Hmmmm concrete n the Spanish times. When I was on the Gloria Pan campaign, I saw a square set aqueduct coming from a side arroyo. It was an Aqueduct aprox. 1 1/2 meter wide with a 'square' raceway of perhaps two ft. One must excuse my vagueness, but it was about 60 years ago.
It was exposed for perhaps 30 ft then ended where it encountered the river. The interesting thing was where it showed in the arroyo it was covered with 6 ft of coarse overburden, yet the exposed area looked as if it had been just made, it wasn't chipped or broken ??
Later a gentleman in Alamos, who claimed to be a structural engr,. told me that he was an expert in cements, so I mentioned the little aqueduct and it's almost prefect condition. He said ""It's entirely possible, we still don't know exactly how to make consistent concrete batches, except while we are using known, calibrated, materials that we can test..
Every source materiel is slightly different, hence constant testing is necessary. The old Spaniards just might have hit upon a perfect comb. of materiel's to make superb concrete.
The early Spaniards , if they needed concrete, they constructed it on the spot. There are very few example available today, that can proven to be of the Spanish exploration era.
I'll never forget the sight of that aqueduct coming out form under several ft of arroyo over burden of river rock.
Incidentally, I never did find where it ended, but it was not part of the Gloria Pan.