Fig. 1. Working principle of OFDR

Fig. 2. Picture of optical cable

Fig. 3. OFDR demodulation instrument

Fig. 4. Schematic of fiber arrangement. (a) Before cracking; (b) after cracking

Fig. 5. Strain curves. (a) 1 cm spatial resolution; (b) 5 mm spatial resolution; (c) 2 mm spatial resolution

Fig. 6. Comparison of monitoring results under three spatial resolutions

Fig. 7. Comparison of measured and real values of crack width

Fig. 8. Experimental scheme. (a) Actual picture; (b) schematic diagram

Fig. 9. Cracking diagram of concrete beam bottom

Fig. 10. Strain of fiber 1 at each loading level

Fig. 11. Strain of fiber 2 at each loading level

Fig. 12. Width of crack 4 at each loading level

Fig. 13. Pictures of cracks. (a) Crack with 0.10 mm width; (b) crack with 0.22 mm width; (c) crack with 0.34 mm width