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 indir ect tension test, and the Marshall test. Four asphalt mixtures with di fferent top stone sizes ranging from 15.8 to 31.5 mm were used. They a re B1 mix (base course), Airport, W3, and W6 mixes (wearing course). S pecimens, 5-in. (127.0 mm) in size, were prepared using the same compa ction equipment as 6-in. (152.4-mm) specimens, designed as by Kandhal, P. S. (''Large Stone Asphalt Mixes: Design and Construction,'' Procee dings of the Association of Asphalt Paving Technologists, 1990). The h eight of the 5-in. (127.0-mm) diameter specimen was calculated based o n 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 e nergy per unit volume ratio of 4- and 6-in. (101.6- and 152.4 mm) diam eter specimens. Similar measured densities of the 5-in. (127.0-mm) spe cimens 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 a chieved. The stability ratios (stability of 6-in./5-in. specimen, stab ility 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 report ed by Kandhal, P.S. (''Large Stone Asphalt Mixes: Design and Construct ion,'' Proceedings of the Association of Asphalt Paving Technologists, 1990). Detailed comparison of flow values for the W3 and B1 mixes wer e 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 we re 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 unde r 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 resili ent modulus and indirect tensile strength was observed. The trend sugg ested that 5- and 6-in. diameter specimens would give more realistic v alues of tensile strength and stiffness for mixes using large stones ( larger than I-in.). When larger diameter specimens are used, the resul ts obtained from the resilient modulus and the indirect tension test a re more representative of the behavior of the mixtures.