Is it feasible to fire-up the heat in the pex before I pour the floor, to keep the concrete from freezing?

NO. Concrete does not 'dry' but 'cure'. It is a chemical process and the longer it takes the stronger the finished product.

Naturally water would freeze if introduced to a sub-freezing slab, so we use an air test and keep the PEX under pressure until we are ready to commission a new radiant slab. If this must be done in sub-freezing weather we use another heat source to keep the slab warm for at least a week before the radiant floor is made operational.

Your radiant floor heating designer/contractor should have let you know these fundamentals.

Posted By BadgerBoilerMN on 05 Jan 2013 11:16 AM
NO. Concrete does not 'dry' but 'cure'. It is a chemical process and the longer it takes the stronger the finished product.

Naturally water would freeze if introduced to a sub-freezing slab, so we use an air test and keep the PEX under pressure until we are ready to commission a new radiant slab. If this must be done in sub-freezing weather we use another heat source to keep the slab warm for at least a week before the radiant floor is made operational.

Your radiant floor heating designer/contractor should have let you know these fundamentals.

I know those things. It is not feasible to heat the garage. I would not want to add so much heat that it would adversely affect the hydration of the concrete. Just keep it in the 50s. Also we will put glycol in the system.

Consider just putting insulation over the concrete and letting the curing process keep it warm.

My concern is the modelling caused by the blankets, and it gets cold in MN.

Then you would not want to add heat. You can cover it as Jon suggests.

Here in Minnesota we typically tent and heat or wait until spring to charge the system. I do it for a living and ended up doing both on my own project a couple years ago. But that is another story...

Me too, my concrete construction company would not take a chance on a customers floor. But for my own, I end up taking chances I would not take on a customers job. I am checking out my options. We prepared the floor in August, but did not get to it and might this month.

As long as you keep the actual slab in the 50s during the cure, you will be fine. There are several ways to accomplish this, but running heated hydronic is not one of them. The first 24 hours is the most critical, but we always provide protection for the first 72 hours even when using an accelerant admixture. Here's a good reference:

Cold Weather Concreting

Some concrete (like dams) use embedded pipes for cooling while curing. I'm curious why using similar pipes for warming is not a good idea (ie, exactly what bad things will happen).

Yes, concrete curing is an exothermic chemical reaction...i.e., heat is generated during the curing process. For large pours like dams, you need to design cooling circuits to prevent having large temp gradients within the structure during the curing process. Temp gradients between the core center and surface will cause cracks to develop. These cracks can also serve as portals for chorides that will have an adverse effect on the reinforcing steel. So, you do not want to add heat to the center of a curing concrete slab...especially if the surface temp will be significantly cooler than the core center.

Well done sailor. Maybe there's hope for you yet.

I have no doubt that some amount of heat can be safely added to curing concrete via embedded pipes. At what btu rate the temp differential within a 4" slab causes a problem is not clear. All curing concrete tolerates some differential.

Correct. Heat can be added, particularly when the slab is losing it as quickly as it does in cold weather. There are chemical additives used that chemically increase the heat of the slab and every time a slab is poured outside it is getting solar input.

The difficulty is finding a feed temperature that will keep the slab warm enough, but not too warm. 60F or 70F would probably be safe enough to start with and you would want to monitor both entering and leaving temperatures.

I looked it up and 35F is a common value (although this is oversimplified and it could exceed 60F). So accounting for the R value of up to 8" of concrete (be conservative, ignore the PEX) and the R value of the air film (assume outdoor and exposed to some wind), I come up with +40F for the safe temperature of the coolant over the ambient temp. So it confirms - pour above freezing or use a tent or insulation.

I think that is the coolant temperature when you want to remove heat from large pours.

I wasn't clear, 35F is the allowed differential within the concrete. So your 60F-70F is a good call (to warm curing concrete, assuming ambient is around 30F). The problem is that with no blanket, the concrete furthest from the PEX may go almost to ambient, so you shouldn't pour at much below 32F, even with 65F circulating liquid.

It is simply a mistake to drive water at any temperature through a slab-on-grade radiant panel. The risk i.e. cracks following the tube pattern will be your first indicator of this common mistake. No speculation is worth the risk.

We use radiant tubes all the time for concrete curing. The key is in monitoring the temperature and having some sense of proper cure temperature ranges. Blankets should be applied as soon as possible, but, you may at times be able to wait until the surface is hard enough to not be scarred by the blanket. We use a flir thermal imaging camera to monitor slab temperature

There are a number of package kits on the market including names like whacker and ground thaw that have units that will thaw ground, cure concrete and even monitor temperature for you. They can be seen working in youtube

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