ Infrastructure Begins Below the Surface
American roads received a D grade in the 2013 Infrastructure Report Card issued by the American Society of Civil Engineers (ASCE). When ASCE released a fresh version of the study in 2017, America’s roads fared no better: another D grade. Short of a “magical genie” granting the wish of an unlimited transportation infrastructure budget, what can be done to lift our roads from D-grade to A+ infrastructure?
A driver’s experience of a road is largely determined by its smoothness. Beyond being a predictor of driving pleasure, road smoothness also is linked to road safety, service life, sustainability and savings. If there were just one lever to pull to improve America’s road quality, smoothness is it.
The Federal Highway Administration (FHWA) assesses America’s road quality primarily by measuring smoothness or, rather, its antonym: roughness. The FHWA especially relies on the International Roughness Index (IRI) as the scale used to evaluate pavement conditions. Along with the 50 states, they regularly measure and record the roughness/smoothness of the country’s roads. Smoothness is an important-enough measure of road quality that contractors are increasingly incented by bonus/penalty payments for new road construction based on the pavement’s IRI score.
The Highway Performance Monitoring System collects data on pavement ride quality on federal-aid highways. Between 2002 and 2012, the percentage of federal-aid highway mileage classified as acceptable decreased from 87.4 percent to 80.3 percent. The national transportation research group TRIP reported in 2015 that “more than one-quarter (28 percent) of the nation’s major urban roads—interstates, freeways and other arterial routes—have pavements that are in substandard condition and provide an unacceptably rough ride to motorists.”
Road smoothness is, by its nature, experienced through contact with a pavement’s surface. However, TRIP’s 2015 report, “Bumpy Roads Ahead: America’s Roughest Rides and Strategies to Make Our Roads Smoother,” noted that to ensure smoother roads longer, focus first below the surface. According to the TRIP report, “A solid, stable and consistent foundation below the surface of a road or highway is critical in maintaining a smooth driving surface. When constructing or reconstructing a roadway, it is critical that the pavement’s base course be adequate to support the roadway surface on which cars and trucks will be driving. If a roadway’s foundation is deficient, it will likely negatively impact pavement smoothness and increase the rate of pavement deterioration.”
One year earlier, WYDOT (using the same road contractor) completed the southbound lanes (only northbound pictured) along the same segment of Interstate 25. At that time, however, they didn’t place geogrid under the road. Nearly 10 years after these projects were completed, the northbound, geogrid-reinforced segment is 30-percent smoother than the non-geogrid southbound segment. In addition, the northbound segment’s IRI rating has improved (by 19.5 percent), while the southbound segment’s rating has declined (by 4.5 percent). Project engineers believe this could be because mechanically stabilized bases become stiffer over time.
To build smoothness into paved roads and highways from the beginning, it’s best to ensure that what’s beneath the pavement is stiff, stable and sustainable, according to Dr. Jorge Zornberg, professor and W.J. Murray Jr. Fellow in Engineering at The University of Texas-Austin. Zornberg conducts research on soil reinforcement/stabilization and geosynthetics, and his research has been sponsored by the National Science Foundation, Federal Highway Administration, Transportation Research Board, Environmental Protection Agency, U.S. Department of Education and Geosynthetic Institute as well as the Departments of Transportation of Texas, Colorado and California.
“For the money, the best way to improve road infrastructure is the use of geogrids,” says Zornberg.
In September 2017, The American Concrete Pavement Association announced that the Colorado Department of Transportation I-25 Lincoln Ave. to County Line Road project received an excellence award for the divided highways urban category. The contractor on the project was IHC, the same organization that built the WYDOT I-25 project. Geogrid lowered project costs, saving taxpayers thousands of dollars. Using geogrid also helped the contractor shorten construction time, making it easier to continue work during inclement weather and enabling them to finish the job six months earlier than expected while maintaining high quality standards.
A geogrid is geosynthetic material used to stabilize soil. Geogrids commonly are used in retaining walls as well as subbases or subsoils below roads or structures. Soils pull apart under tension; compared to soil, geogrids are strong in tension and also confine the soil, allowing them to transfer forces to a larger area of soil than would otherwise be the case.
Geogrids are commonly made of polymer materials, such as polyester, polyvinyl alcohol, polyethylene or polypropylene. They may be woven or knitted from yarns, heat-welded from strips of material or produced by punching a regular pattern of holes in sheets of material, then stretched into a grid.
According to TRIP, 21 percent of the nation’s highways had poor pavement condition in 2015. Minneapolis, for example, had 138,767 miles of public roads, with 15 percent in poor condition. To ensure smoother roads—even in weather-fatigued circumstances—Minnesota has studied and approved the use of geogrids.
“If roads are to perform well, it’s critically important that they have good foundations,” writes Lou Tasa, research fellow, HumanFIRST Program, University of Minnesota, in a 2011 study by the Minnesota Department of Transportation: “Using Geosynthetics to Improve Road Performance.” The study concluded, “What Did We Learn? Results show that geogrids clearly benefit pavement performance, with test sections showing a better ride quality and surface rating than control sections … and the use of geogrids in road foundations clearly benefits pavement performance, potentially leading to longer service lives for roads and reduced maintenance costs.”
According to Zornberg, geogrids address the three most critical “under-the-road” factors that produce smoother pavements: stiffness, stability and sustainability.
A stiffer base course produces a pavement that stays smoother. “From the time of Romans, the basis of road design was a stiff base below the surface,” notes Zornberg. “You want material beneath the surface—above natural ground but below our feet—to be stiff and durable, so when a heavy vehicle traverses the road, its weight is distributed, and the stresses are significantly lower. That reduces cracking, rutting and other types of failure.”
In a 2016 study by Minnesota Department of Transportation, researchers used ﬁeld and lab stiffness testing and software to model pavement base-course performance with and without geogrids. The results showed that geogrid usage improved pavement base stiffness.
Stable subgrade means well-supported and strong subgrade, which reduces the likelihood that, beyond compaction, it will continue to compress, deform or erode after construction, causing pavement cracks, rutting and deformation. Stable subgrades have higher load-bearing capacity and are more resilient to volume changes caused by extreme conditions. As a result, stable subgrades produce smoother roads.
The U.S. Army Corps of Engineers (USACE) has been one of the leading organizations investigating the use of geogrids in pavement construction. USACE validated the effectiveness of geogrid technology for subgrade reinforcement in a 2003 study and Engineering Technical Letter, which noted that geogrids provide significant subgrade improvement by establishing a more-stable working platform and by stiffening the aggregate base course.
There’s also a link between separation and smoothness. Separation usually refers to keeping different aggregate materials (such as a base course and a subgrade) from intermixing. When the two materials intermix, the strength of the material drops. The old expression about what you get when you mix 10 pounds of rock with 1 pound of mud (answer: 11 pounds of mud) is pretty true.
Geogrids, when combined with properly graded aggregate, can act as excellent separators. Geogrids can separate two layers of material that need to have sustained separation to co-exist in a structurally effective manner. By maintaining separation, the strength of the materials will be maintained longer, which supports the pavement surface.
Sustainable roads are those that stay smoother longer without heavy maintenance and remain uniformly smooth across the length of the pavement. And, of course, sustainable roads are environmentally friendly.
Regarding longevity, more states are thinking long term about their road infrastructure investments. In its 2011 white paper, “Releasing the pressure on road agencies,” strategy-consulting firm McKinsey & Company recommended that departments of transportation consider a lifetime cost perspective when building roads.
According to the white paper, “With regard to procurement, some road agencies are shifting aggressively from decision making based on the lowest procurement cost to a model that bases choices on total cost of ownership (TCO). By considering the lifetime cost of any procurement decision, a TCO methodology makes visible lower-cost items that look good at the time of purchase but that will incur high maintenance costs later.”
Many states seem to be testing McKinsey’s advice. States such as New Mexico and Colorado have implemented or are considering longer-term pavement performance warranties with contractors in the 10-, 15- and 20-year range. Michigan is planning a pilot to build roads that last 30 to 50 years.
As more DOTs pursue longer pavement life, road contractors and engineers will face greater pressure to design for long-term smoothness. Again, geogrid technology plays a role by adding enduring stiffness, strength and stability to aggregate base and subgrades, keeping original pavement smoothness in place longer. As a result, geogrid usage helps contractors manage project risk during the project (e.g., enabling construction to continue in inclement weather) as well as after the project (e.g., lowering the risk of early road failure, which triggers warranty terms).
Regarding uniformity, drivers expect consistently smooth pavement over the length of their ride along any road. A stable “platform” of uniform subgrade soil and aggregate base is necessary to construct a smooth, uniform pavement. Geogrid applied across the length of road increases the likelihood of sustained smoothness and a consistently positive driving experience.
Departments of transportation and the contractors that serve them must figuratively “scratch beneath the surface” of road projects to determine optimal pavement design. It’s increasingly clear that they also must ensure optimal pavement design—resulting in sustainably smoother roads—by focusing on how “beneath the surface” is stabilized, strengthened and supported.