Shocking the slab?

JackW

In my old radiant heat system I used a water heater, I'm redoing the whole system and upgraded to a micro boiler. In talking to Jolly Bodger he asked if I concidered temperature control on the water side since the boiler puts out more btu's then the water heater there might be a problem of "shocking" the concrete slab. Is this something I need to be concerned about? Under the concrete floor and all around the perimeter is 2 inches of pink Styrofoam so it's pretty well insulated from the bottom and sides.
Thanks.

hot_rod

It would take a lot of btus and pumping flow to crack a slab on a residential heating system. Possibly a high BTU/ square foot snowmelt going thru a huge delta T quickly?

External restraint cracking can occur when a heated slab tries to expand within 4 concrete walls. Insulation around the edge between slab and wall minimizes that and supply temperature no warmer than required to meet the load.

A concrete pro told me all concrete is prone to cracking. A crack is a free control joint he mentioned. With a smile.

JackW

@hot_rod yes all concrete is prone to cracking, that's why they score it, provides controlled cracking. I don't thing the slab will be that cold, will check the temp before I start, so I should be ok.

nibs

Fresh concrete is best warmed slowly but if it is months or years old.......
Still caution is cheaper than repairs, take a day or two to bring it up to temp, if you can.

GroundUp

BTU output of the boiler means nothing, if the delivery method and water temp is the same. The tubing can only deliver so much heat, regardless of the heat source. I would not be afraid to send proper temp water to a slab that's above freezing, maybe start with 90ish degrees if you can and turn it up after a day if it needs to be higher

hot_rod

I don't know there is an exact definition for slowly when warming a slab?

I think if you run the numbers on say a 40X40 slab 4" thick, about 14.5 yards of mud. About 4000 lbs per yard (3950 in a correct mix) 58,000 lbs! when poured. Water eventually comes out about 30 gallons per yard. Out the top and consumed in the hydration process if you install air tight barriers below

If you were adding 20 btu/ sq ft to that slab on a design day, it will take a day or more to get to design temperature, possibly an 80° top of slab temperature maximum?. Assuming you are adding about 30,000 btu/hr. to that 1600 foot slab.

Calculate the energy required to raise a quantity of concrete 1° per hour for your exact answer, then calculate load as it is warming.. In addition to room wall and roof loss you have edge and bottom loss even with 2" insulation, so a portion of the input goes out.

Heat Capacity of concrete, depending on the mix 0.2 BTU/lb/°

Concrete actually gives off heat as it cures. May be best to just blanket it for a few days before adding any heat?

When the Hoover dam was poured the engineers predicted 125 years for the mass to cool, the excessive heat of hydration was a concern for stress cracking, engineers worried about the integrity of the pour. So 1" steel pipe was cast in the dam and chilled river water pumped during and after the pour. Those pipes were eventually filled with grout.

We poured plenty of slabs in the winter months in the Rockies, with insulated concrete blankets the slabs cured just fine with their own heat being trapped by the blankets.

How would you know if a crack was from too quick of a warming or just typical shrinkage, or restraint crack etc? Shrinkage is the number 1 reason concrete cracks.

Control joints are there to help prevent cracking, more often to cause the crack to follow the control joint and eliminate roadmapping where the crack crosses the entire pour.

A saw cut score needs to be 1/3 the depth of the slab, not easily done with tube in the slab unless you planned around the score cut and didn't cross that area with tube.

If you form up a joint or have seams, best to duck the tube into the ground under those areas, then you are dealing with insulation and vapor barrier penetration in those deep tube areas.

Most finishers fog the pour either the same day or next morning to help with slowing the cure. i'd do that and add some cure admixes with a vapor barrier used.

The class I attended, I took good notes, the instructor claimed "vapor barriers aggravate cracking" top of the slab dries too quickly. Go figure.