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?
In Dr. West’s article, he notes Asphalt Content (AC Content) as being critical to mix durability. In general, mixes with higher AC contents perform better. Without 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, technicians blend specific amounts of aggregates, 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, assuming 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 combustibles 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.
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 gradations 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.
VDOT retrieves asphalt samples from the producer to calculate the same parameters. These are part of the Department’s Independent Assurance program and Verification, Sampling, and Testing program under the Quality Assurance program. While not directly used for payment, VDOT and contractor test results are compared using various statistical measures to identify potential testing or materials issues.
It’s All About the Ovens
For years, it has been recognized that each ignition oven has a unique influence on the OCF. As reported in an article titled “The Importance of Asphalt Content Ignition Furnace Correction Factors” for the AASHTO re:source newsletter in October 2010, “Studies have shown that asphalt and aggregate correction factors are unique to each ignition oven, regardless of manufacturer.” In Virginia and several other states, a single OCF is calculated for a mixture and is listed on the JMF (TL-127). This OCF will be used if the contractor has more than one oven in a lab. VDOT and other third-party labs also use this OCF as part of the Quality Assurance Program. Unlike the process outlined in AASHTO T-308, each lab and oven performing testing would not determine the actual OCF per oven.
The bigger question for this article is—so what? Does it really matter? It does if it results in inaccurate results and pay adjustments or limited production. Both cost the contractor money. The contractor must match their determined OCF with a specific oven if more than one oven exists in a lab. For a lab with multiple ovens, the contractor should determine the OCF for each oven. With enough mixes burned and OCFs determined, the contractor may be able to estimate a standard offset between ovens.
For VDOT and third-party labs, determining an OCF for each mix for each oven would be overwhelming. During any calendar year, contractors will produce more than 100 unique mixes for VDOT to test. Using a contractor supplied OCF for each mix is much easier, but not accurate in determining an AC content.
For more than two decades, ignition ovens have played a critical role in the mix design and acceptance process. Oven Correction Factors not only impact payment, but they assist the certified asphalt mix designer in making adjustments to the asphalt plant. Contractors using more than one ignition oven are strongly encouraged to determine an OCF per mix by oven. Likewise, industry and VDOT should review the impacts of OCF on the Quality Assurance Program and determine what steps should be taken to improve the process.