Well let me jump in with both feet and splain a bit about concrete. By no means a expert but a wee bit of it under my belt.............................
"Concrete professionals" are people who make a living with concrete, experts nope just folks out to make a buck. If it work great if not a million excuses why not. Those stating "it will crack" are the ones who do not fully understand what happens.
Batch design, proper curing, consolidation and proper reinforcement are all keys and all need to be in the upper 85% for it to work as designed.
Most "concrete professionals" and I shall use that term loosely, have no clue as they were taught one way and never change. Most get cracks in everything they pour..........why? There lazy and like to add water. Water makes it easier to work but the more water the more it shrinks during the hydration process causing cracks.! Now you can offset that with chemicals which you will use less water but the mud will be more liquid and more (easier) workable. The trade off is it cost a bit more so it get into the profit margin so it not used unless it specifically called for.
Next is curing, I have 14 day water cured all of my pours including the gunite when sprayed. Why? it cures slower and causes no or at the very most hairline cracks. I have NO cracks on any of my structures.....................................
Design, more sack mix is better than less. 4 or 4.5 sack mix is to weak to use for ponds as the water to cement ratio is almost always compromised. I personally use mimum 6.5 sack mix and prefer 7 sack straight cement NO FLYASH!!!!!!!!!!!!!!!!!!
Reinforcement, got to have it and it must be the proper size and spacing. If your gessing go with more not less. Concrete professionals will tell you one thing and a structures engineer will tell you something else, why? The structures engineer puts his stamp on the line ever time he designs something, and concrete guy does not! Make sure its centered not layibg on the ground or against a wall.............................................. .....
Something to ponder: concrete does give and take, ie flex, move and shake, bounce and or vibrate..........ever sit at a red light and your vehicle is on a bridge???? Ever feel the movement???