I have used compacted fill sand under driveways and pearock (pea gravel) under basement floors.
Pearock is nice because it doesn’t need to be compacted. I would like to use pearock under driveways and garage slabs, but I am concerned about possible settling or slab deflection problems.
Also, which is better: rebar laid in a grid in the concrete, or wire mesh
Replies
Better for what? mesh is (mostly) cosmetic and rebar is (mostly) structural.
I like to compact.
SamT
43357.2 in reply to 43357.1
Better for what? mesh is (mostly) cosmetic and rebar is (mostly) structural.
I am looking to add strenth to the concrete, but also save on labor costs, i.e.: mesh installs faster, but is it comparable to rebar in strenth?
Depends on the mesh. At my parents old place the crew working on the front porch (actually an 8' x 30' patio) used "mesh" left over from constructing a military runway. It was something like 1/4" bar welded at the crossings. Came in panels rather than a roll.
That piece of work has held up well over the past 30 years.
Regular old rabbit fence style mesh isn't really strong enough to hold concrete together if it wants to move.
J,
Concrete has great compressive strength, about like rock. It has a pretty poor tensile strength.
It is weakened by adding too much water in an attempt to make it easier to work.
It cracks because the cureing process pulls it tighter and because of drying, which pulls water out of microscopic voids, also causing it to shrink. If the slab is large enough, the bond to the substrate (even soil) (or the weight of the mass to be moved) and the shrinking forces are both stronger than the tensile strength of the concrete.
Cracking of a large slab like a driveway cannot be prevented, therefore you will want to control the appearance of the cracks using expansion joints.
I'm getting to reinforcement, don't worry.
Cracks will allow water underneath the slab, so you will need a good draining substrate that will not erode out leaving voids underneath the slab.
A common slab will not span any void, so you need to compact the substrate to insure no settling.
With a good cement/aggregate/water ratio on a good substrate, with expansion joints and good curing techniques, a slab does not need any reinforcement.
Air entrainment is good for preventing freeze cracks in cold climates.
Fiber reinforcement is good for controlling small surface cracks on a slick surface when pouring at a time not conducive to a good slow cure. Never to use a slick finish outside.
Mesh is good for preventing deep cracks on a large slab. => 10'x10'. Keep the mesh in the top third of the depth of concrete. Cut 2 of every 3 wires that cross the planned expansion joint.
Rebar in slabs is most usefull where heavy loads are expected or the substrate has issues. See your soil engineer. Rebar when the slab is less than 6" thick is not recommended as there is not enough protection from the elements.
If you do use rebar, stop it 3" from the laid in place expansion material (1/2" asphalted felt) and bridge the joint with slip dowels, 7/8" x 2' smooth rods well covered with grease.
So much for school, now for your original question.
For a standard household driveway only, no soil problems, width about 12', no heavy loads expected, and I would call a trash truck heavy. The occasional delivery truck will be accaptable.
Remove all topsoil, disturbed soil, and previous fill. Level and smooth the subsoil. Fill to grade with engineered fill and compact. In wet freezing zones, design drainage swales. Use welded wire mesh. 2x6 or 6" forms. Not more than 3" of slump in the mud. Expansion joints and slab edges not more than 16' apart.
After finishing, cover with plastic (I don't prefer curing compounds.) If freezing temps are expected cover the plastic with 8"-12" of straw or use curing blankets. If over 110 degree temps, cover with a light layer of straw for shade. Overlap the edges of the plastic and lay some 2xs on the overlap the seal. Seal the outer edges with some dirt.
Curing time can be safely figured by assigning a 0% value to 40f degrees and 100% to 90f and higher degrees (2%/degree difference.) Multiply each hour at temp by the assigned value. When the total effective curing time is 72 hours, the concrete is strong enough for light traffic (cars, not SUVs or delivery vans.) Wait three weeks of comfortable temps before heavy traffic.
SamT
Edit: I forgot to mention, Keep the compacted engineered fill damp, not wet, till the pour. Give time after wetting the EF for all surface wetness to leave.
Edited 5/16/2004 1:49 pm ET by SamT
only thing I can add is , install a vapot barrier bewteen the soil/peagravel and the slab to keep slab from drying out to quick, which creates cracks. all slabs need vapor barrier.
BB,
Your right 'bout one thing.
I forgot to mention, Keep the compacted engineered fill damp, not wet, till the pour. Give time after wetting the EF for all surface wetness to leave.
J was asking specifically about driveways, and I just don't see a need for a vapor barrior there, in fact if there is slightly too much water in the mud, the VB can cause problems, whereas the EF will allow the excess water to drain out of the mix.
SamT
" Expansion joints and slab edges not more than 16' apart."
Done "correctly" you don't even need those.
Look at I-64 in Southern IL. The road will go 3/4 to 1 1/2 miles between expansion strips. That those are at fixed structures, bridges, culverts, etc.
But it is unlikely that you will have the same quality of design, specification control of the concrete, and controlled placement in a residential driveway.
>>But it is unlikely that you will have the same quality of design, specification control of the concrete, and controlled placement in a residential driveway.
Exactly.
So assume something will be wrong and put in expansion joints, huh?
SamT
Hmm just the man I was looking for. And almost the correct thread.
Here's the story, Were in the process of/ finished the pour, into the cure. I don't claim to know it all but do claim to know where to find it out. And you are one of the sources. It is my reinforced theory that spending the time and effort to cure a pour can basically double the performance of the crete. So I'm not trying to get in a pissin contest with the sub but I'm tryin to get some performance. I do like to visqueen the surface where possible, but in this case large sq ' is not the easiest route. I also tried to research a little info on the curing compound sprayed on the finish. It is my theory that if you don't spray sealer on to complete a full surface coat/seal your wasting the seal and making yourself feel good about doing such a good job. I watch the hand pump spray job and it reminds me of calling a Wagner homey spry unit a paint sprayer. I call it a paint spitter. It's my thought that without the proper vaporization over the surface your just not getting the seal. any way that's where it's at and I have been keeping it hosed down now for 3 days and will continue to do so.The pour was Friday and it was 37yds. Any comments and help are appreciated.
Clay
Clay,
>>It is my reinforced theory that spending the time and effort to cure a pour can basically double the performance of the crete.
Almost. Curing is as critical to crete as prep is to painting.
>>It is my theory that if you don't spray sealer on to complete a full surface coat/seal your wasting the seal
Exactly! That is why I don't like sealers. If you get enough sealer on the insure a complete seal, some areas will have seal standing in puddles. A different problem.
Sealers can be used properly, however, not in breezy areas, and not without good equipment like a professional paint sprayer operated by an experienced spray operator. I'ld hire someon who used to spray for Earl Schieb, myself. LOL
>>I have been keeping it hosed down now for 3 days and will continue to do so
What's the weather been like there? Use that cure time formula I gave J. there is enough slack built into it that you can eyeball average 8 hour segments and be safe. Anyway another 2 days of misting morning, noon, and midafternoon will do it.
It doesn't do as much good to just wet it twice a day on warm sunny days. Best to lightly mist before the sun gets hot, in the middle of sun time (noon) and about 4 hours later. It should not be needed in most of the country to wet it for the night.
You have been doing all the right things for cure, now it's up to the mix.
SamT
I Sam. Thanks for the reply.
I get so irritated with the net. Seems like it's getting harder and harder to easily access the info I'm looking for.
I did pretty good search and and after much hunting finally went to JLC search for articles cause I know I saw the info somewhere. and out it pops March 93 issue. So for 5$ I can get the article. Well for 5 min I can go get the issue from my stack of JLC. I'll see if I can get the graph to this message. Hey what's 10G$ worth of E junk good for if you can't use it on occasion.
I was also trying to find application specs on sealer but my mind fried before I could get that info.
I think I got it transferred, and I thought this had a good deal of info that I could interpret.
Clay,
Thanks for taking the effort to share that. It points out the real need for proper curing.
I cropped and shrank the pic for dialup users.
SamT
Sam, you just saved me a real lot of typing. <G>
I'd second everything you said. The only thing I'd emphasize is compaction of a suitable base.
To the original question, base course is just what it sounds like it is. It's an engineered product made of a well-graded mixture of aggregate and fines, specifically intended as road base. Put down about 8" of base course at optimal moisture, compact it thoroughly (95% to 98%), and the concrete can do its job.
If you are over clay or lots of other mushy stuff, a layer of geotextile under the base course couldn't hurt.
As for vapor barriers under the slab, the conflicting opinions are probably all correct in their respective region. Some parts of the country (high humidity) you want the VB or you want to wet the base down good, other parts of the country (low humidity, dry, windy, hot) you don't want the VB, under these conditions it's the most reliable way there is to curl a slab. The slab curls until its own weight exceeds its tensile strength, and you get these huge and otherwise unexplainable cracks that make shrinkage cracks look like nothing.
The ACI has been deliberating about VB for years now, that's their conclusion in a nutshell. It conforms to my experience, so I'm inclined to believe them.
DRC
Hey Dave, maybe you and Bill Hartmann can help me out.
That Time To Cure formula I gave J is something I came up with on my own to give people something to shoot for. Do you guys think it is close enough to use to advise people with??????
Samt
Sam,
It sounds about right to me. The only place I differ is that I'm a little more conservative on light vehichle traffic, perhaps I don't need to be, but I tend to think "better safe than sorry" with concrete.
In the past I've thought of it as 3-day, 7-day, 15-day, and 28-day design strengths, but I like your approach better. It seems to fit the actual site conditions more accurately.
DRC
use the fibermesh and wiremesh, but upgrade the the wire mesh to the next gauge thickness, still available in rolls, very heavy
something haven't seen here, upgrade concrete to 4,000psi the heavier mix has more portland content, microscopically tighter, less water intrusion, longer life don't let them add a lot of juice to the mix when pouring, good for the crete guy, not so good for you
**use the fibermesh and wiremesh, but upgrade the the wire mesh to the next gauge thickness, still available in rolls, very heavy
What gauge is the next thickness?
thanx,
JS
good point... 10 gauge is used in residential garage floors around here someone in another post called it "cosmetic", not a bad description to supersize it, go to 8 gauge, or better yet, 6 gauge
10 ga. runs $100 roll, 6 ga. twice that unrolling and flattening the heavy stuff is a PIA have to straighten it out on grade before the pour feed your crew doughnuts that morning will need good reduction cutter or cut off wheel to cut it to length important for crete guy to use the hook to pull mesh up during pour so it winds up in middle of slab where it's effective
if you can find a supplier for flat stock, comes in wide sheets for highway or commercial work, gets you out of flattening, but has to be delivered
not sure about the pea stone and future compaction/cracking problems as you mentioned in first post might ask around about that or switch to sand or gravel bed i always upgraded mud to 4,000psi, stronger, resists water infiltration
ed
Thanks for the info on the mesh- just what I was looking for.
JS