Evaluating Foamed Asphalt Stability Using Acoustic Emission Techniques

This paper focuses on the feasibility of using acoustic emission (AE) techniques to evaluate stability of foams produced from paving-grade asphalt binders. The study compared asphalt foamability index [expansion ratio (ER) and half-life (H-L)] measurements with AE measurements for the same foamed asphalt produced using a Wirtgen Model WLB 10 S Laboratory Asphalt Foaming Device. Both ER and H-L are used to characterize the suitability of asphalt binders to produce good-quality foamed asphalt binders for pavement construction. However, current methods for determining foamability are considered as empirical, operator-dependent, and cannot be used for continuous monitoring of the foaming process. Two performance-graded asphalt binders, four foaming water contents, and two foaming temperatures were used. Spectral analyses using a test for white noise suggest AE signals resulting from asphalt foaming are not white noise ($p\text{-}\mathrm{value}<0.0001$) and therefore could be useful for evaluating foamed asphalt stability. The evolution with time of AE amplitude during foamed asphalt decay suggests two distinct stages related to foam stability. A clear amplitude dynamics characterized by shorter-duration lower-amplitude foam decay followed by longer-duration higher-amplitude foam collapse was found for all the asphalt binders tested. AE energy, a measure of elastic energy released by collapsing foam bubbles, was found to be sensitive to H-L. The results demonstrate that AE may be useful for characterizing foaming characteristics in a more fundamental manner than existing manual methods. The techniques and results summarized here provide the basis for further investigation into application of AE measurements to monitor and control foamed asphalt stability.