This International Standard specifies the minimum values for expected strength as a function of time and temperature in the form of reference lines, for use in calculations on pipes made of: - polypropylene homopolymer (PP-H); - polypropylene block copolymer1) (PP-B); - polypropylene random copolymer (PP-R); - polypropylene random copolymer2) with a modified crystallinity (PP-RCT).