Holdfast Training Services Trial Blog

Having completed and submitted the tender for the North Eveleigh 11kV relocation project the other week I moved on to two other projects which are both part of the Rail Panel work. This panel consists of three consultants for which SMEC are one that get the honour of submitting proposals for various maintenance and upgrade works for Sydney Trains. Although some jobs are rather chip shop there are plenty of gems on offer and as the panel is a three year agreement it has provided a steady revenue for the rail structures team which I think was greatly appreciated last year.

The first is another tender of what Sydney trains call a Request For Proposal (RFP) for some building modification works on a substation about 50km north of Sydney. I attended the site inspection and even to me semi-trained eyes it looked like a fairly straight forward job. The intention is to remove certain outdated equipment and provide an admin room in its place as well as a seperate extension on the opposite side of the building to house a new DC switch room. I have engaged an architect, surveyor, building services consultant, building code compliance consultant and the pertinent in house specialists and will work on the proposal next week for submission on Fri. Thankfully there is no electrical work to be carried out like the last tender so I hope to control costs a little bit more to get a competitive price together. I’ll update more on this next week I am sure.

The second project moved my focus back to Eveleigh but on the other side of the tracks where a maintenance centre is located. Another consultant from the panel had originally been engaged to conduct an investigation of the basement columns where corrosion and subsequent concrete spalling had been observed. The initial concern was whether there were any structural capacity issues. We were asked to comment on the report findings and state whether we agreed with them. The original report concluded that up to a 50% loss of reinforcement section would be acceptable and that the current imposed loadings were not likely to be near the design loadings. The actual level of corrosion has not been detremined and although I haven’t visited the site the photos I have indicate that it is quite bad in places.

Initial thoughts were that stray current may be the cause of the corrosion. The tracks run parallel to the depot and it was proposed that the piles were picking up stray current from the tracks and conducting it to a point closer to the substation. However, the corrosion has only been observed above ground level and when the ground to the top of the pile cap was investigated there were no signs of corrosion. It must be pointed out as can been seen from the photos that the area of concern is approximately half the footprint of the building and there is no ground slab, the other half of the basement is in good condition (has a floor) and is used as an underground car park.

Apparently, the rate of corrosion from stray current is 9kg/Amp/Year. When iron corrodes by stray current, or by other means, the iron ions have to combine with anions which is commonly oxygen, therefore the corrosion product of iron is generally iron oxides or rust which are insoluable. However, the rate of diffusion of oxygen into concrete can be slow and insufficient to combine with the quantity of iron ions being produced. This means the iron will combine with other anions such as sulphates, chlorides etc which are soluable and so no visible corrosion product exists except a ‘shiny’ surface. The disadvantage of this is that severe corrosion and loss of reinforcement section  and structural capacity can occur before corrosion has been identified. The exposed reinforcement shown in the pictures is all covered in rust and so it was assumed (by a specialist stray current consultant) that stray current was not a major factor in the reinforcemnet corrosion but it could have occured after. As a result I compiled a report (more of a memo really) that proposed firstly a Cathodic Protection system to treat the possible cause of the corrosion and a physical column repair method involving ‘jacketing’ the columns in a 150mm RC layer. A few days later the client responded saying that they do not consider the stray current as an issue (the basement is poorly ventilated and conditions are very humid), therefore they did not want to pursue the CP option (which in their eyes came with an unsustainable electric bill) but they asked for a refernce design for the jacketing option. At present I am working up the details for this option which will assume the existing reinforcement in the columns is not contributing and so additional reinforcement will be placed on the outside. This is likely to increase the current 457mm square dimension of a 2.6m long column to 757mm square which is one short, stocky column.

Going back to the initial structural capcity check, I did not agree with the previous consultants conclusion that 50% loss of reinforcements section could be accepted (I calculated, well mainly an excel SS calculated) that only a 25% loss would be acceptable. I thought this would be a bold call considering a technical director did the initial calcs it appears is was due to the assumed compressive strength of the concrete. They had assumed 25MPa but I had the benefit of using actual core samples that were tested at various locations which using the code (no. of samples, sdev and all that) I used a value of 19MPa. My issues with all this so far is that firstly the actaul state of corrosion or loss of reinforcement section has not been determined and it looks like it will not either so there is a lot of assumptions. The second is the cause, it appears that the basement conditions may be the primary casue but to my mind stray current has not been ruled out and could still be the cause because no intrusive testing has been carried out. The big fat caveat with my design proposal at the moment is that this could be a ‘band aid’ job and unless stray current is conclusively ruled out by measuring the presence of a current flow through the columns and the basement conditions are addressed then at some point in the future the reinforcement will corrode to the point concrete spalling again. In fact any new reinforcement introduced to the columns without addressing stray current will be anodic to the existing reinforcement and thus increase the rate of corrosion in the new reinforcement. I will update again next week on how it all goes.