When first developed, shotcrete mixes were a relatively simple matter of mixing water with cement and fine aggregate in the right proportions to achieve adherence to the surface being coated. In order to not waste materials it was, and is, also important to minimise rebound and excessive slump with both the correct mix for the conditions and skillful operation of the nozzle in maintaining the correct limit angle to the treated surface.
Work underground has arguably made more demands on the available technology than any other application. Specialist considerations include extended materials logistics, often limited space, and dust generation causing visibility problems and health & safety concerns. The early load-bearing capacity of shotcrete has also assumed increasing importance underground with the use of sprayed concrete as an important part of observational methods of tunnel construction, other temporary support applications, and its use in single-shell structures. Shotcrete is suited to the modern practice of tunnel ground control, rather than load supporting, by its possibilities for high initial strength, good bonding to the rock, and (with reinforcement) ductility.
It is also time beneficial for the tunnel construction cycle. Demands on scheduling and structural performances have led to the widespread adoption of fibre-reinforced shotcrete mixes, often replacing or supplementing more traditional reinforcement materials such as steel mesh.
More recently concerns over fire safety in operational tunnels have led to consideration of the role of fibres in the performance of concrete linings subject to intense heat, especially with regard to deterring spalling. Specialist spray-on coatings for fire protection have also been developed, although not yet of structural importance.
Basics
Shotcrete has been defined as gunite with aggregate size greater than 10mm; gunite being mortar applied under pressure of compressed air to form a dense layer on the surface or object being treated.
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By GlobalDataThe selection of aggregate material is as important with shotcrete as it is with all other types of concrete. Sometimes, for environmental reasons as well as economic the material excavated from the tunnel can, with suitable treatment, be used as aggregate for shotcrete and other concrete mixes. This has been a feature of the construction plans for the NEAT tunnels in Switzerland. In the Sedrun section of the Gotthard Base Tunnel excavated rock is lifted to the surface for crushing and grading, and the necessary aggregate grades returned to the lower shaft level for concrete batching. Truck mixers of 4-7m3 capacity then carry the ready-mix concrete for use in Sika–Putzmeister 500 or Aliva-500 shotcreting systems. Sika also supplied additives to meet concrete performance requirements including silica fume to increase watertightness and durability of the placed shotcrete, retarders to adjust the workability time, a solvent to secure water/cement ratios, and non-alkaline accelerators.
Wet or dry?
One of the many debates that has surrounded this technology has been whether the ‘dry-mix’ or ‘wet-mix’ method is best. In the dry-mix process water is added at the nozzle in correct proportion to the flow of dry ingredients. The dry ingredients may be supplied pre-mixed in bags, from silos, or mixed on site (depending on site conditions).
In the wet-mix process most of the ingredients (with the possible exception of certain additives) are mixed wet and conveyed as a slurry to the nozzle. It may be brought to the work area by pumped pipe, ready-mix road trucks, re-mixer silo rail cars, or other specialist vehicles. The means of transport may necessitate the use of additives to deter curing or plasticiser for improved workability.
The apparent advantages of the wet-mix process in bulk transport and improved working conditions has led to its adoption for most underground projects, especially the larger ones. However, despite some authorities stating that wet mix has taken over, dry mix operations still have many applications, especially in smaller sections, for repair work, and variable geology where shotcrete support may be needed only in limited areas.
Why additives?
It cannot be denied that concrete mix additives have widened the applications and flexibility of use in many shotcrete applications. However, some specialists argue that they are being used too readily and are hardly needed in many cases. Despite testing programmes conducted by leading manufacturers, the proponents of minimal additives argue that the long-term structural performance and stability of mixes incorporating many additives has not been proven.
Phil Richardson of the Natural Cement Distribution (NCD) points out that no additives, including accelerators, are required in his company’s products, thanks largely to the use of a special Alpine limestone source. The problem with other mixes, he points out, is that they are based on Portland cement and need to have the set speeded up. Natural cement starts to set in 90 seconds. Consequently it is unlikely to be suitable for wet mix operations, but is ideal for dry mix shotcrete application.
Aggregates slow the setting time to 4 minutes for initial reaction and about 10 minutes for a total set. Citric acid can also be used to retard the set. The required strength is reached rapidly, with NCD Shotcrete 513 achieving 5MPa compressive strength 15 minutes after set (40 mins after mixing), and 9MPa at one hour.
Richardson also states that tests by the UK’s Building Research Establishment (BRE) have shown a continual development of strength, even if excess water may initially slow down setting. Samplers showed a 75% approximate increase in strength in one year (47.5MPa to 83MPa) and a further increase to 89MPa after another two years.
John Aveling of specialist shotcreting contractor Ryan Contracting used NCD Shotcrete 530 dry-mix in the Dudley Canal Stabilisation contract for Morrison Construction. The materials were chosen on the basis of longevity, compatibility with existing construction materials, lack of hazardous chemicals and environmental acceptability. With Shotcrete 530 the binder is natural cement with additives, allowing filtered water from the canal to be used and eliminating the need to transport fresh water. Ryan achieved a successful sequence of lining repairs with minimal rebound compared to other materials tested.
Accelerators
Accelerators are the most commonly used concrete mix additive. As mentioned above they are used to ‘accelerate’ the curing process (hardening rate) or initial setting time, to deter rebound and promote early strength. However, depending on the type used, they can also reduce long-term strength. Their use for ‘temporary’ support immediately after blasting is widespread, in order to provide a safe working area. Their role in forming the permanent structure by shotcreting may not be so important if rebound can be minimised otherwise.
Calcium chloride has been widely used but has the disadvantage of corroding steel in reinforcement, and is banned in many areas, unless no reinforcement is employed. Other types that do not corrode steel are chiefly organic including formates, nitrates and thiocyanates.
Accelerators have also been the cause of health concerns, although safe alternatives are now available. Alkali-based accelerators are banned in many areas due to severe skin irritation that can be experienced by operatives. Sodium carbonate can still be used for limited quick repairs, but at the expense of lower strength concrete.
Although not highly hazardous, the aluminates used in modern accelerators can still present an irritant hazard against which precautions such as personal protective gear need to be employed.
Manufacturers of modern accelerators also point out that their products do not trigger alkali-aggregate reaction and the concrete does not leach, thus reducing the risk of obstructing drainage channels. These accelerators, with minimal health hazards, are available as powders, ready-mixed, or as liquids for easier dosing.
A recently introduced accelerator from Mapei, Mapequick AFK 777 J is a liquid formulation based on inorganic salts for rapid, but adjustable, setting of shotcrete in both dry-mix and wet-mix processes. Mapei claims that this product has almost no loss of strength during long curing times compared to concrete without accelerator.
Retarders
If the concrete mix is required to be workable a long time after initial mixing, whether due to transport or waiting time, then a retardant admixture may be beneficial. A disadvantage of conventional retarders is that that prolonged workability is attained with strong retardation, and this has to be combated with an accelerator to develop early strength, reducing workability.
Using Mapei’s Mapetard SD2000, concrete ‘slump’ is retained with a retarded rate of cement hydration after a short curing time. It is a hydrous solution including 15.5% active polymers. Applications include ready-mix concrete (particularly in hot climates) and use with shotcrete including alkali-free accelerators. It is not recommended for winter use, for which Mapei recommends other products for a quicker development of early strength.
One of the first developed retarders, Delvocrete from UGC BASF, allows concrete to be used for up to three days later if added before concrete is three hours old.
Plasticisers
Another way of retaining concrete slump, or workability, for longer periods is to use a more expensive superplasticiser. Polymers can also be used to reduce rebound as an alternative to accelerators.
Sika has introduced new ‘high range’ water reducer plasticisers of Polycarboxylicether (PCE) type in order to retain workability without the disadvantages of accelerator/retarder combinations. These allow the shotcrete mix to be conveyable for several hours (with stable consistency) and to be accelerated immediately after conveying for application. This works even at high temperatures. Consequently the mixes meet the requirement of prEN 934-5.
Mapei’s Dynamon SR4 admixture is based on modified acrylic resin polymer and is specially designed for ready-mix concrete. Mapei says that, due to its own design and production of monomers it has developed Designed Performance Polymer (DPP) technology in which the admixture’s properties can be modified according to the specific performances required.
Role of microsilica
Fine-particle silica, also know as microsilica or silica fume, is a fine artificial pozzolan and can be used in shotcrete mixes to react with free lime in the cement to block pores in the calcium silicate hydrate, thus promoting imperviousness, strength and durability. It is much more reactive than fly ash, commonly used in ordinary concrete. Its use can eliminate the need for a water reducer or superplasticiser. Depending on availability the cost can vary greatly compared to conventional cement but is used as a substitute in Norway, Canada and the USA where readily available from smelters.
Reinforcement
Fibres made of steel grades and, more recently, plastics, can be added to shotcrete mixes to achieve the required flexural strength, replacing or supplemental to conventionally erected steel reinforcement. The ability to apply almost immediate support adjacent to the face from a secured position is a distinct safety advantage compared to erecting conventional reinforcement where there will be a temptation to work under unsupported ground. An added benefit is that fibre reinforcement offers higher crack resistance and consequently a better bond. A future development by Bekaert is the BP fibre designed for fast-setting shotcrete to achieve a high early-strength combined with improved ductility of the concrete.
Manufacturers of synthetic plastics fibres claim an advantage of being corrosion resistant, although Bekaert says that the corrosion of steel fibres is not a technical issue. Where corrosion could be an aesthetic problem, with rust streaks on public access tunnels for example, then zinc-coated (galvanised) steel fibres can be used.
Barchip fibres are manufactured from polyolefin with a minimum tensile strength of 550MPa. Barchip Shogun and Kyodo fibres from Elasto-Plastic Concrete (EPC) are each 48mm long, and used in shotcrete. EPC also claims that their fibres result in less wear on concrete pumping equipment.
Fire protection
Both plastics and steel fibres can also help with fire resistance of the tunnel lining by deterring spalling, and maybe explosive disintegration, of concrete.
Barchip polyolefin fibres, for example, have a melting point of 150-165°C and so will form small voids early in the development of a fire. Although spalling can occur at temperatures as little as 200°C, the melting of these fibres will take place at a lower temperature. These voids can link to form passages that allow steam to disperse, relieving pressure that might otherwise cause explosive spalling. The dosing of Barchip F to achieve this deterrence of fire damage is 1kg/m3. Others point out that the low melting point of macro-synthetic fibres can also be a disadvantage as their structural effect is then lost, whereas steel fibres retain tensile strength.
BASF UGC’s Meyco Fix Fireshield 1350 is a special cementitious mix of high-strength mortar for applying a passive fire-protection layer to the tunnel surfaces. It can be used for new tunnels and renovating old ones, by manual nozzle handling or by robotic manipulator. Its main components are mineral-based or organic including Portland cement, water, fine-grained aggregates, admixtures and an accelerator. Its fire protection action depends on its high insulation properties to prevent both explosive spalling of concrete and mechanical deterioration of structural concrete and structural steel reinforcement.
What is in the future?
Shotcrete technology has made rapid advances in recent years in terms of application, the use of different materials in its formulation, and the possible successful uses. There has been substantial testing undertaken on individual additives and their results, including recognised panel tests to European Standards, and even on multiple additives mixes from one manufacturer. However those who are not fans of additives point out that little is known about the long-term effects of multiple additive mixes.
Says Phil Richardson, “What reactions are we going to get in years to come because of all the additives in concrete like ferrous sulphate to reduce soluble chromium IV (which can cause dermatitis), air entrainers, waterproofers, etc? What will happen when these additives leak into the water aquifers?”
Even with a basically innocuous ingredient such as steel fibre reinforcement, which is widely accepted as effective, there remain concerns about the design implications. In a recent series of successful seminars led by Bekaert’s Marc Vanderwalle, there were consistent remarks about the lack of design codes for any fibre concrete, and the need for more research on long-term durability performance. Whilst proven durability is claimed, it is not so easy to quantify. This provides justification for the title of Marc Vanderwalle’s book ‘Tunnelling is an Art’, in that skill and experience still counts for much more than numerical data.
Clearly demands placed on shotcrete technology are outstripping the proof available of its ingredients’ performance under all likely circumstances. The emperic ‘proof’ available of what works is more plentiful than the quantified lab-data preferred by most design engineers. A lot more work remains to be done to increase confidence and economy.
First layer of schotcrete sprayed around a lattice arch Shotcrete around arch Barchip modified polyolefin fibre from Elasto-Plastic Concrete Modified polyolefin fibre The Sika PM500 in action PM500
