Page 50

2013FallShotcreteEMag

Goin’ Underground carried by the secondary lining varies from 15 to Reinforcement—Reinforcement of the linings 50%. This is a function of the ratio of horizontal will be provided by structural fibers in the sprayed to vertical stresses, the lining thicknesses, and the concrete matrix in combination with steel bar tunnel shape, as well as the interface properties. reinforcement located around junctions and The load-sharing is less pronounced in the design openings. Fibers—steel or macro-synthetic—add models for real tunnels because of the interaction a modest tensile capacity. This can be incorporated with the ground; notably, the tendency for the into the design using a simplified stress block, for stiffer CSL lining to attract more load overall but example, as described by RILEM8 and shown in at the same time less is applied to the secondary. Fig. 7. Various design approaches have been The loads in the primary tend to remain broadly adopted on different projects, partly reflecting the similar but the reduction of bending moments in confidence of the client or designer, as much as the secondary lining of up to 20% could permit a the state-of-the-art. Traditionally, Design approach thinner secondary lining. 1 was used and no benefit from the fibers was The secondary lining will be structural fiber- assumed. Clearly this is incorrect and unduly reinforced sprayed. Bar reinforcement is generally conservative. In Design approach 2, the fibers are required at openings and some headwalls. seen as guaranteeing the inherent tensile strength Secondary linings are typically designed to carry of the concrete. This approach offers little benefit sufficient residual capacity to resist ground loading in design since the tensile capacity up to first crack after a EUREKA time/temperature fire curve, as is so small. The approach adopted most recently defined in the Technical Specification for is Design approach 3, in which a simplified stress Interoperability–Safety in Railway Tunnels block, with a value of 0.37 fctm.fl, is used, based (TSI-SRT). The EUREKA curve has been developed on RILEM.8 This is conservative itself, because for the rail industry in Germany and is considered the stress at first crack is 20% higher than this the most appropriate to the predicted fire scenarios. value, which corresponds to the residual value at The secondary lining concrete (cast in-situ or the end of a standard beam test. RILEM8 sprayed) will contain micro-synthetic fibers in order recommends limiting the strain to 2.5%; the to limit explosive spalling and maintain structural strains in a standard 75 mm (3 in.) beam test are integrity. The quantity of fibers is typically higher than this at a deflection of 2 mm (0.08 in.). determined by pre-construction testing and a dosage In practice, the Ultimate Limit State does not of about 1 kg/m3 is normal. It has been shown in necessarily govern. Crack widths in the lining extensive fire testing for projects, such as Heathrow should be less than 0.3 mm and this curtails the Terminal 5, A3 Hindhead, and CTRL, that the contribution of the fibers to tensile capacity under inclusion of micro synthetic fibers in high-strength, Serviceability Limit State conditions. The subject low-permeability concrete mixtures significantly of crack widths still requires some development. reduces the risk of explosive spalling when exposed Methods are suggested for predicting crack widths to severe hydrocarbon fires. (such as in RILEM8) but naturally, because this Waterproofing systems—Spray-applied is a new material, the spacing and development waterproofing membranes have been selected due of cracks within fiber-reinforced concrete is not to the benefits they can offer by bonding to both as well understood as in conventional bar the primary and secondary linings. This property reinforced concrete. is advantageous as it offers maintenance and In the past, specifications have often prescribed repair benefits in the long term by preventing the a dosage of fibers; for example, in permanent movement of water, either behind or, should it be linings, typically 30 to 40 kg/m³ (50 to 67 lb/yd3) breached, in front of the membrane. Should a leak of steel fibers. This is at odds with the normal be found on the surface of the secondary lining, practice in most other areas of setting performance as water is not able to move laterally, the source specifications. Following the style of RILEM, will be easily located and treated at that location sprayed concrete can now be specified in the in the primary lining also. following manner: In water-bearing stratigraphy, such as the C28/35 FL 1.7 Lambeth Group or River Terrace Gravels in London This means the 28-day cylinder strength should there is still a tendency for Clients and Designers to be 28 MN/mm2 with a flexural tensile strength of opt for a sheet waterproof membrane. Sprayed more than 1.7 MN/mm2 at a strain of 2.5%, which concrete can be applied to sheet membranes—for corresponds to a central deflection of 3 mm example: Thames Tunnel, UK; Russia Wharf, (0.1 in.) on the standard beam test. EN 144876 Boston, USA; or Dulles Airport, USA. offers another alternative: 48 Shotcrete • Fall 2013


2013FallShotcreteEMag
To see the actual publication please follow the link above