This paper presents the results of research on high performance concretes (HPC) modified by theaddition of polypropylene fibres (PP fibres). The scope of the research was the measurement of theresidual transport properties of heated and recooled concretes: gas permeability and surface waterabsorption. Seven types of concrete modified with fibrillated PP fibres were tested. Three lengths: 6,12 and 19 mm and three amounts of fibres: 0, 0.9 and 1.8 kg/m3 were used. The research programmewas designed to determine which length of fibres, used in which minimum amount, will, after thefibres melt, permit the development of a connected network and pathway for gases and liquids.