VAA Virginia Asphalt Fall/Winter 2020

32 FALL/WINTER 2020 Trenton M. Clark, P.E., President, Virginia Asphalt Association Mike Dudley, Director, Virginia Asphalt Association OVENCORRECTION FACTORS Do They Really Matter? Several years ago, Dr. Randy West of the National Center for Asphalt Technology (NCAT) wrote an engaging and thought- provoking article in Asphalt Technology News . Asphalt Technology News is a semi-annual newsletter published by NCAT that explores various aspects of asphalt and pavements. In his article titled “Don’t Forget the Impact of Basic Principles on Asphalt Mix Durability” (Spring 2015), Dr. West covered several basic areas mix designers and engineers need to remember when trying to improve durability. In the article’s opening paragraph, he states, “The root causes (to durability issues) can often be traced back to failing to follow basic principles of mix design and quality assurance.” He stresses four main areas: AC Content, Lower Ndesign, In-Place Density, RAP and RAS use. Nowhere does he mention Oven Correction Factors, so do they really matter? Asphalt Content In Dr. West’s article, he notes Asphalt Content (AC Conent) as being critical to mix durability. In general, mixes with higher AC contents perform better. With- out enough binder, the mix will have a thin film thickness on the aggregates; the percentage of the binder in the mix will be more susceptible to oxidation; and the mix will be more difficult to compact, allowing in more air and water. So understanding AC Content matters, how do the contractor and the owner ensure the asphalt mix produced has the designed amount of asphalt binder? Mix Design Process: Simplified During the design of asphalt mixtures, tech- nicians blend specific amounts of aggre- gates, additives, and asphalt binder to make an acceptable product. In the lab, the technician will measure the exact weights of each material. These precise weights will be used to calculate the final AC content for the mixture. The mix design is set, assum- ing all aggregate gradations, volumetrics, and performance criteria are met. At this point, the designer knows the exact AC content. However, how can you be sure the asphalt plant is producing the mix they designed? Oven Correction Factor Like a cake or other finished products, it is hard to undo an asphalt mix once it has been produced. Years ago, solvents were used to separate asphalt binders and aggregates. This was an objective approach to verify AC contents in mixes, but the solvents were dangerous to use. In the late 1990s, NCAT developed the ignition oven (Figure 1). Instead of using solvents, high temperatures (1000oF) were used to burn off the asphalt binder (and other combusti- bles in the mix), leaving only the aggregates. By knowing the mix’s weight going into the oven and weighing the aggregates after burning, the AC content could then be calculated. However, with such high temperatures in use, the ovens require proper ventilation. The draft created by the exhaust vent not only removes the burned asphalt but very fine aggregates. Therefore, the weight lost in this process included liquid asphalt binder and fine aggregates as well. Following VTM 102 or AASHTO T-308, mix designers prepare a specimen where the exact weights of aggregates and asphalt binder are known. By placing this specimen in the ignition oven and weighing the remaining aggregates, the Oven Correction Factor (OCF) can be calculated. For example, if the specimen has an actual AC content of 6.0% and, after a burn, the ignition oven records a weight loss of 6.32%, then that asphalt mixture for that ignition oven has an OCF (OCF) of 0.32%. The OCF considers the aggregate loss during the burning process and is assumed to be constant for that mixture during the production season. However, if the aggregate properties change during the year, a new OCF should be determined and used for mix quality control and acceptance purposes. OCF Importance During the production of asphalt mixtures, the contractor and owner are part of a larger Quality Assurance Program. The contractor retrieves and tests samples as part of the quality control and acceptance processes in Virginia. AC content and aggregate grada- tions results are used to calculate the payment for each lot of material. The OCF must be known to calculate the actual AC content of a mixture. Additionally, the contractor determines the volumetrics of the mixture for Go/No Go production decisions. The decisions rely on parameters such as Fines to Effective Asphalt Ratio and Voids in Mineral Aggregate (VMA). Both of these calculations depend on accurate AC contents of production mixtures. If the OCF is incorrect, then wrong decisions may be made. Elias Llano, Asphalt Testing Supervisor, Branscome Paving, preparing a sample. continues on page 34 △