The South African design standards (SANS 2001-CC1 and COLTO) prescribes an elongation variation limit of ±6% and an average variation limit of ±3%. Most often these limits are exceeded in practice. If the elongation variation of a tendon falls outside the prescribed elongation limits it must be assessed by the engineer. This paper analyses data of bonded tendons from post-tensioned structures. The aim of this study is to explain the elongation variance of tendons in post-tensioned structures using selected stress-strain graphs. These structures include a reservoir (Mthatha 1-10) and a viaduct (Gautrain Jean Avenue P80).