In November, we talked a bit about asphalt density, with the general theme that higher density is better. Asphalt density in the overall mat is sometimes subpar (and shouldn’t be), but it is common with longitudinal joints, where one pass of the paver joins an adjacent pass (another travel lane) that was put down perhaps a day or two earlier.
Well, that’s a problem. As discussed in Improving Longitudinal Joint Performance, a TechBrief associated with the Federal Highway Administration (FHWA) Accelerated Implementation and Deployment of Pavement Technologies (AID-PT) program, “premature joint failures are typically the result of a combination of low density, high permeability, segregation, improper overlap, and lack of adhesion at the interface.” All of these contribute to increased water penetration into the joint and while water is good for your body (you should drink lot of it), it’s the enemy of your pavement.
AID-PT concluded that, “Improving joint construction, which improves density and decreases permeability, may be the single most important thing the asphalt industry can do to improve long-term asphalt pavement performance.”
There are three general approaches to forming longitudinal joints (we won’t repeat AID-PT’s excellent illustrations here) – a butt (vertical) joint, a milled or cutback joint, and a notched wedge joint. Remember that in November we talked a bit about questions you should ask during the pre-bid stage or at the pre-construction meeteing – you’ll want to ask your contractor how longitudinal joints will be formed. This is part of how you relate your interest in a quality constructed product. There are arguments for each approach, but milled or cutback joints are less common in our area.
AID-PT discusses practices to consider when specifying your project or talking to the contractor about the approach. Note that a couple of key points are the importance of tacking the edge of the the cold mat for good adhesion and ensuring that the joint is not “starved” of material, a common mistake leading to poor joint density.
They also make mention of longitudinal joint sealants (also called void reducing asphalt membranes, VRAM), where Illinois DOT has been a leader. These are still in an evaluation stage for many agencies but early results are positive and the cost/benefit seems encouraging. We introduced them in our January 2022 e-newsletter if you want to understand them a bit better.
What are the reasonable density expectations for longitudinal joints? Commonly, asphalt specifications will require a minimum of 92 percent density in the mat. What does that mean? This is in reference to the maximum theoretical gravity (Gmm) for a given mix. In other words, a mix is designed, a sample is put together in a laboratory, and the maximum density is determined in a controlled setting. We don’t expect to achieve that in the field application; we don’t need perfection. But we do need to approach that ideal. Nine-two percent is easily achievable with performance asphalts, modern equipment, and qualified contractors. Even with good practices, it may not be reasonable to expect the longitudinal joint to consistently meet the same density as the rest of the mat and AID-PT reports that acceptable joint density is
typically about two percent lower than the minimum mat density specification.
But better is better. Remember that AID-PT concluded that a one percent increase in density above the target rate can extend the service life of the asphalt pavement by ten percent. What does that mean to you? Simplistically, if you expect your asphalt overlay to last 15 years, a one percent increase in density may extend that a year and a half. Now, realize that conditions vary and there are limits on that. You are unlikely to consistently achieve, say, 98 percent, so realistically, you can bump up into the 95-96 percent with many projects, but you can do the math on what that might get you in terms of lifespan of the pavement. So, better is better.
So, how do you achieve better density? Approaches to the “correct” approach to joint construction are vigorously debated. Begin with contractors that have a focus on good joint construction, are eager to describe their approach, and then consistently execute on that plan. Properly tack the cold edge of the joint (we talked a bit about the importance of proper tack in our June 2022 e-newsletter). Establish a proper roller pattern and insist on the proper size and number of rollers to achieve maximum density. Perform quality assurance testing, as we discussed in November.
The Delaware T2/LTAP Center offers a training workshop, Best Asphalt Construction Practices, free to local agency personnel; keep an eye out in the spring for the next session.
Remember that your best contractors will appreciate well-developed specifications and reasonable quality assurance testing to establish a fair bidding environment.
The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have construction management questions or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.
Link to PFD
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