Fig. 1. Schematic of laser acoustic. (a) Thermoelastic effect; (b) Ablation effect

Fig. 2. Diagram of sound wave reflection

Fig. 3. Experimental setup diagram for detection of internal cavities in concrete with laser acoustic

Fig. 4. Schematic diagram of experiment device. (a) Schematic diagram of hammer impact experiment; (b) Schematic diagram of concrete

Fig. 5. Acceleration-time graph. (a) Hammer impacts the area without defects; (b) Hammer impacts the area with defects; (c) Laser impacts the area without defects; (d) Laser impacts the area with defects

Fig. 6. Fourier spectrum of hammer impacting. (a) Area without defects; (b) d=30 mm; (c) d=100 mm; (d) d=150 mm

Fig. 7. Fourier spectrum of laser impacting. (a) Area without defects; (b) d=30 mm; (c) d=100 mm; (d) d=150 mm

Fig. 8. Relationship between vibration energy and laser pulse energy at different spot diameters

Fig. 9. Layout of measurement points. (a) Schematic diagram; (b) Picture of the experimental layout

Fig. 10. Echo energy at different distance between impact position and sensor. (a) Depth=30 mm; (b) Depth=100 mm; (c) Depth=150 mm

Fig. 11. Comparison of vibration energy in the normal area and the defective area