Carbon Fiber Wrapping to repair corroded steel structures

[SIMONMAGSUCANG]

Hi all,

I need to repair the angle steel and I-Beam which are heavily corroded plus metal loss, it is for overhead pipe rack, this is an old LNG plant where these steel structures had never been changed since the last 25 years. I'm currently looking at repairing these steel structure by using Carbon Fiber Wrap; I know this method has been widely used but I just couldn't find any calculation to support my proposal, can anyone here help?

 

[blakmax]

Hi Simon

There are many issues to consider in the use of composites for bonded repairs and design methodology is a small part of the overall repair process. Firstly, for any repair to be effective, there must be good adhesion between the wrap and the structure being repaired. This will require the use of a valid process to prepare the surface of the metal prior to bonding. Importantly that process MUST involve some form of chemical treatment to prevent hydration of the metal oxides on the surface. Do not rely on short-term strength tests to validate your processes. You must test for longer-term environmental resistance. Your process must also remove any existing corrosion products.

Next, you need to realise that carbon fibres when in contact with steel will form a galvanic cell and may lead to more rapid corrosion. You will need to take measures to completely encapsulate the repair material in glass-fibres and resin to prevent galvanic contact.

Other issues you will need to consider include:

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Do you pre-cure the repair and secondary bond it, or do you wet-layup and co-bond? Secondary bonding gives you a chance to throw away defective repairs before you install them, but will also require selection of an adhesive.

​

What are the environmental effects you need to address? (Cold/hot wet/dry) These may drive you towards a higher temperature curing resin/adhesive system so that the glass transition temperature is not exceeded in service. If you need an elevated temperature curing system, then you will need to consider how you are going to heat the repair, and that leads to issues with temperature measurement and control and heating methods.

Your design methodology would probably be based on restoration of section properties (nett section strength, bending stiffness etc.) but must also address stresses in the bond (and please do not use average shear strength unless you apply significant knock-down factors). Also note that for compression members, applying repairs to one side may reduce the buckling strength of the member.

Hope this helps

Max Davis
Adhesion Associates Pty. Ltd.

 

[SIMONMAGSUCANG]

Hi blakmax,

The method that I am planning to do is a field wet lay up method, the resin will be come from manufacture together with the carbon fiber wrap. The wrap is able to full cure under ambient temperature for around 6-12 hours, therefore I don't need to conduct any heating.

Now back to the calculation part, like you have mentioned:
"Your design methodology would probably be based on restoration of section properties (nett section strength, bending stiffness etc.) but must also address stresses in the bond (and please do not use average shear strength unless you apply significant knock-down factors). Also note that for compression members, applying repairs to one side may reduce the buckling strength of the member.".

Yes I am looking at sectional restoration, it will be too costly I believe if I put on Carbon Fiber wrapping on the whole steel structures, I would like to change out the damaged section but it is too costly and the area is too much congested, even crane is not allowed into that area to hold the 4 nos of 30"-36" pipe on the rack for the steel replacement.

How do I perform the calculation in order to make sure that this wrapping method is workable? I have the warp direction and fill direction strength of the wrap candidates, I have most of the data from the technical data sheet, now I just need to start a calculation to ensure this method is able to restore the strength of the corroded section..

 

[berkshire]

SIMONMAGSUCANG (Mechanical)
(OP)

Simon,
You may find that when you factor in the cleaning of these steel beams for getting a good adhesive joint that it may be more effective to put sister brackets into place.
B.E.

You are judged not by what you know, but by what you can do.

 

[blakmax]

Simon

I think that Berkshire's advice is valid, but I stress strongly that the surface is not just to be cleaned. For an adequate bond, the surface must not only be clean, it must also be chemically active, and even then you must treat the surface chemically to prevent hydration of the surface oxides. If you do not stop hydration of the oxides, then the bond will fail over time. Initially a bond to a clean, active surface will exhibit high strength. But as the oxides hydrate, the chemical bonds between the oxides and the adhesive (or resin in your case) dissociate to enable the hydration to occur. This results in interfacial bond failure and eventually the strength decays to zero.

My thoughts are that if it is difficult to replace the brackets, it will be just as difficult to generate the surface conditions which will be essential for ongoing strength. I don't want to convey the impression that bonded reinforcements are difficult to achieve. Indeed I personally was involved in a very large reinforcement on F-111 aircraft wing pivot fittings. But you need to balance the overall cost and difficulty of the processes against the cost of viable alternatives. In the case of the F-111 there was no viable alternative at that time, so the considerable effort was justified. In your case, Berkshire's advice may be more easily achieved than a bonded reinforcement.

Regards

Max Davis
Adhesion Associates Pty. Ltd.

 

[ewh]

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[Compositepro]

I find it hard to envision an effective composite repair that can be done without lifting the pipe out of the way. In any case it seems easier and cheaper add new steel beam supports on either side of the corroded beam. Whatever method you go with, you need to find a contractor who has experience with the methods and techniques and rely on their experience.