Monofire ultra-fine micro synthetic fibers, produced from pure polypropylene, are specially designed for use in concrete mixtures to provide fire resistance in tunneling applications. Monofire is resistant to alkaline substances, non-absorbent, and absolutely non-abrasive. Monofire fibers undergo a special treatment during production and provide excellent mixing and homogeneous distribution within the concrete. The use of these fibers protects the concrete from stresses that cause spalling during rapid temperature rise.