Over the past 100 years, the technology of spraying concrete from innovative to classic has been used in a number of industries to solve various production problems. Moreover, recently, in connection with an increase in the volume of repair and construction works, interest in this technique has noticeably increased. Shotcrete is a layer-by-layer application of stucco or concrete mortars under impressive pressure, due to which interaction of the solution particles with the prepared surface is ensured. Existing cracks and small pores are filled during operation.
Area of use
The use of shotcrete is quite common today. This technique is used for fastening rocks during underground mining and tunneling. Repair of reinforced concrete and concrete structures is not complete without this technology. This may include reinforcing and replacing collapsing material. Shotcrete is used in sealing work as a refractory material. He found his 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 soil fastening in ground construction. This applies to the construction of storage facilities and pits, as well as the strengthening of slopes during the construction of transport interchanges and bridges.
Key Technology Features
When sprayed concrete is applied, the technology differs in certain features, among them - the possibility of applying the composition to uneven surfaces, as well as bases located in any planes. This is due to the high adhesive properties of the mixture and its self-supporting ability. Spraying is carried out at high pressure and speed, which allows for excellent adhesion of materials. After all, the applied layer is compacted independently. The thickness of the surface can be any that will depend on specific conditions and tasks. Sometimes work is accompanied by reinforcement, involving the use of traditional reinforcement or special fibers. The technology allows you to quickly erect load-bearing structures without involving formwork, and also eliminates long waiting times.
Varieties of shotcrete technology
Shotcrete can be applied in many different ways: wet or dry. In the first case, a ready-mixed concrete solution is supplied to the sprayer in the form of a nozzle. This is made possible through special sleeves. The composition is in the form of a continuous stream. In this case, rarefied air and a concrete pump are used. The advantages of this technique are the possibility of using a homogeneous composition, slight dust formation, minimizing the rebound of the material, as well as the possibility of finishing grouting after applying the mixture. The resulting surplus can be used in other construction works. Wet sprayed concrete can be applied in confined spaces.
Disadvantages of wet application
Shotcrete can be applied using a wet technique, but it has certain disadvantages, expressed in the insignificant flow rate of the mortar, which does not allow to achieve the maximum density of the structure. The thickness of the applied layer is limited and does not exceed 3 centimeters in one go. Increased demands are made on the maintenance of the shotcrete machine when it is used with a wet technique. Moreover, the master is tied to the place of production of concrete, and there is no way to use long sleeves. This limits maneuverability.
Features of dry shotcrete
Sprayed concrete can also be applied using dry technology, which provides for the supply of a mixture with fillers in separate hoses from the water. In this case, mixing takes place directly in the nebulizer. Considering the advantages, it is worth highlighting the possibility of delivering the composition over long distances, as well as increasing the speed of the solution flow. Interlayer adhesion and adhesion of the first layer to the surface increase, this can be said about productivity, but the applied layer can have a thickness of up to 60 millimeters. If you use this technique, the base will not need additional primer, the equipment does not provide for the need for washing. It will be enough to blow it with compressed air.
The disadvantages of the dry method
On the site, using the dry sprayed concrete application technique, increased pollution and dustiness will be noted. Workers must be qualified. The ratio of water to dry mix should be adjusted and evaluated during the coating process.
Concrete composition
If you decide to use shotcrete, the composition of the mixture is recommended to be studied first. For preparation, cement and sand should be used 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 cope with mechanical, and in the second - with atmospheric exposure. You should prepare such an amount of the mixture that you can develop in 3 hours, otherwise clumping will occur, cement will lose its active quality.
If you want to work at low temperatures, then plasticizers that are previously diluted in water should be added to the above ingredients. If necessary, prepare a dry mixture, they are added in powder form. Waste generated by the rebound of the material, use for secondary mixing of the solution is prohibited.
Additional ingredients
Some companies sometimes develop additives for shotcrete. A patent for an invention, or rather, an application for it, in this case must be filed. Only then can the products go on sale. As an example, you can consider the application for a patent of the Russian Federation No. 2543233, in which the mixture is supplemented with several complex additives. Thus, in one of them, hydrofluoric acid acts as an auxiliary component . But the main one is a two-component stabilizer - sodium aluminate. Acidic constituents are derivatives of phosphoric acid. In this case, in the first stage, the 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 into it. At the next stage, the main component is introduced.
Composition of shotcrete concrete "Zika"
If you want to know what the composition of Zika shotcrete is, it is worthwhile to familiarize yourself with the characteristics of this mixture in more detail. Thus, it can be used in the winter, which was made possible thanks to special additives and modulating additives. In order to solve different problems, the manufacturer uses a specific recipe. Stabilizers can be added to the cement and sand in the above proportions, which increase the segregation stability of the mixture. Sometimes during hardening, setting and hardening accelerators are used, as well as moderators and additives based on silica fume. The last of these ingredients is necessary for impermeability and increase strength. Shotcrete, GOST, which must comply with the following designation: 26633, characterized by outstanding performance.
Surface preparation
Before the primary layer of mass is applied, the base is protected by a sandblasting hydraulic method. Exfoliations of old paint, plaster, concrete, stains, dirt and dust should not remain on the surface. If the fittings are present, then it will have to be cleaned from rust. To increase the adhesion of the reinforced concrete surface, roughness is imparted. In order to control the uniform distribution of the composition, T-shirts are installed on the surface. Sometimes the future layer is reinforced, for this a 100 mm mesh is used, and if the mesh size is smaller, it will have to be sunk in the first layer. The prepared surface must be purged with compressed air and washed with a stream of water.
Shotcrete technology
If you decide to do the described work yourself, you may be interested in the specific gravity of shotcrete, which is 2400 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, it will cause a large consumption, as well as a slight compaction of the layer. If you exceed the above figure, then the coating will be loosened.
Additives for shotcrete may not be used, but water cannot be abandoned. 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; whereas in the second - 11.2. The optimum ratio of water to cement will be equal to the limit of 0.4. The sprayed concrete is applied in layers, the minimum thickness of each of them should be about 6 millimeters. When spraying concrete, the cement gun nozzle should move along a spiral path. It must be oriented at right angles to the surface. You can change the angle slightly, trying to fill deep sinks or reinforcing spaces.
Wizard tip
Each layer should consist of separate horizontal strips whose height is a meter. It is necessary to lay strips 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.