The traveling public wants smooth, safe, long-lasting pavements. And it’s a well-proven fact that smooth concrete pavements are more durable. To motivate contractors to achieve outstanding smoothness, most states award monetary incentives for achieving it — and they penalize for excessive roughness. Across the nation, concrete pavement contractors take pride in their smoothness numbers.

And clearly, the American Concrete Pavement Association is leading the way toward progress in accurately measuring concrete pavement smoothness using lightweight non-contact profilers. Today, the concrete pavement industry can achieve a much higher degree of accuracy with lightweight profilers than it could six years ago. 

Here’s the issue

To measure smoothness, the International Roughness Index is beginning to replace the California Profilograph. The IRI (see glossary) is commonly measured with lightweight non-contact profilers that resemble a golf cart. In 2002, 12 states used the IRI and 33 states used the California Profilograph to measure smoothness.

“These current lightweight profilers are the state of the art, but most of them don’t do a very good job of measuring smoothness on coarse concrete textures,” says Gerald F. Voigt, P.E., ACPA’s chief operating officer and senior vice president of technical services. The problem: The laser beam used to measure smoothness reads the minute variations in the concrete’s coarse texture. That’s because the laser’s footprint on the pavement is very small — much smaller than a tire footprint. ACPA research has concluded that the lightweight profilers have problems with repeatability and reproducibility of their measurements (see glossary).

“These profilers pick up the texturing, which a car cannot feel,” says Voigt. “The result is a high number that is not accurately reflective of the pavement’s smoothness. When they run the algorithm, the index value is inflated, because the laser picked up surface texture variations — in addition to any bumps that might be felt in a vehicle.”

The ACPA first became aware of the profiler problem in the late nineties. As confirming evidence, a Michigan contractor-member repeatedly ran a lightweight profiler over the same segment of pavement, and the profiler failed to produce the same result each time. At about the same time, the Federal Highway Administration was promoting the concept of lightweight profilers and the IRI, says Voigt. 

So the FHWA commissioned a round-robin procedure by which states were to test results from lightweight profilers. “They had trouble getting good results,” says Voigt. “The states and the FHWA discovered that there were problems with repeatability using these lightweight profilers, but they weren’t sure why.”

Seeking the cause of the problem, the ACPA and its affiliate, the Michigan Concrete Paving Association, commissioned Phase-1 research by Steven M. Karamihas and Thomas D. Gillespie, Ph.D., of the University of Michigan. The primary objective of this project was a comparison of actual profiles, not summary index values. Direct comparison of profiles has the potential to reveal measurement problems using only a few repeat runs, according to the Phase-1 report.

Not the IRI itself

“We had thought the IRI might be the problem, but Karamihas’ research showed that the IRI was not the problem,” says Voigt. “The problem was with the profilers themselves.”

The Phase-1 report was started and completed in 2002. The study, in its words, “demonstrated that high-speed and lightweight inertial profilers were sufficiently repeatable for the measurement of IRI on a moderately rough asphalt site of typical surface texture and a moderately rough concrete site of unusually smooth surface texture.

However, repeatability was compromised on a smooth concrete site with transverse tining, and was inadequate on a smooth concrete site with longitudinal tining. “At present, no reference measurement device can be deemed the most legitimate for verification of profilers on coarse-textured concrete,” the study concluded.

Meanwhile, the ACPA convened a smoothness task force, Voigt says. It began work in earnest in 2002. ACPA’s Highway Subcommittee, working with the association’s smoothness task force, both commissioned the Phase-1 research and developed a Profile Measurement Strategic Plan. Strategies in that plan include: 

	Confirm repeatability and reproducibility problems.
	Quantify impact of texture and joints on indices.
	Work with profiler manufacturers to solve technical issues.
	Test and verify prototype solutions.
	Educate contractors and engineers.
	Devise new standards.
	Implement new specifications.

Key achievements

To date, the ACPA marks four key achievements in carrying out the strategic plan. They are:

New design/construction guidelines have been published, in the form of the document, Concrete Paving: Constructing Smooth Concrete Pavements. Called Technical Bulletin TB006P, it is available at ACPA’s Web site, www.pavement.com. 

The 2002 Karamihas report, discussed earlier, confirmed the repeatability/reproducibility issue.

Newer research (2003) by Karamihas has identified impacts of the non-contact profiler measuring bias on the profile index.

The ACPA has actively pursued improvements in smoothness measurement technology, in cooperation with Ames Engineering and others.

“We retained Karamihas to take the same profiles that were analyzed in Phase 1 and run the profile index algorithms on them to see what the variability of the profiles did to the profile index statistic,” says Voigt. “We found that yes, the non-contact profiler measuring bias does affect the profile index if a zero blanking band is used — but it’s less pronounced if a 0.2-inch blanking band is used.” ACPA’s recommendation: to use a contact profilograph, such as the California profilograph, on longitudinally-tined concrete.

Smoothness measurements with profilers should be taken with a machine separate from the paver — behind the cure application and tining operations, says Ron Guntert, president and CEO of Guntert & Zimmerman Construction Divivision, Inc., a paving equipment manufacturer. “Because of mix variability you can get differential shrinkage across the slab,” says Guntert. “That can affect your smoothness, and so can dowel basket rebound. The paver loads up the dowel baskets, and then they rebound and disturb the concrete surface. That can be a problem. As well, tining can affect smoothness.” 

New results

Today, the ACPA is working with manufacturers of profiling equipment to improve the repeatability and reproducibility of the devices. Following Karamihas’ Phase 1 research, he contacted all of the lightweight, non-contact profiler manufacturers and asked them to modify and improve their equipment to measure concrete pavement smoothness more accurately.

In response to that, Ames Engineering, of Ames, Iowa, stepped forward with two modified non-contact profilers. On October 17, 2003, the ACPA and Ames worked together to test the two modified profilers, and one unmodified profiler, on three test segments in Boone County, Iowa. The three segments were: concrete with a light (smooth) turf drag; transversely tined concrete; and longitudinally-tined concrete.

As tested, the unmodified profiler is called the Ames LISA — Lightweight Inertial Surface Analyzer. It was owned by the Iowa DOT. The second was an experimental profiler owned and operated by Ames Engineering. It was modified to include two optional configurations.

All three configurations showed very repeatable IRI values on the segment with a smooth turf drag and the transversely tined pavement. And the configurations agree with each other on those segments. On the longitudinally tined site, all three configurations still showed good repeatability, but the two modified configurations did not agree with the original configuration at all.

“The LISA profiler is repeatable on coarse transverse tining,” says ACPA’s Voigt. “And LISA’s repeatability is significantly improved on longitudinal tining.”

The bottom line conclusions, says Voigt, are:

“Ames Engineering is the only lightweight profiler manufacturer to make advancements so far.

“The modified LISA profiler is the only equipment to meet the repeatability standard for transverse tining.

“The modified LISA profiler is the only equipment to get close to acceptable repeatability for longitudinal tining.”

Just this past April, the FHWA held another round-up of profiler manufacturers. Several dozen profiler manufacturers’ machines were tested, including GOMACO’s GSI machine (see sidebar).  

As well, the ACPA is working with LMI Technologies on a new laser that would solve the bias problem found in Phase 1. LMI has made the new laser a high priority, and a prototype is to be available this year. Meanwhile, ACPA’s strategic plan is about 50% completed — and the group is marching forward.

 This chart shows reproducibility results for various types of pavement profilers.  (see glossary of terms).

Glossary of Smoothness Terms

Pavement roughness differs from surface texture (see definitions below). Here is a glossary of concrete pavement smoothness terms as defined by the American Concrete Pavement Association and the document entitled Concrete Paving Technology: Constructing Smooth Concrete Pavements (TB006.02P, copyright 2003).  

California profilograph. A 25-foot-long rolling straightedge with a recording wheel at the center of the frame. The wheel measures the vertical deviations in the surface from a blanking band, which in the early years was 0.2 inch. Within the last decade, some agencies have implemented a zero blanking band specification because the 0.2-inch blanking band can hide objectionable bumps. 

Lightweight, inertial surface profilers. Rolling devices that use non-contact laser technology to measure surface roughness. These profilers can get onto the pavement within a few hours of placement and have the speed and ability to compute an array of profile indices. While a number of agencies have begun to employ lightweight profilers, there remain limitations on the accuracy of these devices.  

International Roughness Index. A profile-based statistic established in a study by the World Bank. The IRI is developed mathematically to represent the reaction of a single tire on a vehicle suspension to roughness in the pavement surface. The car is gauged to be traveling at 50 miles per hour. The algorithm of the vehicle suspension system to attain IRI is very complex. IRI is linearly proportional to roughness measured in a profile. 

Profile Index. A smoothness statistic derived from the trace of a profilograph. The number is determined by adding up all of the bumps and dips from the profile trace (in meters or in inches) and dividing by the length of the pavement section in kilometers or miles.  

Repeatability of profiler measurements. A rating of how well the two measurements of the same pavement segment by the same profiler agree with each other.  

Reproducibility of profiler measurements. A rating of how well two measurements of the same pavement segment by different profilers agree with each other. The ACPA is seeking to attain maximum reproducibility among different profilers from the same manufacturer and among profilers from different manufacturers.

Roughness. Minor variations in the vertical elevation of a pavement surface. Roughness is the result of variability in the subgrade, subbase, surveying, placing, equipment, operator technique, and many other factors.

Surface texture. Roughness, or smoothness, does not measure surface texture, which is influenced by the type, gradation, and shape of aggregate used in the concrete mixture; the quality of finishing; the spacing and/or depth of burlap dragging or tining. Generally, texture does not influence the ride of a pavement. The profile indexing procedure should not include texture deviations in quantifying the surface roughness.

New Profiler Can Do Multiple Traces

To address the smoothness measurement situation, Gomaco has invented, and is offering for sale, a new, non-contact profile machine that develops surface profiles for concrete pavement and other paved surfaces. The machine can operate independently, or will mount on a paver.

Called the GSI, for Gomaco Smoothness Indicator, the device has been tested against proven manual profilograph machines. And, says Mark Brenner, a research engineer at the company, “We have good output or graphs compared to machines which have been certified by different states. We also have very good repeatability.”

As the machine collects data, three indices are being calculated continually – the GSI index, the profile index, and the International Roughness Index.

“You can also export an .erd file, a standard file format used for analyzing profile data,” says Brenner.

Gomaco says all GSI components are off the shelf, but that the software is unique. Sensors can be mounted onto the GSI or on the paver itself. “There are a couple of calibration steps for initial setup to calibrate the encoders or the pulse pickups in the motors,” says Kevin Klein, Gomaco’s research and development manager. “This calibrates the distance measuring devices that are on the machine. You drive the machine a known distance and then enter that distance into the GSI computer.”

The distance traveled doesn’t matter. If you overshoot a 100-foot distance, you simply enter the actual distance into the computer, and it will calibrate to that measurement.

To do a single trace, a contractor would use a single-trace unit. That comprises the GSI computer, two sonic sensors, a slope sensor, all the mounting hardware, and cables. With more equipment, the GSI can produce multiple traces. If you are mounting on a paver, you also need two encoders, one for each side. And on the GSI machine there are two CAN networks, one for the GSI and one for the machine control. CAN stands for Controller Area Network and is a network protocol used for mobile equipment controls.

Gomaco reports that it has worked with a couple of contractors in using the GSI, and says they are very interested in the concept. “They’re intrigued by the non-contact concept and by the time savings incurred by doing up to eight traces in a single pass,” says Klein.

10 Tips for Smoother Pavements

A number of factors influence PCC pavement smoothness. Here are 10 key construction factors that influence concrete pavement smoothness as summarized from the ACPA’s Concrete Paving Technology: Constructing Smooth Concrete Pavements (TB006.02P, 2003). Copies are available from the ACPA at www.pavement.com.   

Preparing the grade. Because concrete pavements are usually built in one layer, it is critical that the base and subbase surface be as true to grade as possible before placing the concrete. Automated fine-grading equipment is capable of placing the truest base surface possible compared to other grading methods, according to the American Concrete Pavement Association.  

Producing consistent concrete. Achieving an accurate and consistent slump – and mixture consistency generally — is important, because it directly affects how the paving equipment performs. Control of the mixture consistency is not limited to post-mixing only; quality control must continue during hauling, placing, and finishing.  

Delivering concrete. It is important to maintain a steady flow of concrete to the paving train. One must consider hauling times, the concrete’s strength development speed, the number of batch trucks, the head of concrete in front of the placer-spreader, and more.  

Setting up fixed forms. An accurately set stringline determines the location and height for the forms, if a slipform paver is not being used. Stake spacing for the stringline of about 25 feet will produce good results for straight sections. Each metal form must be clean and in good condition. Forms should be examined with a straightedge or stringline before using them.  

Setting and maintaining the stringline. For slipform paving, the contractor must pay close attention to the position and maintenance of the stringline. The stringline material, stakes, staking interval, splices and repositioning frequency all may have an impact on the resulting pavement surface. The stringline itself may be wire, cable, woven nylon, polyethylene rope, or similar material. The stringline must be checked periodically.  

Operating the paving machine. The paver should be a well-maintained machine, should not stop often, and should not push too much concrete. The primary adjustments a paver operator can make are the machine’s speed and the frequency of its internal vibrators. If the concrete’s plastic qualities vary widely, requiring frequent adjustments in either speed or vibrator frequency will result in surface roughness.  

Paving on vertical grades. It may be more difficult to construct a smooth pavement on grades exceeding 3% than on flatter grades. For steep grades, the contractor may need to lower the slump of the concrete, adjust the profile pan’s angle of attack, adjust the profile pan elevation, and adjust the staking interval.  

Handling dowel bars and reinforcement. Continuous reinforcing steel, mesh reinforcement, and dowel baskets all may disrupt the consolidation pressure during the passage of the slipform paver. Damming and reinforcement ripple are thought to be the most common causes of reinforcement-related roughness. Damming occurs when a basket assembly or transverse steel acts to dam up the concrete on the grade. Reinforcement ripple occurs when either longitudinal or transverse reinforcing steel prevents concrete from being evenly distributed under and over the steel.  

Finishing the surface and headers. In general, it is best to limit hand and mechanical finishing to the minimum needed. Some contractors overuse mechanical longitudinal floats directly behind the slipform pan. It’s a good idea to check the surface with a 10- to 20-foot-long hand-operated straightedge. Experienced finishers can use a straightedge to remove high spots and fill low spots — by employing a scraping motion. Otherwise, they use a long-handled float to smooth bumps and disturbed places in the concrete.  

Educating and motivating the crew. Smoothness doesn’t just happen; people and machines create it. Management practices build a sense of ownership and pride in the finished product. Crew training is vital. For example, the stringline crew needs math skills and a keen eye, while paver operators need to know which equipment functions add or detract from a smooth surface.

Reprinted from Better Roads Magazine
August 2004

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