PP fibres are being increasingly mixed into the concrete for tunnel construction in order to prevent explosive concrete spalling occurring in the case of fire due to the evaporation of the moisture contained in the concrete. The increased fire resistance is based on the general conviction that the melting of the fibres in the case of fire leads to the formation of capillaries, through which the steam can escape. However, the precise mode of action of the PP fibres has not been clarified in detail and there are different model concepts for this. We have already stated in an earlier work that the diffusion of the polymer melting into the concrete matrix is the crucial process here. The results of experiments with new PP and PE fibres, which were specially developed for the further clarification of the mode of action, will now be presented in this work. The results clearly prove that the diffusion process of the melting PP fibres is the only mechanism that leads in the case of fire to the formation of a permeable capillary system for the dissipation of the damaging pore pressure in the concrete. Other processes do not occur.
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