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Backround

I'm going to pour some concrete to anchor a swing set. The base of the swing set that is being anchored is two 4x4s with a 2x4 footer crossed perpendicularly to the 4x4s.

I have a bunch of large softball-sized rocks in my yard. I was thinking I could buy less concrete mix, and get rid of some of these rocks, if I threw some of the rocks into the wet concrete.

I assume the concrete will harden around the rocks and they will set together to become a solid mass.

Question

Is my assumption correct? or will the rocks damage the integrity of the concrete anchor?

Here is a drawing of posts and anchor (with rocks depicted)

Drawing of posts and concrete anchor

Update

I poured the cement in June 2015 (about a year and a half ago). The swingset is still standing strong.

The biggest takeaway from this experience is the consistancy of wet cement. I assumed it was like latex paint, but it was more like thick oatmeal or damp sand. It didn't really pour and fill up all available space. I had to spoon it in with a shovel and pat it down into place around the posts.

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    As a side note, are you using pressure treated timbers?
    – DJohnM
    Jun 18, 2015 at 17:05
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    It really depends on how many stones you add, and whether you're adjusting the concrete mix to compensate. If you just toss a few rocks in there it's probably no big deal, especially if they're evenly spaced and not touching anything. If you replace 1/3 of the volume with rocks, that will substantially undermine the strength.
    – Hank
    Jun 18, 2015 at 20:40
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    @DJohnM yes, it is pressure treated
    – Walter Stabosz
    Jun 19, 2015 at 3:59
  • FWIW, a non-cemented a-frame supports (with screw ground anchors) is going to be much more stable than cemented vertical supports. Remember that a swingset has to take large lateral loads. I don't know how much cement you are using in the above, but you'll likely have to use quite a lot o make it stable.
    – DA01
    Jun 20, 2015 at 15:40
  • A year after the fact, the swingset is going strong. No rot yet. My only mistake was to use 8' 4"x4" instead of 10' ones. This fall, I plan to bolt on some additional 4x4s to increase the height of the swingset. Also, this was my first time pouring concrete. I assumed it was less viscous than it actually was. Instead of soup/bisque viscosity, it was more like oatmeal.
    – Walter Stabosz
    Aug 8, 2016 at 1:47

8 Answers 8

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Concrete is a mix of large aggregate, small aggregate, and cement (a 4:2:1 ratio is a good approximation - though designed mixes will be more calculated than that).

The size of the large aggregate isn't particularly important, unless you are working in very tight spaces or around reinforcement, in which case you want suitably small aggregate.

In this case there won't be a problem at all using softball-sized rocks. Just make sure that they are clean (ie don't have soil stuck to them).

(I remember seeing a section cut through a piece of concrete at the Hoover Dam Visitor Centre, and some of the large aggregate in that was rocks up to perhaps about 8 inch / 200mm or more).

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    It is true that the size of the aggregate doesn't have a direct relationship to the strength of the concrete, but the amount of cement in the mixture does. If you add rocks to an already blended mix of conrete, you will make it weaker.
    – Joel Keene
    Jun 18, 2015 at 19:27
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    @donjuedo There's a little more to Portland cement than limestone, ash, and (or) clay; there's also the heating it to 1500 C to form synthetic minerals that do not exist in nature. A very large part of the strength of concrete is from the gel formed between cement particles and aggregates. Larger aggregates have less surface area to bond with. More aggregates means less cement to form gel. Absolutely adding more aggregates to concrete will make it weaker. If this were not so, the concrete blend would add more rock already, since it's about 1/20 the price of Portland Cement.
    – Joel Keene
    Jun 19, 2015 at 15:47
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    @donjuedo I should mention, for the sake of fairness, that I've been manufacturing Portland Cement for 15 years, and have spent a lot of time in quality and cement chemistry. I'm glad you're a fan of concrete. I am also.
    – Joel Keene
    Jun 19, 2015 at 16:29
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    @donjuedo Yes, it's absolutely true that aggregates increase the strength (and durability) of concrete over straight cement paste, but up to a point. Obviously a mixture of 100% aggregates would not be as strong as some blend of the two, so there's an optimum point. For standard bagged mixtures, the blend is on the side of the optimum point where adding more aggregates will weaken the mixture. In general, a 7-sack mix is stronger than a 6-sack mix, but if all you need is the strength of the 6-sack mix, then why pay the extra money.
    – Joel Keene
    Jun 19, 2015 at 16:33
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    @JoelKeene I'm not 100% sure you're correct. Yes, if you look at standard concrete mix tables, adding additional aggregate will effectively weaken the mix, but those tables are based on the assumption that your aggregate is in small (~10-20mm) pieces. Concrete with larger aggregate requires less cement for the same strength. Intuitively, I suspect that these two factors will approximately balance each other out, so that although you've greatly increased the total volume of aggregate, the average size has increased leaving the strength of the mix basically unchanged.
    – Jules
    Jun 20, 2015 at 20:34
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With regard to using wood posts in concrete:

enter image description here
Image Source

Note that any water that drains down the side of the wood post will drain through to the subsoil. The post should be in a collar, not a cup...

(Feel free to integrate into the answer of @sch )

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    Is everything in the picture necessary? If so, where can I buy cheap bunnies?
    – PlasmaHH
    Jun 19, 2015 at 8:32
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    @PlasmaHH The bunny is necessary to provide extra bracing on the downwind side of the post. If you live in an area with higher winds, it's recommended to use a badger or even a small deer.
    – David Richerby
    Jun 19, 2015 at 10:16
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    It came from here: familyhandyman.com/garden-structures/fences/…
    – DJohnM
    Jun 19, 2015 at 19:31
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    @ZachMierzejewski from the source image (which I've submitted a suggested edit for) it's "cedar with preservative."
    – Matt Ball
    Jun 19, 2015 at 23:45
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    This is correct assuming the earth drains well. I high-clay areas, you likely want to avoid below-grown timber all together.
    – DA01
    Jun 20, 2015 at 15:41
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It is not good practice to bury wood in concrete as you depict. Best to use metal anchored in the concrete and attach the wood to the metal. Your result will be inevitable rot and failure, probably not before the kids outgrow it but maybe. Something like this (http://milspecanchors.com/shop/accessories/6x6-post-anchor-heavy-duty-cast-aluminum-structural-ornamental/) would be better, but you will need some additional diagonal bracing for a swing set. Your design is rigid but short lived even with creosoted lumber. As to the rocks, they neither add nor detract to the concrete, use them at will.

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    Aren't fence posts usually embedded in concrete? How is that different?
    – Walter Stabosz
    Jun 18, 2015 at 22:48
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    Fence posts should have their bottoms buried in dirt (or gravel if you've got money to burn), and then the concrete (or dirt-ment, as I described earlier) is poured in as a "collar" around the post. Treated posts suck up water like sponges, so if the bottom of the post is buried in dirt or gravel the water can drain out the bottom of the post and all is well. But if you put the bottom of the post into concrete the water can't get out, and sooner than you'd like or expect the post will fall apart - it chemicals in the treatment keep it from actually "rotting" but the end result is the same.
    – Bob Jarvis - Слава Україні
    Jun 19, 2015 at 1:09
  • ah yes, you are right. I just read the lowes.com fence building tutorial. Luckliy I happen to have about 3 cubic ft of gravel also just sitting around my yard. For some reason the house came with big rocks, cinder blocks, bricks, and gravel. I turned the bricks and cinder blocks into a nice firepit.
    – Walter Stabosz
    Jun 19, 2015 at 4:04
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Reducing the amount of Portland Cement in the concrete will reduce its strength. That is, adding rocks to an already-blended mixture of concrete will reduce its strength over the same mix without the rocks. If this weren't true, they would just add rock to the original mix, since rocks are way cheaper than Portland Cement.

The other issue, as John mentions, is that the bond with your yard rocks may not be as strong as with the aggregates that the concrete mix people are using, as those aggregates have to conform to certain standards. Any powder, salt, or oil that is on the surface of the rocks will change the bond it will form with the cement.

Having said all that, whether this will impact your specific design just depends on how critical it is that your concrete perform per its design specs, ie, how much you've overengineered your design. However, you are certainly introducing a large unknown by adding the rocks.

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It is pier size that matters. My God, yes to the above answers if you were building a very serious structure. Yes less Portland the less PSI. 6 sack Portland cement is about 4000 psi. You can drive a 10,000 pond bull dozer on it all day. % sack which you can buy at Home Depot is about 3000-3500 psi. It is pier size. Dig a 12" x 2' and add some of your rocks, your swing set will be fine!

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    "[...] the above answers [...]" Please note that the order the answers are displayed in depends on how many votes they receive and/or user preferences. The answers that were above when you wrote the post are not necessarily the ones that you will see above your answer now, and not necessarily the ones that a user with different settings will see above it.
    – David Richerby
    Jun 19, 2015 at 10:13
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People have bound stone in concrete since...well, since they started using concrete. I don't think adding rock is a bad idea in principle but it matters what sort of rock you are adding. Adding round igneous rock isn't as good as adding broken igneous rock but both are better than adding sedimentary or aggregate rock. I'd support using a metal or plastic socket to seat your wooden posts as they will rot otherwise. At least with a socket you can replace the posts over time without having to dig up your concrete anchor. I'd disagree about forming a collar though. Not because it's not a good solution but it's not good for this use case. A collar is just something for the little ones to get hurt on. Indeed, I'd set my post anchors so that there is a few inches of dirt above so I could grow some grass around the posts. A quick splash of creosote up to ground level will help keep the water out of the wood and since you are using a socket you can treat it every 3-4 years.

This is all rather academic. In fact, almost any concrete, with or without stones will be more than adequate. You aren't expecting it to support a great weight or handle huge stresses. It's a swing set, not an antenna array pylon.

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You need the concrete to be strong at the bottom and next to the post, otherwise it just needs to be heavy.

So put some concrete in the bottom, the rocks round the sides and fill the center up with concrete.

You can even brace the post with a few bricks on two sides of the post least most of the post clear, then fill round with concrete. The concrete only needs to grip the post and have enough wight to keep the post stable.

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You don't really show the scale; my concern would be the posts + concrete mass rotating in the hole due to alternating side loading. Depends on how it is used, of course; little kids swinging +/-30..50 degrees of arc seems non-problematic, while a pack of (larger and heavier) teens swinging in sync and going for +/-170 degrees, or full wraparound.

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