For years and decades steel fiber reinforced shotcrete has been used successfully in the tunneling industry. The drawbacks of steel fibers such as the difficult and time consuming handling because of the high weight, the high rebound-rate, the damage of the shotcrete-equipment (nozzles, hoses, ...) as well as the corrosion and the danger of injuries due to fibers on the surface have been accepted due to the lack of alternatives. Although the so far available synthetic macro-fibers (not to be confused with synthetic micro-fibers) can minimize or even eliminate the above problems, they still could not substitute the steel fibers due to the creeping of synthetic materials.
Definition of "creeping" The meaning of creeping is a time-dependent deformation under permanent loads. This becomes most important in the cracked status of fiber reinforced concrete because in this state the fibers are under constant stress. But like one steel cannot be simply compared to the other one (there are soft wires, high-strength wires, galvanized or not galvanized or even Zinc-Alucoated wires) there are also big differences between synthetic materials. Since the determining factor for the creep behavior is not only the fiber material itself, but also its structure and last but not the least, the effective concrete bond.
The bi-component, high performance macro-fiber Concrixas solution
A few years ago a research project supported by the Swiss Government (KTI) was started with the goal to develop a synthetic "High performance fiber" which doesn’t have this creeping problem. After many trials and a long period of experimentation a positive result could be achieved and the creeping with the macrofiber Concrix could practically be eliminated. This is because of a unique bi-component fiber construction, the increase in the degree of crystallinity and the young modulus and the adding of special additives. In addition, the surface was textured to further improve the concrete bond.
Creep performance tests
The long-term tests made by EMPA (Swiss Federal Laboratories for Materials Testing and Research), a prestigious test institute with international reputation, showed distinctly that this new fiber actually prevents creeping. Under constant load, the surface structured bi-component fibers Concrix showed a small increase on the wideness of crack.Under load levels in the range of the occupancy rate indicated for fiber concrete, the mentioned increase on the wideness of crack is absolutely uncritical. In prism tests, no critical values were showed after a period of one year even when loaded with more than twice as much weight. Meanwhile the test went on and more than a year has passed by without any problems.
In the square panel test (dimension according to EFNARC) residual loads with more than 60% of the carrying capacity (maximum load capacity of deflection 2mm) were permanently maintained without any significant increase in deformation.
All of this is possible because of the unique composition of the bi-component Concrix fiber and the optimally structured surface.
Developed in the laboratory, proofed in practice
Since the launch of the Concrix fiber, several projects have been realized, all of them to the full satisfaction of the customers as well as the construction companies. There are many reasons for this: The processing is extremely easy, the fiber bundles guarantee a fast and homogenous distribution throughout the matrix, the machines are not damaged and the fibers themselves are resistant to aggressive ground water und corrosion.