Slab prep - gravel, barrier?
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I would say yes to compacted gravel, and vapour barrier, yes, you should
matt_i
Well-known member
Hello, first I would jumping-jack compact any soil which was disturbed and placed back under the slab. Yes I would place at least 4" thick of "#57 stone" which is secret code for washed crushed limestone 3/4" nominal (no fines = not crusher run). Rough level this, wet down lightly with garden hose and then plate-compact. Then place vapor barrier, I recommend 10 mil so it doesn't get shredded. Then place reinforcing steel. Saw cut the next morning after the pour into 10-12' squares. Wet cure or apply curing sealer to the concrete as soon as it can be walked on without damage (likely late in the day of the pour). Note this isn't the cheapest method and will get you a lot of "we've always done it our way and never had any problems" statements but I think its a reasonably simple formula to get a good job.
The vapor barrier serves two purposes, one to keep ground moisture from wicking up thru the slab long-term, and two, in the shorter-term, to keep water in the concrete mix so it can cure properly. If there's no vapor barrier the water that needs to be included in the crystal lattice structure for the concrete gets pulled away by the stronger force of diffusion into the drier subgrade.
The vapor barrier serves two purposes, one to keep ground moisture from wicking up thru the slab long-term, and two, in the shorter-term, to keep water in the concrete mix so it can cure properly. If there's no vapor barrier the water that needs to be included in the crystal lattice structure for the concrete gets pulled away by the stronger force of diffusion into the drier subgrade.
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Both are absolutely essential. If you plan to heat it, sub slab insulation is essential as well. Clay is a *****...it's unstable and drains poorly. 4" of crushed limestone (also called radon rock in some places) will help stabilize the slab, and will help avoid future moisture issues, but it is expensive. They may try to get you to use class 5 recycle gravel, which compacts really well, but drains poorly (but better than clay) and it much less expensive.
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lakeroadster
Well-known member
Gravel? Well, a properly compacted sub base, yes.
Moisture Barrier? Depends
The more you ask / research / read about concrete slabs the more you realize it's more art than science.
- Re-bar vs. Wire Mesh vs Fiber Mesh vs Wire Mesh and Fiber Mesh,
- Re-bar assures additional cracking vs always use re-bar,
- Plastic Chairs vs Wire Chairs vs Concrete Chairs,
- Thicker concrete at high load areas (2 post lift) vs a uniform area throughout,
- 3,000 psi vs 3,500 psi vs 4,000 psi,
- Vapor Barrier vs No Vapor Barrier,
- Always ask for mix sheets vs you don't know how the hell to read them anyway.
- And my favorite: That link is 10 years old... that's not good information!
In any event here are a number of links to data you may want to review.
Visual Inspection of Concrete
Better Garage Floors
Flatness:
http://www.concreteconstruction.net/how-to/floor-flatness-number-conversion_o
http://www.faceco.com/docs/40 Questions June 2012.pdf
Slabs On Grade:
http://www.bpesol.com/bachphuong/media/images/book/3021r_96.pdf
http://www.constructionknowledge.net/concrete/concrete_basics.php
Plastic Crack Risk Calculator: https://plasticcracks.siue.edu/index.php?unit=Standard&zip=45339
Vapor Barriers: http://www.stegoindustries.com/docs/Vapor Barriers Under Concrete Slabs.pdf
Building Tips for Trouble-Free Slabs: http://www.cement.org/for-concrete-...truction/building-tips-for-trouble-free-slabs
International Building Code Section 1803: http://www.ibc-wiki.com/section-1803/
Cold Weather Concreting: http://www.concretenetwork.com/cold-weather-concrete/curing.html
Control Joints: http://www.concretenetwork.com/concrete/slabs/controljoints.htm
Wire Mesh Standard Practice: http://files.engineering.com/getfil...c=212727627.1.1460754610957&__hsfp=2088101374
Effect Of Rust On Bond of welded Wire Fabric http://rebar.ecn.purdue.edu/wwr/Library\Effect of Rust on Bond of Welded Wire Fabric.pdf
Re-bar Spacing a/ Weight & Materials Calculator: http://www.blocklayer.com/rebar-slabeng.aspx
Concrete Cracking Overview: http://www.cfawalls.org/foundations/cracking.htm
Concrete Floor Slabs On Grade Subjected To Heavy Loads: http://www.constructionknowledge.ne...ts/Div_3_Concrete/Concrete_slabs_UFC_2005.pdf
audioworks04
Well-known member
Perfect explanation!
tapered-pin
Well-known member
dont use 57 stone.. use GAB or crusher run. you can compact it to a consistant elevation and walking on it doesn't create potholes in it the way 57 stone does.
I'd recommend at least 4" thick.
place it and compact in place with water and it will virtually guarantee that your concrete slab is the correct thickness. (not 4" some places and 8" in others)..
vapor barrier goes on top of the stone, any reinforcing (i recommend sheets, not rolls) goes on top of the vapor barrier..
I'd recommend at least 4" thick.
place it and compact in place with water and it will virtually guarantee that your concrete slab is the correct thickness. (not 4" some places and 8" in others)..
vapor barrier goes on top of the stone, any reinforcing (i recommend sheets, not rolls) goes on top of the vapor barrier..
matt_i
Well-known member
My personal thinking on 22A "crusher run" with the fines included is that it can pack very hard (like a road) but you have to wait for it....and sometimes waiting half a year for mother nature to do her work isn't in the timetable before pouring the floor. The fines eventually will settle down and fill up all the voids. Repeated rainwater helps this.
But, if one short-circuits the process and bombs over it once with a plate compactor and pours 2 days later there's a very good chance the fines will start settling *after* the slab is poured and in my thinking that's directionally wrong. The angular washed stone won't settle any more unless the subgrade gives way.
But, if one short-circuits the process and bombs over it once with a plate compactor and pours 2 days later there's a very good chance the fines will start settling *after* the slab is poured and in my thinking that's directionally wrong. The angular washed stone won't settle any more unless the subgrade gives way.
lakeroadster
Well-known member
It seems haphazard to wait for mother nature or to guess. If only there was a way to tell if it was compacted correctly?
Oh wait... there is.. Proctor Compaction Testing of Sub-Grade Material
As a rule, don't use any material for fill unless the aggregate company has proctor compaction test reports on file for that material.
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Not familiar with your soils, red clay. If it expands when wet you need to overdig and replace it with compactable material. Seen many big heaves, cracks do to this, one of the most costly fixes to repair. Check by constructing a small open top wooden box, fill it with the red clay, compacted with a wide face hammer or similar tool. Submerge in water for an hour, pull it out. If it blows out your fasteners don't try to build directly on it.
Been there, repaired that.
Jim
Been there, repaired that.
Jim
Lassen Forge
Well-known member
I can't say it any better than matt... especially the "We've always done it (some other way) and it works fine". I've heard that from more people who did something wrong (even when given specific directions and shown the book requiring it) and every time... well I can't think of one off hand that didn't... fail in one way or the other.
Just my 2¢...
Just my 2¢...
small&rusty
Well-known member
I did a few different slab prep jobs a year or so ago and we used no less then 6" or 3/4- crushed rock with a sand mixture (basically class 5). We'd spread it out and plate compact it, check out heights then add or subtract accordingly. After we reached grade we'd wet the entire pad and plate compact it again. It would dry wicked hard and insure a solid, level base for a slab. The house we're building now they used that pea gravel for the slab prep then put 4" rigid foam on top with a vapor barrier.
If it was me I'd dig that clay out and put in a foot of pit run and then build a gravel pad on top of that. I'd want that slab up at least 6" higher than existing grade, so when it rains my shop floor isn't wet.
If it was me I'd dig that clay out and put in a foot of pit run and then build a gravel pad on top of that. I'd want that slab up at least 6" higher than existing grade, so when it rains my shop floor isn't wet.
tapered-pin
Well-known member
its actually a micatious silt, looks like clay so that is what most people call it.
"georgia red clay" is a term I've heard for over 30 years, but it's just inaccurate.
rburke65
Well-known member
Well the local excavator guy that I used, striped off all the top soil, and brought in 350 yards of clay to raise my shop foot print higher than the surrounding area. That base was like concrete. Vapor Barrie's, ridged foam, mesh, pex, concrete. Dry as a bone. No gravel.
lakeroadster
Well-known member
Gravel is good, it will tolerate some moisture / drainage issues if it gets under the slab.
Clay will wash out easily, leaving voids. And as stated previously, if it's expansive clay you've got problems.
If you never have any drainage issues.. no worries.
For slab sub grade materials it's best to use a fill material that has been processed and that the processor has Proctor Compaction Data on file. That way you know what your getting.
Clay will wash out easily, leaving voids. And as stated previously, if it's expansive clay you've got problems.
If you never have any drainage issues.. no worries.
For slab sub grade materials it's best to use a fill material that has been processed and that the processor has Proctor Compaction Data on file. That way you know what your getting.
no it doesnt
rburke65
Well-known member
Well the reason I went with the clay was his reasoning that they make dams out of clay.....at least his clay in NE Ohio. Had other contractors wanting to do sand. Sand never dried.....always wicking moister. It's ben up 6 years and no problems and my floor is dry.
johnnyradiant
Well-known member
I've got two contractor friends that one swears by "57" the other does not. Most days I tend to lean towards GAB too because it seems to stay put between prep and place.
I would definitely do vapor barrier too
Curious what adding water to a clean 3/4” stone lift will do? There is no fines so the water has no effect except getting the ground wet.
You can used 3/4 Down and wet and pack. If you use clean rock, which I would recommend, then that isn’t required. Clean rock doesn’t pack but you can run over it to have the stones place themselves better to create a solid base. Clean rock around here is the same price and is easier to achieve a uniform base.
Yes lay poly as the post I quoted says it helps cure the cement by keeping water in it longer. Also keeps the water from the ground out of it.
Spend good time on the base or you’ll regret it. Saw cutting might not be necessary depending on size of your slab. Also how what it’s mixture is like before it’s laid and the quality of your base.
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willymakeit
Well-known member
Your vapor barrier will not work if grade stakes are drove thru it during concrete pour.
Have them use a laser to set grade at points needed to screed off of.
Most slabs here are stripped of topsoil, grade built up with compacted select fill,4'' of 3/4 clean, plate compacted, vapor barrier [I like 10 mil. min.] rebar or wire mesh and 4'' 3,000 psi concrete with no air. Use a sealer for moisture retention and a soft cut saw for control joints.
The soft cut saw allows the cut to be completed sooner and reduces chipping of edges during cut.
Have them use a laser to set grade at points needed to screed off of.
Most slabs here are stripped of topsoil, grade built up with compacted select fill,4'' of 3/4 clean, plate compacted, vapor barrier [I like 10 mil. min.] rebar or wire mesh and 4'' 3,000 psi concrete with no air. Use a sealer for moisture retention and a soft cut saw for control joints.
The soft cut saw allows the cut to be completed sooner and reduces chipping of edges during cut.
matt_i
Well-known member
Its probably a point of minutia but I feel like if one is trying to pack the rock as tightly as possible anything that can reduce friction on a micro-level is an aid. My choice of that was a light water spray...not a dousing, just a medium spray to try to achieve above.
lakeroadster
Well-known member
That's great that you've had success with a clay sub grade. And your contractor is right, clay is used for dam's.. and even canal's... but we're not building dam's and canal's though?
For anybody else considering using clay for their slab on grade sub-grade material, don't take my word that it's not a good choice.. Link to Soil Compaction Handbook
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lakeroadster
Well-known member
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Moderate to poor, but not good.
Your chart shows it’s difficult to pack that’s about it. I wouldn’t put a foundation in packed clay. I would add a buffer that can supply an even compaction. If clay is packed properly it will seal out the water and become basically cement. Building practice here is to clean off black dirt to a good undispured clay. Then building up. I would build foundations on undisturbed clay.
lakeroadster
Well-known member
Moderate to poor, but not good.
Undisturbed? The topic here is choice of fill material.
The chart shows that clay is a poor to moderate choice for a fill material.
Your addressing my “building block” response not as fill. I have said I wouldn’t use it as fill. That I would use clean rock.
lakeroadster
Well-known member
Well, you did write "Needs to be packed real good and dry." And that reference is to fill... undisturbed soil is already compacted.
Just clarifying. I'd sure hate for somebody to use clay as a fill, based on the previous threads, when there are a plethora of better choices.
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When clay is used as a base yes packed well and dry.
Not fill to lay a slab on I agree. Up here clean rock is price the same as crushed. On large 200’ plus apron slabs we would use peagravel because it needs no compaction and it won’t sink or expand. It works well. It can be a b!tch sometimes to get nice.