The concrete for high stresses
In the production of fibre-reinforced concrete, fibres are processed in addition to the conventional components of normal concrete: cement, water and binder. Especially components that are exposed to high tensions are stabilised by this additive. In many components, the fibre additive even serves as a substitute for reinforcement. The principle of fibre concrete or fibre cement has been around since ancient times. Romans and Egyptians added straw or hair to the building materials and improved the mechanical properties by this addition. Various fibre materials can be processed in fibre-reinforced concrete. Depending on the installation situation, fibre concretes act, for example, as stabilisers, increase tensile strength or compressive strength or serve as reinforcement, for example in a floor slab made of fibre concrete.
Fibre concrete consists of the following raw materials:
- Concrete admixture (additive)
- Fibres made of glass, plastic, carbon (carbon) or steel
The fibre content changes the properties of the concrete. This happens, for example, because the fibres can absorb tensile stresses from the shrinkage process when the concrete sets. The effect: the concrete sets with little cracking. Further advantages of these concrete types are a constant stress distribution over the tensile area of the concrete component. With fibre-reinforced concrete, there is little or no corrosion, thus spalling is minimised or prevented at the same time. This in turn increases the overall durability of the material compared to reinforced concrete.
Due to the distribution of the fibres in the entire component cross-section and the random orientation of the fibres, an exact force effect is difficult to measure. For this reason, the number of calculation approaches is high, and generally valid calculation methods do not exist. As a rule of thumb for the calculation of steel fibre concrete, the required quantity per m³ of ready-mixed concrete is determined on the basis of the calculated static reinforcement.
The steel fibre concrete mainly used for industrial flooring has an improved working and load-bearing capacity due to the integrated fibre material of steel, the fibres replace the structural steel reinforcement by their effect. A distinction is made between three types of steel fibre, namely:
- Milled steel fibres with crescent-shaped cross-section
- bent wire fibres
- deformed sheet fibres
For concrete that meets the specifications of DIN EN 206-1/ DIN 1045-2, the steel fibres can be used in various forms during processing. This is again specified in DIN EN 14889-1. Distinguish between:
- loose steel fibres
- Fibres bonded in bundles
- Steel fibres in dosage packaging
If the steel fibres are added in quantities of up to 40 kg/m³, the original recipe of the concrete used can be retained unchanged. Steel fibres increase the bending tensile and shear strength as well as the cracking and deformation behaviour. Likewise, this manufacturing process increases the impact strength and reduces the depth of wear, while the thermal conductivity of the component increases because the heat is distributed more evenly and quickly. Mainly steel fibre concrete is used for industrial floors, less frequently it is used in concrete roads, tunnel construction or for floor slabs, foundations and basement walls in residential construction.
Alkali-resistant glass fibres are used as an additive in glass fibre concretes. They serve as statically effective reinforcement, in glass fibre modified concrete their effect comes to bear as micro-reinforcement. Even small additions of glass fibres improve the performance properties, the effect comes into effect from a proportion of 0.4 % by volume, if the glass fibre proportion is 2.5 to 5.0 % by volume one speaks of glass fibre concrete, at lower values the term glass fibre modified concrete has become established.
Due to the corrosion resistance of the glass fibre, which can also serve as reinforcement, concrete covers can be omitted and the material thicknesses of concrete components can be greatly reduced. This makes it possible to manufacture filigree components with high load-bearing strength. Glass-fibre modified concrete can be found, among other things, in façade construction, for roofing panels or for cladding.
Polymer fibres are categorised and classified according to properties in DIN EN 14889-2. Plastic fibre concrete usually has a fibre made of polypropylene added, which reliably prevents the formation of cracks in fresh concrete. However, this only applies to the first curing phase. Polypropylene fibres are therefore always added when crack-free surfaces are to be created, for example in screeds. In fire protection, this special fibre concrete prevents concrete parts from spalling over a longer period of time in the event of a fire. This protects the component reinforcement for longer and the concrete component retains its stability.
Textile or carbon fibres are a variant of plastic fibres. Textile concrete is considered a sustainable building material because steel is saved by using the fibres as reinforcement. The composite material made of fine concrete and high-performance fibres is considered to be extremely innovative and the building material of the future.
- Laying work and installation of reinforcement is omitted in many cases
- Complex shapes are easier to realise
- Storage space on the construction site for reinforcing steel for in-situ concrete components is not required.
- Material savings due to the installation of fibre concrete as shotcrete and omitted concrete cover
- Economical price due to reduced processing times and lower ancillary costs
A disadvantage, especially for steel fibre concrete components, is the extremely difficult machinability. If such a fibre-reinforced concrete component has to be modified subsequently, really heavy equipment is required.