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Filamos – Mining and Construction Technology

Know How Portal

Know How Portal – basic information

General terminology

Advantages and disadvantages of individual methods

Techniques and dry shotcreting

Know How Portal – Basic Information

This Know How Portal covers information regarding the issue of sprayed concrete. The portal will be gradually updated with new information and findings in order to create an information and know how base about shotcrete and related issues.

All information and data covered by this Portal are for information only and Filamos, s. r. o. does not guarantee their accuracy or completeness.

The following sources have been used:

Internal sources of Filamos s. r. o.
Zpravodaj technických informací (Technical Information Newsletter), Volume 1988, Nos 1-2., Collective of authors, VDUP (quotes from this source are marked as follows *.)

Concrete spraying technology

Concrete spraying technology is a method of machine spraying of reinforced and non-reinforced concrete layers on vertical walls, in particular, and ceilings which can be of irregular shapes.

Dry shotcreting

Dry shotcreting is based on pneumatic transport of dry mixtures and their moistening at the moment of actual spraying. The dry mixture is transported from the machine (concrete spraying machines, shotcreting machines) to the place of spraying by transport hoses with spraying jets and gauge water to moisten the sprayed mixture as required.

Advantages of dry shotcreting

Due to the high speed of the transported mixture in transport hoses and the related high discharge speed from the jet (up to 30 m/s), the sprayed on concrete mixture is perfectly compact and there is no need for further compacting by vibrator etc.
By employing the dry technology a thicker concrete layer (15-20 cm, or even more for local overbreaks) can be applied.
Maximum horizontal transporting distance from the machine to the spraying jet is up to 300 m and the vertical distance is up to 100 m.
The transport tube and hose can be adapted for lower pressure (air pressure of up to 0,6 MPa) and smaller inner diameter (max. 50 mm).
Interruption of the working cycle does not require any special cleaning of the machinery and the transport route.
Unused dry concrete mixture can be worked even after a longer break.
Easier and less costly machinery (compared to wet shotcreting).

Disadvantages of dry shotcreting

Handling dry concrete mixture carries the problem of dust. Incorrect maintenance and adjustment of the machine can result in dustiness at the actual place of spraying. Partial dustiness in the spraying jet is caused by the fact that some of the dry concrete mixture cannot be perfectly moistened by the gauge water due to the high speed of the discharge.
Due to the high speed of discharge and other factors, the dry technology has a higher impact on the material and results in falling off of the material (up to 30% of the gravel aggregate).
The water coefficient W/C (water/cement) is regulated on the jet and the regulation is to a certain degree dependent on the experience and skill of the personnel.
The pneumatic transport of the dry concrete mixture places higher demands on a more efficient source of air pressure.

Wet shotcreting

Wet shotcreting is based on the transport of the wet concrete mixture from the machine (concrete pump) by a transport tube or hoses to the spraying jet.

The spraying effect is achieved by mixing the wet concrete mixture at the final stage of the transport and by its acceleration by air pressure. The acceleration can take place either directly in the spraying jet or in a mixing chamber where the liquid hardening agent is added.

Another possibility is the use of compressed air to transport the wet mixture from the concrete spraying machine (the working principle is similar to the wet shotcreting). The stream of concrete is mixed with the hardening agent in the spraying jet.

Advantages of wet shotcreting

Contrary to the dry method, the generation of dust is almost eradicated and the falling off of the material is significantly reduced to about 10%.
It is also possible to adjust the water coefficient W/C more efficiently and the quality of sprayed concrete is not affected as much by the service personnel as they are not required to adjust the water flow.

Disadvantages of wet shotcreting

The discharge speed from the jet is considerably higher compared to the dry method, and therefore the compact effect is smaller and so the final consistency is better. It is not possible to apply more than 5 cm thick layers if spraying overhead. Any thicker layers should be sprayed one at a time.
Further addition of a hardening agent in the spraying jet is difficult as the liquid hardening agents require a fairly high W/C due to pumpability. The wet mixture requires more concrete to reach the same strength compared to using the dry mixture. Usually, setting retarders or hardening agents or plastificators are needed to improve the pumpability in order to comply with the quality.
Only a limited period of time is available between mixing and application of the wet concrete.
It is not possible to interrupt the working process. In case of any longer delays, the entire system must be emptied, cleaned and filled up again before renewing the work.
Another disadvantage is that more complex and more costly machinery including more robust transport pipes are required.

Dry vs. wet shotcreting

During dry shotcreting the dry mixtures is poured into the chute and transported through the transport hoses (or piping) by compressed air to the place of application. The mixture gets moistened in the special spraying jet immediately before the actual application. The required moistening is carried out by the personnel depending on specific requirements.

During wet shotcreting, the concrete spraying machine (shotcreting machine) is filled with a completely wet and premixed mixture. A stream of compressed air can transfer the wet mixture out from the concrete spraying machine (shotcreting machine) or it can lead it directly to the spraying jet (piston pump).

Shotcreting *

Shotcreting is machine spraying of fine concrete and pyroconcrete mixture of the granularity of 4 mm when a particular strength value of the final concrete is not a requirement.

It is used in particular for:

Preliminary sealing with the hardening agent
Primary sealing of concrete and rocks
Grouting brickwork and panels
Plastering bigger concrete areas
Maintenance work in shafts and tunnels
Reinforcement of slopes
Repairs and lining of furnaces, boilers, range of coke ovens etc. in hot or cold environments

Sprayed concrete *

Sprayed concrete is machine applied reinforced or non-reinforced concrete with the granularity of gravel aggregate of up to 16 mm (the most suitable max. size of the grain is up to 8 mm).

It is used in particular for:

Operational securing of mine excavations during tunnelling
Final casing of mine excavations, shafts and tunnels
Concreting walls, tanks etc.
Reinforcement of slopes
Maintenance of mine workings for underground constructions
Filling spaces behind board supports

History of sprayed concrete

The use of sprayed concrete dates back to the beginning of the 20^th century. The American Carl Akeley is regarded as the father of dry shotcreting as he first used this method to create animal plaster models for scientific purposes. The method was quickly applied to construction purposes and due to its major success the term ‘gunite’ was patented in 1911. Pressure tanks were used as engine plants.

In 1950s the technology of wet shotcreting was developed.

The invention of a concrete spraying machine (shotcreting machine) at the beginning of the 1960s, working on the principle of the rotating dispensatory drum, helped to widen the possible application of both shotcreting technologies enormously.

Advantages of shotcreting and sprayed concrete *

Perfect adhesion of the sprayed material onto the base
Reliable filling of all cracks and uneven places
High pressure and tensile strength of ready concrete even after 28 days
High initial strength using suitable hardening agents
Savings in regards to footing forms
High water proofing given by perfect compaction and a low ratio of W/C
Easy selection of the thickness of the sprayed on layer depending on the requirements and situation
Easy reinforcement of endangered constructions, in particular in mining works

Shotcrete

The term „shotcrete“ refers to pneumatic high pressure spraying of concrete or mortar using a spraying jet. The term „shotcrete“ is used as a universal term referring both to wet and dry concrete or mortar spraying. (One can also come across the term “shotcrete” used purely for wet concrete spraying).

The term „shotcrete“ is a derivate of two English words „shoot“ and „concrete“. The term was used and defined for the first time by the American Railway Engineers Association (AREA) at the beginning of 1930s.

Gunite

The term „gunite“ refers to dry concrete spraying when the dry mixture is moistened in the spraying jet immediately before it is applied.

Surface preparation *

It is essential to spray clean surfaces only. Surfaces soiled with dust or other impurities have lower adhesion of the shotcrete or sprayed concrete.

The surface is cleaned using a water jet.

The spray must be applied onto a sufficiently damp surface (but no running water on the stope walls) otherwise there is a danger that the dry surface might take too much water from the applied concrete and degrade the mechanical characteristics of the hardening and hardened concrete, in particular its strength.

Where there is water running down the walls of the stope or there is a localised seepage, it is important to preseal the area or to drain the water away using drain hoses sprayed into the walling. Failure to do so might result in poor adhesion of the concrete to the wall and subsequent slacking.

If soft surfaces are sprayed (clay or sand – walls do not get washed) we would recommend reinforcing the surface with a thin layer of about 1,5 cm of gunite and only then applying sprayed concrete to reach the thickness of about 5 cm. Further layers can then be applied to reach the prescribed thickness. This procedure will prevent the concrete layer from coming off the base (walls of the stope).

Steel netting inserted in one or two layers (stipulated by the project) will increase the strength of walling made of sprayed concrete.

Reinforcement *

The reinforcement wire netting should have a minimal width of mesh of about 100 mm. Smaller aperture can cause reflection of the gravel aggregate (4-8 mm) and does not guarantee perfect adhesion of the sprayed concrete onto the walls (it is difficult to penetrate the netting).

When spraying a wall with inserted reinforcement netting we would recommend spraying on a thin layer before fixing the netting (if possible) which will make the spraying easier.

Spraying onto netting fitted in two layers is not recommended as there are hollow places behind the netting. If the project stipulated that the reinforcement wire netting should be fitted in two layers, it is essential to spray the first netting completely, then to fit the other netting, and finally finish off spraying the concrete to reach the required thickness.

Distance of the jet from the surface *

The distance between the spraying jet and the surface which is being sprayed should be around 1 m. The optimal distance depends on specific conditions (the granularity of the material, air pressure). The increase or decrease of this distance results in more waste. It has been proved in practice that a deviation of ±0,25 m from the optimal distance produces an increase in waste from about 10% to 25%.

The optimal distance from the surface can be affected by the following:

Grain size of the dry mixture (higher granularity = bigger distance).
Air pressure in the jet in correlation to the length of the piping (higher pressure = bigger distance).

The optimal distance for spraying individual mixtures must be determined based on a practical test at the place of application.

Angle of spraying *

It is important to comply with the angle of spraying just as it is important to comply with the optimal distance of the spraying jet from the surface and the determined water coefficient. The dry concrete mixture must always be sprayed across the surface to prevent the reflection of grains that are 4-8 mm big. This reflection occurs if the jet is held under the angle of 15º to 80º to the surface of the sprayed on surface.

Perfect adhesion to the surface and a compact layer can be best achieved by vertical spraying of the concrete mixture.

Spraying technique

When spraying using the spraying jet, it is important to carry out circular movements with a diameter of 10-15 cm under the angle of 8º maximum.

(Gauge) water batching *

The regulation of the amount of the gauge water is done manually using a cock on the spraying jet. Having good results with spraying, in other words having a minimal waste of the concrete mixture depends entirely on the personnel operating the spraying jet. The jet should be operated by a trained and experienced worker who has mastered the technique of spraying. This worker can apply concrete mixtures to various surfaces (dry, wet etc.) adapting to given conditions as necessary.

If the surrounding temperature is below +5 °C , it is necessary to warm up the gauge water.

One can tell if the mixture has the right moisture content considering the following features:

Insufficient moisture content – increased dustiness and dry, undamped particles fall off the surface of the sprayed wall. The concrete is not properly compact.
Correct moisture content – the waste is minimal and the sprayed on layer has a matt surface.
Too high moisture content, (wet) mixture – the mixture runs down, the quality of the spray is bad and stains are formed on the surface.

Material waste *

When spraying vertical areas, the waste should not amount to more than 10-15% of the sprayed on concrete mixture. When spraying ceilings, the waste increases to up to 25-30% of the sprayed on amount of the concrete mixture.

The waste consists of the rough grains from the concrete mixture. The waste is damp on the surface and usually sorted. It should not be reused as by adding it to the dry concrete mixture, the strength is lowered by 20% (tested by the TAZUS laboratory).

The reason for the decrease in strength is the fact that some of the grains of the waste are already covered in cement paste which hardens during further applications and lowers the adhesion to the structure of the sprayed on concrete.

Possibilities of treating the surface – finish *

If gunite or sprayed concrete are applied in more layers and a smooth finish of walling is required, we would recommend adding more water (W/C = 0,5–0,55) to the last 3 cm layer of the spray. The surface must be treated within one hour at the latest. If the gunite or sprayed concrete gets smoothed or ground off, it is important to make sure that the structure of the last layer is not affected and that the sprayed on layer of the concrete layer is not released from the base.

The same rules apply for the gunite and sprayed concrete, in terms of maintaining dampness and water resistance etc., as for plaster and concrete positioned behind the formwork.

Transport and spraying concrete over longer distances

SSB machines can be used for spraying, or for the transport of loose material over longer distances – up to 300 m horizontally and 100 m vertically. When using other material than the standard dry concrete mixture, it is necessary to test the long distance transport possibilities. The lower the ratio of higher sized grains, the lower the achievable distance and height.

Basic conditions:

Maximum moisture content of the material 7%
Sufficient amount of compressed air
Straight transport pipe with no bends if possible, for longer distances one rubber hose is situated behind the spraying machine, followed by straight steel piping and followed again by a rubber hose with a spraying jet
Transport piping and hose must have the same inner diameter along the entire transport route
Piping and tube connectors must not decrease the inner diameter

Concrete tests

In the real world gunite testing and sprayed concrete testing experience certain difficulties. It is not possible to prepare sample cubes using a mould as when spraying into cube moulds the counter-pressure makes the rough material concentrate in corners, whilst the fine materials like sand and concrete are washed out of the mould by the compressed air. Therefore, testing cubes must be obtained by cutting a layer of concrete which has been sprayed on in the relevant thickness. The most successful method has been spraying into vertically positioned crates of 50×50×20 cm and to cut out the cubes needed from these blocks.

Best results are achieved when testing drilled out samples of 100 mm in diameter directly at the workplace.

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