Non-collinear wave-mixing has been shown to be an effective technique for evaluating the amount of oxidative aging in asphalt concrete. Transducers can be mounted on angle wedges, so that wave-mixing can be employed with subsurface dilatational waves, which propagate close to the surface. This allows for inspection to take place when there is access to only one side of the asphalt concrete, i.e., the top surface of a pavement. The incident angles in the previous study were chosen based on known material properties (ultrasonic velocities and attenuations); however, for implementation in the field, the AC pavement properties are not known. This study introduces a modified version of the non-collinear subsurface wave mixing method which allows for practical implementation in the field. One fixed incident angle can be found which is suitable for a range of oxidatively aged asphalt concrete specimens (0 to 36 hours). For the mixture-type used in this study, the most suitable fixed incident angle was found to be 73°. Experimental evidence supporting the validity of this technique is presented. It is hypothesized that the presence of side-lobes, along with the large amount of beam spread, permits the nonlinear interaction to occur and be detected. Selection criteria were used to verify that the observed nonlinearities were a result of bulk wave interaction within the asphalt concrete, and not due to nonlinearities in the testing apparatus.