Foamed asphalt for faster CIR projects
Cold in-place recycling
Foamed asphalt turned a discussion of cold in-place recycling (CIR) into a hot topic at the ninth annual Minnesota Pavement Conference. In a paper session on pavement rehabilitation, Tom Kazmierowski of the Ontario Ministry of Transportation and Curt Bolles of Minnesota's Olmsted County discussed their early experience with foamed asphalt as the binder in CIR operations. Though many agencies do CIR with asphalt cement (AC) emulsions, use of foamed asphalt is a recent innovative development.
The two presenters described similar processes. As usual with CIR projects, the existing pavement is milled and sized. Then, to create the binder, a small amount of water (about 1 percent by volume) is injected into AC that has been heated to 350 degrees Fahrenheit. The result is rapid expansion of the AC to 15 to 20 times its original volume. This foamed product is then mixed with the recycled material at a ratio of about 1-to-100. Finally, the material is laid and compacted using conventional methods.
Advantages and disadvantages
As Kazmierowski explained, the advantage of the foamed AC binder is curing time. "With conventional CIR, a new surface of hot mix asphalt is placed after the mix is cured…," he said. "Typically, the specifications say you cannot overlay for 14 days…to make sure moisture and compaction requirements have been met…[but with foamed AC binder,] the new HMA surface can be placed within two days." Since very little water is used, two days is enough time for moisture to evaporate and secondary compaction to occur. This can allow the construction season to extend into the cooler fall months, when evaporation isn't as rapid.
Bolles said a disadvantage of the new process is that the water tanker must be positioned alongside the paving train to maintain asphalt temperature during expansion. In his project, traffic was stopped for about half an hour while the material was prepared.
Results
Both agencies reclaimed about half the length of a section of pavement using conventional CIR with an emulsion binder, and the other half with a foamed AC binder. Both reported favorable early results:
- As predicted, the foamed AC sections in both projects achieved the specification requirements within two days. By comparison, because of weather conditions in the Ontario project, the conventional CIR section required 30 days to meet the same moisture requirements.
- In Olmsted County, acceptable densities were achieved with the same rolling pattern and number of passes on both conventional CIR and foamed AC sections.
- In Ontario, falling weight deflectometer testing was conducted immediately after rehabilitation on both sections. Mean deflections were 0.29 mm for the conventional CIR and 0.27 mm for the foamed AC. When tested a year later, both sections had mean deflections of 0.17 mm.
- In Olmsted County, the conventional CIR material (CSS-1) cost $15,191 per mile while the foamed AC material (PG 52-34) cost $11,156 per mile.
- In both places, conventional and foamed pavements were uniform in appearance and performed well under traffic while they were curing (prior to overlay).
In Ontario, one year after construction, the conventional CIR surface was found to provide a slightly smoother ride (via IRI measurement) than the foamed AC surface. However, Kazmierowski attributed this to the contractor electing to micro-mill the conventional surface immediately after construction due to concerns with the cross-fall.
—Richard Kronick, LTAP freelance writer