Since 1995 tendons with plastic ducts and electrical isolation are increasingly used in bridges. This enables tendons with better fretting fatigue behaviour, improved corrosion protection and, for the fist time, monitoring. Quality control and long term monitoring are performed with electrical resistance measurements between tendon and rebars. The criteria defined in the ASTRA/SBB guidelines (2001) are specific resistance values (kOhm*m) that take into account the influence of tendon length. Experience with the practical application on nearly 100 objects have revealed two problems: first, on short tendons (< 20 m) the criteria are not systematically fulfilled; second, the time of measuement (28 days after grouting) should be more opened (between some days and months). The guidelines have to be renewed in 2006, thus the project shall adress the above problems. A combination of specially designed laboratory tests and field measurements will allow to define a correction factor for short tendons. A known evolution of the specific electrical resistance of the grout with time will allow to perform the measurements at any time and refer the data to the 28-day value.
The Swiss guidelines "Measures to ensue the durability of post-tensioning tendons in bridges" (ASTRA/SBB 2001) has to be renewed until 2006. The guidelines should be modified taking into account the field experience in the first five years of their use. The main goal of the project is to establish the scientific background for the renewal, especially with regard to the limiting values of the electrical resistance specified in the guidelines. The project intends to study a) anchor heads as possible "defects" in the overall electrical resistance of the tendon (important for relatively short tendons), b) the evolution of the electrical resistance of the grout with time (in order to be able to perform the measurements at any time after grouting and not as now only after 28 days), c) the measured capacitance values of the tendons (a more detailed analysis and elaboration could help to detect grouting voids in the duct).
The research project is methodically based on a combination of laboratory tests and field trials. In the laboratory, the impedance measurement (dependent frequency of the AC electric resistivity) is used for field measurements and easier electrical resistance measurement (constant frequency of the AC voltage).
Electrically isolated tendons (EIT) are now a proven system to enhance the durability of structures with post-tensioning tendons. The polymer ducts reduce fretting fatigue, they form a tight barrier against the ingress of water and chlorides and the electrical isolation at the anchor head allows quality control during construction and monitoring over the whole service life.
Measurements of the electrical impedance on electrically isolated tendons have shown to be an efficient way for quality control of the tendons. Monitoring over time allows the identification of the penetration of (chloride containing) water at defects in the ducts. Thus for the first time, a simple, cost-effective early warning system for post-tensioning tendons is available.
Magnetic flux measurements allow locating defects (short circuits and holes) in the polymer ducts. For optimum success the tendons should be electrically connected at both ends. The interpretation of the magnetic flux measurements requires a good knowledge of the reinforcement present in the structure.
Also electrically isolated tendons that do not reach the 28 days acceptance criteria according the Swiss guideline are protected better against corrosion then tendons with metallic ducts. Unless there is a short circuit all tendons can be monitored over time and tendons that show ingress of water can be identified.
Main factors for success of electrically isolated tendons are high quality material and components, a detailed design focused on the critical aspects, proper testing, the personnel skills and experience in execution and the respect of well established construction procedures.
Della Vedova M., Elsener B., Evangelista L., Corrosion Protection and Monitoring of Electrically Isolated Post-tensioning Tendons, Proc. Third European Conference on Structural Control, 3ECSC 12-15 July 2004, Vienna
Della Vedova M., Elsener B., Enhanced Durability, Quality Control and Monitoring of Electrically isolated Tendons, Proc. 2nd Int. fib Congress, June 5 – 8, 2006, Naples, Italy. Paper 14-9
Elsener B., Long-term m
Renewing of the Swiss guidelines "Measures to ensue the durability of post-tensioning tendons in bridges" (ASTRA/SBB 2001).