5x, not exactly orders of magnitude.
According to some sources1), Nabataea traders in regions of what is Syria and Jordan today used concrete to make concrete floors, housing structures, and underground cisterns as early as 6500 BCE. Allowing for the usual delay contractors have been racking up since the dawn of men I’d round up the total to ~9000 years of using some sort of concrete in construction; close enough.2)
1) I must admit that so far I’ve been mostly looking at Roman use of concrete and have stumbled across the Nabataeas only recently. The sources I’ve come across so far are a bit thin as a lot of them seem to simply copy each other without giving further details. One more item on the list of the things I’ll have to look into when I find the time (i.e. when I’m retired, probably).
2) Yes, this is how civil engineers do maths. The square root of 50 is 7. Because 7x7=49, and 49 is basically 50. As long as you do this in a way that the errors result in giving you an extra margin of safety you’re fine.
