Crack in hearth slab

[paulkjrobbins]

Hello All,
I was hoping to get some advice. I just poured my hearth slab for my 42" pompei build. I did it a little bigger and thicker (5") than the FB plans suggested and added a half cinderblock pier in the middle of my cinderblock walls. I also cemented in all my walls and put rebar all the way up from the foundation slab to connect to the hearth slab every other cinderblock hole. The front opening is supported with two 1/4" x2" pieces of angle iron. The hearth slab has an 11" oc grid of 1/2" rebar in it. My hearth was poured a little under a week ago. We did at 5am since the days are getting really hot here (100 degrees +). The hearth developed a hairline crack down the center of it from side to side after a couple of hours of curing. in the heat. This was despite putting a shade cover over it and dowsing it with curing and sealing compound as soon as I finished floating it. At this point I don't have a way of knowing if it is a surface crack or goes deeper. I formed it with hardiboard so I can't look underneath after I remove the forms. I was thinking of adding on to the center pier, tangential to the crack, to add stability when the oven weight is put on.
I am thinking of proceeding with the build even though this crack is present. Any thoughts? I will upload a picture later.

[david s]

Shrinkage cracking is common if the slab goes off really fast in hot weather. Another problem that exacerbates the situation is the mix being too wet. I always do the pours for my precast concrete slabs in the late afternoon so the concrete won’t early cure too quickly, as I’ve learned that casting in the morning is more likely to give shrinkage cracking (we live in the tropics). As you have plenty of steel in the slab you’ll be fine.