Shotcrete is a mortar or high performance concrete conveyed through a hose and pneumatically projected at high velocity onto a backing surface. It is the force of this spraying action that leads to compaction of the concrete or mortar which then forms layers of concrete to the required thickness .
Since 1900, the year it was invented by American taxidermist Carl Akeley, it has seen many improvements, be it in the equipment or in the specialised techniques required for the pneumatic application of mortar or concrete.
Today it has become a vital material because of it being versatile in shape, high strength, durable, good bonding ability, and in areas of difficult access it can be easily sprayed onto a surface at high velocity, making it handy for operation.
In the mining sector, the demand for shotcrete for underground support has skyrocketed in recent years. The simultaneous working of multiple heading, unusual filling conditions and difficulty in access are some of the problems which are distinctive to underground mining and which require new and innovative applications of shotcrete technology.
The selection of shotcrete for a particular application should be based on knowledge, experience, and a careful study of required and achievable material performance. The success of the shotcrete for that application is contingent upon proper planning and supervision, plus the skill and continuous attention provided by the shotcrete applicator.
Shotcrete can be used to repair the damaged surface of concrete, timber, or steel structures provided there is access to the surface needing repair. The following examples indicate a few ways in which shotcrete can be used in repairs:
Shotcrete repair can be used for bridge deck rehabilitation, but it has generally been uneconomical for major full-thickness repairs. It is very useful, however, for beam repairs of variable depths ,caps, columns, abutments, wing walls, and under decks from the standpoint of technique and cost.
In building repairs, shotcrete is commonly used for repair of fire and earthquake damage and deterioration, strengthening walls, and encasing structural steel for fireproofing. The repair of structural members such as beams, columns, and connections is common for structures damaged by an earthquake.
Damage to marine structures can result from deterioration of the concrete and of the reinforcement. Damaging conditions are corrosion of the steel, freezing and thawing action, impact loading, structural distress, physical abrasion from the action of waves, sand, gravel, and floating ice, and chemical attack due to sulfates.
These problems can occur in most marine structures such as bridge decks, piles, pile caps, beams, piers, navigation locks, guide walls, dams, powerhouses, and discharge tunnels. In many cases, shotcrete can be used to repair the deteriorated surfaces of these structures.
 Spillway surfaces
Surfaces subject to high velocity flows may be damaged by cavitation erosion or abrasion erosion. Shotcrete repairs are advantageous because of the relatively short outage necessary to complete the repairs
Typical underground shotcrete applications range from supplementing or replacing conventional support materials such as lagging and steel sets, sealing rock surfaces, channeling water flows, and installing temporary support and permanent linings.
Shotcrete is also used to permanently cover slopes or cuts that may erode in time or otherwise deteriorate. Slope protection should be properly drained to prevent damage from excessive uplift pressure.
 New structures
Shotcrete is not necessarily the fastest method of placing concrete on all jobs, but where thin sections and large areas are involved, shotcreting can be used effectively to save time. The following paragraphs describe some of the applications involved with construction of new structures.
Construction techniques using inflatable air-forming systems have made the construction of shotcrete shells or domes practical. These large structures have been used for residential housing, warehousing, bridge, and culvert applications.
Cement, water, sand and aggregate are the basic materials used in shotcrete along with various plasticisers and admixtures to enhance its functioning. High water-cement ratio gives slow setting and influences end quality, while moisture content in sand/aggregate is also keenly measured. Optimal W/C ratio is around 0.45.
Air-entraining cement has been used with the wet-mix process and has achieved varied results, with the air content generally much lower than in conventional concrete. Generally, the use of air-entraining cement is not recommended, since in-place air contents are affected by external factors such as air pressure, hose lengths, and equipment type.
- Mixing water: Potable water should be used. In this is not available, the proposed water source should be tested.
- Curing water: No special requirements are necessary for curing water applied to shotcrete. Water for curing of architectural shotcrete should be free from elements that will cause staining.
Lightweight aggregate shotcrete is most practical for the dry-mix process. Since moisture and aggregate contact is initiated at the nozzle, the severe workability reductions common in conventional lightweight concrete production do not occur.
Reinforcing bars for shotcrete should meet the same specifications as for conventional concrete. Because of the placement method, the use of bars larger than 16 mm diameter or heavy concentrations of steel are not practical.
Large bars make it difficult to achieve adequate build-up of good quality shotcrete behind the bar and heavy concentrations of steel interfere with the placement of shotcrete. In general, bar spacings of 150 mm to 300 mm are recommended for shotcrete reinforcement.
 Chemical admixtures
Because of shotcrete equipment limitations, the use of admixtures in shotcrete is not the same as in conventional concrete. Admixtures should be tested in the field prior to use on large jobs to ensure that the desired properties are achieved. Chemical admixtures used in shotcrete should comply with the appropriate requirements.
 Properties of shotcrete
As is the case with conventional concrete, shotcrete properties vary dramatically depending on water-cement ratio, aggregate quality, size, and type, admixtures used, type of cement used, and construction practices.
The proper use of admixtures, fibers, silica fume, and polymers can improve certain properties. Depending on the needs of the particular application, properties of the shotcrete materials and mixtures should be tested prior to final application.
Drying shrinkage is most influenced by the water content of the mixture. Typical values of unrestrained shrinkage range from 600 to 1,000 millionths. Shrinkage is reduced in coarse aggregate shotcrete and increased in shotcrete without coarse aggregate or shotcrete subject to high rebound.
The addition of fibers to shotcrete can result in a product displaying significant load carrying capability after the occurrence of the first crack. The relationship of post-crack load capacity to load capacity at first crack is defined as toughness.
The type, size, shape, and amount of fiber determines the extent of this performance. The use of the toughness index by load-deflection testing, provides a rational means of specifying and comparing performance. However, recent concerns have developed over the specifics of applying this testing procedure (Gopalaratnam et al. 1991).
Most shotcrete is batched and mixed in the field using portable mixing equipment or delivered in mixer trucks from a local ready-mixed concrete plant. Mixing equipment for shotcrete is of the batch or the continuous type.
In the continuous type, individual ingredients are fed to a mixer screw by means of variable speed augers, belt-feed systems, or a combination of both. A hopper is sometimes used in high production units to collect and feed the mixture as required. Water-metering systems are also available to re-dampen the mixture. Batching and mixing equipment must be capable of maintaining an adequate and continuous flow of homogeneous material.
Batching by mass is preferred and will normally be required. Water may be batched by mass or volume. For small jobs, approval may be given to batching by a volumetric container, provided periodic weight checks are made since many shotcrete jobs have a low production rate and are in isolated locations, mixing is often done by a small drum mixer at the jobsite.
 Admixer dispenser
In the dry-mix process, dry (powder) admixtures are usually introduced into the mixture during batching. If a continuous feed gun is being used, they may also be added directly into the gun hopper by a special dispenser, usually an auger-type dry dispenser driven by and calibrated to the gear train of the shotcrete machine. The dispenser should be capable of metering a precise quantity of admixture, usually 1 to 4 % by mass of the cement, into the mixture, and must be capable of accurately varying the ratio of accelerator to cement.
In the dry-mix process, liquid admixtures must be introduced at the nozzle through the mixing water. The admixture may be premixed with water and pumped to the nozzle or added directly to the mixing water at the nozzle.
In the wet-mix process, dry or liquid admixtures may be added to the mixture when batching provided the pumping properties are not adversely altered. As an example, an accelerator would create problems if added during batching, while a high-range water reducing admixture (HRWR) might have beneficial effects. In wet-mix applications, only liquid admixtures may be added to the air supply at the nozzle. They are proportioned to the delivery rate of the mixture through the material hose.
A properly operating air compressor of ample capacity is essential to a satisfactory shotcreting operation. The compressor should maintain a supply of clean, dry, oil-free air adequate for maintaining sufficient nozzle velocity for all parts of the work while simultaneously operating all air-driven equipment and a blowpipe for cleaning away rebound.
A dry-mix nozzle typically consists of a tip, water ring, control valve, and nozzle body arranged in a wide variety of nozzle tips, nozzle sizes, and configurations. Some investigations have shown improved mixing action and less rebound for dry-mix shotcrete when a special pre-wetting nozzle is used and the water ring is placed in the hose 1 to 8 feet before of the nozzle. This has been particularly effective for silica fume shotcrete.
- Type of dry-mix or wet-mix shotcrete appropriate for the work.
- The specific job constraints on the shotcrete work.
- The type of specification: Performance versus prescription, or contractor versus government mixture.
 Mixture proportioning trial batching
Since shotcrete performance is highly dependent on application procedures, trial batching and testing is a critical operation in verifying mixture performance. The batching and mixing of wet-mix shotcrete is practically identical to conventional concrete; only the fabrication of specimens is different. However, dry-mix is a distinct process.
Test panels are particularly important for dry-mix shotcrete because laboratory mixtures cannot duplicate as-shot dry-mix shotcrete. Typically, a performance specification of 12-hour, 7-day and/or 28-day compressive strengths will be specified, along with a grading for the aggregate. Both the wet- and dry-mix methods will yield a higher as-shot cement content and lower coarse aggregate content, due to rebound of the aggregate.
The NMSA depends on several factors. The selection major factors are the allowable shrinkage performance, size of the placement, and the rigidity of the substrate. The amount of rebound, inherent in the shotcrete process, depends on the ability of the substrate and the placed shotcrete to cushion subsequently placed shotcrete.
Shotcrete for thin linings on rock or concrete experiences high rebound. Thicker sections and sections on soil structures experience lower rebound. For placements of thin layers on hard surfaces, coarse aggregate should be minimised or eliminated in the mixture to minimise rebound.
 Wet mix proportioning
Mixture proportioning procedures for the formulation of conventional concrete for pumping applications are applicable for wet-mix shotcrete. The nominal aggregate size is usually 10 mm. The batched cement content will typically range from 350 to 450 kg/m³.
Rich mixtures are common for shotcrete, especially if vertical or overhead shotcrete placement is required. The limiting factor for cement content in a mixture is often governed by the amount of cement necessary for the shotcrete to adhere to a wall or ceiling, not the specified compressive strength.
 Dry mix proportioning
There is no established method of proportioning dry-mix shotcrete. Since it is not practical to perform laboratory trial mixtures for dry-mix shotcrete, field testing of dry-mix proportions is highly advisable, especially if no field data exist for a given dry-mix. The in-place aggregate grading will be finer than the batched grading due to rebound, especially if larger aggregate sizes are used. As with wet-mix shotcrete, the in-place cement factor will be higher also.
 Comparison between dry-mix process and wet-mix process
Shotcrete suitable for most requirements can be produced by either the dry-mix or wet-mix process. However, differences in the equipment cost, maintenance requirements, operational features, placement characteristics, and product quality may make one or the other more attractive for a particular application.
Bond strengths of new shotcrete to existing materials are generally higher with dry-mix shotcrete than with wet-mix shotcrete. Both shotcrete mixtures often provide significantly higher bond strengths to existing materials than does conventional concrete.
Typically, dry-mix shotcrete is applied at a much slower rate than wet-mix shotcrete. Dry-mix shotcrete is often applied at a rate of 1 or 2 cubic yards per hour compared to wet-mix shotcrete applied at a rate of up to 7 or 8 cubic yards per hour. Depending on the application, the in-place production rate may be significantly lower because of obstacles, rebound, and other features which may cause delays.
Rebound is the shotcrete material that 'bounces' off the shooting surface. Rebound for conventional dry-mix shotcrete, in the best of conditions, can be expected to be at least 20% of the total material passed through the nozzle. Wet-mix shotcrete rebounds somewhat less than dry-mix shotcrete.
The use of air-entraining admixtures (AEA) in shotcrete is practical only in wet-mix shotcrete. When batched properly, AEA forms an air-void system suitable for providing frost resistance to wet-mix shotcrete. The formation of an air-void system in dry-mix shotcrete is not possible.
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