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2015SummerShotcreteEMag

Goin’ Underground increasing the bearing capacity of the structural approach by reducing the number of critical dura- system as a whole. The driving design factor is bility problems, thereby allowing much greater hereby moved from the maximum bearable load flexibility in design. to a maximum allowable deformation or rotation Maximum allowable crack widths when using capacity of a plastic joint (crack), which is pro- steel bars or steel fibers are small, because cracks vided by the fiber reinforcement. act as points of rapid salt ingress to the reinforce- ment. Maximum acceptable crack widths are about Structural Considerations for 0.006 in. (0.15 mm) or just 0.004 in. (0.10 mm) as shown by recent in-place tests by Nordström13,14 Final Tunnel Linings and Bernard.15 In contrast, crack width control is For the structural design of final (or permanent) not critical for durability when using macrosyn- tunnel linings similar methods to the above may be thetic fibers since they are not susceptible to corro- applied. However, for final lining applications there sion. Crack width limits might have to be considered are a number of additional factors design engineers though for water-tightness or structural capacity. must consider. While cracking and deformations may well be acceptable for the initial lining or short Embrittlement term ground support, they may be undesirable for Most shotcrete mixture designs focus on dura- final linings. Apart from the structural capacity, the bility and corrosion protection to provide high long-term structural behavior becomes more impor- resistance against chemical attack over their tant for final lining applications. The driving factors service life, which in tunneling is typically herein are durability and corrosion, crack width between 80 to 120 years. To achieve this goal, the control, embrittlement, and retaining the load- mix design often contains large proportions of bearing capacity with age as well as creep. pozzolanic binders, which can develop significant post-hardening of the concrete over time. This Durability and Corrosion leads to embrittlement of the fiber-concrete The durability of a tunnel final lining encom- matrix, which is responsible for post-crack per- passes a number of factors including the permea- formance loss when using steel fibers.15,16 bility of the concrete, concrete strength, durability Embrittlement of FRC with age due to post- of the reinforcement, and control of cracks. The hardening and its detrimental effect on the post- durability of the concrete matrix in FRC is affected crack performance of steel FRC has been known by the same parameters governing plain concrete for nearly 20 years. Numerous research works when subject to the exposure conditions typical of have indicated that aging can lead to a significant an underground environment. However, macrosyn- loss of post-crack performance for steel FRC.15-17 thetic fibers are not subject to corrosion. Typical The change in behavior with age is due to a change issues like chloride ion penetration, carbonation, from a ductile high-energy pull-out mode of post- and to a lesser degree, water impermeability are crack fiber behavior into a brittle low-energy therefore of no concern. This simplifies the design rupture mode of the fiber itself, because of rup- Fig. 5a,b: Corrosion and embrittlement leading to post-crack performance loss in steel FRC with age (left); where neither corrosion nor embrittlement effects occur in MSFRC (right)16 Shotcrete • Summer 2015 51


2015SummerShotcreteEMag
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