Shotcrete in MiningMASTER BUI LDERS ADMIXTURES Shotcrete (Sprayed Concrete) for Mining Applications by A.J.S. (Sam) Spearing Director – International Mining Underground Construction Group Master Builders Technologies A Division of Degussa Construction Chemicals Telephone: +1-216-8397213 Telefax: +1-216-8398801 E mail:
[email protected] January 2002 INDEX 1. An introduction to shotcrete 1.1 Definitions 1.2 History 1.3 Shotcrete as a support element 2. Shotcrete raw materials 2.1 Binders 2.2 Aggregates 2.3Water 2.4 Admixtures 2.5 Fibres 3. Application processes 3.1 Dry process 3.2 Wet process 3.3 A comparison between the processes 3.4 Process selection guidelines 4. Shotcrete design 4.1 Placed strength 4.2 Time 4.3 Shotcrete components and performance 5. Shotcrete mining equipment 5.1 Shotcrete spray equipment 5.2 Shotcrete dosing pumps and systems 5.3 Shotcrete lines and nozzles 6. Shotcrete service needs 6.1 Power 6.2 Compressed air 6.3 Water 7. Logistics 7.1 Slick line design and operation 8. Shotcrete application 8.1 Substrate preparation 8.2 Shotcrete spraying 8.3 Application safety 9. Training 10. Quality control and testing 10.1 Quality control 10.2 Field testing 10.3 Laboratory testing 10.4 Quality assurance planning 11. Costs 11.1 Material costs 11.2 Equipment costs 11.3 Material and equipment transportation costs 11.4 Labor costs 11.5 Application inefficiency costs 11.6 Time related costs 11.7 Other costs 12. Specification and design procedures for shotcrete support 12.1 Observational methods 12.2 The Q system 12.3 The modified Q system 12.4 The ground characteristic curve method 13. Mix design examples and case studies 13.1 Standard dry mix 13.2 Standard wet mix 13.3 High early strength with high single pass 13.4 Fiber reinforced shotcrete for slick-lines 13.5 Fiber reinforced shotcrete for slick-lines in sinking shafts 13.6 A mix to replace bolts and screen 13.7 Shotcrete for kimberlite (blue rock) 13.8 Shotcrete for water inflow areas 13.9 Wet shotcrete for economical reasons 14. The future 14.1.Future advances in shotcrete equipment 14.2.Possible improvements in synthetic fibers 14.3 Future admixture advances 15. References 16. MBT Shotcrete Admixture Products 17. Useful contacts Ø American Concrete Institute Ø American Shotcrete Association Ø American Society for Testing and Materials Ø Australian Shotcrete Society Ø British Standards Institute Ø Deutsches Institut fuer Normung Ø EFNARC Ø International Centre for Geomechanics Ø International Tunneling Association 1. An introduction to shotcrete (sprayed concrete) 1.1 Definitions Shotcrete according to the American Concrete Institute (ACI) is defined as pneumatically applied mortar or concrete, projected at high velocity. Sprayed concrete according to the European Federation of Producers and Applicators of Specialist products for Structures (EFNARC) is a mixture of cement, aggregate and water projected pneumatically from a nozzle into place to produce a dense homogeneous mass. Sprayed concrete normally incorporates admixtures and may also include additions of (steel or synthetic) fibers or a combination of these. Sprayed concrete and shotcrete refer to the same material. The trend especially in Europe is to refer to the product as sprayed concrete. The principles applied to standard concrete technology are no different in shotcrete (hence the growing preference to calling shotcrete – sprayed concrete). This means that shotcrete should be designed to achieve a technically acceptable and a cost effective mixture. This implies that the following technical aspects must be considered: • the material components and overall composition • the application conditions (including access and the availability of services) • the application mode (the dry or wet process) • logistic constraints (mainly as it influences material handling) • health and safety requirements The true final (placed) costs should also always be considered, and this seldom occurs, at present, in the mining industry. The key costs are: • material costs (an easy cost to establish) • logistic costs • equipment capital and operating costs • labor costs (including transportation to site) • application efficiencies (wastage especially rebound) • time related costs (the influence on the overall mining cycle time) Shotcrete is applied by two different processes (methods), defined below (after EFNARC) and explained in detail in Chapter 3: Wet process is a technique in which cement, aggregate and water are batched and mixed together prior to being fed into a purpose-made machine and conveyed through a pipeline to a nozzle where the mixture is pneumatically and continuously projected into place. The mixture normally incorporates admixtures and may also include additions or fibers or a combination of these. Dry process is a technique in which cement and aggregate are batched, mixed and fed into a purpose-made machine wherein the mixture is pressurized, metered into a compressed air stream and conveyed through hoses or pipes to a nozzle where water is introduced as a spray to wet the mixture which is then projected continuously into place. The mixture may also incorporate admixtures or additions fibers or a combination of these. 1.2 History The following time line indicates the development of the support technology: • 1907 - The first machine to spray concrete was developed in Pennsylvania, USA by Carl Akeley for use in construction. • 1910 - The machine and process were introduced to the public at large at the Cement Show in Madison Square Gardens in New York, USA. • 1915 - This concept was later improved by the Cement Gun Company, that later became the Allentown Gun Company. Carl Akeley also registered the term “gunite” for his sprayed mortar mix and this term is still sometimes used. • 1920s - Gunite was used in the USA to fireproof mine drifts (supported by timber sets). • 1930s - The term shotcrete was introduced by the American Railway Engineering Association. • 1940s - Coarse aggregate was introduced into the sprayed concrete mixes. • 1954 - Anton Brunner, an engineer from Salzburg in Austria, replaced heavy steel and timber support with shotcrete in a new diversion tunnel, in squeezing ground, at the Runserau Power Plant • 1955 - The wet shotcrete process was introduced. • 1957 - Little major equipment development took place until in 1957 when the rotary drum was perfected by Meynadier (now MEYCO Equipment) and Aliva. These machines (still in use today for dry shotcreting) have a feed hopper above a rotor with chambers that intersect feed openings allowing some material to enter under gravity. A compressed air stream forces this material into the discharge pipeline and keeps it in suspension until the shotcrete nozzle. This development was probably as a result of the use of shotcrete in Austria as a civil tunnel support in the 1950s. • Around 1965 - Gunite was already used in some Boliden Mines in Sweden. This involved the dry process and in some applications the machine was on surface and the material applied some 300m below surface (at Garpenberg Mine). • 1968 - It appears that the first recorded mining shotcrete application for support was at the Hecla Mining Company in the USA. • 1975 - The Norwegians were the first to realize the benefits of silica fume (microsilica). • 1977 - The Norwegians introduce steel fibers for the first time as a replacement for mesh/screen. • 1994 - The Underground Construction Group of MBT are the first to introduce non caustic alkali free accelerators. • 1999 - MEYCO start field testing the first shotcrete equipment capable to be operated in a fully robotic mode (with no operator necessary). 1.3 Shotcrete (sprayed concrete) as a support element Shotcrete in mining is a very effective support element. For shotcrete to be the productive and efficient support that it can and should be, all aspects must be considered. Too often in the mining industry, technically inferior or inappropriate support systems are used, due to convenience, ignorance, resistance to change and/or expediency. The Table below indicates the type of performance that could be expected from shotcrete underground 28 days after placement: Performance parameter Performance range (at 28 days) Compressive cube strength (MPa) 30 to 80 Flexural strength (MPa) 3 to 8 Bond strength to competent hard-rock (MPa) 0.5 to 1.5 Shotcrete is typically used for the following applications in mines: • As a temporary support. • As a permanent excavation support element (main application). • As the only permanent support (bolt and screen/mesh replacement). • For roadways (major roadways such as declines, ramps or loading/discharge areas). • Pillar reinforcement or even replacement. • Backfill retaining wall construction. • As part of ventilation seals. • As part of an orepass lining support and wear resistant system. • As a permanent final lining in vertical shafts. Shotcrete offers is a passive support on application (by definition). The slightest deformation however generates a significant resistance because it is stiff and has a high Young’s Modulus. In addition it has a micro-reinforcing effect by penetrating and/or bridging micro-cracks in the rock. This is in contrast to the really passive supports such as mesh/screen and most arch systems. In typical mining applications, the sprayed shotcrete lining is thinner than used in typical civil projects. A common thickness range in mining is between 50 and 100 mm, and hence the shotcrete cannot be considered a structural arch. It offers a good rock reinforcement (support) however mainly because it: • Reduces unraveling. • Limits air slack (rock weathering due to moisture and air). • Offers lateral confinement to the rock surface. • Fills fractures in the rock. For optimum performance, where bolts are also used, the bolts should have large base plates (washers) fastened and tensioned to the bolts, against the shotcrete. This causes the shotcrete and bolts to act as a single unit, providing a better support system. Poorly designed and/or applied shotcrete can be hazardous and costly. The main reason for poor shotcreting is: Ignorance ! This can result from • A poorly designed mix. • Unmatched and unsuited equipment and shotcrete infrastructure (mainly supply logistics). • Inadequate services. • Ineffective quality control. • False economy. 1910 - The first shotcrete machine made 1918 – Tunnel in Europe using Guniting Early use of gunite in construction in America Shotcreting underground (date unknown). 1957 – Rotary drum dry shotcrete machine from MEYCO 2. Shotcrete (sprayed concrete) raw materials The material constituents of the shotcrete are important in order to achieve a desired target placed performance at the lowest cost. The materials would include: 2.1 Binders 2.1.1 Cement binders Cement is the bonding material (glue) that holds a cementitious material together. For most shotcreting applications, Portland cement is used. This cement was invented by an English bricklayer – Joseph Aspdin – in 1824. The name was derived from the set materials colour and texture that resembled a local limestone called Portland stone. The cement is produced from mainly a mixture of klinker and gypsum. The klinker is typically produced in a rotary kiln from lime, silica, alumina and ferric oxide. Other types of cement that are commonly used in shotcrete include: • Sulphate resisting cement, that typically has a lower tricalcium aluminate content than Portland cement • High alumina cement (HAC) that is produced by fusing (melting) a mix of bauxite and limestone together. HAC is therefore not a Portland cement derivative, and is often used in refractory applications where it can be troweled in place or shotcreted. For basic applications a cement content by mass of 20% is typical. 2.1.2 Cement extenders (supplementary materials) Cement extenders are commonly used in shotcrete mainly for a cost reduction. Fly ash is the most commonly used and is obtained as a waste product from coal-fired power plants. The fly ash for use in shotcrete is mainly supplied as a blend with the Portland cement already in bags or bulk. Ground granulated blast-furnace slag (GGBS) is another cement extender that can be used. GGBS is a glassy and granular material that is usually produced as a by-product of iron production. The molten slag is rapidly quenched and then finely ground. Fumed silica, silica fume or microsilica as it can be called is also a cement extender but is dealt with under the “Admixture Section” due to it’s very unique and desirable properties. Cement extenders are commonly used to replace about 30% of the cement used (up to as high as 50%), but generally the rate of strength development is reduced, and this is frequently undesirable in mining (due mainly to safety considerations). The same applies to sulfate resistant cements and a better alternative is the use of Portland cement and micro silica that can gain strength rapidly (without an accelerator overdose). 2.2 Aggregates As in concrete, aggregates are used to provide dimensional stability by providing a rigid skeletal structure, reduce the void space to be filled with the cementitious paste and hence reduce the cost. The main parameters to consider are: • Grading and the maximum size • Particle shape and density • The aggregate type especially the presence of reactive chemicals and minerals (for possible alkali-acid reaction) • The compressive strength • Moisture content When applying the shotcrete by the dry process, the aggregates can be selected such that the voids are minimized, as is the common approach when using concrete. As a general guide, the use of +16 mm material is to be avoided and the modern trend is to use a maximum size of 10 mm, due to rebound and wear considerations. Figure 1 gives the aggregate envelope for shotcrete recommended by EFNARC. It is possible to use gradings outside the envelope, but the performance will be reduced, or the material cost increased as additional admixtures would need to be used. A typical dry process base mix by weight would consist of: • 20 to 25 % of cementitious binder • 20 to 15 % of coarse aggregate • 55 to 65 % of sand (natural and wash sand is preferred) Where the dry process is used, the moisture content of the aggregates should be a maximum of 6 %. The mix design needs to be modified when using the wet process because the pumpability is a major issue, and generally more fines are needed to reduce the chance of line blockages, and make pumping easier (at a lower pressure). FIGURE 1 RECOMMENDED AGGREGATE GRADATION ZONE FOR SHOTCRETE COMPARISONBETWEENTHELOADBEARINGCAPACITYOF PLAIN,MESH ANDSTEEL FIBREREINFORCEDSHOTCRETE5 0 4 0 3 02 0De f or ma t i oni nmm Load in kN1 00 051 01 52 02 53 0 Pl ai nshot cr et eFi br er ei nf or cedshot cr et eFi br er ei nf or cedshot cr et e( at ahi gher dose) Wi r emeshr ei nf or cement ( af t er Vandewal l e) F i g u r e 2 COMPARISONBETWEENTHELOADBEARINGCAPACITYOF PLAIN,MESH ANDSTEEL FIBREREINFORCEDSHOTCRETE5 0 4 0 3 02 0De f or ma t i oni nmm Load in kN1 00 051 01 52 02 53 0 Pl ai nshot cr et eFi br er ei nf or cedshot cr et eFi br er ei nf or cedshot cr et e( at ahi gher dose) Wi r emeshr ei nf or cement ( af t er Vandewal l e) F i g u r e 2 12 26 50 72 90 100 90 73 55 37 22 11 4 100 80 60 40 20 0 0.125 (after EFNARC) 0.25 0.5 1 2 ISO sieve (mm) M a t e r i a l p a s s i n g ( % b y w e i g h t ) 4 8 16 100 2.3 Water The water quality can be important and should be free from oils and not heavily acidic. Basically if the water is potable (i.e. drinkable), it is suitable for shotcrete. 2.4 Admixtures The American Concrete Institute in the ACI Manual of Concrete Practice (1999 edition) defines an admixture as: “A material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing.” Many different types of admixtures can be beneficial in shotcrete, depending on the specific application and requirements. Most admixtures can be used in the wet process only (discussed later), but generally only a set accelerator (usually in powdered form) is used when dry shotcreting. In order to achieve the quantifiable benefits from admixture usage, it is critical that the dosing equipment is reliable and appropriate. 2.4.1. Accelerators The use of an accelerator is essential in mining applications. Accelerators ensure that the shotcrete: • Develops a bond with the rock as soon as possible. • Generates internal strength quickly. • Is able to be placed in thick single passes (if needed). Shotcrete accelerators generally fall into the following categories: • Silicates (e.g. water glass or sodium silicate) • Sodium or potassium aluminates • Alkali free accelerators Silicates are not really true accelerators as they only create a gelling effect rather than a rapid early strength gain. Accelerators also tend to reduce the final strength of the shotcrete. This is partly because a slower rate of strength gain results in a finer and a more dense crystalline growth that creates a stronger final product. In a typical dry mix, a powdered accelerator addition of between 2 and 5% based on the weight of the total cementitious addition would be reasonable. Accelerator dose rates are normally expressed as a percentage of the total cementitious content. There is confusion in the mining industry over the definition of an alkali free accelerator. To understand this, the difference between alkalinity and alkali content must be noted. • Alkalinity refers to a “basic” liquid with a pH between 7 and 14. • Alkali content refers to the presence of alkali cations, and a liquid may contain alkali cations but have a neutral pH. To be a true alkali free accelerator according to CEN definitions (Comite Europeen de Normalisation) an accelerator must contain <1 % by weight of alkali (Na 2 O) equivalents. Certain accelerators on the market claim to be alkali free but do not meet this specification and are only low alkali (and hence not as safe to use or as effective). In wet shotcrete, the dose range is about the 3 to 10% based on the weight of the total cementitious content. The latest trend is towards the (non-caustic) alkali free accelerators because they are more environmentally safe, induce more rapid strength gain and tend to cause significantly less long term strength loss. With such an accelerator, a dose rate of around 6 to 8% is normal. Before selecting an accelerator, compatibility tests must be carried out first. Certain accelerators react differently with different cements. 2.4.2. Water reducers (plasticizers and superplasticizers) There are various types of water reducers available and they tend to fall into three broad categories: • Low range (e.g. lignosulphonates) that give about a 15% water reduction, but some can retard the strength gain • Medium range (e.g. melamines) that give about 25% water reduction • High range (e.g. polycarboxylates) that give about a 45% water reduction These admixtures work by charging each cement particle ionically and causing them to separate thereby effectively lubricating the mix and thus being able to reduce the water and still obtain the same consistency (i.e. slump). 2.4.3. Microsilica (silica fume) Microsilica is a very fine and spherical material with a high pozzolanic reactivity. The use of the product in shotcreting has the following benefits: • Improved durability (more resistant to freeze/thaw cycles and improved sulphate attack resistance). • Improved bonding to substrates. • Higher strengths (compressive and flexural) • Reduced rebound • Improved flow in the delivery hose (in the wet process). • Reduced wear in the pump and nozzle (in the wet process). • Improved mix cohesiveness. • Thicker single pass applications A typical dosage would be 5 to 10% by weight of the cementitious binder. 2.4.4. Curing agents/concrete improvers There is an incorrect perception that an underground environment provides good curing conditions for shotcrete. This is incorrect because the ventilation tends to cause premature drying of the shotcrete surface resulting in: • Poor hydration causing a weaker final product • Reduced substrate bonding if the shotcrete layer is relatively thin (less than 75 mm/3 inches) • Significant shrinkage cracking Solutions to this include: • Regular wetting of the placed shotcrete. • The application of an external curing agent (such as a spray applied wax) • The inclusion in the mix (wet process only) of a concrete improver that has the potential to improve curing and increase the bond with the substrate. Regular wetting of the placed product is often impractical and too time consuming. The application of an external curer involves a second (albeit simple) operation and makes it difficult to apply a further layer of shotcrete at a later time (for whatever reason) unless the coating is removed. As with all concrete construction, moist curing conditions are important for 7 days. With accelerated shotcrete, good curing is important basically from after spraying. 2.4.5. Consistency controllers Under certain wet shotcrete applications, the use of a consistency control system can help. The first component is added into the mix (before pumping) to keep the open time and improve pumpability. The second component is added at the nozzle to stiffen the mix and improve the strength development rate. 2.4.6. Hydration controllers The useful life of a wet shotcrete mix can be a limiting factor in underground applications due to logistic considerations. A typical batch of untreated shotcrete should be discarded as waste after between about 1.0 and 2.0 hours (depending on the ambient temperature). Conventional retarders can extend this to about 4 hours, but hydration controllers can effectively put the mix “to sleep” for up to 72 hours. This technology is of great advantage in many underground wet shotcreting applications, because it helps resolve logistic problems. Hydration controllers function in 2 ways: • By acting as an effective dispersant thus keeping hydrating particles apart. • By forming a barrier around all the cementitious particles, thus stopping the hydration process altogether (unlike conventional retarders). This effect is over-ridden when shotcreting, by adding an effective accelerator, and the hydration controller has no adverse effect on the rate of strength gain and the ultimate strength, provided that adequate accelerator is added. If a hydration controller is used, a minimum flow (to BS 1881, Part 105) of 50 cm should be used. 2.4.7. Pumping aids Pumping aids usually also act as water reducers in shotcrete. They improve the pumpability of harsh mixes, frequently caused by poorly graded aggregates. Air entraining agents are a common pumping aid. 2.5 Fibers The application of fibers in construction dates back centuries to the use of horse hair, jute, sisal and cotton. Concrete is by nature a brittle product and is weak in tension. In shotcreting, reinforcement can be provided by the use of screen/mesh or fibers. Fibers have obvious advantages over screen including: • Fibers are more evenly distributed throughout the shotcrete. • Mesh is difficult and labor intensive to apply. It can also represent a safety hazard if manually installed. • The fiber reinforced shotcrete effectively lines the tunnel periphery, and additional shotcrete is needed when screen is used, to fill the depressions (as the mesh is usually fixed at the high points on the tunnel periphery). • Mesh can increase shotcrete rebound significantly (due mainly to screen vibration during spraying). Figure 2 is a comparison between the performance of unreinforced shotcrete and shotcrete reinforced using screen or fibers. Where movement of the rockmass is expected, shotcrete is frequently applied over mesh/screen or fibers are introduced into the base shotcrete mix. The trend is away from mesh/screen reinforced shotcrete to fiber reinforced shotcrete in mining because: • Durability is improved. • Ductility (toughness) is increased. • Impact resistance is increased. • Surface cracking is reduced (not a major concern on mines). • Rebound is reduced. • Compaction is improved (see Figure 3). • Application productivity and safety is improved (major benefit). • Final placed costs are reduced. • Logistics is simplified. Typical fiber reinforcement materials include: • Polyolefin fiber of which polypropylene is the most common (mono-filament or fibrilated) • Carbon fiber • Glass fiber • Drawn steel wire • Slit sheet • Milled steel pieces • Melt extract pieces The most commonly used types in shotcrete are the drawn wire and polypropylene fibers. The most important parameters for fibers are: • The aspect ratio (overall ratio of the fiber length to it’s diameter). • The tensile strength. • The shape. An ideal fiber should have the following: • A length such that it can overlap and bridge at least 2 of the largest aggregate particles used in the mix (typically a length between 25 and 40 mm). • A high aspect ratio (i.e. thin). FIGURE 2 COMPARISON BETWEEN THE LOAD BEARING CAPACITY OF PLAIN, MESH AND STEEL FIBRE REINFORCED SHOTCRETE 50 40 30 20 Deformation in mm L o a d i n k N 10 0 0 5 10 15 20 25 30 Plain shotcrete Fibre reinforced shotcrete Fibre reinforced shotcrete (at a higher dose) Wire mesh reinforcement (after Vandewalle) FIGURE 3 THE ADVANTAGES OF STEEL FIBRE REINFORCED SHOTCRETE OVER MESH REINFORCED SHOTCRETE MESH REINFORCED SHOTCRETE Plain shotcrete Potential voids or poorer compacted shotcrete Welded wire mesh Mesh pinned to rock Cover to mesh Rock Maintains contact and bonds with the rock over the entire surface STEEL FIBRE REINFORCED SHOTCRETE Steel fibre reinforced shotcrete Rock (after Vandewalle) • A high tensile strength • A shape that results in a good anchor particularly at the fiber ends in the shotcrete. The use of steel fibers and particularly synthetic fibers in dry shotcrete is not generally recommended due to the high fiber loss found in the rebound (significantly more fiber loss than the overall measured rebound and in the +50% range). Where steel fiber is used a dosage rate of between 30 and 50 kg/m³ (0.4 to 0.6 % by volume) is generally used. The potential use of polypropylene fibers in shotcrete for mining applications has increased dramatically recently with the development of high performance polymer fibers. The high tensile strength and crimped shape have resulted in performances very similar to that obtained with steel. Dosage rates of between about 7.0 and 13.5 kg/m³ (0.75 to 1.5 % by volume) are typical. Figure 4 shows the wide variety of fiber performance that can be obtained using different fibers. Typically high performance steel fibers have higher residual loads immediately after failure but less load at high deformations than high performance synthetic fibers. The key to the cost effective use of fibers is to consider the: Cost/J of energy In the future, more use will be made of polymer fibers, possibly blended with steel fibers, in high performance mining shotcrete applications. FIGURE 4 ROUND PANEL TEST RESULTS WITH DIFFERENT FIBRES 35 30 20 25 15 5 10 0 5 10 15 20 Deflection in mm 25 30 35 40 0 L o a d i n k N Plastic fibre A Plastic fibre B Plastic fibre C Plastic fibre D Steel fibre X Steel fibre Y Examples of Shotcrete Fibres DD fibre from Synthetic Industries Collated Dramix from Bekaert HPP fibre from Synthetic Industries Xorex fibre from Synthetic Industries Barchip fibres from Hagihara STRUX fibres from Grace Forta fibre GSF fibre from Grace 3. Application processes There are two application processes in shotcreting and the selection depends on the specific application and the site conditions. 3.1 The dry process In this application, the particulate material is conveyed (pneumatically) in a basically dry state from the pump to the nozzle, where the water is added. A typical section through a dry machine is given on Figure 5. The nozzleman is the key to the successful application because he controls the vital water addition. Too much water causes the shotcrete to sag away from the rock and reduces the strength, and too little water causes higher rebound and can also lead to a strength reduction. The moisture content of the mix (prior to water addition at the nozzle) should be between 2 and 5% to minimize dust production at the pump. More than 5% water can cause blockages in the line. A pre-dampener can be used to reduce the dust at the machine and is relatively successful although the unit is large. In a typical dry application, the water to cement ratio should be in the 0.40 to 0.45 range. 3.2 The wet process Until the last decade, the dry process was the most common method of application, but the wet process is rapidly gaining popularity in mining, in line with the general move in mines to mechanize underground operations for safety and productivity reasons. In the wet process, the entire mix (including the total water) is fed into a hopper and then pumped to the nozzle. The mix needs to be fluid enough in order to be pumped and therefore the introduction of water reducers into the mix is generally essential in order to maximize the strength gain and reduce the overall costs. The two main pump arrangements for wet machines are shown on Figure 6. The nozzle design in wet shotcrete is important because compressed air is added to produce the necessary spray velocity, and generally accelerators are also added to improve the early strength gain. A water cement ratio around 0.40 to 0.45 is typical. FIGURE 5 THE ROTOR PRINCIPLE OF A TYPICAL DRY-SPRAYING MACHINE Feeding hopper Pressurized air Rotor Rotor Outlet FIGURE 6 WET SHOTCRETE PUMP TYPES WORM PUMP PISTON PUMP (With S tube) 200 O 125 O A D S (after Vandewalle) 3.3 A comparison between the processes The Table below compares the main differences between the dry and wet processes. Wet process Dry process Little dust (about 10% of dry) Considerable dust Low maintenance cost High maintenance cost High capital cost Low capital cost Low rebound (typically about 5 to10%) High rebound (usually 25 to 40%) Moderate to high placement rate (between 4.0 and 25m³/hr) Low to moderate placement rate (up to about 6.0m³/hr) Low transport distance (up to about 200m) High transport distance Moderate to high placed quality Moderate placed quality Low operator sensitivity High operator sensitivity Better suited to high application volumes Better suited to low application volumes and stop/start operations 3.4 Process selection guidelines The selection of the most appropriate application process should be site specific and based on total cost, performance and monthly consumption. As a general guideline, if a mine places more than 3000m³/year, wet shotcrete should be considered (unless the spray areas are numerous and access between them is difficult. There can be no doubt that the overwhelming trend in mining internationally, is towards wet shotcreting, due mainly to performance, productivity and overall cost considerations. This trend has mainly occurred because of the improvement in the equipment and the development of high range water reducers and hydration control admixtures. These developments have resolved most of the logistics issues on the mines associated with shotcrete transportation from the batching plant to the spray equipment. There is also no truth in the commonly held view that dry shotcreting is the best option, where the overall worker skills level is low (i.e. in developing and third world countries). Wet shotcrete has proved itself a cost effective method all over the world, if the placement volume is high and the performance requirement high. The main considerations when selecting the most appropriate shotcrete application process for a specific application are: • The overall volume needed for the application and the time available to spray it. • Logistic considerations (can bulk bags be handled, or is wet shotcrete available via a pipeline for example). • The performance required (fiber usage for example is only a reasonable solution in the wet process). • Overall cost considerations. • The location of each spray site and the requirements. • If fibers are needed for most applications, then the wet process is the obvious choice due to the high fiber losses in the rebound if the dry process is used. 4. Shotcrete (sprayed concrete) design The final strength of shotcrete, like concrete, is mainly dependant on the water to cement ratio and the air content after placement. Much is available concerning the design of shotcrete, but simple aspects are frequently overlooked initially, and these can create major losses, costly delays and final sprayed linings that do not meet the desired performance requirements. Shotcrete design must include more than creating a laboratory mix that meets the strength gain requirements, with locally available raw materials (such as cement, sand, stone and water) in adequate supply. Whilst this is important, other equally vital aspects must not be ignored: • the fact that the strength must be achieved on the rock (not in the lab) • the shotcrete must bond adequately to the rock • time available for spraying a given volume • the placed cost of the mix 4.1 Placed strength The strength gain (and general performance) of the shotcrete needs to be reliably achieved as sprayed. This means that the mix must be pumpable, bond well to the substrate (with the minimum of rebound), build up desirable thickness in few passes and usually gain strength rapidly. This generally implies the need for a cohesive mix, with an initial high slump and finally a low slump on placement. Such needs would not have been evident during lab trials. Achieving such requirements is governed by the use of admixtures and additives: • micro-silica for cohesion, rebound reduction and durability • superplasticizers for slump and water to cement ratio control (in the wet mix process) • accelerators for early strength development and high single pass application thickness (e.g.: with alkali free accelerators overhead single pass thickness of between 30 and 50 cm are possible) • concrete improvers (internal curing admixtures) to achieve long term strength, better bond, less cracks and improved durability (in the wet mix process) 4.2 Time Time is often a scarce commodity in mining, and when shotcreting, it presents two main problems: • Supplying adequate volumes of material to the shotcrete site for spraying. • Actually spraying the area in the time available to fit in with the mining cycle. Adequate material can be obtained by many means, but the most exciting is the transport of the material as a slurry down a pipe. Untreated shotcrete has an open (useful) time of only 1 to 2 hours, and hence it is often essential to use a hydration control admixture to increase the open time to that needed for the safe operation and application of the shotcrete, often around 10 hours or even more. The time to actually spray the area is not a problem, if the shotcrete machine can be adequately supplied with material. With dry mix machines the volumes placed can be up to 8 m³/hr, and with wet shotcrete machines, up to 25 m³/hr can be sprayed. Shotcrete components and performance The following Table (modified from Munn, 1997 by Garshol, 2002) gives the typical material ranges and properties that can be expected from normal dry and wet process shotcrete: Parameter Dry process Wet process Strength range (MPa) 20 – 60 20 – 80 Slump (mm) N/A 50 – 250 Maximum aggregate size (mm) 10 10 Maximum coarse aggregate (%) 25 40 Cement content (kg/m³) 300 – 450 350 – 500 Water to cement ratio 0.40 – 0.45 0.36 – 0.50 Typical rebound without micosilica (%) 25 – 50 10 – 15 Typical rebound with microsilica (%) 20 – 40 5 – 10 5. Shotcrete mining equipment Shotcrete equipment must be well matched to the specific application underground. High quality equipment is needed to produce a high quality sprayed product on the rock. Shotcrete is safety related and all aspects must be well considered, implemented and checked. 5.1 Shotcrete spray equipment The spray capacity of shotcrete equipment has traditionally been given in m³/hour of water in most literature. The throughput of shotcrete is usually about 20% less than this figure. The requirements for shotcrete equipment in mining are usually different from those in underground civil applications because in mines: • Space is frequently a constraint. • A shotcrete machine will need to travel more (between development ends) and on poorer surfaces. • Logistics are difficult. • Quantities needed per blast are much less (smaller cross sectional area and smaller thickness applied). The choice of equipment depends on the following: • The placement rate needed during a shift (to minimize the mining cycle if necessary). • The specific excavation size and the general mining tunnel dimensions. • The quantity of shotcrete to be placed per month. • The number and location of areas needing shotcrete. • The ease of access between the different areas requiring shotcreting. • The overall time that shotcrete will be needed (i.e. for on-going routine support work, or only a specific project area in a mine). It is important to note that in many applications where the advance rate of an excavation needs to be high, the utilization of the shotcrete equipment tends to be low, but the overall utilization of the mechanized equipment suite (drill rig, LHD unit etc.) is increased. The Table overleaf outlines the equipment specifically available for mining from MEYCO Equipment, based in Switzerland. Other equipment is available but is more suited to shotcrete repair work or major underground civil projects and is therefore not given. Machine Process Shotcrete spray volume (m³/hr) Conveying distance (horizontal/vertical in m) Drive MEYCO Piccola Dry <3.5 500/100 Electric, air or diesel MEYCO GM Dry 3.5 to 10 500/100 Electric, air or diesel MEYCO Rambo Wet 5.0 100 horizontal Electric MEYCO Mamba Wet 20 N/A (boom to ± 9m high) Electric, diesel MEYCO Cobra Wet 15 N/A (boom to ± 9m high) Electric, diesel MSV 2100 Wet 20 N/A (boom to ± 9m high) Electric, diesel The MEYCO Cobra and the North American version the Mine Shotcrete Vehicle (MSV) both have on-board compressors. Other equipment has also been used on the mines made by MEYCO including the Suprema and the large spray mobile. All concrete pumps can transport wet shotcrete, but when wet shotcreting a custom designed pump should always be used. The reason for this is that the pump pulsation needs to be minimized to ensure a consistent quality product is sprayed onto the substrate. Pulsation is not a concern usually when pumping concrete. 5.2 Shotcrete dosing pumps and systems This is clearly only an issue in the wet process. Various pumps can be generally used but when using the alkali free accelerators, the type of pump is important because the accelerators are generally heavily saturated suspensions and obtaining a consistent and adequate dose rate is important. To achieve this only 2 types of pumps are suitable: • Mono (Moyno) pumps. • Peristaltic (hose) pumps. The mono pump is probably the best option as it is pulsation free. The capacity of the dosing pump is also important and a rate of 10% of the cement content by weight could be required as a maximum. Accurate dosing is important and some of the shotcrete rigs from MEYCO have a system that monitors the shotcrete throughput and doses the accelerator at the correct level at all times, using a program control system (PLC). This is called the MEYCO Dosa TDC System. 5.3 Shotcrete lines and nozzles The nozzle design is important as it effects: • The compaction of the sprayed shotcrete (the higher the better). • The bond of the shotcrete to the rock (the higher the better). • The rebound during spraying (the lower the better). • The consistency of the mix when dry spraying. The accelerator needs to be well mixed into the shotcrete for optimum results. The hardware to help achieve this is therefore important. In the dry process, the water ring and assembly is critical to ensure thorough wetting of the mix. MEYCO have a system that also splits the water feed to the nozzle to help the wetting process. The air ring and general housing design for the wet mix is important for good propulsion onto the rock and accelerator introduction and mixing. Safety chains holding flexible hoses are essential and should be judiciously placed. Full write-ups on this equipment can be obtained from the following reference sources: • MEYCO Equipment Tel: +41-52-2440700 Hegmattenstrasse 24 Fax: +41-52-2440707 8404 Winterthur Switzerland • The MEYCO web site is http://www.meyco-equipment.ch Wet shotcreting equipment The MEYCO Rambo for hand spraying Variations of the Rambo, including a pan mixer for converting a pre- bagged shotcrete product into a wet mix The MEYCO Mamba The MEYCO Cobra with on-board compressor. A MEYCO boom with the Logica robotic system fitted (refer to Chapter 14) Close-up of the laser scanner (part of the Logica system) 6. Shotcrete (sprayed concrete) service needs For a successful shotcrete application the following is needed: • adequate mix design for spraying and performance • an adequate material supply at the machine • matched equipment and infrastructure for the application • trained crews • correct preparation prior to spraying • adequate services (e.g.: power, air and water) • correct application technique • appropriate quality control and remedial actions 6.1 Power A reliable and well earthed electrical power supply at the correct voltage is needed. 6.2 Compressed air A well maintained supply of compressed air is needed with adequate pressure and volume that depends on: • The particular equipment specification. • The condition of the equipment. • On site operating conditions. • The hose length and diameter. As a guideline, the typical air requirements (the most critical service) are as follows: • For dry shotcreting about 5 m³/hr, the air consumption is about 15 m³/minute at a pressure between 3 and 6 bars. • For wet shotcreting about 15 m³/hr, the air consumption is about 12 m³/minute at a pressure of about 6 bars. Mine compressed air pressure (if available) is frequently too low and hence the larger mining shotcrete rigs are often fitted with compressors. 6.3 Water As mentioned earlier, the water quality can be important and should be free from oil and not heavily acidic. Potable (drinkable) water is suitable for shotcreting. 7. Logistics Getting the shotcrete to the equipment in adequate quantities and when required is the key to the successful introduction of shotcrete in mines. It is also frequently overlooked in the design and planning stages leading to bottlenecks and inefficiencies later on. There are many ways used to get shotcrete to the site including: • Vertically down a slick-line and into some form of pressure dissipater before discharging it into a concrete pump, or some form of agitator vehicle (transmixer). • Down a cased borehole, then as above. • Down a decline in concrete trucks. • In dry product bulk bags (containers) then if wet mix, to an underground batching plant then into agitator vehicles. • Small bags (dry mix only). The choice of transportation method for the shotcrete material depends mainly on: • The mine infrastructure. • The material handling system. • The location of the working places needing shotcrete and the demand per shift. • The daily shotcrete requirement. 7.1 Slick line design and operation A slick line is the only shotcrete material transportation method that needs to be elaborated on. The following is a guideline for a slick line: • A 150 mm diameter pipe appears suitable, although a smaller diameter pipe (say 100 mm) may be more appropriate in order to reduce the velocity. • If the vertical drop is high, allowance should be made in the pipeline for expansion due mainly to the high temperature generated by the shotcrete falling in the pipe. • The wear rate is high due to the free fall velocities, and this potential problem is further exacerbated if the pipe column is not installed vertically. Generally it is advantageous to have a second pipeline installed as a back-up. • If the mix is optimally designed, segregation is not a problem except at the start of a pour. Before the first batch is dropped down the line, the column must be lubricated with water and then possibly a cement and water paste. Unless this is done, the fines will adhere to the sides of the pipe and only the aggregate will fall to the bottom. • Some form of pressure dissipater is needed at the bottom of the pipe to control the exit of the shotcrete from the pipe. This is generally achieved using a “kettle” of some suitable design. This has a sacrificial wear plate of solid steel, tungsten or ceramic as shown in the examples in Figure 7. The kettle can also perform the function of a re-mixer, in case any minor segregation has occurred. • A spare kettle should always be kept as a replacement if needed. FIGURE 7 SHOTCRETE PRESSURE DISSIPATORS Pipe flanges Sacrificial wear block Dead ends (i ) (i i ) (i i i ) 8. Shotcrete (sprayed concrete) application 8.1 Substrate preparation Substrate preparation in a key element in the successful application of shotcrete. The substrate should be free from loose materials, dust and films (such as oils). This can generally be achieved by using a combined water and compressed air jet. Cleaning should start at the high point (the excavation roof, back or hangingwall) and work down systematically to the low point (excavation floor or footwall). Adhesion onto weak structured materials (such as shales and mudstones) is frequently poor, and should be considered when designing an appropriate support system. Spraying onto a surface that can vibrate (such as screen/mesh) can cause problems such as poor placed density (and even voids) as well as increased rebound. 8.2 Shotcrete spraying Good application techniques are the key to ensuring compliance with target specifications. The following should be undertaken whenever possible: • Caution must be taken not to incorporate rebound lying on the tunnel floor against the sidewall, into the shotcrete applied to the sidewall, as this impacts the in situ shotcrete strength very negatively. To avoid this, it is common practice to start shotcreting on the sidewalls (ribs) and move upwards to the tunnel roof. Rebound should always be discarded and never considered for re-use. • During spraying, it is beneficial to rather fill cracks instead of bridging them. This has a beneficial effect on the overall rock mass stability by tending to stabilize the rock. • Larger voids or cavities should be carefully filled first before shotcreting the surrounding area. 8.3 Application safety Particular attention should be given to operator eye protection, mainly associated with rebound. Preferably goggles that seal around the eyes should be worn. All operators should wear at least dust masks, but the problem becomes severe in the case of the dry process. Skin and eye protection becomes critical if caustic accelerators are used. This becomes less of a concern if alkali free accelerators are used, but the cement is still alkali and can cause minor irritation. Long durable gloves should be worn at all times (except by the operator if using a remote control). Lighting should be adequate not only from a safety point of view, but also to improve the properties of the final sprayed shotcrete, and this is frequently overlooked (unless rigs are used). 9. Training The nozzleman is the key to successfully placed shotcrete whether it is by the dry or wet process; or manual or robotic placement. An aspect that is frequently overlooked, is that if hand held spraying is to be successful, the effective volume sprayed should be limited to between 4 and 8 m³/hr. Shotcrete nozzle-men should also be rotated regularly during a shift, as the work is arduous. Should higher volumes be required, the use of a spray manipulator (robot) is essential, and offers other benefits such as higher placed quality, especially overhead. The correct shotcrete application technique is shown on Figure 8. Figure 9 shows the correct hand spraying method needed for spraying at different sidewall heights. Various training courses are available internationally for nozzlemen including: • ACI Nozzleman Certification for hand spraying (ACI 506.3R-91). • CUC Certification for robotic spraying planned for possible introduction late 2002. Most of the training courses are in two main parts: • Shotcrete mix design theory and testing. • Shotcrete practical application. Both aspects are very important. Further information on training and related matters can be taken from the following sources: • ACI web site at www.aci-int.org • CUC web site at www.icguc.com FIGURE 8 CORRECT SHOTCRETE APPLICATION Extreme rebound High rebound Low rebound MANIPULATING THE NOZZLE TO PRODUCE THE BEST SHOTCRETE SHOTCRETING ANGLE FIGURE 9 CORRECT HAND SPRAYING POSITIONS 10. Quality control and testing Poorly applied shotcrete can create a more hazardous condition than no shotcrete as it can give a false sense of security. There is also no benefit to be derived from having a good quality management system in place, unless there is a feasible and safe contingency action plan if measured limits are not met in the shotcreting process. 10.1 Quality control Quality control is an important and generally overlooked, parameter needed to ensure safe, consistent and cost effective support. Quality control tests should be: • reliable • meaningful • timeous • simple • relatively inexpensive The main targets checked in any shotcrete quality program should be associated with design compliance (bond and strength) and sprayed design thickness. It is however totally unacceptable to have a quality control system in place, but fail to take adequate actions, if non-compliance is identified. The absolute minimum parameters that should be checked regularly during a spraying shift are: • mix design (including water content) • services (e.g. air volume and pressure) • strength (e.g. by using sprayed panels) • thickness (can use pins into the rock, that need to be covered during spraying) It should be noted that depending on the compressive strength test shape used, different results are obtained with the same mix, tested at the same time. There is however a relationship between the more common shapes tested (the cube, the cylinder and the cylindrical drill core). EFNARC gives the relationship in the Table overleaf: Height to diameter ratio Of the drilled core Equivalent cube strength factor Equivalent cylinder Factor 2.00 1.15 1.00 1.75 1.12 0.97 1.50 1.10 0.95 1.25 1.07 0.93 1.10 1.03 0.89 1.00 1.00 0.87 0.75 0.88 0.76 10.2 Field testing of shotcrete This commonly can consist of: • Slump tests (BS 1881: Part 102) • Hilti gun • MEYCO penetration needle • Schmidt hammer tests • Drilled cores (minimum diameter of 50 mm) • Thickness marker tests • Pull-out tests • Bond/adhesion strength tests (EN 1542) • Sprayed pre-fabricated panels (BS1881: Part 120) The thickness marker tests are important operational tests, during and just after spraying. The operation of the Hilti Gun is shown in the photographic sequence overleaf. 10.3 Laboratory testing of shotcrete Laboratory test methods for shotcreting are really only useful for comparative purposes as none of the common methods take account of the adhesion to the substrate (rockmass), and this is the critical performance parameter. For small scale testing of the shotcrete’s workability, the flow method (spread table) is useful and relatively quick. This method is cover in BS 1881, Part 105, and for certain high performance superplasticizers, it gives more meaningful results than the slump test. Shotcrete in many mining applications frequently performs under post failure conditions (if fiber reinforced) and thus lab test methods that utilise low deformations are considered of less relevance in mining (such as the ASTM C1018 beam test). Hilti Gun early shotcrete strength testing • The Hilti gun method is suitable for shotcrete strengths between 2 and 15 MPa. • Different lengths of nails are used for different strength shotcrete. • The nail is shot into the fresh shotcrete. • The shotcrete layer should be around 15cm thick if possible. • Caution should be used as the gun is dangerous if not used correctly. • A nail shot into the shotcrete to a depth depending on the strength of the shotcrete and the embedded length is recorded (using the shoulder on the nail as a reference). • To obtain a more reliable result at least 8 bolts should be used at each time period where the strength is needed. • A nut is screwed fully onto the nail as shown. • The test nut on the nail. • The pull tester is placed securely over the nut. • A direct pull-out load is recorded on the dial that can be converted into a compressive strength. • The equipment should be kept in good condition, stored in a safe place and regularly serviced. The strength of the shotcrete is then calculated using the depth of the specific nail into the shotcrete, the pull out force and a correction factor supplied by Hilti. The following are however considered appropriate for most shotcrete mining applications: • The round determinate panel test. • The European Federation of Producers and Applicators of Specialist Products for Structures (EFNARC) panel test. 10.3.1 The round determinate panel test The round determinate panel developed by E.S. Bernhard (1998) is rapidly gaining acceptance as a suitable method for testing different shotcrete mix performance for mining applications in particular. A photographic sequence showing the preparation and testing of the round panel specimens is show overleaf. Details on this method can be found in the following references: • Bernhard, E.S. 1998, Measurements of post-cracking performance in fibre reinforced shotcrete. Australian Shotcrete Conference 1998 – Sydney, Australia – October 8 and 9 1998. • Bernhard, E.S. 1998, The behaviour of round steel fibre reinforced concrete panels under point loads. Engineering Report CE8, Dept. of Civil and Environmental Engineering, University of Western Sydney – Australia – 1998. • Bernhard, E.S., Pircher, M. 2000, The influence of geometry on performance of round determinate panels made with fibre reinforced concrete. Engineering Report CE8, Dept. of Civil and Environmental Engineering, University of Western Sydney – Australia – January 2000. 10.3.2 The EFNARC panel test This test is also widely accepted particularly in the underground construction industry in Europe. With this test a point load is applied to: • A 600 by 600 mm shotcrete panel at least 100 mm thick for hand spray application tests. • A 1000 by 1000 mm shotcrete panel for robotic application. The failure mode is generally flexural, but sometimes punching can occur. The normal testing age is at 7 and 28 days. The center point load is applied typically over a 100 by 100 mm surface. Round determinant panel testing • Considerable equipment and services are needed to shoot the round panels and cleaning the equipment and hoses is important between different mixes. • At least 3 panels are used per test mix. • A standard shotcrete mix must be used each test for fiber comparison studies. In this case accelerators are not necessary. • For comparative mixes the results for poured (properly compacted with a vibrator) and shot panels are similar. • The advantage of spraying however is it indicates whether the mix with the specific fibre is easy to spray. • For actual mix design tests, the full mix including accelerator must be used. • The finish is important (especially ensuring a near uniform thickness). • Panel condition before final finishing. • Curing the round panels is important. • They should be kept damp and moved as little as possible for at least a full day. • Recording the average diameter of the round panel is important. • An average thickness for the panel is critical (and the differences in the measurements should be as small as possible). • The round panel in the frame prior to testing in a press. • The load and deformation data is automatically recorded so that the energy etc. can be calculated after the test. • Close-up of one of the failure cracks during a test using a synthetic (plastic) fibre. • Examination after the test can reveal whether the fibres tended to pull out or snap, and how well ditributed through the mix they were. • The normal and desired mode of failure is a 3 pointed star. • Occasionally only a single crack is formed. This can be due to using a poor fiber, too low a dose rate or from a poorly sprayed panel. EFNARC specifies 3 types of shotcrete toughness (energy absorption) based on the panel test at a 25 mm deflection: • Class a 500J • Class b 700J • Class c 1000J Bernhard (1999) has established an approximate linear correlation between the EFNARC panel tests and the round determinate panel test as follows: 1000J at 25mm deflection in an EFNARC test = 400J at 40mm deflection in a round panel test Details on this method can be found in the following references: • EFNARC Association House 235 Ash Road Aldershot, Hampshire, GU12 4DD, UK Tel: +44-1252-342072 Fax: +44-1252-333901 • www.efnarc.org (publications can be downloaded at no cost). 10.4 Quality assurance planning Successful shotcreting requirements a comprehensive quality management plan, that checks all aspects of the process and takes effective and appropriate action immediately any problem is identified. The Table overleaf could be the basis of a quality assurance plan for shotcrete: Main parameter Test Parameter Comments Pre-spraying Composition Pre-determined based on testing Grading Sieve analysis Stock levels Buffer stock essential Storage conditions Keep dry Equipment condition Preventive maintenance and daily inspections Services Must be checked each shift Safety Key at all times During and after spraying Substrate condition Must be prepared Accelerator level Set based on conditions Early strength Thickness A critical issue Rebound Mainly a cost issue Visual Skilled operators make good use of this. Safety Always paramount Days after spraying Shotcrete performance Sound for bond etc. Support effectiveness Monitoring 11. Costs Shotcrete placed costs are seldom if ever accurately estimated. The same is evident for most underground support systems. The following are some of the more significant cost elements: • Material costs • Equipment costs • Material and equipment transportation costs • Labor costs • Application inefficiency costs • Time related costs • Other costs 11.1 Material costs The material cost is generally relatively easy to establish, and is higher for wet mixes. Material costs can be judiciously reduced by careful raw material selection, blending and admixture usage (e.g. water reducers). 11.2 Equipment costs The capital cost for the shotcrete and ancillary equipment is also easy to determine, and varies significantly depending on the capacity and on the application process. Wet machines are always significantly more costly than dry machines. The maintenance cost of the equipment is generally overlooked, but can be substantial. Dry equipment maintenance per m³ sprayed is between 2 and 4 times more than with wet equipment. In Canada and the USA, maintenance (and replacement) costs for the dry machine, hoses and nozzle are typically around $14.00/m³ sprayed. Depending on the tax laws in a specific country, capital equipment is written off in between 3 to 10 years usually. Assuming a volume of shotcrete to be placed each year, a depreciation figure can be readily estimated. 11.3 Material and equipment transportation costs This important cost is generally ignored or avoided for any support system, but it can be very significant and should be considered. Mines that take the time to investigate this aspect are generally able to justify the installation of a shaft pipeline with pumps or agitator-cars (transmixers) for the horizontal transportation. This assumes that the monthly volumes needed are regular and relatively large. 11.4 Labor costs The true labor cost involved with the entire process needs to be considered. When comparing shotcrete against other supports, any rehabilitation costs associated typically with any support system should also be estimated. For example, the useful life of unprotected mesh/screen is typically between 3 to 5 years (depending on the conditions). 11.5 Application inefficiency costs The rebound must be considered when shotcreting, and is not too difficult to estimate. Quality wet shotcreting should be 15% or less (10% or less if applied with a manipulator), and quality dry shotcreting should be 25 to 40%. This must be accounted for in any shotcrete costing exercise. 11.6 Time related costs These need to be considered if the mining cycle is critical for a specific excavation development. The effects of lost blasts, due to the support installation taking too long is very significant in many cases. This aspect is frequently ignored, but considering this aspect usually more than justifies the capital expenditure of a large self contained robotic spray rig for many mining projects. 11.7 Other costs These include the cost of: • Services • Down time on overall mining production and development An adequately detailed sprayed concrete costing spreadsheet is available from Mr. Knut Garshol at
[email protected]. 12. Specification and design procedures for shotcrete (sprayed concrete) support The fact that shotcrete (especially fiber reinforced) is a very effective support liner in mines is well accepted, based mainly on experience. The design of the performance (including thickness) requirements for shotcrete under given rock engineering conditions in mines is still relatively poorly understood. Detailed geotechnical examination in all the developing tunnels and excavations is not feasible or justified. Frequently on mines therefore, the reasonable assumption is made that if a support system has worked in the past under similar conditions that it will work again. This method however tends to result in a “worst case” design that is seldom the most cost effective. There are however methods that can be used if needed. Methods that can be used include: • Observational Methods (mainly NATM). • The Q System developed by Barton (1974). • The Q system modified for toughness by Grant et al (2001). • The ground characteristic curve method by Speers and Spearing (1996). 12.1 Observational methods Rock masses are very complex systems and are further complicated in mining because the stress tensor typically changes during the operational life of most excavations. Predicting future rock mass responses is therefore difficult at best. Observational Methods involve field observations and monitoring to make predictions on rock mass stability and the effectiveness of any support. The most critical parameter measured is closure (convergence). The most common and widely accepted system is the New Austrian Tunneling Method (NATM), used mainly in the underground construction industry. It was developed in the 1960s with the aim of stabilizing a tunnel in the most cost effective manner, therefore making the most of the rock’s self supporting ability. The name was chosen by the main developer of the system – Prof. L.V. Rabcewicz.. Rabcewicz and Muller used the sprayed concrete based support system in 1964 in the Schwaikheim Tunnel. The first NATM application in the USA was designed for the Pittsburgh streetcar transportation system by the Urban Mass Transportation Administration (UMTA) in 1983 (Kirwin, 1985). This method was selected over a more conventional approach due to the potential cost savings. This is generally an iterative process involving the following repeating steps: 1. Gather relevant on site rock and stability data. 2. Analyze the data using any method considered suitable and determine a rock stability or classification. 3. Design an appropriate shape for the excavation profile taking account of the final function of the excavation and the local conditions as dictated by the rock mass properties and stress regime (tensor). 4. Develop a support system that usually involves bolting and shotcreting. 5. Install the support in the developing excavation, after each round (if blasted) with the aim to limit rock movement (unraveling). 6. Monitor the rock stability with the installed support (with the application of shotcrete it is sometimes difficult to identify local geological features and rock types etc.). This always involves closure measurements but could also include extensometer measurements and physical load measurements in the anchors using load cells. 7. Start at step 1 again and update the information and redesign/alter the support if necessary. 12.2 The Q system This system is a refinement of the Rock Quality Designation (RQD) developed by Deere et al (1969) and takes account of the presence and properties of joint sets and the state of stress. The use of this system for shotcrete design is flawed because it does not include a toughness requirement. 12.3 The modified Q system to include toughness The Q system was modified by Grant et al (2001) to incorporate the shotcrete toughness performance levels (TPL) developed by Morgan (1990). The EFNARC panel toughness (J) is specified for each ground condition identified using the Q system as follows: Rock class Description EFNARC panel toughness (J) F Extremely poor +1400 E Very poor 1000 – 1400 D Poor 700 – 1000 C Fair 500 – 700 B Good < 500 A Very good 0 Grant et al suggest a rock class of G where the ground movement is expected to be so large than even fiber reinforced shotcrete will be inadequate. They define this when the ground movement is expected to exceed 0.05 times the bolt spacing. Under this condition they recommend the following be considered: • Steel sets or reinforced shotcrete ribs (as per the Norwegian Concrete Association Publication No.7 of 1993). • Conventional mesh reinforcement. • Secondary linings. 12.4 The ground characteristic curve method This method was developed by Speers and Spearing (1996) and is useful in high deformation applications. The method utilizes the 2 or 3 dimensional FLAC program (ITASCA, 1992). It uses the Mohr-Coulomb criteria for brittle failure, and although it is known that rock failure can seldom be classified by a single criterion (Stacey and De Jongh – 1977), it is a useful design tool that is especially powerful qualitatively. In essence the numerical model is used to derive a relationship between the pressure on a tunnel periphery generated by the support and the corresponding rock displacement. This relationship is called the ground reaction curve, the ground characteristic curve or the required support line (Brady and Brown – 1985). Input parameters for the support (such as cable bolts, grouted bolts, mesh and shotcrete) can be obtained from the published literature and the manual for the computer code. Full write-ups on these methods can be obtained from the following references: • Rabcewicz, L.V. The New Austrian Tunneling Method. Water Power – November/December 1964 and January 1965. • Barton, N., Lien, R., Lunde, J. 1974, Engineering classification of rock masses for the design of tunnel support. J.S.Afr.Inst.Min.Metall. vol 74, no. 8. pp. 312-320 – South Africa – 1974. • Grant, N.B., Ratcliffe, R., Papworth, F. 2001, Design guidelines for the use of SFRS in ground support. Shotcrete: Engineering Developments – Hobart, Australia – 2 to 4 April 2001. • Speers, C.R., Spearing, A.J.S. 1996, The design of tunnel support in deep hard-rock mines under quasistatic conditions. J.S.Afr.Inst.Min.Metall. vol 96, no. 2. pp. 47-54 – South Africa – March/April 1996. 13. Mix design examples and case studies The case studies cover the applications listed below and are given as a guide only. Any shotcrete mix should first be lab tested to ensure that it meets the performance criteria set for the specific application, and then field tested. Results will vary dramatically depending on the raw materials sources and no support should be used (including shotcrete) without the approval of a suitably qualified and responsible person (e.g. a geotechnical or rock engineer). Case studies will deal with the following applications: • Standard dry mix • Standard wet shotcrete mix • High early strength mix with high single pass build • Fiber reinforced shotcrete mixes for slick lines especially in sinking shafts • A mix to replace bolts and screen/mesh • Shotcrete for kimberlite (blue) rock • Shotcrete for water inflow areas • Wet shotcrete for economical reasons Not all the case studies are from mines, but the applications are typical. 13.1 Standard dry mix The water to cement ratio is typically between 0.40 and 0.45. To better wet the mix and reduce dust formation at the nozzle, the water ring can be placed between 1 and 3m back from the nozzle. A standard typical mix could be per m³: - Portland cement 350 kg - -8 mm aggregate 350 kg - Fine sand 1400 kg An accelerator dose of between 3 and 4% is typically used. This can be added as a powder or a liquid. A water to cement ratio of 0.45 to 0.50 is typical. 13.2 Standard wet shotcrete mix There really is no standard mix design, but a basic mix could be: - Portland cement 425 kg - -8 mm aggregate 300 kg - Fine sand 1400 kg - Water reducer 4 kg An accelerator dose between 4 and 6 % is typically used. A water to cement ratio around 0.40 to 0.45 is typical. Clearly this mix can be readily modified at a nominal cost to become more effective as illustrated in the other examples below. 13.3 High early strength mix with high single pass build Where high early strength and thick single pass layers of shotcrete are required, the following mix could be a guideline per m³: - Portland cement 520 kg - Microsilica 25 kg - Aggregate (0 to 8 mm) 1600 kg - Water reducers 6.5 to 7.5 kg - Internal curer 5 kg - Hydration controller 2 kg - Accelerator 8 % - Steel fiber (25 mm) 50 kg - Water:cement ratio 0.45 - Average thickness 20 cm The above mix was used at North Cape Tunnel in Norway (Melbye, 2001) and produced the following results: • compressive strength of > 2 MPa at I hour > 7 MPa at 7 hours > 30 MPa at 1 day > 40 MPa at 28 day • rebound of around 5 % • average single pass thickness of about 250mm. It should be noted that the poor rock quality and the arctic weather conditions lead to the relatively high cement content. 13.4 Fiber reinforced shotcrete mixes for slick lines especially in sinking shafts Fundamental aspects need to be considered when designing support systems for use during shaft sinking, and these include (Spearing and Nel – 1999): • Space and access is always a major constraint. • Dust can be a concern, especially due to the limited space and ventilation availability. • Hoisting capacity is limited. • Time is a very precious commodity, because the costs associated with delays are huge as the time to access the orebody is directly effected. • Humidity and temperature are generally high. The use of shotcrete as support can be highly beneficial under such constraints, because: • The shotcrete machine can be placed out of the way on one of the platforms of the sinking stage, or lowered when needed onto the shaft bottom. • Dust can be virtually eliminated by using the wet application process. • The shotcrete mix can be transported via a pipe range in the shaft, making it independent of hoisting. • Shotcrete can be applied very rapidly. • The effects of temperature and humidity can be negated. The design of the shotcrete mix is critical, and must be considered carefully. The shotcrete mix should contain much the same constituents as any other shotcrete, but more care is needed, as indicated below: • For standard shotcreting, typical cement contents of around 400 kg/m³ can be used, but in shaft sinking, more than 400 kg/m³ is recommended mainly to limit segregation during transportation. • Micro-silica is frequently used in shotcrete, but is considered very necessary for shaft sinking, for its ability to improve the cohesiveness of the mix (in the slick-line). Typical additions between 30 and 50 kg/m³ should be considered. • Hydration controllers are frequently used in shotcreting operations, mainly to extend the useful life of the mix prior to spraying. In shaft sinking it is vital, to reduce the effect of a concrete pipe blockage in the shaft. • Accelerators are commonly used, and when shotcreting in shaft sinking operations they are important to gain early support. Alkali free accelerators are becoming more frequently used in shafts mainly because they cause no problems to people working in the vicinity, and cause less final strength reduction. • Effective water reducers are needed to help the mix flow in the pipe, and still give a high rate of early strength gain, by having a low water to cement ratio (less than 0.4). • For permanent shotcreting during shaft sinking and related operations, the use of fibre reinforced (wet) shotcrete (FRS) is highly desirable, because the rock tends to move considerably during the useful life of the shaft and nearby infrastructure, and the potential for rockburst damage is often a consideration. 13.4.1 Vaal River Operations – 11 Shaft Moab Khotsong (South Africa) A typical mix per m³ used for sinking down to 7500 feet (2300 m) below surface, using a batch plant on surface at the 11 Shaft of Vaal River Operations in South Africa (Buckley, 1998) was: - Portland cement 450 kg - Microsilica 40 kg - -8mm stone 1400 kg - Sand 320 kg - Water reducer 10.7 kg - Internal curer 5.0 kg - Hydration controller 2.8 kg - Accelerator 5 % (on total binder content) - Steel fiber 50 kg The above mix was batched on surface and sent vertically underground in a nominal 6 inch (150 mm) diameter pipe. 13.4.2 Western Deep Levels – South Mine (South Africa) In order to access gold ore reserves in excess of 4000m below surface, a replacement ventilation shaft was sunk between 84 and 109 levels (between about 2580m and 3300m below surface) as shown on Figure 10. A novel raise boring method, utilizing the “V” Mole (see Figure 11) was used in the development of the 7.0m diameter shaft, by a South African contractor - RUC. Sidewall support during the final sinking, consisted of split sets and SFRS installed concurrently with the advance. The shotcrete was installed using a manipulator, attached on a steel beam that was fitted around the sinking stage. The shotcrete was mixed at a batching plant located on 84 level, and was fed via a 150mm diameter pipe to a 1.0m³ re-mixing tank that was located on the second deck of the stage. The shotcrete design mix was as follows per m³: - Portland cement 480kg - Micro-silica 48kg - Total aggregate 1525kg - Steel fiber 40kg - Water reducer 5.5kg - Internal curer 5kg -.Hydration controller used part of the time - Alkali free accelerator up to 5% - Water 207litres FIGURE 10 SOUTH SHAFT DEEPENING PROJECT Main Shaft SS 11 Level 14 Level 19 Level 108/99 Level V e n t e r s d o r p C o n ta c t R e e f C a r b o n L e a d e r R e e f 109 Level 113 Level 116 Level 120 Level 123 Level 126 Level 130 Level 132 Level Shaft bottom -4117mbd SS1SS2 SSV Shaft C a r b o n L e a d e r R e e f b o u n d a r y V e n t e r s d o r p C o n t a c t R e e f b o u n d a r y FIGURE 11 SECTION THROUGH THE SHAFT 50 ton winch support structure Kibble Stage Drill rig “V” mole Mixer and pump Robotic arm and shotcrete spray nozzle The mine personnel estimated that the total rebound was about 5%, although physical measurements could obviously not be made. The shaft development that passed through the Ventersdorp lava sequence, with an unconfined compressive strength (UCS) of over 300MPa, proceeded as planned. In the weaker Witwatersrand quartzites, with a UCS of about 200MPa, excessive sidewall slabbing (dog-earing) occurred behind the cutter head, before the shotcrete could be applied. This caused some delays, and it was concluded by the project team on the mine, that the SFRS should have been applied even sooner (right up to the face). 13.4.3 Alp Transit Tunnel – Sedrun, Switzerland The Alp Transit tunnel system for cars and trucks is aimed to accelerate transportation through Switzerland, and reduce pollution. This will be achieved by the use of railway transport carrying motor vehicles through the Swiss Alps. The Gotthard Tunnel will be used mainly to transport trucks, on rail cars at high speed (about 140km/hr). The Sedrun section of this project consists of an access tunnel to a shaft that is being sunk some 800m, with a diameter of 7.75m, as shown on Figure 12. Shaft Sinkers is part of the consortium involved with this preliminary phase. The shaft will be used to provide additional starting locations for the main tunnels, of about 57km length. Once these tunnels are complete, the shaft and access tunnel will be used to provide services and ventilation to the main tunnels below. The shotcrete is batched on surface and taken to the shaft platform via a 150mm diameter steel pipeline, fitted with a kettle (energy dissipater) at the bottom. A custom designed kibble (bucket) with a 4.5m³ capacity is used to transport the shotcrete material from the kettle to the shaft bottom (about 40m vertically). The use of this kibble frees the hoisting system for other operations, whilst the shotcrete is being sprayed. The shotcrete is then applied using a small air powered wet mix pump (Allentown AP10), as a temporary support, before the final lining is cast. The mix design is as follows per m³: - Portland cement 450kg - Micro-silica 40kg - -8mm aggregate 670kg - Sand 1080kg - Water reducer 5.9kg - Accelerator 4 to 8% The water to cement ratio was 0.47. FIGURE 12 DETAIL OF SHAFT AND EQUIPMENT AT SEDRUN SEDRUN Temporary accommodation Pipeline for cement transport Access tunnel aprox. 990m Silos Shaft bottom Concrete plant Gotthard base tunnel Headgear Ventilation shaft Ventilation structure Hoisting cage Passenger lift The above mix produced the following results: • compressive strength of > 3 MPa at 4 hour > 11 MPa at 12 hours > 27 MPa at 1 day > 40 MPa at 28 day • rebound of around 8 % • thickness of about 100 to 150 mm. 13.5 A mix to replace bolts and mesh – INCO’s Stobie Mine - Canada A mix that has been successfully used to replace bolting and screening in drifts at INCO’S Stobie Mine in Sudbury, Canada (O’Hearn and Buksa, 1997) is as follows per m³: - Portland cement 400 kg - Microsilica 40 kg - Coarse aggregate 350 kg - Fine aggregate 1275 kg - Water reducer 4.0 to 5.0 kg - Steel fiber (30 mm length) 50 kg - Water:cement ratio 0.40 to 0.45 - Average thickness 65 mm The above mix produced the following results: • Slump of 65 mm (150 mm is recommended generally) • Bond strength of >0.18 MPa at 8 hours >0.45 MPa at 1 day > 1.2 MPa at 28 days • Compressive strength of > 9 MPa > 40 MPa > 44 MPa 13.6 Shotcrete for Kimberlite (blue) rock Kimberlite pipes are frequently associated with diamonds. Kimberlite (a variety of mica and peridotite) is hygroscopic and tends to pull water from a shotcrete mix. This causes the shotcrete to kimberlite interface surface to weaken to such an extent that the shotcrete bonds very poorly (if at all) to the rock. Some form of seal is therefore needed to stop weathering from occurring in longer term excavations. Where shotcrete has been needed in the past, the general approach has been to apply a thin membrane (liner) first, and then spray the shotcrete onto the membrane. This works technically but is costly and involves a two stage operation. Dry shotcrete has been found to work, but according to Storrie (2001), wet shotcrete has advantages such as higher productivity, less rebound and less dust. For rock engineering considerations, the shotcrete needs a 28 day strength of 45 MPa and an EFNARC energy absorption of 750 J. Based on successful underground trials, the following wet shotcrete mix (Storrie, 2001) was found to be successful: - Cement 500 kg - Microsilica 40 kg - Sand 1558 kg - Water reducer 5.4 l - Plastic fiber 8 kg - Accelerator 5% - Water:cement ratio 0.43 13.7 Shotcrete for water inflow areas Clearly shotcrete cannot be placed over substantial water outflows, but with the correct mix design and especially the correct accelerator and dose rate, shotcrete can be sprayed over surprisingly high water outflows (if the pressure is low). The ideal solution is always water sealing (injection) if required, or at least a well considered water management plan. The following is an example from a rail tunnel – Blisadona Tunnel in Austria (Melbye et al – 2001). There was general heavy water ingress and therefore the sprayed concrete had to gain strength rapidly, in order not to fall off the rock. The mix design and results were as follows per m³ of material: - PZ375 cement 420 kg - Total aggregate 1750 kg - Water reducer 0.7 % - Hydration controller 0.4 % (for 7 hours open time) - Alkali free accelerator 7.5% (average) - Water:cement ratio < 0.5 Compressive strength: 6 minutes (Hilti gun) > 0.3 MPa 10 minutes (Hilti gun) > 0.4 MPa 30 minutes (Hilti gun) > 0.6 MPa 12 hours (Hilti gun) > 8.0 MPa 7 days (cores) 25.0 MPa 28 days (cores) 31.0 MPa 13.8 Wet shotcrete for economical reasons The support system used on any mine must first protect people but also must be cost effective. The total installed cost of support (including any longer term possible rehabilitation costs) must always be considered. The economics of a dry process versus a wet process should always be the main deciding factor. Telfer Gold Mine, part of Newcrest Mining Ltd in Australia, compared the economics of both wet and dry shotcreting for their application (Cepuritus, 1996). The wet shotcrete process (produced on site) proved to be about 10% cheaper than the dry process (transported to site in bulk), mainly due to transportation cost savings as the mine is very remote (and other considerations such as rebound reduction). - Portland cement 425 kg - Microsilica 40 kg - Aggregate (-7 mm) 500 kg - Coarse sand 1000 kg - Dune sand 250 kg - Water reducer 4 liters - Stabilizer 3 liters - Water:cement ratio 0.49 - Average thickness +50 mm More detailed information on shotcrete (in English, German and Spanish) can be obtained from the following sources: • www.ugc.mbt.com • Americas Tel: +1-216-8397500 MB Inc. Fax: +1-216-8398801 23700 Chagrin Boulevard, Cleveland, OHIO 44122, USA • Asia/Pacific Tel: +65-8607305 MBT Singapore Fax: +65-8630951 33 Tuas Avenue 11, Singapore 639090 • Australia Tel: +61-2-96244200 MBT Australia Fax: +62-2-96247681 11 Stanton Road, AUS – Seven Hills, NSW 2147, Australia • Europe Tel: +41-1-4382211 MBT Schweiz Fax: +41-1-4382246 Vulkanstrasse 110, 8048 Zurich, Switzerland • Japan Tel: +81-3-35828814 NMB Fax: +81-3-35833800 16-26, Roppongi 3-chome Minato-ku Tokyo 106-0032, Japan • Latin America Tel: +1-305-6674239 5941 SW 47 th Street, Fax: +1-305-6670793 Miami, Florida 33155-6028, USA • South Africa Tel: +27-11-7541343 MBT Mining and Tunneling Fax: +27-11-7541105 11 Pullinger Street, Westonaria 1780, South Africa 14. The future The future for shotcreting in mines is positive and is likely to grow significantly. This is not only because of the on-going advances in particularly wet shotcrete, both from an equipment and product point of view, but also safety concerns associated with the installation of mesh/screen (the most common alternative). These advances will include: • Shotcrete equipment automation. • Advances in the performance of synthetic fibers. • Advances in admixture performance. 14.1 The future of shotcrete equipment Mines typically apply shotcrete at between 50mm to 100mm thick. This is in contrast to most underground civil applications were the typical thickness would exceed 100mm. A development (Tschumi, 1998) that should have a most significant impact on shotcrete in the future, and quality control in particular, is the current on-going development of a semi-automatic (or automatic) spray manipulator (robot) that can: i) Measure the tunnel profile using a laser scanner. ii) Shotcrete to a desired thickness basically automatically, keeping the spray angle and distance at the ideal settings (thus minimizing rebound). iii) Check that the design thickness has been achieved, re-spraying any areas that are under-sprayed. The first prototype has been evaluated by INCO at it’s test mine in Canada (Runciman et al 2001). These tests clearly showed the potential for such a system. The thickness variation in “fully automatic” mode was found to be only 7.5mm and this is a significant achievement. This development will permit support systems that mainly rely on fiber reinforced shotcrete to be designed and implemented in underground mines. The safety aspects of this are obvious because it should help resolve the most difficult quality control issue: that of achieving the designed placed shotcrete thickness. Less obvious are the potential cost benefits of the equipment such as: • Reduced rebound (achieved by maintaining the correct nozzle to substrate distance and angle at all times). • Reduced maintenance costs and increased equipment life due to the more precise and less erratic control of the manipulator under computer control. • Less operator training needs (but more computer literate maintenance needs). The rebound in well controlled manipulator applied wet shotcrete is typically between 10 and 15%. The rebound using the Logica has been measured at between 5 and 10%, so a reduction of 5% rebound is possible. The complete Logica control system costs less than $110k, and hence can pay for itself just with the rebound reduction after only 11000 m³ sprayed. This is a simplistic approach and assumes a cost of fiber reinforced shotcrete to be US$200/m³. This calculation assumes that the use of a conventional manipulator (boom) mounted shotcrete rig has already been justified and only the Logica technology needs to be justified as an “add-on”. In the near future, mining booms will be routinely available with and without the Logica technology, but the future will clearly include Logica. Full write-ups on these methods and equipment can be obtained from the following reference sources: • Tschumi,O. 1998, State of the art of the latest generation concrete spraying robot. 100 th CIM Conference – Montreal, Canada. • Runciman, N., Rispin, M., Newson, G. 2001, Tele-operated shotcrete spraying with the MEYCO Robojet Logica. Shotcrete: Engineering Developments – Hobart, Australia – 2 to 4 April 2001. • The MEYCO web site is http://www.meyco-equipment.ch 14.2 Possible improvements in synthetic fibers Improvements in the handling and performance of synthetic (plastic) fibers will continue, and the trend will be away from steel fibers. This is because: • In mining, shotcrete frequently operates post first crack, and with the steel, corrosion of the fibers can be an issue. • The plastic fibers are not a minor hazard that can cause minor cuts to personnel after placement (as the steel fibers do that stick out from the shotcrete). • The plastic fibers seem able to perform such that the shotcrete can deform more before being totally ineffective. An issue that still needs to be fully resolved is that associated with the density of the synthetic fibers. They tend to float and hence can cause problems with sump pumps etc. 14.3 Future admixture advances Many mine operators are calling for even faster early strength development so that they can re-enter a freshly sprayed end safely but quickly. At present many operators want at least a compressive early strength of 2 MPa after 2 hours, but even higher strengths would be desirable. The bond early strength is actually the important parameter rather than the compressive strength but this is not specified at all at present, probably due to practical field testing issues. Future accelerators will tend to develop strength even faster and still have only a little effect on the final strength. Alkali free accelerators will be the only approved shotcrete accelerator due to the issues of health and safety. 15. References American Concrete Institute, Guide to shotcrete, Report 506R-2. Almgren, G. Rock mechanics and the economics of cut and fill mining. Application of Rock Mechanics to Cut and Fill Mining – University of Lulea – Sweden – June 1980. Barton, N., Lien, R., Lunde, J. 1974, Engineering classification of rock masses for the design of tunnel support. J.S.Afr.Inst.Min.Metall. vol 74, no. 8. pp. 312-320 – South Africa – 1974. Bernhard, E.S. (1998) Measurements of post-cracking performance in fibre reinforced shotcrete. Australian Shotcrete Conference 1998 – Sydney, Australia – October 8 and 9 1998. Bernhard, E.S. (1998) The behaviour of round steel fibre reinforced concrete panels under point loads. Engineering Report CE8, Dept. of Civil and Environmental Engineering, University of Western Sydney – Australia – 1998. Bernhard, E.S. (1999) Correlations in the performance of fibre reinforced shotcrete beams and panels. Engineering Report CE8, Dept. of Civil and Environmental Engineering, University of Western Sydney – Australia – 1998. Bernhard, E.S., Pircher, M. (2000) The influence of geometry on performance of round determinate panels made with fibre reinforced concrete. Engineering Report CE8, Dept. of Civil and Environmental Engineering, University of Western Sydney – Australia – January 2000. Brady, B.H.G., Brown, E.T. (1985) Rock Mechanics for underground mining. George Allen and Unwin – London, UK. Buckley,J.A. (1998) The application of wetcrete as a support medium. Shotcrete and its Application – SAIMM – Johannesburg, South Africa Cepuritus,P.M. (1996) Economic advantages of on-site wet-mix shotcreting. Shotcrete – Techniques, Procedures and Mining Applications – Kalgoorlie, Australia Comite Europeen de Normalisation – www.cenorm.be Deere, D.U. (1969) Engineering classification of in situ rock. AFWL Report TR67- 144 – Washington D.C., USA. Galinat,M.A. (1998) High performance polymer fibre reinforced shotcrete. Australian Shotcrete Conference – Sydney, Australia. Garshol, K (2002) Personal communication. Golser, J. (1976) The New Austrian Tunneling Method (NATM). Shotcrete for Ground Support – ACI Publication SP-54 – October 1976. Grant, N.B., Ratcliffe, R., Papworth, F. (2001), Design guidelines for the use of SFRS in ground support. Shotcrete: Engineering Developments – Hobart, Australia – 2 to 4 April 2001. Itasca (1992). FLAC – Fast Lagrangian Analysis Continua. Version 3.0/3.2. Kirwin, J. (1985) NATM: The wave of the future in rock tunnel construction ? The International Magazine on Transportation in Cities – Vol XII, No.5 – May 1985. Melbye,T.A., Garshol, K.F. (1999 and 2001 edition) Sprayed concrete for rock support. © Master Builders Technologies – Zurich, Switzerland. Morgan, D.R., Chen, L., Beaupre, D. (1990) Toughness of fiber reinforced shotcrete. Munn, R.L. (1997) Basic technology for sprayed concrete. 3 rd National Shotcrete Workshop (Rock Technology Ltd.) – Australia – November 1997. Norwegian Concrete Association - Technical Specifications and Guidelines; Publication No. 7 - 1993. O’Hearn,B., Buksa,H. (1997) Boltless shotcrete. 1 st South African Rock Engineering Symposium – Johannesburg, South Africa Peck, R.B. (1969) Advantages and limitations of the observational method in applied soil mechanics. Geotechnique 19, pp 171-187. Runciman, N., Rispin, M., Newson, G. 2001 Tele-operated shotcrete spraying with the MEYCO Robojet Logica. Shotcrete: Engineering Developments – Hobart, Australia – 2 to 4 April 2001. Sauer, G. (2002) Light at the end of the tunnel. www.dr-sauer.com Spearing, A.J.S. (1995) The potential for shotcrete as a tunnel support in gold and platinum mines. Innovative Mining and Support Systems for Safety and Productivity Colloquium – SAIMM – Randburg, South Africa – May 1995. Spearing,A.J.S. (1998) Practical guidelines on shotcrete application. Shotcrete and its Application – SAIMM – Johannesburg, South Africa Spearing,A.J.S.,Chittenden,N. (1998) Design,application and quality control to ensure safe and cost effective shotcreting. Rock Mechanics and Productivity – SANGORM (ISRM) – Carletonville, South Africa – October 1998. Spearing, A.J.S., Naismith, W.A. (1999) The rationale, design and implementation of steel fibre reinforced wet shotcrete, in hard rock tunnels, where rapid advance is needed in South Africa. Rock Support and Reinforcement Practice in Mining – Kalgoorlie, Australia – March 1999. Spearing,A.J.S., Nel,P.J.L. (1999) The design, transportation and application of wet shotcrete for the support of vertical shafts and related development. 3 rd International Conference on Sprayed Concrete – Gol, Norway. Speers, C.R., Spearing, A.J.S. (1996) The design of tunnel support in deep hard- rock mines under quasistatic conditions. J.S.Afr.Inst.Min.Metall. vol 96, no. 2. pp. 47-54 – South Africa – March/April 1996. Stacey, T.R., De Jongh, C.L. (1977) Stress fracturing around a deep level bored tunnel. .S.Afr.Inst.Min.Metall. vol 78, pp. 124-133 – South Africa. Storrie, A. (2001) Wet shotcrete onto kimberlite. Shotcrete and Membrane Support Colloquium – SAIMM – Randburg, South Africa – April, 2001. Tschumi,O. (1998) State of the art of the latest generation concrete spraying robot. 100 th CIM Conference – Montreal, Canada. Vandewalle,M (1997) Tunnelling the world. © N.V.Bekaert S.A. 16. MBT Shotcrete Admixture Products The list below is not exhaustive and certain products are not available in every country. For more detailed information, please contact the local MBT operation or the Underground Construction Group web site at: www.ugc.mbt.com The Technical Data Sheets for most of the products listed below can be downloaded directly from the above web site above. Another useful source is the “Jobsite Manager”, available from the local MBT office. For assistance with any shotcrete mix design or shotcrete related problem, contact the nearest MBT office. Never use admixtures without first insisting on some form of testing. • Dry shotcrete accelerators Traditional accelerators (typically 3 to 5 % dosage by total binder weight) Ø Accel-A-Set 2000 Ø Accel-A-Set 120 Ø MEYCO SA 100 Ø MEYCO SA 120 Ø MEYCO SA 500 series Ø MEYCO SA 160 to 170 series Alkali free accelerator Ø MEYCO SA 545 (typically 4 to 8 % dosage by total binder weight) • Wet shotcrete accelerators Traditional accelerators (not usually recommended) Ø MEYCO SA 100 (chloride free) Ø MEYCO SA 120 (chloride free) Ø MEYCO SA 430 (chloride free) Alkali free accelerators (typically 4 to 8% dosage by weight of total binder) Ø MEYCO SA 160 Ø MEYCO SA 161 Ø MEYCO SA 162 Ø MEYCO SA 170 • Water reducers (plasticizers and superplasticizers) Ø Polyheed SG (can be used with TCC system below) Ø Pozzolith series Ø Rheobuild series Ø Glenium series • Microsilica (typically 5 to 10 % by weight of cement) Ø MEYCO MS 610 Ø MEYCO MS 660 (slurry) Ø MEYCO MS 685 (nano-silica suspension) • Concrete improvers (concrete curing agents) Ø TCC 735 (dosage at 5 kg/m³ of shotcrete) • Consistency controllers (Total Consistency Control – TCC) Ø MEYCO TCC 765 and 766 Ø MEYCO TCC 780 • Hydration controllers Ø Delvocrete Stabilizer (typically 0.4 to 2.0 % by weight of total binder) • Pumping aids (also part of TCC) Ø Water reducers in general Ø MEYCO TCC 780 Ø Air entraining agents (e.g. Micro-Air 100) • Steel fibres • Synthetic (plastic) fibres There are also combination admixtures, not listed above that act as superplasticizers and hydration controllers (e.g. Glenium T801 and T803). 17. Useful contacts Ø American Concrete Institute (ACI) formed in 1904. P.O. Box 9094, Farmington Hills, Michigan 48333-9094, USA. Tel: +1-248-8483700 Web site: www.aci-int.org Ø American Shotcrete Association (ASA) formed in 1998. 38800 Country Club Drive Farmington Hills, MI 48331, USA. Tel: +1-248-8483780 Fax: +1-248-8483740 Web site: www.shotcrete.org Ø American Society for Testing and Materials (ASTM) – formed 1898. 100 Barr Harbor Drive, West Conshohocken, PA 19428 USA. Tel: +1-610-8329500 Fax: +1-610-8329555 E mail:
[email protected] Web site: www.astm.org Ø Australian Shotcrete Society C/o Jetcrete Australia 30 Waratah Street, Kirrawee, NSW 2232, Australia Tel: +61-2-95218733 Fax: +61-2-95218992 E mail:
[email protected] Ø British Standards Institute (BS standards) – formed 1901. 2 Park Street, London, W1A 2BS United Kingdom Tel: +44-1-6299000 Fax: +44-1-6290506 Ø Deutsches Institut fuer Normung (DIN standards) – formed 1917. (German Institute for Standards) Burggrafenstrasse 6, Postfach 1107, W-1000 Berlin 30 Germany Tel: +49-30-2601362 Fax: +49-30-2601231 Ø European Federation of Producers and Applicators of Specialist Products (EFNARC) Association House, 235 Ash Road, Aldershot, Hampshire, GU12 4DD, UK Tel: +44-1252-342072 Fax: +44-1252-333901 Web site: www.efnarc.org Ø International Centre for Geomechanics (CUC) Rheinstrasse 4, Postfach 64, CH-7320 Sargans, Switzerland. Tel: +41-81-7253150 Fax: +41-81-7253140 E mail:
[email protected] Web site: www.icguc.com Ø International Tunnelling Association (ITA) C. Berenguier – Secretary General CETU 25, Ave. Francois Mitterand, Case No. 1-69674, Bron-Cedex, France Tel: +33-47-8260455 Fax: +33-47-2372406 E mail:
[email protected] Web site: www.ita-aites.org ® Registered Trademark MBT Holding AG © Copyright 2002 Master Builders, Inc. Printed in USA 2/02 #1023014 Edition 1 MASTER BUILDERS, INC. United States 23700 Chagrin Boulevard Cleveland, Ohio 44122-5554 Phone: 800-MBT-9990 Fax: 216-839-8821 Canada 1800 Clark Blvd. Brampton, Ontario L6T 4M7 Phone: 800-387-5962 Fax: 905-792-0651 www.masterbuilders.com