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How to make a large concrete block lighter?

I am planning on using a 12" H x 18" D x 20' L concrete block as the Hearth of a contemporary wood burning fireplace. The problem with using such a large unit of concrete is the weight of the block. I was thinking of using a high density Polyisocyanurate or Polyiso foam block in the center of the form to take some weight out of the Hearth. I am not sure going with route will cause me any problems down the line and would appreciate any comments you might have.

My goal is to make reduce the weight of the concrete block so any suggestions you have would be appreciated. ( I have read about using ground up plastic instead of rock but I have not found anyone in Salt Lake City using this method.)

Thank you

Asked by dennis levine
Posted Oct 24, 2011 8:48 PM ET
Edited Oct 25, 2011 5:39 AM ET


12 Answers

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You provide both the "height" and the "depth" of your proposed concrete hearth, which is confusing. It sounds like your proposed hearth is either 12 inches thick or 18 inches thick. Why?

I think that about 2.5 inches or 3 inches thickness is all you need. Include a little mesh or rebar if you want. But you don't need a hearth that is 12 or 18 inches thick.

Answered by Martin Holladay
Posted Oct 24, 2011 8:56 PM ET
Edited Oct 24, 2011 8:57 PM ET.


The hearth is 18" in depth and 12" H to meet the bottom of the firebox. In other words the hearth is a block of concrete 20' long, 12" in height fron floor to fire box and 18" in depth.

Answered by dennis levine
Posted Oct 24, 2011 9:29 PM ET


I agree wtih Martin unless you are trying to create a raised hearth or other condition where the height is an important factor. You also do not say whether this hearth will be on a supported floor or on grade. If on grade, the weight will make little difference other than the cost of concrete. If supported, you should make sure the supporting framing is adequate for whatever you end up placing. Also, concrete on wood framing, or even on grade, is very likely to crack, especially when it is this size. Therefore Martin's suggestion of reinforcing is a good one unless cracks are not critical to the finish design. Finally, forming on high density foam or polyisocyanuate is not needed for forming purposes as almost any foam will support 3" to 6" of concrete with capacity to spare. I've done passive homes where we put the entire weight of the house on high density foam to create a thermal break and I don't see how your condition would ever equal this kind of loading. Now if you happen to be looking for a sizeable thermal mass that is thermally isolated from exterior temperatures, we are into a whole new set of considerations.

Answered by Corian Johnston
Posted Oct 24, 2011 9:46 PM ET


Frame a box with steel studs and sheathe it with cement backerboard. Make the box 9 1/2 inches high. Then pour your hearth slab (2 1/2 inches thick) on top of the box.

Answered by Martin Holladay
Posted Oct 25, 2011 5:32 AM ET


It might be helpful to study how they do concrete countertops

Answered by Keith Gustafson
Posted Oct 25, 2011 10:22 AM ET


Would hypertufa also be acceptable for this application? I believe chimineas (chiminaeae?) withstand the heat without much trouble.

Answered by Mojave Disaster, 3B
Posted Oct 25, 2011 1:05 PM ET


I like Martin's suggestion for several reasons:

• It's the most straightforward way to make it far lighter, if you're worried about weight.

• It'll take much less concrete. As well as saving you money, concrete is an energy-intensive product which produces a great deal of CO2 in its manufacture. This is a green building forum after all. A 2 1/2" slab will provide plenty of thermal mass, if that's what you're after.

• A solid mass will be the devil to remove if you or a subsequent owner ever decide that it's the wrong thing in the wrong place. We do a lot of remodeling - you'd be surprised how many times what seemed a great idea at one time turned out to be old, ugly and in the way later on.

Answered by James Morgan
Posted Oct 25, 2011 5:36 PM ET


Thank you all for your suggestion.....I know concrete will crack, any issues with cracking with Martin's suggestion? Any way to reduce cracking problems?

Answered by dennis levine
Posted Oct 27, 2011 10:11 PM ET


There are two issues here:

1. You need the steel framing of the box to be strong enough to bear the weight of your hearth, and to be rigid enough not to wiggle.

2. Your concrete hearth will need to be reinforced with mesh, fibers, or rebar.

If you don't have a lot of experience placing thin concrete slabs, you can research concrete countertops on the Web, or hire a contractor with experience installing such slabs.

Answered by Martin Holladay
Posted Oct 28, 2011 3:58 AM ET


Martin, I'm building a concrete "stand" for my new heat pump. Because I will eventually be building a deck around the heat pump, I want to raise it to the height of the finished deck to save moving it later. The measurements of the concrete stand will be around 47" wide x 20" high x 20" deep, giving - I think - a total weight of around 1700lb! I have been told that I should try and lighten this by building an inner form and fill this with sand when I do the pour. Given that sand is at least 2/3 the weight of concrete could I not substitute, say, hollow PVC pipes (standing vertically, but sealed and covered at top and bottom by a 4" layer of concrete), or styrofoam or would these compromise the integrity of the concrete (the heat pump weighs around 350lb!)?

Answered by Paul Christie
Posted Sep 26, 2012 9:53 AM ET


Once you have poured your footing below frost level, you can built up your proposed support with a rectangle of concrete blocks (CMUs). Once you get within 4 inches of the top, you can pour a 4-inch-thich slab on top of the blocks.

Answered by Martin Holladay
Posted Sep 26, 2012 10:12 AM ET


Heat pumps can be set on metal or pressure treated stands direct on the ground. Forget the frost footer and concrete. The hearth, frame up and tile like a hot tub enclosure. If mass is desired, then start with concrete at the footer and standard chimney work, stack blocks.

Answered by aj builder, Upstate NY Zone 6a
Posted Sep 26, 2012 1:35 PM ET
Edited Sep 26, 2012 2:05 PM ET.

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