By Geoff Dillon
In an asphalt pavement, longitudinal joints occur where one paving lane (also called a “mat”) is placed adjacent to an earlier placed paving lane. The localized area of the longitudinal joint is more difficult to compact and, as a result, has higher in-place air voids compared to the interior of the mat. While in-place void levels at the interior of the mat are typically 6-8 percent (92-94 percent compaction), void levels at the joint can be 10 percent or greater.
At these high void levels, the pavement becomes permeable where water and air intrude the joint which accelerates deterioration. Not only does the asphalt mat deteriorate faster, but underlying layers are also weakened from water intrusion at the opened longitudinal joint.
Deterioration along the longitudinal joint is often cited as a leading cause of premature pavement failure. While the centerline joint often comes to mind when discussing longitudinal joint failures, longitudinal joints also typically exist between driving lanes as well as wherever asphalt matches up to shoulders, curbs and gutters.
Why does the mat near the longitudinal joint typically have higher air voids? The first HMA mat is compacted with an unconfined edge, so it is a challenge to achieve density 6 to 12 inches from the edge. The adjacent mat (also called second pass) is then placed against the exposed edge of the first mat, which has typically been compacted and cooled. The cooled edge from the first mat provides confinement during compaction of the second mat along the longitudinal joint. Thus, density on the “confined side” of the joint should be greater than the “unconfined side” of the joint, assuming best paving and compaction practices were used.
There are many approaches that have been used to improve longitudinal joint performance. The remainder of this article describes five longitudinal joint construction methods as well as a materials-based approaches to improve joint performance.
The notched wedge modifies the joint from a nearly vertical plane to a sloped plane with a longer contact area. The first paver pass features a taper at the edge on to which the second paver pass adds material to provide an overlap. Notches are critical at both the top and bottom of the wedge so as to provide adequate thickness of the mat to allow aggregate realignment and packing during compaction.
Joint re-heaters strive to improve adhesion at the joint by pre-heating the cold side of the joint with a propane-fired or infrared heater. Pre-heating allows more intimate contact with the new material, thereby increasing tensile strength at the joint. The joint re-heater seeks to replicate the superior adhesion and density of echelon paving, where two adjacent lanes are paved simultaneously and the joint does not cool before compaction. Care must be taken with the joint re-heater to not over-heat the asphalt binder, which can cause premature aging.
The joint maker seeks to improve density near the joint. It is a pre-compaction screed mounted in front of the main screed at the end. It is typically four inches wide. The joint-maker creates a strip of higher density material immediately behind the screed at the edge of the mat. This serves to prevent lateral movement of the mixture when it is being compacted under the roller.
Restrained edge is a mechanical approach to improve density by improving confinement. On the first paver pass, material at the unconfined edge of the mat will move slightly sideways under the weight of the roller causing low density. A device is attached to the side of a roller which “restrains” the edge providing lateral resistance and hence increased density. The greatest challenge of this device is the skill of the roller operator to accurately hold it on the edge of the mat.
Cutting back or milling the joint
Airport pavement specifications (both FAA and military) require the contractor to cut back the outer few inches of the low density unconfined edge prior to paving the second pass. On airfields, this is done while the mix is still warm and plastic with a cutting wheel (think pizza cutter) on a long wheelbase vehicle such as a road grader or a roller.
Eliminating the low density material by the unsupported edge of the joint can also be accomplished by milling and paving one lane at a time rather than milling all adjacent lanes before paving begins. By doing so, unconfined joints are eliminated as there is always a confined edge to pave against. Some agencies will even mill a few inches into the new adjacent mat to ensure paving against new material that has been well compacted.
These methods do discard a little extra new mix into the RAP stockpile, and this cost should be weighed against long-term benefits from superior joint performance.
Some road owners have explored materials-based solutions to reduce air voids at the joint, as opposed to the mechanical-based solutions discussed above. Materials-based methods may include asphalt materials laid prior to the paver pass which migrate up into the joint to reduce permeability. Or they may use post-applied asphalt material applied after the joint has been constructed. In certain circumstances, longitudinal joint sealants may be used successfully with a mechanical-based approach. Fog seal methods may be applied at the top of the joint to reduce permeability from the top of the surface as well.
As joint deterioration continues to be one of the most frequently cited causes of premature pavement failure, driving toward improved longitudinal joint performance represents a critical challenge for the asphalt industry. The methods highlighted in this article discuss potential solutions to the longitudinal joint challenge. The Asphalt Institute has been actively involved in researching this challenge and recommending best practices.
Dillon is the President of Asphalt Materials, Inc. in Indianapolis, Indiana.
While best practices for longitudinal joints is a complex topic, an excellent source of information is asphaltinstitute.org/engineering/longitudinal-joint-information/. This webpage includes a wealth of material covering best practices for constructing and specifying longitudinal joints, as well as innovative methods and materials used to improve longitudinal joint performance.