Corrosion detection and monitoring in post-tensioning steel

[Ingenuity]

I am a little familar the nondestructive evaluation technique that has been developed that is capable of determining the location and severity of corrosion of embedded or encased steel rebar and strands (for concrete strucutres) where it utilizes time domain reflectometry (TDR). By applying a sensor wire alongside of steel reinforcement (such as a prestressing strand), a transmission line is created. Physical defects of the reinforcement will change the electromagnetic properties of the line. Both analytical models and small-scale laboratory tests have shown that TDR can be effectively utilized to detect, locate and identify the extent of damage in steel reinforcement in this manner.

BUT, this is fine for new construction where you can place the sensor wire during initial construction. My question is - does anybody know of a NDT (or semi NDT) technique that can be used to DIRECTLY detect (and/or monitor) corrosion in EXISTING steel stands of concrete structures WITHOUT having to attach a continuous sensor wire adjacent to existing embedded strand, chip concrete, or use chloride contents etc.

Maybe i am being too idealistic!


TIA.

 

[rustbuster1]

I believe your last statement is true. A corroding tendon is not even that easy to differentiate from a non-corroding one, depending on your threshold definition of "corroding". Locating the position of localized corrosion along a tendon with any reliability is not feasible. One might even ask why this is necessary? The tendon's ability to provide the required load is what matters, not how many local corrosion sites it has or where they are.

I'm curious to see if anyone else has contrary experience to mine.

 

[Ingenuity]

rustbuster1,

Thanks for the response.

I understand that research at the University of Delaware was had some positive results on being able to determine relative corrosion magnitudes and corrosion types along a tendon length using TDR and a trace wire - but that is for new construction. I also understand that actual field application in ground anchors over 10 years of monitoring has confirmed the research.

My main focus on post-tensioning tendons is related to unbonded monostrands that are commonly used in the USA in building structures, and more specifically the repair of such tendons.

A common repair technique is to repair where a tendon is broken (after some invasive testing), but falling short of checking other "hot spots" within the same tendon it is possible that corrosion is present at sufficiently high level/s that shortly after repair at one location (and its subsequent re-stressing) the tendon still has a high probability of failure due to corrosion at another location and it most often goes visually undetected.

I agree that the "tendon's ability to provide the required load is what matters" BUT the more corrosion sites on a tendon the more probability that the tendon force will be lost - and this force is lost over the complete tendon length in the case of unbonded PT - the more lost tendons, the more sensitive the element is to structural distress or failure.

 

[LittleWheels]

Hello Ingenuity,

Have you considered using acoustic emission monitoring of reinforced and prestressed concrete corrosion? There is some interesting work being done on this subject at the moment.

I've used it for reinforced concrete condition evaluation recently but I understand that there is currently more published work involving prestressed concrete.