I have no idea how long they let the concrete cure. The rule of thumb is for one month, 30 days, and the way you let it cure also affects the hardness. Keeping concrete moist/wet while it is curing makes it much harder. I learned that when I was a kid way before I was actually trained/certified to do that kind of stuff as my Dad was an engineer and an architect and I helped him pour footings, driveways, and such for or family and ourselves.
Reading through all the different threads here on FR it seems there was no re-bar in the concrete and that just seems really dumb. no matter how hard concrete becomes it just cannot take loads without some kind of support, reinforcement, or both. I know it was tensioned but, as we can see, that didn't do too well.
Why the builder thought it was an OK idea to put that part of the bridge up either without the cable stays or temporarily shoring it where the road median was I have no clue.
If rebar was used we should see some twisted ends sticking out of the debris. But photos have not been focused in enough out of respect, while people are still being recovered. Their bodies anyways.