How to get perfectly smooth concrete?

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  #1  
Old 05-10-12, 03:26 PM
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How to get perfectly smooth concrete?

I am going to break apart a small corner of my garage that is badly cracked, then refinish it. I have always wondered how they get that perfectly smooth concrete look that you see in garages. I have done outdoor projects with concrete. No matter how much I smooth it out, it winds up with a bit of a rough texture. What am I doing wrong?
 
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Old 05-10-12, 04:05 PM
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The super smooth concrete you have seen was probably finished with a power trowel. As the concrete sets up and the water leaves the surface it is just starting to set- it's almost too hard to work by hand, but it can still be worked with a power trowel. You could do the same thing with a steel trowel in your garage, but you need to resist the urge to work it too soon while it is still wet. Float your cement then let it set up a little, so that when you work it with the steel trowel, it doesn't get all watery and soft. If it does, that's okay... but you need to let it set some more before you give it a final finish. You will just kind of have to watch it until it's the right consistency.
 
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Old 05-10-12, 04:14 PM
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So after I dump the concrete in there and level it, how much time might I expect to pass before going to work with a steel trowel? How much smoothing by hand does it take to get that smooth look? And would I need to come back perhaps once per half hour to smooth it over again and again, or should I able to do it once and then be done?
 
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Old 05-10-12, 08:18 PM
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A lot depends on the consistency of the cement as it comes out of the truck. If it's wet it will take a lot longer to set up. If it's a good mix, it might begin to set up in an hour or two. Generally you will place the cement, strike it off with a board or some other form of screed, which is often enough for starters. Don't work it again until it appears quite dry. Depending on what you have for finishing tools- a frisco or bull float (on a long pole), you would use them to smooth the finish once it's getting firm- when working it no longer brings up large quantities of water. Once the cement seems pretty firm- maybe another hour or so has passed- then try your hand with the steel trowel. Don't scrape it, but do hold it at a very slight angle so that the leading edge doesn't drag, and most of the pressure is downward. This will pack the cement at the surface, creating a smooth surface. If it starts getting slurry, it is still too wet.

A lot depends on the weather and the mix, so you will just have to kind of play it by ear. Something in the sun and wind will set up pretty fast in a dry climate. But if its cool and shady, and the mix is wet, it can be up to 4 hrs.

Perhaps a pro will chime in soon with some additional advice. I've finished my share of cement but it's not like I do it every day.
 
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Old 05-11-12, 12:32 AM
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Concrete (not cement, that's the gray fluffy powder) is ready to hit with a steel trowel when pushing your thumb into it doesn't leave an imprint. Bearing down on the trailing part of the trowel will bring up the fines and result in a smooth finish, while raising the leading edge just a very small amount. Use broad, sweeping strokes instead of choppy ones. Done properly, you will actually hear the mix "sing" as the trowel makes its passes.
 
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Old 05-11-12, 02:26 AM
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There is another tool you use before the steel trowel. It's called a hand float and it's made of wood or magnesium and is thicker than a trowel. That's what you use to get the freshly placed concrete smooth enough for a steel trowel later. For larger areas you can't reach by hand, a bullfloat is used as X Sleeper said.
Sequence: pour the concrete, screed it off with a straightedge to get it flat, float the slab to close it up, edge it if desired, wait until it's as hard as Bridgeman said, steel trowel it, wait awhile, steel trowel again, repeat until it's as smooth as you want it.
 
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Old 05-11-12, 01:19 PM
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A steel trowel like this one?

Or maybe one with rounded edges like this one?
 
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Old 05-11-12, 01:26 PM
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either one would work fine
 
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Old 05-11-12, 01:42 PM
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No way am I a concrete finisher but from what little bit I've done - I think the rounded corners are a little more diy friendly.
 
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Old 05-11-12, 02:07 PM
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Concrete Finishing

Does anyone use a jitterbug after the concrete is placed and leveled?

The aggregate needs to have sand in it to give a smooth finish. Some mixes around here do not contain sand. If the pros think differently, please correct me
 
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Old 05-11-12, 05:31 PM
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Every concrete mix contains sand. What it may not have is enough cement. Jitterbugs have fallen out of favor, at least around here. You need rocks near the surface to get a good, hard floor. A jitterbug knocks the rocks down and leaves a weaker layer of sand and cement on top. It may be easier to finish, but it's weaker.
 
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Old 05-11-12, 06:17 PM
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The cracked garage area that I'm replacing is only about 4 feet X 4 feet. Would I need one of these "jitterbugs" or could I do all of this by hand with a steel trowel?
 
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Old 05-11-12, 07:59 PM
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Don't waste your time or money trying to find and then learn how to use a jitterbug. Just not worth it. I bought a used one more than 40 years ago, and after one or two jobs it sat in a corner, collecting dust. Far more work than it was worth. Finally sold it after getting sick of moving it around the garage/shop.

In the several hundreds of yards of concrete I've placed and finished (both truck-mixed and hand-batched) since 1969, I've never seen a mix that didn't contain any sand. Permeable concrete doesn't contain sand, but I don't think you want that in your garage.

Even if it's 5" thick, your 4' x 4' patch will only contain less than 1/4 C.Y. of mud. Meaning you'll be batching it in a small mixer or even using a shovel in a wheel barrow. Quikrete 5000 contains sand and rock and works well, and you'll need either 12 bags (80-lb.) or 15 bags (60-lb.). More than a half-ton total after you count the weight of mix water. Striking it off uniformly to grade with a cantilevered 2 x 4 will assist in getting the smooth finish you desire, otherwise you'll be fighting yourself trying to eliminate lumps and dips. Either set a temporary false form along one of the back faces (pulling it after strike-off and filling the resulting void) or nail an expansion felt along both back faces set to finished grade. Use short sawing strokes with the strike-off, bearing down while keeping a small roll of mud ahead of the strike-off, and filling in any low spots that develop. Immediately after strike-off, hit the mud with a wood float first, then wait for the bleed water to evaporate off before using a mag or resin float on it, followed by a steel trowel after the thumb-print test. Better results will be achieved using a 14"-long steel trowel instead of the "baby" 10-inchers pictured. Broad, sweeping semi-circular passes work best, with the heel (close end) of the trowel depressed slightly and the toe slightly raised during each pass to avoid marring. You want the trowel to sing to you (a high-pitched, hissing sound) as it brings up the fines. Contrary to what Pecos suggested (repeating and then repeating the steel trowel passes), DO NOT over work it, or you will weaken the surface, resulting in a potential for eventual scaling.
 
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Old 05-12-12, 04:39 AM
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His request for "perfectly smooth concrete" was what prompted my suggestion to trowel it over and over again. I used to be the "edge man" on a slab crew finishing tens of thousands of square feet per day. As such, I had to get the edges and around pipes and drains finished the same as the rest of the slab which were burnished with a riding trowel machine. To do this, you HAVE to hit it over and over again, starting with an 18" trowel and finishing up with a 6" cut trowel. By doing so, I can make concrete look like a mirror, hence the "perfectly smooth". I doubt his existing floor is quite that smooth, and he probably wouldn't need to burnish it.
I agree that a novice would most likely overwork the slab by starting to finish it too wet. However, if you know the timing required and don't start hitting it until the bleed water is gone and there's no chance of knocking the aggregate down, the top isn't getting any weaker by repeated hard trowelling.
 
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Old 05-14-12, 12:06 AM
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Maybe there's a good reason for you never graduating from being an "edge man" to a full-fledged finisher.
 
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Old 05-14-12, 04:08 AM
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Ooh burn!
There are a lot of different ways to do things and no one knows it all including me and you. This is an internet forum and everything on it is someone's opinion. The guy asked how to get "perfectly smooth concrete". I gave my opinion from a 26 year veteran concrete finisher's perspective and you gave yours from a semi-retired bridge/structural engineer's perspective (your profile). He can take my advice or yours, whichever he chooses. No need for a pissing match. I generally agree with your posts, just not on this particular point. Let's agree to disagree.
 
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Old 05-15-12, 10:01 PM
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If my making a simple statement was construed to be a "pissing match," then I apologize. Can we kiss and make up? Or at least let me buy you a beer the next time you get to Oregon? We could probably share a lot of work experiences 'til closing time.
 
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