New concrete slab thickness / mixture?

[Overachiever]

I just tore out the concrete from one of my bays. It looks like the existing slab was 8" thick. Can I get by with 6" or is 8" some sort of car wash standard? Is there anything you mix into the concrete to make it better for the car wash environment?

Also, how would you pour it for the new floor heat? I noticed the old slab had the floor heat at the bottom of the 8" slab which doesn't seem right... no wonder it didn't work well. I've heard some people saying to put sheets of insulation on the bottom and others say its not necessary.

 

[MGSMN]

What I would do: Put 2" foam down (tape seams), use plastic staples to hold the heat tubing to the foam, pour 6" minimum concrete over that. You can safely drill into this 4" or less. I would also use 2" foam on the foundation walls (frost wall).

What you could do: Put 2" foam down, lay wire mesh on top of the foam, tie heat tubing to the wire mesh, pour 6" minimum concrete over that and pull the wire mesh up into the middle of the concrete as its poured. You cannot drill into this.

IMO: I would never place the tubing directly on the earth and pour over that. You are wasting energy trying to heat the earth as well as the concrete slab. The earth will act as a heat sink. I believe the average ground temp is 55 degrees year around, the minimum boiler temp setting I've seen on boilers is way above that. A boiler like your furnace heats until the thermostat is satisfied then shuts off. If the 55 degree ground temp was adequate to keep concrete above freezing there would be no need for additional heat or salt for your walk ways.

 

[soapy]

I agree with foam around the foundation edges. When they use foam around here it always has the foil backing. 4 inch pour is standard and 6 inch for heavy trucks etc. I always added fiber mesh and used 4500 lb. Concrete mix vs. Standard 3500 lb. Wire mesh will do little to prevent cracking.. Rebar on 2 foot centers minimum but I always went with rebar on 1 foot centers tied into the walls. Figure your slope at 1\8 or 1\4 INCH per foot. You should check your local building requirements first.

 

[Roz]

This summer we poured 5 new bay floors as the 30 year old floor heating system leaked and the tubes could not be repaired. Poured 4" on top of 4" of packed crushed stones, no insulation or reflective pads necessary according to our concrete person because this is not a bathroom where you want the heat to rise above the floor level - we just need to keep concrete above 32. We did use a stronger rated concrete that cured faster and stronger than the average concrete - for a slight up charge. You want to price and spec the project in advance of signing a contract to avoid change orders.

Some places required 6-8" to have the floors slope slightly toward the pits. Used a wire mesh 4" below surface. Zip tied heating tubes to the mesh in rows (16 rows per bay) that are 12" apart. Very important, plumber kept constant 80lbs of air pressure on all the tubes while the concrete team was pouring to make certain they did not mess up his work. Used rebar 12" apart in concrete section that is above a water holding tank for added strength (not in a bay). Heating tubes go to equipment room via the front of the bay aprons which are new so the aprons should not freeze too. I have pics of the project that I can send you if you want or you are welcome to call me if you have any questions. I forget the name of the heating tube used but plumber made a point of mentioning the type as some used in heated floors in the past did not last and were part of some large class action lawsuit.

Hope this helps. Good luck, it is a process that takes several days and is not cheap however I expect the investment to keep all our bays open this winter.

 

[wash4me]

I don't think you need insulation under the concrete. Like someone said you are trying to keep it above 32. You are actually getting heat out of the earth part of the time and the heat that goes down will come back up slowly. It's easy to find online advice/pics for hydronic heating but very little info on carwash heating systems. Don't start thinking it's like a snowmelt system either. In that case you are letting the concrete get below 32 then rapidly heating it back up and to do this you need insulation. Totally different than maintaining above 32 at the carwash. IF you want to improve something and spend a bit of money go to 6 inch centers on the tubing. You have to keep the lowest temp section between tubes at 32. There is a much more even heat if the tubes are closer allowing you to run cooler glycol.

 

[piperken]

One thing to keep in mind is you need volume of heat in the floor and the bigger the tubes the more heat you can get to the floor to do the job. Never put a tubing system in that is smaller than 5/8" and pesonally i wouldnt go below 3/4". We had a company put in 1/2" tubing and when it goes below -8 the bay freezes up bad and have to shut it down. Im in the process of researching how to design a carwash tubing system that eliminates dead spots in a floor. We have a bay that was redone with 3/4" in tubing but there is a dead spot that freezes up or doesnt get enough heat to thaw it out. cheers