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1 Accurate determination of asphalt content and aggregate gradation is critical for controlling the quality of asphalt mix- tures during construction. Most state specifications require quantitative evaluation of the asphalt content of mixtures as a criterion for acceptance. The ignition furnace test procedure specified in AASHTO T 308, âDetermining the Asphalt Binder Content of Hot Mix Asphalt (HMA) by the Ignition Method,â is required or allowed by most state departments of transpor- tation (DOTs) for determining the asphalt content and aggre- gate gradation of asphalt mixtures. AASHTO T 308, which was developed and implemented in the mid-1990s, involves removing the asphalt binder from laboratory mix samples by burning them in an ignition fur- nace at a high temperature, typically 1,000°F (538°C). The test is complete when the change in mass does not exceed 0.01% for 3 consecutive minutes. Since ignition of the asphalt binder also results in some loss in aggregate mass, the procedures require the determination of an asphalt content correction fac- tor for each asphalt mixture and for each ignition furnace used. Developing the correction factor involves the preparation of two samples of the asphalt mixture at the design asphalt content. The correction factor (CF) samples are tested, and the CF is calculated as the difference between the actual and measured asphalt binder contents expressed as a percentage of the mass of the asphalt mixture. Some states that have aggregates with high mass loss dur- ing ignition do not allow the use of the ignition test due to con- cerns about its accuracy. It has been shown that high CFs result in more variability in the measured asphalt content, making it less appropriate to use for acceptance/rejection of produced asphalt mixture. It has been standard practice for some agencies/contractors to share CFs between ignition units. For example, there have been cases where a CF was developed during mix design and the same correction factor used for more than one ignition test unit during the quality control/quality assurance (QC/QA) testing of the asphalt mixture. This approach can significantly reduce test time where there is more than one unit being used in a laboratory. However, this approach violates AASHTO T 308, which indicates that a correction factor should be estab- lished for each mixture and for each ignition test unit. There is a need to identify and quantify the consequences of sharing correction factors between units/mixtures and also to identify testing parameters that affect the measured correc- tion factors. By identifying possible causes of variation, the test procedure could be adjusted where possible to make the correc- tion factors more consistent between ignition furnaces. Also, it should be possible to reduce the difference in correction factors between all types of equipment. First, factors affecting the cor- rection factor must be determined, and then procedures must be implemented to optimize these factors for best results. 1.1 Objective The objectives of this research were to (1) determine the significant influences that affect the variability of asphalt and aggregate correction factors for ignition furnaces; and (2) develop guidelines for the installation, operation, and main- tenance of ignition furnaces to minimize the variability in correction factors between furnaces. 1.2 Scope This research was divided into three phases: Phase I encom- passed a literature review on the use of ignition furnaces, a survey of state DOT and industry practices regarding the use of ignition furnaces, and the development of an experimental plan to accomplish the research objectives. Phase II involved the execution of the experimental plan to determine the sig- nificant factors that affect the asphalt and aggregate correc- tion factors for ignition furnaces. Phase III produced this final report and a proposed AASHTO practice for the installation, operation, and maintenance of ignition furnaces. C h a p t e r 1 Introduction
