The days of applying an off-the-shelf repair technique to deteriorating pavement are quickly coming to an end. New engineering methods, a changing economic landscape and a strong commitment to research and development are forces working together to bring more targeted and effective solutions to road repair and preservation. This means that road owners are tasked with analyzing road repair projects on a case by case basis and must educate themselves on the current best repair solution for a given stretch of road.
With the intention of capturing the best available benefits for drivers combined with the maximum value for their dollars, the state of North Dakota recently installed its first stretch of the Next Generation Concrete Surface (NGCS). Primarily, “they were looking for ways to give local drivers an even greater edge in the severe winter driving conditions,” explained Terry Kraemer, president of Diamond Surface, Inc. The grooves associated with NGCS offer increased safety over other pavement surface textures, allowing the road to maintain its texture longer and creating a safer road long-term. NGCS offers several other benefits over traditional technologies, as well. The surface can significantly decrease tire/pavement noise compared to transversely tined pavement. It also provides a smoother, more uniform ride.
Between June and October 2015, the North Dakota Department of Transportation (NDDOT) initiated a concrete pavement preservation (CPP) project on I-29 near Grand Forks, ND. This section of interstate runs north-south just inside the western boundary of the city and serves as a major transportation corridor. The total project value was $7,619,426.00. Work was done in both the north- and south-bound lanes along a 10.6 mile stretch beginning near 32nd Avenue and extending north beyond North Grand Forks Interchange. The project included joint resealing, bridge repairs and bridge painting. NDDOT chose the NGCS as part of this pavement rehabilitation project in order to determine the viability of improving upon the skid resistance, as well as smoothness and noise level, of a typical concrete pavement.
From the beginning it was clear that in addition to improving pavement serviceability, the purpose of incorporating the NGCS on this project was to learn more about the high-performance surface with the intent of using it on other roads across the state, pending a favorable result. NDDOT worked with neighboring DOTs and the contracting community pre-bid to come up with the best plan and specifications to help ensure a well thought out project. They extensively measured existing IRI, skid numbers and noise level prior to construction of the NGCS and at the end of the project, documenting quantifiable benefits for future consideration.
What Makes The NGCS Safer?
The NGCS is a diamond saw-cut surface that provides a consistent profile with a predominantly negative texture, absent of positive or upward texture elements. The surface resembles a combination of diamond grinding and longitudinal grooving and is created using 1/8 inch wide longitudinal grooves saw-cut to a depth of 1/8 to 3/16 at 1/2 to 5/8 inch centers. Prior to diamond grooving, the surface is flush ground with 1/8 inch wide blades with 0.035 inch spacers, resulting in a very fine, corduroy-like surface finish.
The fact that lower decibel levels, and therefore quieter travels, are experienced on an NGCS is well established. But the NGCS also offers improved wet weather driving, which is especially helpful in hazardous conditions, since automobile accidents are known to increase when there is a loss of friction between the tire and pavement surface due to slippery conditions.
Often, especially during cold weather, it is not just one hazard that drivers confront, but an unpredictable combination of several hazards, ranging from ice that melts and refreezes to blowing wind and snow drifts. And with every new variable, accident risk is compounded. In Grand Forks, ND, which is located in the upper half of the state, the cold season lasts from late November through early March, according to data compiled by the Grand Forks Air Force Base. During that time, the area experiences an average daily high temperature below 28 degrees Fahrenheit and average lows close to zero degrees Fahrenheit. Any improvement that can be made in the skid resistance at the road/tire interface is, therefore, extremely beneficial.
Longitudinally grooved pavements have long been associated with reduced accident rates in real-world conditions. Puzzling, however, is the fact that results for isolated ribbed tire friction values on longitudinally textured pavements showed little or no significant improvement in skid number. This led the International Grooving & Grinding Association to contract with MACTEC, Inc. to conduct friction testing using California Test Method 342 on a number of longitudinally grooved surfaces. The test apparatus used allowed friction to be measured at various angles to the centerline (specifically, at zero, 15, 30, 45 and 90 degrees). This approach enabled the measurement of anisotropic friction, that is, friction that would have different values when measured in different directions. The grooved textures tested exhibited an increase in friction as the device was oriented at increasingly greater angles to the direction of travel. The findings suggest that during actual driving conditions, when there is a loss of vehicular control and the vehicle begins to move at an angle to the longitudinal grooves, traction is enhanced, tires can re-establish friction and the vehicle can regain control.
Bearing this out are further studies conducted by the California Department of Transportation (CALTRANS) in Los Angeles. Results of the study, conducted on 322 lane-miles of longitudinally grooved concrete, showed that the largest reduction in wet pavement accidents was in sideswipe and hit object accidents. The CALTRANS tests compared the longitudinally grooved pavement to control sections of 750 miles of ungrooved concrete pavement. The average daily traffic varied from 60,000 to 200,000 vehicles on these freeway research sections. For the grooved sections, the total traffic exposure was over 4,000 million vehicle miles on dry pavement and approximately 70 million vehicle miles on wet pavement. Wet pavement accident rates decreased an average of 70 percent on all the grooved pavements studied as compared to the control sections, where there was only a 2 percent reduction in accident rates.
Considering the proven benefits of grooving, the NDDOT believed that installing an NGCS on I-29 near Grand Forks would greatly enhance the road's safety. "This section of roadway regularly suffered challenging driving conditions," explained Kraemer, "especially during winter snowfalls that resulted in slippery pavement surfaces. When looking for a solution to this problem, the NDDOT discussed the benefits of an NGCS with the Minnesota Department of Transportation (MnDOT), who had applied NGCS to a section of I-35 near Duluth, Minn. several years ago. MnDOT had been looking for a very smooth, quiet pavement but also realized there was an added benefit to an NGCS in terms of safety during winter snow conditions. Minnesota's demonstrated success with the NGCS encouraged the NDDOT to install it in their own state."
The International Roughness Index (IRI) of the pavement prior to the construction of the NGCS surface measured 61.94 inches per mile, a very smooth pavement by any measure. The IRI of this section following NGCS construction measured an ultra-smooth 36.42 inches per mile. The surface became less noisy, as well. Using an Onboard Sound Intensity (OBSI) tire/pavement noise measuring device, the NGCS reduced noise by an additional 3 dbA beyond the already quiet longitudinally textured surface. These benefits are sure to be noticed and appreciated by the motorists and homeowners on and in the vicinity of this vital roadway.