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2016SummerShotcreteEMag

Goin’ Underground any strength testing inside a lab and so this requires real time field testing. Field Application Field trials were undertaken during primary shotcrete lining works at Whitechapel station, being constructed by BBMV, a joint venture of Balfour Beatty, BeMo Tunnelling, Morgan Sindall and Vinci Construction. The scope of field application was limited to collecting real-time thermomechanical data. Mechanical Testing Due to the importance of early age strength development, stringent testing criteria requiring in-situ testing, such as needle penetrometer and stud driving (BS EN 14488-2 2006), were specified. These tests were conducted separately on the panels sprayed immediately after the lining spray was finished. Table 1 describes the typical details of the testing methods used during the field testing. Thermal Imaging The temperature variations in the early age of concrete, mainly caused by hydrating portland cement, can be measured by thermal imaging using a camera with the capability of detecting infrared (IR) radiations. A FLIR E60bx camera was used. Figures 3(a) and (b) show digital and thermal images, respectively, of a shotcrete lining section demonstrating how thermal imaging can measure the temperature remotely. For the shotcrete panels only the top surface was imaged whereas for the lining, surface areas of key locations such as crown, shoulders, and axis level are monitored. Site Testing The following testing procedure was adopted for the field trials in order to validate the method (this will not be the procedure when SMUTI is used for systematic monitoring): 1. Select appropriate lining section; 2. Prepare five shotcrete panels, corresponding to lining section, for mechanical testing and thermal imaging purposes; and 3. Thermal imaging of lining section. The shotcrete mixture design is shown in Table 2. Results and Discussion The following section discusses results corresponding to testing and thermal imaging of a Table 1: Tests Performed on Shotcrete Panels Stage Test Type Strength Range (equivalent cylinder compressive strength) Time and Frequency Typical Test Apparatus 1 Penetration needle (panels only) 14.5 to 145 psi (0.1 to 1.0 MPa) Up to 1 hour Minutes – 15, 30, 60 Meyco Penetrometer 2 Stud driving (panels only) 290 to 2320 psi (2.0 to 16.0 MPa) Up to 24 hours Hours – 3, 6, 12, 24 Hilti DX 450-SCT (a) (b) Fig. 3: (a) Digital and (b) thermal images taken during the early age of shotcrete lining demonstrating the ability of thermal imaging to measure temperatures remotely 50 Shotcrete • Summer 2016


2016SummerShotcreteEMag
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