So to quote a commenter by the name of vigilant fish...

"Concrete armour should have more health than light, but very slightly less than heavy armour with A LOT MORE weight to it. As it is, there is no point to concrete, with even less health than light armour and so much gravel needed (one block needs 2.500kgs of gravel, are you ♥♥♥♥♥♥♥♥ me) that it is just impractical compared to anything else. It's obsolote."

Personally I don't mind the cost, but make it so that the concrete is healthier, more bulky, while it shouldn't be as tough as heavy armor, its still bloody stone you know, give it the weight and toughness to boot.