Fiber-reinforced Sprayed Concrete
Fiber-reinforced sprayed concrete has now become much more important due to the development of new and more effective types of fiber, its increasing availability and its inclusion in various standards. It can be considered the perfect combination with sprayed concrete. Like conventional concrete, sprayed concrete is a brittle material with limited tensile and bending strength but very good compressive strength. It is certainly possible to reinforce sprayed concrete with conventional steel reinforcement, but its installation is very labor intensive, timeconsuming and frequently in conditions that are still safety critical. Also, reinforcing bars are not well adapted to the flexible layer thickness design of sprayed concrete. This is why it makes sense to use fiber-reinforced sprayed concrete.
Well distributed fibers in concrete
In principle, all fiber types and materials are suitable for sprayed concrete, where the material is used in tunneling, steel fiber is generally most appropriate. Carbon fiber has ideal properties but is completely uneconomic for use in conventional sprayed concrete. Glass fiber is only suitable for special fine-particle applications and has to meet special requirements for its long-term behaviour. Polymer fiber is mainly used for concrete repairs because it improves the internal cohesion of the sprayed concrete and reduces shrinkage cracking during early strength development. Plastic fiber improves the fire resistance of concrete in general. Modern generations of plastic fibers are now appearing in the traditional steel fiber applications.
Steel fibers for improving energy absorbtion of concrete
Steel fiber surpasses reinforcing bars and mesh on cost-performance in nearly every case.
The following guidelines apply to fiber-reinforced sprayed concrete production:
- The fresh concrete consistency must be more plastic so that the fiber-reinforced sprayed concrete can be pumped.
- Due to the larger surfaces, the lubricant and adhesive film requirement is greater and therefore the binder content must be increased.
- The adhesive properties are improved by the use of silica fume.
- The point for adding the fiber depends on the type of fiber and can be changed if problems occur (e.g. hedgehog formation).
- Remember that fibers are also lost with the rebound and therefore the content and efficiency of the sprayed concrete are the determining factors, not the theoretical steel fiber dosage.
Energy absorption classes according to EN 14487-1