Corrosion induced losses in pre-stressed tendons

The diameter of single tendons in pre-stressed concrete is typically around 5mm, corrosion combined with high levels of pre­stress can lead to a catastrophic, sudden rupture if not discovered in time. However, even if corrosion is present but not severe enough to cause rupture, a loss in pre-stress may result which may influence the load carrying capacity of the member This paper investigates the loss in pre-stress of tendons subjected to different levels of corrosion and different level of pre-stress. Twelve timber moulds were manufactured as a base for applying the pre-stressing technique. Ungalvanised tendons measuring 5.4mm diameter were pre-loaded to two extreme target levels of stress, namely 30% and 80% of ultimate tensile strength (UTS) and surrounded by mortar. Accelerated corrosion of 0% (control), 3% and 6% target loss of cross-sectional area were applied to replicate in-situ conditions. The strain in the tendons was continuously monitored throughout the corrosion process via vibrating wire strain gauges. The results show that the lightly pre-stressed tendon (30% UTS) exhibited a negligible change in pre-stress for 0% and 3% corrosion which increased to 17MPa when the degree of corrosion was doubled to 6%. Similarly, a decline in the initial high target level of pre-stress of 80% UTS was recorded as 9 MPa for 3% corrosion but when the corrosion doubled to 6%, the loss in pre-stress increased by a factor of 7 to 63MPa reduction. This indicates that higher degrees corrosion coupled with higher levels of pre-stress leads to higher losses of pre-stress and this should be accounted for as an additional loss at the design stage.