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moisture barrier or not

1K views 14 replies 9 participants last post by  DSW 
#1 ·
I'm tired of reading on this. I have yet to find a definitive answer.

You guys that no concrete what's your take on this subject?

5.5'' slab for shop.

Should I use a barrier or not.
 
#9 ·
In Memphis, TN (lots and lots of humidity and sweaty garage floors), most of the better concrete guys use really thick visquene for the moisture barrier and the smallest diameter rebar for reinforcement. If you can find the epoxy coated stuff....:grinpimp: Most all the concrete plants around here use fiber mesh in their mixes.

IMO, I would build a footer along the perimeter of the floor as that's were most fractures develop. I would even go further and install cradles and rebar too. But, that's only if $$$ was in budget and you really want a great floor.
 
#10 ·
I have a bach of Architecture and 3 years of structural eng. before I changed majors. 10 years in concrete const. Wire or steel is only good if you can keep it in the bottom 1/3 of the slab AND keep it a min of 1 1/2" from the outside 2" would be better. Roll wire is junk stay away from it, use wire mats or bar on chairs. We use 1 1/2" or 2" chairs with 4 gauge wire mats not the light 10 gauge you often see. in heavy trafic areas such as driveways we often ad #4 bar on 18" cent. as well.

We always use fiber mesh in almost any floor or driveway. There are several type of fiber mesh. Very small fine fibers that are good on floors that will have a very smooth finish. Larger strands that are about the thickness of a mech pencil lead that are used in structural applications where you won't put a fine finish on the surface. For most floors we use the fine fibers. If they power trowel you will have a hairy looking floor. A quick pass with a propane weed torch will burn off the fine hairs. Do this after the floor has had time to cure and don't over heat the conc.

Concrete will always crack. The trick is to make it crack where you want it to. 100' of concrete will shrink 5/8" when it dries. This tension in the concrete is what causes the cracks. Wire, bar, fiber will limit this to a certain level. So will thicker conc. Saw cuts, control joints ( the grooves you see in sidewalks) and expansion joints also either limit cracks or cause them to crack where you want them to, ( in the bottom of the saw cut or control joint) Concrete will also crack where it is unsupported ( poorly compacted sub base), Where it changes thickness, Like when they pour a thin slab 4" down on to footings and it become 8" thick at the outside, or where it is locked from moving, outside slabs tied to walls in freeze thaw areas.

I would personally rather have 8" of conc over a good 8-12" of well draining sub base and no steel (my driveway saw cut in 10' squares), than all the wire or steel in a 4-5" slab over a thin 4" stone subbase and only a few jointer marks, my neighbors. BTW not a crack in mine and his is a disaster.
 
#12 ·
Make sure they take out the steel when they put the vapor barrier in. A vapor barier full of holes is not worth a whole lot. You will probably have some perimiter cracks where the concrete turns down and gets thicker becoming the footings. A saw cut full depth of saw (3 1/2" - 4 1/2") around the outside, lets say 1' from the forms, where the concrete thins into the floor will help to limit the cracks. Also they should divide the floor up into aprox 15'x 15' (250 sq ft max) areas with saw cuts aprox 1"-2" deep to limit shrinkage cracks. We prefer saw cuts to jointer marks because you get a deeper more garanteed place for the floor to crack and you don't have to work about them as you finish the floor as you cut them in later that day or the next depending on conditions. Saw cuts are also less likely to catch wheels of tool boxes or jacks.
 
#13 · (Edited)
I have been reading on the Internet about this subject to try and educate myself. Still confused! I read about curling caused from moister barrier and it worries me. Would it be better to just leave it out and apply some sort of sealer to the concrete once dry? Does curling happen often?

Edit to include area.

Northern CA. Ione / Sutter Creak area.
 
#14 ·
We've never used a moisture barrier on our pours and we've never had any problems. I think your geography and soil/moisture type play the biggest role of its necessity.
 
#15 · (Edited)
This will be long so bear with me. I want to cover the main points in a way that will make the most sense to you.

I doubt that curling will be an issue in your floor. After reviewing some info I have found the following... Curling is primarily caused by changes in moisture and drying. As this is an interior slab, you can rule out enviromental changes such as daily temp changes due to sunlight, surface moisture due to rain, if you use a vapor barier, sub soil moisture. The other causes are due to concrete placement practices that if done properly are not a consideration these are.

1)Use the lowest practical water in the slab. Many contractors like to order concrete stiff to meet specs but then add water to make it easy to work. Propely ordered concrete is easy to pour yet looses no strength.

2)Use largest practical maximum size agregate and or highest practical coarse agregate content to minimize drying shrinkage. Many contractors order pea gravel / pump mixes with small stone because they are easier to place. The larger the stone, the stronger the concrete in general. Again properly ordered concrete eliminates this.

3) Avoid higher than necessary cement content. Goes hand in hand with #2. More small stone requires higher amounts of cement.

4)Take precautions against excessive bleeding. Propely ordered concrete will have limited bleed water. Excesive water used in finishing is asklo the same. Water added to the surface weakens the surface. Proper finishing techniqes prevent this. Finish after bleed water has evaporated and yet not so hard as to be unworkable. A fine line but easily doable for an exp. crew. Similar to #1.

5) Cure concrete thoroughly. In hot climates wet or damp curing is used to prevent surface drying. Usually mist spraying or damp burlap to retain surface moisture. Curing compounds can also be used. This is different that #1 and #3 as the concrete is " hard" and the water will not dilute the mix.

6) When minimizing curling is critical use joint spacing not to exceed 24X slab thickness. 6" x 24= 12' so blocks should be aprox 12'x12' after saw cutting. Critical use would occur on super flat floors for robotic forklifts. I listed 15' in the post above, still reasonable but I wouldn't go larger. I'd make that call on the size of the building.

7) Use thicker slab or increase thickness at edges. Already done as I see in the pict.

8) The use of properly designed slab reinforcment may help to reduce or eliminate curling. Already done as per picts. Bar spacing may be a little wide but I think it looks about 18"-24" which shouldn't be to bad.

9) Avoid using poly vapor barier unless covered with 4" of stone. As told to me by a structural eng., This is a great idea in theory but poor in practice. To place 4" of stone over the vapor barier and NOT damage the barier is in reality impractical. A vapor barier with holes is not worth much. Also many inspectors will not allow this as the are unable to verify the vapor barier is in place and intact prior to the pour.

Now if you don't have soil moisture problems in your area I feel that the vapor barier may be unnessary. A well draining sub base will limit moisture getting to the slab. Much of this depends on your subsoil type and its ability to hold / retain moisture. However local code may require it. That said I don't feel the vapor barier will hurt you either. Remember most floors will sweat due to temp difference / humid air. A glass of ice water sweats in the summer because the glass is cold and warm moist air condenses on it, not because the glass lets the water thru. The glass is your "cold" concrete.

I'd stay away from most sealers. They chip, flake and break down over time and look bad. That said, my driveway is sealed with a sealer designed for stamped concrete. This is a solvent based sealer that unfortunately is nolonger available. It resists oils, gas, and chemicals. I don't know if the new sealers are as good. This is not stuff you buy at Depo or a paint store. The stuff I used came from Increte and the new stuff I've see comes from Butterfield. These are penetrating sealers, Xylene/ tolulene based, may not be avail in CA due to enviro. regs. ( this is NOT Thompsons water seal) You still have to renew every few years but as they penetrate and are clearish they are not as bad. Down side they will make the floor more slippery when wet and will darken the color of the concrete from white to tanish. PM me if you think this is a way you might go and I'll try and get you some more info.

Just to finish if I was to order the concrete for this job, I would specify the following.... 3000- 3500psi concrete, 3/4" agregate, 5-6" slump, fiber mesh. This is our std pour mix. If it was going to be super hot and sunny over 75 deg I would add 1 -2 oz of retarder unless nite temps would fall below 40 deg. I would also plan on using surface moisture curing if temps go past about 80 and or if it is super windy and above 65. If temps fall below 35 deg I would plan on covering the slab with blankets and if extremely cold I'd either hold off the pour or add non chloride accelerator, assume max daytime temps below 50 deg. A good concrete guy will know all of this.

Disclaimer: Concrete is not an exact science. Even with perfect conditions, perfect placement, perfect mixes concrete WILL crack. The best you can do is to try and eliminate the variables. Treat this infomation for what you paid for it. I also do not mean that all or most contractors skimp on the steps above, but I have seen many that do in order to make the job easier or meet a price point.
 
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