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2014FallShotcreteEMag

Variability of Compressive Strength of Shotcrete in a Tunnel-Lining Project By Lihe (John) Zhang RSpace Association (ITA) about what shotcrete were as follows.Performance requirements for the wet-mixecently, questions have been asked by theInternational Tunnelling and Underground variability can be expected for compressive Specifications strength for shotcrete (sprayed concrete) in ground • Compressive strength: 1450 psi (10 MPa) at support projects. The compressive strength of 3 days, 2900 psi (20 MPa) at 7 days, 5000 psi concrete and shotcrete is mainly controlled by (35 MPa) at 28 days tested to ASTM C1604/ mixture design parameters, including the water- C1604M; cementitious material ratio (w/cm). With shotcrete, • Boiled absorption: <8% tested to ASTM C642; other factors, including the skill set of the nozzleman and applying the shotcrete, can also affect compres- • Volume of permeable voids: <17% tested to sive strength. The shotcrete method used—wet- ASTM C642. versus dry-mix and accelerator addition at the To meet the performance requirements, a wet- nozzle—may also affect compressive strength mix shotcrete with silica fume was designed, as and variability. shown in Table 1. During recent shotcrete ground support projects, This mixture design used 10% silica fume by the author conducted routine quality control (QC) mass of cement to achieve the durability require- testing for compressive strength of shotcrete. Data ment. A hydration-controlling admixture was used from these projects showed that the compressive to extend the plastic life of the shotcrete for up to strength is dependent on the application method 8 hours. To apply overhead shotcrete with minimal used. For one tunnel project, over 6500 yd3 fallout, an alkali-free accelerator was added at the (5000 m3) of accelerated wet-mix shotcrete was nozzle at dosages of 6% by mass of cement. The applied and over 700 sets of cores (two cores for higher dosages of accelerator were used in wet each set) were tested for compressive strength ground conditions and where thicker shotcrete at 7 and 28 days of age. This paper provides a encapsulations were required. statistical analysis of the shotcrete compressive strength test data from a civil tunnel project in Shotcrete Quality Control Testing western Canada and shows that the variability in The project required hand nozzling and all of compressive strength is influenced by a number the nozzlemen were American Concrete Institute of factors. (ACI) Certified Nozzlemen. Two levels of qualifications were required: Level I—each Introduction nozzleman was required to shoot a vertical test During the construction of a tunnel project in panel and an overhead test panel, and cores were western Canada, a total of over 6500 yd3 (5000 m3) extracted from each panel and tested for compres- of shotcrete was applied as the final lining. The sive strength, boiled absorption, and volume of tunnel was lined with shotcrete reinforced with permeable voids as required by the project one or more types of reinforcement, including specification; Level II—each nozzleman was rock anchor bolts, steel mesh, steel sets, or ring qualified specifically to the underground rein- beams, depending on the ground conditions. Wet- forcement shooting, underground ring beams, mix shotcrete with accelerator added at the nozzle steel girders, and wire mesh reinforcement. was used for the project. The project design During the construction stage, one QC testing required shotcrete to be applied to an average panel was shot for each nozzleman for each day thickness of 4 in. (100 mm), with a minimum 2 in. when shotcrete was applied. Four cores with (50 mm) cover over steel sets or ring beams. diameters of 2.75 in. (70 mm) were extracted for 22 Shotcrete • Fall 2014


2014FallShotcreteEMag
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