Appendix II. Transmission channels from teacher salaries

The relative importance of different material parameters for sprayed concrete depends on the type of stability problem. Thin layers applied to hard rock to prevent loose stones and wedges from falling out, depend mostly on adhesion. The compressive strength in such a case is of minor importance. The compressive strength, on the other hand, is the main factor when a thick closed ring support of soft ground is consid-ered. In this situation the adhesion is of no interest at all.

Compressive strength can be used as an indirect indication of durabil-ity factors. The concrete shall be of satisfactory long term durabildurabil-ity in the environment where it is applied. There may be a difference between a road tunnel with heavy traffic and a water transport tunnel in this respect. In most cases the sprayed concrete must meet a 35 MPa strength class according to a normal national standard test procedure.

In Norwegian sub sea road tunnels this requirement is now a grade 45 MPa concrete.

Adhesion to the rock surface is generally an important parameter. The problem is that it is complicated to measure accurately and it varies a lot within short distances. Very often people are reluctant to specify the required adhesion in a contract, because the control results may cause more problems than is worth while. In our opinion the control

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focus should be kept on compressive strength, application technique and cleaning of the surface in advance. In this way, the best possible adhe-sion that the surface allows, can be achieved.

The tensile strength of sprayed concrete is not so important. In design considerations this strength cannot be included anyway, because there is always a chance of shrinkage cracks in critical sections. Across a crack there is naturally no tensile strength. The same applies to the flexural strength of the sprayed concrete material itself.

It is important that the required compressive strength is achieved by a mix design that gives a minimum shrinkage. There are two reasons for this:

< Low shrinkage improves adhesion.

< Low shrinkage reduces cracking and improves durability.

To produce a low shrinkage, the content of fines and cement should be low, the w/c ratio should be low (generally less than 0.45) and applica-tion technique must be correct (good compacapplica-tion and spraying at right angles). A curing compound after application, water spraying, or the use of a concrete improver (such as MEYCO^® TCC735) should be a routine part of the work.

The thickness of the sprayed concrete layer is a design question. The contractor shall distribute the necessary concrete volume to meet the requirement as closely as possible. This is a practical problem, espe-cially if the specified thickness is large (200 mm and more) and the full thickness is placed on a limited area during one operation. Under such circumstances the tendency is that the walls get more concrete than required and, of course, the roof gets less. This is the opposite of what is wanted, from a stability point of view.

This leads to a very important parameter in sprayed concrete applica-tion, the short-time strength development. Safety and economy are improved if the strength gain within the first minutes and hours is high.

High early strength is possible when using accelerators. Economy is improved to a maximum if it is possible to build full thickness in one continuous operation, even on a limited area.

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8.7 Reinforcement

The traditional reinforcement in sprayed concrete is steel wire mesh (normally 3 to 6 kg / m² and square openings of 100 to 150 mm). It is also named welded wire fabric (WWF). This product should never be replaced by the kind of nets used for fences (chain link mesh). Chain link mesh usually has a wire thickness of 2 to 3 mm and openings of 50 mm.

Chain link mesh must not be used in sprayed concrete, due to the small openings and fluffy behaviour, which causes high rebound, a build-up on the net, and leaves voids behind it.

Shing Mun Tunnels, Hong Kong

Contractor: Gammon, Dragages, Skanska

50 m² tunnel Cumulative hours

One cycle SFRS, . h

One cycle WWF, 1.0 h

Figure 69

Installing WWF is highly manual work, and it is very hard to improve on its efficiency. The cost of reinforcement by WWF is hence constantly increasing, because the production capacity is fixed. The direct erection cost for WWF per m² is in the range of USD 16 to 24. The substantial increase in overall tunnelling capacity by switching from WWF to steel fibre reinforced sprayed concrete (SFRS) is shown in Fig. 69.

It is important to be aware of the purpose of reinforcement in sprayed concrete. In rock support there is the constant possibility of unexpected loads and deformations. The best possible safety margin is achieved by the highest possible fracture energy in the sprayed concrete layer.

The fracture energy (toughness) is represented by the area below the load deformation curve, when testing beams and panels under flexural

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loading, see Figure 70. Today, modern test procedures, such as the EFNARC and the ASTM C 1550 test methods, are based on sprayed panels testing.

Steel fibre reinforced

Unreinforced

Deformation

Load P

Figure 70

8.8 Tunnel support methods

Sprayed concrete has traditionally been considered a temporary sup-port in most countries. Due to increasing pressure on economy, the interest for one pass tunnel linings based on sprayed concrete, has increased substantially during the last few years. High performance wet-mix sprayed concrete, where necessary combined with steel fibres, has become the first choice for this approach.

Figure 71

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The most commonly used support combinations are: Rock bolts (some-times with steel straps), sprayed concrete (usually steel fibre reinforced) and cast concrete using steel shuttering.

Rebars Rock bolts

0.5–1.0 m 2.0–5.0 m 150 mm

Figure 72: Support with ribs of fibre reinforced sprayed concrete

Spacing between rock bolts Q=0.1 Q=1.0 Q=10 Q=30 Areas without sprayed concrete 1.2 m 1.4 m 2.0 m 3.4 m Areas with sprayed concrete 1.3 m 1.6 m 3.0 m 4–5 m In recent years it has become more and more common in very poor rock conditions to replace the traditional cast concrete by steel fibre reinforced sprayed concrete combined with rock bolts and steel bar reinforced ribs of sprayed concrete, see Figure 72. This is a more adapt-able solution than the pre-made lattice girders. In cases of substantial overbreak, the volume of sprayed concrete will be far smaller, and one layer of typically 16 mm bars is much easier to spray over with a good compaction. Net effect is saved time, better durability and improved economy.

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9. Permanent sprayed concrete tunnel linings

9.1 Development of permanent sprayed concrete tunnel linings

Conventionally, tunnels constructed using sprayed concrete have been based on a temporary sprayed concrete lining to stabilise the opening after excavation and to contain short to medium-term loads. When this lining has fully stabilised, a permanent cast in situ concrete lining has been installed to contain long-term loads, and provide durability and watertightness, either by the use of a waterproof membrane between the temporary and permanent linings, or by the use of steel reinforce-ment to reduce crack widths to 0.2 mm to allow autogenous heal-ing. This shall be referred to as the double shell method. Since 1994, sprayed concrete technology has improved dramatically in terms of stable admixtures and application methods, particularly with the wet-mix process to give a durable, high performance concrete.

In 1996, both the Jubilee Line Extension and Heathrow Express Rail Link projects constructed tunnel linings using permanent steel fibre reinforced sprayed concrete instead of conventional in situ concrete within the temporary sprayed concrete linings, lowering costs and sig-nificantly reducing the construction time, particularly in sections of complex geometry.

The current state-of-the-art sprayed concrete technology equips the tunnelling industry with a considerably more economic tunnel lining system in the form of a single pass of permanent sprayed concrete, pro-viding a structural lining that is also durable, watertight and can be sur-face finished to a degree that is similar, if not equal, to cast concrete.

The essence of the Single Pass Tunnel Lining method (SPTL) as described in this chapter, will be to maintain the design philosophy of the temporary sprayed concrete lining, but to enhance the mate-rial performance and construction control. This will permit the primary SPTL sprayed concrete lining to be considered as a permanent, durable structural element that will fulfil the structural requirements both during construction and throughout the designed life of the structure. This can

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be performed either as a true single shell, or, if required, acting mono-lithically with an additional sprayed concrete layer installed later during the construction process.