Impact echo measurements
With railway and motorway tunnels which, using the blast excavation method, are built as a twin-shell vaulted construction with an inner shell of vaulted concrete, the reference thickness of the tunnel inner shell must be 2-dimensionally inspected for reasons of quality assurance, whereby the apex area and the block joints are analysed in particular. We use the impact echo measurement process for surveying the tunnel inner shell thickness.
In the past, leakages occurred in several tunnel building works, the cause of which was not traced to a defective seam but to deterioration due to exposed reinforcement.
Mined tunnels with sealing on the rock side tend to leak where the reinforcement is exposed on the rock side. The damage then always occurs when the seal is pressed onto the exposed reinforcement by the outer pressure which is present. If the reinforcement cage is correctly built in, then this damaging mechanism can only occur where the necessary minimum thickness/reference thickness is not achieved.
The basis for the implementation of the survey and the qualification of the personnel is the German "Guideline for the Application of Non-Destructive Testing of Tunnel Linings" (RI-ZFP-TU).
The measurement raster covers two measurement tracks each at the start and end of the block in the area 3 o'clock to 9 o'clock. In the apex area, five measurement tracks are implemented over the entire block length. The maximum distance between the measurement tracks may not exceed 80cm as per the guidelines.
The rate of spread of the acoustic waves in the concrete is determined at open apex cleft sockets (spy holes). Non-destructive measurements are first carried out at various points on each block, then manual thickness measurements are performed. The acoustic velocity is then calculated with v= 2d * f (m/s).
After concluding the survey the measurement report per block in accordance with RI-ZFP-TU is made available with graphic and tabular illustration of the results in addition to an evaluation of the survey with recommendations for further action. In the evaluation, the acoustic velocity for the respective block was used or respectively a value corresponding to the concrete age was used with blocks which could not be calibrated.