Hot mix asphalt mixtures that have tender characteristics still have to be compacted to a minimum level of density or a maximum level of air void content. Because of the lack of internal stability in the mix, however, the compaction of a tender mix is often very difficult.
It has been found that a mix that has tender properties – that moves or shoves excessively under the weight of the compaction equipment while being rolled – may be able to be compacted to the proper density level by taking advantage of the three temperature zones which normally exist in that tender mix.
In the first, or upper, temperature zone, the asphalt concrete mix is relatively stable during the compaction process. Within the temperature range from laydown (320°F to 285°F, depending on the use of polymer modified binder and other factors) down to about 240°F, the HMA material is stable under the applied compactive effort. In this upper temperature range, the mix will not shove or check under the rollers regardless of whether a vibratory roller or a static steel wheel roller is used.
The lower limit of this upper temperature zone is not an exact value – it depends on the characteristics of the mix, the rate of cooling of the mat, the thickness of the layer, environmental conditions, and the type of roller used. In some cases, this value may be as high as 250°F or higher while in other cases it could be as low as 230°F or lower.
The middle temperature zone – the problem range – extends from approximately 240°F down to about 195°F. In this temperature range the mix will move, shove, and check under the applied compactive effort. A bow wave will form in front of the steel drums of the roller and the mix will crawl longitudinally. The mix will also move laterally or transversely and the mat will widen out if the edge of the roller is not positioned properly over the unsupported edge of the asphalt concrete mat by at least 6 in. In this middle temperature zone, the HMA material lacks the internal stability to support the weight of the steel wheel compaction equipment.
The lower temperature zone extends from the bottom of the intermediate temperature zone down to approximately 160°F or even lower. Within this temperature range the mix is cool enough to regain the internal stability necessary to support the weight of the compaction equipment. It is possible, but usually very difficult, to achieve the required degree of density within this relatively narrow temperature range if the mix has been de-compacted during the rolling of the mix when the mix temperature was in the middle temperature zone.
To compact a hot mix asphalt mixture which has tender characteristics and moves under the rollers in the middle temperature zone, a contractor has one of two choices.
• All of the compactive effort can be applied in the upper and/or the lower temperature zones when the mix is internally stable and does not move, shove, or check under the steel wheel rollers. In this case, no compactive effort is applied to the mix when the mix temperature is within the middle temperature zone.
• If it is necessary to roll the mix while the temperature is within the middle zone, a pneumatic tire roller can be used because the mix will typically not shove in front of the rubber tires (as it will in front of a steel wheel roller drum in this middle temperature zone).
In order to compact a HMA mix that is stiff, it is necessary to roll it as close behind the paver as possible. In order to compact a HMA mix that is tender, it is also necessary to roll it as close behind the paver as possible in order to take advantage of the upper temperature zone in the mix.
For most asphalt concrete mixtures that exhibit tender characteristics, the tender mix must be compacted to as high a density as feasible, as quickly as possible. For most tender HMA mixtures that means placing two rollers on the mat within the upper temperature zone.