In this report, work carried out for the Bridges Division of the Transport and Road Research Laboratory (TRRL) into the feasibility of detecting and mapping internal cracks within concrete structures caused by the alkali-silica reaction (asr) is described. A literature search was carried out and several techniques identified, and briefly described in this report. Three methods: ultrasonic technique, impulse radar technique and computerised tomography appeared to fulfill the criteria for the technique, which were: (1) the ability to detect and map cracks within concrete members and give their positions and orientations relative to the member's surface; (2) to discriminate between cracks, reinforcement irrespective of other embedded materials; and (3) be a non destructive technique. The ultrasonic technique was assessed using a commercial ultrasonic tester, wide and deep cracks could be defined, but ambiguous results were obtained for cracks whose depth gradually increased. The impulse radar technique, a medium cost solution, had operation advantages in rapid survey time, but would require modification to achieve higher resolution and hence determination of the crack dimensions. The radar showed a capability of imaging internal reinforcing. Computerised tomography, the slowest and most expensive technique met nearly all the technical requirements, and was shown to have high potential for imaging very small cracks, even when close together and near steel reinforcement.

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