Hiding aggregate in concrete pour

[fourrings]

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 --

TIA, Dennis

[charlie]

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.

[BobK207]

Dennis-

Charlie is correct. I have formed concrete in plywood & waxed cardboard (SonoTube).

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 glass, 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 smooth surface.

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.

cheers Bob

[fourrings]

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 suggestions?

[dpb]

It ain't gonna' be glossy out of the mold, even w/ melamine.

Also, ain't nothin' like a small practice piece or two (or three)...

[Pete C.]

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.

[gfretwell]

Vibrate the forms with a rubber hammer, it will bring the cream out and knock the gravel back.

[BobK207]

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

OP-

Per dpb comments...if finish is so important how about some test runs?

cheers Bob

[oldtimer90]

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....

[Pete C.]

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.

[fourrings]

...

Solid plan. I will be making a test run (or two) to check out results of the surface prior to final pour.

Thanks to all for advice!

[charlie]

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.