Development of a semicircular bend (SCB) test method for performance testing of Nebraska asphalt mixtures.
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2015-12-01
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Abstract:Granted that most distresses in asphalt (flexible) concrete (AC) pavements are directly related to fracture, it becomes clear
that identifying and characterizing fracture properties of AC mixtures is a critical step towards a better pavement design.
This report examines the testing variables of a reliable and practical semicircular bending (SCB) test for evaluating the
fracture characteristics of asphalt concrete mixtures at intermediate service temperature conditions. The first part of this
report investigates the repeatability of the SCB fracture test method by integrating a statistical-experimental approach to
identify testing variables of the SCB test that result in repeatable test results. Toward this end, five testing variables (the
number of testing specimens, specimen thickness, notch length, loading rate, and testing temperature) of the SCB test were
investigated due to their significant effects on mixture fracture characteristics. After statistical analysis of 18 specimens
tested with a typical testing variables, approximately, five to six specimens/replicates were found to be a reasonable sample
size that could properly represent asphalt concrete fracture behavior using the SCB test method. The coefficient of variation
(COV) of the mixture fracture energy was used to evaluate the effect of each variable on the repeatability of test results. A
range of 1 mm/min. to 5 mm/min. for the loading rate, a notch length from 5 mm to 25 mm, and a specimen thickness of
40 mm to 60 mm and a testing temperature of 15-40°C showed a good repeatability of fracture energy with small COVs
(≤15 %). The second part of this work is to investigate the sensitivity of the SCB test using the previously determined
testing variables. Fourteen different asphalt concrete (AC) mixtures collected from 12 field construction projects in
Nebraska were used in this task. The ANOVA test showed statistically significant differences between mixtures at a 95%
confidence level. Tukey’s HSD multiple-comparison analysis found similarities within mixtures of same types and
differences between mixture types. In addition, the fracture energy of bituminous mixtures increased with increasing amount
of virgin asphalt content in mixture. Overall, the SCB test method developed herein proved to be repeatable and sensitive
to changes in mixtures, and thus a promising tool for evaluating the fatigue fracture resistance of AC mixtures.
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