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Monitoring of a concrete bulkhead

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Ultrasonic monitoring techniques have proved to be highly effective for the monitoring of critcal concrete structures. Transducers can be surface mounted or embedded within the volume prior to pouring the concrete.

The example described here is from a concrete bulkhead where an array of 24 transducers were installed within the volume prior to pouring of the concrete. These transducers were used for both passive monitoring of acoustic emission (AE) activity and active ultrasonic surveying. The system was used to monitor the change in material properties during the curing process and the subsequent behaviour of the bulkhead in response to pressure and temperature changes during its operation.

The diagram below shows the growth of a fracture that was identified from the location of AE activity. Each red dot represents an AE event. This fracture started to grow around 8 days after the concrete was poured and its nucleation and growth are clearly imaged by the AE locations. This information was used by the operators to plan and execute a programme of remedial grouting that succesfully stabilised the crack. For scale, the final fracture was around 3 m long.

At Fracture Nucleation
During Fracture Propogation
After Complete Fracture Growth
At Fracture Nucleation
During Fracture Propagation
After Complete Fracture Growth

The array of 8 transmitters and 16 receivers was used to carry out a series of ultrasonic velocity surveys at regular intervals. These monitored the variations in P and S wave velocity and amplitude during the curing of the concrete and was used to assess the change in material properties of the concrete and its subsequent behaviour.

The graph below shows the variations in P and S wave velocity and amplitude for a 30 week period after the concrete pour for a raypath that intersects the fracture decribed above. The increase in signal velocity and amplitude during the initial curing is clear, as is the sharp drop that coincides with the opening of the fracture. This fracture was of sufficient aperture that the signal could not be detected along the ray-path interesecting it until the grouting had been completed. Subsequent surveys (week 22 and beyond) show the P-wave velocity has returned to a value compatible with the continued curing of the concrete and the amplitude has returned to around 40% of the pre-fracture value. The amplitudes were smaller due to greater attenuation over the grouted fracture plane.

Concrete Survey Results

The concrete was poured in September 1998 and in June 2002, nearly 4 years later, the array continues to operate and provide essential data for monitoring the response of the key to temperature and pressure changes in the tunnel behind.

We thank AECL for their kind permission to use this data on our website.


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