I have not done much with concrete, but I have made lots of things out of many other materials over the years, and in general I have found that making things - and making them come out the way you want them - is never as simple as it looks. "Reality has a surprising amount of detail", it was once wisely written:

http://johnsalvatier.org/blog/2017/reality-has-a-surprising-...

This project is not to my personal taste but I respect the work which went in to it, and I'm glad its creator got what he wanted.