cracked floor slab with in-floor pex

CanadaKeith

I am a home inspector in the Toronto area. My question is about tolerance of pex in-floor piping to cracking and slight movement of the basement floor slab. Recently I came a cross a basement in-floor radiant system, and there was cracking of the concrete slab and some hollow sounding areas of the slab. This is pretty typical of the poor quality basement slabs here, so it normally is not a big concern, but I was wondering at what point does the cracking and possible slight movement put the pex at risk of damage? The house was about 10 years old. Crack width was perhaps 2-3 mm at the top surface.

hot_rod

most concrete cracks, as long as the slab doesn't move at the crack you should be okay. Parser test would assure you have tube integrity

As far as hollow sounds, could be the sub-grade wasn't prepared properly, or you have ground water flowing below the slab, washing away the gravel below?

Mark Eatherton

I've seen slabs with 1/4" vertical differential that didn't pinch or restrict flow. I've also seen 5/8" PEX that was stretched apart on a 300 foot side walks (snowmelt). The slab moved horizontally creating about 3/4" gap and literally stretched the tubing in two. There is a metal access plate there now, along with numerous repair couplings. I call it the continental drift theory...

ME

CanadaKeith

Thanks Gents I really appreciate the responses. The warranty spec here (Tarion warranty) for new construction allows a 4mm wide crack in a floor slab and builders are not in the habit of installing expansion joints ( or rebar) . I am not aware of any tighter spec if PEX is installed . I am looking thru PEX manufactures installation specs but have yet to find reference to any specific concrete quality recommendations or reference to allowable movement. Good to know the PEX can stretch a bit

Canucker

What did they attach the pex piping to before the slab was poured? There must be something in it, if it has tubing installed

Gordy

I would be more concerned with vertical shifting, and larger separation dimensions. As Mark noted.

Paul48

The slab has to be stabilized, or the pex will probably fail. They might be able to map the tubing with IR, and pump foam under the voids. It would serve 2 purposes.......stabilize and add insulation.

CanadaKeith

Thanks again. Unfortunately the nature of home inspecting is I don’t get to see when the house is being built or get to be involved in the follow up. I believe the client bought the house. I guess the risk depends partly on the interaction of the PEX within the concrete . Can the PEX slide within the concrete, or is it gripped by the concrete?. Do installers these days use regular concrete or a special mix around the PEX?

Gordy

The bond of the concrete to pex is poor if any. The show stopper is the returns with in a loops length locking in the pex for longitudal movement to slip through the concrete. So it's the elasticity of the tubing that would make up for movement. Vertical displacement of a slab crack is going to be the same effect. Once elasticity of tubing meets its limits the tube will fail.

John10

I designed my house with a 7" thick slab to use as a heat battery. It sits on R24 of foam. I haven't yet used the PEX to heat up the slab and I have a question on the maximum temperature changes which might be safe.

The slab is typically 11C which is the ground temperature here. It gets up to 22C when we have a few days of really strong sun. If I use the PEX to heat it up further, how high can I go before there is a risk of cracking?

I thought maybe 30C might be the top end from a practical point of view and maybe 25C much more common with my heating system. With repeated cycling between say 11C and 30C would I be asking for trouble?

If the slab is 24' wide, how much would it expand at the temperature range of 11C to 30C?

Gordy

Proper control is key. That’s mostly to control the fly wheel effect of mass. 7” thick is a lot of mass to control with abrupt outdoor temperature swings. Low, and slow. Just like ribs😁.

You don’t mention control joints in your slab. Is there?


30* C or 86* F shouldn’t present any issues, but the average water temp, and flow rates should be dictated by the calculated heat load. Using outdoor reset, with indoor feed back, and slab sensor is best.

Zman

Is there living space over this slab? If so, the slab would be closer to room temp than 11C on the low end. The room would also be very uncomfortable when the slab reaches 30C.

I don't think there would be an issue with that temperature range as long as there is some expansion joint at the edges. You could plug your numbers into this if you want to get exact. https://www.engineeringtoolbox.com/linear-thermal-expansion-d_1379.html

nibs

How well cured is your slab?, If new, probably should wait for at least 28 days, & if the slab is warming for the first time bring it up to temp as slowly as you can. If the slab is too green, you could get edge curl from uneven drying and cracking because of uneven heating causing shrinkage cracking. The longer the cure time, the stronger the slab.

hot_rod

John10 said:

I designed my house with a 7" thick slab to use as a heat battery. It sits on R24 of foam. I haven't yet used the PEX to heat up the slab and I have a question on the maximum temperature changes which might be safe.

The slab is typically 11C which is the ground temperature here. It gets up to 22C when we have a few days of really strong sun. If I use the PEX to heat it up further, how high can I go before there is a risk of cracking?

I thought maybe 30C might be the top end from a practical point of view and maybe 25C much more common with my heating system. With repeated cycling between say 11C and 30C would I be asking for trouble?

If the slab is 24' wide, how much would it expand at the temperature range of 11C to 30C?

Where is the tube located in that 7" slab? If it is at the bottom, hopefully not, it will take higher SWT to get the output you need.


https://www.supplyht.com/articles/98799-tubing-depth-does-affect-the-performance-of-heated-slabs