I worked in concrete repair for half a year, I mostly only used the jack hammer and angle grinder but I paid some attention.
Here is what I think I know:
When rebar corrodes it expands and that means even more air, water and salt can reach even further into the construction, meaning even more rebar is exposed and starts corroding. Edit: this probably weakens the concrete as well both because of exposure to the atmosphere and because of the cracking action but this last one is just an educated guess from my side.
How we worked was our foreman went first with an ultrasound device to detect the rebar and how much cover it had. He'd then sample the concrete by cutting into it, knocking out one side of the cut using a hammer and spray indicator fluid (I guess fenolftalein) on it. If the concrete turned pink it was healthy. You could measure how far out it was healthy.
If the measurements rebar was out in the damaged concrete we would expose it and apply protection before closing the wound. Sometimes we would also use a sledgehammer to adjust the rebar somewhat.
A number of factors influence how long concrete lasts before it needs maintenance:
- How far rebar is buried.
- The quality of the concrete as it leaves the mixer
- If the driver had lunch break or got stuck in traffic after loading the truck and added water to prevent anyone from noticing. Concrete starts curing immediately after mixing so you have limited time before it needs to be in the mould
- How well the concrete was vibrated to make sure it filled all gaps and that no significant air pockets was left
- How close you are to the sea
There's always someone on HN who knows something about everything and hopefully this was one of my turns to satisfy some of the curiosity for someone :-D
Edit: working with those older guys was really interesting. Seing a 70-year old bloke throw around tower lift parts that I figured I didn't have a chance of lifting. All the stories. The time I played with a bunch of cards and one of them askes to borrow it and started showing off crazy tricks; turned out he had a bronze medal from Nordic magic for juniors (and since he was now close to retirement and his hands were large and swollen I also got a chance to see what was really happening :-)
It was also the first place I know that actually practiced HSE. Others laughed but this team had 1-2% extra profit margin and I remember we got a nice bonus too. Ended up at around $15/hr which was close to twice what I used to get for much harder and dirtier work.
This was the spring of 2001. They liked me so when I left for school I got a nice job cleaning the work sites.