Fibres for reinforcing concrete come in many forms, shapes and sizes, and are manufactured in many different ways. This provides variations in the characteristics of the fibres and, in turn, influences the performance of the fibre reinforced concrete.
In construction there are two types of fibre generally used, steel and micro polypropylene, although a variation of the polypropylene fibre is now on the market, know as ‘structural’ or ‘macro’ fibre, which will be discussed later in this article.
The micro type of polypropylene fibre has been used very successfully over many years, in applications such as floor slabs, general precast items, external hard-standings, etc. The benefits of these fibres range from replacement of anti-crack steel fabric mesh, to early age drying shrinkage crack resistance, crack control(1), a measure of impact resistance, and it is claimed some ductility.
Steel fibres make brittle concrete highly ductile and have been commercially available for around 30 years. They came into use in general construction in the UK some 12 to 14 years ago. This quickly spread into industrial flooring markets as the additional technical benefits of greater crack width control and increased load bearing capacity gave designers the opportunity to reduce slab thicknesses and reduce costs. Additional benefits of being able to remove both layers of conventional anti-crack steel mesh from these types of slab, gave the contractor a considerable saving on time.
There are many variations on these two types of fibre available to the tunnelling and construction industries, and there are a number of documents that describe them in detail(2). There appears to be a great deal of confusion regarding which fibre is most suitable for a specific job or function, or how performance can be measured to give a cost effective solution for a particular application. Performance can usually be measured in the form of the traditional beam test for SFRC(3&3a), or by a slab test(4). Both of these are well established test methods, however, whether or not they are fully understood is open to debate.
Fibres and fire
In recent years polypropylene fibres have come into the tunnelling sector, used to enhance the fire protection of exposed concrete surfaces in new tunnel construction. Recent major projects in the UK employing these micro polypropylene fibres include the Channel Tunnel Rail Link (CTRL) and the Terminal 5 works at London Heathrow, as well as various utility tunnels.
Extensive research into the behaviour of fire in tunnels has been undertaken, both before and after the Channel Tunnel fire. The resulting evidence has shown that adding a relatively small quantity of polypropylene fibre provides a substantial reduction in the explosive spalling effect of the concrete elementsä. The research was undertaken with micro polypropylene fibres, not the macro type, and to the author’s knowledge, there appears to be very little fire research into these fibres.
The three specimens in the above picture clearly show varying degrees of spalling due to fire. These were very basic tests, carried out before moving onto more detailed testing at a later date. Lightweight aggregate was used in these particular specimens and the moisture content was fairly high, to mimic a worst-case scenario. The results were more than interesting. From left to right: steel and polypropylene fibres (low dosage rate) – some spalling but no cracks; steel and polypropylene fibres (higher dosage rate) – some surface crazing, no spalling or cracking; steel fibre only – specimen fell apart shortly after removal from the furnace. The photograph on the right is a specimen with no fibres at all, the test was stopped part way through as the specimen had begun to spall in the furnace.
New fibres
The new type of macro polypropylene, or ‘structural’, fibre that has appeared on the market in recent years, is becoming established in what were once considered traditional steel fibre market areas. These fibres display good ductile behaviour at what is considered larger crack widths, and are beginning to be used in deep mines where there are large deformations, and also in temporary structures determined to be sacrificial, such as primary tunnel linings.
However the latter of these is becoming increasingly complex. As new methods of applying primary linings become more widespread, and contractors wishing to progress works more efficiency are looking to invest in primary linings as part of the final designed structure, the contractor cannot afford a sacrificial element in the overall design of the tunnel structure.
The toughness values achieved by macro (structural) polypropylene fibres are considered low (24% – 50%) at dosage rates of between 4.5kg/m and 9.1kg/m, when compared to steel fibres with dosage rates of 20kg/m to 30kg/m, depending on aspect ratios. Bekaert has found that 20kg/m of steel fibre, with a high aspect ratio (>65), will out perform most macro polypropylene fibres (Figure 1). There is little detailed information available regarding the behaviour of macro polypropylene fibres in fire and in creep.
The new generation of high carbon steel fibres being used in high strength concretes (>100MPa) generate significant toughness values, giving high ductility. This is especially useful when considering the use of sprayed concrete as a one pass lining system, saving time and increasing productivity.
Steel fibre sprayed concrete
Steel fibres have been used extensively around the world in most types of tunnel construction, from some of the first precast elements in Eastern Sicily for water supply, to the current CTRL project and extensions of the Piccadilly Line and Heathrow Express into T5. Cast in-situ concrete linings using steel fibre reinforcement have also been successful, such as the Thames Tunnels*.
Sprayed concrete with steel fibres has been used in many forms, from simple repair materials in Europe in the early to mid 1970s, through to the present day high carbon steel fibres used in high strength sprayed concrete, such as the LaserShelltm method currently proposed for use on several major projects in London(6).
If we take sprayed concrete as the lining method, most tunnelling designs tend to use the self bearing capacity of the ground as much as possible. Therefore a method of support has to be applied that allows the ground to move in a controlled manner, until it reaches a new equilibrium by creating a self supporting arch in the ground. This will generally consist of sprayed concrete and a combination of steel reinforcement, rockbolts, and arches to support the ground.
The sprayed concrete has to have an elasto-plastic behaviour in order to follow the imposed ground deformations without failing in a brittle manner. As a ductile material, steel fibre reinforced sprayed concrete provides a bearing capacity, allowing the ground to become self-supporting and provide shear capacity. If there is potential for falling rock blocks, the sprayed concrete has to have a strong bond to the strata. This bond enables the sprayed concrete and the ground to work together, to form the self supporting arch. However, as sprayed concrete is cementitious based it is subject to shrinkage stresses that can cause debonding of layers, and thereby weaken the support mechanism. Crack control is therefore a very important benefit that steel fibres bring to the lining. As with all concrete, the lining must be effectively cured.
Steel fibre sprayed concrete can be used in final linings providing excellent impact resistance and crack width control, which helps to improve the overall durability of the lining. However, there maybe a risk of surface fibres that may be deemed as a safety hazard, or will discolour if wetted. Both of these issues can be eliminated by spraying a simple finishing coat of un-reinforced material over the surface.
Steel fibre precast segmental linings
The use of steel fibre concrete for precast segmental lining has been in use in the UK since 1994, when the first rings were installed at the London Heathrow Baggage tunnel by Miller Tunnelling (now Morgan EST). When designed with high aspect ratio steel fibres (>75), an overall reduction in lining thickness of 50mm was achieved, thus reducing the quantity of excavated material. The segments were also very robust** in nature and this minimised damage from demoulding, handling, transportation and construction operations, reducing time needed for repairs, etc.
Current developments
Within the last two years extensive research has been carried out into Self Compacting Concrete (SCC) and segmental lining production. CV Buchan has recently completed the manufacture of tunnel segments for the Dunfermline Duplicate Sewer, currently under construction by AMCO Donalon for Scottish Water.
Some of the main advantages of SCC are that the use of vibration equipment is completely eliminated from the production process, saving energy, reducing noise levels in the factory, and reducing the risk of Vibration White Finger. The properties of SCC provide a dense concrete matrix and a highly durable concrete.
CV Buchan has just started production of steel and polypropylene fibre reinforced segments for the cable tunnel in Dartford under construction by Amec for National Grid.
Conclusion
There are many benefits to be gained from the use of fibre reinforced concrete in the tunnelling industry: from the recent use of micro polypropylene fibres for fire resistance enhancement; to the relatively new macro polypropylene fibres, which display similar properties to steel fibres in sacrificial or temporary structures. Steel fibres provide excellent crack width control and this provides enhancement to long term durability, as the micro cracks are controlled and the potential for autogenuos healing to occur(8).
The reduction or elimination of traditional steel reinforcement can provide cost and time savings. However, this also offers the potential of increased safety, by enabling miners to spray concrete without risk of entering freshly excavated areas of tunnel(6).
*Steel fibre used in conjunction with conventional reinforcement
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