The Smooth - Quiet Pavement

Dec. 28, 2000
The National Quality Initiative Survey of drivers across the country found that ride quality is one of the primary criteria in pavement rehabilitation priorities with the traveling public.
The National Quality Initiative Survey of drivers across the country found that ride quality is one of the primary criteria in pavement rehabilitation priorities with the traveling public. The users of America’s roads want a smooth and quiet pavement and hot-mix asphalt (HMA) is the most popular product used to meet that need.

Rough pavements create a variety of problems for motorists, including poor ride quality, potentially unsafe driving conditions and the development of dynamic conditions for trucks, which may cause further damage to the pavement and damage to the cargo and trucks themselves. All of these problems have costs associated with them.

The Federal Highway Administration has published a report that indicates smooth pavements last longer and are more cost-effective. The study indicated that a 25% increase in smoothness yields a 9% increase in live life of the pavement.

Measuring ride quality

For years, the highway industry has worked to achieve smooth pavements. The construction equipment to produce a smooth pavement is currently available. However, there is not a universally accepted way in which to measure ride quality.

In the past, the effect of roughness on ride quality has been rated by panels who developed indexes of ride quality. Test values from specific equipment were then correlated to the ratings developed by the panel. Recently, profiling techniques have been developed to describe the pavement surface independent of the equipment. Examples of profiling techniques include California profilograph-type equipment and profilometers.

The profilograph is a device that develops a trace of the pavement surface and it is pushed along the roadway surface by technicians. Blanking bands are sometimes used to eliminate the “noise” or garbage in the trace. The information data from this trace is either manually reduced or is entered into a computer program to develop a pavement rating roughness index.

The inertial profilometer is a device, which develops a “picture” of the road in terms of wavelength and amplitude and measures and records genuine elevation profiles of the road from which different roughness indexes can be calculated. The device contains non-contact sensors that measure the vertical distance to the pavement from the device.

An inertial reference corrects the readings for the bounce of the survey vehicle. The device contains non-contact sensors, which measure the vertical distance to the pavement from the device. The device has several different possible configurations and can be mounted on the bumper of a vehicle or may be designed integral to the vehicle.

The profilometer is operated by a technician driving the vehicle at speeds varying from 20 to 60 mph. The actual operating range varies by manufacturer. It is important that the operators follow proper procedures in calibrating and operating profilometers to reduce errors.

The National Cooperative Highway Research Program (NCHRP Project 10-47) is developing guidelines for longitudinal pavement profile measurement for computation or the International Roughness Index (IRI) and Ride Number.

At the recent U.S. Hot Mix Asphalt Conference in Portland in October, Don Popejoy of Ritchie Brothers Construction Paving Inc., talked about the myths and misconceptions (M & Ms) for achieving smoothness. His M & Ms are:
•Smoothness is well defined;
•Smooth roads don’t cost more;
•To get smoothness, get a bigger stick;
•An overlay will smooth up anything; and
•Anyone can build a smooth road.

Defining smoothness
Let’s talk about each of these M & Ms.
Is smoothness well defined?

While everyone will agree on the importance of smoothness to the performance of the pavement, the industry has no uniform, industry-wide standard. From the contractor’s perspective, the owner’s definition is the only one that matters because that’s who pays the bills. Depending on the agency’s perspective, various blanking bands may be used for profilographs and the results can be quite variable. Many in the industry feel that the use of the profilometer is the wave of the future. If the industry moves towards the use of profilometers, some of the variation in the analysis should be reduced or eliminated.

How much do smooth roads cost?
There are quite a few operational techniques that the contractor can use to achieve smooth pavements. These include keeping the paver moving, not allowing the truck to bump the paver, windrowing the mix, using material transfer devices and decreasing the lift thickness. All of these approaches can improve pavement smoothness but all take time and will result in additional cost to the contractor.

To get smoothness, get a bigger stick?
Popejoy states that “nothing motivates a contractor so much as a chance to earn a bonus.” Smoothness specifications must quantify the value of the increase in quality to both the owner and to the contractor who must achieve it. As specifications are being developed, it is important to include the contracting industry in the process so that the bar can slowly be raised. Too tough a specification at the beginning may undermine the very improvement that you are trying to achieve.

An overlay will smooth up anything?
While the HMA may be a great product, there is little magic in the placement of an overlay. New, black and shiny does not equal smooth. In some cases, DOTs are forced to stretch their overlay dollars by placing overlays that are too thin to achieve the improvement in smoothness desired. A thin overlay will simply not fix an extremely rough surface. The use of a leveling course or milling may be necessary to achieve the final goal. The key message here is that the design engineer must fit the cure to the illness. It is important to get enough overlay to meet the smoothness requirements of the public.

Anyone can build a smooth road?
While automation has advanced the production and placement of hot-mix asphalt, it is as true today as ever that high-quality people build smooth pavements. Both the company and the individual must be committed to building a high-quality pavement.

Appropriate training of everyone in the production chain is critical to achieve the goal of smooth pavements. In addition, appropriate incentives must be made available to the employees to provide motivation commensurate with the high-quality expectations for the pavement.

In summary, achieving pavement smoothness is not a mystical event. It just requires appropriate design, good specifications, appropriate equipment, true incentives and quality people. Given these tools, the contractor can achieve the optimum hot-mix asphalt pavement—both smooth and quiet.