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Posted: Wed Nov 14, 2007 7:14 am
by TheVictorianCobbleCo
Hi guys, just to say I'm back after being away for a while, but secondly - have started a branch in Cape Town making slabs etc, have always used 10 -13mm stone but the quality here is so bad that I had to move down to 6mm stone - looking at the detail on this website, a slight change in proportions is all thats needed to maintain similar strength - am I correct? I aim for 30 -40Mpa and typically cast 37mm thick, tho my cobbles are 50mm thick. Last question, on the surface of some cobbles, once demoulded, I get a "diamond mesh " effect - like you can see the outline of the stones just below the surface, With time it fades - I'm assuming my guys add a bit too much water or leave the mould on the vibrating table too long. Any comments?
Posted: Wed Nov 14, 2007 1:09 pm
by Ted
With the slabs only being 37mm thick, I think a 6mm aggregate may be better. I was once told that the maximum aggregate size should not exceed 20% of the distance between form faces.
Also a 6mm aggregate should produce more surface area than say a 13mm aggregate for the cement paste to attach itself too, thus increasing MpA through increased cement usage.
South Africa is a very competitive market though is it not? I know loads of people in Mpumalanga, KwaZulu-Natal and Gauteng who are into precast slabs. Many don't even use a course aggregate in a situation such as this.
Have you got any photos of this "diamond mesh" problem?
Have you tried this place for information on sourcing good quality aggregate - www.aspasa.co.za..??
I've never heard of people having aggregate problems in SA. Most of the people I know are currently more concerned about the cement shortage. Is this affecting you? All the decent cement is being used up for the World Cup construction work I hear...
Decent aggregate is available... have you called PPC Cement in Pretoria? www.ppc.co.za
Posted: Wed Nov 14, 2007 10:37 pm
by lutonlagerlout
i was under the impression that most pre-cast slabs in the uk were made using <10 mm aggregate,like you say bill it needs to be right to get the thing strong enough
cheers LLL
Posted: Fri Nov 16, 2007 9:54 am
by TheVictorianCobbleCo
Thanks to both of you, appreciate the answers. My first load of 13mm was fine, the second load had anything from below 6mm up to 19mm, and with the oddest shapes, very little uniformity. the supplier eventually admitted to having holes in his dividing screens. I've settled on the 6mm now with good results. Cement is still reasonably available, tho I'm small time in comparison to the bigger guys, and buy it in 50 kg bags, both rapid hardening and normal, all from PPC. SA is big on these slabs, Indian sandstone still unknown, the guys are not keen on anything much bigger than 450 x 450, and cladding in various forms extremely popular and a lucrative product. As soon as I have a pic of the diamond effect on the cobbles I'll post it. Cheers