Coating a concrete bridge to stop rust
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  1. #1
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    last night on the History Channel I saw the tail end of a special on Highway 101 on the Pacific coast.(repeats saturday 11:00?). they showed rust prooffing a old concrete bridge. I think it was over the Rouge River in Oregon. They explaine dproblen is water rusts the rebar and it spalls the concrete. So they knock off any spalling concrete and claen it all up. Then they use wire fed zinc plasma guns to coat the concrete with molten zinc 15-60mils thick. Conect it electrically and the rebar is protected fo 20-50 years or so. Just like hot dip galvanising I guess. Also showed another concrete bridgre that used stainless steel rebar and water proff cement.
    Bill D.

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    Havent heard about the metal spraying technique, but I can vouch for the stainless rebar. I read an article in a trade mag about the stainless rebar being used by the Oregon DOT, and it listed the steel distributor. I called em up and bought a bunch of their stainless rebar drops. I had a project where I wanted all stainless construction, and so I used the stainless rebar, tig welded to the rest of the piece, and then imbedded in non shrink quick dry grout. Didnt like the idea of a galvanic corrosion situation happening under the base plate.
    That stainless rebar is pretty slick stuff, but man is it hard. Very high tensile strength- it put a nick in the rod cutting die of my ironworker.
    I had a little left over, and I made a knife out of it, but unfortunately it is just a 3 series, so it wont hold a very permanent edge- more of a decorative knife than a real workhorse.

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    That stainless rebar would make nice garden arbor or trellis. I have seen 20 foot pieces with both ends stuck in the ground about 10 fett apart do this each way in a ten foor square and tie the centewr for a nice arbor. Extend it as long as you want. The one I saw used iron rebar inside of plastic pipe..
    Bill D.

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    I suppose dipping the bridge in plastic tool handle goop is out of the question.

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    Interesting that they couldn't just connect the rebar to a sacrificial anode, and let that sucker dissolve.

    There is definitely a difference in highway construction...on one hand they deliver mild steel rebar to a staging area in May and then install it in August. On the other hand there's the green primed rebar (even the ends) which goes immediately into the ground.

    -Matt

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    For cathodic protection to work, the entire structure, or portion to be protected, and the anode, must be immerse in the same electrolyte.

    I used zinc metal spray nearly 50 years ago to protect all the spillway gates, and immersed structure, on our 7 dams and just about everything else that was exposed, including the transmission towers. It would be very good in the re-bar application.

    Spent most of my career as a corrosion engineer

    Blair

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    Forrest, dipping the bridge in goop would be a tad difficult, but what about feeding the stuff to seagulls? It would take a while but eventually they'd have it fully coated.

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    One of the problems with stainless steel used in concrete bridges, is the cost of the material, and the fact that the cables (if we are talking prestressed concrete) that are typically used for prestressing are high tensile carbon steel. The use of S/S of a grade which will meet the modulus of elasticity requirements is VERY Cost prohibitive. Only people that I have ever seen that could afford it was Uncle Sam, (via the Corp of Engineers) and even they didn't buy much of it, compared to what they did of the plain stuff.
    However, back in the late 70's,early 80's, the use of epoxy coating on rebar (probably that green primer looking stuff mentioned earlier) gained popularity, and the search was on to find some that would work on prestressed cable. (The stretching the cable takes, tended to peel the coating off) Finally one was found, but it was slick as glass on the outside when cured, a definite drawback on prestressed, but worked great for post tensioning. Another idea, of using a glass sand in the coating turned it into a viable product.
    Bill D
    Actually the zinc coating you talked about more resembles electroplate, instead of hot dipped. Electroplate is attached by electricity, and hot dipped creates a chemical bond, by means of emersion in a zince bath.
    David from jax

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    Although the work seldom makes the news, spall repair has become a very big busness.

    Millions are spent repiaring the floors of parking garages. The spalling gets so bad that it causes holes clear through the floor.

    The concrete work on ordinary highway bridges is another area that requires almost constant attention.

    I guess that contractors never realized the importance of making sure that ordinary steel rebar is entirely free of rust when the original pour is made.

    The way I have seen it done is that the spall is completely chipped away and then the rebar is sand blasted clean and eoxy coated.

    I have been told that rebar won't rust in concrete if it isn't rusty when it goes in.

    If it has rust on it, the rust will continue under the concrete causing a "pop-off" or spall.

    The Green Stuff seems to be the ticket. The bar is nice and rough and the epoxy is already on it.

    The grouting outfit I made high pressure pumps for got into "concrete restoration". It is really messy work but grouters don't mind.

    Sometimes the joke was that grouting and comcrete restoration was a license to steal, the profits were so high.

    Think of it - there is a lot of messed up concrete out there and the job can't be sent off shore.


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    Bill,
    it would be interesting to hear if this is actually the rebar rusting or if it is a "cheap" grade of concrete, that has/had ground/crush SLAG in it, as in the midwest. When one drives along in missouri, you'll see clad bridge pillers and HUFE rust stains on the roads and all over the place.

    It turned out that the builder used a cheap aggregate, and it's rusting... causing viods, staining and spawling...

    As the hoover dam has the same basic construction (rebar and agg) I wonder if the "wet" side has the same trouble?

    jeffe

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    You guys that don't have the freeze and thaw cycles like we do in the midwest are lucky. Ox and his damn snow are the problems up here. Water gets in the small crack, freezes, expands, and up comes the concrete. Every summer the crews are out fixing bridges. There are a lot of universities that are getting research money for this.

    JR

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    Regarding the aggregate in the concrete I remember when they were building the new jail in Oakland California.(probably PIC to call it a "jail") some brick dust got mixed in with the concrete. They had to go in and remove that set of pours at a great cost. I understood the concrete would have been easy for the inmates to break through.
    Bill D.

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    There are premix concrete that you can use that is perfect for rust.

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    bloody vikings

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    bloody vikings
    Calm down now.

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