How to earn smoothness incentives.

Road agencies and contractors alike have made it their business in recent years to specify and build measurably smooth pavements, and for good reasons. The traveling public gives ride quality a high priority. Numerous studies have shown that pavements built smoother will last longer. Plus, smoother roads are safer, and they require less maintenance.

Use of a material transfer vehicle avoided touching the paver with another machine and helped produce a smooth pavement on this section of I-15 in Utah.

As a result, most states are willing to pay extra for smooth pavement in the form of incentives written into the smoothness specifications. And virtually all states require that the contractor either correct a pavement that doesn’t meet a specified smoothness level or accept a payment reduction.

States select the projects that will have smoothness incentives or penalties. In Oklahoma, for example, about half of the state’s asphalt overlay projects have a smoothness specification, and all full-depth projects do.

North Dakota has a Provisional IRI Specification that uses the International Roughness Index as a smoothness criterion (see sidebar). The state pays an incentive of $3,500 per lane mile for new pavement with an IRI of 42.0 inches per lane mile or less. The payment is $1,500 for an IRI of 42.1 inches to 47.0. An IRI between 47.1 and 57.0 will result in neither an incentive nor a penalty, and more than 57 on the IRI scale puts the contractor in a penalty situation.

Achieving smoothness

Smooth asphalt pavements result from attention to detail in subgrade preparation, hot-mix consistency, paving technique, and compaction. An example comes from Haskell Lemon Construction Company, which recently built a 2-mile-plus length of four-lane divided State Highway 152 in Oklahoma City. The four-lift asphalt paving project won quality awards from both the Oklahoma Asphalt Pavement Association and the National Asphalt Pavement Association.

A grade sensor located just behind the tow-point cylinder is the best position for smoothness, because it produces a slow change in screed angle.

"Our final subgrade preparation sets the stage," says Jay Lemon, president of Haskell Lemon. "We put stringlines up and trimmed the subgrade with a grade trimmer." The project had four bridges in the alignment, and the contractor ran the stringline across all four bridges to match the grade with the profile of the bridges.

Lemon says his crews constructed the subgrade and first lift of asphalt in phases, but did not pave any of the second lift until the entire first lift was in place. "We made sure we could pave from bridge to bridge, so that we had no cold transverse joints between the bridges," says Lemon. "We paved 16-feet wide on the inside pass, 12 feet on the center pass, and 10-feet wide on the outside lane."

The contractor used a paver with an automated grade-control system. A material transfer vehicle fed mix to the paver and kept it moving at a constant speed. "The paver never stopped," says Lemon. "The plant was 4 miles from the beginning of the project, and we used eight to 10 tandem-axle trucks hauling 15 to 17 tons per load."

Haskell Lemon worked with the Oklahoma DOT to schedule the finish paving work near the end of last summer. "That way we got longer days and warmer temperatures, which are more favorable for quality paving," says Lemon.

The smoothness numbers were exceptionally good. Each lane averaged under 1 inch of deviation per mile on the profilograph, with a 0.2-inch blanking band (see sidebar). "We achieved the maximum incentive payment," says Lemon.

Don’t touch the paver

Rubblized concrete pavement formed the base for 22 miles of hot-mix asphalt pavement on I-15 in Utah, a project that Staker & Parson Companies completed in the spring of 2006. The contractor paved three 2-inch lifts of hot mix topped with a 1.5-inch wearing course of stone matrix asphalt.

Running grade left and grade right with two averaging devices is the best method to achieve a smooth asphalt pavement.

Belly-dump trucks dropped a windrow of hot mix in front of the material transfer vehicle, which picked up the asphalt and transferred it to the paver. "Nothing touches the paver, and I think that’s one of the keys to smoothness," says Gary Lindley, project manager for Staker.

The first 2-inch lift was a leveling course on top of the rubblized and compacted concrete. "Then we put down another 2 inches and opened it to traffic," says Lindley. "We paved the next two lifts under traffic. With each lift, the pavement got progressively smoother."

Staker & Parson Companies earned 93% of the available smoothness bonus on half of this I-15 project and 9% of the smoothness bonus on the other half.

Staker used a track-mounted paver with a factory-mounted 40-foot averaging ski for automated grade control. "We had passes of 15, 12, and 11 feet," says Lindley.

On one half of the 22 miles, Staker earned 93% of the available smoothness bonus, and on the other half, the contractor received 97% of the available bonus. "We had to achieve 5 inches or less of deviation per mile on the California profilograph," says Lindley. "We have a crew that has been together for quite a while. Their pride in workmanship made all the difference."

The rollers made slow, gradual stops on the mat when necessary. "The less you can stop on the mat, the better off you are," says Lindley. When stops are necessary for more time than changing direction, compaction experts recommend stopping the roller on a cooled portion of the mat — not right behind the paver where the asphalt is hot.

Mill and overlay

Lehman-Roberts Company earned the maximum smoothness bonus on a 4.8-mile mill-and-overlay stretch of I-40 in Memphis, Tennessee. "The project was done entirely at night. We used a material transfer vehicle and the paver never stopped," says Rick Moore, president of the company and a former chairman of the National Asphalt Pavement Association.

The contractor did not use the same trucks for milling and hauling mix. One group of 15 trucks hauled recycled asphalt pavement away from the milling machine, which worked about 1.5 hours ahead of the paving train. A separate set of 12 trucks hauled mix to the paver.

"We felt it worked better with two sets of trucks," says Moore. "For safety in traffic, it’s best to minimize the number of times the trucks are ducking in and out of traffic at night. Plus, the truckers are more conscientious about what they’re doing if they only work in front of one machine."

Production averaged about 1,500 tons per night. "Nothing broke down during the entire project," says Johnny Driver, general superintendent for Lehman. "It went along just like we bid it."

With a zero blanking band, Lehman-Roberts averaged 12 inches of deviation per mile on the profilograph. "Twenty to 30 inches is acceptable, and anything above 30 you have to fix," says Driver.

He said the plant was able to provide a very consistent mix, which helped achieve the excellent smoothness numbers. The paver moved at 35 to 40 feet per minute. Compaction was accomplished with a vibratory roller in the breakdown role, a rubber-tired roller in the intermediate position, and a vibratory tandem-steel-wheel roller operating in static mode as a finish machine. The use of Tech Shield release agent on the rubber-tired roller prevented the polymer-modified mix from sticking to the tires.

Paving fundamentals

At the World of Asphalt conference in Atlanta last March, David Shelstad of Caterpillar Paving Products presented a number of fundamentals that lead to smooth asphalt pavements. He said the following factors affect the paving screed, and they all need to remain as constant as possible to produce a smooth pavement:

- Paving speed.
- Head of material in front of the screed.
- Screed adjustments.
- Mix design and temperature.
- Air temperature.
- Grade temperature.

The screed’s angle of attack is normally between 0.125 and 0.250 inch, Shelstad said. If the screed angle is too high, or tilted up too much, the screed will compact the mat with its trailing edge and you will notice a shiny surface on the mat. A screed angle that is too low will produce an open texture in the mat and cause excessive wear on the leading edge. Screed extensions need to be adjusted to run at exactly the same height as the main screed.

Haskell Lemon Construction achieved the maximum smoothness incentive on this section of SH 152 in Oklahoma.
Your mix-hauling capacity needs to match the paver speed. If you have too few trucks — or if the paver moves too quickly — the paver will need to stop, the screed will settle, and you’ll produce a bump wherever the paver stops. If you have too many trucks, the mix will cool in the trucks and mix temperature will be inconsistent. The paver may be tempted to speed up to catch up with the trucks, and that change of speed is not good.

Paving for smoothness requires teamwork, attention to detail, and pride of workmanship. It’s not rocket science, but your paving and plant crews need to care enough to do it right.  

How Smoothness is Measured

The two most commonly used smoothness indices are the International Roughness Index and Profilograph Index, says The Transtec Group, an Austin, Texas-based consulting firm.

The International Roughness Index can be determined using measurements from any valid profiler — inertial profiler, inclinometer-based device, rod-and-level, etc. — that generates a profile trace showing the true shape of the pavement surface. This pavement profile is fed into an algorithm that determines the IRI value for the pavement. An inertial profiler measures the pavement profile using an accelerometer (acceleration sensor) to form an inertial reference and a height sensor to measure the pavement surface height relative to that reference.

A high-speed profiler is an inertial profiler mounted to a vehicle operated at prevailing highway speed. A light-weight profiler is an inertial profiler that is relatively light-weight compared with high-speed profilers. It is usually mounted on a golf cart-type vehicle or all-terrain vehicle and works at a much slower speed than prevailing traffic.

The Profilograph Index is generally measured with a profilograph, either California-type or Rainhart, although some software programs can compute PrI from a profile trace produced by an inertial profiler. PrI is determined by counting the number of scallops in the profile trace that fall outside of the specified blanking band. The most commonly used blanking bands in the U.S. are 0.2-inch and 0-inch, although a few states use a 0.1-inch blanking band as well, says the Transtec Group. PrI is sometimes called the Profile Index.

Both the IRI and PrI are reported in units of inches/mile or meters/kilometer. However, Transtec cautions, these measurements are not directly correlated and cannot be directly interchanged. In general, profile traces are collected in either one or both of the wheelpaths within a pavement lane, although some states require a quarter point measurement instead. Some states average the values from the two wheelpaths, while other states use only one wheelpath to assess smoothness.