Volume 23, Issue 6 (November 1995)
Specimen Size Effects on the Diametrical Mechanical Testing of Cylindrical Asphalt Mixes
Specimens, 4-in. (101.6 mm), 5-in. (127.0 mm), and 6-in. (152.4 mm) in diameter, of asphalt mixtures were prepared in the laboratory to test for the specimen size effect on the resilient modulus test, the indirect tension test, and the Marshall test. Four asphalt mixtures with different top stone sizes ranging from 15.8 to 31.5 mm were used. They are B1 mix (base course), Airport, W3, and W6 mixes (wearing course).
Specimens, 5-in. (127.0 mm) in size, were prepared using the same compaction equipment as 6-in. (152.4-mm) specimens, designed as by Kandhal, P. S. (“Large Stone Asphalt Mixes: Design and Construction,” Proceedings of the Association of Asphalt Paving Technologists, 1990). The height of the 5-in. (127.0-mm) diameter specimen was calculated based on the diameter over the height ratios for the 4- and 6-in. (101.6- and 152.4-mm) specimens. The compaction level required was based on the energy per unit volume ratio of 4- and 6-in. (101.6- and 152.4 mm) diameter specimens. Similar measured densities of the 5-in. (127.0-mm) specimens compared with the densities of the 4- and 6-in. (101.6- and 152.4-mm) specimens indicate that the same level of compaction has been achieved.
The stability ratios (stability of 6-in./5-in. specimen, stability of 6-in./4-in. specimen and stability of 5-in./4-in.) were also analyzed. They were found to be close to the theoretical values reported by Kandhal, P. S. (“Large Stone Asphalt Mixes: Design and Construction,” Proceedings of the Association of Asphalt Paving Technologists, 1990). Detailed comparison of flow values for the W3 and B1 mixes were made. The B1 mix gave an unacceptable flow/diameter ratio caused by the effect of large stones. The value was too low in the case of the 4-in. (101.6-mm) specimen for the B1 mix. However, acceptable values were found for all sizes tested using the W3 mix, which had a top stone size less than 1-in. Hence, large stones (stones with diameter greater than 1-in.) in 4-in. specimens tend to produce lower deformation under loading, which also accounts for the higher modulus measured in the resilient modulus test.
The influence of the ratios of the diameter of specimens to top stone size were determined for the resilient modulus and the indirect tension test. A general trend of reduction in resilient modulus and indirect tensile strength was observed. The trend suggested that 5- and 6-in. diameter specimens would give more realistic values of tensile strength and stiffness for mixes using large stones (larger than 1-in.). When larger diameter specimens are used, the results obtained from the resilient modulus and the indirect tension test are more representative of the behavior of the mixtures.