jon hergert
09-10-2006, 06:35 PM
Just finished pouring three stones this weekend. Did some research with a concrete foundry, got some special admix and came up with about the nicest looking stones that I have ever seen and I have seen quite a few. Thought i would share my learning experience.
Started off with Slaters hardware 18, 20, and 21" molds. I had a lot of space available so i dug a hole in the native sand for each mold and buried each half way. When i used to use a tire i always got a ring on my stones from the bead and with really heavy stones, i have collapsed the bead before. If you have the space, i find digging a hole for the mold is much better.
Also, i ran out of the silicone release spray and ended up using a light coating of wd40. Not sure if it was the mix or the wd40 but the molds released a lot easier and cleaner than they ever had in the past.
Rented a cement mixer and bought 1/2 pallet of quikrete 5000. Using the mixer, i made up an extremely dry mix, 1/2" slump, so dry the concrete was forming little concrete balls in the mixer. This is such a dry mix that you couldn't practically do this by hand, mixer is really required. Also, using a mixer, you are going to spill some concrete and waste some, so this will definately require more bags of concrete plus i suspect you get a denser concrete mix as well.
With the mixer going, i added about 4-5 cups of super plasticizer to the mix. Super plasticizer is a chemical compound i got from the concrete foundry that temporarily binds with the cement in the mix, imparting a charge to the cement molecules, causing them to repel one another, forcing the cement to evenly distribute itself and making the mix much more inviscid. The mix within a couple of minutes changed from dry scratchy little concrete pills to pancake batter.
Then poured the mix into a five gallon bucket and then poured into the molds. If you are planning on suspending a weight in the center of the mold, you need to have it at height before you start to pour as within just a few minutes after the agitation of the concrete mix stops, the mix will revert to a consistency similar to cold tacky and from experience, you will not be able to move the weight off the bottom.
With a high early strength concrete mix like quikrete 5000 plus the super plasticizer, this mix goes off really early. The concrete that i spilled on the ground, I was standing on in less than 15 minutes without leaving footprints. Within an hour, the concrete had already settled and a little bit of water had risen to the top of the mold.
When i used to pour stones with a much more typical 3 or 4" slump, i would have 6" of water at the top. Now with this very dry mix, the water is only 2 or 3".
To finish off the very last part of the stone, i would add 1 cup of dry concrete mix at a time to the top of the mold and poke it with a piece of rebar to mix it into the upper 3" of the mix. When it got to be thick, i stopped added material and just let it set.
I stripped the molds today and had stones without any voids whatsoever, night and day difference to way my previous stones turned out. Concrete was hard enough so that stripping in less than 24 hours isn't a problem, which would have allowed me to easily pour two sets this weekend if i wanted.
Started off with Slaters hardware 18, 20, and 21" molds. I had a lot of space available so i dug a hole in the native sand for each mold and buried each half way. When i used to use a tire i always got a ring on my stones from the bead and with really heavy stones, i have collapsed the bead before. If you have the space, i find digging a hole for the mold is much better.
Also, i ran out of the silicone release spray and ended up using a light coating of wd40. Not sure if it was the mix or the wd40 but the molds released a lot easier and cleaner than they ever had in the past.
Rented a cement mixer and bought 1/2 pallet of quikrete 5000. Using the mixer, i made up an extremely dry mix, 1/2" slump, so dry the concrete was forming little concrete balls in the mixer. This is such a dry mix that you couldn't practically do this by hand, mixer is really required. Also, using a mixer, you are going to spill some concrete and waste some, so this will definately require more bags of concrete plus i suspect you get a denser concrete mix as well.
With the mixer going, i added about 4-5 cups of super plasticizer to the mix. Super plasticizer is a chemical compound i got from the concrete foundry that temporarily binds with the cement in the mix, imparting a charge to the cement molecules, causing them to repel one another, forcing the cement to evenly distribute itself and making the mix much more inviscid. The mix within a couple of minutes changed from dry scratchy little concrete pills to pancake batter.
Then poured the mix into a five gallon bucket and then poured into the molds. If you are planning on suspending a weight in the center of the mold, you need to have it at height before you start to pour as within just a few minutes after the agitation of the concrete mix stops, the mix will revert to a consistency similar to cold tacky and from experience, you will not be able to move the weight off the bottom.
With a high early strength concrete mix like quikrete 5000 plus the super plasticizer, this mix goes off really early. The concrete that i spilled on the ground, I was standing on in less than 15 minutes without leaving footprints. Within an hour, the concrete had already settled and a little bit of water had risen to the top of the mold.
When i used to pour stones with a much more typical 3 or 4" slump, i would have 6" of water at the top. Now with this very dry mix, the water is only 2 or 3".
To finish off the very last part of the stone, i would add 1 cup of dry concrete mix at a time to the top of the mold and poke it with a piece of rebar to mix it into the upper 3" of the mix. When it got to be thick, i stopped added material and just let it set.
I stripped the molds today and had stones without any voids whatsoever, night and day difference to way my previous stones turned out. Concrete was hard enough so that stripping in less than 24 hours isn't a problem, which would have allowed me to easily pour two sets this weekend if i wanted.