About 25 years ago when I was first taught masonry by an older guy he explained why chimney caps would crack, and how to build them better.
The chimney cap, that structure that overhangs the bricks at the top of many chimneys, with the ceramic flue sticking out for your fireplace or furnace, is made of concrete.
What happens is a battle between the slightly different rates of thermal expansion of the clay bricks, the soft mortar between them, the hard concrete cap, and the ceramic flue. Something finally gives, and sometimes it’s the cap.
Why? Because many masons make the cap in place using a wood form, pouring the concrete right on to the bricks so it melds with them, and the flue. So all four of these different materials are connected, and that’s the battle ground.
His solution was inexpensive, fast, and simple. He would put a layer of 6 mil plastic on the bricks, under the edges of the wood form, and then pour the concrete. Effectively providing a bond break so that the bricks and mortar could live without a fused connection to the concrete cap and the flue. No more battle, for a more enduring cap.
In 2014 the Ontario Building Code finally caught up with him, and now requires a bond break to be installed between concrete caps and brick or stone chimneys. My clients were better served by this wisdom learned long ago.