There is described a system for determining air content of fresh concrete received in a drum of a concrete mixer. The system generally has a high energy photon source mounted to the drum and emitting high energy photons towards a photon path rotating about a rotation axis as the drum rotates, the photon path is immersed in the fresh concrete during rotation, a photon detector mounted to the drum, the photon detector counting high energy photons received from the photon path; and generating a signal indicative of a number of counted events; a computing device determining an air content value of the fresh concrete based on the generated signal and on reference data; comparing the determined air content value to an air content threshold; and generating an alert based on the comparison to be displayed.