20 years of fibre concrete linings in the UK
John Greenhalgh, Manager Underground Sales UK & Ireland, Bekaert Ltd (UK) | october 2010 |
About 10 years ago the story of fibres in the UK market covered applications for industrial flooring, external pavements and in some tunnel works(1). Now, almost 20 years on, there is much more to tell. A lot has happened regarding the use of fibres in the tunnelling industry, not just in the UK but globally. More research, new guidance documents, the advent of European Standards for fibres, developments in fibre concrete and the relevant testing criteria, evolving performance-based specifications, the CE markings, and of course, many more projects providing the global industry more valuable experience.
The UK's first steel fibre segmentally lined tunnel was the 1.4km baggage tunnel at London Heathrow International Airport in 1995. This was a design-build project by Miller Tunnelling (now Morgan Sindall) in which the traditional reinforcement cage was removed completely and Dramix® steel fibres used instead. By using the additional load bearing capacity that the high aspect ratio of steel fibres gives to the concrete, the lining thickness was reduced from the standard to only 15cm, thus reducing the volume of concrete required, the excavated material to be disposed of and the load being brought onto the lining
Further projects followed as the industry wanted to understand this material. After further investigations a section of the Jubilee Line Extension of the London Underground was built using steel fibre concrete segments manufactured by Charcon Tunnels, now Tarmac Building Products. The twin 1.2km bores ran from London Bridge Station to Southwark Station. When final inspection took place, the contractor, Costain-Taylor Woodrow, was able to report that damage to the segments was surprisingly low. Costain-Taylor Woodrow also used Dramix® steel fibres for the first time in sprayed concrete temporary works primary linings and for the permanent lining in a ventilation tunnel.
One of the most prestigious projects undertaken in London was refurbishment of the Brunel Thames Tunnel originally completed in 1843. Operating as part of the London Underground East London Line, little had been done in terms of real maintenance over the years. By the early 1990s it was beginning to show its age and had areas of leakage that would only get worse with time. It was decided to refurbish the tunnel in its entirety.
One of the main concerns for the refurbished tunnel was the potential of a train derailment and its impact on the new lining. To address the concern, it was decided that a lining incorporating steel fibres together with traditional reinforcement would be the appropriate solution. The lining would also benefit from resistance to potential thermal cracking, and while maintaining a very small crack width, would provide excellent impact resistance. With consideration for long term durability, a corrosion resistant zinc-coated steel fibre was used. The fibre concrete was produced on site and pumped into the specially made shutters provided by Taylor Woodrow’s production facility in West London. The final finish, we believe, would have impressed Brunel himself.
Many more projects using steel-fibre reinforced segment linings followed through the 1990s, mainly in the utility sector, and the new millenium opened with two major applications were accomplished. First the tunnelling works for the new Terminal 5 (T5) building at Heathrow Airport and the long twin tube running tunnels under the northern suburbs of London for the Channel Tunnel Rail Link (CTRL) into the St Pancras terminus of the trans-European train services. Both projects used steel fibre concrete in precast segments for lining the running tunnels which are of larger diameters than most previous rail tunnel projects in London. The Heathrow Express extension tunnels to T5 are more than 5m i.d. and the CTRL running tunnels are 7.15m i.d.
Both projects also used steel fibre for sprayed concrete lining works. For CTRL, two major access shafts, at Wayside and Corsica Street respectively, were designed with steel fibre sprayed concrete linings. At T5 Morgan Sindall used a sprayed concrete lining method called Lasershell for the first time. As part of the design, ribs and support arches were eliminated completely and replaced with a high strength sprayed concrete lining containing high carbon Dramix® steel fibres with a tensile strength of greater than 2,400 MPa. This provided for a highly ductile sprayed concrete lining. Normal low carbon fibres could not have coped with concrete of such high strength.
Fibre for fire resistance
At the time of these projects, and many more since, a significant development took place to provide tunnel linings with the resistance to explosive spalling due to fire. After the first fire event in the Channel Tunnel link between the UK and France in 1996, a major test programme was undertaken by Peter Shuttleworth of RLE (the Rail Link Engineering design group) for the CTRL project.
It demonstrated clearly the benefits of using micro polypropylene fibres-monofilament in concrete to minimise the explosive spalling effect. This work has since benefited tunnelling projects worldwide with micro polypropylene fibre fire resistant coatings now specified for most public use tunnels and underground space structures.
In this respect there have been some significant projects where steel fibres, usually in combination with micro polypropylene fibres, have been used with and without traditional reinforcement cages, to provide effective protection against concrete spalling due to fire in some especially large diameter tunnels.
A major study undertaken by the client shortly after completion of the main CTRL tunnel works showed a remarkable lack of damage to the segments during manufacture, transportation and installation thus reducing repair time and costs significantly. These benefits are consistent throughout the tunnelling industry.
As the manufacturer of Dramix® steel fibre and a supplier of polypropylene fibres, Bekaert works closely with all involved on projects in order to offer the most economical design combined with the highest performance possible for the project solution. Based on this experience the key requirements proposed for precast segments, for example, are mainly the following:
• Fibres should comply with the European Standard EN 14889-1
• Fibres should have European conformity the CE marking and be System 1 fibres for structural use
• Fibres should be of drawn steel wire with a steel tensile strength of ≥ 1,200 MPa
• Dimensional tolerances should be according to EN 14889-1
• Fibre length: 60 mm
• Aspect ratio l/d =80
• Glued fibres should be used to ensure a good distribution and homogeneity in the concrete.
• Steel fibres should be added by way of an automatic dosing system
• Type of concrete should be according to project requirement and at least of C40 to C60 grade
• In some ground conditions a corrosion protected steel fibre may be considered
In 2003 a design model from Rilem(6) was published for laboratory testing of steel fibre concrete. Today there are national and international recommendations on the sizing of the structures or structural elements made up of testing these materials as well for validating the quality of steel fibre concretes.
Furthermore, clear test methods are now available for the designer to determine the minimum dosage for each project. For example, the minimum average residual flexural strength is measured with European Standard EN 14651 titled: ‘Test method for metallic fibered concrete - Measuring the flexural tensile strength (limit of proportionality (LOP), residual)’. FR1 is the residual flexural tensile strength corresponding with CMOD (crack mouth opening displacement) = 0.5mm which is the key requirement for SLS (serviceability limit state) design.
Published recently is The Model Code for Concrete Structures. Produced by the International Federation for Structural Concrete or fib (fédération internationale du béton) and is intended to serve as a basis for future codes. From it design criteria for establishing new standards and new specifications for design of fibre concrete structures will be developed. The Code concentrates on the long-term life cycle considerations for the design of concrete structures and includes a section on the design codes for fibre concrete.(7)
For the concrete mix design, Bekaert takes into account all the relevant standards, as well as the specific loading requirements of each tunnel project . This results in a specific steel fibre type and dosage tailored to each project, and guarantees the most economical and qualitative solution. The UK tunnel industry has gained a great deal of experience with steel fibre concrete over the last 15 years or so.
If one adds up the total number of steel fibre reinforced concrete lined tunnels in the UK the total length would exceed 100km. The firm prediction is that this will double easily in the next 10 years, perhaps in less than 10 years. Projects including the Crossrail under London and the Thames Tideway sewerage tunnels for Thames Water also in London have specified or are developing design criteria for steel fibre reinforced segmental linings.
Steel fibre concrete in tunnelling has been embraced in most countries around the world and it is only a matter of time before the option becomes the first option discussed at early project meetings, and is specified as the standard solution - maybe not in all cases, but certainly in most.
- J Greenhalgh; Concrete magazine, September 2001; 10 years of fibre concrete - a manufacturer’s viewpoint
- M Vandewalle; Tunnelling the World First Edition pp186; Naples Metro; Metrosud - Construction 1990/91; Lotto 3 Salvator Rosa – Vanvitelli section,
- N Varley; Concrete magazine, February 1998; Concrete Tunnel Linings at London Bridge
- P Shuttleworth; NAT Conference Proceedings, Washington DC, USA, October 2001; Fire Protection for Concrete Tunnel linings
- H Davies, E Woods, P Shuttleworth; Tunnels & Tunnelling International, March 2006; Focusing on Fibres : The CTRL Experience
- Rilem Vol. 36 October 2003 pp560-567; TC 162 _TDF Test and design method for steel fibre reinforced concrete
- International Federation for Structural Concrete-fib; Model Code for Concrete Structures 2010 - First complete draft, Volume 1 and Volume 2