Shotcrete is a process by which concrete is projected at high speed onto a surface, using a hose where it is propelled by compressed air, to form various construction elements in buildings. The mixture used for this type of concrete is relatively dry and is consolidated by the force of impact, developing a compressive force similar to normal concrete or high-strength concrete, depending on the dosage used.
Properties
They are similar to those of normal concrete in terms of apparent density, and resistance to compression, tension and shear. On the other hand, thanks to its particular structure, shotcrete is more waterproof and more resistant to frost than traditional concrete of the same composition. In addition, shotcrete has a notable characteristic: it adheres to the application surface and allows the desired surface shape to be obtained.
Dosage
It is generally recommended to dose materials by weight. The composition curve must have a granulometry that fits into the corresponding granulometric zone, normally 0-8 or 0-12. Thus, as a first approximation, the cement dosage will be about 400 kg/m³, which can be lowered if silica fume or alkali-free accelerators are used "Base (chemistry)").
In the case of the wet route, the water/cement ratio will generally be between 0.40 and 0.50, depending, among others, on the variation of the fineness module of the aggregates "Aggregate (mining)") and its nature, in order to achieve an adequate consistency for the spraying machine (cone between 12 and 18). The upper limit should not be exceeded to ensure that the chemistry of the accelerators and superplasticizers, essential in this application, works properly.
The usual dosage of setting accelerators is 4 to 5% of the weight of the cement, both in powder and liquid, except for silicate-based accelerators, now out of use, which require dosages of 10 to 12%. The dosage of the superplasticizers and stabilizers will be established through tests on the same job, and will depend on the aggregates, the cement, and the workability time.
The fume-based addition of silica powder will be added in a dosage between 4 and 10%, and fly ash in a percentage not exceeding 15 or 20%, depending on the type of cement.
It is always necessary to carry out previous tests on the same job in order to adjust dosages of aggregates, cement, water, additives and additions according to the existing conditions, to meet the project requirements.
Shotcrete (Shotcrete/Gunited)
Introduction
Shotcrete is a process by which concrete is projected at high speed onto a surface, using a hose where it is propelled by compressed air, to form various construction elements in buildings. The mixture used for this type of concrete is relatively dry and is consolidated by the force of impact, developing a compressive force similar to normal concrete or high-strength concrete, depending on the dosage used.
Properties
They are similar to those of normal concrete in terms of apparent density, and resistance to compression, tension and shear. On the other hand, thanks to its particular structure, shotcrete is more waterproof and more resistant to frost than traditional concrete of the same composition. In addition, shotcrete has a notable characteristic: it adheres to the application surface and allows the desired surface shape to be obtained.
Dosage
It is generally recommended to dose materials by weight. The composition curve must have a granulometry that fits into the corresponding granulometric zone, normally 0-8 or 0-12. Thus, as a first approximation, the cement dosage will be about 400 kg/m³, which can be lowered if silica fume or alkali-free accelerators are used "Base (chemistry)").
In the case of the wet route, the water/cement ratio will generally be between 0.40 and 0.50, depending, among others, on the variation of the fineness module of the aggregates "Aggregate (mining)") and its nature, in order to achieve an adequate consistency for the spraying machine (cone between 12 and 18). The upper limit should not be exceeded to ensure that the chemistry of the accelerators and superplasticizers, essential in this application, works properly.
The usual dosage of setting accelerators is 4 to 5% of the weight of the cement, both in powder and liquid, except for silicate-based accelerators, now out of use, which require dosages of 10 to 12%. The dosage of the superplasticizers and stabilizers will be established through tests on the same job, and will depend on the aggregates, the cement, and the workability time.
To prepare the concrete mix, both dry and wet, it is recommended to use a plant with a mixer, preferably with a vertical axis, since the technical requirements and support characteristics require a homogeneous preparation and mixing of the components, especially with the incorporation of additions and additives, fundamental in shotcrete technology.
Many of the causes of malfunction of spraying machines are caused by mixing in dosing plants without a mixer, or the incorporation of additives and additions in the application pit without adequate mixing.
Shotcrete being a material widely used in the support of tunnels and slopes, its application should not be left to chance, but on the contrary, it requires well-studied facilities, which allow the development of an adequate mix and transport according to established standards, to achieve the final characteristics of the concrete in accordance with the requirements requested by the designer.
It should not be forgotten that the support of a tunnel or slope has, in addition to the constructive purpose, a responsibility for the safety of the equipment and human resources involved in the work.
Another basic aspect is the study of the characteristics of the aggregates; granulometries, density, humidity, and absorption coefficient. Control of these parameters is essential both in the initial phases of mix design and in the execution phase.
Uses
Shotcrete has been used successfully, since its invention in 1911, for a wide variety of uses in construction, both in residential buildings and civil works and for lining tunnels, bridges, containment and stabilization systems for slopes and tunnels, silos, tanks, swimming pools, dams and canals, rehabilitation of structures in general, fireproof protection for steel, on horizontal, vertical surfaces or curved structures.
Although its use is barely becoming known worldwide, it has already been used for several decades in some countries, with great success, since it does not need formwork and the surfaces on which it can be applied can be uniform or irregular.
Modern rapid house construction systems incorporate this method for more effective weather, fireproof and water-repellent insulation, since the concrete is projected on a steel reinforcement that is linked to polystyrene. When concrete sets, it hardens and forms a stable structure and can have any type of conventional finish.
Application mode
Normally two or three operators are needed, one who holds the injector or hose head and decides which way it is projected, another who helps hold the hose and a third who watches the concrete mixer, in case the mixture is prepared on site.
The injector will be oriented perpendicular to the surface to be projected, at a distance between 60 and 180 cm. In most cases, concrete can be sprayed in a single phase or pass, with a thickness that can vary between 2.5 and 5 cm.
Once the concrete is projected, different finishes can be made: the rustic one, which would be the natural one that is formed when projecting it; finishing with a broom or brush that creates a straw-like surface; the semi-rustic that is achieved by passing a trowel; or smooth, which is achieved using conventional methods to smooth the surface of a mortar.
The fume-based addition of silica powder will be added in a dosage between 4 and 10%, and fly ash in a percentage not exceeding 15 or 20%, depending on the type of cement.
It is always necessary to carry out previous tests on the same job in order to adjust dosages of aggregates, cement, water, additives and additions according to the existing conditions, to meet the project requirements.
To prepare the concrete mix, both dry and wet, it is recommended to use a plant with a mixer, preferably with a vertical axis, since the technical requirements and support characteristics require a homogeneous preparation and mixing of the components, especially with the incorporation of additions and additives, fundamental in shotcrete technology.
Many of the causes of malfunction of spraying machines are caused by mixing in dosing plants without a mixer, or the incorporation of additives and additions in the application pit without adequate mixing.
Shotcrete being a material widely used in the support of tunnels and slopes, its application should not be left to chance, but on the contrary, it requires well-studied facilities, which allow the development of an adequate mix and transport according to established standards, to achieve the final characteristics of the concrete in accordance with the requirements requested by the designer.
It should not be forgotten that the support of a tunnel or slope has, in addition to the constructive purpose, a responsibility for the safety of the equipment and human resources involved in the work.
Another basic aspect is the study of the characteristics of the aggregates; granulometries, density, humidity, and absorption coefficient. Control of these parameters is essential both in the initial phases of mix design and in the execution phase.
Uses
Shotcrete has been used successfully, since its invention in 1911, for a wide variety of uses in construction, both in residential buildings and civil works and for lining tunnels, bridges, containment and stabilization systems for slopes and tunnels, silos, tanks, swimming pools, dams and canals, rehabilitation of structures in general, fireproof protection for steel, on horizontal, vertical surfaces or curved structures.
Although its use is barely becoming known worldwide, it has already been used for several decades in some countries, with great success, since it does not need formwork and the surfaces on which it can be applied can be uniform or irregular.
Modern rapid house construction systems incorporate this method for more effective weather, fireproof and water-repellent insulation, since the concrete is projected on a steel reinforcement that is linked to polystyrene. When concrete sets, it hardens and forms a stable structure and can have any type of conventional finish.
Application mode
Normally two or three operators are needed, one who holds the injector or hose head and decides which way it is projected, another who helps hold the hose and a third who watches the concrete mixer, in case the mixture is prepared on site.
The injector will be oriented perpendicular to the surface to be projected, at a distance between 60 and 180 cm. In most cases, concrete can be sprayed in a single phase or pass, with a thickness that can vary between 2.5 and 5 cm.
Once the concrete is projected, different finishes can be made: the rustic one, which would be the natural one that is formed when projecting it; finishing with a broom or brush that creates a straw-like surface; the semi-rustic that is achieved by passing a trowel; or smooth, which is achieved using conventional methods to smooth the surface of a mortar.