where Eu is the dynamic modulus (MPa), h the sample height
(mm), W the sample mass (kg), d the sample diameter (mm), t the propagation time (ms), and n the Poisson’s ratio.
Finally, the objective of this study is to measure the dynamic modulus of two asphalt mix types (dense and porous) applying ultrasonic direct test. These are compared to values of dynamic modulus obtained with the standard dynamic test applied in Spain, which is more expensive, difficult and time consuming, by obtaining correction factors.
2. Experimental procedure
The tasks carried out to physically characterize the dense and porous asphalt mixtures used in the tests are described below. Further, the procedures used to obtain dynamic modulus thereof are described, following both the ultrasonic direct test and standard dynamic test.
2.1. Physical characterization of asphalt mixes
The following physical characteristics were identified: relative densities, air voids, stabilities and plastic deformation, the last being applicable only to dense asphalt mixtures. The determina-tion of apparent relative densities and percentage of air voids in compacted asphalt mixtures was obtained following the Spanish standard NLT-168/90 [8]. The materials used to manufacture the samples corresponded to dense (D-12 limestone) and porous (PA-12 dolerite) asphalt mixtures. The granular structure of these asphalt mixes can be observed in Fig. 1. In addition, Tables 1 and 2 show the design gradation curve of the mixtures used in this research.
Calculation of air voids, density and stability was performed on 12 cylindrical samples of 101.6 mm diameter and 63.5 mm height (six of each mixture type). Samples were prepared according to the procedure specified in the Spanish standard NLT-159/00 [9]. This procedure consisted of kneading and compaction using a Marshall hammer at a temperature of 150 1C. The number of blows per face was 75 for the D-12 mixture samples and 50 for the PA-12 type.
The determination of density of D-12 samples was also performed pursuant to the aforementioned standard, whereby the mass, in grammes, per sample was obtained under the following conditions: sample in water, saturated sample super-ficially air-dried and dry sample. In this way, density values were obtained from weight differences. In the case of the PA-12 mixture, sample densities were determined from the sample geometry. This was carried out by taking four height measure-ments for each sample, which when averaged gave a reference value for the exact sample height. Hence, knowing the mean