Fibre shotcrete

In a number of respects, the properties of fibre shotcrete may be quite different from those of ordinary FRC [65], In particular  

1 Aggregates in fibre shotcrete tend to be smaller than those in ordinary FRC. 

2 The aggregate content of fibre shotcrete is much lower than that of ordinary FRC ( 30% by mass, compared with 50-60%), because of both placement difficulties and high rebound (see later). 

3 Fibre shotcrete has much higher in situ cement contents, approaching 600-700 kg/m in some dry process applications. 

4 Because of the pneumatic compaction of the fibre shotcrete, the internal voids tend to be different from those in cast FRC, in terms of both distribution and range of sizes. 

---

Shotcreting Using a modification of normal shotcreting techniques, it has been found possible to produce steel and polypropylene fibre shotcretes, for use particularly for lining of tunnels, and for stabilization of rock slopes. With this method, too, relatively high volumes of fibres can be added to the mix. Pulp type processes For asbestos cement replacements (cellulose or other fibres are used as a replacement for the asbestos), the fibres are dispersed in a cement slurry, which is then dewatered to produce thin sheet materials. These can be built up to the required thickness by layering. This process yields fibre contents of typically from 9% to over 20% by volume. 

EFNARC The European Federation of Producers and Contractors of Special Products for Structures (EFNARC) has proposed a plate test [38] to quantify the toughness of FRC, and in particular fibre shotcrete. The test involves a 600 mm square plate, 100 mm thick, simply supported on all four sides on a 500 x 500 mm span, and loaded at the centre. The toughness (absorbed energy) is determined from the load vs. deflection curve out to a deflection of 25 mm. Bernard [39] found a good correlation between this plate test and the round panel test. The EFNARC test is sometimes used in Europe, but rarely in North America. 

For Steel fibre reinforced shotcrete (see Chapter 14), different considerations apply, and most mix designs are still arrived at empirically. Typical mix designs for steel fibre shotcrete are given in Table 7.2 [22]. 

Thus, different fibre geometries and fibre-matrix interactions can affect the flexural behaviour of SFRC. This is demonstrated in Figure 7.18 [41], which shows the effects of different fibre shapes on the flexural behaviour of steel fibre shotcrete. These data suggest that the aspect ratio (//d) concept, which was developed for smooth, straight fibres, is not really useful when applied to deformed fibres,... 

F i bre shotcrete i s parti cu larly effective because the very nature of the appi ication ensures that the fibres are preferentially aligned in two dimensions. It has been shown by Morgan and Mowat [64] that steel fibre shotcrete, for instance, can provide better load-bearing capacity than plain wire mesh-reinforced shotcrete at small deflections, and at least equal capacity at large deflections (Figure 14.13). 

Figure 14.11 Steel fibre shotcreting. Photograph courtesy of B.H. Levelton and Assocoiates, Vancouver, Canada.

Figure 14.12 (a) The BESAB system for the production of steel fibre shotcrete (b) wet spray nozzle and (c) dry spray nozzle [63]. 

Figure 14.13 Load vs. deflection curves for steel fibre shotcrete panels, compared with plain shotcrete and wire mesh shotcrete [28].

Because of the fact that fibre shotcrete is often not fully consolidated, and because of rebound effects, the water/cement ratio law that governs the strength of cast concrete does not work very well, and so there is generally a poor correlation between w/c and compressive strength, as shown in Figure 14.14 [65],... 

In general, fibre shotcrete may be specified and tested in the same manner as ordinary FRC and plain shotcrete. Flowever, the European Federation of Producers and Contractors of Specialist Products for Structures [66] have suggested some particular specifications and tests for what they term sprayed concrete . In particular, they recommend that the toughness of the fibre shotcrete be specified either by a residua strength class Ixom a beam test) or an energy absorption class from a plate test), though they go on to state that the two tests will not give values that are comparable. 

As stated earlier, both steel fibres and synthetic fibres have been used successfully in fibre shotcrete, though steel fibres tend to lead to greater improvements in toughness than do synthetic fibres [71], The most rapidly growing area of application... 

Fibre shotcrete is also particularly wel I suited for repairs to deteriorated concrete structures. The key to successful fibre shotcrete repairs is proper preparation of the substrate, including removal of any damaged material. If this is not done, it is all too common to find perfectly good fibre shotcrete repair material lying beside the structure after debonding from an improperly prepared surface. As well, the surface of any structural or reinforcing steel should be blast cleaned to achieve a good bond. 

Finally, it is absolutely essential that the nozzlemen be properly trained, since it is they who control the final quality of the fibre shotcrete application. In North America, the American Concrete Institute has developed detailed policies for the certification of shotcrete nozzlemen [74]. For major projects, it is wise to require all the nozzlemen proposed for the shotcrete applications to shoot preconstruction test panels, constructed with the same reinforcing and other details as specified for the structural repair itself. This enables them to become fami I iar with the materials and procedures to be used on thejobsite, and permits an evaluation of their skills. 

Two good examples (out of a great many reported in the literature) of the use of fibre shotcrete in repairs in Canada are the rehabiiitation of berth faces at the Port of St. John [75], and repairs at the Port of Montreal [76]. 

It has also been possible to develop very high performance fibre shotcretes [77], comparable in performance to cast engineered cementitious composites (ECC). Though such materials require close control of the particle size distribution, and of the necessary chemical and mineral admixtures, the resulting materials exhibit proper pumpability and shootability in the fresh state, and strain-hardening behaviour in the hardened state, as shown in Figure 14.17. 

---