Concrete Calculations

[di11on]

Re: Concrete Calculations

Thanks again Dave.

I'm wondering, since my oven is cast in sections - would it make sense to do some curing before I mortar them together? There'd be a lot more surface area for moisture to escape.

In this case, are there any special precautions I should take? Otherwise, I could just follow the standard instructions with my un-mortared dome and then some again once I've mortared. Would that make sense?

[david s]

Re: Concrete Calculations

Using an alternative fuel is a very good method, but we're not allowed to discuss it on the forum. You may be able to find some good solutions on the curing thread or PM me.

[di11on]

Re: Concrete Calculations

Thanks David, really appreciate the advice.

I did add burn-out fibers. I cut up some polypropylene rope. Not much though, and I didn't separate the fibers very much. Nice weather here in the South of France... I might just let it dry naturally for a good while before firing up. Since mine is in sections, I could, for example, turn them upside down in the sun to dry the inside.

Here is the advice from the Ciment Fondu website:


It is advised that the first firing does not take place
before 48 hours after casting. Then to carry out a
gradual rise of 50?C/hour and holding for at least 3
hours at 110?C before then continuing up to 500?C.

What do people generally use to control the temperature of initial firings?

[david s]

Re: Concrete Calculations

By casting your dome in sections you are telling it where you want the expansion stresses to occur. If you cover over the dome with another layer any expansion cracking will take place where you have designed it, rather like grooves in a concrete slab.

Re driving out the water, firstly, did you add any burn out fibres to your home made castable? If not then you will need to proceed extremely slowly. Recommended firing schedules for commercial castles (that do include burn out fibres) are in the range of 25 to 50 C /hr Particularly for the 100- 300 C range where steam explosions are most likely to occur. The temperature of the castable will vary wildly between the top of the dome and the base as the base is way wetter and cooler. You can't take it too slowly, but you certainly can take it too fast. If you are having fine weather then leave it exposed to the sun and wind for a couple of weeks to do much of the drying for you. It's a bit like trying to fire a clay pot that hasn't dried. It can be done, but is way more risky.

The firing will not add extra strength to your castable unless you fire it to around 1000 C. Do not attempt to do this as rapid chemical changes take place around 550 C that can damage the refractory and firing with wood is impossible to control in a slow enough temperature rise to do this safely at this range.Your mix should perform ok, but will never be truly sintered.

[di11on]

Re: Concrete Calculations

Ok, so I've cast my dome... using a ciment fondu/grog/sand mix. Just waiting for the last section to cure.

What's the deal with curing fires? Do they just speed things up, or do they lead to better strength/performance?

[di11on]

Re: Concrete Calculations

You can add on another layer straight away, but it won't be as good as if if it were cast in one go. The other alternative is to leave the casting as it is and thicken it up when the oven castings are all assembled.

Thanks David. When you say thicken it up one assembled, won't I risk cracking if I cover over the assembled sections?

[david s]

Re: Concrete Calculations

You can add on another layer straight away, but it won't be as good as if if it were cast in one go. The other alternative is to leave the casting as it is and thicken it up when the oven castings are all assembled.

[di11on]

Re: Concrete Calculations

Hi folks,

So I've actually cast the first of 4 sections of my dome. Everything going well so far. I used iced water and had plenty of time to work with the mix. The batch I prepared left me a little short by 1 to 1.5 cm of thickness (I was shooting for 7cm). Is there any problem with adding another layer? If so, do I need to wait until the previous layer has set? I presume I would need to use the same mix?

[david s]

Re: Concrete Calculations

Calculate the volume of your dome (using outside radius to calculate total volume, subtract inside volume using inside radius) get the volume required, but don't forget to halve this because it's a hemisphere, then add the 20% for reduction on mixing. The extra required where the oven door sticks out at the top will be about the same as what you lose in the actual door opening.if you also want to cast an entry you'll need more castable for this too.

[Toomulla]

Re: Concrete Calculations

When I mixed concrete in a mixer on site rather then ready mixed the accepted ratio was 21= 18 which is about 85% or a 15% reduction this mix has aggregate so the mixing of a brew with more fine aggregate May result in less volume.

My experience is that more material is consumed by wastage and additional volume then measured , having poured many concrete slabs experience is that you always use more then the theoretical calculation so be conservative

[di11on]

Re: Concrete Calculations

Wet volume is approximately 80% of the dry material

Just looking into this a bit more...

In most of the forums I've read that you multiply the desired volume by 1.54 (or 1.64 if allowing for 10% wastage). But I guess this is for regular portland based concrete. 80% as you quote would be equivalent to multiplying your desired volume by 1.25 which is quite a bit lower. Is the 80% figure specific to refractory cement?

My calculations show that I will need just less than or just more than one 50kg bag of grog (depending on which calculation you go with), so I'm wondering if I need to get a second 50kg bag or not (costing me an extra €75). What would the impact be of less grog and more sand? Less thermal mass? Weaker structure? At the same time, an advantage of getting a second bag of grog is that I would get a different granularity giving me a better distribution.

Thanks in advance

[di11on]

Re: Concrete Calculations

The density of castable refractory also varies depending on brands and what type you get. Go for dense castable not insulating castable for the inner dome. The stuff I use is approx 2 Kg/litre or quoted as 2000 Kg/m3 Check the specs on the stuff you are getting.

Hmm... I was gonna go with this stuff... CIMENT FONDU? but it says it has a bulk density of 1100kg/m3

[david s]

Re: Concrete Calculations

The density of castable refractory also varies depending on brands and what type you get. Go for dense castable not insulating castable for the inner dome. The stuff I use is approx 2 Kg/litre or quoted as 2000 Kg/m3 Check the specs on the stuff you are getting.

[di11on]

Re: Concrete Calculations

Wet volume is approximately 80% of the dry material

Thanks for that - using that formula says I need about 10% more than using my assumed density formula - I hadn't account for any error, so that gives a good number to work with.

[Toomulla]

Re: Concrete Calculations

Hi folks,

I'm trying to work out the weight/volume of ingredients I'd need for a given volume of concrete. I presume the volume of concrete you end up with is not the combined volume of the ingredients?

Edit:
I know the volume of the casts...

Do I just assume a certain density for the finished concrete and thus calculate the finished weight and then assume that the combined weight of the ingredients should equal this weight?

Wet volume is approximately 80% of the dry material