Cathodic Protection of Corroded Pre-stressed Tendons

The corrosion of reinforcement in concrete, whether conventional or pre-stressed, remains a significant cause for the loss of durability of reinforced concrete structures. Corrosion costs the UK economy between 3% to 4% of Gross National Product. This is, in turn, has resulted in the development of greatly enhanced methods of remediation and life extension. One possible approach is to use Cathodic Protection (CP) to control further deterioration. CP of reinforcing steel in concrete structures has proved to be effective for preventing or controlling corrosion and been used successfully for over 25 years. CP is able to stop corrosion in a reliable and economical way where the environment has caused reinforcement corrosion and subsequent concrete damage. However, concerns exist about the ability of CP to avert deterioration in pre-stressed structures due to hydrogen generation and subsequent embrittlement of the tendons. This research investigates the performance of pre-stressed steel tendon exposed to an impressed current cathodic protection (ICCP) at varying potentials on a long-term basis to establish its effect on strength and establish optimised criteria for CP that can be safely applied to deteriorated pre-stressed structures. Twelve timber moulds were manufactured for applying the pre-stressing technique to test specimens. Tendons measuring 5.4mm diameter were selected in both the galvanised and ungalvanised state. Two levels of pre-stressing have been investigated, namely low level (300-400 MPa) and high level (800-1200 MPa). Three different degrees of corrosion Stage I, II and III with target losses of cross-sectional of 0-1 %, 2-4 % and 4-7 % respectively, were employed to replicate in-situ conditions. The actual degree of corrosion was verified gravimetrically by weighing the tendons both before and after testing. The tendons were pre-stressed in two types of electrolyte, namely a saline solution and a sand/cement mortar representing mortar. Upon completion of the corrosion phase using an anodic impressed current method, Impressed Current Cathodic Protection (ICCP) was applied to the tendons at two levels of polarization, normal protection (ICCP-N) in the range of -650 to -750 mV vs Ag/AgCI/ 0.5M KCI and over protection (ICCP-O) ranging between -850 to -1300 mV vs Ag/AgCI/ 0.5M KCI for an extended period to both ungalvanised iv and galvanised pre-stressed steel tendons, to investigate its effect. The potentials of the pre-stressed steel tendon and potential decay resulting from the application of ICCP were monitored and analysed. The strain in the tendons was also monitored throughout the corrosion and ICCP phases to establish pre-stress losses. Finally, the mechanical properties were investigated and the tendon surfaces and fracture modes inspected using an Infinite Focus Microscope (IFM) and Scanning Electronic Microscope (SEM). The results confirmed that accelerated corrosion is a reliable technique for generating the corrosion of steel. ICCP can be used in the corroded pre-stressed tendons as the long-term application shows there is no significant effect on the surface or damage of the both types of tendons with low or high levels of pre-stress. There has been a long term loss in service stress which due to corrosion, due to ICCP or a combination of both. From the results, the loss is more likely to be corrosion induced rather than ICCP. A higher degree of corrosion leads to a higher loss in pre-stress in highly pre-stressed tendons, which is an additional loss that should be accounted for at the design stage.