Pavement texture fundamentals

August 2013 » Features » REBUILDING AMERICA'S INFRASTRUCTURE
Engineering smoother, safer, quieter pavements that also reduce gas consumption
Robert Otto Rasmussen, Ph.D., INCE, P.E.

Life is full of bumps in the road, but the most important ones might be smaller than you think. Pavement texture – bumps and dips – can affect everything from ride quality to braking distance. Texture affects road noise and even gas mileage.

As so many important things are affected by pavement texture, why do pavement engineers sometimes overlook it? Two main reasons: Until recently, engineers weren't able to measure texture in a meaningful and accurate way. Perhaps more importantly, there wasn't a full understanding of the links between pavement texture and friction, noise, rolling resistance, and smoothness.

A lot has changed in recent years, and texture is now becoming an integral aspect of pavement engineering.

Figure 1: The geometry of pavement texture can be broken down into texture wavelengths, including microtexture, macrotexture, and megatexture. Roughness (not pictured) is texture with wavelengths in excess of 500 mm.

Texture fundamentals
Pavement texture is defined by the irregularities on a pavement surface that deviate from an ideal, perfectly flat surface. The World Road Association (PIARC) has established standard categories of texture (Figure 1) classified by texture wavelength (the distance between peaks). These include microtexture (wavelengths up to 0.5 mm), macrotexture (0.5 to 50 mm), megatexture (50 to 500 mm), and roughness (wavelengths larger than 500 mm). Real pavement textures are a complex mix of all of these texture wavelengths. To separate them out, engineers need mathematical tools and a texture measurement (profile).

Figure 2: Pavement texture contributes to most of what defines pavement functional performance (sometimes called pavement surface characteristics). In some instances, more texture is a good thing (shown as green bars), in others, less texture is better (bars shown in red).

As shown in Figure 2, different sizes of texture will affect pavement surface characteristics in different ways. Small texture affects friction, while large texture affects ride quality. Noise and rolling resistance are principally controlled by macrotexture and megatexture.

Measuring texture profiles requires specialized equipment that measure pavement surface elevations on a very small scale. Instead of reporting elevations in meters, texture measurements are reported in fractions of a millimeter. Given the need for both precision and accuracy, lasers are commonly used for texture measurement. Most texture profilers use a single point laser, but 3D laser profilers are gaining favor because they can gather a greater amount of information about a pavement's texture. One such device is RoboTex (Figure 3), a remote-controlled texture-measuring robot. With a complex array of on-board sensors, the device is capable of measuring texture in three dimensions, as small as 0.1 mm from peak-to-peak, and 0.01 mm deep.

Figure 3: RoboTex is an example of a 3D texture profiler that is capable of digitizing texture elevations with sub-millimeter accuracy. The device is remotely piloted and to date has evaluated hundreds of miles of pavement around the world.

RoboTex has been used to collect texture on thousands of pavements worldwide. The texture profile database developed using the 3D measurement system has been used to relate subtle nuances of texture geometry back to smoothness, friction, noise, and rolling resistance. Predictive models have emerged, which are now being used to engineer a newer generation of pavements.

A smooth, safe, quiet ride
It's relatively easy to visualize the effect that texture will have on ride quality. In Figure 2, the red bar illustrates what we already know: the greater the texture, the worse the ride.

The public demands smooth roads. Recognizing this, the highway industry has been migrating toward using the International Roughness Index (IRI) as a quality indicator. The IRI converts a pavement texture measurement (profile) into a single number, which has been well proven as a reproducible and relevant measure of ride quality. The Federal Highway Administration (FHWA) recently identified IRI as the principal means to monitor the performance of pavements nationwide, and state highway departments continue to adopt it into their standard practices for construction quality assurance (Figure 4).

Figure 4: Pavement smoothness is commonly reported using the International Roughness Index (IRI). In this figure, IRI data are shown for a highway in the Arkansas. Areas shown in red indicate localized roughness, while dark blue areas represent smooth pavements. Visualizing data in this manner is an important aspect of a performance management system.

While ride quality is important, so is our ability to stop when we need to. Braking distance is a function of tire-pavement friction, and friction is a function of the pavement texture. While smooth roads are important for ride quality, some bumps and dips – albeit small ones – are needed for pavement friction.

Friction is affected by the texture we impart into concrete pavements and the way we select and size the aggregates for asphalt pavements. The materials used affect friction immediately after construction and during the life of that pavement.

Figure 5: High Friction Surfaces (HFS) like this one in Texas are an important new tool for resurfacing existing pavements where friction demand is high, such as in tight radius curves.

In some cases, highly specialized pavement surfaces have been engineered for high friction. The FHWA has been showcasing so-called High Friction Surfaces (Figure 5), which have been demonstrated to be a viable surfacing technique for existing pavements where additional friction demand is necessary (such as at intersections and on horizontal curves where run-off-the-road accidents are prevalent).

Engineering pavements to be smooth and safe is paramount, but in recent years, the once minor topic of quieter pavements has received a lot of attention. Ideally, we would drive on roads that are smooth, safe, and quiet. Living and working in a community with quieter pavements would mean less traffic noise and an improved quality of life.

The complexity in engineering quieter pavements is that an optimized texture is necessary – particularly a texture that can maintain adequate levels of friction. In recent years, work sponsored by FHWA, state transportation departments, and the paving industries have lead to quieter pavement options among all pavement types.

Figure 6: The ideal texture for quieter pavements is a very fine texture that does not protrude above an otherwise "flat" surface. A chip seal surface treatment is an example of a loud pavement, while a fine-graded asphalt pavement can be very quiet by comparison.

As illustrated in Figure 6, the fundamentally optimum pavement surface has small (fine) texture that appears relatively "flat" on top, but has a network of random, narrow, closely spaced grooves. For concrete pavements, the quieter options are commonly diamond ground or drag textured surfaces. For asphalt pavements, small-aggregate and open-graded mixtures have both proven quiet.

Engineering for energy savings
Energy policy remains an important issue. The new federal CAFE fuel economy goal is 54.5 mpg by 2025. Ask your friends in Detroit how they plan to achieve this, but be forewarned, the response you get may make you blush.

The highway industry can help. The prospect of engineering a pavement to improve fuel economy is not only intriguing, but also very real. Fuel economy improves when the rolling resistance of a vehicle decreases, and pavement texture again plays an important role in this.

Of course, it is important for your vehicle to be able to slow down when you hit the brakes. However, if you've accelerated to your intended speed, it would be ideal to maintain that speed without a whole lot of help from the engine. Beneath the car, the pavement texture is constantly interacting with the tires as they roll, and energy is consumed as a result. If this interaction could be changed so that less energy is lost, then the vehicle will keep on rolling and fuel economy will improve. There is of course a sustainability perspective to this – with improved fuel economy comes reduced emissions, cleaner air, and a happier and healthier planet.

While elusive for some time, the ability to engineer a pavement with low rolling resistance is now within reach. Final touches are being sought through numerous efforts including international studies such as MIRIAM (www.MIRIAM-CO2.net) and COOEE (www.COOEE-CO2.dk). Efforts in the United States include those at the Mn/ROAD facility, sponsored by Minnesota Department of Transportation.

In simple terms, smooth roads will generally have better rolling resistance. However, smooth roads as measured by IRI are not enough. Much of the texture that affects rolling resistance will not affect ride quality, and thus a new measure (beyond IRI) is needed if we are to evaluate the quality of roads for energy efficiency. An emphasis on megatexture is needed, for example. Ultimately, we may someday look at building to a new International Rolling Resistance Index (IR2I). Until that time, we can specify materials and construction methods that are known to produce less megatexture, and as a result, improve fuel economy.

Coming together
Can we have our proverbial cake and eat it too? Fortunately, yes we can. Pavement engineering for optimal smoothness, friction, noise, and rolling resistance is not only possible, but it is slowly working its way into standard practice.

Some caution is needed, however, since prudent pavement engineering requires that we continue to target both long-term structural and material performance as well as cost. The good news is that optimization of all of these factors is also possible.

The thought of "saving the world" rarely crosses the mind of a pavement engineer. But it is possible to improve society through these practices – and these improvements are now a little bit greater through pavements that ride smooth, get you home safely, contribute to a quieter society, and reduce our dependency on oil.

Robert Otto Rasmussen, Ph.D., INCE, P.E., is vice president and chief engineer for The Transtec Group, Inc., Austin, Texas. He can be contacted at robotto@thetranstecgroup.com.

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