The processes used to produce paved asphalt surfaces are little changed from those used 20 years ago. “When we compare the paving laydown processes of today with those of the past, one would say little has changed,” notes John Mooney, product manager for paving and milling products, Volvo Construction Equipment. “We still use both wheel and track pavers, hydraulically and manually extended screeds for laydown, dump trucks and transfer vehicles for material delivery and vibratory and pneumatic rollers for compaction and finish.”
What has changed are the mix designs. Superpave, warm mix asphalt (WMA) and stone matrix asphalt (SMA) are just a few examples of recent developments. “Mix designs are changing rapidly,” says Mooney. “Fundamentally, they are processed the same. But it becomes a fine art in how the paver processes that material.
“In general, mixes become more sophisticated, designed for specific applications and performance characteristics,” he continues. “To deal with these very technically advanced mix designs, the pavers and rollers have evolved, as well as the skill requirement of everyone in the paving industry.”
The result is improved surface quality. “The quality of today’s asphalt surface is much smoother, with an overall improvement in surface texture consistency and joint quality,” says Laikram “Nars” Narsingh, product manager for Wirtgen America’s Vögele line of asphalt pavers. “Also, the deterioration of the surface takes much longer, due to improved paving processes such as reduced mass and temperature segregation, and higher joint density.”
Consistent Speed Delivers Optimal Smoothness
Since its introduction in the 1930s, the free-floating screed has become the industry standard among asphalt paver manufacturers. Its design remains largely unchanged, yet mat quality has continued to improve for several reasons, one of which is enhanced paver technology. “Improved control technology such as CANBUS with digital display, non-contact sensors and improved hydraulic controllability has allowed better control,” Narsingh points out.
Modern asphalt pavers are more flexible and easier to operate, thanks in large part to the incorporation of advanced electronic control systems required to meet diesel engine emissions regulations. These systems have enabled key paver functions — the conveyor, auger, tram system, steering modes, etc. — to be tied together for more precise controllability.
For example, the drive control systems on the Volvo P7110 and P7170 are designed to take the pavers from a stop position to full speed with very “delicate” control and feel. An electronic paving speed controller allows the operator to ease the paver into and hold a preset speed throughout the paving process.
“The paver drive controls play a very important role in producing a consistent, high-quality mat,” Mooney states. “If an operator has the ability to feather the start and stop, and the ability to set a consistent, predetermined speed, the crew is on the path to developing a bonus performance payday.”
“Precise propel controllability, especially on track pavers, allows the operator to maintain the proper straight lines that improve smoothness and joint quality,” adds Narsingh. He notes that the single-steering joystick on Vögele pavers, combined with the ability to set the desired paving speed via a digital display, allow operators to significantly control the pull force. Wirtgen also offers a screed “hold and freeze” function that automatically engages with the propel lever to prevent screed settling and hump due to stops and starts.
The ability to smoothly transition into turns further minimizes the potential for irregularities in the mat. “New technology allows operators to automatically set the turning radius of the paver into turns, which is one of the most challenging operations for operators,” says Narsingh.
Managing Material Flow
There have also been significant advancements in how material moves through a paver. “Anti-segregation components and designs built into material-handling systems of the paver are the biggest improvement over the last handful of years,” says Tom Travers, sales and marketing manager, Carlson Paving. “The less the material is agitated, the better it looks and the longer it lasts. Improved tunnel design, fewer sharp corners, smaller auger drive boxes or outboard auger drive boxes, tunnel kits, independently adjustable auger systems and reverse auger segments have all improved material handling, causing less segregation.”
“Features of modern feeder systems that ensure material consistency include independent auger and conveyor design that allows better material flow under the auger drive box,” says Narsingh. Material deflector plates and strategically positioned auger flights also allow for better material consistency in front of the screed. “Non-contact auger sensors and potentiometers (instead of flow gates) to control material delivery to the augers — coupled with digital display of the material height — allow operators to improve controllability,” he adds.
Twenty years ago, the speed of the augers and conveyors were regulated manually. Paddle switches were routinely used to control the on/off of the augers and conveyors, and flow gates controlled the conveyor volume. “Today, easy to adjust, no-contact sonic sensors control speed and material volume. The screed operator can infinitely adjust speed and material volume, creating a consistent head of material,” says Mooney.
Volvo offers a four sonic material feeder system on its P7000-Series pavers. Two sonic feed controllers manage the conveyor’s material load; two others in the auger channel control the amount of material in front of the screed. Hydraulically extendable tunnels (based on the original Blaw-Knox design) help control the head of material across the full screed width. “This is critical to create a controlled flow path so the quality and texture of the mat is the same the full width of the screed, even when fully extended,” Mooney explains.
The introduction of the tapered screed has further aided material flow control. “Carlson introduced the tapered screed in order to help with fluid material movement across the front of the screed,” says Travers. “With the patented 2° taper, the EZ screed has been able to help in reduced tearing and segregation of the asphalt, while at the same time aiding in the fluid outboard flow, whether the augers are running or not.”
A berm builder on the EZ screed incorporates a curb into the pavement to help direct water runoff to appropriate drainage points. “The heated endgate helps in joint construction, as well as fluid movement of the material in the extension,” Travers adds, “aiding in better joint quality due to the 500° endgate.”
Machine Automation Advantages
The transition to a less skilled workforce as experienced operators move out of the market has necessitated the move to greater equipment automation.
“All pavers should have and use a material feed sonic or paddle feed system,” Travers emphasizes. “A crew member can’t possibly control this function as well as an automated system that is furnished from the manufacturer of the paver you purchase.
“Grade and slope systems are also essential for quality pavements, and normally will pay for themselves by decreasing material over runs and maintaining yield,” he continues. “There are quite a few choices for high-quality systems for the grade and slope function. These systems take the human error out of the equation and allow the focus on the screed personnel and paver operator to maintain speed, width, material interface and other critical functions of their job.”
For example, the Vögele Navitronic system provides the ability to automatically control the width and direction of the pavement by automatically steering the paver, says Narsingh. He notes, “Today’s integrated automatic grade and slope control systems make it much easier to implement automatic grade, as the systems are native to the paver and do not have to be transported to the jobsite. Also, these integrated systems allow operators to monitor grade and slope without being in automatic control mode.” The lack of complicated parameter changes makes them easier, and more likely, to be used, as well.
Volvo pavers are designed to accept most manufacturers’ automation systems as plug and play. “Automatic grade and slope (AGS) systems are changing at breakneck speed,” Mooney comments. “Paving crews have their preferences and no one supplier owns a superior position, so most paver manufacturers provide connection with major AGS systems.” Volvo also plans to introduce an updated version of its Blaw-Kontrol system in the near future.
Refined by Design
According to Travers, there are still basically three types of screeds in the industry: the fixed screed (wedge lock); the front-mounted extension (power extendable); and the rear-mounted extension (power extendable).
Narsingh adds that the three types of vibratory screeds most commonly used in the U.S. include:
- Rear-mount screeds — main and extension screed plates are equal length (18 in.) front-to-back; paving width averages 10 to 19 ft. 6 in.
- Equal-width front-mount screeds — main and extension screed plates are 18 in. equal front-to-back; paving widths average 10 to 18 ft.
- Unequal-width front-mount screeds — main screed plate is 24 to 28 in. on average, and the extension screed plate is 10 to 12-in. on average, front-to-back; paving width averages 10 to 19 ft. 6 in.
Like the pavers themselves, key refinements over the past 20 years have left a lasting impression on screed design. “The biggest improvement is the heating system,” says Travers. “Carlson introduced the electrically heated screed in the early ’90s and has been improving the design ever since. Most other OEMs have followed suit and are now fully electric on the majority of screeds they offer.”
Another major advancement is the adjustable screed plate. “The adjustments of the screed plates are the single most important improvement in the industry,” Travers asserts. For example, Carlson’s completely adjustable EZ screeds provide the ability to adjust out high and low spots created by wear over the life cycle of the plates. “The easier these adjustable points are to reach gives the crew the ability to increase life cycle and maintain proper flatness and smoothness in the mat quality.
“We also went to a higher grade of steel for our screed plates about five years ago,” he continues. “We use Hardox 450 in all of our plates as the aggregates in asphalt mixes has gotten more abrasive.”
Some refinements are right at the operator’s fingertips. According to Mooney, Volvo screed and paver controls and switches are color-coded, grouped by function and ergonomically placed for quick access. In addition, built-in functions such as auto prime, paving speed, screed height hold and more are designed to enhance performance and ensure consistently high-quality paved surfaces.
Versatility has also become increasingly important in screed design. “With today’s economy, most contractors involved in multiple applications — and with one crew for all applications — prefer a versatile screed,” says Narsingh.
As such, manufacturers are working to build in added flexibility. Take the Vögele VF series, for example. “The VF series screed is an unequal-width, front-mount screed designed with the versatility required for multi-variable applications,” says Narsingh, “and the weight and rigidity required to maximize smoothness and density.”
Slow Yet Steady Progress
The paving industry is unique in that it’s not uncommon for a 20+-year-old paver to still be in active service in a contractor’s fleet. While it may not be the primary unit, it may see periodic use during the paving season.
“We have an industry where the cost per mile can be upwards of a million dollars, and the only way it gets done is by a paver. Would I put a 20-year-old paver through that type of production? No, but I might have it as a backup,” says Mooney.
This creates challenges for paver manufacturers going forward. “We have an industry that is still using equipment that may be 20 years old and we’re trying to integrate new, modern equipment that is electronically sophisticated and delivers all these benefits,” Mooney states. “The crews must be able to move between them and do as good a job with the new machine as they do with the older machine.”
As a result, dramatic changes in the immediate future are unlikely. Rather, more gradual improvements to enhance performance and, consequently, mat quality are likely to continue. “The equipment of yesterday performed very well,” says Mooney. “Now it’s the refining of that [equipment] — and there’s a lot of refinement that’s going on.”
Such refinements, while incremental over time, are generating substantial benefits. “All of these features, when used effectively, improve smoothness and density, resulting in higher bonuses, reduced downtime and overall lower operating costs,” says Narsingh.