Over the last 10 years developments in sprayed concrete technology have progressed rapidly and, as a consequence, the use of shotcrete has increased dramatically worldwide. Today more than 8Mm³ of sprayed concrete is applied in ground support every year(1). Current advancements are being driven by evolving underground construction techniques and a ‘holistic’ approach to creating high-quality permanent sprayed concrete structures, considering a design life of 100 years or more, using modern materials and application techniques.

Durability is the key issue. “Creating permanent sprayed concrete structures cannot be achieved unless you look at a number of important durability factors,” says Ross Dimmock, technical director for MBT UGC and technical chairman of the European Technical Committee for Sprayed Concrete (EFNARC). “The advancement of automated application techniques and shotcrete materials provides huge potential, but in order to meet this, equal attention must be paid to the development of buildable designs and the training and accreditation of applicators.”

Quality in control

Until the mid 1990s, the dry-mix process was the dominating method of applying sprayed concrete. Today 70%-80% of sprayed concrete is applied using the wet-mix method(3), and with rapidly increasing tendency. “Within the next five years wet-mix is anticipated to represent 80%-90% of all sprayed concrete works worldwide,” estimates Dimmock. The wet mix is an attractive option, as the concrete is mixed and water added under controlled and reproducible conditions. The w/c ratio is under control. “One often forgets, however, that the equipment makes heavy demands on the fresh concrete, first of all in terms of pumpability,” Dimmock points out. “Furthermore the method requires larger amounts of fast setting admixture materials, which may lead to overdosing and loss of strength in the final product.”

Considering the rapid trend from dry to wet-mix shotcrete, it is clear that future developments will be centred on the wet-mix process. It therefore comes as no surprise that manufacturers are focusing on the continued improvement of additives, machinery and equipment for wet-mix methods.

Admixtures and additives

With the use of wet-mix comes the requirement for various admixtures to control the properties of the concrete. For Dr Gustav Bracher, specialist materials engineer for Sika Tunneling & Mining, the clear development over recent years in shotcrete additive technology is the introduction and advance of alkali-free accelerators. Replacing caustic aluminate-based accelerators is now a requirement not only in developed ‘high-tech’ countries, but also in ‘low-tech’ developing countries. Such aluminate-based accelerators, unless handled very carefully, cause severe skin burns and blindness if splashed into the eyes. They are also caustic to the environment and markedly increase the alkalinity of the concrete, causing alkali-reaction deterioration of the shotcrete lining with time. “However, they were very fast reacting,” says Bracher. “It was the search to produce an alkali-free accelerator with comparable properties that drove research.

“Today there are two generations of alkali-free accelerator – the first are less expensive products, but these can prove more expensive in the long run through the need for high dosage rates and a higher rebound rate due to slower reaction times. The second generation products are more expensive, but can save time and money in the long run.”

Dimmock agrees: “Due to their complex chemistry, alkali-free accelerators are legitimately much more expensive than traditional accelerators. However, accelerator prices have very little influence on the total cost of in-place sprayed concrete. Of much larger consequence are the time and rebound savings achieved, the enhancement of the quality and the safe working environment.”

Another significant consideration when choosing a shotcrete accelerator is its compatibility with superplasticisers. The development and application of superplasticisers to improve workability and extend the ‘useful life’ of concrete has taken a high profile in wet-mix shotcrete technology. “At present there are superplasticisers that will extend concrete mix life to 2 hours or so, but after that two products are needed – the superplasticiser and a stabiliser, to increase workable life to 6+ hours. What we are all searching for at the moment is a single product that will do the work of both additives for longer term life,” says Bracher. “We don’t give a figure for the longest possible extension of workable life for concrete because this is a meaningless concept in reality.”

Extending the life of wet-mix concrete is a function of the particular logistics of the contractor and of the job site set-up, as well as the shotcrete mix ingredients. Today, contractors often make use of mobile batching plants, which greatly reduce the uncertainty of delivery times in congested urban areas, as well as the long distances from the batching plant to the tunnel site. “The bigger concern is producing a wet-mix shotcrete at the batching plant that is efficiently and effectively compatible with the accelerator introduced at the nozzle. An extended life of 8-12 hours without slump loss and using a single rather than a two-part product is the primary target for most contractors,” says Bracher.

Equipment development

It is now no longer possible to talk of shotcrete or concrete additives without considering pumping and placing equipment. The same is also true in reverse. Application and development of shotcreting equipment cannot take place in isolation, separated from simultaneous concern and development of additives. This has been a reality in the industry for some time and the major additive producers have developed their own ranges of application equipment, and concrete equipment manufacturers have tried developing their own ranges of specialist shotcreting equipment.

Sika Tunneling & Mining’s long term association with, and subsequent take over of, the Aliva range of shotcrete equipment has been highly successful. Development of a wet-mix pump however was a more complex undertaking. Finally, through a series of unsuccessful partnerships with other wet mix pump manufacturers, Sika has established an alliance with leading concrete pump manufacturer Putzmeister. Rather than competing with each other, the Sika/Aliva/ Putzmeister alliance aims to cover all essential components of modern, high production, high quality wet-mix shotcreting. The new Sika-PM 500 shotcrete manipulator was the first system to be developed for tunnelling by the alliance. The machine has a compact design and incorporates the WETKRET 133 shotcreting boom. It can be additionally equipped with two 1000lt admixture tanks and a 600lt water tank. For smaller tunnel sections, the Sika-PM 400 is also now available.

Another result of manufacturers adopting an integrated approach to equipment design can be seen in Meyco‘s Suprema and Potenza models. Meyco, the equipment division of MBT UGC, has developed these machines in order to offer pulsation-free conveyance of wet-mix from the pump to the nozzle. An electronically controlled push-over system is integrated into the output adjustment and brings the pulsation of the flow to a minimum. An integrated programmable control system (PLC) co-ordinates and controls all functions of the machines. Importantly, a dosing unit, the Meyco Dosa TDC system, for liquid admixtures is integrated into the drive system of the machines and is connected to the PLC system. This synchronises the dosing of accelerator to the concrete output of the pump. “This is of critical importance when constructing permanent sprayed concrete structures, so as to reduce the risk of reduced strength and durability with over dosing,” says Dimmock.

Automation and integration

One of the most significant moves towards consistent high-quality sprayed concrete linings has to be the continued automation of placing equipment. Meyco’s Robojet Logica is one the first of a new generation of fully automatic computer controlled manipulators. The computer controlled system measures the profile of the tunnel with a laser scanner and applies the material to preset values. This kind of automation improves consistency of application and reduction of rebound, but also, more importantly, eliminates the exposure of risk to miners working in unsupported ground.

It was this risk of exposure to falling (or sliding) ground, that prompted specialist shotcrete contractor, Morgan Est, to develop the LaserShell lining method(4). In the six year development of this technique, Morgan Est Tunnelling (formerly Miller) worked with numerous manufacturers and equipment suppliers to refine a sprayed concrete lining technique that would effectively remove the risk to workers at the face. Combining research into concrete mix designs, high carbon steel fibre fibre support, sprayed waterproofing membranes, automation of application and a training accreditation course for shotcrete technicians, has resulted in a truly ‘holistic’ approach to advancing the method.

Training and modern specifications

Although modern technology goes some way towards improving the consistency of spraying application, the quality of the end product is still significantly reliant on the skills and experience of equipment technicians. “The number of specialist contractors working with sprayed concrete has increased over the last few years,” says Dimmock. “Some of these contractors, such as Morgan and Veidekke, have developed specialist training courses for their operators and this has raised the quality of application globally. What we now need to address is the need for international standards for nozzlemen training.” It is with this view in mind, that EFNARC is now working on the development of a training and accreditation scheme, which is in line with the Sprayed Concrete Specification (1996)(2), and which can be adopted by national training bodies (such as the Construction Industry Training Board (CITB) in the UK).

Meeting the potential

Wet-mix shotcrete, applied using modern environmentally safe admixtures and high performance equipment, provides the industry with an economical tool to construct permanent, high strength, durable concrete structures.

Advantages such as flexible logistics, safer working conditions and reduced environmental impact, offer further potential for the technique. The application process has also become highly automated, significantly reducing the degree of human influence that has, in the past, prevented some clients from considering sprayed concrete as a permanent structural material.

The time and cost saving potential in the application of wet-mix fibre-reinforced shotcrete as permanent support is in most cases substantial, and sometimes dramatic. It must be ensured that design methods allow the use of such permanent support measures.