Primary areas of development over recent years in the field of sprayed concrete include improvement in operator safety, such as by using non-toxic accelerators and reducing dust, better early-age concrete performance, wider use of fibre reinforcement (polymer artificial fibres as well as steel), better operator training, and also mechanised application further developing into robotic devices with sophisticated controls.
In addition, modified sprayed concrete mixes and lining designs have been developed to achieve the desirable properties for fire resistance and waterproofing or control.
In the absence of sprayed concrete design standards, albeit with agreed testing procedures (see EFNARC European Speci-fication for Sprayed Concrete & European Standards), specifications are dependent on designer preferences, which manufacturers and contractors have to meet. Product developments, both in materials and applicators, have to some extent permitted safety factors to be increased or economies to be made as efficient application and performance can now be more assured.
For example ‘shadows’ and possible voids should be a thing of the past when using correctly programmed robotic equipment with swivel nozzles to work from all relevant angles onto the surface being lined, including any installed reinforcement. Correct mixing without separation in transmission is also necessary to achieve a homogenous lining.
Ingredients
A range of safe concrete additives is available from leading suppliers to achieve the required properties of sprayed concrete both in placement and curing. These include retarders during transport, plasticisers, silica fume for increased density and waterproofing, and accelerators for binding to the target surface and early load bearing.
Mapei offers a range of over nine Mapequick liquid accelerators for wet-mix and dry shotcrete mixes. A recent introduc-tion applicable to the repair of tunnels is Mapegrout T60 sprayed concrete mix. Tests have been carried out of the material used in conjunction with Mapequick AF1000 alkali-free accelerator to avoid slump and achieve a build-up of 150mm in a single application.
Two layers of small-diameter reinforcing mesh were employed. The last 20mm thickness was sprayed without accelerator to form a floated face finish. Mapegrout T60 is sulphate-resistant, thixotropic and fibre-reinforced mortar for wet or dry-process application. Appropriate accelerator dosage can achieve the required curing time from instant to over 24 hours.
The costs questions
Something that is looming large in the minds of contractors, but has yet to make an impact with most design engineers, is the effect of soaring raw materials and energy costs. The latter, of course, affect all aspects of manufacturing and contracting.
Steel has soared in price over recent months, and it is a world-wide phenomena affected by seemingly remote factors such as the shortage of electrical power in China due to a particularly severe winter, and the consequent need to close arc furnaces, resulting in substantial loss in production. As for the steel fibre reinforcement of sprayed concrete, it is as much the uncertainty of price and supply as the higher cost that can play havoc with contract estimation unless a very large purchase contingency has been built in.
A representative of a major steel fibre supplier claimed: “We have softened the blow as much as we can by increasing the price for fibre by US$80/t twice in two months, rather than US$160-200/t in one go, as suppliers of rebar have done. But normally we cannot afford to guarantee price for longer than a month.”
Delays in shipping and the world-wide price increases mean there is little point in looking further afield unless special negotiation is successful. According to a trade bulletin the world rebar price hit US$764/t this month whilst rebar prices in China have also hit record prices, Turkish scrap prices rose sharply and export rebar hit US$880/t, and suppliers in Southern Europe and Japan all lifted prices the previous month for rebar and wire rod.
Our source said, “Cute users have been buying up stocks of fibre and we are virtually cleared out, with production going straight out the door. We can fulfil smaller short-term orders but long-term (over a month) fixed-price orders cannot be accepted without special approval and a substantial project. All the indications are that this is not a temporary condition, and we will all have to manage high prices for the foreseeable future. Whilst most contractors have taken the current situation on board as they are directly affected by the increased prices, realisation has yet to filter through to most consulting engineers, perhaps because they consider it to be a contractors’ problem. Unlike previous occasions when steel went up in price, the synthetic fibre suppliers are also affected due to increases in the price of petrochemical products like polymers.”
Vice-president of the ITA, Eivind Grov, and research manager for rock and soil mechanics with SINTEF of Norway says he is not too concerned about the increasing steel costs, insofar as they are related to sprayed concrete use, although there is a shortage of steel for rock bolts, he reports.
“I don’t think the current situation will be to the disfavour of sprayed concrete as a permanent lining,” he said. “The use of massive cast concrete is likely to suffer more because of the greater amounts of materials used.”
A downstream supplier of fibre-containing mixes commented, “We are holding our prices so far, and have not been pushed yet. There is high demand though from tunnelling and mining with a boom in rock support, especially in Asian infrastructure projects.”
Whilst steel-fibre-reinforced sprayed concrete, for example, is relatively light on materials affected by soaring prices, other forms of necessary reinforcement can greatly increase the cost of the whole lining.
Welded mesh and rockbolts are common ways of improving the strength of the lining and the surrounding ground, whilst lattice girders are commonly used in poor ground. Not only are they relatively economical in the use of steel, their use in preference to standard girder sections means that they interfere less with the application of the sprayed concrete.
Heintzmann lattice girders are available in triangular and rectangular sections for primary shotcrete lining. A ‘strong wave’ triangular section utilises a thicker steel chord on the apex. Lengths and bending radii are produced as required with connections by butt plates or overlapping. In addition the Heintzmann Group offers yielding TH-arches for extreme conditions, which, when reverse bent, give only a small spray shadow.
Similarly, Pantex lattice girders supplied by the Alwag division of DSI and designed by TAT, are available in arches of triangular and rectangular sections of different construction. ‘Wallplate beams’ for connecting arches longitudinally are also available. Alwag says that Pantex lattice girders have been tested for stability and load bearing even when not bonded into the shotcrete layer. Once covered in concrete the lattice structure in a tunnel lining allows porous zones and ‘shadows’ to be avoided for better load bearing and water-flow deterrence. The good bond between lattice girders and well-placed sprayed concrete allows the girder arches to be integrated into the design of the sprayed concrete.
Going back to steel reinforcement fibres, there is claimed to be a clear case for not buying cheap, even setting aside durability issues. Bekaert, for example, offers two types of premium Dramix fibres for underground concrete spraying; the RC-65/35-BN and the ZP 305. Bekaert’s John Greenhalgh commented: “Our premium fibres are usually more expensive but careful use can save money. Use of fibres with a lower aspect ratio (than the 64 of the RC-65/35-BN Bekaert fibre) can mean you need more to get the same performance – say 35kg/m3 instead of 20kg/m2 with ours, resulting in a total cost increase; added to which we have a long-term commitment to performance.”
Current topics
Papers and presentations scheduled for the Fifth International Symposium on Sprayed Concrete, to be held in Lillehammer, Norway, on 22-24 April reflect current developments in underground sprayed concrete technology.
Anders Ansell of Sweden’s KTH research body will present practical guidelines for shotcrete use on rock exposed to blasting, whilst Ø Bjontegaard of Norway will cover testing of the energy absorption of fibre-reinforced sprayed concrete. Ansell will also present with Jan Holmgren on tests on restrained shrinkage of shotcrete with steel fibres and glass fibres.
A wide range of conditions for sprayed concrete use are recognised from around the world, naturally including a paper on the use of wet-mix sprayed concrete in permafrost rock from T Beck of Norway. Another environmental topic from Norway, by Per Hagelia of the Norwegian Pubic Roads Administration is the deterioration of steel-fibre-reinforced sprayed concrete by the action of sulphate-enriched groundwaters and bacteria.
There are to be other papers covering ingredient performance considerations, especially on accelerators and reinforce-ment fibres, although there have been few substantial developments in the ingredients themselves in recent years. Performance studies and the most effective use of sprayed concrete are the main foci of development, with quite a variation in expertise between countries.
Eivind Grov has commented that Norway and other Scandinavian countries, who were the front-runners in the development and application of wet-mix shotcrete, appear somehow to be caught up by the rapid development of shotcrete in other countries. This may be due to the general rock conditions there but Grov thinks it is more a matter of approach. “From when I attended a conference in Tasmania in 2001 I noticed a difference. The people there have really been quite advanced in the use of sprayed concrete. They have a thorough theoretical approach, whereas the Scandinavian approach has always been empirical. We can improve by the theoretical approach to design, and in other aspects of sprayed concrete. We are developing tests to learn more about the abilities of sprayed concrete and in the application of materials knowledge. Fibres are being tested in many countries. Taking the best from both approaches will make sprayed concrete a winner, which is indeed required in the tunnelling industry.”
“We in Scandinavia, including the Norwegian tunnelling industry, may be becoming too laid-back. We will try to ‘kick the legs’ of tunnellers to prevent them from falling asleep and keep up the drive in improving, testing and documenting shotcrete application. This is important for all participants in the tunnelling industry.”
Of course there will also be reports on the work of ITA Working Group WG12 from Animateur Tarcisio Celestino, Themag Engeharia, Brazil and former Animateur Knut Garshol, now of the USA, who will present the recent “State-of-the-art report on shotcrete” prepared by the working group members, collecting practices from the member nations of ITA.
Hindhead Tunnel
The UK’s 1.9km Hindhead Tunnel will probably use one of the most advanced sprayed concrete linings to date in that the process will be even more closely tied to careful profile excavation, as far as ground conditions will allow. Excavation with a Liebherr backhoe plus sprayed concrete lining using three BASF Meyco Logica robotic spraying units (see below) has been selected rather than a TBM. The key is working close to the planned profile using Amberg 3-D profile monitoring integrated with the machinery control systems.
Application
The Meyco Division of BASF offers a wide range of concrete spraying equipment for all sizes of tunnel, portals and various site conditions. The Meyco Potenza mobile spray unit has been developed from the Spraymobile and retains its main features but with central power and control. These include the Meyco wet spraying machine, TDC dosing system, Robojet manipulator and a 4×4 Dieci chassis. Six Potenza units have been delivered for use on the 6.5km high-speed rail Lainzer Tunnel, in Vienna, Austria . The main purpose is producing a thick primary lining in unconsolidated rock.
The Logica is Meyco’s most technically advanced system with robotic control to maintain an accurate profile and lining thickness. The hardware has been further developed with automatic control and georeferencing. The system scans the surface of the tunnel with external georeferencing tracked by a total station. This prepares a 3-D model of the tunnel section to be lined. The concrete spraying is controlled automatically by robotics integrated into the spray vehicle. Three units are to be used at Hindhead.
CIFA has two main mobile sprayed concrete systems; the self-propelled, twin-axle CSS-3, and the PCS 209 single-axle trainer unit. The former carries its own water and liquid accelerator tanks. The Uniflux EAS 2 accelerator dosing system is configurable in two capacities to meet different flow rates, and has electronic control management. Depending on the boom configuration the CSS-3 can cover tunnels from 2 to 16m radius. The remotely operated nozzle head swivels to 45 deg either side of the longitudinal axis and through 180 deg in the vertical plan.
The PCS 209 trailer can be towed at 6km/h, powered by diesel or electric drive.
Sealing
As a rapid means of surface stabilisation on rock or soil UGC International (BASF Europe) introduced Masterseal 845A/855 thin spray-on liner (TSL). This provides weathering protection and well as acting as a base layer for further sprayed concrete lining with which it forms a good bond to allow single-shell linings. Curing takes place within a few hours. It is generally only one part of a final support system and chiefly aimed at mining applications. Another grade of Masterseal is used in tunnels primarily for waterproofing. Masterseal uses dry powder spray pumps with water added at the nozzle.
Aesthetic support
Under discussion is the acceptable finish of a final sprayed concrete lining. Is a rough finish acceptable for pubic access tunnels and others where lighting is important? Can the finish be made more acceptable by decorative panels or additional coatings? Or is it necessary to smooth the surface by an additional layer or mechanical process?
Former head of technical management of BASF UGC International, Ross Dimmock, reports that the use of sprayed concrete lining is of growing importance and interest in underground urban railway construction for permanent as well as temporary lining, due to improved capabilities. The finish is naturally a consideration. “In the Santiago Metro, Chile, for example, there are huge spaces supported with a permanent sprayed concrete and membrane sandwich construction. The finish is ‘as sprayed’ and over-painted. There are also decorative panels including glass sections and fascias. The Helsinki Metro also has a sprayed finish that looks really modern.”
“A drawback of a smooth finish is that it shows imperfections such as cracks and seepage more clearly.” says Dimmock.
Neither should a spray finish in running tunnels be a problem, even for high-speed railways. “With AlpTransit (in Switzerland) it was thought that eddy currents in the air could be a problem in deciding between a cast or sprayed final lining,” continues Dimmock. “It was found, however, that only the tunnel services presented a potential problem as long as the sprayed concrete was applied to the correct profile.”
“A two-way perspective is necessary,” commented Grov. “It involves two different things – one related to the need for rock support and the other to architectural aesthetics. It’s a long story. In Scandinavia we are utilising the capacity of the rock itself, only supplementing its self-standing capacity until you have sufficient. Other countries use it as a temporary phase only and add another permanent lining to finish. In Norway we try the ‘self-standing’ approach for all sorts of ground, but there is a limit. We can also use sprayed concrete followed by bulk cast concrete, but that amount is very small. There are a large number of projects each year that use no concrete at all, with cast-in-place used in very bad ground only. We always try to find a support solution that will replace cast-in-place concrete, such as by using steel-fibre-reinforced shotcrete.”
Elevated walkway on Line 4 of the Santiago Metro in Chile Elevated walkway on Line 4 of the Santiago Metro in Chile Glass panels over rough sprayed concrete on Line 4 of the Santiago Metro Glass panels over rough sprayed concrete on Line 4 of the Santiago Metro Screen dump from the Meyco-Amberg profile control monitoring system used in the Logica automated spraying system Screen dump One of six BASF Meyco Potenza mobile spraying systems used in the Lainzer rail tunnel in Vienna, Austria BASF Meyco Potenza mobile spraying system Spraying the face on the Gotthard HSR Tunnel in Switzerland Spraying the face on the Gotthard HSR Tunnel in Switzerland
