I'm unable to find any information pertaining to "floating" concrete
when it is poured into a form. My ultimate goal is to hide all coarse
aggregate from all surfaces when poured into a mold. I plan to use
melamine surface for the mold.
So far, I've come up with two alternatives: Only use sand as
aggregate and option two is to pour standard concrete mix and then
after braking mold, cover all surfaces with mortar mix. While I plan
to have a rebar skeleton, I'm unsure of sand-only concrete strength.
And option two is not ideal as I'd like to bypass additional steps
post mold braking, if possible.
What I'm making are table legs that will be about 6"x3"x40". Surface
finish I'm looking for is similar to this --
i formed some concrete countertops using melamine as a form just using a
high strength sack mix. i formed them upside down so that the surface
against the bottom melamine was the top surface after flipping. i didn't
have any aggregate showing in either the sides or tops. i think the trick
was to vibrate the concrete when the mold was half filled, and then again
after it was full.
Charlie is correct. I have formed concrete in plywood & waxed
The resultant finish (smoothness) on the concrete is determined by
the smoothness of form & the amount of vibration.
The waxed tubes often resulted in a concrete surface as smooth as
plywood forms yielded a much rougher surface.
The cement paste "cream" adheres to the plywood & pulls off the
concrete mass yielding the rougher (almost a wood grain finish)
Since you're using melamine as a form surface, you've got a really
On the form open face, vibration will cause the aggregate to settle
below the surface so hand tamping will most likely not be needed.
On our formed laboratory models, we used a 3/8" gravel mix that was
70% sand, 30% gravel. This mix typically yielded 28 day compressive
strengths in the 4000 to 5000 psi.
Vibrating during pour sounds like the ticket. I'll have to wait and
see the results. Thank you both!
I will however need to reconsider the melamine choice as I'm not
looking for a glass-smooth surface. The goal is having a surface to
the like of 1000 grit sandpaper: smooth, yet not glossy. Any
Sonotubes can generate a glass finish..... interior designers love it.
The key to a glass finish is super smooth form material (hard to beat
waxed cardboard) and super low adhesion so as to not pull off the
glass finish when removing the forms.
The melamine will most likely give a smooth but not glassy finish
Per dpb comments...if finish is so important how about some test runs?
it was smooth and flat, but not glossy. i used a handheld sander with
diamond pads up to 3000 grit, then a couple different grades of buffs and
finally polish to get them reflective.
i doubt melamine will get you to a 1000 grit finish because it has some
roughness to it. you may have to hand finish the molds or the concrete to
get that smooth.
as others have said, do a couple 1' square samples using different mixes,
different amounts of vibration. you might even want to polish the melamine
with a hand sander. go easy with the sander, use very high grits and some
sort of dust extraction, as it's not a very thick layer of melamine. don't
breath the dust.
Perhaps spray or pour and tumble a coating of cement only slurry to coat
the form and let it set a bit before pouring the regular cement in to
finish the pour. A bit like fiberglass with the "gel coat" they spray on
the mold before they spray the fiberglass.
Isn't melamine the stuff that is killing people and animals from
getting into food? I dont know what it really is, but I would not be
messing with that toxic stuff. At least not without finding out more
about it. How is is toxic? For example, touching it, breathing the
dust, getting it into your mouth, vapors from it, etc. How can it be
disposed? This may be very dangerous....
Melamine is basically a plastic and is perfectly safe in it's intended
uses i.e. laminate on shelving, molded utensils, etc. All the problems
have resulted from melamine powder being added to foodstuffs
intentionally as a way to cheat quality control testing.
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