[KTL]

Concrete placing is definitely not as easy as it looks. Not necessarily hard, just some experience is helpful.

Super is correct about how concrete sets-up. It's not the drying, it's the reaction going on with the portland cement (if that's the type of cement in the mix). You can place concrete underwater and it will cure and fully harden- i've found "lab samples" (concrete truck washout and excess dumping) in stormwater ponds and sewers to prove it! Concrete is kind of a living thing in that it's constantly curing, LONG after it's been placed

Point being is if you're not mixing consistently, you can end up with a crumbly mix (too little water to thoroughly complete the reaction) or a mix that takes long to cure (not necessarily a bad thing). That's where experience comes in, to judge how your mix is behaving and add water or admixtures to steer it in the desired direction. Bottom line, concrete mix is a science in and of itself. However less critical applications like curbs, sidewalks, general flatwork, etc. are not a big deal due to the low liability. Bridges, dams, buildings, etc. are another story!

And that's just the mixing issues. Placing and finishing is a whole 'nother story. Working the mix too much weakens the curing process on account of it breaking the bonds created by the reaction, plus disturbance of the air content of the mix. People who finish the crap out of the placed concrete can sometimes pull too much of the paste to the surface and segregate the aggregate from the paste. Without that aggregate, you lose some strength and can invite surface deterioration- spalling.