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  Sprayed concrete strength monitoring using thermal imaging at Bond Street Station Upgrade

  As a sprayed concrete lined tunnel is advanced, it is important to monitor the strength development of sprayed concrete to ensure the workers are not at risk from falls of sprayed concrete, and that the lining has sufficient strength to support the ground loads and maintain stability of the tunnel. This is usually achieved using a combination of needle penetrometer and stud driving tests in a panel sprayed immediately after the lining has been sprayed. The biggest problem is that these tests are local tests on very small areas of sprayed concrete, and it is assumed that they are representative of the whole tunnel lining. This paper describes a new method of monitoring sprayed concrete strength gain, using an Arrhenius equation based maturity method that can calculate strength based on temperature history. Laboratory and on-site calibration tests are used to provide the parameters for the software, and a thermal imaging camera is used to measure the temperature. So the strength of the whole lining can now be monitored from a safe position as easily as taking a photograph. This paper will give details and results of a full-scale beta trial at London Underground's Bond Street Station Upgrade..

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