This article is the second of a four-part series on paving provided by Volvo Construction Equipment.
Successful asphalt paving projects don’t happen by accident. They are the result of careful planning and proper preparation. From proper paver setup to achieving optimum mat temperature and conducting the rolling pattern at just the right time — each step in the process will affect the next. And attention to detail throughout the process can mean the difference between getting a bonus and being delayed.
In the previous installment, best practices for paver setup were shared. After ensuring the paver is in top working condition, the crew must then focus on the factors that will yield the best quality pavement. Material level and paver speed both play a significant role in determining pavement quality.
Laying a Quality Mat
On almost every asphalt paving project, the crew can spend from a few minutes to nearly an hour setting up equipment, depending on the condition of the equipment and the complexity of the project. Pavers, compactors and MTVs must all be prepped for the long hours of use to help ensure uptime. Once the equipment is in use, the paving crew can fine-tune operations in order to lay the best quality pavement.
The two most important factors for laying a quality mat are all about consistency:
1. Consistent head of material in front of the screed
2. Consistent paving speed
More than 95 percent of all material faults in asphalt paving are due to an improper head of material in front of the screed. The “head of material” is the mass of paving mix that lies directly in front of and spans the width of the screed. If the head of material in front of the screed fluctuates widely during the course of paving, it is nearly impossible to produce a level and smooth pavement surface.
“Too much or too little material affects how the screed rides and that translates to poor rideability,” said Chris Connolly, operator training specialist of Volvo Construction Equipment.
If the head of material increases, then more material is being forced under the screed, causing it to rise. This fluctuation in mat depth translates into a wavy surface. If the head of material decreases, there will be less material passing under the screed and supporting the weight of the screed. The screed will gradually drop, reducing the mat thickness and causing major flaws in the mat.
The correct head of material should be constant across the entire width of the screed. A good visual cue is whether the material is even with the center of the auger shaft. While monitoring material levels is the responsibility of the screed operator, many machines are now equipped with automatic feed systems that use sensors to automatically regulate the proper delivery and flow of material in front of the screed to ensure smooth and consistent paving.
Paving speed also has an impact on the quality of mat. In an ideal paving operation, the paver will operate nonstop throughout the day because a change in paving speed can directly affect the ability to lay a uniform mat.
When selecting a paving speed, several considerations should be made. These include the number and size of trucks, volume and output of the mixing plant, rolling and compaction rates and crew capabilities, plus the distance from the mixing plant to the paver. The hot mix material needs to arrive at the site hot to allow for proper laydown and compaction. It is important to select a speed that falls within the limits of these considerations. Once speed is selected, it must be consistently maintained. Ripples, waves and irregular mat depth can occur when paving speed doesn’t remain relatively consistent.
Speed limiters are available on some paver models to control paving speed. The function is similar to cruise control for an automobile. A preferred paving speed is selected on the speed limiter — 30 to 40 feet per minute for example — and the paver is limited to that speed.
Changes in speed also create challenges with timing the delivery of hot mix material in order to keep the hopper at least a third full. Consequently, changes in paving speed are commonly caused by trucks inconsistently arriving at the job site, or bumping the paver during the truck exchange. Even the smallest bump can create a depression behind and beneath the screed. Subsequent rolling may not fix this surface defect and can often be felt by traffic after the fact. To avoid negatively impacting the mat, truck drivers should stop 1 to 2 feet in front of the paver, allowing the paver to initiate contact.
With the paver properly set, and the mat laid with perfect head and consistent speed, it’s time for the compaction operation. Stay tuned for Part 3 of the series, which will cover best practices for compaction.
Part 1: Precision Paving Begins With Proper Equipment Setup
Part 2: Precision Asphalt Paving 101: Laying a Quality Mat
Part 3: Precision Asphalt Paving: Best Practices for Compaction
Part 4: Precision Asphalt Paving — Rolling Pattern, Safety
Part 5: Past, Present and Future of Intelligent Compaction
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