In this large-scale field study we use a prototype impact-echo scanner to detect delaminations on a concrete bridge deck surveying in total over 17,000 m 2 . Delaminated bridge sections are known from manual sounding and coring. A large-scale damage assessment is necessary to identify the need of repair work. Based on first results, two lanes of the bridge are repaired and in subsequent tests the bonding of fresh and old concrete is examined. It shows that delaminations found on the bridge deck surface are unevenly distributed with more defects on the southbound lanes. This indicates constructional problems during the concrete placement. The developed scanner excites stress waves by dropping steel solenoids on the surface and recording the impact-echo frequency by air-coupled microphone arrays. It is pushed at a speed of 600 m/h, hitting the surface 300 times per square meter. The recorded data is preprocessed on site with an automated delamination detection threshold implemented. Found delaminations are transferred to a bridge map to allow an overall damage assessment.

This study investigates the non-destructive detection of delaminations in concrete plates using non-contact laser ablation, instead of the conventional hammer excitation, as part of the impact echo method. We performed tests on five concrete specimens of different sizes, two of which contained artificial delaminations. A range of steel ball hammers was used as reference impulse sources, the responses of which were compared with wave excitation generated by a 7 ns pulsed 1064 nm Nd:YAG laser with 150 mJ pulse energy. Signals were recorded by surface-mounted accelerometers and two contactless methods: microphones and a laser Doppler vibrometer. The laser generates frequencies across a broad range of frequencies (0 to 150 kHz) but with much less energy than the hammers' narrower frequency spectra; the laser pulse energy transferred into the specimen is 0.07 mJ, corresponding to about 0.5 ‰ of the impulse source energy. Because of this, the thick intact plates' characteristic thickness stretch resonance frequency can be reliably detected by the hammer excitations but not when using laser excitation. However, the laser can excite low-frequency flexural vibration modes over a shallow delamination at 3 cm depth. The low-frequency flexural vibration results are verified by numerical natural frequency analysis.

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