A Method for Determining Gyratory Compaction Locking Point of Asphalt Mixture Based on Inter-Particle Contact Stresses

Abstract:

This paper proposed a novel method for determining the stress change rate index R_s and the locking point of stress （LPS） based on a new granular sensor （SmartRock） to test the contact stress of particles inside the asphalt mixtures during rotary compaction. Three asphalt mixtures， AC-13， AC-20， and SMA-13， were selected for rotational compaction to test the contact stress of coarse aggregates. The interlocking state of mixture samples was investigated and identified from the perspective of micro-mechanics. In addition， the results of the identifications were compared with those from the traditional rotary compaction method. The results indicate that the test signals of the SmartRocks located at the top and bottom of the specimen have a spectral aliasing and harmonic interference. It is recommended to arrange the smart sensors in the middle of the specimen with less interference. For the AC-13 mixture， three compaction stages could be found. In the initial compaction stage， the coarse and fine aggregates contact and squeeze each other， and the stress and R_s of the coarse aggregates increase rapidly. As the fine aggregate and asphalt binder suspend around， the coarse aggregate begin to bear the load， and the stress of the coarse aggregate and R_s decrease and enter the creeping stage. When a stable internal structure is formed between the aggregate and the asphalt binder， the stress reaches an interlocking state. For the AC-20 mixture， the coarse aggregates quickly form an occlusal skeleton and the contact stress continues to increase under the action of external load to enter the tightening stage， resulting in the increase of stress and R_s. When the aggregates form a stable skeleton load-bearing structure， the interlocking state is reached. For the SMA-13 mixture， it would segregate due to the improper temperature. At a mixing temperature of 170°C， the particle stress amplitude fluctuates and increases non-linearly during the tightening compaction phase. Compared with the interlocking points results from the traditional method， the LPS determined based on R_s all lags behind the volume interlocking point.

Key words:asphalt mixture;gyratory compaction;SmartRock;stress change rate;locking point