Re: [ATM] portland cement; grind. etc.

["dune" <dune@shadow.freeserve.co.uk>]

The perfect recipe for making any concrete structure whether it be a 
grinding disc or building is control of water. Too much or too little will 
reduce the strength of the unit.

As a guide the concrete should have enough water to allow the full hydration 
of the cement within the unit but not enough so water capsules are left 
entrained. If larger discs are required you may place chicken wire in the 
uppermost surface which will become tension members.

Compaction of the concrete should be undertaken to expel all air. A 
reasonable tamping rod would be a 1" x 1" flat timber stick. I would suggest 
that after a minimum of three minutes compaction all air entrainment should 
be expelled.

As the concrete will be cast on top of the mirror with reasonable formwork 
about the disc the top surface should then be floated smooth.

After 24 hours drying in a warm room dislodge the disc and submerge it in 
water. Leave it in the bath for a minimum of seven days. This will ensure 
that the concrete will attain its maximum 7 day strength. The disc will not 
gain its 98.8 % strength until 30 days, so its up to you whether you leave 
it in the water till then, but if your like me you will want to use it 
sooner rather than later.

If anyone is interested I can teach you how to calculate your concrete mix 
by weight so you can end up with a known quantity.

Cheers
Glynne


----- Original Message ----- 
From: "Malcolm Macdonald" <toliman@wave.co.nz>
To: "atm" <atm@atmlist.net>
Sent: Sunday, June 25, 2006 12:35 PM
Subject: Re: [ATM] portland cement; grind. etc.


>
> ----- Original Message ----- 
> From: "KR" <kreid@primus.ca>
> To: <atm@atmlist.net>
> Sent: Sunday, June 25, 2006 9:24 AM
> Subject: [ATM] portland cement; grind. etc.
>
>
>> Greetings,
>>     The cheap grinding, polishing tool made from portland cement has a
>> limited life.
>> I've had two break unto pie shaped wedges after a years time. Just a note
>> to
>> those who
>> like to spend eternity perfecting.
>>                                                                regardes
>> KR
>>
>>
> Sounds like they are  shrinking as they dry out. How do you make them? eg 
> Do
> you bake them to dry them or use some kind of rapid drying agent.
>
>
>       I have been making and using concrete tools exclusively for about 
> ten
> years now and never had any problems.  I have so many in the shed now that 
> I
> contemplated making a garden path with them, but are too shiny and 
> slippery
> for that.
>
>
> My method :- Put a piece or paper/plastic on the glass blank, make a dam
> around the edge with Duct tape or anything water proof.  Arrange your
> Stones/glass offcuts/tiles, then pour over your cement. Tap the edge of 
> the
> class, vigorously, all round with a piece of wood  to remove air and make 
> a
> nice smooth back. Then leave a day or two to dry.  After a year they 
> Should
> be hard and ping like glass when tapped with a hammer. I use them time 
> after
> time and they seem to last forever.
>
>                               Cheers                       Malcolm.
>
>
>
>
>
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