Building the Unsupported Longitudinal Joint
BY Benjamin Everett
How we build the unsupported longitudinal joint
The subject I want to bring to you this month is about another long-standing issue. In my view, we have not yet properly or adequately come up with a sustainable solution to guarantee long-term joint life. I’m referring to what is known as the leading edge or unsupported longitudinal joint, which is the first pass paved on a carriageway requiring two or more paving runs side by side.
Typically, most roads I encounter (though not all) are crowned in the center on a carriageway of two lanes or more. In other words, the highest part of the road is in the middle along the centerline to provide a means to ensure water runs off it.
I understand that in some territories roads are built instead with a slope or straight grade from one side to the other, which also achieves the same result. After all, we don’t want standing water on any part of the road.
Whichever way the road is built (crowned or sloped), oftentimes it will need to be paved in two or more passes. Here we have a longstanding issue that crops up. That is the unsupported longitudinal joint (aka the “open side” or “leading edge”) that has to be compacted then matched with the next paving run. We continue to see ongoing problems of premature longitudinal joint failure.
Specifically, the problem is when we try to properly compact an unconfined or unsupported pavement edge, whether that’s a longitudinal or transverse joint, invariably it resists the compactive forces imparted from the roller in a vertical plane and tries to “squeeze out” laterally. This is a common, everyday issue all paving contractors have to contend with. We have to ensure we get adequate longitudinal joint density without distorting the edge and causing target mat depth to suffer when the material refuses to stay put.
There are various methods that have been trialed and are somewhat successful, although only a few would I consider to be the perfect solution. These include the next five ideas, which you may be familiar with, and an idea I’d like to propose from outside the box.
We have to ensure we get adequate longitudinal joint density without distorting the edge and causing target mat depth to suffer when the material refuses to stay put.
Building
You could anticipate how much “squeeze out” or lateral displacement is likely to occur during compaction and factor in that loss of depth by placing the outer 12-18 inches thicker than the rest of the mat. You do this so after compaction the pull levels out at the right finished height. This avoids a low area at the joint where water can pond and ensures finished height compliance.
If you have a screed with either a berm builder or extenders that can be sloped negatively or “up,” you can do this. I use this method and find it pretty effective.
As an example, on a 2-inch compacted mat, I might add another 10% to the outer edge. In this case that would be about ¼ inch. For those who have never done this, it might sound crazy, but it works to solve the thickness issue along the unconfined longitudinal joint. However, it doesn’t always or consistently ensure target mat density or air voids are where you want them to be. So, it’s not a perfect solution, in my view.
Wisconsin Job Exceeds Longitudinal Joint Density Expectations
Avoiding
Many contractors default to staying away from the unsupported edge during compaction until the mat has cooled off enough to be more stable. The problem with that technique is that by the time the mat has cooled off enough to become stable under the rollers, it’s oftentimes too cool to be able to achieve the required target density and air voids. I don’t recommend this method as it’s counterintuitive in my view, and in my experience not a successful methodology.
Cutting
In Europe, most tandem steel rollers are fitted with an edge cutter and pressing device to try to both confine the unsupported edge during rolling and to trim off the low-density outer edge after rolling is complete but while the mix is still warm enough to allow this. Traditionally, on U.S. airport paving projects, a “pizza cutter” or disc mounted to a grader blade has been used to cut away and waste the unsupported longitudinal joint by at least 3-4 inches.
The issues with both of these cutting methods are that they add quite a lot of additional time to the operation and waste a lot of material that shouldn’t really need to be cut off were it compacted well enough in the first place. Plus, it requires a lot of effort, energy and expense to pick up the discarded mix, thoroughly clean the cut back joint face (which can often cause the edge to be inadvertently damaged in the process with a skid steer, etc.), and then tack the entire joint face. It’s basically adding in a whole lot of extra work, time and expense you could avoid with a different joint-building method.
It wouldn’t be very difficult to create an automated longitudinal joint confinement system, without the manual steps of laboriously laying out, securing and removing a temporary joint confinement method.
Overhanging
Some pavers use certain compaction techniques such as having the breakdown and finish steel drum rollers overhang the unsupported longitudinal joint by approximately 4-6 inches to consolidate that leading edge in a vertical direction and prevent it from displacing laterally. I have used (and I continue to use) this methodology where necessary, and while it is reasonably effective to a degree, it’s not perfect. At the end of each breakdown roller pass there is a conundrum encountered when the steel drum compactor has to turn in toward the center of the pass as it approaches the screed to stop on an angle to avoid leaving a bow wave as it stops and reverses direction. In both directions as it comes inside the unsupported edge and goes back out to overhang on the return pass, I notice some degree of edge distortion can occur.
Also, although rolling while overhanging the joint is a fairly reasonable method, I find that there is still some degree of edge “squeeze out” (although a lot less than other methods) with the resultant issue of air voids and density typically slightly less than target along with a slight but observable loss of overall mat height along the joint.
Lastly, this methodology requires that the important secondary compaction stage undertaken with a pneumatic tire roller is unable to get closer than typically 8-10 inches or more from that unconfined longitudinal joint, as a pneumatic tire roller creates a significant amount of lateral compaction versus a steel drum compactor. This means that the important intermediate rolling stage isn’t able to be undertaken directly on that longitudinal joint until the second paving run is pulled. By then, the joint is often too cool to achieve additional compaction. All in all, that’s really a problem hiding in plain sight, and one I believe we as an industry need to face up to, confront and overcome.
Schellinger Construction Hits Notch Wedge Joint Density Specs
Echelon or Full-Width Paving
Obviously, the best solution to overcome the problems highlighted above is to pave the entire width in one continuous pass. This can be done with a single paver built out wide with screed, auger and mainframe extensions, or by paving in echelon with two or more pavers with a hot-on-hot joint overlap. To me, that’s an ideal solution and one I like to do if I have the resources and ability to do it.
However, as we all know, it’s not always possible to use these methods. Typically, on heavily built up urban and city streets we have to share the rest of the carriageway with motorists, preventing us from having the entire roadway to pave in one go.
There has been a lot of research, money, time and effort dedicated to understanding the dilemma we continue to see manifesting at the longitudinal joint, as well as solutions to overcome or improve the issues described above. But I don’t feel we are there yet.
There have been excellent industry articles posted online for more best practices on longitudinal joint construction, many of which are well over 10 years old. Many I would recommend are difficult to find today. Check out “How to Exceed the Long Joint Density Spec” and “How to Build a Long Joint in Cold Weather” at TheAsphaltPro.com for two more recent items. I’ve also included a variety of joint construction video links in the sidebar below. Yet we still seem to struggle with realizing consistently compliant, high quality, unsupported longitudinal joints on the first pass.
I think our industry is missing something here.
A Supporting Idea
Here’s a solution to ponder. Carefully look at what Japanese paving contractors do. I notice they build timber confinement along the entire length of the longitudinal joint to produce temporary confinement.
This appears to overcome all of the described issues—low joint density, missing target air voids and low finished mat depth on the edge. The very act of confining the material forces the mix to comply and to be properly compacted, which is a simple but clever idea.
Now I can already hear our U.S. paving brothers hollering at me, “But we pave three times faster than the contractors in Japan! They only pave at about 13 feet per minute, and we want to do at least 32 feet per minute or more!”
Sure, I hear you.
Solve the problem as I am. It wouldn’t be very difficult to create an automated longitudinal joint confinement system, without the manual steps of laboriously laying out, securing and removing a temporary joint confinement method. I’ve already designed it. If you want to talk about it, just reach out.
Benjamin Everett is the owner of About Asphalt Ltd., and has worked in the asphalt industry for nearly four decades. For more information, contact him at bjeverett@aboutasphalt.co.
Here are some links to videos that showcase joint construction methodology:
Part 1 of the Asphalt Institute presentation of “Best Practices for Specifying and Constructing HMA Longitudinal Joints” to UDOT University, February 2017
https://www.youtube.com/watch?v=SgPCGjw6MzY
Part 2 of Asphalt Institute’s presentation
https://www.youtube.com/watch?v=VlhMwb4mRMQ
Part 3 of Asphalt Institute’s presentation
https://www.youtube.com/watch?v=FEdHmsje71k
A 5-minute step-by-step look at the “Maryland Method Longitudinal Joint Compaction” from Marshall Klinefelter
Maryland Method Longitudinal Joint Compaction – YouTube
Timelapse video of Japanese job site begins around the 20-second mark here
https://www.youtube.com/watch?v=p2hk4qDrSbM
A Japanese paving site with wood-confined edge
https://www.youtube.com/watch?v=xlDC_VbquJA&t=6s
A Japanese paving site with endgate riding on wood structure to confine edge