Concrete chinney Issue

[Vtktm50]

Hi Folks, First post here. I have a concrete chimey with a clay flue I built approx 4 yrs ago. I have a bad crack on the top block mortor joint. I have tried to mortor it but keeps cracking and shifting at winter time. I live in Vt and chimney is used for a wood stove. My guess is frost is heaving on the flue. I have a rain cap on it as well. All other blocks and joints look fine. Any repair suggestions would be great, thanks.

[HayZee518]

Scrap out loose mortar and press in some hydraulic cement. Hydraulic cement expands as it cures. what has happened is that water gets into a crack and expands when it freezes.

[Vtktm50]

Chimney Issue

Thanks HeyZee,

I will agree on the water and freeze possibility. I had an old mason tell me to grease the flue before I filled in the cap. Never heard of that myself. I will give the hydro cement a try.

[jamaicajoe]

chimney

the top flue liner needs to be able to move
put some type of expansion material around the flue so it doesnt make ci=ontact with the cement cap