Quick posting.
Concrete is a special chemical mix of gravel, sand, water and cement. Get the mix right and this material becomes incredibly strong but brittle. To combat this brittleness add in some strong steel rods and this gives something for the concrete to bond around.
Chemically all the ingredients slowly react with each other and glue together. Over hours the concrete becomes hard enough to walk upon, over weeks it starts to reach its full strength. After a month the concrete is about 85 percent and slowly cures for the next fifty years ! ! !
After 50 years the concrete slow starts to debond; remember humans live 100 years, so the versatility of concrete is it only has to used once in a lifetime generally. But after one hundred years concrete is almost worthless functionally.
What a role and job concrete performs for that 100 years : )