Do you take time to think about what you are going to pave today and how you will set up your paver? Or do you go through the same setup routine every day, without thought, and hope what worked yesterday will work again today? Whether you answered yes or no, you’re right! When it comes to setting up your paver at the beginning of a shift, some things should be routine every day, whereas other aspects of the paver setup will vary based on the type of paving that day. Some of the routine items to check every day (often at the end of a shift) are maintenance items such as grease points, fluid levels, filters and general cleanliness in areas where asphalt mix accumulates and affects mat quality and/or mechanical performance and wear. These items are all covered in the Operations & Maintenance Manual for your machine. If you do not have this manual, ask for it.
Maintenance intervals and areas to check are often summarized in a chart that can feature be photocopied, laminated and kept on the machine or somewhere easily accessible to the crew. Most mat quality problems are a result of improper setup and operation of the paver and screed. Other problems are related to poor maintenance and cleaning. A few key areas to keep clean are the screed pivot points, drop arm area, deflector plate, pre-strike off plate and the tracks and bogey wheel areas. The remainder of this article will focus on setup factors that affect mat quality.
After moving the paver to the starting location for the day’s paving, the front face of the screed should be lined up where the new mat will begin at the transverse joint. The screed should be raised or lowered to about one foot off the ground. The first step is to turn on the screed heat. It will take 20 to 40 minutes for the screed and extension plates to come up to full temperature to prevent the mix from sticking when the paver takes off. The second step is to set the left and right tow point heights. The goal here is to establish a straight line of pull from the tow point to the pivot point of the screed when the paver takes off. This will ensure a “flat” or horizontal transverse joint after it is compacted.
A common, but easily avoidable mat defect, is a bump or dip at the transverse joint that results from tow points set too high or too low when the paver takes off (image #1). Next, set the paving width using the hydraulic screed extensions, or in some cases, by attaching bolt-on screed extensions. Whenever it is practically possible, use equal extension on both sides to balance the forces acting on the screed. Now, set the crown (break point in the center of the screed) to the job specifications. In most cases, the crown is set to zero, meaning the main screed is paving flat for its full width. Next set the extender height. Setting extender height is one of the most critical steps to proper setup because it will determine the angle of attack that the screed rides at all day long. This is what we call our “Equilibrium Angle of Attack” (image #2). The only way to change the equilibrium angle of attack is by picking up the screed, re-setting the extender height and lowering and re-nulling the screed. Set the extender slope (if any) on both sides. Extender slope is often used when paving mainline with a shoulder break. In most other situations, there is no slope in the extenders, resulting in a “flat” or straight mat across the full paving width.
All the steps up to this point are done with the screed in the raised off the ground position. Trying to make any of these adjustments with the screed on the ground introduces stresses in the screed that can unload once paving begins and cause the screed to behave erratically. A screed under stress is similar to pre-stressed steel beams that are flat when they are loaded, and ‘rebound’ into an arched shaped when the stresses are removed (unloaded). A screed that has stresses induced in it on the ground will “unload” when mix is put under it, and the paver takes off. Next, select some starting boards to lower the screed onto. The starting boards must be 36” to 48” in length (for most screeds) to support the full depth of the main screed and extenders. If the boards do not support the full depth of the screed, the screed could roll forward (nose over) as the paver takes off.
When selecting the proper thickness of starting boards, always remember that the mix will roll down about ¼” per 1”. For example, a loose mix thickness of 2.5” behind the screed will roll down to approximately 2.0” in thickness after the rollers achieve density (image #3). Lower the screed onto the starting boards by using the float switch (this removes all lift cylinder forces on the screed) and inch the machine forward to take out any slack at the tow point. The pin at the tow point should be in contact with the tow arm on both sides. This minor, but important step of taking out the slack at the tow point, is often overlooked. Once the extender height is set, and the screed is lowered to the ground with the slack at the tow point removed, there will be some stresses on the screed.
To alleviate these stresses, it is time to null the screed and remove any stress by turning the depth cranks on both the right and left sides until they are loose and no resistance or tension is felt. The screed is now totally relaxed and able to float freely on the mix once paving begins. A freefloating screed performs the best when it is nulled out before taking off. A common mistake is to put additional “turns” into the depth cranks after the screed is nulled out. In doing this, the operator just induced stress back into the screed. To ensure a good longitudinal joint, end gates should now be lowered to contact the grade or adjacent lane (joint matching). End gates confine the edge of the mat for maximum joint density during compaction. Set the auger height at 2” above the mat surface. Auger height may be adjusted slightly once paving starts. Position the sonic or paddle (mechanical contact) feed sensors. Sonic sensors should be aimed perpendicular to the live, moving mix, approximately 18” from the mix and 18” from the outermost auger flight. Paddle sensors should contact the mix approximately 18” from the last auger flight and “rest” at a 45° angle on the mix. The feeder controls – conveyor ratio and mix height controls – should be set at 40 percent and 60 percent respectively. For machines with dials on them, this is the “10 o’clock” (conveyors) and “2 o’clock” (mix height) positions.
Finally, it’s time to fill the auger chamber with mix! The ideal mix height is ½” auger height. The top half of the augers should be exposed and not buried in the asphalt mix. Buried augers become tunneling machines, causing segregation and the large head of material is harder to move, causing the screed to behave more erratically. Augers should be no further than 18” from the end gates on a front-mount screed or to 36” (maximum) on a rearmount screed. Always have mainframe extensions (tunnels) in front of any auger extensions (image #4). To properly manage filling the auger chamber, the mix should be alternately conveyed back from the hopper and then augered out until the auger chamber is filled to ½ auger height all the way to the end gates. This often requires someone to shovel mix the final distance past the end of the augers out to the end gates. If the paver operator force feeds mix to the end gate using the augers, it is likely that mix will be pulled underneath the tractor by the conveyors and an excessive head of material will result. Eighty to ninety percent of all mat defects are related to the head of material in the auger chamber.
The final thing to set up is automatic grade and slope control. It is highly recommended to connect automatic grade control cables when the paver is off. If the paver is running, it is possible to get a short while connecting the grade control cables and ruin a wire harness. Many crews connect the grade control cables before starting the paver and then set up the position of the grade sensors later. All makes of automatic grade control operate on the same principle of placing a single sonic (or contact) grade sensor in line with the auger shaft for joint matching and precise thickness control or up near the tow point to place a smoother mat. The slope sensor acts like a carpenter’s level to measure and control the cross slope of the mat. Position your grade sensors along the length of the tow arm based on the screed reaction you want to achieve, adjust sonic sensors to approximately 16” above grade and then “bench” the grade control system before taking off. This is normally done by pressing and holding a button until the system beeps and displays a green light. Many crews like to manually reach the desired mat thickness and slope in the first several feet of paving before engaging the automatic grade and slope controls. This approach may work better when the steps outlined in this article are not followed in order. When these setup steps are followed in order, take offs are consistent and taking off in automatic is reliable and accurate.
Finally, pull off the starting boards and accelerate quickly to the planned paving speed. Now you take a quick look back at the beautiful transverse joint you left for the rollers to compact flush with the existing mat! Great job team! As paving continues, focus on managing the head of material at ½ auger height and keeping the augers constantly turning at 20 to 40 rpm. Constant auger movement may require re-positioning feed sensors. If you did everything in order, there’s no need to stop the paver and wait for handwork at the joint – keep going and the rollers will be right behind you. Any stops and starts should be made by quickly pulling the propel lever to neutral and then accelerate back up to full speed by quickly moving the propel lever full stroke forward. Do not adjust the speed control. Gradual starts and stops cause dips in the mat that IRI will notice. Be sure to think about changing feed system settings (conveyors and augers) to maintain a constant head of material if it is necessary to change paving speed. If you maintain your equipment and follow these basic steps every time as a team, your take offs will be consistent and smooth!