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 2×4 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)
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.
Best Answer
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).