Evaluation of interface fracture in model concrete

The effectiveness of forced vibration testing to detect interface fracture within model concrete is examined. Model concrete cylinders containing spherical glass aggregates or voids were cast. Quasi-static load was applied to induce interface fracture between the aggregates and mortar. Resonant frequency was measured from longitudinal standing vibration of the cylinders. Results indicated that the resonant frequency decreased as the amount of voids or load increased. Wave propagation analyses were then used to interpret the experimental results. The calculations indicated that the waves diffracted around the voids during standing vibration, and the diffracted waves decreased the resonant frequency. A similar result was also shown in the cylinders containing interface fractures, since they act as discontinuities along the aggregate-mortar boundaries. Furthermore, the damage conditions within the composite material were also evaluated using these testing and calculation methods. In the load tests, internal cracks were relatively stable in the plain mortar cylinders in the vicinity of maximum load, whereas internal damage of the cylinders containing the glass aggregates developed quickly. The effectiveness of forced vibration testing was demonstrated not only in identifying interface fracture within composite materials, but also in measuring the degree of resulting damage.