The construction of two partial-depth (cold in-place) recycling projects was monitored and the quality control results were analyzed to compare gradations produced with single- and multi-unit recycling trains, to assess the effect of recycling train forward speed on gradation, and to compare differences between emulsified and foamed asphalt recycling agents in partial-depth recycling applications. The research highlighted the challenges with variability and small sample sizes associated with in-place recycling but provided representative and consistent results between projects. The results showed that there was no discernable difference in the density and strengths of partial-depth recycling layers produced with the single- and multi-unit recycling trains. The main benefit of the multi-unit train was better control of maximum aggregate size by the on-board screens and crushing unit, however, the crushing unit did not appear to change or improve the finer portion of the gradation (i.e., material passing the 5-mm sieve), which has a known larger influence on compaction density, air-void content reduction, strength, moisture resistance, and stiffness. The recycling train type and forward speed did influence the coarse portion of the gradation (i.e., >5 mm), but did not appear to influence compaction density. Test results indicated that the sections treated with emulsified asphalt had higher densities than those treated with foamed asphalt. However, there was considerable variability in materials and pavement structure along the length of the project, which probably had an influence on all results. The findings permit agencies to specify that recycling train and recycling agent choice can be the contractor’s decision.
Louw et al. (Sun,) studied this question.