Investigation of tender zone in compaction of coarse-graded Superpave® hot-mix asphalt mixtures

Tender hot-mix asphalt mixtures have been observed and experienced by paving contractors for many years. However, during the field compaction of coarse-graded Superpave(R) mixes, a "tender zone," not a true tender mix, is sometimes experienced. The tender zone is a range of mix compaction temperatures during which the mixture exhibits instability during roller action. Many possible causes of the tender zone have been presented, including differences in laboratory and production absorption, mix moisture, a low dust-to-asphalt ratio, increased asphalt binder film thickness, and a temperature differential with the lift. A study was conducted to document and evaluate field mixtures exhibiting the tender zone to determine the possible cause(s) of its occurrence. Documentation included mix-, production-, and construction-related items. Laboratory evaluation consisted of mixture gradation and volumetric testing along with Superpave asphalt binder testing with the project asphalt binder before and after steam distillation. Project results failed to clearly identify one particular reason for the occurrence of the tender zone. However, it is believed that the tender zone was a result of field absorption being less than the design absorption and increased asphalt binder film thickness acting in conjunction with an inherent temperature differential within the lift.