Shotcrete or sprayed concrete: process technology

Recommendation points

• Concrete in the hose. The technique of applying mortar by gunning
• What construction problems are solved by gunning concreting
• Manufacturing of critical structures from shotcrete
• Special properties of shotcrete and their application
• Application of technology within IZHS

The reader often hears about the technology of shotcrete or sprayed concrete by spraying, but not everyone can imagine the possibilities of this method. We will reveal the secrets of shotcrete in this article, we will teach the reader to use the maximum possibilities of technical progress.

What does the contractor choose: new technologies or traditional methods? Today, concrete structures are erected without formwork: shotcrete solves an impressive range of tasks – from pouring load-bearing concrete structures to facade finishing.

Concrete in the hose. The technique of applying mortar by gunning

The essence of the method is simple: the cement slurry is fed into a spray gun, where, carried away by the pressure of compressed air, it is sprayed onto the wall. During spraying, it is also possible to moisten the dry mix or add additives. A whole range of equipment is involved in the process:

1. Raw material hopper – cone tank.
2. Compressor and Water Pump – Special or General Purpose.
3. Connection hoses – abrasive resistant.
4. Sprayer: hose, hopper bucket or robotic arm.

There are two main methods of gunning: dry and wet. They are used in different types of construction, the equipment is also significantly different.

Dry method	Wet method
Dry mix of sand, cement and filler. enters through the hose into the mixer hose, where it is moistened and carried away by a jet of compressed air to the sprayed surface.	The raw material bin is filled with a pre-prepared mixture, which is delivered by a pump to the sprayer, where, under the action of compressed air, it turns into a directed jet.
Differs in high speed of application.	Convenient when working at height and in confined spaces.
The dry mix is ​​easier to deliver to the job site.	Needs preliminary preparation of the mixture.
Water source required.	Bulky equipment.
Increased dust formation.	Industrial use of construction.
A thicker coat can be applied immediately.	High quality finished surface.
No preliminary surface preparation required.	Small rebound of the sprayed composition.

Almost any material is used in wet shotcrete, only the maximum size of the aggregate fraction is limited. For dry shotcrete, concrete mix and powdered additives.

What construction problems are solved by gunning concreting

Shotcrete solves the problem of fast erection of thin-walled structures: walls, partitions, panels and ceilings. From a purely technical point of view, shotcrete is characterized by high productivity – up to 50 m can be processed within 1 hour² surfaces.

The possibilities of gunning during the reconstruction of architectural monuments are appreciated at their true worth. Solid particles of the mixture can be brought to the size of the dispersed phase, forming an aerosol during spraying, ejected from a nozzle at a speed of up to 200 m / s. This composition completely fills microscopic cracks, preventing the further spread of atmospheric and biological erosion..

Another advantage of shotcrete is used not only by reenactors, but also by designers. The fact is that the shape of the processed surfaces can be any (for example, domed houses), therefore, the finishing of spherical profile structures has been greatly simplified..

Industrial construction remains a truly promising direction for using the technology, because gunning allows you to quickly create reliable technological containers: tanks for storing liquids, swimming pools, rooms with a completely isolated environment. But you can spray not only cement, but also fireclay and silica solutions, so gunning has become more in demand in metallurgy than the production of refractories.

Manufacturing of critical structures from shotcrete

From the point of view of reliability, the shotcrete method is not only not inferior, but also many times superior to traditional concreting in strength and strength. A particle of dry matter flies out of the nozzle at a speed of up to 250 m / s, the composition is compacted in a humid environment so strongly that atomic forces of static stress come into play.

In the conditions of the test site of JSC TsNIIS 48 samples of shotcrete were tested for strength using an IP-100 machine. The test results showed the compressive strength on average from 40 to 70 MPa in tensile and about 11.5 MPa in bending. Even vibrating concrete with a very fine filler does not possess such characteristics..

For comparison, the average compressive strength of control samples of concrete products used in road construction is about 36–47 MPa *, including products that have undergone post-treatment with sulfonation and calcination. Shotcrete also distinguished itself in terms of water absorption (up to 2%), water permeability (not lower than W12) and frost resistance in a wide range from F300 to F1000.

Only one conclusion can be drawn: technically, gunning is suitable for any known and many promising sectors of housing or industrial construction..

* From the dissertation of Safina O. M. “Vibro-compressed road concrete products with increased operational characteristics”.

Special properties of shotcrete and their application

Shotcrete uses a wide range of additives to give the formulation over 40 different properties and even more combinations. Additives are used both for special and for ordinary concrete.

Additive class	Practical action	Additive example
Cement hydration regulators	The composition sets and hardens faster / slower.	Asilin (48 rubles / kg) Proston-D18 (32 rubles / l)
Liquid separation blockers	No moisture released during application. The thick layer of the composition does not “slip”.	Conwisol SM-11 (16-20 rubles / kg) SikaLatex (165-180 rubles / kg)
Air entraining agents	Reduces the density of concrete. Reduces the thermal conductivity of the layer.	Mapeplast LA (120-140 rubles / l)
Plasticizers	Increase the modulus of elasticity. Increases tensile and flexural strength. Increase the mobility of the mixture.	“Hardplast” (110 rubles / l) SikaPlast-520 (135-160 rubles / kg)
Waterproofing penetrants	Reduce water permeability. Reduce adhesion.	Kalmatron-D (70-75 rubles / kg) Sika-1 (300 rubles / kg)
Structural and PCA modifiers	Reduces concrete shrinkage. Strain concrete. Reduce water permeability.	“Parade” RSAM (65-75 rubles / kg) Cement NC (25-30 rubles / kg)

Application of technology within IZHS

Let’s consider the application of shotcrete technology on a practical example. Suppose a standard bathroom with dimensions of 4×2.4 m with a ceiling height of 3 m is being renovated on the high-rise floor of an apartment building.It is necessary to fill in a waterproof concrete jacuzzi bowl 1.5×1.5 m and the floor. And at the same time to correct the geometry of the room – to plaster the walls with a mixture of concrete with low thermal conductivity, so that the tiles would not “cry” either. What is the solution to the problem offered by shotcrete?

The shotcrete machine rises to the apartment in disassembled form on a regular lift. The inner wall of the future bowl is assembled from OSB, then sprayed with the required thickness (no more than 5–7 cm, the bowl will turn out to be thin but strong). For the floor and bowl we use waterproof concrete with the addition of Penetron ADMIX water repellent.

With the walls, everything is also simple – we install the beacons and spray the mixture, remove the excess with the rule, fill the pits and level again, getting a continuous wall covering with foamed heat-insulating concrete. For the walls, concrete with expanded clay aggregate and Mapeplast LA foam additive is used. We will strengthen both compositions with polypropylene fiber – we will increase the strength class of concrete by two.

Materials	Consumption	The cost
Expanded clay concrete M-100 V75	Walls – 1.3 m3	1.3 m3 x 3300 rubles. = 4290 rubles.
Concrete brand M-550 B40	Bowl – 0.3 m3 Floor – 0.77 m3	1.07 m3 x 4800 rub. = 5136 rub.
“Penetron ADMIX”	0.8 kg / m2	0.8 kg x 310 rubles. = 248 rubles.
Mapeplast LA	0.5 kg / m3	0.535 kg х 140 rubles. = 74.9 rubles.
Polypropylene fiber 8 mm	1.3 kg / m3	1.69 kg x 150 rubles. = 253.5 rubles.
Cost of work: Shotcrete machine rental:	8000 ~ 12000 rub. x 2.37 m3 = 18960 ~ 28400 RUB. 4000 rub / day
Total:	RUB 28,962.4. RUB 14002.4.

In most cases, doing the work yourself significantly reduces the cost of the project. When gunning, you can cut the financial costs by half, and the work itself is done cleanly and fairly quickly – in half a working day.