Steel fibre-reinforced shotcrete

Steel fiber concrete is a building material from the base concrete with steel fibers added to existing composite material. He is a homogeneous building material, which are subjected to both pressure and train bend and thus can be applied to all types and loads. For dimensioning own calculation methods (design ) can be used depending on the type of load. The deterioration by corrosion plays only a minor role in this composite building material.

  • 5.1 corrosion

History

Fibers in cement based materials have long been used. However, a calculation of static point of view was not possible because to do so was no mechanical basics. It was only in the 1990s (eg, the Association of German steel fiber manufacturer ) were created foundations for building applications by various associations. Followed guidelines and also a design for foundation slabs, wide application, however, was not yet in place. With the leaflet " steel fiber reinforced concrete " of the German Concrete and Construction Technology Association from October 2001, a general standard for the design, production, processing and testing of steel fiber reinforced concrete was set in Germany for the first time. Bernhard Wietek developed in 2008 another design approach of SFRC. This made it possible to measure the components according to the occurring load effects.

In the GDR existed since 1988 calculation rules for fiber-reinforced concrete.

Application

Generally, steel fiber concrete is applicable for almost all components of a building. The necessary dimensions to be constructed, as well as the economy. In contrast to steel concrete steel fiber concrete is produced in a single operation, leading to much less labor time needed for processing. This is a crucial economic advantage of the building material steel fiber concrete.

The following pictures show a selection of such applications.

Trafohaus

Warehouse poured 30x6x6 m

Foundation for diagonal supports at a mountain station

Precast anchor wall

An additional use for steel fiber concrete is helpful when shotcrete. Although in this case, a higher engine wear is given, the overhead for this is compensated for by the simple, labor-saving application. The following examples show some typical applications.

Rock support vernageltem steel fiber shotcrete

Secured house wall masonry

Components

The composite building material steel fiber concrete consists mainly of concrete and steel fibers. By mixing the two parts of the steel fiber concrete is produced. Both the individual materials concrete and steel fiber are subject to large variations, as well as the mixing ratio of these two ( called dosage ), it is crucial for the properties of the final product.

Concrete

The properties of concrete and its terms are defined in national standards and also in the Euro code. For the static analysis of the material, concrete can with the material laws of Christian Otto Mohr and Charles Augustin de Coulomb ( Mohr- Coulomb failure criterion ) are considered. The result is there for materials the active voltage surface in which all stress states are permitted outside acting stress conditions lead to failure of the material.

These characteristic values ​​are now an output for determining the strength of the composite steel-fiber concrete.

Steel fiber

The number of steel fiber types available on the market is very large. For a calculation of components, however, the material strengths must be verifiable known for the different types of fibers in the composite with concrete. There are therefore listed only a few fibers with their characteristics. The characteristic values ​​are determined in bending tests, in which case the influence of steel fiber is evaluated on the flexural load-carrying behavior. This requires the investigation of concrete beams and steel fiber reinforced concrete beams under standard test conditions.

With this series of experiments, the values ​​for the individual fibers to be examined.

Upset form

As a result, can be specified for the already studied steel fibers following list.

To these characteristic values ​​and the corresponding values ​​of the respective dose strength steel fiber concrete can now be calculated with the Mohr- Coulomb contexts.

Dosage

Underdosing means the amount of steel fibers which is incorporated in the concrete. Unit as kilograms per cubic meter is specified (kg / m³).

Depending on the dosage, the influence of the steel fibers can be controlled by the properties of the composite. In the calculation is determined by the angle of the fiber, the proportion of the tensile strength due to the existing amount of steel fibers. This Zuganteil can be determined according to the Mohr -Coulomb constitutive law of the respective train and compressive stress for this dosage. This gives a new representation in the voltage circuit and the allowable voltage area. With these permissible stresses now, the design of a cross-section are performed.

With increasing dosage in non-cracked state, the load carrying capacity is only moderately increased, however, is the influence in partially or completely cracked cross section very much (that is, the steel fibers hold the flexural mainly ). It turns out that in contrast to the concrete steel fiber concrete has a Nachrissverhalten, the very positive impact.

Structural behavior

For the design of structural components made ​​of steel fiber concrete is available in Germany in 2010, for example, general building approvals by the German Institute for Construction Technology in Berlin, as on the subject of approval foundation slabs of steel fiber concrete. In this approval, the user must operate at a rated by 75 pages Bemessungsnomogramme until it comes to a solution. The relatively complex procedure is used only by specialists.

There are also the Bemessungsgregeln the DBV Journal. Another solution, which is applicable for all load conditions and building structures, developed Wietek, which is shown below. In general, the rated cross -section is divided into three different states:

Partially torn state

Fractured cross-section

Corresponding to the respective state of the calculation can then be performed. As a result, the variation of the crack depth and the dosage the image can be seen in Chapter dosage.

Now the individual load cases can be calculated in the static calculation. These are:

  • Bend
  • Bending with axial force
  • Print - Teal
  • Shear - shear force
  • Breaking out of a support

Can thus be determined with relatively simple calculations, the dimensions of the respective sections necessary. It is generally advised to work with as a concrete quality and dose of a steel fiber in a building. Thus, the structural analyzes considerably easier because there will be only the cross-section values ​​to calculate.

Conservation and durability

The maintenance and the durability of steel fiber concrete can be directly compared with the concrete of the building material. There are the properties of the concrete predominant, so that no additional measures must be taken. The only difference is in the corrosion of steel fibers, which are to be considered specifically.

Corrosion

By a carbonation or chloride penetration of the concrete surface here is a risk of corrosion of the steel fibers. A volume increase in the corrosion of iron to iron oxide (or other corrosion compounds ), which is, however, suppressed by the concrete is formed. Since the steel fibers in cross section usually less than a millimeter, locally only relatively small forces can take over the concrete entirely created here. Thus, corrosion of the steel fibers from the concrete strength prevented and there is no further propagation of the corrosion in the component. Only on the surface corrosion spots can occur, but which reach only one to two millimeters into the component. For corrosion to components is not a static problem, only one appearance on the surface, which can be easily prevented with an appropriate surface treatment.

Regulations

There are no standards in Germany and Austria available for steel fiber concrete. As in Germany, there is no building authorities introduced technical building code, the general application of steel fiber concrete in civil engineering is possible only with specific approval of the supreme building authority of the respective country at the request of the owner or with a general building inspectorate approval.

With steel fiber concrete, among others, the following regulations deal:

  • German Committee for Reinforced Concrete: Directive "Steel fiber reinforced concrete ", 2010.
  • SIA 162-6; SN 562162-6, February 1999 SFRC
  • Leaflet steel fiber concrete of the DBV ( German Concrete Association )
  • Directive fiber concrete of ÖVBB ( Austrian Society for Concrete and Construction Technology)
  • General Technical approvals of the German Institute for Construction Technology in Berlin
Dose (biochemistry)
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