Fibers modify the non-linear behavior of structural concrete
The incorporation of fibers into the concrete matrix modifies the non-linear behavior of structural concrete, especially in tension, preventing the opening and propagation of fissures
The UNE-83510 standard specifies the conditions to which the fiber must be subjected. The test consists of stretching the fiber until it breaks. It should be remembered that before breaking metallic fibers undergo plastic deformation.
The force before breaking or deforming is called tensile strength and is given in stress values per unit area of the fiber cross section (N / mm2). A low or medium carbon fiber, it is characterized by having a tensile strength between 400-1500 MPa.
The dosage is defined as the amount of fibers (kg) per cubic meter (m3), the more fibers incorporated in a concrete, it will have greater benefits. However, exceeding the necessary amount can be counterproductive due to the formation of hedgehogs and low workability due to the reduced fluidity of the paste; For these reasons, it is necessary to calculate the optimal dosage for each case using computer software.
In order to use the software, the residual resistance results of the characterization tests were previously incorporated into it, according to the UNE-EN 14651 standard
Subsequently, for each type of work, the values of the terrain and the hypotheses of load that the floor will support, such as racking, distributed load, truck and truck traffic, are incorporated
Regarding the dosage, it is important to highlight the effect that the fibers have on the consistency of the concrete. The fibers increase the viscosity of the cementitious paste due to its structural rigidity, modifying the values of the Abrams cone (test that allows us to know the consistency of concrete due to its water / cement ratio)
The solution to the loss of fluidity is the use of superplasticizing additives; they are responsible for increasing the flow capacity of the concrete paste without modifying the water-cement ratio to a maximum of 5% of its weight. These additives modify the consistency of the concrete, thus improving its workability
Kneading is a critical phase for fiber-reinforced concrete since there is a risk of generating unwanted agglomerations such as urchins
As previously discussed, with low dosages, kneading problems are reduced in the same way as with fine aggregate content and lower fiber slenderness. The order of filling is decisive, as a general rule the fibers are incorporated next to the aggregate or just after introducing it at a slow speed, 20 kg and 60 kg per minute with the maximum speed of rotation for the mixer, with the intention of ensuring the maximum homogeneity of the concrete.
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