Short-term elongation variation of posttensioned tendons

When tension is applied to steel tendons, they elongate in proportion to the tensile force, as predicted by Hooke's law. This elongation is used by the South African standard on concrete structural works and the South African standard specification for road and bridgeworks to determine the adequacy of the force applied in a tendon. The standards prescribe an elongation variation limit of ±6% and an average elongation variation limit of ±3%. According to these standards, if the elongation variation of the tendon falls outside these prescribed limits, it must be brought to the attention of the engineer. The scatter of tendon elongation results is often greater than the range prescribed by these standards. This usually requires the contractor to retension the tendons at huge financial costs. In most cases, the results obtained after retensioning are the same. This paper analyzes tensioning data obtained from a variety of projects that have been completed in South Africa in recent years. The aim of this investigation is to determine the causes of variation in elongation and suggest adjustments, if any, to the current elongation variation.