This study investigated the cause(s) of the excessive delamination and shoving distresses observed in NCDOT Division 13. Two possible causes of these distresses were identified to be: 1) the intermittent purging of the baghouse fines in in-situ asphalt mixtures, and 2) improper selection and/or application of the tack coat, i.e. the use of CRS-2 emulsion versus the PG64- 22 asphalt binder.
Laboratory performance test results on field cores and asphalt mixtures showed that baghouse fines had a stiffening effect on mixtures and these mixtures were more resistant to rutting. However, mixtures containing baghouse fines were found to be moisture sensitive with tensile stress ratios below that specified by NCDOT. The performance test results for the evaluation of the bond strength of the in-situ cores, showed that the PG64-22 binder used as tack coat provided a better interfacial bonding compared to the CRS-2 emulsion.
Results of this investigation suggest that the delamination and shoving distresses in NCDOT Division 13 could be attributed to the combined effect of intermittent purging of baghouse fines in asphalt mixtures and the use of CRS-2 emulsion as tack coat. Due to intermittent purging of baghouse fines, some in-situ mixtures may contain significantly higher proportion of baghouse fines compared to regular fines. Although the NCDOT JMF requires use of an anti-strip additive, the dosage does not appear to be sufficient to offset the increased moisture damage leading to in-situ mixture deterioration and, consequently, loss of strength and stability. Once the moisture damaged mixture is susceptible to shoving under traffic loading, the CRS-2 emulsion may not provide the tacking strength necessary for the surface layer to remain bonded to the lower layer, hence, leading to delamination.
