4. Conclusions
This paper discusses a laboratory process that can potentially be used for the measurement of dynamic modulus (quality control aspects and structural design) of asphalt pavements by direct ultrasonic testing. Two mix types (dense and porous) were analyzed to evaluate in the study the frequency dispersion of wave velocity in asphalt material. However, to implement the process effectively, more varieties of asphalt mixtures should be considered for the development of standard specifications in Spain. Finally, the following remarks can be made on the basis of this research:
Dynamic modulus calculated by ultrasonics is higher than that obtained by dynamic test (taking as a reference the result obtained at 10 Hz). The increase of moduli magnitude can be associated with an increase in frequency of the type of test used but may also be due to different testing methods, varying the frequency dispersion of wave velocity in asphalt material.
Measurements of dynamic modulus by ultrasonics may re-place the low frequency standard dynamic test, which is more expensive, difficult and time consuming, by just applying a correction factor on the calculated modulus. For the two mixes investigated these correction factors were 0.34 (dense mix) and 0.40 (porous mix). So these values can be used as a reference value for dynamic modulus of asphalt pavements at low strain.