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2015SpringShotcreteEMag

Shotcrete Corner Table 1: 11-Day Tensile Strength of Bond Test (First test: MICHELS Corporation) Specimen Result Failure Average 1A 132 Glue plane Three layers, prep scratch finish 1B 185 Glue plane 164 4 in. (100 mm) each lift 1C 174 Glue plane 2A 95 Glue plane Two layers, prep scratch finish 2B 179 Glue plane 141 6 in. (150 mm) each lift 2C 148 Glue plane 3A 95 Glue plane Three lifts, prep nozzle finish 3B 47 At layer interface 76 4 in. (100 mm) each lift 3C 84 At layer interface 4A 99 Glue plane Two lifts, prep nozzle finish 4B 32 At layer interface 58 6 in. (150 mm) each lift 4C 42 Glue plane Between second and third layer 1D 248 Glue plane Three layers, prep scratch finish 1E 215 Glue plane 200 4 in. (100 mm) each lift 1F 138 Glue plane surface caused uneven stress with the test U-frame scratched, and we have proposed to do this on that may have contributed to the lower tensile these MTACC projects. bond strength. The nozzle finish may have better Following the pull test, Superior performed the bond in other situations. The procedures followed mockup in layers. With all overhead work being and the criteria met the guidelines of ACI 506R- performed in layers, each layer was prepared and 05, “Guide to Shotcrete.” With these full-scale shot with a 2- to 14-day time lapse between lifts. tests, we have proven that layers produce structur- After the mockup was performed, cuts were ally monolithic sections when the surface is made through different locations. As you can see, the encapsulation of the reinforcing bars and water stop was excellent. Layering was not evident, and with the pull test data, this allowed Superior Gunite to proceed with the work. A follow-up test was performed for the East Side Access MTA project CM006 with another general contractor (GC), Frontier Kemper. The same procedure was followed, but only a scratch finish was prepared in the two boxes. More of the J-B Weld adhesive for gluing on the steel pucks was used on this second test to try to obtain better results. Although the additional glue raised the Fig. 5(a): Picture of glue plane failure test results, the failures were still in the glue and not between the shotcreted layers. Table 2 shows the test results (in psi). A larger mockup was performed for this project and cuts were made through the shotcreted arch to evaluate the encapsulation. In all the tests with a roughened, scratched surface preparation between layers, we were never able to break the bond between layers with the test because all the tests failed at the glue adhering the steel puck to the concrete surface. Conversely, our tests showed that shooting a subsequent layer Fig. 5(b): Picture of layer interface failure on top of an unfinished, nozzle­finished surface 36 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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