by Jason Brazer, P.E.
Asphalt leveling course is defined as a layer of asphaltic concrete pavement of varying thickness spread on an existing pavement to compensate for irregularities prior to placing the surface or wearing course. As you may or may not know, an asphalt pavement section or structure is made up of multiple and different asphalt layers (lifts or courses). The lower layers are constructed for strength and support, while the top layer is constructed for smoothness and longevity. The layer or course that is often overlooked is the leveling course (middle).
The leveling course can be installed over new pavement, existing pavement, or milled pavement. When installing the leveling course, it is important to follow best paving practices ensuring the pavement is clean, dry, and a tack coat has been properly applied. The thickness of a leveling course will vary across the paving mat. For instance, if the leveling course is being used to improve surface drainage, it may be 1.5″ thick at one edge to get water to shed away from the building and only 1/2″ thick where this particular paver pass is ending.
Leveling installed to promote drainage away from the building.
The leveling course may also be utilized at an average consistent thickness across the entire project. This may be performed on roadways or larger parking lots where drainage is good overall, but the pavement structure needs to be improved. Some roadways can be milled 1″ in depth to remove the shallow cracking and then a leveling course, placed at 1.25″ loose (1″ compacted) depth, will be installed along the entire roadway. Not only did this add an extra inch to the total pavement section, it also gave the crew a smooth, even surface to install a pavement interlayer fabric prior to placement of the wearing/upper course. Had the roadway crown needed to be increased, a thicker leveling course could have been installed down the centerline as well.
Asphalt overlay system including a leveling course and geotextile interlayer (fabric).
Another benefit to installing the leveling course is that it helps to identify unstable areas within the milled surface so the proper corrective action can take place prior to the final lift being applied. The last thing you want to see is a repair in the final lift of the new parking or roadway surface. Repairs that require cutting will affect longevity, deterioration rate, and ride quality.
When it comes to paving the surface course or final lift of an asphalt structure, consistency in regards to proper slope and smoothness of the mat is a key factor to proper drainage. The leveling course assists the crew by making sure low areas left from the milled surface or prior deficiencies in the pavements being overlaid are filled in prior to the final lift being installed. On some projects, the surface course is paid per square yard versus per ton. Therefore, the leveling course ensures that the contractor is being paid per ton for the correction to the grade while the owner is getting the proper thickness on the surface without a surprise compliant or change order from the contractor saying they installed more tonnage than anticipated.
There is a science to creating a good, consistent asphalt mix for any project. Typically, the surface mix and leveling course are the same product. Therefore, by paving a leveling course the day before the surface course, the producer is able to run proper quality control and quality assurance testing on the mix itself. That way, if something is out of tolerance (i.e., improper asphalt binder content, too much dust in the mix, or a variance in the air voids), the plant has an opportunity to correct issue(s) and make the proper adjustments prior to material being used for the final surface.
Proper density plays a key role in the longevity of a good quality asphalt pavement. The density of a surface mix is dependent on many variables. Two of the more important factors are consistency on depth as well as building on a firm foundation. As referenced above, the leveling course assists in both of these areas when constructed properly.
When constructing a project, contractors want to get paid for what they do. The leveling course gives them a way to be compensated for unforeseen issues like asphalt chunking up when being milled out of a low area by a building that is failing to drain. It is also valuable for a new concern that needs to be addressed by the client and responsible contractor which is the ADA (American’s with Disabilities Act) parking regulations. Slope requirements are federally mandated for all ADA parking stalls, as well as walkways leading to and from parking stalls. When a parking lot is updated, it is necessary to make proper adjustments to get slopes in the ADA stalls to fall within regulations. If there is not a way in the project documents for the contractor to perform this work and be compensated, there is likely going to be an extra/change order submitted. The use of a per ton leveling course pay item eliminates this. The photograph below shows how the leveling course is being placed in multiple lifts and then transitions to a zero thickness on the right side once the crew is outside of the ADA stall location.
Correction of slope through the use of an asphalt leveling course.
Slope verification shows non-compliance in ADA stall.
The final benefit of the leveling course involves longevity of the pavement. The additional layer of asphalt helps to reduce and prevent reflective cracking from the existing pavement that was overlaid. Because of the leveling course, there is additional pavement for the crack to work through and also a separation in the lifts that can act as a stopping point for the crack, or at least delay the crack from migrating up the surface. If the leveling course is used in conjunction with a pavement interlayer fabric and a surface course installed at the proper thickness, there is a possibility to slow or even stop reflective cracking as a whole.
To summarize, a leveling course is defined simply as a variable thickness asphalt layer within a pavement structure, but it has many more uses and benefits than that. Not only does it help to simplify some of the unforeseen items that can develop from an overlay, but it also keeps the owner and contractor aware of how these unforeseen items will be paid for. It helps to level the paved area, create proper ADA slopes, assist in drainage, and create a better/smoother final surface. With proper testing, the plant can improve mix consistency, the final density on the surface mat will increase, and it can slow the deterioration caused from existing pavements below. When looking at the size and scope of many paving projects, simply adding a cost-effective leveling course item can provide many benefits that will enhance the value of your asphalt pavement asset.