MAKE UP OF CONCRETE FLOOR FOR GARAGE
toycept
| Posted in Construction Techniques on
I’m having a concrete floor put in a newly built garage. I’m looking for suggestions of things I should request when hiring a contractor to pour/install the concrete floor. For example, is it necessary to use wire mesh? Is it advisable to have a plastic sheet vapor barrier (some contractors suggest it slows the curing time and does bad things to the cement)… is it recommended to have stones in the concrete mix…. what PSI should the mix be. I’m not planning on having any really heavy vehicles other that a car on the floor. The floor will be 21X25 feet….. are expansion joints necessary on a floor that size?…. any suggestions for me to ponder over would be very much appreciated.
Thanks in advance.
Replies
I'm so glad I put a heavy vapor barrior directly under the concrete of my garage floor. It's always nice and dry, if you lift anything up you never see moisture. Recently I bought a heavy 15 mil poly barrior, I think it's called monster barrior- I'd have used that if I was to do it again. Just make sure you put it directly under the concrete- if you put it under the stone you'll get pooling water and it could actually accelerate moisture transmission.
Concrete guys don't like it because the barrior stops water runoff of the concrete so the setup takes longer til they can finish the concrete. If they pour a stiffer mix it will be a stronger floor as lower psi results from more water in the mix anyway. When I have a moisture barrior in place (with wired mesh in place) it's all setup and ready to go in the early AM so the slower setup isn't a problem.
rock, naw dont need no stinking rocks
Are you calling rocks and aggregate the same thing? Why no aggregate?
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Fiber/mesh is not an either/or question - you need BOTH.
The fiber reduces microcracking, which can weaken the concrete, particularly if it is not wet cured.
The wire mesh provides structural strength and holds the pieces of concrete together when it cracks.
FWIW, I use wire mesh, fiber, and pea gravel when I do a garage floor. No expansion joints up to 20x24, 4" thick, keep it wet for 2 weeks. Haven't had a crack yet.
I,m sorry please disregard any question I have answered on concrete, for i know nothing of the subject. I will kindly go back and deleted all asnwers I have made for the last seven years on the subject.
I guess he told you, Brownie. Shame on you for trying to answer the question.
:)
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Why so "testy"? We are all here to learn from other's experience and knowledge.
The misconception that fiber can replace wire mesh is endemic among the trade. No matter how much publicity the concrete association puts out, no matter how much training the unions give the finishers, the myth persists.
The problem is that the fiberglass fibers are not long enough to provide structural strength - and can't practically be made long enough.
I use the fiberglass fiber, too - it's great for preventing microcracking and the associated weakness in the slab, particularly when the slab is not or cannot be properly wet cured.
I also use wire mesh, because it will hold the pieces together if/when the slab cracks.
But what do I know, I've only been doing this 30 years ;-)
Hello. I certainly don't want to get into the middle of the wee pi$$ing contest that seems to be brewing, but I'll lend a hand. I've been a "concrete guy" for 20 years.
As mentioned, poly DIRECTLY under the concrete, and wire, and rebar etc.... but on top of the gravel, will slow down the drying time. This is good.
Mesh helps the concrete hold together after it cracks, but does little or nothing to prevent cracking.
6 bag mix minimum, or 4000 psi concrete, is what you should use. On my new shop floor, I poured 6 bag mix, with micromesh re-inforcement (what brownbagg was trying to recommend), with #10 mesh AND rebar. Rebar DOES strengthen concrete, and helps prevent it from cracking.
On a floor your size, I would do a softcut into 4 sections. One down the middle in each direction will control future cracking. As a company, we have repaired over 20,000 concrete surfaces, and I think I'm qualified to tell you that without control joints, a floor that large is likely to crack any which way it wants without them.
You want the surface trowelled for a finish. We like to 'burn' the floors in, which makes them very hard, and almost polishes the surface. Some do this by hand, and some use power trowels. And, seal the floor within 24 hours with a good sealer. Talk to your contractor about all of these issues, and ask how he does it himself.
In my opinion, if he's skipping steps, run like hell. Find a new guy.
Jeff
We like to 'burn' the floors in, which makes them very hard, and almost polishes the surface. Some do this by hand, and some use power trowels.
Some believe this "power troweled" polishing effect makes a floor a poor candidate for subsequent acid staining.
Without a doubt, it definately does. I didn't see in the original post where he was going to acid stain the floor. If so, I wouldn't have recommended it.
Burning a floor in makes it harder and more durable, as well as less likely for automotive oils to stain the floor badly.
I wasn't hypothesizing about all the available options that someone could do to concrete, I was just answering his questions.
Jeff
Have you had any issues with the poly causing "curling" of the slab, due to uneven moisture levels on the top and bottom? The concrete engineers say that's why poly shouldn't be used under the slab - or if used, should have 4" of sand over it to equalize the moisture levels.
Haven't tried it, just curious if anyone has seen this problem.
I can comment on that one. After talking to quite a few people locally, here on BT, and elsewhere, my shop slab is as follows: compacted base, 6-mil poly, 2" XPS, 2" sand layer, and 6-sack concrete. The concrete was a 3" slump recipe modified with a water reducer to flow easily during placement. After placement I kept it wet for several days by spraying it down periodically. We had some rain during that period also, and the surface never dried out.
Along one edge and in one corner I got minor slab curl. The top of the slab was originally perfectly flush with the top of the stemwall. Now it's ~1/4" up. Since then I've read that the correct thing to do would have been to cover with poly and keep it wet underneath for several weeks. My mistake. It's not going to cause any problems at all for me, but in some situations it could. Apparently slab curl can be quite severe sometimes, and cracks occur due to stress caused by the floating sections.
To be honest with you, I only put so much stock into what 'engineers' say about something I've been doing all my life, and my father-in-law (my mentor) for 50 years before me. We have almost no callbacks, so I can only 'guess' we're doing it right. New technologies are always great, but pouring a garage floor is about as simple as it gets, as far as I'm concerned.
We pour very stiff, power screed, check everything with a laser for pitch, and finish. I suppose that if you pour real wet, you may have issue's with variances in moisture content (sounds like I'm talking about wood lol), but we haven't had any issues with 'curling'.
Jeff
Jeff, What do you use for a screed? What do you think of these lightweight wet screeds like the Wacker, VibraStrike, etc?
If you use real stiff mix, do you strike it close to level before you power screed it?
Do you screed off of pipes and forms or place pins at correct level?
I'm always curious as to how someone with your experience does stuff!
I use vibrastrikes, with 2 Honda motors on each. I would guess that, like everybody else, I use a similar technique to get the surface 'flat'. I set the grade and snap lines around the perimeter, and in the center of the floor, set a stake with 4 nails, one on each side, one for each direction. This breaks the floor into manageable grids, and in the case of a 2 car garage, into quads. I then set the corners and walls by hand float, and connect them with parallel 'rails' of concrete, all at the correct height, with the screed. Screed the concrete in between the rails, and move on to the next 1/4 floor until the job is done. Float, finish trowel, and edge. Done. Wow, I wish it was that ez.
A good friend of mine has a much larger concrete company than mine, with 150 guys pouring very large jobs, all commercial. He has a couple of those nifty laser screeds that do all of this automatically. I've seen it in action on a couple of large projects (shopping malls) he did last year, and they're pretty impressive, but overkill for small jobs like garages and shop floors.
Jeff
I know a lot of people who watch concrete being poured and finished and comment that it really isn't that hard. " The next time I need to pour, I'll get a couple of buddies and a few six packs and do it myself."
Then when the truck pulls up, they start to find out it isn't as easy as they thought and the floor looks like the surface of the moon when they are done!
I hear that - and I remember once being that guy.
Fortunately, one of the "buddies" was a union concrete finisher with a LOT of experience. He ended up troweling pretty much the whole floor. Watching him work, I realized he only made it LOOK easy. He really saved the day - and a neat guy, to boot.
Took a lot of years to learn to finish well and get comfortable with it. Still, I call that guy, who even though he is up in years, does a better job than I probably ever will.
Like in any trade, nothing tops experience. Great finishers usually are guys (and gals) that take a great deal of pride in the finished product. They've been taught their trade by the old timers, and after a while, become the wiley ole' veteran's that they once learned from.
My mentor was my father-in-law. He was a GC who worked all day on his commercial projects, went home, had a 1/2 hour dinner, and went to his spec house that he was building EVERY SINGLE DAY for 35+ years. He taught me everything I know about building, concrete, carpentry, etc.... because he did almost every job himself. That's the way it was done 25 years ago if you wanted to make any money. If he saw you skipping a step, he'd wack you real hard like a bad dog. 2nd attempt at shoddy work, and you were HISTORY.
Unfortunately, he passed away a few years back. Now, I can sit around with his 2 sons and my wife and kids, and we tell stories until we can't stop smiling.
Jeff
That story reminds me of my Dad, who literally worked 7 days a week at two different jobs.
When I was in my early thirties, I worked like him and put in many a day from dawn till dusk and then worked in the shop fixing equipment after dark.
One day he sits me down and says " you're working too much...Those 16 and 17 hour days are not good for you....you shouldn't be working more than 14 or 15 hours!!"
I still laugh every time I think of that lecture and his idea of a "normal work day"
To be honest with you, I only put so much stock into what 'engineers' say about something I've been doing all my life, and my father-in-law (my mentor) for 50 years before me. We have almost no callbacks, so I can only 'guess' we're doing it right.
That's sort of the age old debate, isn't it? Contractors generally have more "real world" experience than engineers, actually doing the work, but the engineers understand the hows and whys better than the contractors.
It's great that you don't have many callbacks, but I've found in residential construction that often the consumer is not informed enough to know when to call back, especially for more minor issues. For example, most contractors assert and consumers generally believe that "concrete will crack".
Engineers will tell you that concrete cracks due to improper curing or lack of reinforcement. It's certainly possible to place concrete that does not crack, but many contractors don't do it - mostly because they don't really know how, but also because they feel it is not cost effective for them.
The discussion about fiber vs mesh is a good example - many contractors believe that fiber can replace mesh. Any engineer can explain that it doesn't - that they serve different functions. But the contractor, who doesn't see the result of leaving out the mesh when the concrete cracks 5 years down the road, doesn't see the difference and discounts the engineer's information.
When I am looking for a new concrete sub, one of my questions for them is "does concrete crack, and why or why not?". Most subs say yes, concrete cracks. The ones I hire say some thing like "it can", and then express an understanding of why, how, and how to prevent it.
Someone posted something like "concrete is simple" - and it is, and can be. However, it's also very complicated. Given a good mix and a little experience, one can place a floor that won't crack. But how do you know if the mix is good, how do you know how to specify the mix for your conditions, and how do you cure it to prevent all cracking? That's the realm of the engineer.
To each his own, I suppose.
Well, a bit more info would help, too.
Like, is this a pure slab-on-grade, or does it have a "mat" edge, or "turn down" edge? What does your AHJ require? (Some want frost-depth foundation walls, and inspections, and the like.)
If it were in my area, I'd specify a 12x12" "mat" edge, with a mid-span 12x12 'beam' cast in. The perimeter beams would get 1 top and 2 bottom #4 continous, the mid-spans 2 #4 at bottom, both stirruped with #14 @ 24" OC; and then #4 at 15" O.C.E.W. using 3000psi with fibre and flyash admixtures (all subject to agreement by the engineer I have to have sign the drawings before I could get a permit to do any of the work). Typically, that all can be cast-in-place as long as it's in undisturbed soil, infill would be well-graded sand with 6mil poly over that.
But, that's my area--others differ.
One other thing I haven't seen mentioned is to put a small step @ the garage doors. My cement sub does this by pouring a piece of angle stock into the floor, leaving a 3/4 or so lip, so the floor in front of the garage door is slightly lower. keeps driving rain etc. from blowing under gasket. He doesn't use fiber-mesh, but he does cut all the floors the day after he pours. I've had very good luck w/ him, and very few (hairline) cracks.
my .02
Bing
Compaction of the dirt will prevent major cracks. If you drive and park your car on the dirt, it will show how much compaction was done. My driveways will hold a fully loaded cement truck without cracking , cause I alway drive on the dirt first to compact it.