The word concrete causes lots of problems, and sometimes can be the source of unintentional humour. Concrete in its most common form is the building material that you use for roads and houses – in Czech, beton. So, if you say, “There’s one concrete lady at my company who I really like,” it is a very strange and funny sentence. Instead, you need to use the word, ‘specific’, or ‘particular’. So, correct is: “There’s one specific lady at my company who I really like.”
We do, however, use the word ‘concrete’ to mean something that is ‘fixed’, i.e. for a plan that has firmly been decided or an idea that is solid. For example, you can say that you don’t have any concrete plans for the weekend, meaning that nothing has been arranged 100%. Or at a business meeting, the boss could say, “I need some concrete marketing ideas!”, meaning some ideas that are useable or solid, as we say. Here’s an example of concrete and specific used together:
A: Do you have any concrete plans for the summer?
B: No concrete plans, but there’s a specific town in Moravia that I’d like to go to.