The phrase "a concrete indication of" is correct and usable in written English.
It can be used when you want to specify a clear and definite sign or evidence of something.
Example: "The increase in sales is a concrete indication of the success of our marketing strategy."
Alternatives: "a clear sign of" or "a definitive proof of".
"This violation is a concrete indication of Russia's acts aiming to escalate problems, despite the clear warning by our country and by Nato," Turkey's foreign ministry said.
"It's a concrete indication of a real commitment on the part of the administration," said Dana Gunders, staff scientist at advocacy group Natural Resources Defense Council and author of the Waste-Free Kitchen Handbook: "The goal is very aggressive – it could have been 10% or 30%, so they really went all the way by committing to halving the amount of food waste produced".
This can be a concrete indication of a good reputation.
Turkey strongly condemned the incident as a concrete indication of escalating hostilities between the two nations.
"The wall was a concrete indication of a new reality," said US Secretary of State John Kerry on Saturday.
Using this metric gave a concrete indication of the number of controls required to yield significant results.
The doctor gives a concrete indication to the seriousness, the expected duration of the complaint and the course, with or without treatment.
Unable to reach the crew, the intruders then left after an estimated 50 minutes without taking anyone or anything, and without giving any concrete indication of why they had come.
"Ireland is arguably the country in the world most scarred by abuse, and if he comes here and doesn't offer any concrete indication of how accountability will be delivered at long last, I think there will be an eruption.
Although Mrs. Whitman has not given any concrete indication of her plans beyond the consulting business, she has the built-in advantage of statewide name recognition and a popularity, at least in New Jersey, that has more or less held steady through her worst travails.
