Steel Fibre Shotcrete in Tunnelling

1 Initialization 

With demands for safety, process technological streamlining, and decreasing construction time, which facilitates the use of the material as well as costs, which are saved compared to a conventional reinforced concrete structural solution, the readiness to apply steel fibre shotcrete in tunnelling increases. Steel fibre concrete has the benefit of eliminating the requirement for the exceedingly difficult and time-consuming installation of reinforcement when using the shotcreting technique (Fig. 1). 

The building process is accelerated. Additionally, the rock is first secured at a very early stage. Given the challenging geological conditions, rock loosening happens between driving the cross-section and activating the support. Early bearing effect activation of the support helps achieve the aim when there are bad rock conditions. The quick installation process and the early setup strength enable this early bearing behaviour. Typically, dosing units for the ready mix in the concrete factory are used to add steel fibres to the mix during the shotcreting process. When using steel fibre shotcrete, it’s important to take into account rebound, which results in a somewhat lower fibre content in the tunnel shell than with ready-mix. Due to the driving process, it is evident that the fibres are oriented perpendicular to the direction of placement, which is something that may be positively evaluated in terms of the bearing characteristics. In the case of steel fibre shotcrete, the two effects indicated above prompt the production and assessment of test specimens to determine the energy absorption capacity under conditions identical to those present at the ensuing construction. The more favourable an impact the steel fibres have on the steel fibre shotcrete’s ductility, bearing capacity, and energy absorption capacity, the longer and thinner the steel fibres are. To ensure processability, the fibre length should be kept to about 35 mm and should not be longer than 2/3 of the hose diameter. 

2 Steel Fibre Shotcrete’s Material Characteristics 

In addition to boosting the bearing effect, steel wire fibres are added with concrete to increase the concrete’s various mechanical qualities. The following crucial traits are significantly enhanced by the inclusion of steel wire fibres: 

greater ductility under pressure and tension 

increased impact resistance 

enhanced durability, less tendency to spall, and reduced fatigue behaviour 

Greater bending tensile strength in all three spatial dimensions at narrower fracture widths in the operating state 

When used in conjunction with the shotcreting method for tunnelling, steel fibre concrete application offers definite advantages over traditional reinforced concrete application. These specifically pertain to improved industrial safety, cost savings for the reinforcing operations, and streamlining and speeding up the entire work cycle. Because no spraying shadows result when spraying via reinforcement, the sprayed layer is more homogenous. Additionally, because it is easier to follow the contours of the rocks, less overbreak must be sprayed. 

3 Creating a Standard for Steel Fibers in EN 14889-1 

Steel fibres used in concrete applications in Europe must bear the CE label. The standardised norm EN 14889-1 [6] outlines the minimum specifications for steel fibres. The standard lays out specifications for concrete, mortar, and grouting mortar for use in bearing and other applications. Steel fibres for bearing purposes (System 1) and steel fibres for other purposes (System 3) are the two separate systems for certifying conformance. 

The term “bearing purposes” is defined in the standard as “the application of fibres for bearing purposes where the added fibres contribute towards the bearing capacity of a concrete element.” So in almost all pertinent circumstances, a System “1” certification of conformity is required. Thus, in order to avoid any misinterpretation, only steel fibres monitored and certified in accordance with System “1” with the applicable EU certificate of conformity should be used. The norm includes the allowable limits for the fibre qualities that apply in each situation. The influence on the strength of the concrete is tested on a reference concrete in order to clearly demonstrate the differences in the capabilities of the various types of fibres. A test procedure in accordance with DIN EN 14651 [5] defines the minimal amount of additional steel fibre required to achieve residual bending tensile strengths of 1.5 N/mm2 for a crack opening width of 0.5 mm and of 1.0 N/mm2 for a crack opening width of 3.5 mm. 

EN 14487-1, the European Norm for Shotcrete 

The application of fibres is defined as well as the essential test procedures to ascertain the ductility and capacities of various fibre concretes in the European shotcrete norm EN 14887-1 [2]. This standard includes two separate test procedures: statically defined beam tests to determine cross-section bearing capacity in accordance with EN 14488-3 [3] and statically undefined slab tests to evaluate energy absorption capacity (system bearing capacity) in accordance with EN 14488-5 [4]. As a result, the EN 144887-1 [2] refers to additional European test technique regulations, which are discussed and clarified in the next chapter. 

Area of deformation 

Strength category (minimum value in [MPa]) 


D1 0,5 to 1 D2 0,5 to 2 D3 0,5 to 4 S1 S2 S3 S4 

Table 1: The residual strength classes are defined. 

According to EN 14488-3 [3], strength classes and pertinent deflection are classified in accordance with Table 1. It is crucial to note that at any point in the corresponding deformation area, the chosen strength value (S1-S4) does not undershoot the load-deformation curve (D1-D3). The classifications for the energy absorption capability in Table 2 are defined by EN 14488-5 [4]. 

class in energy absorption 

Energy absorbed in joules with a maximum deflection of 25 mm 

E500 500\sE700 700\sE1000 1000 

Table 2: Classifications of Energy Absorption 

The outcome of beam tests enables the cross-sectional bearing capacity to be determined should the statically undefined slab tests allow a conclusion to be made regarding the system bearing capacity (appropriately determined from typical backanchored shotcrete shells) (appropriately applied in a calculation, in which the crosssectional bearing capacity is taken as material resistance). The test procedures and the outcomes they yield vary in this regard. If the cross-sectional bearing capacity is included on the resistance side, the results of slab tests cannot be used for dimensioning (usual procedure for bearing structure dimensioning). 

5 Test Methods for Capability Determination 

Slab Test following EN 14488-5 

The deformation-controlled test method uses a quadratic test specimen with dimensions of 600 x 600 mm and a slab thickness of 100 mm to calculate the energy absorption for fiber-reinforced slab-shaped test specimens (please also see Fig. 12). To account for the impacts of fibre orientation and fibre rebound, this is constructed under site conditions (Fig. 2). The test specimen is configured to allow for free rotation during the test, with deformation being controlled by a single force delivered to the centre of the slab with a maximum deflection of 30 mm (evaluation takes place at 25 mm). The load-deformation curve is continuously recorded. The outcomes are then shown in a graphic showing the absorbed energy as a function of deflection. 

After EN 14488-3, statically designed bending beam tests 

The deformation-controlled test method pertains to a notched beam 500 mm in length, 125 mm wide, and 75 mm high, which is cut from a previously sprayed quadratic slab. It is used to determine the bending strengths for fiber-reinforced slab-shaped test specimens. The test specimen is mounted on two independently spinning rollers, and deflection is regulated by applying two separate loads, one at each of the third places along the span width, with a maximum deflection of 40 mm. The load-deformation curve is continuously recorded. 

6 Fibre Concretes’ Long-Term Behavior 

When applying shotcrete, it’s crucial to keep an eye on the fibre concretes’ long-term behaviour as well. Numerous creep tests have demonstrated that using steel fibre concrete does not cause significant creep deformations or creep failure. On the other hand, plastic fibre concretes can be expected to experience significant creep deformations. This behaviour is described in various publications. For instance, the Austrian Guideline on “Fibre Concrete” [10], whose appendix includes the results of creep experiments, is mentioned. The majority of creep experiments were conducted using test objects in the shape of beams. 

Similar findings about the propensity of plastic fibre concretes to creep were obtained from studies of the long-term behaviour on slab-shaped test specimens with dimensions in accordance with EN 14488-5 [4]. (Fig. 3). Although the tendency of the plastic fibre concretes to creep was noticed after a reasonably short period of time with a variety of genuine temperature changes, the load level for the long-term tests was only set at 60% of the allowable load calculated from the short-term tests (Fig. 4).

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