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2015SummerShotcreteEMag

Goin’ Underground turing of steel fibers at crack widths, which exceed maximum crack widths when used in combination the elongation capacity of the fiber. The fibers with conventional reinforcing bars. break rather than being pulled out of the concrete matrix. This effect leads to performance degrada- Conclusions tion by primarily affecting the capability to react The structural performance of MSFRC or to a change of loading conditions while the struc- MSFRS is able to meet or exceed that of welded- ture is aging. Typical examples for changed wire reinforcement or steel fiber-reinforced con- loading conditions in tunneling are nearby under- crete for tunnel lining applications. Design methods ground or subsurface construction, seismic for the structural load-bearing capacity are similar. loading, or changes in hydrological conditions or Macrosynthetic fibers are easy to pump and tidal effects. For this reason, satisfactory perfor- apply using the shotcrete process, reduce the wear mance at early ages (around 28 days) does not and tear on pumps and slick lines and are easier guarantee an acceptable performance of the aging and safer to handle than steel fibers. steel FRC. The performance of steel FRC at crack Macrosynthetic fibers are not susceptible to widths in excess of 0.04 in. (1.0 mm) can decrease corrosion and do not have to meet stringent crack by as much as 50% compared to the optimum width limitations for durability. High-perfor- exhibited at early ages; thus, a performance- mance macrosynthetic fiber reinforcement is ideal reduction factor should be applied to the long-term for aggressive exposure conditions and guarantees flexural resistance of steel FRC.15,16 durable performance over the design life cycle MSFRC is largely unaffected by this phenom- without suffering matrix embrittlement and per- enon because post-hardening or changes in paste formance loss with age. hardness make little difference to the behavior of The inherent isolated creep properties of mac- the fiber within the composite beyond the first few rosynthetic fiber reinforcement play a subordinate days of hardening. The performance of MSFRC role in the long-term performance of tunnel linings evident at 28 days is therefore unaffected over time. because compression forces typically govern overall behavior. Due to the advantages discussed Creep Considerations in this article, it is to be expected that macrosyn- In general, the magnitude of creep deformation thetic fibers will become more common in tunnel in uncracked shotcrete does not depend on the shotcrete and cast-in-place tunnel linings in the type of reinforcement and is similar for a centrally near future. layered light steel welded-wire reinforcement, steel fibers, or macrosynthetic fibers.18 In cracked References concrete, however, the load ratio (applied creep 1. Bernard, E.S.; Clements, M.J.K.; Duffield, S.B.; and load over static capacity) governs creep defor- Morgan, D.R., “Development of Macro Synthetic Fiber Rein- mation. There is only a minor difference in the forced Shotcrete in Australia,” 7th International Symposium on Sprayed Concrete – Modern Use of Wet Mix Sprayed Concrete performance of FRC in combination with rein- for Underground Support, Sandefjord, Norway, June 16-19, 2014. forcement with a light steel welded-wire rein- 2. Ireland, T. J., and Stephenson, S., “Design and Construc- forcement or macrosynthetic fibers up to load tion of a Permanent Shotcrete Lining – The A3 Hindhead ratios of 50% during a loading period of 100 days. Project, UK,” Shotcrete: More Engineering Developments, E. The requirement for long-term testing of S. Bernard, ed., CRC Press, Boca Raton, FL, 2010, pp. 143-152. MSFRC is only necessary when long-term tensile 3. Gonzalez, M.; Kitson, M.; Mares, D.; Muir, B.; Nye, E.; and Schroeter, T., “The North Strathfield Rail Underpass stress is expected to be imposed on a cracked sec- – Driven Tunnel Design and Construction,” 15th Australian tion in service. However, this loading regime Tunnelling Conference 2014, Sydney, Australia, Sept. 17-19, seldom exists in tunnel linings, which are typically 2014, pp. 369-374. loaded under compression. Thus, the concerns 4. Carlson, J.E., Wotring, D.C., Auber, R.J., and Vitale, which have been raised about the long-term per- M.G., “Synthetic-Fiber Reinforcement for the Cast-in-Place formance of macrosynthetic fibers in respect of Final Tunnel Liner at the Euclid Creek Tunnel Project,” Proceedings, Rapid Excavation and Tunneling Conference, creep and the associated consequences for crack June 7-10, 2015, New Orleans, LA, pp. 882-889. width development with time under sustained 5. Shotcreting in Australia: Recommended Practice, flexural loads appear to be significantly overstated. second edition, Concrete Institute of Australia, Sydney, Aus- The results of recent research19 show that the inclu- tralia, 2010, 67 pp. sion of macrosynthetic fibers in the concrete has 6. Barton, N., and Grimstad, E., “The Q-System Following only a minor effect on the flexural strength of the Projects,” Rock Engineering - Theory and Practice, Proceed-Thirty Years of Development and Application in Tunnelling cross section, but the fibers reduce time-dependent ings of the ISRM Regional Symposium EUROCK 2004, Sal- in-service deformations and significantly reduce zburg, Austria, 2004, pp. 15-18. 52 Shotcrete • Summer 2015


2015SummerShotcreteEMag
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