I posted this over at house chat but was told that this was a better place. I am building a radiant slab house and would like to minimize cracks.
What I proposed was 6″ slab, 6 bag mix fiberglass reinforced, and 6×6 #10 mesh.
What has been suggested so far is a dry mix, (4″ slump), over EXP insulation and a vapor barrier, then covered after the pour to retard evaporation.
Another suggestion was to saw the slab under the internal walls to force any cracks into non-ritical areas.
Any other great suggestions?
What should I specify for soil compaction?
Also, I thought that pumping required wetter concrete. Is this true or is it just more convenient for the pumper? If true, I’ll have to open up more area around the slab.
Thanks,
Phil
Replies
http://www.findarticles.com/p/articles/mi_m0NTF/is_1_13/ai_n8706165
Jon Blakemore
Phil,
That is a good article Jon sent you, except I personally would never use a vibrating foot compactor.
If you have hinky soil, consider replacing the top 2' with compacted in place engineered fill.
SamT
Phil, Do you need the mesh for structural reasons? Is it being used as temperature steel? There are fibers that are sold as secondary reinforcement or as a replacement for wire mesh, such as TUF-Strand SF by Euclid Chemical Co. Possibly these could work in your case?
One of the reasons concrete cracks is because of the change in volume. There is never more water in the mix then when it is first placed, as water evaporates the concrete volume decreases. Concrete needs water to hydrate and for finishing, yet to much water can increase the number of cracks and is a detriment to strength.
A four inch slump is not a dry mix. Three to five inch concrete can be pumped. Pump owners do not like to pump "stiff" concrete because they believe it strains the equipment. A High Range Water Reducer (HRWR) can be added to the mix to increase the slump and keep everybody happy, except you because it costs money. If you go this route, add the HRWR at the site since it has a limited life. Do not let the concrete pump operator put a hose in the hopper while concrete is being delivered for your project. This causes segregation of the mix, then the pump line will clog and the pump operator will blame you that the mix is too stiff.
Concrete wants to shrink the same amount in all directions, basically, into a square, so when adding joints try and keep this in mind. Joints are a way to control where the concrete will crack. What is the size of your slab?
Curing is extremely important, start as soon as possible. On a windy day with the sun shining, water is being lost quickly, do not admire your beautiful finish and start curing at your leisure. Cracks will form that you won't see until it is to late. Try to avoid "blessing" the concrete to get a nice smooth finish, this weakens the top layer of paste.
What is the finish flooring? If you use a curing compound, keep this in mind.
On major construction projects the maximum density of the soil would be determined by performing a proctor and a nuclear gage used in the field to verify the % required compaction. A local lab could perform this task for you. What was existing on the site? If it was a building of comparable size, the soil may be sufficiently compacted.
Here's what I did for my (just poured) radiant slab. It's been six days since we put it in.
- Base course of 12", yes 12", of 'compacted' 3/4" to 1-1/2" crushed stone. You can't really 'compact' crushed stone, but you can settle it and provide a flat surface for insulation.
- "Slab-Shield" foam blanket insulation - two layers of 1/4" open cell foam insulation bonded to a foil radiant/vapor barrier.
- 6x6 welded wire mesh.
- Mr. Pex PEX-A oxygen barrier tubing. 1/2" dia, five loops of 300 feet each, exactly. Spaced 12" on center.
- 5" slump, 3000 pound mix with 1% accelerator (your mileage may vary) with 3/4" long NYLON fiber additive. Not the hairbrush bristles that the batch plant usually sends out around here. Slab depth varies a bit, but not less than 4" and not more than 5".
Bullfloat, power trowel and cure. No sealer, no curing agent, no expansion joints, no plastic, no nothing. So far it's looking awesome. This is, however, in a dried-in basement with ICF walls and a floor deck above. I plan on getting some heat into the slab in a few weeks, even in midsummer, to get some of the moisture out and allow the slab to adapt to temperature.
We'll see how it holds up, but I think it will do fine. Minor hairline cracks are to be expected, no matter what you do. Concrete cracks as a result of curing, and it is usually pretty tough to prevent it 100%.
Jon B ;
Thanks, great article!
SamT;
The soil is sandy loam with a clay base about 2' down. I plan on hitting the clay with the footings but the only unfortunate thing is that it is on a slight slope. So short of plaining the whole slope off I'll need stepped footings. This means that the soil will vary in thickness before it hits the clay. Not ideal, right?
Dan241;
I am not married to wire mesh if there is a better product out there. IS TUF-Strand SF superior? The one thing I will need it for is to tie wrap the pex tubing.
How expensive is HRWR? Since everything else is going to rest on this slab, I don't mind spending a little more to get it right. I don't believe in 'overfinishing' the concrete either. The flooring will be a combination of slate/tile with engineered wood, (either floating or glued; not sure yet).
By the way, my slab is 58' square, (foursquare house), so I've got that going for me!
Jon:
I am planning on 1/2" pex as well. Did you buy it locally or off the web? Any good sources?
Thanks all!
Phil
Limiting structrural cracks depends on having the best base materials you can - we compact 18" of gravel because we get a lot of water and have clay soils. If your base moves, the crete will too and that will lead to cracking.Limiting shrinkage cracks means limiting the amt of water in the mix to the very minimum that can be pumped, placed, and finished. The more water there is, the more shrinkage there will be.I don't like cutting control joints in a radiant slab. something in me says I will cut the PEX. Maybe it's Mr Murphy whispering in my ear...But one trick is to use a control joint by putting the asphalt impregrnated strips around the perimeter between slab and basement walls, and keep a layer of sand between the footer and the slab. That way, the crete can shrink towards cenbter, into itself. if it is adhered to the footers and walls, it will crack in the middle and the crack will be wider.
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Anyone have experience and sources for control joint material to put in before or during the radiant slab pour? I'd like to avoid saw cutting.
There's a sheet metal product in use around here that I think is very slick. It's got a tight V-shaped profile and is installed into the slab with the legs pointing downward. The bent edge is braced up to be level with the top of the slab--a good way to do this is to pour puddles of sack mix every so often and then set the metal in that, tamping it down to the right level. The puddles get embedded in the main pour.
Anyway, these form screeds during the pour and then serve as control joints afterward. I've been on a couple of jobs pouring slabs with them and they are not visible after finishing. I am told that a very straight hairline crack will develop over them after a while, as the slab shrinks.
Someone around here must know what they're called...
Phil,
sandy loam is good, IF, you have no organics mixed in.
The trick with clay is the maintain the moisture level forever.
Here is what I think is ideal:
Lay out the outside corners of the footing, then scrape off the sandy loam level at 1' below the highest point of grade below the slab. This means that you might not touch the soil at the low point of your lot. Compact the remaining soil out to a foot or so past the footing lines. lay in and compact in 6" lifts sandy loam and/or EF to bring the footprint level and at desired grade.
When this is done you will have a level; pad ready to receive the slab, but you won't have any footings.
Now, accurately layout the insides of the footings. Remove all soil down to clay from these lines out 10', sloping away at 1/8"/foot (1% grade,) being careful to maintain the lines so you can use the undisturbed compacted fill as the inside form. Place a stout vapor barrior where the footings will be formed and poured. I would form the footing so it would reach from the sloped graade clay up to the sill grade. You said "slight slope"; (=<18"/58'.) More than that, step it in 12"-18" increments. Be sure and seal all seams in the VB at any steps.
As soon as the footings are poured, fold the VB over it to seal all moisture in for at least 7 days. If you have the time, wait 14 days. Then add enough VB to extend to the full 10' width you excavated around the footing. Place a perimeter drain system to open air at the outer edges of the VB, lay 3"-6" 3/4" clean gravel inside that to the footing, cover the gravel with filter fabric to keep silt out of the drain system and backfill with whatever non clay soil you have, lightly compacting as you go.
The next step is to run a Jumping Jack around the footing to insure no voids, fill and level as needed. Once again compact the entire footprint and place a sealed seam VB over all. Insulate immediately to protect the VB from tears.
The goal is to keep the moisture content of the clay under the house footprint as homogenous and as constant as possible.
In my mind WWM has two purposes; 1: gives concrete subs a cheap way to appear to be doing something important. 2: Has specific applications that require special techniques to place properly. Especially with RH, I would use #4, (#3 if pushed,) 1'OC both ways. Chairs under the top layer and tie the PEX to the bottom layer. Use chairs tall enough to take the bottom of the bottom layer of rebar to 2" from finished surface.
If you use 'dobies for chairs, soak them thouroughly the day before the pour, cover with visqueen and shade, then place them immediately before pouring.
As soon as the slab has been finished, cover it with visqueen with a two foot overlap of adjacent strips and insulate it with 6"-12" straw or hay or 1/12" foam if you have that laying around. Let it cure at least 7 days. 14 days is better. You can recycle the straw onto your landscaping later.
While the statement "Concrete is garunteed to crack" is technically true, it is not garunteed that you will be able to see any cracks without a magnifying glass or stains.
As you intend to place finish flooring over the entire slab I would not use fiber mesh.
I may use thickened slab footings under bearing walls. Depends on location and where I expect cracks. Cut expansion grooves at one edge of non bearing walls. Use thickened slabs around any column footings and double up the rebar there-at.
I would not step the footing to support the edges of the slab, and I would use insulating rigid closed cell foam under the slab and as a break at all edges. Cut the top of the break at a 45* so the slab does not appear to stop 1"-1 1/2" from the footing.
SamT
Phil:
You've received some good ideas; my 2 cents:
subgrade prep. thoughts are on-track - use 800lb vibratory plate compactor, if possible. if subgrade "pumps" (maybe ask knowledgeable contractor) then remove and replace.
ensure you have 3-4" of crushed stone leveled to install and place slab upon.
the reinforcing steel (whether rebar or net) should be located in the upper 1/3 of the slab to be effective, per ACI (American Concrete Instutute).
for industry, spec'ing rebar set on "chairs" is much easier to check workmanship.
I'd agree that curing is sensitive - best application & curing weather is cool and wet, so get out the sprinkler or place plastic over concrete to prevent excessive drying too quick.
Good luck
been trying to stay out of it but yall know I cants. 99% of early cracks are shrinkards crack due to heat of concrete. Fiber work great but is not a cure all. I remember seeing this old finisher that would build a dam around his placement. and then flood with water, kept slab under water for weeks. never saw a crack on his projects.
On your third point is this for slabs on grade? I was told to put rebar in the lower part of concrete and when going over walls to put it in the upper part. Sort of like putting tape over a row of dominos ,when you pick it up it sags , Flip it over and it will lay flat.
With a five inch slump and no curing agent, you will have more than minor cracks. Like you said, it's only been six days. Look again in six months. You don't give the size of your slab, but I'll bet that you see a good quarter inch shrinkage crack in a forty foot slab especially after adding heat.
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I doubt it. First, I have ICF walls, so it's likely to pull away from the walls. Second, with fiber and mesh, it's not likely to crack. Third, it's protected. NO exposure to direct sunlight.
I WILL check in six months and either eat my words or say "I told you so". I'm not willing to cut it because of the radiant tubing.
Jon, you mention 1/4" double layer of some type of foam. How does this compare to 2" dowbd as far as heat isolation.
Why are you not slow curing this slab?
When you poured did you pump up the radiant system with air to show any drops in pressure during the placement of the concrete?
thanks.Remodeling Contractor just outside the Glass City.
Quittin' Time
That pex is predy tuff stuff . What would you use in flat work that would punch a hole in it?
don, beats the #### outta me. But stranger things have happened.Remodeling Contractor just outside the Glass City.
Quittin' Time
Jon, you mention 1/4" double layer of some type of foam. How does this compare to 2" dowbd as far as heat isolation.
I was told that the Slab-Shield insulation was almost equivalent to 2" of rigid foam. Possibly an R-8.
Why are you not slow curing this slab?
No interest in putting a sealer on a floor that will have a variety of future floor coverings, and none at all in places. Additionally, the floor gets zero sunlight, so it is not drying fast at all.
When you poured did you pump up the radiant system with air to show any drops in pressure during the placement of the concrete?
Nope, although it was recommended. That PEX is mighty tough, and we took great pains to protect it during the pour - plywood sheeting and boards over the tubes for wheelbarrow runs, no direct pressure on the pipes, etc.
ummm... dave.. i'd sure like to know more about your sheet metal screeds.. i'd use those in a heartbeat.. both for control and for screed..
something similar.. we used a vinyl screed that was set into the concrete after it was poured and bullfloated... then you finished with it in place.. after the initial cure you were supposed to "zip" the top off and it would leave two pieces of vinyl as a control joint
in actual practise it was hard to pull the tops off... i think most of them stayed in placeMike Smith Rhode Island : Design / Build / Repair / Restore
Mike, I web-searched my axx off using every term I could think of, found lots of interesting stuff but not this material. I am going to the lumberyard this morning and it's possible they sell it but I'm not sure. Bottom line, my buddy who used it, his number is unlisted and so I will swing by his house on Saturday. Gimme another 24-48.
It's definitely something I want to use in my own shop slab.
If PEX is anything like the polybutyline I used long ago, it certainly is tough. The big thing is not to use any tools with sharp edges. It's a little more work but no big deal.
On the poly, it was pressurized to 70# with water so if there was a break, the pressure dropped and you saw an immediate 'fountain'. I assume I can do the same with PEX.
Since this is a slab on grade and I have two brearing walls spliting the slab into 3rds, either I tie everything together as one big slab or have three floating slabs right?
One would be ~58x14, the middle one would be ~58x20 and the last would be ~58x24.
Unfortunately, the portions that I could easily score are all wood whereas the ones that have no natural breaking point, (e.g. 18x33) are all slate.
It sounds like the most important keys are properly compacted base, the dryest pour reasonable, and a long curing time.
PEX is crosslinked so it has a memory and is about ten times as tough as PB
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Well, I'm bringing this post back up - I said that I'd check my floor in six months and it's been over 2 years now.
I've been all over my basement slab looking for cracks, and the only thing that slab has done (through two winters worth of radiant heating and summer cooling, I might add) is draw away from the ICF basement walls by about 1/16" to 1/8". There are no cracks anywhere in the field, which means that the entire assembly is acting as a monolithic floating unit within the constraints of the basement walls. Much better performance than most, and about what I expected. I suppose I could seal that joint at the wall, but there's no point - the basement is as dry as any I've ever seen.
The key to a solid slab is a near-perfect compacted stone or gravel base, low-slump concrete, and proper reinforcement.
What did you use.... rebar, mesh, fiber...?
But you will prove to those "That all concrete crack" are liers. you dont want that do you. you wont be able to sleep
I've an 8 x12 front porch that was added in the '40s.
Water from roof runoff had funneled under the corners of the slab next to the house until about a foot was washed away underneath each.
Removed the old linoleum and there are zero cracks to be found anywhere on that uninsulated beach sand slab sitting above ground level with no footer.
be 50+ years My furniture, part of which I made myself — and the rest cost me nothing of which I have not rendered an account — consisted of a bed, a table, a desk, three chairs, a looking-glass three inches in diameter, a pair of tongs and andirons, a kettle, a skillet, and a frying-pan, a dipper, a wash-bowl, two knives and forks, three plates, one cup, one spoon, a jug for oil, a jug for molasses, and a japanned lamp. None is so poor that he need sit on a pumpkin. That is shiftlessness. There is a plenty of such chairs as I like best in the village garrets to be had for taking them away. Furniture! Thank God, I can sit and I can stand without the aid of a furniture warehouse.-Thoreau's Walden
concrete is going to crack. period. in fact, we had a project where the floors were stained concrete on metal deck and structural steel frame. by far the funniest comments made by people walking through the building were "Is the concrete supposed to do that?" "Is that ok?" etc. Over every beam line there meandered a nice little bit of character. It is part of the allure *wink*.
i think the simplest answer to your question, however, is this:
minimize cracking by placing saw cuts at proper intervals and at all the places where it is most likely to occur, such as outside corners and anywhere there are differential materials (column pads, etc.)
thickening the entire slab is a good way to gain added strength without adding a heck of a lot of cost. but even better is adding a better sub-grade and compacting the heck out of it. consider undercutting your soil and replacing with engineered fill.
but the question remains, if you aren't going to have a finished, exposed concrete floor, what are you worried about?!?!?! it won't affect your radiant heating system, so let it be and don't spend the extra money.
we just had one installed (heat mats) for a community room in a larger apartment building. the slab was 4" on 4" granular fill with 6 x 6 welded wire fabric (which is highly suspect even with the best of concrete placers and finisher -- it gets stamped down to the point where it does NOTHING in almost every situation). by placing saw cuts at about 10' intervals and making sure to cut from the block foundation outside corners, the slab is virtually crack free -- and yes, it will be finished as exposed, stained concrete. it CAN be done, and done well.
good luck!
j
By the way, curing agents applied to the surface will prevent the proper adhesion of many types of paints and some other coatings. If you plan on using anything stuck to the slab, you're better off skipping the curing agents. One of the most effective methods is to lay burlap over the slab, soak it completely sopping wet, cover it with plastic, and keep it out of the sun. Keep it wet for as long as possible adding more water as needed -- a week is not too long.
Wow, it's enough to make you go back to crawl spaces! Just kidding.
This is really great information. I like the idea of going with #4 rebar but I need the 6x6 mesh to tie down the pex. Any problem with using them both? They will definitely be in the middle of the slab as I'll be there monitoring for breaks in the tubing as it's poured.
If I use a floating wood floor then I'm indeed not concerned, but I will be using slate, and may want to use glued down engineered wood so what ever I can do to eliminate expansion would be helpful.
Since I'm in the Pacific Northwest it is pretty cool and wet here and I will be in a trailer on the property so I will be able to keep it well watered.
I just had a couple of slabs poured. The heating contractor I used said wiring pex to mesh is the old way, but not the good way. He used a stand-up stapler that shot arched headed plastic staples over the pex and into the foam insulation. Theses staples have legs about two inches long and barbed heads. They hold just good enough.
No mesh. He said walking on the pex while pulling up on the mesh during a pour always seemed to him to be a risky business.
Ron
ok let try again. Rebar and wire mesh has too different function. Wire mesh, fiber controls shrinkard cracking. rebar controls load factors. rebar does nothing till a load is applied to the slab. rebar cannot replace wire mesh, and wire mesh does not replace rebar. can they coexist in a foundation, yes.
phill, after you get all your preliminaries figured, concentrate on the pour. Floating floors require being flat. Ups and downs are a real pain to deal with if you want a solid job (as solid as floating permits).
With our radient slab there have been no noticable cracks, entire downstairs is ceramic. Poured with mesh and fibre on well compacted base. Used 1" dowbd as the expansion against the foundation. Kept slab wet and under plastic 3 weeks.
However, nothing about concrete is a given. We have expansive clay here. Someone mentioned the consistent moisture content of native clay. It is important. A slab can sink when the clay dries out. Divorcing the slab from the perimeter foundation keeps the heat in/cold out. However, with no mechanical connection, on certain clays that divorce might allow up to a 1/2" of sinkage. If you can somehow include the footing in its support, you might come up with a fool proof method.Remodeling Contractor just outside the Glass City.
Quittin' Time
I'm doing a slab on slab refit of my attached garage, 24x24. I'm puting down 2 inch Dow 250 and than the pex and new slab. I want a stained finish (the room will be a family room until the kid count goes down) and of course no big cracks. Ideally I'd like to have a grid of lines cut in the slab and grout them after staining for that tile look. Anyway, my question is how thick does the slab have to be to resist cracks while driving on it, fibers, mesh, rebar or all?Ted
like this, I used fiber. 2+3=7
Beautiful, that's exactly what I'm after. When did you cut the grid? I'm also a little slow - what's 2+3=7 deal? Ted
here. 2+3=7
Careful with wood floors over concrete. Concrete gives off moisture for a long time -- like a year. I don't know all the particulars, but I know to caution you to consult the flooring manufacturer to see how long after concrete is placed before it's dry enough to put down wood. If you are building this summer, by next spring, having gone through one winter with the heat the slab will probably then be as dry as it is going to get.