Variability of Ignition Furnace Correction Factors (2017) / Chapter Skim
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22.1 Background Accurate determination of asphalt content and aggregate gradation is critical in the quality control of asphalt mixtures during construction. Historically, the most common method has been extraction using any of a number of different solvents such as trichloroethylene, methylene chloride, or trichloroethane.
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3 The current AASHTO T 308 procedure consists of burning the asphalt from laboratory mix samples in an ignition furnace at a high temperature [1,000°F (538°C)
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4only sieve outside the limits, the aggregate correction factor must be applied to that sieve only. 2.2 Types of Ignition Furnaces There are essentially two ignition furnace types (based on heating mechanism)
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5 function of time. As an example, a typical profile may look like a set of steps of different lengths and amplitudes, where amplitude on the fan would represent speed (air flow)
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62.3.2 Brown and Mager, 1995 (9) Brown and Mager conducted a round-robin study (RRS)
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7 and three sets of material for testing, consisting of three types of aggregates and one binder type. The aggregates were granite, limestone, and trap rock.
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8for typical Virginia aggregates. A total of 10 Superpave mix designs from nine aggregate sources were used and included 12.5, 19.0, 25.0, and 37.5 mm nominal maximum size aggregate blends.
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9 conducted to assess the effect of fines' variability with other aggregates, particularly dolomites. 2.3.8 Williams and Hall, 2001 (18)
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10 samples) , and the variability was measured in terms of the standard deviation of the results.
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11 ing) , most aggregates would lose some mass during the burning process; hence, a correction factor had to be determined and used to account for this aggregate mass loss when calculating the asphalt content.
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12 and the highest temperature that gives a correction factor of less than 1% is selected. For the four types of furnaces investigated, significant variation was found in the asphalt correction factors for these aggregates with high mass loss.
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13 tests. The following paragraphs summarize some of their key findings.
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14 the standard test temperature of 1,000°F (538°C)
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15 2.3.12.6 Thermogravimetric Analysis Thermogravimetric analysis was also conducted as part of this study. The results indicated that the sample dolomites that were exposed to high temperatures in the ignition furnace had decomposed partially; hence, less mass loss in the thermo gravimetric analysis was recorded.
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16 results showed very similar results. The current Indiana DOT test method to determine asphalt content by ignition allows the test to be conducted at 800°F (427°C)
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17 RAS typically contains 20% to 30% asphalt binder, which is much greater than the 5% to 6% that normally exists in HMA. If the sample size is too large, the equipment may be damaged as combustion begins due to the large amount of asphalt binder.
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