Warm-mix asphalt (WMA) is one of the most beneficial improvements among paving technologies in the last several decades. It is often thought of as a recent innovation, because its widespread application in the United States has occurred since year 2000. However, it was being used in Europe and had seen certain applications in North America at earlier dates. At this point, there is a considerable volume of experience with WMA.
My purpose in this writing is to offer some clarification concerning erroneous information presented in the Portland Cement Association (PCA) report “PCA Marketing Intelligence Report Concerning Warm-Mix Asphalt Potential.” I will address only that about which I possess applicable information, some of which may be later or more detailed than that used for the PCA report. These topics will include incomplete drying, moisture sensitivity, the need to add RAP to provide necessary stiffness to the mix, and the cost benefits of WMA.
Warm-mix technology does not result in incomplete drying of aggregate.
An important point of the referenced PCA report is that the making of WMA causes incomplete drying of the aggregate, which results in pavement quality problems. The author of the PCA report further states that hot-mix asphalt (HMA) is normally made upward of 340 degrees F (171 degrees C) to assure complete drying of the aggregate. The author correctly states that additives, such as hydrated lime, may be required to enable good bonding of the asphalt to the stone when moisture susceptibility is a problem.
I have been in the industry since 1989, long before WMA use became widespread. For several years, I was in a hands-on position working frequently at HMA plants in the field all over the country and world, as well as designing them. I have seen mix made at 340 degrees F (171 degrees C), but that is definitely not the norm. The typical HMA production temperature is 300 degrees F (149 degrees C). Unless a dryer is experiencing some malfunction, that temperature is more than sufficient to accomplish the necessary drying. Higher temperatures are sometimes used for other reasons, such as with certain special mix types, but are rarely necessary to accomplish proper drying. High drying temperatures can be used to overcome dryer deficiencies, but good dryers in proper condition can do an excellent job of drying almost any aggregate at temperatures substantially below 300 degrees F (149 degrees C).
Warm mix asphalt (WMA) is one of the most beneficial improvements among paving technologies in the last several decades.
Residual moisture is always internal (inside individual aggregate stones) and is typically only found with porous aggregates. But most dryers today are capable of effectively removing internal moisture at temperatures in the range of 250 degrees F (121 degrees C) and even lower in many cases. It is more a matter of how the stone is handled in the dryer than of final temperature. Internal lifters (flights) should lift all the aggregate material and provide good exposure of aggregate to hot burner gases inside the rotary dryer. Worn or incorrectly designed flights can cause incomplete drying by failing to sufficiently expose all of the aggregate. But then I would question whether a worn or incorrectly designed concrete plant makes the same quality of concrete, as does one in proper operating status. It is basic to all industry to operate good equipment that is in proper condition.
I have seen lime and other additives used to address moisture sensitivity concerns, but the truth is that certain aggregate types and certain climatic conditions impact that decision. Those states where such conditions are common already require additives as a basic part of their DOT specifications even with HMA. So, that cost is not associated with whether the mix is HMA or WMA. There are few cases where an additive is required just because of a change to warm mix. So, it is not correct to indiscriminately decrease the anticipated savings associated with WMA based on an assumption that an additive will be required. Even in those cases where an anti-stripping additive is used in WMA where it is not used in the corresponding HMA that does not change fuel consumption.
My wish is to let the real facts speak for themselves.
Many of the benefits of WMA as mentioned have obvious cost benefits, but those returns have not yet been fully determined because that will take some time; but it is abundantly clear that a little fuel savings is just the “tip of the iceberg.” Any dollar-estimate of the benefits in terms of fuel savings alone is likely to be conservative to the extreme.
While we at the Astec Industries, Inc. family of companies make both asphalt plants and concrete plants, I do not think it is our best interest or that of the public for one material to be used in preference over another based on erroneous information.BACK TO ISSUE
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