Monitoring and evaluation of self-healing in concrete using diffuse ultrasound

The progression of self-healing of cracked concrete is monitored in situ by diffuse ultrasound, in a novel application of this technique. The diffuse ultrasonic measurements are conducted on uncracked, tensile through-thickness cracked, and flexure partial-thickness cracked concrete specimens. Specimens made from three different mix designs are unbonded post-tensioned with a precompression force of 6.2 MPa that allows for the generation of cracks with widths less than 200 μm that should exhibit self-healing and are then exposed to a simulated marine environment. To estimate the extent of crack healing over time, two diffuse ultrasonic parameters are measured: an effective diffusivity and an arrival time of maximum energy (ATME). In addition, microscopic measurements of crack width on the specimen surface are taken over a 4 month exposure time period. Initial cracking causes an increase in the ATME and a significant decrease in diffusivity as compared to those of the reference, uncracked control specimens. With the progress of self-healing, the ATME of both tensile and flexure cracked specimens decreases, while the diffusivity increases and recovers the initial values found in the uncracked control specimens. These changes in the ATME and diffusivity suggest autogenous healing of the cracks. Comparisons of experimental results for these two parameters show that the diffusivity predicts self-healing trends in a more effective way than the ATME. A simple model is used to evaluate healing rates in different concrete mixes. The healing rates determined for the three mix designs are consistent with the results of independent observations. Finally, it is concluded that the present diffuse ultrasonic method is capable of monitoring and evaluating the self-healing of concrete.

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