Ingenuity
Structural
- May 17, 2001
- 2,356
Does anyone have any references or research documents available that address corrosion rates of steel prestressing strand that is coated with a lithium-based grease, within a HDPE sheath?
The project is an existing post-tensioned concrete structure constructed with 1/2" diameter 7-wire, unbonded prestressing strand tendons, with a grease coating (lithium-based) that is inside a HDPE extruded sheath.
The tendons are stressed to a tensile stress of 80% of UTS, and minimum UTS is 270 ksi.
The max permissible water content of the grease at time of manufacture is 0.1% by weight. The structure is approximately 6+ years old (building was not completed due to 2008/9 financial crisis) and we are conducting an investigation of potential corrosion damage to the steel strands. The ends of the steel strand have been left exposed for this duration and water has entered the encapsulated system. We have tested the water content of the grease and it is more than 3 times the max permissible at time of manufacture.
We have extracted several strands from the structure and conducted visual observation and whilst the grease has emulsified in some locations, there is no evidence of corrosion - no surface corrosion, no pitting.
So question is: given a measured water content of 0.3% within the grease, and capping off the ends such that no further water is capable of entering the tendons, and assuming that oxygen is able to enter the system, what are corrosion rates that can be anticipated over the service life of the structure, assumed to be 50+ years?
The project is an existing post-tensioned concrete structure constructed with 1/2" diameter 7-wire, unbonded prestressing strand tendons, with a grease coating (lithium-based) that is inside a HDPE extruded sheath.
The tendons are stressed to a tensile stress of 80% of UTS, and minimum UTS is 270 ksi.
The max permissible water content of the grease at time of manufacture is 0.1% by weight. The structure is approximately 6+ years old (building was not completed due to 2008/9 financial crisis) and we are conducting an investigation of potential corrosion damage to the steel strands. The ends of the steel strand have been left exposed for this duration and water has entered the encapsulated system. We have tested the water content of the grease and it is more than 3 times the max permissible at time of manufacture.
We have extracted several strands from the structure and conducted visual observation and whilst the grease has emulsified in some locations, there is no evidence of corrosion - no surface corrosion, no pitting.
So question is: given a measured water content of 0.3% within the grease, and capping off the ends such that no further water is capable of entering the tendons, and assuming that oxygen is able to enter the system, what are corrosion rates that can be anticipated over the service life of the structure, assumed to be 50+ years?
