Overview
Measurement of corrosion protection of tense anchors with insulated electrical resistance has been introduced in Switzerland in 1990. The experience with this measuring method are generally good.
However, there are always measuring results that give cause for debate. On the one hand occur in individual anchors on successive electrical resistance, the material safely or are system dependent. On the other hand, occur also unexplained variations in the measurements that need to be measured for technical reasons. Especially the second set of measurements often affects the confidence of builders in the method.
The main objective is to clarify the reason(s) for the fluctuations of the electrical resistances in certain anchors.
An additional desired goal is to determine if the successive drop of the resistance values at individual anchors is material technically or constructive or execution due to technical reasons.
Physical explanation for the causes of fluctuations of the electrical resistance of isolated, post-tensioned anchors.
Clarification of the causes for the gradual decline of electrical resistance of certain anchors.
Elaboration of rules for the execution and the valuation of the measurement of the electrical resistance of isolated permanent anchors.
Eventually proposal for improvements of anchor systems (construction, material, execution).
The research objectives to be achieved by three strategies:
a) Statistical evaluation of all available measurements of electrical resistance values of isolated anchors.
b) continuous measurements of the electrical resistance of three anchors with striking variations in the electrical resistance over a representative period with simultaneous registration of all possible influencing factors (temperature, humidity, etc.).
c) laboratory tests to those used for electrical insulation materials as a function of temperature, water content and humidity.
Funding
Results
The strong fluctuation of the electrical resistance is caused by condensation processes within the anchor head. The anchor strands act as heating elements in winter and cooling elements in summer, since they are in contact with deeper areas in the soil that exhibit a constant temperature during the year. The effect of this heat flow through the strands on the resistance of the anchor is depending to a significant degree on the design of the anchor head.
This influence can cause an opposite seasonal dependence of the resistance of anchors on
the same structure. However, it was found that the durability of the tensile member is not
compromised by these fluctuations, even when the resistance decreased below the threshold
of 0.1MΩ.
Strong fluctuations of the electrical resistance should be eliminated in order to avoid problems in the interpretation of the monitoring data. This should be achievable with an electrical insulation of the load cells.
Weaker fluctuations of the electrical resistance in the order of one decade can be caused by temperature variations and are irrelevant.
The permanent decrease of the electrical resistance is caused by a defect in the electrical
insulation on the bond length lv, in the transition zone lv/lfr, or in the upper sealing of the inner anchor head seal. A possible corrosion risk is given in the prior two cases, while a defect in the upper sealing of the anchor head has no detrimental effect on the strands.
Technical Implications
The present concept of corrosion protection by means of an electrical insulation and the respective normative rules are reasonable and adequate, although some deficiencies may still occur impeding the monitoring.
Policy implications
There is a need for an improvement of the anchor systems in order to prevent the decrease of the electrical resistance after the installation of the anchors. There are clear indications that the systems with an inner anchor head sealing by tubes made of PE have to be improved.
A new measuring method is proposed for locating a defect in the insulation. However, this method should be tested regarding its reliability with additional investigations, which could not be integrated in the present project.