AGB2004/010 - Quality control and monitoring of electrically isolated post-tensioning tendons in bridges
Overview
Background & policy context:
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.
Objectives:
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).
Methodology:
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).
Share this page