Cast heat riser evaluation

[Jura]

Dear practitioners

I'm an absolute greenhorn  to casting but I have cast my first heat riser (18 cm)  from a high alumnia concrete. 

(5 kg of the cement for every 20 kg of aggregate)

A three of us had a marvelous 30 min-lasting dance around the cast form trying to vent it.

I believe it must have kinda reminded a native americans Death Dance around a torture  stake ;-)

But try as we might we didnt get a totally uniform surface :-(

We got such result.


It seems the gap/cavities created by meandering air bubbles are only at the outward surface.
(We have not seen any irregularities in the concrete structure while cutting out  the port)

What could be  the reason of  such gaps forming in this area?

[drooster]

Were your inner and outer mold forms made of two different materials?

[Vortex]

Casts need to be vibrated quite vigorously to get the bubbles to rise out, how were you doing it exactly? Also the amount of water you use is quite critical, as you need enough to make the mix thin enough for the air to rise out but not so much the mix is to wet. If you cast it vertically then I would think the air maybe had to far to travel to get out before it started setting. If you cast it horizontally then maybe they were trapped and couldn't get out?

[fzurzolo]

Yes to what Vortex just said. I've seen a lot of concrete (I get paid to be a civil engineer) and those voids look like pretty standard air bubbles to me. The one you are pointing at seems a bit rough but that could just be that some of the cement broke out upon demolding.

 If I was you, I wouldn't worry one bit about it. It looks quite good actually.

If you really hate air bubble voids and can't vibrate well then use a fabric form.  It makes for a really lovely finish as well.  But it might be hard to make a perfect cylinder with just fabric.

[Jura]

Thanks you all Guys!

Your words brought some balsam to my soul.

Apr 6, 2017 7:51:44 GMT -8 drooster said:

Were your inner and outer mold forms made of two different materials?

Yes. The outer layer of the cast form was a tube of strong cardboard dedicated to casting concrete pales (25cm in diameter)

The inner tube we used was a standard a 16cm PVC sewage pipe. (so we had to wrap it with some old PVC to get the desired 18 cm.)

We vibrated it in a vertical position with an electrical driller-hammer with a wooden tip and hammers in both hands.



(that's why I mentioned the Indians' death dance ;-))

As we were jumping around the vertically placed cast form wielding hammers as axes


I'm afraid you are right as to the concrete density.

We probably didn't add enough water I was discouraged by all the sources warning against adding too much water.

So the air bubbles had a hard way to make it to the cast surface

[pigbuttons]

Casting concrete is more of an art than a science and requires a large experience pool to draw from. Your casting looks pretty good and if those are the worst of your air pockets you may have a usable piece. Too much water is far worse and high alumina cement is very easy to over water, it likes to be on the thicker side. That requires more finesse to get it defect free, starting with having to vibrate the casting at about 4" intervals ( also known as lifts ), no more than 6". You don't mention what kind of aggregate you used, if stone then it is easier to vibrate, if perlite or some other light weight material then it gets much more difficult. A concrete vibrator is of great value with stone aggregate but not at all with light weight stuff. You can also use a 1/2" dia. rod to stir and pogo stick ( up and down action instead of circles ), and especially around the edges near the form surfaces. Then add another 4 to 6 inches and repeat. It is also helpful if the forms are moist when starting so the cement mixture doesn't want to stick and that will allow the air bubbles against the form to move more freely to the top. A simple homemade vibration table can be made by attaching a small orbital sander to light gauge work bench which will work as well if you make even thinner lifts. There is of course dangers with vibrating the mixture with stone aggregates as well, in that if you over vibrate the stone will settle to the bottom or light weight stuff will float to the top. This is where experience comes in to know how much is enough and how much is too much.

Another option with high alumina cement is to leave it much thicker, just barley plastic, and pack it down like you would rammed earth. In this case you would do it in 2" lifts instead of the 4" lifts above. Because it isn't really a liquid it won't trap air as easily but you have to be thorough and patient.

The mixing paddle on the end of the drill is probably counter productive and would more likely add air pockets rather than remove them in my opinion. The rod being pogoed up and down is a much better option for most applications.

[grga]

This is a nice cast! Small air pockets at the surface are harmless and are always there especially if you have low-water mix. From your figs I would say that you have probably more water than just the right quantity of water.



I have some experiences with regular concrete and made a few tests with CAC. Regular and CAC cement is stronger (compressive strength) if you use less water, however if to dry it is impossible to make dense and compact cast (at home with the drill vibration) and then by my opinion is not mechanically strong enough. So probably more water is better but then you need to carefully dry it and you can have some small thermal cracks. And thermal crack are also by my opinion something which is probably impossible to avoid if having bigger and complicated concrete parts as yours. Your riser would very probably crack when heated very hot. If it would be composed of two halves cracking maybe be avoided.



Other thing which would be nice to know is what aggregate you used. Silica send should be good to 500C, granite 800C, schamote 1100. For regular sand it depend on its structure ?



Do report when you fire it, I keep my fingers crossed.

[Jura]

Apr 7, 2017 1:39:36 GMT -8 grga said:

This is a nice cast! Small air pockets at the surface are harmless and are always there especially if you have low-water mix. From your figs I would say that you have probably more water than just the right quantity of water.

We added a really minimal water amount to obtain some plasticity of the mixture although two friends of mine seeing the consistency of the obtained mass were pushing to add more (they have some experience with normal concrete cast)& I had to use a lot of skills to convince them to stay on the "minimum side".

Apr 7, 2017 1:39:36 GMT -8 grga said:

I have some experiences with regular concrete and made a few tests with CAC. Regular and CAC cement is stronger (compressive strength) if you use less water, however if to dry it is impossible to make dense and compact cast (at home with the drill vibration) and then by my opinion is not mechanically strong enough. So probably more water is better but then you need to carefully dry it and you can have some small thermal cracks. And thermal crack are also by my opinion something which is probably impossible to avoid if having bigger and complicated concrete parts as yours. Your riser would very probably crack when heated very hot. If it would be composed of two halves cracking maybe be avoided.

Thanks very much! for sharing knowledge!
We considered the issues. and decided for an optimistic approach ;-)
Peter touched the issue in the theoretical part of his website.

Thou the cracks appeared it did not hurt much as it started to act as a dilatation.

We will fire it a few times before installing it in the bell and leave a inspection bricks so as to be able to peel of insulation and peek into it after a year o two.

Apr 7, 2017 1:39:36 GMT -8 grga said:

Other thing which would be nice to know is what aggregate you used. Silica send should be good to 500C, granite 800C, schamote 1100. For regular sand it depend on its structure ?

ugh!
Now I can see I forgot to mention the used aggregate type (shame on me).
Here we decided to use the andalusite one, which was considered to have a better thermal shock properties, and higher thermal resistance (up to 1500 C)

Apr 7, 2017 1:39:36 GMT -8 grga said:

Do report when you fire it, I keep my fingers crossed.

Thanks.
I Promise you that!

[Jura]

Apr 6, 2017 19:34:08 GMT -8 pigbuttons said:

Casting concrete is more of an art than a science and requires a large experience pool to draw from......

Hmmm ...

To me as a novice in casting your post has been of great importance and IMHO its content shall be put in a FAQ sticky chapter at the beginning of this thread ("Materials castings, refractories etc" I mean)

So herewith I'd like to propose creating and placing there the info you provided.