Use of ultrasound for the evaluation of density homogeneity in trapezoidal asphalt concrete test specimens

Abstract

In the French methodology, two of the most important tests for the elaboration of asphalt concrete mixtures are: the fatigue test and the complex modulus test. These tests are made with trapezoidal samples extracted from pre-compacted blocks, either extracted from the field or prepared at the laboratory, which must have uniform granular distribution and void homogeneity. Generally, the method used to evaluate the homogeneity of the block is the gamma densitometer; however, this equipment is not available. For this reason, with the ultrasound technique, we look for a way to evaluate the homogeneity of the blocks while addressing the precision and statistic limitations showed in the test procedures for fatigue and modulus. This work evaluates the use of the ultrasound method by the direct transmission mode, on asphalt concrete blocks to determine the wave's propagation speed. One of the choosing criteria for the trapezoidal samples which will be used for the fatigue and complex modulus tests is the void percentage. The resulting values of the void percentages for the trapezoidal samples were compared with the wave speeds obtained by the ultrasound test. With the purpose of facilitate the data analysis, the results are presented in a graphic form.
This study verified that, even though the ultrasound test can detect the homogeneity variation in the asphalt concrete blocks, the precision level of the equipment being used, doesn't allow yet the use of this test with the objective of selecting the blocks to perform the fatigue and complex modulus tests, because the selection of the final test samples is very rigorous.