Cemforce's Ingredients

Well, to my perhaps more-than-slight disappointment, the ingredients Cemforce used were different to what I was using. For a start, they are using a Cement:Sand ratio of 1:1, whilst I was using 3:1, which means they are using 3 times more cement in their mix than I was. Like me they add a plasticizer to the mix to reduce the water content, whilst at the same time making the concrete runny enough to spray it into the mould, but they also add an accelerator, which allows the concrete to cure faster.

For the reinforcement, they use alkali-resistant glass fibre rather than polypropylene, as they found that the polypropylene tended to stretch before reaching its full tensile capacity, during which time the concrete began to crack (I have some glass fibre mesh, but haven’t got round to testing it yet). The fibres are chopped, rather than woven, cos its cheaper that way, and they can be sprayed into the mould, rather than having to be placed by hand (see the the video below).

The Cemforce loo

Cemforce are using a panel made up of a mix of 28mm concrete and polystyrene mix sandwiched between two layers of Glass Fibres mixed with concrete, as you can see below.
These panels are then assembled into latrines. Their factory in Kimberley is semi-automated, and has about 120 production workers, producing, an average of 4000 loos per month. They sell complete units, including the pit lining, toilet slab, and toilet house, to Contractors who have won government contracts to build latrines for the poor.

Here's a little video of one of the early stages of the panel production line:



A layer of wet concrete is sprayed onto the mould, then a mix of wet concrete and glass fibres is sprayed on. This produces the first Glass reinforced Concrete layer. The next stage of the production process is that a layer of polystyrene and concrete mix is packed in by hand, and then another glass reinforced Concrete layer added.

It was pretty cool to see the factory in action. It wasn't a huge place, and it seemed like it was something that Esibayeni could handle. Wicus was happy to give us plenty of info, and I almost broke my pen trying to scrobble down all the stuff he was telling us!

Slab Success!

Haha! It Works!! I made a new slab, this time with bigger stones, and 50% thicker. I went a bit over the top with the thickness, but I wanted to be sure that I wasn’t wasting my time and that it was actually possible to make a dome shaped slab that was (a) unreinforced, (b)less than 100mm thick, and (c) could still hold at least people without breaking.
Also, I put a post up on an online toilet forum (yes, I’m not the only sad g#t on the net trying to make loos!) and a South African guy, who’d made PLENTY of these dome-shaped slabs before (see what he had to say by clicking ‘comments’ below). Anyway, he’s not only provided me with some handy hints and tips and contacts, but has also offered an old fibreglass mould for the dome shaped slab – sweet! This means that me and Siyabonga won’t have to make a mould out of sand every time – and its recycling as well! (my sister and steve will be proud of me!)

The Case of The Missing Concrete

When I began making up my Concrete mix, I used a Cement:Aggregate ratio of 1:3 by VOLUME, then added enough water to make the mixture as runny as I wanted. For example, I used 4L cement and 12L of sand, and 4.2L of water. Which I assumed would give me 4+12+4.2=18.2L of concrete.

Big mistake!

The finished test panel seemed quite thin, so I measured the ACTUAL volume of mix that I had made up. To my surprise I found that I had only 12L of concrete!? I had lost 6.2L of Concrete! Then I had a think...(never a good thing)

Between the Cement and Aggregate (and to a tiny extent the Water) particles there are air gaps. When you mix them together, the particles fill these gaps.

And so that is why the concrete seemed to be disappearing.

So the ACTUAL volume of concrete produced was about 1/3 LESS than the CALCULATED volume of ingredients put into the mix.

So as long as this pattern is the same every time I mix the concrete (which is pretty likely as long as I use the same cement:aggregate:water ratio and the same type of aggregate). Then all I need to do is increase the volume of ingredients by 1/3.

Crisis over! Hurrah! :-)

Pouring and Curing the Panels

Its amazing (for me at least!) to think that despite having spent 3 years at university studying Civil Engineering, and spending about 1/3 of that time learning about the wonders of Concrete, I’d only actually made the stuff once before. And so with a little trepidation I began casting the Textile Concrete Panels…

Just to warn you, if you’re not that interested in Concrete (ha ha, as if!) then read no further!

Concrete has 3 ingredients which you mix together: Cement, Aggregate and Water. I always used to get the words Concrete (the end product) and Cement (an ingredient) mixed up, but I digress… The Cement and Water react with each other over time and become hard. Aggregate is basically stone or sand, (both of which are a lot cheaper than Cement) and provide bulk to the Concrete. The ratio of Cement, Aggregate and Water you mix has a HUGE effect on the strength of the finished Concrete. Briefly…

Cement

The more there is, the stronger the Concrete, but remember, it’s the most expensive ingredient!

Water

It’s a bit like Goldilocks and The Three Bears. You gotta get the porridge just right!

Too much water: Makes the mix runny and easy to work with BUT as the mix dries, lots of air gaps are left AND the reaction between the Cement and Water is too quick so you get a WEAK CONCRETE. :o(

Just enough water: The mix is easy to pour AND there is enough Water for all the Cement to react fully so you get STRONG CONCRETE. :o)

Not enough water: The mix is stiff and so can be hard to work with AND there is not enough Water to react with all the Cement, so you get a WEAK CONCRETE. :o(

Aggregate

For the Textile Concrete Panels I need a runny mix that it will seep between the fibres of the textile, so I've been using use a fine Aggregate (sand). At the moment I've had to use a combination of fine sand from a nearby riverbank, and coarse sand from a big pile that happens to be lying around near to where I've been working.

I've been experimenting with different mixes to find the right Cement:Aggregate:Water mix, which takes time, but once I know these mixture ratio’s, the rest is easy! (I hope) :o)

Curing the Concrete

Once mixed and poured into a mould, you need to ‘Cure’ the Concrete. This is a way of slowing down the reaction between the Water and Cement in order to make the finished Concrete as strong as possible (anyone remember growing crystals at school, cos its got something to do with that). It was a surprise to me just how important the Curing process was, but after writing almost a page on how and why I cured the panels I realised was a bit too much to include in this blog, so if you really want to know, please leave a comment!

PHEW!