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2015SummerShotcreteEMag

Goin’ Underground the preferred performance assessment tool used in which are eventually used as a basis to supplement the design of temporary tunnel linings based upon the stress-strain relationship of the concrete on the Q-chart and other similar design approaches. the tension side using defined procedures.10-12 The ASTM C1550 round panel test uses a 3 in. This approach generally provides structural thick by 32 in. (75 mm thick by 800 mm) diameter gains for FRC and FRS versus unreinforced con- round shotcreted specimen. A conversion factor crete, but typically the load-bearing capacity is less of 2.5 was found to correlate the results from the than the maximum reached in the elastic stage. The different panel tests.9 This means that, for example, advantage of FRC versus unreinforced concrete is a result of 740 ft-lb (1000 Joules) in an EFNARC that it offers considerable loadbearing capacity in panel relates to approximately 300 ft-lb the post-cracking phase due to the elasto-plastic or (400 Joules) in an ASTM C1550 panel. ductile failure behavior of FRC, 10-12 which is A classical design approach for fiber-rein- similar to steel welded-wire-reinforced shotcrete. forced concrete tunnel linings in civil tunneling In tunnel linings, which are statically highly inde- applications is the use of Moment-Normal Force terminate, this ductile behavior allows for load (M-N) interaction diagrams, which are obtained redistribution, thereby increasing the structural by equilibrium iterations on a given cross sec- capacity of the structure as a whole. tion.10-12 The approach adopts and modifies clas- An economic, state-of-the-art tunnel lining sical design methods from unreinforced and design considers the load redistribution in the reinforced concrete structures. The FRC material ground and the ground-support interaction. Load properties are herein derived from beam tests, redistribution induces deformations and a ground support that is flexible enough to withstand this deformation. Historic lining designs were struc- turally stiff and attracted a lot of loading, which in return required heavy reinforcement. Modern designs allow for controlled deformations resulting in much thinner and softer linings. How- ever, the structural integrity of the deformed lining has to be considered. To take full advantage of the material properties of FRC, the design approach of tunnel linings has to move from an elastic to an elasto-plastic approach, similar to the consideration of plastic moments commonly used in steel-structure design. After reaching the elastic capacity of the lining, the lining cracks but is still able to provide plastic bearing capacity. The load is hereby redistributed within the tunnel lining by Fig. 3: Stress-strain curve of FRS Fig. 4: M-N interaction diagram of FRS 50 Shotcrete • Summer 2015


2015SummerShotcreteEMag
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