Replies

1. 
   G.LaLonde | Jul 10, 1999 03:35am | #1

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   I share your concerns. I can't see any practical way to use 4 slump mix and work it through a maze of re-wire , re-bar and heating tubes and get it to have good contact with both the rod and the heating tubes in a 4" slab. That would be very difficult with a juicier mix.

   What is the slab sitting on?? In practical terms you are going to be confined as far as space is concerned to fit a grid of re-bar and another grid of heating tubes and mesh in a 4" slab without having problems with the tubes being too high or the rod being too low.

   Sounds like one of those things that looks good on paper to me!

1. 
   Rob_Dykeman | Jul 10, 1999 06:04am | #2

   *If I were pouring that slab I'd use the 4" slump, but add a midrange plastisizer an air entrainment so to get the flowability around everything.

2. 
   Guest_ | Jul 10, 1999 07:26am | #3

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   Dear T.H.

   I have only one question.

   Do you want the slab to fail?

   If you want it to fail, then do water down the concrete.

   You hired a professional to give you the benefits of his or her experience and you should allow them the responsibility for the fininshed product.

   We pour 1000's of cubic meters of concrete with 4" slumps and around all kinds of intricate re-rod and piping without any problems. Let the pros do their job and you will end up with a product that will last longer than the payments.

   Gabe

1. 
   Guest_ | Jul 10, 1999 07:52am | #4

   *You have very good advise from Rob & Gabe. Keep the 4" slump as designed & go with a plastisizer. Get with your design pro. & your ready-mix producer. We use several different types of flowable mixes and the work very well.Ron

3. 
   G.LaLonde | Jul 10, 1999 07:55am | #5

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   Gabe, You better step back and take a look at this one. He's talking about doing this in a slab 4" thick. That is a whole different thing than what you're talking about. To be at all effective the re-rod needs at least an inch or more of concrete on all sides of it. On top of this he is adding a layer of mesh and heating tube that will consume at least 1 to 1 1/2 inches taking into account the tendency of mesh not to lay flat. He's likely going to end up with the tubes at or very close to the surface, which could damage them during the finishing process or lead to surface cracks in the concrete.

   If 4 slump concrete is actually required for strength, I would spend a few more dollars and make a thicker slab using smaller aggregate. If you have actually seen anyone do what he is describing , I would be interested in knowing the end results.

   By the way, Have you ever met an engineer or architect who recommended adding water to concrete? Almost every redi-mix operation that I have worked with delivers slump 4 concrete to the job. This is basically the strongest mixture that they can get to flow out of the truck. Everyone knows that you must add water at times to concrete and millions of structures are built every year this way. They are not all doomed to failure. There is no doubt that adding water to concrete is THE single worst thing you can do to weaken it.

4. 
   Guest_ | Jul 10, 1999 09:56am | #6

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   Hi G.Lalonde,

   There's one word I didn't miss seeing and that is "structural".

   The question was "what is the proper slump for a structural slab?"

   It would be better to have more coverage but not more water.

   I have to stick with my original advise.

   Gabe

5. 
   G.LaLonde | Jul 10, 1999 05:00pm | #7

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   T.H. A few more things to consider.......

   I pour a lot of these slabs and have never seen one spec'd like this. Ask your architect about the function of the re-rod in the slab . Perhaps he could use fiber-mesh , 6 or 7 bag mix and accomplish the same thing. This would make it much, much easier to pour. You would still use the re-rod in the beams. If he insists on re-rod, you must make the slab thicker.

   I am totally opposed to the idea of using any vibrating equipment such as a vibrator or power screed on slabs with PEX tubing in them. The potential for puncturing the tubing with sharp aggregate or other contact is too great. We walk on tip toes when doing these jobs

   It is very important to get good consolidation of the concrete around both the re-rod and the heat tubing. This may require thinning the concrete slightly, depending on slab thickness and weather conditions etc. A rich mix of slump 7 concrete, poured in a thicker slab ( 5" minimum- 6" preferable) and using fibermesh would be more than adequate for almost any residential floor I can imagine. This would add only a few hundred dollars to the "average" house slab.

   Don't forget the control joints. Random cracking can be very damaging to heat tubing. Plan your joints and protect the tubing at these locations with sleeves under the joints. Allowing heat tubing to get closer than an inch or so to the top of the floor creates weak spots that act as joints and allow cracks to develop.

   There is always more than one satisfactory way to do something like this and I really think you need to sit down with your architect and come up with a more practical approach than his original plan.

6. 
   T._H._McGinn | Jul 12, 1999 05:07pm | #8

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   What is the recommended slump for a structural 4-5 inch radiant heated slab? The hydronic lines are over 6x6 mesh, over #4's 2 Ft. o. c. each way.There are 4 grade beams with a trench drain running through one of them.

   The architects spec's call for a 4 inch slump and a vibrator to eliminate the voids in such a stiff mix.

   My concern is the potential damage to the hydronic lines from the vibrator and or being caught between the vibrator and one of the pieces of steel.

   I believe a 7 inch slump would eliminate the need for a vibrator and not have the potential for voids.

   Has anyone been down this road?

7. 
   Guest_ | Jul 12, 1999 05:07pm | #9

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   I think that you should put the tubes under the wire , tye the wire to the rebar ,but not tight,that will keep the tubes from floating to the top . The wire will help move heat through the concrete .