How to Build and Compact the Longitudinal Joint
BY John Ball
The key to proper joint construction is in the compaction. You know you’ll get good compaction of the joint if you perform your quality control through best production and best paving practices. You must double-check to make sure the equipment is in good condition, to make sure you’re paving properly, to make sure the mix is the right temperature, to make sure the mat is the right height, and so on. Don’t take any piece of the puzzle for granted. Check it to be sure. Let’s get down to basics to see how to build and compact the longitudinal joint for your best chances for full pay.
We’ve discussed before in the pages of AsphaltPro the importance of using a clean and well-maintained machine. The level of importance does not decline because it’s been a few months since we reminded you. The mechanic and/or paver operator must ensure all maintenance is performed correctly so the paver has its best chance to respond correctly to automation. You will want to run the joint matcher on the screed so you can bring up the thickness by 1/16th of an inch. Automation is the guarantee that you’re making the right adjustments, but that miniscule adjustment requires a precise response from the equipment.
By keeping the paver clean, you give material a clear path for flow from the hopper through the head of material and under the screed plate. A strike-off plate with no divots or chunks of mix stuck to it builds a flat, smooth mat over which the screed floats. A clean foot on a spring-loaded endgate rides on top of the joint smoothly, adjusting correctly for down-pressure as needed; whereas, a cooled endgate with a messy foot drags and catches on the mat edge, tearing at the joint and creating pockets and holes that the crew has to fill in with shovelfuls of mix. That is not the way to build a joint.
It has been my experience time and again that the ideal temperature for the mix leaving the plant is 300 degrees. For a project I assisted in the Northeast, the trucks were arriving at random enough intervals that the mix charging the hopper was no longer 300oF. That kind of inconsistency is not acceptable when you’re trying to build longitudinal joints. To solve the problem, we communicated with the distributor to work out a consistent haul truck delivery to accommodate our paving speed.
Our paving speed was 22 to 23 feet per minute. Even though project managers are often pressed to go faster, get more tons down and pave further each night, you don’t want to race the paver down the mainline. By paving at a rate of 22 fpm, you can establish a good head of material that doesn’t fluctuate up and down. You can receive trucks and charge the hopper at a steady, even rate.
Once the material is flowing through the auger box, the screed has to float over it smoothly. You want to start by balancing the screed. Take the straight edge, which can be something as simple as an 8-foot wooden level, and place it on the screed to make sure it’s level all the way across. (Take a look at this month’s “That’s a Good Idea” department on page xx for one crew’s answer to storing the straight edge during paving.) A metal level can warp from heat, so be aware of the condition of the straight edge you use. Also make sure the endgates are loose and reactive to the automation.
As stated above, it takes a clean and well-maintained screed with clean and well-maintained endgates for the type of precise response to automation that we want. Ideally, you will want heated endgates to minimize pickup of material; ideally, you will want spring-loaded endgates to ride and adjust easily as the automation directs the system.
To achieve the correct mat thickness after compaction, you will place the lift a bit “taller” than final spec. In other words, if the mat is to be 1 inch thick, you will place a lift of 1.25 inches. That extra quarter of an inch is known as the fluff factor.
You will use the automation to dial in the height of the mat you place, but it’s vital to double-check your work. The automation is the guarantee that you’re making the right adjustment at the screed, but your level is your double-check. It’s your peace of mind. Take the level to the back of the screed and lay it transversely across the joint. Use a tape measure or folding ruler to measure the distance between the existing mat—the one you’re matching—and the base of the level. That measurement is your fluff factor. That measurement must reflect the exact amount you’ve built in for compaction.
With the paver placing the lift with the perfect fluff factor, the breakdown roller must get onto the mat with perfect timing. For the project in the Northeast, hitting the mat with the rollers at the right time was critical to success. Once we had the truck drivers delivering mix in a timely fashion, we had temperatures we could work with.
For the first pass with the breakdown roller, have the operator come up to the screed about a foot on the inside of the joint and back. This allows the drum to move the material toward the joint, which has a cold, supporting edge preventing it from escape.
For the second pass with the breakdown roller, have the operator set over so he overlaps the joint 2 inches, but is still rolling on the hot side.
The breakdown roller needs to be used in vibratory mode and needs to be in the 300 to 180-degree temperature zone. Once the mat has cooled to less than 180 degrees, it’s time to let the pneumatic-tired roller get into the intermediate zone.
You can build excellent longitudinal joints and you can achieve full, specified density of those joints for any mix design a state agency requires. You just have to put your best production and best paving practices into play. This article spelled it all out from the production temperature to the gentle tapping of the lute artist to the exact rolling pattern you want to see. Timing is critical to ensure all steps take place during optimum temperature zones, and that attention to timing is the benchmark of your top quality crew constructing and compacting top quality joints. <endmark>
John Ball is the proprietor of Top Quality Paving, Manchester, New Hampshire. He provides personal, on-site paving consulting services around the United States and into Canada. For more information, contact him at (603) 493-1458 or email@example.com.