Page 159 - AWSAR 2.0

Corrosion happens all over the surface of steel and over time, the lost steel section can affect the performance of the structure. In PC, theinitialrustproductscangetfilledupinthe gap between the 7 wires. By the time enough rust is formed to crack the concrete and flow out, the structure may be too unsafe to use. Corrosion is also reported to be a lot more localised in this case than the former. This has happened in New Zealand. When rust stains were noticed in a girder of Tiwai Point Bridge, a detailed investigation was done. It was found that at 40 years of age, the girder had a 60% section loss and had to be decommissioned. So, visual investigation of PC structures gets a huge thumbs down! “What are you waiting for? Go! Install scaffolding everywhere. Stop the traffic. Inspect all the bridges immediately using all the sophisticated techniques you can think of.” I wish it was that easy! Though our government has enough technical expertise to do so, it is just not practical.
There are way too many bridges.
That is when I started
working on my research
topic electrochemical characterization and corrosion
assessment of prestressed
concrete bridges. I started a
long-term study on 3-metre-
long pre-tensioned concrete
specimens with two different
concretes to understand the
consequence of delayed
corrosion detection on the
engineering properties and microstructure of prestressing steel. The specimens have been exposed to diffusing chloride ions for about 15 months now and will be investigated after rust stains are visible on the surface. I plan to do a case study based on the obtained results, which can help sensitize us the end users and stakeholders
Ms. Dyana Joseline || 135
on the seriousness of this issue and make way for changes in our current assessment methods. I also did an experimental study to understand whether the currently adopted half-cell potential based corrosion detection criterion (ASTM C876) is suitable for assessing PC systems. I found that the criterion has to be altered to ensure that no false-negative interpretations (concluding that structure is not corroding when it actually is) are made.
Since corrosion is an electrochemical reaction, the best way to understand corrosion is by electrochemical methods like half-cell potential, linear polarization resistance and impedance spectroscopy techniques. These techniques can help in understanding what happens at the steel-concrete interface in a non-destructive way. I began laboratory experiments to understand the way in which the embedded prestressing steel starts losing its metal ions to corrosion when exposed to
salt water (sodium chloride solution). I developed my own test setup, which can be used to assess the effect of multiple factors on chloride-induced corrosion initiation, namely, stress level and cement type. I selected relevant binder types and stress level so that what I do in the lab can be taken to the field. As an engineering outcome, I am experimentally determining the amount of chloride ions that can cause corrosion initiation (also known as chloride threshold) taking
the variable nature of corrosion into account using statistical principles. This is one of the inputs needed to calculate how much time is left before corrosion can begin. I am basically trying to develop a framework that can be followed by the government to assess the existing PC bridge-stock. A routine preliminary
In PC, the initial rust products can get filled up in the gap between the 7 wires. By the time enough rust is formed to crack the concrete and flow out, the structure may be too unsafe to use. Corrosion is also reported to be a lot more localised in this case than the former.