Volume 4, Issue 1 (January 2007)
Criterion Test for Method Selection in Determining the Bulk Specific Gravity of Hot-Mix Asphalt
A number of test methods are available to measure the bulk specific gravity of compacted hot-mix asphalt (HMA). The most commonly used test method, ASTM D 2726 , was designed for use with specimens that do not contain open or interconnected air voids such as dense-graded HMA mixtures. Coarse and open-graded HMA mixes are known for their open or interconnected void space. The current method for addressing coarse and open-graded mixes, ASTM D 1188 , can lead to erroneous bulk specific gravity measurements. Thus a nondestructive vacuum sealing method (ASTM D 6752)  was developed to address HMA mixtures that contain open or interconnected air voids. The objective of this research is to define the point at which the vacuum sealing method (ASTM D 6752)  more accurately measures the bulk specific gravity of an HMA specimen as compared to the conventional test (ASTM D 2726)  utilizing the saturated surface dry (SSD) method. Permeability (ASTM PS 129-01)  measures an HMA mixture's ability to allow moisture to pass through it, which in turn is related to the mixture's connectivity of void space. For this reason permeability was chosen as the fundamental property to investigate. ASTM PS 129-01  is hypothesized as a criterion test for method selection in determining the appropriate bulk specific gravity test for coarse-graded HMA mixtures. One hundred and thirty-four field compacted roadway cores from 28 paving projects throughout the State of Michigan were evaluated using the vacuum sealing method and conventional SSD method of determining the bulk specific gravity of compacted HMA. These cores were then tested using a falling head permeability apparatus. It was determined that the bulk specific gravity of compacted HMA fine-graded mixes using the vacuum sealing method and conventional method are statistically the same. There is a difference in bulk specific gravity measurements when using the two bulk specific gravity tests on coarse-graded mixtures for roadway cores when the air void level is above 5.1 %. Thus the air void/permeability relationship was used to determine the coefficient of permeability at which the vacuum sealing method provides a more accurate measurement of the bulk specific gravity. This value was found to be 3.7×10-6 cm/s.