Special concreting methods. Gunning

Shotcasting refers to the technological process of applying thin layers of a cement-sand mortar or fine-grained concrete to a concrete or other surface with the help of a special installation — a cement gun for a cement mortar and a concrete syringe machine for a concrete mix. To do this, a dry mixture of sand, cement and coarse aggregate is mixed with water under the action of a jet of air and applied to the surface of the structure to be processed. The solution in this case is called gunning, and the concrete mix applied by the concrete-syringe-machine, in turn, is called nabryzgbeton or "syringe-concrete."

Due to the large kinetic energy developed by the particles of the mixture, the solution (concrete) applied to the surface acquires enhanced characteristics in terms of density and strength, water resistance, frost resistance, adhesion to the application surfaces.

The composition of gunning includes cement and sand, the composition of sprayed concrete in addition to cement and sand includes coarse aggregate up to 30 mm in size. Mortar or concrete mixes are prepared on Portland cement not lower than M400.

The process of applying a layer of gunning (sprayed concrete) involves two stages: in the first stage, a plastic layer consisting of a solution with the smallest aggregate fractions occurs on the surface of the application. The thickness of the layer of cement milk and fine fractions, which is able to absorb the impact energy of large particles of aggregate and capable of holding large particles, is 5 ... 10 mm; in the second stage, partial penetration of grains of a larger aggregate into the mortar layer and thus the formation of a shotcrete layer or sprayed concrete occurs.

The shotcrete is usually accompanied by the loss of a certain amount of material bouncing off the application surface (the so-called “rebound”). The magnitude of particle rebound depends on the conditions of work, the composition of the mixture, the size of large particles of the aggregate and the kinetic energy of the particles upon impact. In the initial stage of application, almost all particles of coarse aggregate bounce off the surface, and only cement and grains of small aggregate fractions are retained on it. Therefore, the initially applied layer up to 2 mm thick consists mainly of cement paste. As the thickness of the applied layer increases, larger aggregate particles begin to linger in it, after which a constant percentage of rebound is established. Quantitatively, the magnitude of the rebound when gunning vertical surfaces is 10 ... 20%, and when gunning ceiling surfaces — 20 ... 30%. Reducing the volume of rebound is achieved by selecting the optimal speed of the mixture from the nozzle and the distance from the nozzle to the surface of the gunning or sprayed concrete.

Currently, there are two types of application to the surface under pressure of working compositions - dry and wet.

In the dry method, the initial dry mixture in suspension is fed to the nozzle (nozzle), where the mixture is mixed with mixing water, i.e. gunning In the nozzle, the mixture is stirred, followed by supplying it under compressed air pressure to the concrete surfaces.

With the wet method , a ready-mixed concrete or mortar enters the nozzle under compressed air pressure. In the nozzle, the mixture goes into suspension and under pressure is applied to concreted surfaces. The applied mixture is called pneumatic concrete, which is associated with working installations - pneumatic installations and pneumatic superchargers.

The dry method is used for applying gunning, and wet method for gunning and splashing concrete. Each of the methods is characterized by its technical means and features of the operations.

The main technical means for gunning dry mixes are a cement gun and a concrete syringe machine. The installation includes a unit for applying the mixture, a compressor, a nozzle, hoses for supplying a dry mixture of air and water to the nozzle (Fig. 9.1) and, if necessary, is supplemented with an air cleaner, water tank, and a cylindrical tank for dry mixture. The principle of operation of the units is the same.

The dry mixture is loaded into a cylindrical tank and through a conical gate enters the lower part of the tank, from where under the pressure of air from the compressor is fed through a flexible hose into a cement-gun nozzle, to which water is also fed through another hose. In the cement-gun nozzle, water wets the mixture of cement and sand, and in the concrete-syringe machine — also coarse aggregate; The final mixing process is completed at the nozzle outlet. The wet mixture ejected from the nozzle at a speed of 100 to 140 m / s is applied to the surface to be treated, forming a layer or spread over the solution on it.

The solution or concrete mix is ​​applied to the surface in 2 ... 3 times layers with a thickness of each layer up to 25 mm. For the concrete mix for the first applied layer, the maximum size of the coarse aggregate fraction should not exceed 10 mm. Subsequent layers are applied after the previous setting, the total thickness of the rough is 50 ... 75 mm, a solution of the composition from 1: 2 to 1: 4.5 is applied. If provided for by the project, these units can be applied to the surface and waterproofing of waterproof cement mortar with a layer of 5 ... 10 mm.

Typically, the distance from the cement cannon to the surface to be treated is 0.7 ... 1.0 m, for a concrete syringe machine — 1.0 ... 1.2 m. For better adhesion to the applied composition, the surface is pre-cleaned with dry air or sand under pressure from a cement gun, and then the surface is cut.

The direction of the jet is usually taken perpendicular to the surface. The air pressure in the cement cannon and the concrete syringe machine is from 0.15 to 0.35 MPa, depending on the distance, the type and size of aggregates, and the requirements for a particular concrete layer. Water to the nozzle is supplied under pressure, 0.05 .. .0.1 MPa above the air pressure for the dry mixture.

Sand and fine crushed stone with a grain size of up to 8 mm are used for gunning, and crushed stone with a particle size of up to 25 mm is used for sprayed concrete, cement for gunning is accepted only by top grades.

A break in work is allowed for 1 ... 2 hours, concreting joints are arranged in a razbezhku, grout produced before the cement begins to set. Shelter and watering is performed as in ordinary concrete, vapor-proof film coatings can be arranged.

Using one unit per shift, you can apply gunning with a layer of 15 ... 20 mm on a vertical surface with an area of ​​280 ... 320 m with a capacity of up to 1.5 m3 of mixture per hour.

In domestic practice, two-chamber cement guns (SB-117 and SB-67A with a dry mix capacity of 2 and 4 m3 / h, respectively) are mainly used as a unit for applying the mixture. Bell gates of the upper and lower chambers provide sluicing. While the dry mix from the lower chamber is fed by the feeder to the discharge opening and compressed air is carried into the material hose, the upper chamber is filled with a new portion of the dry mix. Thus, continuous gunning is ensured.

Technological sequence of operations with this method:

• loading the prepared dry mixture into the cement-gun;

• dosed supply of dry mix to the cement-gun discharge device for pneumatic conveying it through hoses;

• transporting the dry mixture in a jet of compressed air and through the hoses to the nozzle;

• dosed supply of pressurized water to the nozzle and mixing of the solution in the nozzle;

• application to the shotcrete surface of the ready-made mixture coming out of the nozzle with a torch at high speed.

For shotcrete using a dry method, clean sand is used with a moisture content of not more than 6%, with a particle size of 2.5 ... 3 with a maximum grain size of individual grains of 5 mm (gravel with a maximum grain size of 8 mm is allowed). The size of the aggregates should not exceed half the thickness of each gunned layer and half the mesh size of the reinforcing mesh. The ratio of the ratio between the mass of cement and sand
1: 3 ... 1: 4,5. The cement content in the gunning is 600 ... 800 kg / m3 with the actual water-cement ratio at the exit of the nozzle 0.32 ... 0.37. With a smaller W / C, spraying and insufficient wetting of the dry constituents take place; with large W / C, the deposited layer creeps in. In the course of work, no sagging is allowed in heights greater than 1/2 the thickness of the gunned layer. Installed fittings should
be protected and secured from displacement and vibrations.

The overpressure of the air in the cement cannon is usually taken 0.2 ... 0.3 MPa, which ensures that the humidified mixture leaves the nozzle at a speed of 100 m / s. To obtain a dense layer of gunning uniform thickness nozzle when applied keep at a distance of 0.7 ... 1 m from the surface of the application, move it in a circular motion, and the mixture stream is directed perpendicular to it. In order to avoid bloating, the thickness of each layer applied by gunning should be 3 ... 5 mm when applied to horizontal (bottom-up) or vertical non-reinforced and reinforced surfaces. If there are several layers, the next layer is applied with an interval determined from the condition that the previous layer is not destroyed by the action of a jet of fresh mixture (determined experimentally).

The main technical means for the wet method of shotcrete are blowers (pneumatic installations and various pumps).

In domestic practice, with the wet method of shotcrete, mortar mixes are used on fine sands with the addition of fine stone fraction of 3 ... 10 mm in an amount of up to 50% of the total mass of the aggregate. For applying the mixture on the surface using the installation "Pneumatic" of various modifications, which include: receiving-mixing device with a forced action mixer; vibrating screen with cells 10x10 mm; feeder; material pipeline; air duct; nozzle for applying mixtures. As a feeder of the “Pneumatic Concrete” installation, serial mortar pumps C-683, C-684 and C-317B are used with nominal flow rates of 2.4 and 6 m3 / h, respectively, converted to a direct-flow circuit and additionally equipped with a mixing chamber. The air to the mixing chamber is supplied under a pressure of 0.4 ... 0.6 MPa, which ensures the release of a jet of mixture from the nozzle at a speed of 70 ... 90 m / s and the formation of a sprayed torch.

Technological sequence of operations with this method :

• loading of a prepared mortar or concrete mix into the supercharger;

• injection of the finished mixture through the hoses to the nozzle;

• supply of compressed air to the nozzle, which ejects the finished mixture flowing through the hoses to increase the speed of its exit from the nozzle;

• application of the ready-mix to the gunned surface of the plume.
For high-quality deposition of concrete (mortar) layers, the “Pneumatic Concrete” installation is guided by the following: the nozzle when applying the mixture is positioned perpendicular to the surface (deviation of the nozzle is allowed at a small angle when filling the space behind reinforcing bars with a diameter of more than 16 mm); the nozzle should be located at a distance of 0.7 ... 1.2 m from the working surface in order to minimize the “rebound”; on vertical surfaces the mixture is applied from bottom to top; the thickness of the applied layer at a time should not exceed 15 mm when applied to horizontal (bottom to top) surfaces, 25 mm when applied to vertical surfaces and 50 mm when applied to horizontal (top to bottom) surfaces. When there are signs of slipping of the mixture, it is necessary to reduce the thickness of the applied layer; when applying the first layer on the formwork or hardened concrete, use fine-grained mixture, which reduces the loss of materials on the "rebound"; the thickness of this layer should not exceed 10 mm; to obtain a flat surface after setting the last applied layer of cement, the surface is additionally trimmed with a solution on fine sand, which is immediately smoothed out.

Concrete shotcrete is generally not competitive with traditional concrete work technology. This process is relatively expensive, time consuming and inefficient. It is used when it is impossible to erect constructive elements several centimeters thick using traditional methods of concreting (especially when using pneumatic formwork), when it is necessary to obtain a material of enhanced properties, for applying tunnel lining, when arranging protective layers on the surface of prestressed tanks, to repair and reinforce reinforced concrete structures, monolithing joints, etc.

The main scopes of shotcrete are tanks, vaults-shells, thin-walled structures with increased strength and water resistance. The method is successfully used in the correction of concreting defects, improving the water resistance of existing structures and structures, when concreting thin-walled reinforced concrete structures on the reinforcement cage.