Understandably in view of the recent economic situation, development of hardware has been minimal lately. One of the most advanced concepts, the Meyco Logica automatic application robot, was introduced a few years ago and seems to be most often applied to spraying on waterproofing membranes or fireproofing mortars, but is a valuable addition to tunnellers’ capabilities for more accurate and consistent linings at lower cost.

Hardware manufacturers have been concentrating on improving customer service, gaining experience and increasing their share of an increasingly global market. What hardware development there has been is mainly in improved control systems and monitoring of performance, as customers usually want records of the work done. The Logica now has an improved ergonomic and automatic control system to operate the Robojet-based manipulator with eight degrees of freedom. A laser scanner is used to measure the tunnel profile and consequently allow calculation of the standoff distance and nozzle angle.

For medium-to-large tunnelling projects requiring anything but localised shotcreting there is little alternative to wet-mix, mechanised sprayed concrete to meet production requirements. Equipment such as Meyco’s wide range for different excavation dimensions and deliveries, Normet’s Spraymec range and the Putzmeister/Sika models are mainly designed for efficient underground use.

Service first
Frequently improving customer aftercare involves co-operation with a major, complementary manufacturer in a noncompeting activity. The joint marketing of Sika (materials) and Putzmeister (equipment) is now well established. Last year Meyco BASF made a global cooperation agreement with Sandvik Mining & Construction for tunnelling and mining. Initially this covers co-operation on sprayed concrete machinery servicing.

Normet has also been strengthening its international coverage, particularly in tunnelling. Normet’s build-up over recent years produced its best ever sales year in 2008 and, despite the subsequent downturn, is well positioned technically and logistically to serve tunnelling more widely.

Which materials?
Similar to hardware, materials manufacturers have come up with little new, although Mapei of Italy, which operates a specialist underground division (UTT), maintains seven major research centres for all products. Some 5 per cent of its sales revenue and 12 per cent of employees are in to r&d. Most suppliers have been concentrating on marketing, and meeting customer expectations, including specifications and codes. Exceptional development trends include more use of polymers.

After several years of trying to persuade specifiers of the viability, if not advantages of macro-synthetic fibres (as opposed to micro-synthetic fibres for fire resistance) in shotcrete, suppliers such as Elasto-Plastic Concrete (EPC) and Propex (Fibermesh 650) are now beginning to gain major orders for both primary as well as secondary reinforcement. EPC’s Andrew Ridout reports more large orders from the US and Norway, as well as Japan and Australia.

Of course steel fibre suppliers such as Bekaert are not taking the challenge lightly and emphasise certain performance characteristics which, they say, make steel fibres a better choice for shotcrete reinforcement.

There are now many standards covering the ingredients, methods of application and testing of sprayed concrete (shotcrete) structures. The most recent include:
• BS EN 13670:2009 – Execution of concrete structures.
• ISO226966:2009 EDTN1 – Execution of concrete structures.
• BS EN 934-2:2009 – Admixtures for concrete.
• BS EN 1504-9:2008 – Products and systems for the protection and repair of concrete structures.

In addition, the European Federation for Specialist Construction Chemicals (EFNARC), drew up a specification and guidelines in 2008 for thin spray-on liners for initial stabilisation and sealing.

There has been much discussion over many years about how shotcrete should be used in tunnel lining. The fully designed sprayed concrete lining (SCL) approach was developed in response to the more ad hoc observational approach of traditional NATM in deciding what type of support (albeit pre-designed) to use, usually involving various levels of reinforcement and thicknesses of shotcrete.

A particular extension of SCL is the CombiShell lining developed by Miller Tunnelling (now Morgan Est) and Beton u Monierbau (now Alpine BeMo Tunnelling). Used on the London Heathrow Airport Baggage Tunnel, the concept has been further developed by these two partners with Mott MacDonald as LaserShell, used on the Heathrow Terminal 5 Tunnel and London King’s Cross rail station redevelopment. It employs TunnelBeamer laser profiling of the excavation and shotcreting to achieve the designed tunnel lining profile in soft ground.

Even though mechanised shotcreting has the advantages of virtually tireless application from comfortable operator positions, it still requires considerable operator skills, unless the process can be totally computer controlled in a robotic operation. Even then manual intervention may be necessary.

Below-par skills have led to ‘shadows’ in applied shotcrete around obstructions such as around support materials or rock projections. There may also be a lack of uniformity in application of layers, requiring return visits to meet the specification.

For these reasons, in addition to health & safety, proper education of operators has become a major priority for contractors and promoters. The result is a drive to establish nozzleman training and certification in various parts of the world. The efforts have been mainly national with, for example, the American Shotcrete Association organizing certification with the American Concrete Institute running training and including schemes in other countries such as Singapore. At the end of last year, the Sprayed Concrete Technical Committee of EFNARC launched its own Nozzleman Certification Scheme. It does not cover training but an examiner assessment course is being run by CUC for EFNARC at the Hagerbach Test Gallery, Switzerland.

Alternative approaches
At the start of a project the twin emphases in providing sprayed concrete have to be on long-term design performance and cost control. However, with the many unexpected problems that can develop during construction, suppliers are often called upon to provide fast solutions. This may involve re-examining other approaches.

For example, the dry-mix shotcrete process has been largely restricted to small tasks, repair work, and small excavation dimensions that make the use of wet-mix equipment difficult.

In addition to promoting the benefits of not using the apparently ubiquitous shotcrete additives, Phil Richardson claims other advantages for the dry-mix Natural Cement (NCD) shotcrete that his company distributes. “It’s ironic that our shotcreting capabilities have been largely ignored,” he reports, “but now we are being called in to help with waterproofing problems.”

The company’s Shotcrete 513 and 530 mixes are designed for fast setting and high early strength, especially in wet and cold conditions. “Thus where other products have failed in the presence of flowing water,” claims Richardson, “ours are able to adhere to the surface with early, and continuous, strength build up. As well as helping to deal with water, the Shotcrete fast cure can save projects a lot of time. With Shotcrete 513 we can achieve 5MPa compressive strength at 15 minutes – that’s about 25 minutes after spraying – with 65MPa at 28 days. Compared to other dry-mix processes there is less rebound and dust make. ”

Its inertness has resulted that Shotcrete 513 has met strict specifications for use in association with water supply and aquifer work. Galeforce Engineering used it to stabilise limestone in canal tunnels in the English Midlands. Managing director John Aveling said, “In addition, due to its waterproofing and water-stopping properties, it has also been utilised for areas of substrate with severe water egress such as piled walls and tunnel linings and has proved very effective in difficult circumstances, even as far as having been sprayed onto flowing water.”

NCD Shotcrete mixes have been used in the Hindhead Tunnel lining by Balfour Beatty to cover leaks from end-of-shift joints in the support lining. Also, in trials with contractor Morgan EST the material stopped leaks of up to 2-bar pressure.

Richardson claims distinct advantages from not employing additives in the right concrete mix. “With no modification to the mix, we can eliminate the ‘idiot factor’ in preparing for shotcreting,” he claims, “and because the material is natural we have no waste disposal problems.” He also says that serious consideration has to be given to the effects of all current concrete additives may have on future generations.

Natural Cement Distribution generally recommends the Lancy Tubaflow shotcrete equipment and, when shotcrete reinforcement is required, Bekaert Dramix steel fibres to increase tensile strength. The compact Lancy Tubaflow air-operated, trailer-mounted unit for both wet- and dry-mix spraying, has pumped over 650m.

A Meyco Logica 15 automatic spray robot applying a Fireshield membrane One of two Putzmeisetr-Sika SPM 500 PC wet-spray concrete manipulators used on the B29-Schwäbisch Gmünd bypass tunnel for a thick shotcrete lining sealing below the water-table