Attend the education session “Laydown Best Practices” on Wednesday, March 11, 2020 from 1:00 p.m. – 2:30 p.m. at CONEXPO-CON/AGG.
Every paving contractor wants to achieve a smooth mat. A smooth mat is a sign that the asphalt is well-placed and will result in a smoother ride, less wear and tear on vehicles, proper water runoff, better traction, and safer commutes.
“There are three basic factors that affect laying a smooth mat that have stood the test of time—paving speed, the head of material and the angle of attack of the screed,” says Jeff Ensell, Division Manager – Asphalt Construction-Milling, C.W. Matthews Contracting Co., Inc. “If any of those three factors are out of whack, it will affect your smoothness. This has been taught industry-wide since the conception of pavers; it is a basic fundamental that holds true.”
1ST PRINCIPLE: KEEP A CONSISTENT PAVING SPEED
Perhaps the most important of the three principles is paving speed. Ideal paving speed is about 25 to 30 feet per minute, but paver operators should know the tons per hour of asphalt they expect to receive and be aware of any conditions that may affect their desired paving speed.
“One of the main ways to maintain smoothness is to keep the paver running in a consistent manner. Factors such as the length of your haul and how many trucks you are running will determine your paving speed,” says Ensell. “You don’t want to stop. Every time you stop, you are opening yourself up to a potential problem. Keeping everything flowing is the key to operation.” Moving at a consistent speed also means the milling machines, cleanup crew, and rollers need to move consistently forward to achieve flow.
When the paver stops, the free-floating screed will settle, which can leave a definite deviation where the screed sat. The longer the paver sits, the more temperature deviation you will have in your mat.
A screed is several thousand pounds of steel and free floating. When the paver is stopped or moving slowly, the screed will settle into the asphalt. When the paver moves forward again or speeds up, there is an impression at the position where the screed was when the paver stopped or slowed.
asphalt paving2ND PRINCIPLE: MAINTAIN A CONSISTENT AND ADEQUATE HEAD OF MATERIAL IN FRONT OF THE PAVER
The head of material is the paving mix in the hopper of the paver and may be used to refer to material coming out of a material transfer vehicle.
“Once the material is trucked to you, your first line of defense is your dump personnel, who is often the least trained member on the crew, which is kind of backwards,” says Ensell. “Here is where you need consistency. It sounds simple but, even making sure you are receiving the same mix is essential. Less fluctuations in the material moving through the machine will result in more consistent temperatures and gradation which will help with your compaction side of things.”
If the amount of mix in the head of material fluctuates widely during paving, it becomes extremely difficult to produce a smooth surface. A greater head of material increases mat thickness and can result in a wavy surface, while a lesser head of material reduces mat thickness and can lead to asphalt failure.
To help keep a consistent head of material, Ensell recommends using a material transfer vehicle. A material transfer vehicle can maintain consistent flow better than just trucks and they don’t make contact with the paver, consistently moving forward. The less the head of material is fluctuating and the material moving through the machine is uniform, then you are going to get more consistent asphalt behind the screed. These steps limit segregation, temperature issues and changes in paver speed.
asphalt mix3RD PRINCIPLE: FACTOR IN THE BEST ANGLE OF ATTACK
Since screeds are free floating, they glide across the asphalt at an angle and height that is determined by several factors including paver speed and head of material.
By moving faster, the material will move faster under the screed and the screed will fall; by moving slower, resistance builds on the back of the screed, and the screed will rise.
Likewise, when there is little head of material, the screed will fall; if there is significant head of material the screed will rise.
“If the attack of the screed is not correct, you will not achieve desired compaction and/or mat quality will not be where it should be,” says Ensell.
DON’T FORGET THE MIX
Of course, if your mix isn’t right, following the three basic principles of paving won’t matter. For example, ensuring the right asphalt cement (AC) content and gradation will give the material the viscosity it needs for the material to flow under the screed properly.
“Having the right mix will decrease voids, segregation issues and improve with compaction, so make sure the people at the lab are watching your mix,” says Ensell. “Your AC content is essential to a smooth mat. If AC fluctuates, this is when you start having problems with compaction and material tear or compaction numbers go down.”
MAKING THE GRADE
Grade control is what maintains depth of material. It is used on milling machines and asphalt pavers for keeping depth consistent. “To get a smooth road, you must start with a smooth surface. Grade control is not a fix-all for all problems, mill and pave with both using averaging systems. Don’t just plug them in; use them. If your milling isn’t done right, smooth and have a good pattern, it really sets you back on the next lift for achieving smoothness,” says Ensell. “Fundamentals work, but they are just words without the follow through. Take the time to observe, correct and teach your people, they will react to it and your projects will benefit from it.”
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