Fiber-reinforced projected concrete for tunnel applications
Solutions to reduce thickness and increase productivity
For many years, steel fiber-reinforced concrete has been successfully used worldwide in projected concrete applications. This reinforced concrete provides multiple advantages such as:
- Material and runtime savings
- Reinforcement ductility
- Easy application
- Safe-working environment
As in conventional concrete, projected concrete is a fragile material, with limited traction and bending resistance, and with very good compression resistance.
To turn it into a soft material, projected concrete can also be reinforced with conventional steel, however the installation is complex, it takes a long time and sometimes it is not possible to guarantee the minimum safety conditions. In addition, this type of reinforcement does not adapt well to the design of the thikness of the flexible layer of the projected concrete.
The following aspects should be taken into account for the production of fiber-reinforced projected concrete:
The consistency of fresh concrete should be more plastic so that fiber reinforced shotcrete can be pumped. The use of fibers in concrete can cause a loss of workability, the magnitude of which will depend on the type and length of the fiber used, as well as the amount. This factor should be especially considered when requesting the consistency of the concrete on site.
Dosage depends on the type of fiber and can be replaced by another reference if problems occur (e.g. hedgehog formation).
There are fibers that get lost in the rebound so the determination of the content and efficiency of shotcrete are the determining factors, not the theoretical dosage of the steel fibers.
When a tunnel is excavated, earth movements occur inland, which is what is known as convergence
Support is used to maintain the stability and bearing capacity of the ground. The supports may be needed temporarily or permanently. Concrete sprayed with steel or gunite fibers forms a ring.
There is another system for the sustenance such as the use of a ring of segments. It is very common to make temporary and permanent holding systems in combination. This support works in the opposite direction to the force exerted by the terrain, exerting radial forces.
Structural use of fibers occurs when the addition of fibers is designed to contribute to the load-bearing capacity of the concrete. This standard covers fibers intended for this use in all types of concrete and mortar, including concrete for paving, precasting, in-situ, repairing concrete and shotcrete.
Final, secondary or definitive coating
The final coating is the most important stage in the execution of a tunnel. Represents and determines the durability and structural strength and is also the support for infrastructure installations for lighting and ventilation.
In traditionally excavated tunnels, our steel fibers are used to structurally reinforce the final coating of shotcrete or pumped directly on the formwork carriage; this formwork carriage must have a series of windows that allow concreting and vibrating. This coating can be used for several reasons: To improve ventilation, because of the presence of water or even for aesthetic reasons.
In cases where traditional mesh reinforcement is reduced and even eliminated, productivity increases and the thickness of the lining can be decreased. Steel fiber reinforced concrete is stronger, more ductile and less permeable than traditional concrete.
In tunnels excavated using modern automated TBM (Tunneling Boring Machine) excavators, the lining of voussoirs (precast concrete) can also be used as a final coating and is one of the fastest growing applications today.
Primary o temporal lining
The primary coating provides stability to the tunnel structure while providing also immediate support to the excavated terrain. This support cladding is temporary and commonly executed in shotcrete reinforced with steel fibers.
When necessary, this phase also includes the installation of additional reinforcement elements consisting of sections in steel arches (trusses) in the shape of the tunnel contour, resting on the ground, which resist the stresses of the terrain. It is necessary to execute a running beam or a footing in cases where there is a risk that the truss might get sticked into the ground.
At rimsa we have extensive experience in the application of steel fibers and synthetic macro fibers in shotcrete. Contact our technical sales team to analyze your specific case.
