Paving roller speed also is governed by the lateral displacement or tenderness of the HMA. If the mixture moves excessively under the rollers, the speed of the compaction equipment should be reduced. Roller speed should be kept constant. If the paver speeds up and the rollers also speed up, less density will be obtained in the mix for the same number of passes of each roller over each point of the pavement surface. It is very important that both the paver and the roller maintain a consistent speed to obtain consistent density.
The actual number of passes needed over a point in the pavement surface by each of the rollers is a function of many variables. The type of compaction equipment is one primary variable. Three-wheel, static steel-wheel rollers apply different compactive effort than tandem static steel-wheel rollers; pneumatic-tire rollers and single- or double-drum vibratory rollers all apply different compactive effort, as well.
However, these capabilities vary with layer thickness, mix temperature, mix design (asphalt content and aggregate gradation) and environmental conditions. In addition, the number of asphalt roller passes required of a particular roller depends on its position in the roller train-breakdown, intermediate or finish rolling. It may be possible, for example, to obtain a significant increase in the density of the mat for the same number of roller passes when a pneumatic-tire roller is moved from the intermediate position behind a vibratory roller to the breakdown position in front of that same vibratory roller.
To determine the minimum number of roller passes needed to achieve the required density level, a test strip should be constructed at the beginning of the project. Typically, only one combination of rollers is tested with one combination of passes of each roller. To pick the most efficient and economical number of roller passes, it is suggested that more than one test strip be constructed, with each test area using different rollers in different positions behind the paver.
Roller passes must be distributed uniformly over the width and the length of the HMA layer. Most often, the center of the paver lane receives more passes from the rollers than do the outside edges of the lane. The number of roller passes applied by each roller must be the same over each and every point in the pavement surface to obtain consistent density.
