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concrete mold keeps cracking, why?

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lovetoskate
Posted: Fri Aug 05, 2011 8:17 pm Reply with quote
Joined: 18 Dec 2009 Posts: 38
I can usually just get about 15-20 boards per mold. Why does it keep cracking? Ive tried using quikrete fiber reinforced, quikrete non shrink grout, and nothing seems to be working for the long run. I just dont get it. I'm using 40 tons of pressure on them, and have a piece of 2x10 on top of them so that the channel steel isnt touching the concrete directly, and NOTHING WORKS grr!
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lox
Posted: Fri Aug 05, 2011 8:45 pm Reply with quote
Joined: 22 Jul 2011 Posts: 75 Location: phx
using minimum water in the mix and maximum cure time?
keep them wet for a few days after casting, damp towels for a few days after that, cover them in plastic if you live in a dry place
maybe?

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lovetoskate
Posted: Fri Aug 05, 2011 9:55 pm Reply with quote
Joined: 18 Dec 2009 Posts: 38
they just never look smooth and pit free either. I dont understand why. Should I seperate the mold a few days after and let the 2 halves cure? or should I do what I've been doing and just let them sit in the box for a month?

I always develop spider web cracks, and thats when I know i only have a few decks left to make before the mold blows up. It frustrates me. I try to use the least bit of water as I can, but its really hard to mix. I usually just add enough water so that I can get everything mixing.

What kind of consistancy should the crete be before pouring? mine is usually kinda like a slushie in that it stays somewhat together but can still be poured.

I do also get some air pockets... not sure how to vibrate the air out of the mixture.
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kirb-slider
Posted: Sat Aug 06, 2011 12:21 am Reply with quote
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Crete should be way "thicker" or viscous. More like a hella thick breakfast oatmeal.

Vibrate using an electric sander with no paper.

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lovetoskate
Posted: Sat Aug 06, 2011 1:33 pm Reply with quote
Joined: 18 Dec 2009 Posts: 38
how would i vibrate it with the sander? I do have one on hand i could use
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BartD
Posted: Sat Aug 06, 2011 10:59 pm Reply with quote
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Hold the sander on the outside of your form, like you would be sanding the outside and it will vibrate the crete inside the form.

Do you have pictures of how your molds keep cracking?

BD

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lovetoskate
Posted: Sun Aug 07, 2011 12:27 am Reply with quote
Joined: 18 Dec 2009 Posts: 38
I threw them all away, but they seem to crack directly to where the 'pockets' of the decks are, also in line of where i put the threaded rods. should i use some rebar or wire mesh next time?


BartD wrote:
Hold the sander on the outside of your form, like you would be sanding the outside and it will vibrate the crete inside the form.

Do you have pictures of how your molds keep cracking?

BD
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jsntarrant
Posted: Sun Aug 07, 2011 7:08 am Reply with quote
Joined: 29 Jul 2007 Posts: 315
lovetoskate wrote:
I threw them all away, but they seem to crack directly to where the 'pockets' of the decks are, also in line of where i put the threaded rods. should i use some rebar or wire mesh next time?



yes

you could even use both and fiber mesh...and for best strength, your consistency think chunky oatmeal, not slushee


Last edited by jsntarrant on Sun Aug 07, 2011 10:03 am; edited 1 time in total

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alldayphila
Posted: Sun Aug 07, 2011 9:10 am Reply with quote
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I have no experience in molds, but wonder if the following would eliminate the cracking.

latex additive
latex modified thinset
epoxy thin-set mortar
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lovetoskate
Posted: Sun Aug 07, 2011 10:35 pm Reply with quote
Joined: 18 Dec 2009 Posts: 38
when the mold is cured, would adding a "skin" of hydraulic cement or fiberglass to the outside help? i just dont understand why they crack.

I made a new mold today with some 5000PSI quikrete, and had an oatmeal consistancy. Hopefully in 28 days itll work
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