Caracterização viscoelástica linear de misturas asfálticas por meio de ensaios ultrassônicos

Abstract

With the evolution of pavement engineering towards increasingly mechanistic methods of design, non-destructive tests to characterize properties related to performance will be necessary to control the evaluation of asphalt mixtures in the field. In this dissertation, ultrasonic tests were applied to asphalt mixtures with the objective of characterizing the linear viscoelastic behavior, through the parameters known as dynamic modulus and Poisson's ratio. Ultrasonics velocity pulse tests produce mechanical waves that propagate in samples of known geometry. The waves’ flight time is measured to calculate their speed. For a given type of wave and isotropic linear elastic materials, the velocity depends only on the specific gravity, modulus of elasticity and Poisson's ratio (Wave Equation). Knowing the specific gravity value and the calculated waves' velocities, we have an equation as a function of the modulus and the Poisson's ratio, with these two variables dependent on each other. Such an equation can be adapted for isotropic linear viscoelastic materials, with the main difference being the influence of phase angle values on wave attenuation. Typically, tests are performed with compression wave transducers (P waves), and fixed values of Poisson's ratio are adopted to estimate an approximate value of modulus (first analysis). Alternatively, a second equation must be produced, by means of measurements with another type of wave, such as the time of flight of shear waves (S waves), in order to simultaneously determine the modulus and Poisson's ratio (second analysis). The results were systematically validated with those obtained from the classic complex module test (DNIT, 2019), whose absolute value is represented by |E*|, calibrated using the 2S2P1D rheological model. A sensitivity analysis of the required rheological parameters of the Wave Equation was performed, verifying the results of |E*|. Simplifying hypotheses were assumed, which involve considering the null phase angle and the Poisson ratio constant value and adopted by the hypothesis (values from 0.25 to 0.40), which resulted in satisfactory errors, considering variations of temperature (28.6oC, 8.3oC and 0.7oC) and the investigated frequency (54kHz). In the second analysis, it was studied the obtaining of rheological parameters of mixing, from tests of propagation of waves of Compression and Shear simultaneously. It was observed that the propagation time of the S wave, as long as it is properly determined (adopting the value of time of flight until the second perturbation) is decisive in the value of Poisson's ratio. The process adopted resulted in acceptable errors, allowing to obtain the |E*| without the need to adopt fixed and unverified values of Poisson's ratio, as in the first analysis. This work contributed by presenting strategies for non-destructive characterization of asphalt mixtures that can be adapted for technological control and field evaluations. It is expected that with the adoption of increasingly mechanistic methods, the control can evolve to evaluate composition, provide measures directly related to performance, as the dynamic modulus of asphalt mixtures.