Reading (spying on) what T-Rex has been writing while I'm out of town, I've come to realize that nobody understands anything about engineering. The contractor who was arguing with me about following the procedure certainly doesn't understand, and its clear that T-Rex doesn't understand either. So, without further setup, here is how post-tensioned concrete works:
First you form up the shape of the thing that you are going to make out of concrete. In this current case, its a 5 foot tall, 13 foot diameter circle. Next you put the required rebar into in. In this case, its #8 bars (1" diameter rebar). You put in ducts or sleeved post-tensioning strands as well. Then you pour the concrete. This is where the waiting game comes into play. When you pour the concrete, you take samples. You test these samples at various times to try to find out when the concrete gains enough strength to tension the rods. In this case, the specifications said that the concrete should obtain a strength of at least 6000 psi in 28 days. It also said that the concrete should obtain a strength of 5000 psi before you tension the rods. They screwed up the mix big time, so it will only obtain maybe 4500 psi in 28 days, meaning that it will probably never get to 5000 psi. At 5 days, the concrete had only obtained 3600 psi. So, we went back to the engineer who did the calcs, and asked, "what is the bare minimum you NEED, not how much you WANT". He said, the bare minimum he could accept is 4500 psi (OK) and the minimum we need for tensioning is 3000 psi.
Basically, as soon as we heard that, we realize that we could tension as soon as we could mobilize out there to do so. So, it was first available plane out there to tension the rods.
Tensioning the rods is VERY critical to the overall strength of the final product, so they needed an engineer's representative to be on-site to make sure they followed procedure. The tension on each rod (rock anchor in this case) is 240,000 pounds for this project. That is a lot of force on each rod.
The procedure for tensioning is very specific. The first rod should undergo the following: Tension to 45,000 pounds, measure the elongation, release to zero pounds, measure any residual elongation, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, release to zero pounds, measure any residual elongation, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, tension to 135,000 pounds, measure the elongation, release to zero pounds, measure any residual elongation, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, tension to 135,000 pounds, measure the elongation, tension to 180,000 pounds, measure the elongation, release to zero pounds, measure any residual elongation, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, tension to 135,000 pounds, measure the elongation, tension to 180,000 pounds, measure the elongation, tension to 217,000 pounds, measure the elongation, release to zero pounds, measure any residual elongation, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, tension to 135,000 pounds, measure the elongation, tension to 180,000 pounds, measure the elongation, tension to 217,000 pounds, measure the elongation, tension to 240,000 pounds, measure the elongation, hold for 10 minutes, measuring the elongation every minute. If there is no additional elongation after ten minutes, lock off the load.
For every additional rod, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, tension to 135,000 pounds, measure the elongation, tension to 180,000 pounds, measure the elongation, tension to 217,000 pounds, measure the elongation, release to zero pounds, measure any residual elongation, tension to 45,000 pounds, measure the elongation, tension to 90,000 pounds, measure the elongation, tension to 135,000 pounds, measure the elongation, tension to 180,000 pounds, measure the elongation, tension to 217,000 pounds, measure the elongation, tension to 240,000 pounds, measure the elongation, hold for 10 minutes, measuring the elongation every minute. If there is no additional elongation after ten minutes, lock off the load.
The first rod then takes about an hour. The process of setting up the jack on each rod, then going through the loading, measuring, and locking off process on each additional rod takes more than 20 minutes per rod. The jack alone weighs something like 150 pounds, and is not the easiest thing to move.
Anyway, it takes time. Its not "waiting for concrete to cure", its tensioning rods. If I were waiting for concrete to cure, I would be there for days. Also if the concrete isn't strong enough, putting these kinds of loads onto the concrete would split it. That would be very dangerous to anyone within, I don't know... half a mile? When a one and three quarters inch rod that is holding 240,000 pounds lets go, lets just say that you don't want to be in the same zip code.
That isn't the only danger involved. There is also the crane over your head that they use to move the stupid jack around. There is also the danger of dropping one of the 50 pound wrenches on your toe. Ever seen a one and three quarter inch rod's nut? It ain't small. Not to mention flying around on dirt runways in a Cessna Caravan.
Well, enough ranting. Everyone should understand the basics of prestressing, and trust your engineers. And pay them more money. And pay them on time (GDM Architects, I'm looking in your direction).