Getting ready to pour the main floor next month on my new house. I remember one of you Radiant contractors out of the twin cities has done a lot of light weight concrete applications. I am looking for the right formula to give the concrete company for my radiant applications. Any suggestions or formulas for "Light Weight" concrete would be appreciated!! Thanks Hank P.S. Have used "Gypcrete" in the past and don't want to pay the premium expense!

Why not use plates and get a more responsive radiator?

Lightweight concrete has terrible heat diffusion and represents a significant loss in efficiency.

If this is over wood, then light weight = lower strength and lower strength means greater risk of cracks.

Posted By ICFHybrid on 22 May 2013 12:26 AM
Lightweight concrete has terrible heat diffusion and represents a significant loss in efficiency.

Very true, but how many Radiant contractors do you know that can even spell efficiency much less properly address it when doing a design/install?  Seems like many struggle with basic fluid dynamics and heat transfer principals.

Thanks for the responses, but I know there is a contractor with a very good reputation from the twin cities area that has poured a lot of light weight concrete over radiant tubes. My floor is already designed and built for the extra load of gypcrete or concrete & 75% of the floor space is going to be tile. I'm not a Radiant specialist (but have a HVAC specialist on the job) I am only a building contractor; but I had gypcrete in my last house I lived in for 11 years w/ 21 ft vaulted ceilings and liked the comfort. * I feel like the trades are just like everything else in life - "we all play to are strengths". I am spending money on the insulation and building envelope. It may be better to have gone a different route, but given my choice of equipment and lack of natural gas and fuel prices this is the direction I went. I was the 3rd in my co-op 12 years ago to install a full geo- heat pump and I would never do that again, but I did like the way the main floor with gypcrete performed.

Posted By WI HANK on 22 May 2013 06:17 PM
Thanks for the responses, but I know there is a contractor with a very good reputation from the twin cities area that has poured a lot of light weight concrete over radiant tubes. My floor is already designed and built for the extra load of gypcrete or concrete & 75% of the floor space is going to be tile. I'm not a Radiant specialist (but have a HVAC specialist on the job) I am only a building contractor; but I had gypcrete in my last house I lived in for 11 years w/ 21 ft vaulted ceilings and liked the comfort. * I feel like the trades are just like everything else in life - "we all play to are strengths". I am spending money on the insulation and building envelope. It may be better to have gone a different route, but given my choice of equipment and lack of natural gas and fuel prices this is the direction I went. I was the 3rd in my co-op 12 years ago to install a full geo- heat pump and I would never do that again, but I did like the way the main floor with gypcrete performed.

Like you say - there is lots of radiant in lite weight concrete all over north america. Is it the absolute best way? Maybe not but I think the difference is too small to worry about.

As a rule the concrete company will have worked out the specs for a mix that they know will work for what you need. Take a little time to talk to them, explain the project to them and see what they say. Most of the bigger concrete batching companies have full time engineers on staff to work these things out. If you tell them what you want, they will mix it but you will warrantee it. They have probably done this many times. If you are using a different pump operator then one supplied by the batching company, bring him into the conversation as well. Some of the mixes are based on porous materials that like to jamb up in the pump. Know what type of pump is being used helps the batch man in designing the mix.

There is no reason lite weight should be weaker or crack more than normal concrete and it will certainly be strong than gyp.

Did you put tile over gyp in the last house? Normally we would not do that. As the thinset dries, its greater strength will usually pull up the top skim of gyp.

Just poured 2,700 square feet. Have poured a lot over the decades.
This time we did something different, it was my personal building so no client, I am left to my own devices.
On the advice of my flat worker an concrete man (we also have worked together a lot over the years) I placed a 4 mill plastic sheet over the platform, then attached pipe.
This worked great to minimize cracks, 3 weeks later and no cracks.
I will use this as a shop space so the 1 1/2" cement will function as finished floor for the next few years.
I placed stove black in the cement truck, the floors are a good dark grey/black color.
I will tile in several over this, for now it is a temp floor and looks good.
The mix for a 2 inch line pump;
7 1/2 sack Portland cement
Pea gravel
fiber mesh if you are not concerned about small bumps (in this application I skipped it)
a little soap for slip.
Your pump/flat worker will know the slump he needs.
I put a sealer on the next day to slow drying time.

In the Seattle area we pay about 1.50 sq ft placed 1 1/2" concrete.
The plastic worked really well, building is dry and closed in. no moisture issues.
I have as said poured a lot of mud over the years, the plastic is a good trick.
I spray paint my pipe pattern to the plywood in orange. Basic guide dots. Then used clear 4 mil plastic, then stapled the pipe down over plastic.
So it went
Dan

Dan - do you happen to know the weight per cu. ft. of that mix? It doesn't sound like lite weight if it is pea gravel based. To get the weight down you would have to have some porous/expanded type of aggregate.

There is no reason lite weight should be weaker or crack more than normal concrete....To get the weight down you would have to have some porous/expanded type of aggregate.

That's why it's weaker.

Maybe not but I think the difference is too small to worry about.

9% here, 4% there, 7% over there; pretty soon it actually adds up to something.

but how many Radiant contractors do you know that can even spell efficiency much less properly address it

Those would be the ones who need even more details operating in their favor.

That's why it's weaker.

Why do cracks matter, again?

Gypcrete (the Maxxon Co. trademark) or Gypspan (the Hacker Co. trademark) is gypsum concrete. While gypsum concrete doesn't have the heat transfer efficiency of slab-on-grade, it is still significantly better than plate above-floor and below-floor assemblies. Gypsum concrete is commonly and successfully used for 1.5" thin slab hydronic radiant floor heating.

Gypsum concrete has a density of about 115 lbs per cubic foot. Slab-on-grade concrete has a density of about 150 lbs per cubic foot. So gypsum concrete may be considered by some to be lightweight concrete as compared slab-on-grade concrete. However, competent contractors do not consider concrete to be true lightweight concrete until the density is about 30 lbs per cubic foot. This low a density is achieved by using special lightweight aggregates such as vermiculite or polystyrene beads in lieu of crushed stone. Unfortunately, true lightweight concrete has a R-value about 16 times higher than slab-on-grade concrete and about 8 times higher than gypsum concrete, and it should NOT be used for hydronic radiant floor heating projects...at least if one has any understanding of heat transfer efficiency and considers it important.

Like any other concrete, undesired gypsum concrete cracking can be avoided by using water reducers, fiber mesh, crack control methods, and proper hydration time (sealer). And like everything else, if you know what you doing, you will have a very successful outcome.

"However, competent contractors do not consider concrete to be true lightweight concrete until the density is about 30 lbs per cubic foot."

I think you meant to say 30 lb lighter then regular concrete!

"The American Concrete Institute (ACI) defines Structural Lightweight Concrete (SLC) as concrete with a minimum compressive strength of 2,500 PSI (17 MPA) and an equilibrium density of between 70 and 120 pounds per cubic foot (1,120-1,920 kg/m3). It consists of entirely lightweight aggregate, or a combination of lightweight and normal-density aggregates."

Please note the compressive strength!!

Its always good to check facts before poaching outside your discipline (;=))

For those of you who WANT to know, heres a good read http://www.structuremag.org/article.aspx?articleID=605

No FBBP, I did indeed mean 30 lbs per cubic foot and NOT "30 lb lighter then regular concrete" as you indicated. The ACI definition you quoted is applicable to gypsum concrete. We often use true lightweight concrete (with densities as low as 25 lbs per cubic foot) in our large condo or office building construction projects for increased fire resistance and decreased sound transmission. It is a great concrete product for this application, but it should NOT be used for hydronic radiant floor heating projects...because you will have about a 22% heat transfer efficiency loss as compared to using regular concrete. If you also doubt the R-values I stated, please consult Sieg's Modern Hydronic Heating, Figure 10-45. So perhaps you should consider taking your own advice...

If I understand the weight correctly.... If
1 1/2"x 12x 12 Gyp creat is about 15 lbs sq. ft
1 1/2" x 12 x 12 concrete 7 1/2 sack mix pea gravel is about 17 lbs sq ft
I am not an engineer and I am super thank full for spell check, but these are the numbers I know to be about right.
I am sure these numbers will be corrected if wrong.....
I suggest you confirm by checking the weight of a yard of dry concrete, back it down to cubic inches.
You will be in the hunt.
The only difference between concrete and light weight as I understand (Big if ) is that it is air infused at the pour, as we do in the field at pour, this will lighten it some what.
Dan

Posted By sailawayrb on 23 May 2013 12:50 AM
No FBBP, I did indeed mean 30 lbs per cubic foot and NOT "30 lb lighter then regular concrete" as you indicated. The ACI definition you quoted is applicable to gypsum concrete. We often use true lightweight concrete (with densities as low as 25 lbs per cubic foot) in our large condo or office building construction projects for increased fire resistance and decreased sound transmission. It is a great concrete product for this application, but it should NOT be used for hydronic radiant floor heating projects...because you will have about a 22% heat transfer efficiency loss as compared to using regular concrete. If you also doubt the R-values I stated, please consult Sieg's Modern Hydronic Heating, Figure 10-45. So perhaps you should consider taking your own advice...

Sailor - you are right that the product you are talking about is use for thermal protection and sound deadening. It is also use for flower pots and planters but never in the hydronic floor industry. I'm not sure it would even stand up to foot traffic.

What is used by competent floor contractors is STRUCTURAL light weight concrete as define by ACI and defined in the article I referenced in my last post.

No, it is not the definition of gypsum concrete. Gypcrete interestingly is made with gypsum while light weight concrete is made with portland, two very different products. One with high strengths and water resistance, the other not so much.

The best thing about gypcrete is that if someone damages the tubing all you have to do is pour a puddle of water on it and wait about 20 minutes. After that you can use a spatula to scrape away the gyp and make the repair. Gyp should never be used in moist environments or where there is a chance of water spillage. It should not be used under products with a higher tensile strength. It can not be patch with high strength patching compounds such as Ardex's feather fill.

Some of the more astute gyp contractors will mix up to a bag of portland to three bags of gyp and the sand to produce a stronger, more water resistant product that still maintains some of gyp's self levelling, self finishing properties.

Dan, the difference between 1.5" thick 15 lbs sq ft (120 lbs per cubic foot) Gypsum concrete and 4 lbs sq ft (30 lbs per cubic foot) true lightweight concrete is that the aggregate in Gypsum concrete is still crushed stone (Gypsum) and the aggregate in true lightweight concrete is vermiculite or polystyrene beads. Regular concrete would be 18 lbs sq ft (150 lbs per cubic foot), but we don't use regular concrete for 1.5" thin slab hydronic radiant floor heating...we would recommend Gypsum concrete for this application.

Posted By Blueridgecompany.com on 23 May 2013 01:31 AM
If I understand the weight correctly.... If
1 1/2"x 12x 12 Gyp creat is about 15 lbs sq. ft
1 1/2" x 12 x 12 concrete 7 1/2 sack mix pea gravel is about 17 lbs sq ft
I am not an engineer and I am super thank full for spell check, but these are the numbers I know to be about right.
I am sure these numbers will be corrected if wrong.....
I suggest you confirm by checking the weight of a yard of dry concrete, back it down to cubic inches.
You will be in the hunt.
The only difference between concrete and light weight as I understand (Big if ) is that it is air infused at the pour, as we do in the field at pour, this will lighten it some what.
Dan

Dan - you are right that air entrainment will lighten normal concrete marginally, however structural light weight is made up of aggregates other than gravel. (expanding clays, shales and slates as well as some volcanic products)