Whatever one’s preference in philosophy and terminology for how sprayed concrete linings are used, both the tunnelling design engineer and contractor need to be sure of the properties of the lining material at all stages of its application. This includes everything from the delivery of the materials to site, through mixing, materials workability, effects on working environment, losses through waste, quality of application, and both short-term and long-term load-bearing properties.
All can be affected by the materials used, how they are combined and the means of application. Today there is much less tolerance of over-specified linings, waste, hazards both during construction and within design life, slow and variable processes, and even poor aesthetics in public areas. This results in new, or previously unrealised demands.
It is essential that designers and audit teams can make valid tests of the lining materials before and after application to check that they comply with specifications and that the correct performance can be expected or, if not, remedied.
Agendas
Recognising the special importance of sprayed concrete, or shotcrete, the International Tunnelling Association (ITA), set up a Working Group on ‘Shotcrete Use’ (WG12). It’s many topics and activities can be hampered by the long periods between meetings for decision making, a fact that has been recognised by a move to increase the number of meetings as much as possible. This includes group meetings at the recent ‘Shotcrete for Underground Support X’ in Canada last September (T&TI October 2006, p30). An ITA Internet Discussion Forum is also available for WG members.
Fortunately many other bodies, both national and international, industrial and academic, have strong interests in the efficient use of sprayed concrete linings underground, many with overlap in terms of leading practitioners and activists.
Other interested organisations include EFNARC (European Federation of Producers and Applicators of Specialist Products for Structures), AFTES (French tunnelling society), the UK Sprayed Concrete Association, etc.
Human factors?
A shotcrete nozzleman’s job is one of the most difficult, and potentially hazardous, in tunnelling. Manual handling is laborious, visibility and breathing can be difficult, and there may be health concerns about the content of the materials. In order to produce the required coverage around irregular rock surfaces and reinforcement structures, especially when providing temporary support, the nozzleman may be tempted to place himself under poorly supported ground. Various systems are available to avoid these problems but the potential hazards remain.
The ITA Executive Council has charged WG12 with tackling the issue of accidents in shotcreting. The Group is collecting information on actual cases including the deleterious consequences of some chemical additives, and excavation instability due to the lack of early support capabilities of the shotcrete.
A nozzleman who is tired, uncomfortable and worried about his health and safety is not going to produce a good lining, however dedicated. Therefore his task needs to be improved by measures such as personal protection, better working position and access, lighting and control of potentially hazardous ingredients in the shotcrete mix.
The main concern over shotcrete ingredients has been alkali-based accelerators to achieve early curing to prevent slump on the coated surface. Due to their corrosive and irritant nature they are largely banned now. However, the alternatives also carry hazards with them, albeit lesser in practice. A recent data sheet from BASF UGC on the Meyco SA 167 alkali-free, liquid accelerator points out that it is acidic, an irritant and can damage eyes. Most problems can be cured, however, by applying plenty of water. Traditional accelerators included sodium silicate, which is not generally hazardous but is less effective. Safety authorities also have concerns over the amount and size of silica particles in shotcrete. There are also likely to be physical hazards such as scratches from steel fibre reinforcement, material rebounding from the face, and dust. Consequently the necessary protective equipment such as safety gloves, eye protectors and respirators should be worn when necessary. With additives, as with all forms of shotcreting, there must be adequate ventilation.
Ultimately automation is a probable, but maybe only partial answer to the problems with manual operation. Perhaps one of the most advanced expressions of mechanised concrete spraying equipment is the Meyco Robojet Logica laser-guided spraying robots to give accurate concrete layer control and, through programmed uniform movement of the nozzle, proper coverage to avoid voids and spray around obstructions. Spraying can be carried out automatically. Using a joystick remote control unit, the robot can operate with eight degrees of freedom. The laser scanner ensures that the sprayed material is applied to the tunnel surface to pre-set values automatically, or guided by the operator. Tolerances of 3-4mm have been achieved in projects. The system keeps the distance and angle to the application surface correct to reduce material rebound to a minimum. Although the mix composition, especially the use of accelerators, has a major effect on rebound, in general the nozzle should be held perpendicular to the surface being coated, and never more than 45 degrees from the perpendicular.
An alternative, or complementary factor, to remote-control or manual nozzle operation is a long boom for correct placement. BASF UGC Meyco recently has just introduced a new, larger Roadrunner with the Meyco Maxima spray manipulator. The equipment is mounted on a Mercedes-Benz Actros 4-axle carrier truck for travel between sites. The boom can be extended from the park position to the most awkward spraying position for semi-automatic operation.
Training and certification
Both for safety and quality control reasons, nozzlemen need adequate training and certification to prove it. As with many skills qualifications, appropriateness and uniformity can be problems. Many countries require nozzlemen to be certificated. Other interested bodies are also working on international certification programmes. The ITA WG12 is working on standards of nozzlemen certification incorporating reports from France, Norway and Brazil. Other countries’ input is being sought.
Reinforcement questions
There has been much discussion, if not argument, between proponents of the three main forms of sprayed concrete reinforcement available; i.e. steel fibres of various design, ‘synthetic’ fibres of suitable plastics, and pre-erected steel structures such as lattice girder arches and interlinked welded mesh sheets. In relatively poor ground the need and efficiency of rock bolting also has to be taken into consideration since a well-designed programme greatly assists the fresh shotcrete in its task of early support.
Although major suppliers now offer both steel and plastics structural reinforcement fibres (as distinct from plastics fibres for fire damage control), the claims for each can still be difficult to unravel. Independent testing under realistic conditions is therefore of great importance. The ITA Shotcrete Use Working Group is running a test programme at the Hagerbach test galleries in Switzerland using three different types of fibres.
A great advantage of fibre-reinforced sprayed concrete over using unreinforced shotcrete on conventional reinforcement is labour and time saving. Bekaert cites a Swiss example where the use of 100mm thick steel fibre reinforced shotcrete lining was chosen to replace the designed 130mm thick lining of sprayed concrete reinforced by welded mesh. There was a 25% time saving reducing the cycle time from 12 to 9h. Both lining types included rockbolts, and the maximum shift allowed is nine hours.
Controlling input
It is important for concrete workability, curing performance, and possibly safety, that the mix and its ingredients are as close as possible to the correct mix design. Several automatic dosing devices have been developed, but these are getting more accurate and easier to use. Normet’s mid-range sprayer, the Spraymec 7110 WPC, is now available with a new configuration of the Nordoser 900 EH 4G accelerator dosing system. The system monitors and maintains the accelerator flow proportional to actual concrete delivery. The dosage rates can be preprogrammed according to the mix design. To withstand underground use it is housed in a shock-resistant IP65 enclosure.
The improved features of the new configuration Nordoser 4G include a reduction in control modules from three to two, multiple data storage capacity, a colour display module and USB data transfer. There are separate displays for settings and event registration. The operator can select an intermediate report showing cumulate data for concrete and accelerator used to that time. Pushbutton control allows such data production for more than one location. Registration is automatic if the accelerator or pump rate is reset. Data can be removed from the machine at the end of the shift using a standard USB-stick for supervisory checks.
Correct inclusion of accelerators can provided improved strength both early and in the long term. Using the BASF UGC range of other additives as examples, workability can be adjusted using superplasticisers for a low water/cement ratio (UGC Glenium and Rheobuild), while hydration control admixtures prolong the workability if there may be a delay in the use of the mixture during long transport, etc. (UGC Delvocrete). Low permeability, high-strength sprayed concrete can be achieved using additives based on micro- and nano-silica technologies (UGC Meyco MS range).
In constructing the Athens Metro the points of use of sprayed concrete were located at many points considerable distances from each other and the batching points. Consequent transport delays and cyclical delays meant that conventional mixes could be beyond their usable life at the point of use. Although the dry-mix process was specified, contractors became convinced of the benefits of wet-mix spraying. The sites layout meant that there could be interruptions of 3-4h between applications of 3-4m3 of sprayed concrete. The time from batching plants to use could be 3-72 hours compared to 1-2 hours for conventional sprayed concrete mix with accelerator. Using UGC Delvocrete Stabiliser at up to 2% of the binder weight, hydration could be delayed for up to 72 hours.
Testing for quality
The shotcrete mix and its application can be tested at various stages in the process including sampling of the mix (wet or dry), checking for voids and coverage after spraying, laboratory testing of cured panels for load-bearing properties, and, of course, the monitoring of ground movements after spraying by observational methods.
Various forms of panel tests have been adopted, but the ITA Working Group on Shotcrete Use states that the current standards available for testing panels do not properly cover specimen preparation. The consequence is a scattering of results, which can be improved by standardisation of specimen preparation. The Group is also collecting information on problems arising from current procedures.
It is difficult to simulate underground conditions for a sprayed concrete lining in laboratory tests due to the irregular rock surface, variable concrete thickness, and other variabilities, but it can be argued that a test sample of thin slab sprayed concrete is more accurate for fibre-reinforced shotcrete than beam testing. Based on a Norwegian test programme from the ‘80s, Rivalain and Durand of SNCF and Alpes Essais Laboratory respectively developed the flexural/punching square slab text. This is designed to simulate the behaviour of the lining under rock pressure around a rockbolt. The test is also included in EFNARC recommendations and the European CEN standard for sprayed concrete. The test slab is supported on four edges and the load applied at the central point. The load deflection curve is recorded until a deflection of 25mm at the central point is reached.
Is the job done?
Once the concrete has been sprayed on, the job does not end there. The lining should be tested for voids and the cured concrete tested for ‘final’ load bearing. This is distinct from convergence measurements, etc. in observational methods of construction.
Another approach is to log concrete spraying performance to ensure that certain specified parameters have been complied with. This is facilitated by the control systems available on the more advanced mechanised spraying equipment. The ability to prove performance, both in concrete spraying and of the applied lining, can help convince clients that are not persuaded by the likely cost-savings alone. If the client has been affected by bad experiences in the past, they may be persuaded by a highly professional approach and the inference that something was basically wrong with the procedures rather than the sprayed lining methods.
The additional properties of fire resistance and ‘waterproofing’ that may be imparted to sprayed concrete linings can have economic benefits, even if it may be necessary to incorporate a ready-made plastics membrane lining for impermeability. Other approaches include microsilica additives (see above) and spray-applied waterproofing membranes such as UGC BASF Masterseal. ‘Single shell’ sprayed linings are being used increasingly, offering more cost savings over a two-stage approach.
Conclusion
Tunnel designers and contractors, under continuing economic pressures, cannot wait for all-encompassing guidance and often have to carry out their own trials for each project or set of conditions encountered. One trouble is that this can be wasteful overall by duplication of effort and resources. The answer is a free flow of information on experiences and test results with minimal commercial distortion. Thus conferences, study groups and research programmes do have important roles to play provided they are frank and run efficiently to meet industry needs within a reasonable time frame.
