Shotcrete
Shotcrete is concrete that is conveyed to the installation site in a hose line, where it is pneumatically applied from a spray nozzle and compacted by the impact energy.
The manufacture and use of shotcrete is based on European DIN EN 14487 and the national application standard DIN 18551. The recipe largely corresponds to that of normal concrete in accordance with DIN 1045-2.
Although it has been known and used in Germany since 1920, mostly under the name Torkretbeton , the use of this process only increased in the second half of the 20th century, even if its share in the total concrete production is still relatively small.
From 1921 onwards, American cement guns and Tector devices from the Torkret branch in Lugano were in operation in Switzerland. They were used, for example, in the pressure tunnels of the Amsteg power plant and in the SBB tunnels of Coldrerio and Massagno . The cement guns had asserted themselves, which is why the term "Gunit" (English "Gunite") has become common in Switzerland.
history
In 1908 the American animal taxidermist Carl E. Akeley applied for a patent for the invention of an “apparatus for mixing and applying plastic materials”. With this device, known as the “Cement Gun”, a dry mixture of concrete and cement mortar was blown through a delivery hose to the installation site. A pocket wheel located at the bottom of the filling chamber was used to feed the material evenly into the conveying hose, which was then evenly moistened with a water ring located at the end of the hose while flying through the spray nozzle. Two pressure chambers arranged one above the other, which were alternately ventilated with compressed air, allowed the chambers to be continuously filled with mix.
In 1919 the German-American Carl Weber, who had experience with this device as an engineer in America, registered his own patent for a dry injection molding machine in Germany. He founded a company to build and sell the machines, the Deutsche Torkret Baugesellschaft. As a result, the designation Torkret machines became established for these machines, and the term Torkret method, or Torkretieren in general, was used for the process. The company later renamed Torkret GmbH (today's Torkret AG), based in Berlin and from 1956 in Essen, was a leader in the use of machines for many years. The Torkret method became particularly important in the repair of the existing concrete structures, which had been badly damaged during the war.
The wet spraying process was developed from the mid-1950s. Sprayed concrete was now also used to build tunnels and caverns . The New Austrian Tunneling Method , developed at the time , in which a (reinforced) concrete shell was sprayed onto the exposed tunnel areas for support and consolidation immediately after the tunnel hole had been cleared, became generally accepted.
The increasing use of sprayed concrete for civil engineering structures of all kinds required the standardization of this process. With the first version of DIN 18551 (shotcrete) published in 1974, the term shotcrete was introduced for the method instead of the previously common name Torkret method.
application
Today, shotcrete is used primarily for the repair and reinforcement of concrete components, for terrain and rock consolidation, for temporary shoring on large construction sites and in tunnel construction, as well as for the creation of natural-looking surfaces on leisure and sport climbing facilities .
Due to the special features of the process and the required equipment, shotcrete is usually carried out by specialized companies. According to the ZTV-ING, the deployed personnel must have a so-called nozzle driver's license.
The shotcrete method has the advantage that no formwork or only one-sided formwork is required , that there is no need to compact after the concrete has been applied and that, in general, very good adhesion is achieved. Overhead concreting is also possible. Application areas may have to be pre-treated.
During the injection process, part of the material to be sprayed bounces off, the so-called rebound. Since the coarse parts of the aggregate in particular bounce off the solid ground at the beginning, a concrete layer with an increased fine-grain and cement content remains. The fine parts of the first layer claw into the substrate due to the impact energy, which results in a stable bonding bridge for the subsequent layers.
With careful nozzle guidance, the application process leads to very good compaction of the concrete. The largely void-free concrete is tight, resilient and protects the reinforcing steel it contains from corrosion. These properties were used as early as the 1920s to build thin and wide-span shell structures based on the Zeiss-Dywidag shell construction.
In the United States and Australia, screed and wall surfaces are often provided with decoratively colored and structured surfaces that imitate , for example, paved natural stone surfaces or stone walls . A method for designing decorative wall surfaces with shotcrete has also been known since 1999. For example, a net-like matrix can be attached to the raw surface, which is removed again after a thin layer of colored shotcrete has been applied. In this way, the impression of the joint pattern of a brick walled natural stone or brick wall can be achieved.
Manufacturing process
Dry spray process
In the dry spraying process, cement , aggregates and powdered additives are mixed dry and put into the concrete spraying machine and conveyed in a compressed air stream (thin stream conveyance) through the pipe or hose to the spray nozzle. Only in the nozzle area is the dry mixture provided with the necessary added water and, if necessary, liquid additives and accelerated into a continuous jet.
This process is used, for example, to repair the joints of natural stone masonry on historical buildings.
Advantages of dry spraying
- Low investment
- Flexible
- the amount of water can be adjusted, for example to improve the adhesion of the concrete to overhangs
- lighter hoses with a smaller diameter are easier to guide by hand
- Work interruptions are easily possible
- Increased processing time by holding the dry mix longer
- Spraying possible in confined spaces
- Funding over distances of up to 1400 meters (around 100 meters are ideal)
- Little cleaning effort for the hoses
Disadvantages of the dry spray process
- Defined degree of moisture of the aggregate used is necessary
- Dust development with dry mix
- High rebound
- Wear on the spraying machine, delivery hoses and spray nozzle
Wet spray process
In the wet spray process, cement, aggregate and water are mixed together and conveyed to a spray nozzle by means of a concrete pump (dense phase conveyance) or compressed air (thin flow conveyance).
With the wet spray process, it is easier to achieve a uniform quality (with a constant w / c value ) throughout the entire spraying process .
Advantages of the wet spray process
- Very little dust generation
- Larger order output
- Better homogeneity of the mixture
- Less loss due to rebound (10 to 20%, compared to an average of 20 to 25% with dry spraying)
- Constant water content through defined w / c value
Disadvantages of the wet spray process
- High weight of the hose in the dense phase process - usually the use of controlled manipulators is necessary
- The machines take up a lot of space
- Short processing times of the finished mixture, less flexibility
- The mixture cannot be adapted to the local conditions as with the dry spray process
- Higher cleaning effort
- It is necessary to determine the required mixture quantity in advance in order to avoid residual quantities
Dense phase process
The finished mixture is conveyed through the hose with a piston pump or a screw pump . At the spray nozzle at the end of the hose, air is added at around seven to nine cubic meters per minute at a pressure of seven to nine bar, in order to accelerate the mixture to the required exit speed. A superplasticizer must be added to the concrete mix to promote it.
Due to the soft consistency, it is also necessary to add a liquid accelerator to the spray nozzle, which is dispersed by the supplied compressed air and thus distributed in the mixture.
Thin-stream process
In the thin-stream process, too, compressed air is fed to the mixture in the spray nozzle in order to achieve the required start-up speed. However, it is possible to dispense with the preparation of a previously flowable consistency.
properties
The workability required for conveying and spraying the concrete and the required compressive strength of the sprayed-on concrete are decisive for the composition of the initial concrete (the mix design ). The processing determines the water requirement ( water binding agent value ) on which the compressive strength depends.
With the dry spray method, the spray operator can adjust the amount of water to the needs on site during the concreting process.
In the wet spraying process, the starting concrete is produced in a plastic consistency ( slump 35 to 41 centimeters). The water requirement also depends on the grain shape and grain composition of the aggregate. It can be reduced by using liquefiers or superplasticizers.
The (initial) rebound of the coarse parts of the aggregate results in an increase in the cement content. If accelerators are used, the 28-day compressive strength may be lower.
Standard mix
A typical mixture consists of Portland cement (CEM I 42.5 R) and aggregate with a maximum grain of 8 mm and has a water cement value of 0.5. This results in an initial compressive strength of at least 20 MPa after 2 days and a final value of 42.5 MPa after 28 days.
According to ÖN B 4710-1, up to a maximum of 25 percent fly ash can be added to improve pumpability
Binder content
When producing the mix for dry sprayed concrete, the amount of binder is usually between 320 and 460 kilograms per cubic meter of concrete. In order to determine the actual cement content of the installed shotcrete, the rebound must be included. Compared to the initial mixture, the rebound mainly leads to a loss of the coarse-grained aggregates and thus to an increase in the cement content. With a typical standard mix of 350 kilograms of cement per cubic meter, a rebound of 25 percent by volume results in a cement content of around 450 kilograms per cubic meter in the end product.
fertility
With a 25 percent rebound, 1000 liters of dry mix results in around 555 liters of adherent shotcrete. (If a quarter of the dry mix is lost as a result of the rebound, 750 liters of 1000 liters of dry mix are left. These 750 liters are compressed by 1.35 times to 555 liters. The ratio of the volume of the shotcrete to the dry mix is therefore 1 : 1.8.)
Water-cement ratio
The water-cement ratio is a decisive factor for the quality of the concrete. In the dry spraying process, the total amount of water is made up of the added water at the nozzle and the inherent moisture contained in the aggregates.
In contrast to the wet spraying process, the water-cement ratio varies with the dry spraying process, as the amount added is determined by the nozzle operator. This is often seen as a disadvantage. In practice, however, the result is a relatively even value, since too little water is added immediately in the form of excessive dust formation or increased rebound, while too much water allows the shotcrete to flow down. If done correctly, the water-cement ratio should be below 0.5 in this way.
Moisture content of the surcharges
An important aspect of the dry spray process is the natural moisture of the aggregates. If the mixture is too dry, too much dust will be created when spraying. If the inherent humidity is too high, this can lead to problems: The shotcrete throughput drops sharply, the machine and delivery lines become encrusted, and blockages occur. The inherent moisture content of the mixture should be between three and six percent. In addition to the mixes produced on site, the use of ready-made dry mixes, which are supplied in sacks or silos, has also established itself in recent years. Properly manufactured and stored, these mixtures do not contain any inherent moisture.
To prevent dust formation, it is advisable to wet the aggregate material before it gets into the machine. Specially equipped conveyor systems or wetting nozzles can reduce dust generation by up to 60%.
Additives
Various admixtures are available on the market to control the properties of the shotcrete. The most important are the setting-accelerating additives, which reduce the setting time (setting accelerator). The shotcrete sets faster and achieves higher early strengths. This allows the subsequent layers to be applied more quickly and in larger layers. In large construction projects, solidification accelerators contribute significantly to an increase in production capacity and are important prerequisites for many applications. For example, early strength is a basic requirement for underground structures and building pit securing. Common accelerators are sodium water glass and calcium chloride , with calcium chloride having a disadvantageous effect on the corrosion resistance of the reinforcing steel. Other additives are superplasticizers, sealants, retarders and stabilizers.
When using solidification accelerators (especially water glass), strength reductions of 20 to 50 percent can occur. If necessary, the draft mix for a shotcrete of the general strength class C 25/30 must be based on a concrete of the strength class C 35/45 to C 45/55. The achievable values must be determined by a test. In addition, accelerators can lead to a decreasing modulus of elasticity and increased shrinkage and creep.
In the dry spray process, dust reducers can be used to reduce dust generation. As with all additives, precise dosing is required to meet quality requirements.
Fibers
Steel and synthetic fibers can be added to the shotcrete. This results in higher work capacity (energy absorption capacity) and impact resistance as well as improved shrinkage behavior of the shotcrete.
Steel fiber concrete in the form of steel fiber sprayed concrete is used in particular for construction pit sheeting and slope stabilization. Conventional steel reinforcement can be dispensed with.
Shotcrete cement
Shotcrete cements (SE cements) are characterized by early setting with little dust and reduced rebound. It is a low-gypsum cement that can be used in the dry spray process and solidifies within a very short time when water is added, so that no accelerator is added. The use of a shotcrete cement can therefore be expected to have a higher final strength.
literature
- Amos: The Torkret method and its application . In: Zentralblatt der Bauverwaltung , Volume 42, No. 88 (November 1, 1922), pp. 541–542.
- Günther Ruffert: Shotcrete. Beton-Verlag, Düsseldorf 1991, ISBN 3-7640-0282-4 .
- Bernhard Maidl u. a .: Handbook for shotcrete. Ernst, Berlin 1992, ISBN 3-433-01196-6 .
- Bernhard Wietek : Steel fiber concrete. 2nd Edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8348-0872-1 .
- Johannes Beranek: "The New Austrian Tunneling Method": Investigation of shotcrete made of fiber-reinforced and mat-reinforced shotcrete and their possible applications, AV Akademikerverlag, Saarbrücken 2013, ISBN 3-639-45838-9 .
- Rupert Springenschmid: Concrete technology for practice. 1st edition. Bauwerk Verlag, Berlin 2007, ISBN 978-3-89932-161-6
- Peter Grübl, Helmut Weigler, Sieghart Karl: Concrete. Types, manufacture and properties. Ernst & Sohn Verlag, Berlin 2001, ISBN 3-433-01340-3 .
- Wilhelm Scholz, Wolfram Hiese (Ed.): Knowledge of building materials. Werner-Verlag, Cologne 2007, ISBN 978-3-8041-5227-4 .
- Harald Schorn, Richard Sonnenberg, Peter Maurer: shotcrete. Publication series Spezialbetone Volume 6. Verlag Bau + Technik, Düsseldorf 2005, ISBN 3-7640-0452-5 .
- Klaus Eichler, Claus Flohrer, Walter Pichler: Shotcrete technology. Binding agents and processes - Construction technology and the environment - Reinforcing and repairing - Securing and protecting. Volume 641. expert verlag, Renningen 2003, ISBN 3-8169-2155-8 .
- DIN 18551: 2014. Shotcrete - National application rules for the DIN EN 14487 series and rules for the dimensioning of shotcrete structures. Beuth Verlag, Berlin 2014
- DIN EN 934-5: 2008. Admixtures for concrete, mortar and grout - Part 5: Admixtures for shotcrete - Definitions, requirements, conformity, marking and labeling; German version EN 934-5: 2007. Beuth Verlag, Berlin 2008.
Web links
- Building materials science : shotcrete (bibliography)
Individual evidence
- ↑ Pietro Teichert: The history of shotcrete. Swiss engineer and architect magazine. Volume 97, 1979
- ↑ Explanation of the procedure on Torkret.de; accessed in December 2016
- ↑ Grübl, P .; Karl, S .; Weigler, H .: Concrete. Types, manufacture and properties . 2nd Edition. Ernst and Son Verlag, Berlin.
- ↑ Maurer, P .; Schorn, H .; Sonnenberg, R .: shotcrete . In: Series of Special Concrete . tape 6 . Publishing house Bau + Technik, Düsseldorf.