Compaction must be achieved while the mix is still hot enough for the applied compactive effort to reorient the mix's aggregate particles to reach maximum density. If the HMA is stable under the rollers, the rollers should operate as closely behind the paver as reasonably possible. Both the breakdown and intermediate rollers should be within 500 ft of the laydown machine.
With a stable mix, fewer roller passes are needed to obtain a given level of density when rolling is accomplished directly behind the paver-where the mix is the hottest. More density is usually obtained with one pass of the roller when the mix temperature is 250 F than with a similar pass when the mat is at 220 F.
If the mix is tender and moves either longitudinally and/or transversely under the compaction equipment, the breakdown rolling is often delayed to avoid excessive shoving or checking of the mix by the rollers. This, however, is the wrong solution to the problem.
The properties of a mix that cannot be compacted immediately behind the paver need to be modified. It is very difficult to obtain the required degree of density with a tender mix-the rollers can't compensate for a poor mix design. When a tender mix is encountered, the mix design-not the compaction process-should be changed.
Until the mix design can be changed, often the best way to compact a tender mix is to use a pneumatic-tire roller in the breakdown position, directly behind the paver. A vibratory roller can be operated in the intermediate position, some distance away from the laydown machine. While most of the required density is obtained with the pneumatic-tire roller, care should be taken that the vibratory roller does not operate too close to the paver and cause the mix to move-shove or check-and thereby cause a density reduction instead of the desired increase in density.
