Over the past 100 years, the technology of spraying concrete from innovative has turned into a classic one and is used in a number of industries to solve different production problems. Moreover, recently in connection with the increase in the volume of repair and construction work, interest in this method has increased significantly. Shotcrete is a layered application of plaster or concrete solutions under impressive pressure, due to which the particles of the solution interact with the prepared surface. The existing cracks and small pores are filled during work.
Scope of Use
The use of gunned concrete today is quite common. This technique is used for fixing rocks in underground mining operations and tunnel construction. It does not do without this technology and repair of reinforced concrete and concrete structures. This includes strengthening and replacing the collapsing material. Shotcrete is used in sealing operations, as a fireproof material. It has found its wide distribution in the color, black, chemical, cement and glass industries, each of which is associated with high-temperature processes. It is applied to create a protective wear-resistant coating. Quite often this method is used when creating objects of artistic purpose. The technique is used for fixing soil in land construction. This concerns the construction of storage facilities and excavations, as well as the strengthening of slopes in the construction of transport interchanges and bridges.
Main features of the technology
When sprayed concrete is applied, the technology differs in certain features, among them - the possibility of applying the composition on uneven surfaces, as well as the bases located in any planes. This is due to the high adhesion properties of the mixture and its self-supporting ability. Nabryzg is produced at high pressure and speed, which allows for excellent adhesion of materials. After all, the applied layer is compacted on its own. The thickness of the surface can be any, which will depend on the specific conditions and tasks. Sometimes the work is accompanied by reinforcement, which involves the use of traditional reinforcement or special fiber. The technology allows you to quickly erect load-bearing structures without using formwork, and also eliminates long waiting times.
Varieties of shotcrete technology
Concrete can be sprayed in various ways: wet or dry. In the first case, a ready-mixed concrete solution is supplied to the atomizer in the form of a nozzle. This becomes possible through special sleeves. The composition is in the form of a continuous stream. In this case, diluted air and a concrete pump are used. As advantages of this technique is the possibility of using a uniform composition, little dust formation, minimization of material rebound, as well as the possibility of finishing grouting after application of the mixture. Formed surpluses can be used in other construction works. A wet method of spraying concrete can be made in enclosed spaces.
Disadvantages of wet application
Shotcrete can be applied by wet technique, however, it has certain disadvantages, expressed in a low flow rate of the solution, which does not allow achieving maximum density of the structure. The thickness of the applied layer is limited and does not exceed 3 centimeters in one approach. To service the shotcrete machine, when it is used in a wet technique, higher demands are made. Moreover, the master is attached to the place of production of concrete, and it is not possible to use long sleeves. This limits maneuverability.
Features of dry shotcrete
The application of gunite-concrete can also be carried out by dry technology, which provides for the supply of a mixture with fillers on separate hoses from water. The mixing takes place directly in the nebulizer. Considering the advantages, it is worth highlighting the possibility of feeding the composition over long distances, as well as increasing the velocity of the solution flow. Interlaminar adhesion and adhesion of the first layer with the surface increase, this can be said about the performance, but the applied layer can have a thickness of up to 60 millimeters. If you use this technique, the base will not need additional priming, the equipment does not provide for the need for washing. It will be enough to blow it with compressed air.
Disadvantages of dry method
On the site, when using the dry method of applying gunite-concrete, there will be increased contamination and dustiness. Workers must have the appropriate qualifications. The ratio of water to the dry mix should be adjusted and evaluated during the coating process.
Composition of concrete
If you decide to use shotcrete, the composition of the mixture is recommended to be studied in advance. For the preparation should be used cement and sand in a ratio of 1: 3 or 1: 4, everything will depend on the operational requirements for the design. In the first case, the surface will perfectly handle the mechanical, and in the second - with the atmospheric effect. It is necessary to prepare such quantity of a mix which you can develop for 3 hours, otherwise there will be a clumping, cement will lose active quality.
If you want to work at low temperatures, plasticizers should be added to the ingredients listed above, which are previously diluted in water. If it is necessary to prepare a dry mixture, they are added in the powder state. Waste generated during the rebound of the material, used for secondary mixing of the solution is prohibited.
Some companies sometimes develop additives for shotcrete. A patent for an invention, or rather an application for it, in this case must be submitted. Only then can the produce go on sale. As an example, we can consider RF patent application number 2543233, in which the mixture is supplemented with several complex additives. Thus, in one of these, hydrofluoric acid acts as an auxiliary component . But the main is a two-component stabilizer - sodium aluminate. Acidic constituents are derivatives of phosphoric acid. In the first stage, water is heated to a temperature of 60 to 110 degrees, and then aluminum sulfate and hydrofluoric acid, diethanolamine and the acid component of the stabilizer are introduced thereinto . In the next stage, the main component is introduced.
Composition of shotcrete "Zika"
If you want to know what the composition of shotcrete concrete of the company "Zika" is, it is worth to familiarize in more detail with the characteristics of this mixture. Thus, it can be used in winter, which became possible due to special additives and modulating additives. In order to solve different tasks, the manufacturer uses a certain recipe. To the cement and sand in the above proportions, stabilizers can be added, which increase the segregation resistance of the mixture. Sometimes, during the shutter, accelerators for setting and hardening, as well as retarders and additives based on microsilica are used. The last of the listed ingredients is necessary for impermeability and strength. Shotcrete-concrete, the GOST of which must comply with the following designation: 26633, is distinguished by outstanding performance characteristics.
Before the primary layer of the mass is applied, the base is protected by a sandblasting hydraulic method. On the surface, there should be no peeling of the old paint, plaster, concrete, stains, dirt and dust. If there is an armature, then it must be cleaned from rust. To improve the adhesion of the reinforced concrete surface, roughness is given. In order to control the uniform distribution of the composition, T-shirts are installed over the surface. Sometimes the future layer is reinforced, a 100 mm grid is used for this, and if the size of the cells is smaller, it must be drowned in the first layer. The prepared surface must be purged with compressed air and rinsed with a stream of water.
If you decide to do the described work on your own, you may be interested in the specific gravity of shotcrete-concrete, which is 2,400 kilograms per 1 cubic meter. This must be taken into account when calculating the thickness of the future layer. The distance between the nozzle and the surface will directly affect the amount of flaking material. As practice shows, a distance of one to two meters is optimal. If it is reduced, this will cause a large flow, as well as a slight compaction of the layer. If you exceed the above figure, the cover will be loosened.
Additives for shotcrete-concrete may not be used, but water can not be refused. It should be added in a certain ratio. For example, to obtain a strength of 26.4 MPa it will be necessary to add water and cement in a ratio of 0.35. If you want to reduce the strength to 22.1, then the proportion should look like 0.5. In the first case, the percentage rebound will be 19.1; While in the second - 11.2. The optimum ratio of water and cement will be equal to the limit of 0.4. The application of gunite-concrete is done layer by layer, the minimum thickness of each of them should be about 6 millimeters. When applying spray-concrete, the cement-gun nozzle must move along a spiral trajectory. Orient it at a right angle to the surface. Change the angle can be insignificant, trying to fill deep shells or reinforcement spaces.
Council of the master
Each layer must consist of individual horizontal strips, the height of which is a meter. Strip one another on each other by 20 centimeters. If you intend to use a clean solution without the use of additives, then the second layer should be applied two hours after the first.