Sprayed concrete is used as the first cover layer in the construction of some tunnels; it generally is not required to perform any other function. Two examples of successful waterproofing of tunnel constructions are cited below. The first example describes a mine-access tunnel with water ingress from the mountain, while the second outlines the waterproofing of a pressurised water tunnel. In the tunnel sections under consideration, waterproofing was achieved by polymer modification. An inspection after a two-year service life revealed that impermeability was undiminished. The second example describes the renovation of a pressurised water tunnel, where water loss from the tunnel into the mountain was reduced. After two years of operation, an inspection showed that the sprayed-concrete shell was still intact.
Requirements for sprayed concrete linings
Concrete shrinks, and this basic property must be taken into account when constructing sprayed-concrete shells. Free shrinkage is influenced by various factors, such as the water/ cement ratio, the solids content, the atmospheric humidity and the aggregate type. The conventional concept of shrinkage does not factor in the adhesion to the substrate i.e., adhesive bonding between the rock and the concrete shell. When considering bonding, one invariably thinks of bonding materials or composites. However, concrete is in itself already a composite, comprising additional components which include an aggregate, cement and reinforcing steel, (a mixture of different components constitutes a bond). The European standard EN 1504-3 regulates concrete renovation and describes and examines multi-layered composite materials. In contrast to a concrete member that is subject to free shrinkage, the shrinkage described in the standard is restricted by the adhesive bond. Since deterioration of the adhesive bond is the decisive factor in the application, the shrinkage test in the standard was replaced with a tensile adhesion test. Reprofiling material applied to old concrete not only has to comply with a classified compressive strength; its tensile adhesive strength must also be adequate. This material bonding also applies to sprayed-concrete applications: the rock and sprayed-concrete layers and spray membranes (if used) or shell concrete also constitute a bond within the single-shell tunnel construction. Although economic considerations have raised awareness of the value of single-shell tunnel construction, conventional construction methods are generally used. Yet, while the deformation of the mountain subsides with time, and the sprayed concrete layer is disregarded in terms of structural analysis, the inner shell can be calculated accurately, because it is unaffected. The rough sprayed surface of the initial layer suffices to allow for good adhesive bonding between the sprayed concrete layers; the concrete structure can therefore be regarded as monolithic.
Conventional construction methods using sprayed concrete as the initial cover layer, and a film sealant followed by a concrete shell or precast segment, are based on the assumption that the safest waterproofing possible is thereby provided.
Water impermeability of tunnel constructions
Tunnel constructions must be leak-proof commensurate with their use. Here the term ‘leak-proof’ or ‘impermeability’ must be examined more closely. Leakage rates are a standard category in building requirements stipulated by STUVA (DE). The unit used for water leakage is liter per square meter and day (lt/m2 per day) per 100m tunnel length. The SIA 272 (CH) (Swiss Society of Civil Engineers and Architects) classification for impermeability ranges from ‘small moist areas’ to ‘flowing water’ [6]. The common concern was to define classes and create categories which enable the contractor to propose or implement an appropriate waterproofing process, and for the customer to specify and monitor the impermeability required for his building projects. The fundamental question to be asked in the development of a construction system is whether to assess a sample or a building component. The advantage of samples is that sampling is performed under highly controlled, reproducible conditions; consequently, after successful testing, the sprayed concrete sample can be classified as leakproof.
It is far more difficult to test building components, since they are influenced by many boundary conditions. Built-in sprayed concrete, in particular, is not only subject to purely material fluctuations; here factors such machinery, spraying procedure, climate conditions and even the nozzleman must be taken into account, as they all determine the quality of the sprayedconcrete shells. The quality of the application can also be improved by training the machine operator to qualify for nozzleman certification, since the human factor and materials both influence quality equally.
Three tensile adhesion strength and cracking in composite materials
Tensile adhesion strength is one parameter used to describe material bonding. A comparison between typical values according to EFNARC (European Federation for Specialist Construc- tion Chemicals and Concrete Systems) and EN 1504-3 is listed in Table 1. EN 1504-3 is of particular interest because shrinkage is not viewed as a single property. Instead, bonding, adhesion and toothing of interfacial layers are taken into account.
When sprayed concrete is applied, factors relating to stresses, such as the temperature of the concrete and the ambient temperature must be taken into account. When cement hydrates, the temperature rises, see figures. 1 and 2, above. However, the progressive strength development and subsequent cooling result in tensile stress. The combination of sprayed concrete and an accelerator causes the temperature of the fresh concrete to rise.
The nozzleman can exploit the hydration heat as it indicates whether the concrete formulation has started reacting, i.e., he observes or measures the temperature of the sprayed concrete surface. The concrete development serves to harmonise the strength development, tensile stress, tensile adhesion strength and shrinkage, such that cracking does not occur. The strength curves J1 to J3 specified in the directive for sprayed concrete are frequently implemented. However, formulations with a high early strength range of J3 should only be used in special cases, as they also cause higher thermal stresses.
In addition to the thermal stresses occurring in the fresh concrete, the physical shrinkage of the hardened cement must be taken into account during aging: the sum of the forces causes cracking if cohesion within the concrete is exceeded. Shrinkage alters the length, and the force exerted inside the concrete is changed into tensile stress. An example of the force behavior in dry shrinkage is illustrated in Figure 3.
Development of a test specimen
The shrinkage at a tunnel surface is much more complex; material roughness and applied thickness are only some of the impacts. To get more information about the shrinkage behavior we developed a test specimen to combine some of the aspects. The inlay specimen is made out of a solid but porous concrete, it’s comparable to a drainage concrete. The size is a triangle of 150mm by 60mm, height in the middle is 700mm. This concrete fragment is placed in the middle of a 150mm by 150mm by 100mm mold and is filled up with concrete.
The shrinkage and shear force is quite complex, we have different heights and angels see Figure 4. Storage condition: The surface was covered for three days with a plastic sheet and stored at 23°C, followed by 25 days 23°C / 50 per cent relative humidity level. After the drying time, the surface of the specimen and marked the cracks were scanned. The observation was that some hair cracks in the center of the specimen were found, following the direction of the stress directions. For the proofing of water tightness a test according to DIN 1048-5 was performed. At the beginning of the water pressure test the concrete samples had a similar water absorption but after increasing the pressure to 5 bar and longer test time the reference sample continued to absorb water, while the polymer modified concrete reached a maximum.
Polymer-modified SCL
Polymer-modified sprayed concrete has waterproofing properties. The following application examples and tests show that this process reduces the tendency to crack, thereby increasing watertightness.
The copolymer used in the investigation was based on Vinylacetate Ethylene, having a high flexibility with a glass transition temperature below 0°C.
Tests performed on coredrill samples taken from sprayed concrete slabs showed that sprayed concrete is also watertight. The results of concrete core drill samples were stored for three days at a water pressure of 5 bars, in accordance with DIN 1048, are listed in Table 3.
A higher penetration depth was noted for polymerisation exceeding five per cent; however, a reduction of the building component’s susceptibility to cracking is ample compensation for this.
The erection of a crack-free and therefore impermeable structure is the actual challenge facing waterproof sprayed concrete, and building component or individual tests only reflect some of the requirements.
All types of cracks can be avoided more effectively with waterproof single-shell or, alternatively, monolithic construction methods. The use of polymer binders reduces shrinkage by improving bonding and cohesive forces in the actual concrete mixture.
A crack appeared in the reference concrete applied to a 0.5m by 0.5m raw limestone slab, which was manufactured without polymer. The crack was 0.6mm wide. The reference samples with polymer had no cracks, or cracks of 0.2mm maximum width.
The traditional concern regarding shrinkage in sprayed concrete is to increase adhesion or bonding. When adhesive bonding is good, shrinkage forces, which lead to tensile stress in restricted shrinkage, are reduced by the adhesive force.
The sum of all the forces remains intact. This principle is also the basis of the shrinkage test as is specified in the standard EN 1504-3, according to which structural bonding only exists when the levels of adhesion between the interfacial layers are high.
The improvement of the tensile adhesion strength measured on sprayed-concrete samples on a concrete slab is shown in Table 4. Higher levels of adhesion can promote resistance to cracking effects. Polymer binders are used in Polymer Cement Concrete (PCCs).
The reduction of the elastic modulus and the increase in the forces causing cracking are described in the test.
Application in mining access road, the Clara tunnel
The successful waterproofing of the mining access, the Clara tunnel in a salt mine in Stetten (Germany) using polymermodified sprayed concrete was presented at World Tunneling Congress 2010. The sprayed-concrete formulation comprised 7.5 per cent liquid polymer (3.8 per cent solid).
After a two-year service life, an inspection was conducted to determine impermeability.
The area in question measuring 400 to 500m was inspected, and the documentation thus showed that impermeability had not deteriorated.
In 2008, the unmodified concrete, here at 430m, showed typical lime efflorescence caused by water penetration, small hair cracks and also large flaws.
These typical lime deposits which have formed on the surface are a good indicator. Through the use of the SIA 272 classification, they indicate class two for polymer-modified concrete (considered almost watertight – classed as watertight) and class four for standard concrete (considered highly susceptible to leaks).
The pressurised water tunnel in Hintermuhr, Austria
Pressurised water tunnels, like those used in pumped-storage hydropower plants, have become common building constructions. Their purpose is to minimise influences on the water equilibrium of the mountain concerned: they should not loose water, nor draw from the ground water.
The pressurised water tunnel in Hintermuhr, Austria, erected between 1988 and 1992, was largely left in its natural state due to good geological conditions; problem areas were treated retrospectively.
Since the tunnel is completely filled with water during operation, and is pressurised by a water column of approximately 15m, renovation became necessary to reduce water loss. Notwithstanding injections performed during previous renovation measures, approximately 50m in total still leaked to such an extent that inner rings consisting of 100mm sprayed concrete were installed in several locations. During renovations in April 2009, polymer-modified dry-mix sprayed concrete was used. The polymers were added to the mixing water in the injection nozzle.
Another advantage, apart from the waterproofing, was reduced rebound. The construction period could be shortened considerably as a result, and far less material was used. The restoration was successful and the tunnel is still watertight and complies with the relevant regulations. An inspection conducted in May 2011, after two years of operation, demonstrated that the sprayed concrete shell had remained intact, and the spray pattern of the concrete applied in 2009 was still clearly visible in 2011.
Conclusion
The formation of cracks and, therefore, their prevention, are of crucial importance to waterproof, single shell tunnel construction. Both practical examples demonstrate that polymer-modified sprayed concrete has lasting waterproofing properties when used for pressurised water applications. Polymer binders increase adhesion to the substrate and improve the adhesive bonding strength.
Due to the influence of flexible polymeric properties in otherwise rigid cementitious concrete structures, stress factors can be averted or counterbalanced, and the sprayed concrete becomes more resistant to cracking.
