Rayleigh surface waves for characterization of the air void system in fresh concrete

The durability of concrete pavement to freeze/thaw cycles is mainly dependent on the air void system. Air entraining admixtures are used to provide a beneficial air void system. Large entrapped air voids reduce strength and make it insufficient to simply characterize the porosity by the air content; void spacing and specific surface parameters are also important. Ability to perform quality assurance testing - nondestructive evaluation - of concrete pavement soon after placement is highly desirable. Thus, laboratory experiments have been conducted to investigate Rayleigh surface waves for characterization of porosity in fresh concrete. A mediator mounted onto a Plexiglas wedge is used to introduce waves from an ultrasonic transducer onto the surface of the concrete. The challenges are that fresh concrete is highly attenuative and that the material properties evolve as the concrete sets. Rayleigh wave speed is shown to be sensitive to porosity by simple micromechanical modeling, and results are presented for normal concrete with large aggregate, sieved concrete, and mortar. Wave speeds are significantly less (10-22% depending on time after placement) for concrete with approximately 5% porosity relative to concrete with no air entrainment admixture.