Services Required for Spraying

Electrical Power

A reliable and earthed electrical power supply at the correct voltage should be provided to electrically powered machines in accordance with the relevant site standards. Care should be taken to ensure adequate protection of all electrical cables and trailing cables.

 

Water

Water quality and temperature will affect the shotcrete performance. The water should be of potable quality and of suitable temperature, typically 18–25 °C for shotcreting applications. In mining, water temperatures can exceed 40 degrees, and if the water cannot be chilled then the use of sufficient stabiliser may be required. Water quality should comply with AS 1379. It is also important to have an adequate supply of water with sufficient pressure for the spraying application, as well as curing (when carried out), and cleaning.

 

Lighting

Adequate lighting is important to safety and helps the crew spray a quality product with the correct thickness and minimum rebound. The intensity and positioning of the lighting are important to reduce shadows and assist the operator’s view of the workplace.

 

Ventilation

All enclosed areas need to be well ventilated due to the dust, fumes and other airborne contaminants created during the process of shotcrete application. In tunnelling and underground mining applications, well designed quality ventilation is essential to dilute and remove machinery fumes, dust and chemicals from the area being sprayed.

 

Compressed Air

A supply of clean, dry compressed air is needed for spraying operations with adequate pressure and volume. The supply depends on the particular equipment specification, the condition of the equipment, on-site operating conditions, and the hose length and diameter. As a guide, typical air requirements are:

1. For wet-mix shotcreting, the air consumption is about 12 m³/min (425 cfm) at a pressure of about 7 bar.
2. For dry-mix shotcreting, the air consumption is about 15 m³/min (530 cfm) at a pressure of around 7 bar.

Inadequate air pressure and volume can result in poor compaction of shotcrete. Excessive air pressure can result in too much rebound.

 

Preparation for Spraying

Substrate and Surface Preparation

The particular means of surface preparation required depends on the condition and nature of the substrate against which shotcrete is to be placed. In all cases, where flows of water could interfere with the application of shotcrete or cause leaching of cement, the water should be sealed off or diverted prior to spraying by pipes, gutters, strip drains or sheets to points where they may be plugged off after spraying.

In underground construction, pre-injection of surrounding strata using cementitious or chemical grouts is often used to prevent water ingress. Most importantly, all substrates or surfaces should be clean, free of dust, oil, excessive water and other contaminants which might interfere with bonding of the shotcrete. Pre-damping of most surfaces other than steel and impervious formwork is essential to minimise loss of moisture from the shotcrete.

In tunnelling applications, all surfaces to be sprayed should be prepared prior to shotcreting. Adequate surface preparation is particularly important to achieve optimal adhesion between the shotcrete and the substrate. Seepage water entering the tunnel should be controlled and prevented from adversely affecting sprayed concrete placement. Water inflows which might cause deterioration of the sprayed concrete or adversely affect sprayed concrete adherence should be diverted by channels, strip drains, weep holes, chases, pipes, drainage mats or other appropriate means away from the area receiving shotcrete.

It should be noted that the long term bond strength between shotcrete and substrate cannot be relied upon to support the shotcrete, particularly when drying shrinkage and cracking is likely to occur. While bond may be relied upon for short-term support of linings, it cannot be relied upon in the long term. The B82 Shotcrete specification produced by TfNSW now requires the long-term bond strength to be taken to be zero for structural purposes, thus a secondary means of attachment between shotcrete and substrate must be included in lining designs.

 

Earth Surfaces

Proper preparation and compaction of the earth are essential to prevent erosion during application. The earth surface is then trimmed to line and grade to provide adequate support and to ensure the design thickness of the shotcrete. A moisture barrier may be installed which will prevent movement of moisture from the newly-placed shotcrete into the earth.

 

Rock Surfaces

The substrate should be free from loose materials, dust, thin dried layers of previously applied shotcrete or over-spray, and films (such as oils). This can generally be achieved by using a combination of water and compressed-air jet. Wet sandblasting can also be considered. In underground tunnels and mines, “scaling” is often carried out by mechanical hydraulic pick hammer, or high-pressure hydro scaling to remove loose rocks and scats. Cleaning should start from the top working downwards. If the rock surface has dried up post initial scaling and washing down, it should be pre-wetted prior to the application of shotcrete to prevent dehydration of the shotcrete at the rock boundary as this can substantially reduce bond.

 

Formwork

There are two types of formwork commonly used with shotcrete. Non-rigid formwork is used where the appearance of the back of the shotcrete is of no importance. Examples include hessian, or fine-gauge expanded metal, attached to a light framework. It should be firmly fixed and held taut to minimise vibration or flapping so that sagging is avoided and good compaction of the shotcrete can occur.

The alternative is rigid formwork. Timber or steel formwork is commonly used where a defined surface is required. Rigid formwork should be coated with a purpose-designed release agent to prevent absorption of moisture and adhesion of the shotcrete. Additionally, it should be adequately supported and strengthened to prevent excessive vibration and deflection. Plywood is generally sufficient for rigid formwork. Smooth-faced materials need only be employed when the face is to be accurately positioned and a “fair-faced” surface provided.

 

Concrete Surfaces

Shotcrete is often used to repair damaged concrete surfaces. Prior to spraying, all loose, cracked or deteriorated surfaces should be removed and broken back to sound concrete. Water blasting, chipping, scabbling, light hydro demolition or other mechanical means should be used to remove any contaminated concrete, form release chemicals, oils or corrosion products. Where reinforcement is exposed, it should be free from loose rust, scale or other deleterious matter likely to affect durability and bonding. If required, a chemical bonding agent or slurry coat can be applied to the surface of steel reinforcement. However, it has generally been found that ‘bonding’ agents between shotcrete and old concrete do not guarantee adequate long-term bond and therefore should be avoided.

Where shotcrete is to be placed against a smooth concrete surface, it should be roughened or abraded using either of the aforementioned mechanical methods. After the above preparation is carried out, the concrete surface should be pre-wetted prior to the application of shotcrete.

 

Shotcrete Surfaces

To achieve a monolithic lining, the bond between applied layers must be frictionally tight and form fitting. Shear reinforcement between two layers should be avoided (except for lattice girders and steel sets in tunnels) since shear reinforcements substantially increases the shadowing and poor compaction risk of the final product. The shear and tensile bond between subsequent layers can generally be achieved with the surface roughness of the initial layer to provide an effective interlocking surface.

To achieve optimum bond between subsequent shotcrete layers the application surface of an existing layer should be prepared by:

1. Removal of any damaged or disintegrated sections.
2. Cleaning by high pressure air and water jetting to remove any dust.
3. Removal of grease, oil and diesel soot using a detergent.
4. Surfaces should be damp, but not saturated before the application of subsequent layers.

Bond is improved by applying successive layers as soon as possible (that is, as soon as strength development is sufficient to support the combined self-weight of multiple layers) after the initial layers have been placed. Bond may be disrupted by shrinkage of the secondary layer which can lead to cracking and curling. It therefore follows that bond can also be enhanced by using a shotcrete mix that exhibits low drying and early thermal shrinkage, and by facilitating an adequate curing process. In general, a monolithic lining can best be achieved by spraying the second and subsequent layers no more than 3 days after the initial layer(s).

 

Environmental Conditions During Spraying

The environment prevailing at the point of spraying can have a substantial effect on the quality and performance of in place shotcrete. The following guidelines should be adhered to in regard to permissible environmental conditions during spraying.

 

Temperature

Shotcrete or mortar should not be applied if the temperature at the time of application is less than 5 °C or more than 35 °C, unless adequate precautions are undertaken. When necessary to do so it is essential to seek advice from a qualified concrete technologist to achieve the desired results.

 

Windy Conditions

It can be difficult to spray shotcrete in windy conditions. If there is a likelihood of extreme conditions, provision should be made to screen the nozzle, the jet and the surface to be treated to prevent the mix from being blown out of the jet. Particular consideration should be given to stray paste or mist particles that can easily travel with wind settling on surrounding surfaces, or in high wind situations, some distance away.

Wind an also promote cracking by rapidly drying the fresh concrete. Screening of the applied surface should be provided where possible and evaporation retardant considered. Curing procedures should be applied as soon as possible. Exposure to dry winds and strong sunshine is one of the most common causes of damaging and unsightly cracking in freshly sprayed shotcrete surfaces.

 

Rainy Conditions

Unless adequate protection is provided, shotcrete should not be placed during rain or when rain appears imminent. On exposed sites fresh shotcrete must be protected against rain. Heavy rain falling on freshly-placed shotcrete may cause it to slip or run compromising finish and appearance and will, at least, reduce its final surface strength and durability. In contrast, light drizzle may aid in the curing of freshly sprayed shotcrete.

 

Spraying Techniques

Spraying of shotcrete is usually achieved by one of two methods: hand (or manual) spraying, and mechanised (or remotely controlled) spraying.

 

Hand Spraying

The procedure, equipment and personnel involved in manual shotcreting should produce an end-product that is dense, homogeneous, without segregation of aggregate or fibres, and that does not display sloughing, collapsing, excessive rebound or other visible imperfections. Shotcrete should be able to be applied in layers up to 75 mm in thickness with adequate adhesion to the surface or bond to previous layers of shotcrete without sagging or slumping.

Shotcreting should be stopped in situations which may adversely affect the quality of the finished product. Shotcrete that is segregated, loose, porous or otherwise poorly compacted should be removed prior to the application of additional shotcrete layers.

The concrete pump should be regulated to evenly deliver the wet mix shotcrete at the rate required for the shotcrete application. The shotcrete should emerge from the nozzle in a steady, uninterrupted flow. Where the flow becomes intermittent for any reason, it should be directed away from the work until it becomes constant. This most commonly occurs when the swing arm changes over during pump operation. The nozzle should be held perpendicular to the receiving surface except that, where necessary, an angle of 0° to 45° to the perpendicular may be used.

Where a layer of shotcrete is to be covered by a succeeding layer, the first layer should be sharply tapered at joints. Feathering of the tapered joints should not be permitted. The second layer should be placed on the tapered surface. Curing compounds and bond breaking materials should not be applied to surfaces that will be covered by an additional layer of shotcrete.

For vertical and near vertical surfaces, application of shotcrete should commence at the bottom of the surface with the full thickness applied before applying any shotcrete to overhead surfaces. Rebound must be removed and is not to be integrated into the shotcrete linings. Distance from nozzle to the receiving face should be between 0.5 to 1.0 m for hand spraying to achieve the highest degree of compaction and lowest rebound. The nozzle should be held perpendicular to the face at all times. Manipulation of the nozzle to place shotcrete during spraying should be a circular or oval motion.

The sprayer should firstly fill all over-breaks and zones of weakness such as fissures, faults, gravel zones and soft spots if applicable (this process is normally limited to rock/soil surfaces). Spraying should then commence from the lower sections moving methodically upwards. If accelerator is used, dose rates may be marginally increased as the shotcrete application moves from the base up the wall and overhead. In some cases, it may be prudent to apply a series of thinner layers rather than attempting to spray the entire thickness in one pass of the nozzle. It is important that no subsidence or sagging of the shotcrete occurs.

 

Encapsulation of Reinforcing Bars

If used, steel reinforcement should be designed to facilitate spraying around the bars. This is important to achieve proper encapsulation of the bars and prevent voids forming behind the bars. When spraying through reinforcing bars the nozzle should be held closer to the work and at varying angles to permit better encapsulation and to facilitate the removal of rebound. This procedure forces the shotcrete behind the bar while minimising build-up on its front face. Where bars are closely spaced and it is impractical to spray one layer at a time, more than one layer of bars may be sprayed concurrently, provided the nozzle changes position to ensure the shotcrete is placed behind the bar while minimising build-up on its front face.

Encapsulation of reinforcing bars is very difficult using set accelerated shotcrete, so use of accelerated shotcrete should be avoided wherever possible when spraying bar-reinforced members. Similarly, inclusion of fibres in a mix will interfere with encapsulation of bars and is best avoided. Given the difficulty of properly encapsulating bars, mechanised spraying is seldom used when producing bar-reinforced sprayed structures such as linings and walls.

 

Construction or Expansion Joints

End-of-day joints and construction joints are very important in the satisfactory use of shotcrete for construction and protection. An unformed end-of-day or construction joint should come to a tapered edge, over a width of 200 to 300 mm for thicknesses up to 75 mm, and with a proportionately greater width for greater thicknesses. The surface on the taper is brushed to remove laitance and rebound, and allowed to set, but is not to be cut or trowelled in any way. Before shotcreting recommences, the taper is cleaned with an air-water blast and wetted. The whole taper is covered with fresh concrete as soon as possible and the thickness built up from there. Where the joint is expected to transfer compressive load the joint should be formed as a butt joint. For example, this would typically occur in a longitudinal joint in an arch or wall. See the Recommended Practice: Shotcreting in Australia (3^rd edition) for further guidance on joint selection and spraying.

Contraction joints may be provided by the pre-positioning of full-thickness strips, usually wood or steel, which are left in place, or by saw cutting the shotcrete shortly after it has achieved final set. The spacing of contraction joints depends on the application and should be designated on the plans. In practice, the spacing usually varies between 5 to 10 m on expected movements.

 

Mechanised Spraying

Mechanised spraying is used extensively in underground and open pit mining and in civil tunnelling and slope stabilisation activities. Mechanised spraying (most commonly using the wet mix system) allows the application of higher volumes of shotcrete and has the advantage of remote application where the machine operator can guide a boom mounted nozzle to reach areas that would otherwise be inaccessible from a position of safety. The spraying boom is usually mounted on a specially made shotcrete chassis that can have rubber tyres or tracks, although for some applications of very high wall slope stabilisation projects an adapted concrete boom pump can be used. The advantages of mechanised spraying include safety, higher output which can reduce cycle times, cost savings due to reduced labour and rebound, improved quality and improved working conditions for the sprayer.

When shotcrete is applied in areas where there is no or insufficient ground support existing, the risk of rockfall should be assessed and a safe position within supported ground should be ascertained for the rig to be set up. In an underground environment, the ventilation to the work site should be assessed for its adequacy to remove the dust and fumes that will be generated during spraying from the work area. Access to the work area by other personnel and equipment not related to the shotcrete process should be restricted through use of signage and barricades.

 

Inspecting for Hazards Prior to Spraying

The area to be sprayed should be inspected before any preparations are made for spraying in the first instance if the excavation was done with explosives a suitable qualified person should check for misfires and deal with these accordingly, If mechanical scaling is used this should be completed and the ground checked for suitable stability ensuring there is no obvious potential for rockfalls, rock movement, note made of any groundwater seepage or issues and a decision on how to deal with it to avoid potential pressure build up behind the shotcrete, and where necessary faces logged and mapped by the geologist and structurally signed off by the geotechnical engineer or responsible person before it is handed over to the nozzle operator.

The nozzle operator must satisfy him or her-self that the services such as ventilation, lighting, water, compressed air and power are adequate. The nozzle operator must also familiarize themselves in the case of underground works, with routes of egress, closest refuge chambers, first aid stations or supplies and or closest muster points.

The surface to be shotcreted should be examined by the supervisor or geotechnical engineer for any, loose ground, water seepage and any signs of ground movement. Once the area has been assessed and signed off, the shotcrete operator can move the machine into position. The shotcrete operator should also take the opportunity to identify any areas that will be difficult to spray (such as shadows). In an underground environment, adequate lighting is critical to ensure that these hazards can be identified by the operator and a high-powered hand-held torch is recommended for inspections.

 

Set up of Machines

The spraying rig must be parked, wheels chocked and stabilized using its on board hydraulic jack legs where available, in a position where the boom is able to reach the area to be sprayed whilst the operator is able to stand in a safe position with full visibility of the area they will be spraying. This must be under fully supported ground if spraying underground. The nozzle should be accessible when the boom is retracted and some means of ensuring if the nozzle blows off during the application it can be retrieved or replaced without any person venturing beneath “unsupported ground” at any point before, during or after spraying.

While there are spraying rigs on the market that are self-sufficient with their own shotcrete on board most rigs require supply from an agitator truck or, in extreme cases, by some other form of transport. The receiving hopper on the spraying rig must be in an accessible place and the delivery equipment should be guided into place by a spotter or the nozzle operator who must ensure they are in full view of the delivery operator at all times. Staff must ensure neither themselves or any other personal stand between the delivery vehicle (usually a concrete agitator truck) and the spray rig or any other potential crush point at any time until the agitator truck or other delivery vehicle is immobilised and chocked. Communication is of paramount importance, especially in an underground environment where it will be dark and may be noisy.

 

Preparation of Substrate

Preparation and thorough cleaning of the substrate is one of the most critical steps to the successful application and subsequent performance of the shotcrete. This process must take place immediately before the application of shotcrete and if there is a delay then the substrate should be re-cleaned. A thoroughly cleaned substrate will greatly reduce the fallout that can waste both time and material, and improve the bond, whilst reducing dust during the application. It is important that every single part of the substrate where it is intended to apply shotcrete is cleaned thoroughly. The substrate should be cleaned using water and compressed air blown through the nozzle, although if a hydro-scaler is available this works even better. In places where the substrate contains clay, silt, sand and other loose material, the amount of water can be reduced or, if necessary, use only compressed air to blow over the substrate. Cleaning should always take place starting at the top closest to you and work our way across from one side to the other slowly moving further away till the entire back has been cleaned, then proceed to do the vertical walls working away from yourself towards the face, and then down and back towards yourself till you finally end up at the bottom. The idea is that you clean in such a way that no loose material can get knocked onto the boom.

The purpose of cleaning is to remove any residue, loose material dust or any other foreign material that will inhibit the bond to the substrate. Surfaces must always be damp prior to shotcrete application. By cleaning the entire substrate that is going to be sprayed this also gives the nozzleman an opportunity to check the machine is correctly positioned so that the nozzle can reach the entire area to be sprayed at the required nozzle distance of minimum 1 metre and maximum 2 metres and remain perpendicular to the substrate.

 

Hydro-Scaling

Some spraying rigs are equipped with a hydro-scaler. Hydro-scaling improves bond strength in addition to removing lose ground. Hydro-scaling uses a high-pressure water jet to remove loose rocks and dust from a surface. The water pressure will typically be kept between about 3000 psi and 6000 psi to be effective. Hydro-scaling pumps are usually fitted to shotcrete spray machines to enable the same piece of equipment to both hydro-scale and spray. The hydro-scale nozzle is located at the head of the boom, close to the spray nozzle.

When hydro-scaling an area, the shotcrete adjacent to where the current application will occur should also be hydro-scaled at least one metre back from the fresh rock to ensure adequate bond of the overlapping shotcrete to the previously applied shotcrete. The operator should then progressively scale the rock to be sprayed from closest to farthest and from top to bottom such that the boom is never exposed to unscaled ground. If large, unstable blocks are visible that may pose a hazard to the boom during spraying and cannot be removed using hydro-scaling, mechanical scaling may be required before shotcrete application commences. If an area is mechanically scaled, it is still advantageous to hydro-scale to ensure complete removal of any remaining dust and films.

 

Preparation for Spraying

Prior to spraying, the shotcrete machine should be carefully coated with a layer of form oil to assist with cleaning of the machine after spraying. The shotcrete nozzle should be checked for cleanliness and wear. Both of these factors can affect the shotcrete velocity through the nozzle and hence the compaction that is achieved. Most nozzles have some form of wear marker inherent in their design which will indicate when they are required to be changed. Accelerator lines must also be checked before spraying is commenced. The flow of the accelerator can be checked from a gauge on the pump or assessed through timing the discharge into a calibration jug.

The accelerator dosage is normally recommended as a percentage of the cement content of the mix, in calculating the dosage since the accelerator only reacts with the C3A component of ordinary Portland cement it vital that only the ordinary Portland cement faction of the mix is considered and any supplementary cementitious product are ignored. If a blended cement is being used the blend ratios or percentages should be know for the accelerator dosage calculation. Overdosing of accelerator is extremely detrimental to the final strength and long term durability of the shotcrete.

Before filling the hopper with shotcrete, the shotcrete line needs to be lubricated to avoid blockages. This can be done with a grout where 15–20 kg of cement is mixed with 10 litres of water. Alternatively, a lubricating agent that is mixed with water can be used. Fill the grout directly into the reduction pipe, connect the pipe and then after checking the cleanliness of the grid lower it and request the concrete agitator truck driver to give the shotcrete a good mix for a minute or two at mixing speed before starting to discharge the shotcrete into the hopper. Fill the hopper with shotcrete.

A visual inspection of the shotcrete mix’s consistency and slump should be assessed by the nozzle operator who should also check that if it the mix is meant to contain fibre then that it does contain fibre.

Once the hopper is two thirds full then start pumping at a low output with the line disconnected from the nozzle. Pump until the grout has passed through the line and shotcrete is starting to come out of the line, stop pumping, connect the line to the nozzle, do a final check that the accelerator is at the correct setting. Just before spraying commences, the sprayer should ensure the position chosen to spray from is under supported ground and has good visibility of the ground intended to spray. Move the nozzle to the starting point, but point the nozzle away from the substrate, turn on the air and then the accelerator and do a visual check that the air stream from the nozzle contains accelerator. Check the air pressure and volume is correct (6 bar at a minimum of 10–12 m3 per minute), avoid spraying plain accelerator onto the substrate as this may cause bonding issues. Start the concrete pump at a slow speed and after a few strokes adjust it to normal operating speed.

 

Mechanised Spray Technique

To minimise rebound and maximise compaction during mechanised spraying, the nozzle must always be kept a minimum distance of one metre and the nozzle must never exceed two metres from the surface being sprayed. The correct nozzle angle is also important and should be as close as perpendicular to the surface as possible. Overdosing of accelerator adds to rebound exponentially.

The operator should start at the lowest point and work forward in a horizontal oscillating pattern to spray an even layer of shotcrete onto the surface. The shotcrete should be applied in the same fashion that one would build a brick wall, spraying in a horizontal strip away from the operator then moving up and spraying in a horizontal strip back towards the operator. Only when both sidewalls have been sprayed should the operator then start to spray the backs (roof), this time working from one side to the other starting at a point closest to the machine and slowly retreating away from the machine.

It is very important that the oscillating function on the machine is working and used as this gives a good even spread of shotcrete and greatly reduces unnecessary nozzle movements which not only gives an uneven spread of shotcrete but also unnecessary wear on the nozzle and extra work for the sprayer.

Shotcrete is generally applied in layers of approximately 25 mm (especially when being applied overhead) to prevent fallout. Ideally, the operator should wait ten minutes between layers to ensure adequate set of the first layer before applying the second. Most mining applications require shotcrete thickness of between 50 mm and 100 mm and civil applications commonly require a thickness exceeding 100 mm. Thickness control is important to ensure not only that adequate thickness is being achieved, but also that the application is of even thickness and that shotcrete is not being wasted due to excessive thickness. The best way to ensure even thickness understand the output of your machine and to spray as described above in an orderly manner completing one wall at a time slowly building up from the bottom, avoiding fast movements adjusting movements to the output of the concrete.

If there is a fissure or fault present or any pockets that need to be filled and it is determined that this should be sprayed first then once it has been sprayed and sufficient time has passed for the shotcrete to have set then the entire area to be sprayed must be cleaned properly with compressed air and water before any general spraying begins.

If the sprayer experiences a fall out of shotcrete then they should avoid spraying that area immediately, rather wait to let the surrounding shotcrete set. When filling fall-outs apply thinner layers with time between to allow setting.

If spraying an area where access is required to continue tunnel advance, it is common to spray a “re-entry panel” of shotcrete on an area of wall under supported ground. This panel can be marked with the date and time of spraying and a penetrometer may then be used to check the strength development of the shotcrete without entry into the area sprayed being necessary. Alternatively, a historical record of beam end tests is carried out to provide a good guide to re-entry times according to the required specifications. All sprayed areas should be barricaded, or a sign used to indicate the hazard of wet shotcrete.

 

Thickness Measurement and Control

Methods of thickness control during and after spraying include using metal probes of a set length mounted on the end of the shotcrete boom to check the depth of the wet shotcrete and the use of stick on depth indicators which may be applied before spraying commences. Both methods have some disadvantages: boom mounted probes can cause damage to delicate boom hydraulics if they are not used carefully, do not

provide an indication of excessive thickness, and do not provide a permanent record of thickness. Stick on depth indicators are time consuming to apply, are often dislodged by the force of spraying and may be obscured by the spraying. Both methods only provide point data, and when shotcrete is applied to rough surfaces this can be far from representative.

Shotcrete thickness can also be measured by several methods post spraying. The most common method in use is the drilling and measuring of probe holes, though the small number usually drilled combined with the fact that they only provide point data suggest this method is of questionable value.

More representative data can be obtained through the generation of before and after three dimensional surveys of the areas being sprayed. This has been achieved using laser scanners and more recently has been achieved through photogrammetry. A survey must be taken after hydro-scaling and then one after spraying. The two surveys can then be compared and a “thickness map” generated.