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Fibre Bragg grating sensors are embedded into concrete cylinders to monitor cracking deep within the specimens. Loading cycles and loading till failure tests are performed, with measurements from the sensors collected up to, and during, failure. Calibration of the off-axis angle of the optical fibre sensors is performed and the spread of this angle noted. The use of the bandwidth of the gratings to indirectly measure the differential strain across the grating, and thus the amount of internal cracking within the specimens, is proposed and tested. The results accord well with the theoretically predicted behaviour of bulk concrete. © 2008 IOP Publishing Ltd.