The most common and most likely answer to this crossword clue is the 6 letter word GUNITE
G
U
N
I
T
E
Buddy explains!
The straight part
We know that "Concrete" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concrete = gunite.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GUNITE mean?
GUNITE noun- A form of shotcrete in which a dry cementitious mixture is blown through a hose to the nozzle, with water injected only at the point of application.