question about rocket mass stove in ground piping

I am new to the site and new to rocket stoves. I have been reading and watching videos. It all makes sense other than the pipes that run in the ground or thermal heat bank. I see that most seem to have duct work pipe. How does this hold up or is it only form for cob fill. Can you use concrete instead of cob?

John, from what I can surmise, 1) the gases have cooled considerably by that point 2) duct work pipe is less expensive than stove pipe, 3) yes it is mainly a  form for cob fill.  If the pipe fails the cob is still there.  

> Can you use concrete instead of cob?

Probably, I don't see why not.  Cob or gravel seem better though.  

In ground pipe is usually disadvantageous.
Unless the bit if earth that you want to heat is insulated from the rest of the soil,and protected from precipitation, the rocket will never adequately heat the thermal mass.

At least, that's  what I've read😏

Thank  you for the fed back.  I can see cob and concrete holding up in the cooler heat runs for the thermal mass and holding shape after the duct work has rusted out or such.  If I were to use gravel or rocks around the duct work, what is the life span of it, if I build a "good" rocket stove?

I think the lifespan of ductwork in a gravel mass depends so much on atmospheric/environmental factors that there is no one good answer. In a dry environment it might last decades; in a very humid environment it might rust out in a few years.

I would figure 5 years minimum on galvanized duct pipe  30 ga.
And 10 years minimum on galvanized chimney 26 ga.

If the exhaust temp of your barrel never exceeds 750F the galvanizing will not be burned off.
Then all you need to worry about is keeping the ash/debris in the system from attracting moisture,
and turning into either acid or caustic soup. So after your last burn of the spring, clean it all out, and seal it up.
In moist environments there will be dew forming inside the pipe when it is 95degrees and 95% humidity outside.
The mass of the RMH will be cooler than the outside air, and will attract moisture to interact with the soot.

The figures on time duration of vent and stove pipe are from trash burner use, 2 year for vent and 5 year for stove pipe.
RMH mass pipes are under a lot less stress.

TYVM  for all the information. I think I will just use the pipe as a form and use cob or concrete.  could i use chimney liner and then surround it with cob. I am a guy who likes to build it as "bullet proof" and durable as i can.

Chimney liner meaning stainless? If so, I would expect it to be very durable.

No. As in the square or round concrete liner for chimneys. It is kinda brown or tan in color.

So terra cotta flue pipe... That is resistant to high temperatures, but not to rapid temperature changes. In a RMH core it would have a very short life as it would crack up and probably disintegrate; in the mass it would last longer, but would crack at least near the core. Ductwork surrounded by cob would probably last about as long before leaking while being considerably cheaper.