A few questions about a pour-in-place concrete crown

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

Richon

New Member
Aug 9, 2022
3
Atlanta, Georgia
I am getting ready to pour a new crown for my brick chimney and I have an assortment of questions I would appreciate some guidance on.

The interior of the chimney chase is filled with loose brick to a level that is mostly even with the top course of chimney bricks. I plan to use some of the smaller concrete chips and rubble from the old crown as additional fill where needed. I will use sill seal foam to create a bond break/ expansion gap around the flue liner. The new crown will measure 55"x 34" (extending 2.5"beynd the brick chimney in each direction), will be sloped for good drainage, and will have be 3" thick at the outside edges. I'm planning to use Quikrete Commercial Grade Crack Resistant Concrete Mix based on guidance from Quikrete's support desk. I've made my concrete forms using 2x4 and 2x6 pine lumber and incorporated a piece of quarter round to create a drip-edge when the concrete is poured.

I live in metro Atlanta, Georgia and it is August - which means the weather is hot and humid. Recently, daily temps start out in the mid 70s and climb to a high in the mid 90s temp usually breaks 90 degrees around 1:00 pm. Humidity levels start the day at 99% and the daily average humidity is 85%. There is a chance of a Thunderstorm pretty much every day.

The good news is that they are calling for a break in the weather this weekend, with daytime highs only in the mid 80s and less than 50% chance of rain. I plan to take advantage of the cooler temps and do my pour this weekend. I will start mixing and pouring concrete around 8:00 a.m. to take advantage of cooler morning temperatures.

My Questions:
  • Bond Break Material: What material should I use as a bond break between the brick and the new concrete? I've got some plastic but is a layer of 6 mil plastic enough? I also thought about using aluminum flashing typically used in roofing. Another suggestion I saw was to use cement board. Hardee makes a waterproof cement board but I don't know how well that would hold up in this situation - or how waterproof it would be after I cut it to size.
  • Filling the gaps: There are a few places where there is a small gap of 1/8”to ¼” between the edge of the brick and the forms for the concrete. What should I use to fill these gaps so that my concrete mix doesn’t leak down the sides of my chimney?
  • Release Agent for Forms: What should I apply to my forms so that the forms come off cleanly and that will not stain/discolor the concrete? I've seen WD-40 used. I’ve heard cooking oil works but some types of cooking oil may cause discoloration in your concrete. A You-Tube video said to use motor oil. I guess a clean synthetic might work without staining but I’m not sure about traditional motor oil. What do you recommend?
  • Concrete Finishing: Do I need to edge the crown as part of my finishing work? If yes, what tool should I use? The edging tools I have seen at the local big box store all have a 90 degree bend which I can't use with the slope my crown will have. I've tried to think of something I can make or modify that would work but I've not come up with a solution yet. Maybe try to round the edges with my margin trowel?
  • Curing: I've been worried about proper curing given the temperatures I am working in. I am thinking about using a Quikrete product called Quikrete Acrylic Cure & Seal - Satin Finish. I spoke with Quikrete Support and they tell me that you can apply this stuff after the bleed water evaporates, and not have to think about curing. It sounds too good to be true! As an alternative to the Cure & Seal product, I have some plastic so I could lay plastic over the crown and cinch it tight around the chimney. I also have some burlap I could use to hold moisture to the surface but I worry I wouldn't be able to keep the burlap wet enough if the temps shoot back up during the cure time. Does anyone know if the curing products live up to their claims? What approach to curing would you use? Whatever cure method I use, I will put a canopy over the concrete to protect it from any rainstorms we might get.
My experience working with concrete is limited so I'm a bit nervous about the project. However, I've got more time than money these days so I decided to take this on as a DIY project. I'm doing it now in the heat of summer because I had some wind damage to my roof, so with a new roof in my immediate future, I want to get the chimney work done before getting the new roof installed.

Sorry for the long posting. If you need more information from me, please ask. I appreciate any guidance I receive.

Richard
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
29,146
central pa
6 mil plastic is fine.
Run the plastic out over onto the forms to seal it up.
I use wd40 on our metal forms and it is fine.
I always just wrap the whole top of the chimney in plastic to help it cure properly.
And yes a standard edger should work fine
 
  • Like
Reactions: FixerUpper

Richon

New Member
Aug 9, 2022
3
Atlanta, Georgia
6 mil plastic is fine.
Run the plastic out over onto the forms to seal it up.
I use wd40 on our metal forms and it is fine.
I always just wrap the whole top of the chimney in plastic to help it cure properly.
And yes a standard edger should work fine
Thanks Bholler for the quick response and concise answers to my questions. I had read the advise you provided in several other postings before I posted my list of questions and I was hoping you'd weigh in on my post.

I went back and looked at the standard edger again and see that it will indeed work fine. Originally I was thinking that both wings of the edging tool were about the same length from the curved corner of the tool if that makes sense.

Three follow-up questions related to curing:

1. Once I have poured the crown, I let the bleed water evaporate from the surface, and then wrap in the plastic, correct?
2. My online research says I should I unwrap the crown and moisten the surface once a day for 7 days and can remove plastic wrap after day 7. Is this correct? Would moistening more than once a day beneficial?
3. How long should I leave my forms in place?

The weather is looking much improved for me to get this done. I plan to pour tomorrow and the high is forecasted high is 87. After tomorrow we've got at least 4 days of slightly cooler (low to mid 80s for daily high temps) and significantly less humid air.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
29,146
central pa
Thanks Bholler for the quick response and concise answers to my questions. I had read the advise you provided in several other postings before I posted my list of questions and I was hoping you'd weigh in on my post.

I went back and looked at the standard edger again and see that it will indeed work fine. Originally I was thinking that both wings of the edging tool were about the same length from the curved corner of the tool if that makes sense.

Three follow-up questions related to curing:

1. Once I have poured the crown, I let the bleed water evaporate from the surface, and then wrap in the plastic, correct?
2. My online research says I should I unwrap the crown and moisten the surface once a day for 7 days and can remove plastic wrap after day 7. Is this correct? Would moistening more than once a day beneficial?
3. How long should I leave my forms in place?

The weather is looking much improved for me to get this done. I plan to pour tomorrow and the high is forecasted high is 87. After tomorrow we've got at least 4 days of slightly cooler (low to mid 80s for daily high temps) and significantly less humid air.
1. I wrap right after it sets up enough that I can finish it.
2. I never wet it afterwards but it should increase strength slightly if you do .
3. I have pulled them as little as 12 hours later. But that's risky