By James A. Scherocman, P.E.
The durability of longitudinal joints in asphalt pavements is a major problem on many roadways across North America. After a short period of time under traffic, these joints tend to ravel. In some cases, the raveling is severe enough to completely erode the mix at the joint leaving a gap between the lanes. (Photo 1)
There are a number of factors during construction that directly affect the durability of a longitudinal joint. The first is the compaction of the unsupported edge of the first lane of mix placed. The second is the amount of mix from the second lane that overlaps the top of the first lane. The third factor is related to raking the mix at the joint. The final factor is compaction of the mix at the joint when the second lane is placed adjacent to the first lane.
Compaction of the First Lane
One key to the construction of a durable longitudinal joint is proper compaction of the unsupported edge of the first lane of the pavement placed. The mix placed by the paver will have a slope on its outside edge. The amount of slope, typically about 60 degrees, depends on the type of end plate on the paver screed. This wedge does not receive the same amount of compaction as the rest of the mix due to its shape and position.
The type of roller used and its position on the unsupported edge of the pavement significantly affects the amount of density that can be obtained. Longitudinal joint compaction is best accomplished by using a steel wheel roller. The proper location for the edge of the steel drum is approximately 6 inches over the unsupported edge of the first lane.
Several commonly used practices produce poor results. If the steel wheel roller is operated inside the unsupported edge of the first lane, the mix will have a tendency to widen out or move in a transverse direction. The amount of movement that occurs is dependent upon the properties of the mix; a tender mix will shove more than a stiff mix. Additionally, a crack will typically form at the edge of the roller drum.
Another poor technique is to place the edge of the drum directly over the unsupported edge of the pavement. When this occurs, the mix will still move sideways under the roller, resulting in poor density of the mix at the unsupported edge. Similarly, a pneumatic roller cannot be used within about 6 inches of the unsupported edge without pushing the mix sideways due to the high pressure in the rubber tires.
Overlap of Mix from Lane to Lane
The amount of mix overlapped from the second lane to the first lane is the next critical key in the construction of a durable longitudinal joint. If an excessive amount of mix is placed over the edge of the first lane, it will have to be removed by raking the joint or it will be crushed by the rollers. If too little mix is placed over the edge of the first lane, a depression or dip will occur on the second lane side of the joint. In either case, the joint will not perform well under traffic. The proper amount of overlap is about 1 to 1-1/2 inches.
When matching a joint of a milled asphalt pavement, the amount of mix overlap must be controlled very carefully. The milling machine forms a vertical face that has a significantly different slope from that formed by the end plate of the paver screed. The amount of mix overlap from the new mat to the milled lane should be about ½ inch, maximum.
Raking the Longitudinal Joint
If the proper amount of mix is placed in the correct place, no raking of mix at the longitudinal joint is necessary. If excess mix is placed over the top of the first lane, it should be removed with a shovel instead of pushing or raking the excess mix over the top of the new lane. Thus the third key to a durable joint is to not rake the joint during construction.
When a joint is raked, some of the mix needed at the joint is pushed onto the hot mix on the second lane. This makes the mix too low on the second lane side directly at the joint and too high on the second lane a short distance away from the joint.
Essentially, the mix ends up at the same elevation on each side of the joint. If the joint is raked flat, the rollers will not be able to compress the mix at the uncompacted side of the joint since it is at the same level as the compacted mat of the first lane. This results in low density on the second lane side of the joint.
Compacting the Longitudinal Joint
The final key to constructing a durable longitudinal joint is the location of the rollers during the compaction of the mix at the joint. The most efficient location to place the rollers, either pneumatic tire or steel wheel, is on the hot side of the joint with the center of one outside tire or about 6 inches of the drum extending over the top of the joint, over the first, compacted lane.
This type of rolling pattern will result in higher compactive effort being applied to the mix at the longitudinal joint and thus achieve higher density at the joint. Additionally, by rolling from the second lane side, the hot mix is being compacted at the same time as the mix at the joint, resulting in a more efficient overall compaction operation.
Proper construction of the longitudinal joint between pavement lanes consists of four primary steps. First, the unsupported edge of the first lane must be compacted by placing the drum of a steel wheel roller about 6 inches over the unsupported edge. Second, only 1 to 1-1/2 inches of mix should be lapped over the top of the first lane when the mix in the second lane is placed. Third, the mix placed at the joint when the second lane is constructed should not be raked but should remain where placed by the edger plate on the screed. Last, the mix at the longitudinal joint should be compacted from the hot side of the joint with the center of the outside tire of the pneumatic roller directly over the joint or the drum of a steel wheel roller extending 6 inches past the joint.
Durable longitudinal joints are a workmanship issue. Proper construction techniques will provide for a long-lasting joint without raveling or deterioration.
Jim Scherocman is a consulting engineer specializing in pavement design and construction. He can be reached at (513) 489-3338.