Glass fiber reinforcement in thin slab mix???
 

Any opinions or knowledge of glass fiber in a thin slab mix for fracture prevention in a radiant hydronic floor being a poor choice? Glass is a good heat conductor as windows prove. My thin slab will also have rewire in it to secure hydronic tube to. I like how fibers have performed in other concrete floors I have done. Slab does not need to be light weight material(subfloor designed to hold it). Joists on 9"cc to fit pole barn steel siding subfloor which has 9"cc formed ridges.

Hi Drake,

I have no personal knowledge about using fibers as reinforcement but you say yourself you have good experiences with it.

This statement though is not valid.
Glass by itself is not a supergood insulator but when compared to some metals it is a lousy conductor. Copper conducts heat about 400 times as good as glass, aluminium 250 times, carbonsteel 50 times and stainless steel 25 times. Most types of concrete conduct heat about 2 - 3 times better than glass.

The glassfibers being thin and only a small percentage of the floor the thermal characteristics won't hurt the performance of the floor much, I would not recommend against using fiberglass because of it not being a great heatconductor.

What I would use is fiber reinforced mortar with polymer hardener and don't walk on it for at least a week, 2 would be better.

I use polymer and polypropylene fiber in my mixes with very good results. I would think glass fiber will work if it is thoughlying encapsulated by the mix. Are you using a commercial mix or mixing your own?

I have not yet chosen a thin slab mix. It will be 3/8' rod reinforced, 1" thick. It does not need to be self leveling or light weight. I am leaning toward a small arg premixed HS concrete as it is available bagged and I can use my mixer. What ratio of fiber do you add?

Concrete is well known for its exceptionally bad insulating properties. You would have to do a lot to it to make it a decent insulator. Things such as use additives that increase foaming during mixing or add Styrofoam beads to it. A lot of Styrofoam beads.

This is for a passive solar heat sink and hydronic heated floor. Highly conductive material is desired not insulation. Insulation will be below floor to direct heat loss into living space.

For a floor 1 inch thick, I would reconsider using 3/8 rebar. As a rule of thumb, rebar will need concrete 3/8 thick surrounding it, which is more than 1 inch. You could use 6x6 remesh in flat sheets. Remesh will still require some bending to get it flat. Alternatively, you could use metal stucco lath.

I used a mortar mixer to make the 3/4" ferrocement roof on my house. It has no reinforcement other than fiber.

For 6 cubic feet of mix I use 30 lbs portland, 150 lbs sand, 2 lbs dry polymer, 1 ounce of polypropylene fiber, water reducer/plastitizer, and 1.25 gallons of water. The polymer is actually ethylene vinyl acetate (VAE). VAE is found in everything from paints to coatings on solar panels and phones. It's also the main ingredient in Elmers wood glue and Tite Bond 2 glue. 1/2 cup of glue per 6 cubic ft of mix. Avoid paints with poly vinyl alcohol as it won't bond well. 1/2 gallon Latex paint base will work provided it is the last layer. Otherwise, it will cause delamination. I use electronic scales from Walmart to weigh each component.
At this point, I have used this mix for floors, walls and roofs. Water content is the key. You want the mix to be like peanut butter.

Drake,

Is the surface of your thin slab going to be the final floor? If so, do not use fibers as they stick up and are very difficult to get a "clean" finish on the concrete surface.

An alternative is to put the fibers in the concrete, roughly level it out and within 12 hours pour on a 1/4 inch (4 mm) concrete floor leveler. The initially almost liquid floor leveler pours virtually flat and bonds exceptionally well to concrete if done with a few hours of the initial concrete pour.

But practice is key with floor leveler as it sets up VERY quickly. It can go from very liquid to a putty like substance in just 10 minutes. I love it as you can put on a perfectly smooth finish without all the finishing that a concrete floor surface requires. In fact, a "rough" surface is great as the floor leveler really grips into that rough concrete surface.

And the floor leveler can have a smooth almost glass like surface with no work. In fact, if you work floor leveler, it sets and then you don't have that smooth surface.

As you can tell, I love floor leveler and have used it in a garage, in a basement and on an outside patio. The cost savings was high as you don't have to pay for the expensive labor to smooth out the surface.

Steve

The slab will be covered with dark(for solar)porcelain tile which I can install myself so rough surface will be good for adhesion. I have used fiber in other slab projects just not with radiant floor.