Theme Park Enginerd: Corrosion in Concrete

[imagineerz]

So this is a pretty specific and detailed question. But for those of us who aren't materials engineers, here's some background first.

 

Most, if not all, coasters use concrete footers. And these footers are often reinforced with steel rebar, due to concrete's terrible tensile strength. However, because steel is not a naturally occurring metal, rather a refined version of iron ore, it is thermodynamically unstable. This means that it wants to return to its natural state, iron oxide, also known as rust. This is where we get "corrosion." Because the makeup of concrete can, in essence, serve as an electrolyte, rebar in concrete can easily corrode. There is a two fold problem here: 1. The more the rebar corrodes, the less structural integrity there is, and 2. Rust is exponentially bigger than steel, which leads to concrete cracking, leading back to problem 1.

 

So here's my question: How do ride manufacturers prevent corrosion in concrete? Of course, there are solutions (ie cathodic protection), but they tend to be decently expensive, and in the example of cathodic protection, even if only one anode is placed in each footer, the amount of footers can lead to insane costs. Anyone know any more?

[certifiedcoastergeek]

^Magic is the answer. Ride magic. There wasn't a tinfoil hat smiley

[Shavethewhales]

There's nothing special about how the amusement industry treats concrete, it's the same as any other application. Most, if not all, rebar used today is coated with Epoxy to hinder corrosion. Sometimes Epoxy isn't even necessary if your design doesn't have a terribly long service life or critical implementation, but I think at this point the cost of epoxy is fairly negligible at this point so it's just kind of standard now.

 

There's also galvanization. Here's a resource that talks a little about it, but consider that it is a bit of a biased source for galvinization. https://www.galvanizeit.org/uploads/publications/Galvanized_Rebar_vs_Epoxy_Rebar.pdf

 

I think everything I've ever designed with concrete has used epoxy coated rebar, but I've only done a couple of footers and a small parking lot. It's not exactly a specialty of mine, but you can make a career out of specializing in concrete design and technology.

[Evan1127]

Rust doesn't form in any non-negligible amount if the concrete remains uncracked. As long as the foundations don't crack, it's not a problem. And if cracks are found quickly enough, they can be sealed.

 

Shavethewhales, I have to disagree with you on the use of epoxy-coated rebar. Most of the foundations I've seen (which, incidentally, are in the amusement industry) don't have any sort of protection on the rebar, and that's how I remember learning it. Protective coatings on rebar are generally used where there are other factors that may make rusting rebar worse, such as road salts, etc. Epoxy reduces the bonding between rebar and concrete, so it's not ideal to use.

 

Edit: It's worth noting that uncracked concrete provides sufficient rust protection because the alkalinity of concrete helps to maintain a passive protective layer around the rebar. Cracked concrete loses that protection (and also allows water intrusion).

[imagineerz]

Gotcha. Then I'm assuming there isn't much concern about the concrete's chloride content over time?

[Evan1127]

The American Concrete Institute limits the chloride ion content in concrete. A quick search led me to this discussion that helps to explain it.