By R. Michael Anderson, P.E.
Since its publication as a standard in 1939, ASTM D244 Standard Test Methods and Practices for Emulsified Asphalts, has principally relied on the distillation procedure to recover the base asphalt from an emulsified asphalt.
This procedure, now described in ASTM D6997 Standard Test Method for Distillation of Emulsified Asphalt, involves heating a 200-gram sample of emulsion in an aluminum alloy still to a temperature of 260°C (500°F). This distills the water from the sample and leaves the technician with an asphalt residue that has physical properties that are supposedly representative of the properties of the base asphalt.
While this practice has worked well for many years as a method to obtain residue from emulsified asphalts, many asphalt technologists were concerned that the temperature far exceeded the temperatures that would ever be seen by the emulsified asphalt during storage and application. As such, it was possible that the properties of the residue after recovery could be different from the properties of the original base asphalt. This is particularly true for polymer-modified asphalt emulsions where studies have shown that the high temperature and extended recovery time in the standard distillation procedure may significantly degrade the polymer.
In response to the concern regarding the distillation procedure, asphalt technologists have focused on alternative techniques for recovery of residue. One technique that may be considered for polymer-modified emulsions is the evaporation procedure described in ASTM D6934 Standard Test Method for Residue by Evaporation of Emulsified Asphalt. In this procedure, three beakers, each containing 50 grams of emulsified asphalt, are heated for two hours in a 163°C (325ºF) oven, and then removed, stirred and replaced in the oven for an additional hour.
After three hours in the oven, the beakers are removed and allowed to cool to room temperature. The beakers are weighed to determine the residue percentage. If the residue is needed for testing, the beakers are replaced in the oven until the residue is fluid enough to pass through a 300 µm (#50) sieve. The ASTM standard notes that this procedure “…tends to give an asphaltic residue lower in penetration and ductility than the distillation test method (D6997).”
Another technique for obtaining residue from an emulsified asphalt is described in ASTM D244 as the low temperature vacuum distillation procedure. This procedure is particularly suitable for asphalt emulsion residue properties that may be altered by standard, high-temperature distillation (i.e., polymer-modified emulsified asphalt). In this procedure, the emulsion is heated while using an applied vacuum of 88 kPa below atmospheric pressure. The emulsion is heated for 45 minutes until it reaches 135°C (275°F) and is then held at this temperature for 15 minutes. The ASTM standard notes that the low temperature vacuum distillation procedure is “…not intended to produce residues equivalent to the D 6997 260°C (500°F) distillation procedure.”
Finally, ASTM has recently published a new procedure – D7497 Standard Practice for Recovering Residue from Emulsified Asphalt Using Low Temperature Evaporative Technique – that takes two days to complete, but has a maximum temperature of 140°F (60°C).
Until such time that an acceptable recovery method can be identified that would be suitable to all asphalt emulsions, the question won’t be “Got Residue?”, but rather “How’d You Do It?”
Mike Anderson in the Director of Research and Laboratory Services at the Asphalt Institute.