Preliminaries

The construction industry calls for industrial, well mechanizable procedures rendering the realization easier and safe.
In the past 50 years the tunnel construction developed revolutionary. This development manifests in design, calculation procedures and in mechanization of the construction as well. Disregarding the eventuality of the receiving rock properties, the tunnel construction can be mechanized. Whole industrial branches were founded on this market. The single tunnels of long- and medium length are generally constructed by shield tunnelling method and are provided with single shell, watertight walling. In case of larger or shorter tunnels the state-of-art variant of shotcrete technology is widespread. These tunnels have external walling realized by boring and cutter machines, robot shotcrete, and inner shell structure realized by tunnel form panel concrete.
The “secondary” service structures related to tunnel construction (connecting and ventilating tunnels, junctions, etc.) can be supported in a primary phase by shotcrete technology adapted, but the permanent structure requires form panel technology.
However, this already belongs to the past. The modernization of the final step has arrived and the installation of the permanent structure became mechanized.

Application of steel fibre in tunnel construction

In reinforced concrete structures we use reinforcing steel for tension. It can be simply realized in tunnels with woven steel fabric provided that one component of the surface is straight. For complex surfaces, steel fibre concrete can be applied, in which case the short steel wires mixed into the concrete can transform the section into “homogenous” material absorbing the tension. The steel fibre concrete was applied at first for industrial floors, and due to its advantageous properties it spread in other fields too. For its use in tunnel construction, the earlier knew procedures were to be integrated in a new application sphere. By application a steel fibre batched inner curtain wall we obtain an identical load bearing capacity than in case of wire mesh reinforcement, however the construction technology is much simpler and much less expensive. The behaviour of steel fibre reinforced concrete walling differs from the one of regular reinforced concrete and there is not sufficient design and execution experience in Hungary yet. In our survey we examined the technological design, verifying, testing methods in compliance with the European standards. Our task was to collect and apply the prevalent regulations and standards, than to help the Client and Authority to overcome their fear, finally to execute the construction.
We need many conditions to be completed to generalize this method. One of them is a pilot project realized in parallel with the development work. The theoretical and practical issues provided us a lot of further information that we could adapted.
The installation of steel fibre was realized in the structures of ventilating system of Délbuda Rákospalota metro line in Kálvin and Rákóczi subway stations. Beside the technical problems we also needed the approbation of Client and Authority. This kind of structure has not been installed yet as permanent structure in tunnel in Hungary. Prior to the installation we had studied the technical literature and had listed the necessary facilities. The Hungarian Technical University Reinforced Concrete Chair was involved into the research.
The structure had to satisfy the special demand of metro. The necessary watertightness of structures of metro stations needed the installation of a damp-proof layer too. The life-time of tunnel shall exceed 100 years, and the fire resistance is to be over 2 hours beside a temperature of 400° without important damage.
Due to the regulations and standards the walling is to be constructed in 3 layers. The external one shall grip on the insulation and protect the insulating foil. It is made from regular shotcrete mix layered by dry or wet procedure. This layer with a thickness of 2-3 cm is not considered into calculation of the load bearing of structure. The central, load bearing, layer is a steel fibre reinforced concrete walling constructed with wet procedure in a thickness according to the calculations. The inner layer is a plastic fibre shotcrete with multiple functions: ensures the concrete covering of steel fibres according to standards in order to preventing it from corrosion; increases the fire resistance of the structure, and finally gives a more esthetical appearance by its smooth surface.

Research phase of development

As the first step of development a review of special literature and regulating environment was realized and the search and completion of domestic regulation. The main aim was the adaptation of the applied international regulation by Client and Authority in order to have the necessary future basis and references.
The research deals with the following issues:

- requirements of components of concrete
- requirements of durability
- components of mixes
- execution of shotcrete
- requirements for finished products
- test methods
- quality assurance
- safety
- shotcrete tests

There are many requirements of concrete components.
In course of our development work, we gave the detailed definition of these requirements only enumerated as follows :
aggregates, cement, mixing water, accelerators, other aggregates, durability requirements, mix design, shotcrete execution, quality assurance, safety.

Requirements concerning finished product

According to EFNARC, EN206 the compressive strength of shotcrete is to be determined between C24/30 and C48/60.
Bending, tensing and compressive strength tests are to be realized on the samples.
In case of special requirements, flexibility modulus, adhesion, fibre content frost resistance and water-tightness can be also tested in compliance with the prevalent standards.

Testing methods

The setting time of concrete – in comparison with the control mix - is tested in the first phase by needle penetration method at young concrete, later by nail shooting.
The test of hardened concrete is realized on samples enclosed by in-situ test panels. The samples ensure that compressive strength, density, bending and permanent strength, energy absorption, flexibility modulus, adhesive strength, permeability, frost resistance, fibre content can be tested. In case of special demands fire resistance can be also tested.

Execution phase of development

The fulfilment of mix tests in compliance with the international standards enabled the construction to start. At first, the installation was realized at ”secondary” structures. The support during breakthrough of line tunnel of metro station Rákóczi square was realized from steel fibre concrete. The aim of experimentation was the realization of large-scale production of the test mix, put in service of logistical system, put in operation of nail shooting and preparation of specimens. The staff was skilled in dry concrete nail shooting, however the steel fibre and the wet procedure needed acquisition of further practice. They carried out with care all tests on the experimental surfaces realized and on specimens in order to prepare themselves for the effective future task. According to the test result the design and the document for the Authority were elaborated and finally on the basis of successful test the amendment was approved.

See realization in our image gallery annexed.