The words “concrete” and “particular” get commonly confused by English learners. Of course, we are not talking about “concrete” in the sense of “made of concrete” (the building material), as in “a concrete wall”. We are talking about “concrete” as in “a concrete example”.
Statistically speaking, if you are reading this, the word you are probably looking for is “particular”, which is about 10x as common as “concrete” in English. It is used to emphasize that you are talking about just a single individual or thing, for example:
Have you already agreed on a particular date? (correct)Have you already agreed on a concrete date? (wrong)I couldn’t find the particular book you were talking about. (correct)I couldn’t find the concrete book you were talking about. (probably wrong)
“Concrete”, being much less common than “particular”, is most commonly used in the sense of “tangible, real” when talking about evidence and proofs:
They had just a suspicion; no concrete evidence was found. (correct)
They had just a suspicion; no particular evidence was found. (unnatural)
or in the sense of “not abstract, specific” when talking about examples:
Your example is very abstract; could you make it a bit more concrete? (correct)
Your example is very abstract; could you make it a bit more particular? (unnatural)