Lab Testing Materials with the Vacuum Oven
Testing asphalt pavement materials such as aggregate, reclaimed asphalt pavement (RAP) and mixture specimens often requires that samples be dried to a constant mass. Most labs accomplish this using convection ovens or fan drying, depending on sample type. The National Center for Asphalt Technology (NCAT) recently completed a study to determine if vacuum ovens are an economically feasible alternative to convection ovens and fan drying in an asphalt pavement lab. The underlying premise of a vacuum oven is that at low pressures, water converts to steam at a much lower temperature, which should enable more efficient drying.
The cost of the vacuum oven (Figure 1) used in this study was approximately twice that of a standard convection oven of similar size. Circular holes were drilled into the shelves to better facilitate air flow and moisture drainage within the vacuum oven. Per the manufacturer’s instructions, the vacuum line was closed once target pressure (26-27 atm below atmospheric pressure) was applied in order to prevent moisture damage to the vacuum pump.
The vacuum oven was evaluated relative to conventional drying methods for three classes of materials: aggregate, recycled asphalt materials and compacted HMA specimens. Six aggregate materials, representing both coarse- and fine-graded stockpiles with a range of absorption values, were included in the study. Recycled materials included both coarse- and fine-graded RAP, as well as recycled asphalt shingles (RAS). All aggregate and recycled material samples were wetted by adding distilled water (2 percent by weight for coarse materials and 6 percent for fine) and dried at 110°C in both vacuum and convection ovens.
Compacted HMA specimens were also used to evaluate the performance of the vacuum oven. Samples represented three different specimen geometries used for common lab performance tests: dynamic modulus and flow number using the Asphalt Mixture Performance Tester (AMPT), the Hamburg Wheel-Tracking Test, and creep compliance and indirect tensile strength (IDT). All samples were prepared using the same plant-produced 12.5 mm Superpave mix and were compacted to 7.0 ± 1.0 percent air voids. All compacted specimens were submerged for four minutes (according to AASHTO T166) and dried to a saturated surface-dry (SSD) condition prior to vacuum oven or fan drying. To minimize sample aging, the vacuum oven temperature was set at 40°C.
For each material, the average drying time was defined as the time of measurement beyond which no more than 0.1 percent sample mass was lost, per AASHTO T255-00. Average drying time for each material was used to determine if the vacuum oven performed better than conventional drying methods. Figure 2 shows an example of the data generated during the experiment.
Average drying time for fine aggregate—vacuum oven vs. conventional oven drying.
For the majority of aggregate materials tested, the vacuum oven did not result in faster dry times than the conventional oven. The one exception was low-absorption, coarse, rounded gravel. Theoretically, the vacuum oven should be effective on highly absorptive materials with extensive pore space, but these data did not support this theory.
The vacuum oven provided only a slight drying benefit for the RAS material, which is considered minimally absorptive, and none for the coarse and fine RAP. Again, the only drying benefit with the vacuum oven was observed for low-absorption material. However, RAS is fine-graded, whereas in the aggregate evaluation the drying benefit was observed only with a coarse-graded aggregate. Thus, there appears to be no correlation between gradation and vacuum oven effectiveness.
With the compacted HMA specimens, the vacuum oven showed an improvement in the rate of drying versus fan drying. However, no comparison was made between the vacuum oven and a conventional oven set to a low temperature for drying compacted HMA samples. It is also unknown whether oven drying would detrimentally age compacted performance specimens.
During the course of vacuum oven testing, large amounts of moisture evaporated from samples and re-condensed inside the vacuum oven chamber. Since the condensed moisture was unable to escape the sealed chamber, the samples were being dried in a high-moisture environment which could be a reason why the vacuum oven was ineffective in drying most aggregate and recycled asphalt materials. Two modified procedures were tested in an effort to address this problem.
First, a large container of desiccant was placed on the vacuum line, allowing the line to remain open during drying. As the escaping moisture was pulled through the vacuum line, it was trapped in the desiccant before having an opportunity to damage the pump. The high-absorption coarse and fine aggregates were re-tested using this retrofit. While the desiccant was effective in removing moisture, it became saturated quickly and this procedure did not result in an appreciable improvement in drying time.
A second modified procedure involved re-testing the high-absorption aggregates at ambient temperature, using only vacuum pressure to remove moisture from the materials. This modification was also not effective in reducing drying time.
The vacuum oven did not improve the drying rate for aggregate and recycled asphalt materials, except for those with the lowest absorption values. While the vacuum oven did offer an improvement in drying time for compacted HMA samples compared to fan drying, questions remain regarding this practice. Thus, the performance of the vacuum oven does not warrant its additional cost relative to conventional lab drying equipment.