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Thread: Anyone here know about concrete?

  1. #1
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    Anyone here know about concrete?

    I'm having a new back patio and front sidewalk poured tomorrow. I'm wondering if I should apply a sealer after the concrete has cured. I've read to wait about 30 days. I've also read that most sealers sold in retail stores are junk and a waste of money. Does anyone here have any information they can share?


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  2. #2
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    All I know about concrete I learned from Steve Ash."It Dries hard and it cracks"
    "There’s a lot of work being done today that doesn’t have any soul in it. The technique may be the utmost perfection, yet it is lifeless. It doesn’t have a soul. I hope my furniture has a soul to it." - Sam Maloof
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  3. #3
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    Vaughn has some expertise in this matter. Wait for him to weigh in.
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  4. #4
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    Tom, 30 days is about right depending on the thickness of the concrete. 30 days should be good if it's about 4" or less. In my limited experience with this I was told and used just regular Thompson's Water Seal and it seemed to do quite fine. A concrete man I knew told me to use that. Ain't gonna swear that it's the best thing to use, but it worked fine for me.

  5. #5
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    Dunno about sealer, but for the summer heat, keep it damp for the first few weeks for it to cure evenly. Just mist it with a garden hose often.

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    Darren

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  6. #6
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    Never heard of waiting 30 days around here the last step before the floor guys leave is to seal it.
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  7. #7
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    Around here they pour sidewalks, driveways, and patios with concrete so watered down that it almost levels itself. Adding too much water, like this, makes it easy for them to spread and finish, but it weakens the concrete significantly. Every pour like this breaks up and the surface chips off as the years go by.

    When the concrete comes off the ready mix truck chute it should form a steep sided pile and not flatten out by itself. If it comes off the chute like muddy water, stop the job and make them return the load. You don't want it that way. The concrete installation contractor should have to pull it around with special rakes to spread it and level it. A 10' by 10' area can be done without moving the chute, but an area 2 X or more in size should require the chute and/or truck to be moved to place the concrete properly. Before the pour I always put plastic down to hold the moisture in the concrete and keep it from going into the gravel or ground. I also cover the pour after it has been surfaced and edged with a sheet of plastic. Over the next two weeks this plastic is removed and the concrete is sprayed with additional water, then re-covered with the plastic. Slowing the curing process this way will produce a much stronger and reduced stress product that will last much longer. All concrete is going to shrink and crack, so have the concrete contractor make 1" stress relief saw cut lines across the concrete every 10' with a concrete saw before covering the pour with plastic and before they leave the job. When cracks do form, hopefully they will form along these lines that were cut. For an additional charge, the ready mix company can put reinforcing fibers in the mix. These will make the concrete much stronger, less likely to crack, and will reduce or eliminate the need for steel reinforcement.. If you don't go with the fibers, you should have a steel mesh centered in the concrete pour for reinforcement. The crew should be working the edges against the forms as they level the mix by tapping the sides of the forms with a hammer or vibrator to remove air bubbles too. The forms should be left on for 24 hours, but can stay longer. When removing them, be careful, as the concrete can be chipped easily at this point.

    After the two week cure under the plastic you can remove the plastic and let the concrete dry. I would wait a few more weeks before putting any sealer on it. I hope this helps.

    The Boulder (Hoover) Dam has refrigeration lines in it to remove the heat from the concrete curing process. It was built in the 30's and the concrete is still curing. They are still removing the heat from it. The slower the cure, the stronger the concrete.


    Charley
    Last edited by Charles Lent; 07-11-2016 at 12:29 PM.

  8. #8
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    Thanks all for the advice. The pour went well today. They made quite a mess in my yard but other than that I'm hoping for good results. They will return tomorrow to remove the forms. We have rain expected over the next couple days so I'll have to ask them if it needs to be covered.


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  9. #9
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    You've gotten good advice so far.

    Quote Originally Posted by Darren Wright View Post
    Dunno about sealer, but for the summer heat, keep it damp for the first few weeks for it to cure evenly. Just mist it with a garden hose often.
    Quote Originally Posted by Chuck Thoits View Post
    Never heard of waiting 30 days around here the last step before the floor guys leave is to seal it.
    In basic terms, concrete gets stronger the longer it is kept wet. When they talk about a 3000 psi mix, they mean the concrete will reach or exceed that strength if kept moist for 28 days. As Charley pointed out, keeping it moist also helps prevent surface cracks from forming.

    Around here, virtually all professionally-placed concrete is sprayed with a curing compound as soon as it's finished. The curing compound is essentially a sealer that helps hold the moisture in the concrete for the initial cure...generally a few days to a week. It doesn't looks like the concrete is sealed, but it does the job. Covering with sheet plastic and occasionally moistening the surface (as Charley described) is also effective, although a lot of commercial outfits prefer the curing compound, since they're no need for follow-up visits to the jobsite. A bit of concrete science: Concrete never really dries, it cures. With the exception of a little bit of surface moisture, the water in the mix doesn't evaporate. Instead, the water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in the cement powder form chemical bonds with water molecules and become hydrates or hydration products.

    Quote Originally Posted by Tom Baugues View Post
    Thanks all for the advice. The pour went well today. They made quite a mess in my yard but other than that I'm hoping for good results. They will return tomorrow to remove the forms. We have rain expected over the next couple days so I'll have to ask them if it needs to be covered.
    No need to cover it to keep the rain off after about the first 24 hours. The rain will be good for it, and occasional moderate misting with a garden hose (if it's not raining) doesn't hurt either.

    How well the surface holds up over the years is largely controlled by how wet the mix was to begin with (as Charley described) and also on the techniques used to finish it. If the guys who placed it are pros, I suspect they knew how to do it right. On the subject of soupy, or watery-looking mix coming off the truck, Charley is right about the more water the weaker the finished product. There are some exceptions to this, though. If they use additional cement powder in the mix, that can make up for the extra water. Another way to have soupy, yet strong concrete is to use chemical additives called plasticisers. I've worked on commercial projects that used plasticisers resulting in a mix so soupy it looked like fresh cow poop, but once cured, it was very strong. That said, the chances of your contractor using extra cement or plasticisers is pretty slim, since both of those add to the cost per cubic yard.

    Personally, for an outside slab, I wouldn't bother with any sealer beyond the initial curing compound.

    On the subject of control joints (the saw cuts or tooled joints Charley mentioned), a good rule of thumb is to have the joints spaced at 24 to 36 times the thickness of the slab. So a 1" slab would have joints every 2 to 3 feet. A 4" slab would typically have joints every 8 to 12 feet. (So the 10' number Charley gave would be right on the money.) Another thing to keep in mind is that by their nature, concrete slabs want to be square. Because of this, it's best to try to place the control joints so as to create square shapes (or something close to a square). For example, on a 3' wide sidewalk, ideally the control joints would be every 3 to 4 feet, even though the "rule of thumb" says you can go up to 12 feet between joints (assuming a 4" slab). But on a 24' x 12' patio slab, you could probably get away with a single control joint, splitting the slab into two 12' squares. (Personally, I'd probably make it into eight 6' squares instead of two 12' squares, though.)

    I hope this helps.
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  10. #10
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    Thanks for confirming my post Vaughn.

    Tom, if it's going to rain now that the concrete is over 24 hours old, let the rain fall on it. It will do the concrete a lot of good. Cover it with plastic when the rain stops and it will harden very well. You want it to be kept wet. Don't let it dry out for a week or more and it will be the hardest and most stress free pour that you can expect.

    Charley

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