Too massive Mass-Heater

First, lets remember the J-Stove / Rocket Stove project: https://youtu.be/V2q4g-5P8Js

To capture the heat from the J-Stove, a secondary stove pipe connects into a separate mass, the pipe then exits the mass and rejoins the main J-Stove stove pipe to flow out the chimney. This is the mass form I designed and made. It will warm to a few hundred degrees durning a 40 minute burn of the J-Stove and radiate heat for 8 to 12 hours.

The side vent of the stove fits into the hole pictured below- the mass form is upside down and will have a welded metal platform to align it with the side vent.

This is the mass heater form / mold, inverted for pouring in the mass. It will have stove pipe connections and tubing added, along with rebar and expanded metal to add strength to the mass of aircrete (foamed concrete). This form provides a large mass and a bench for warm seating.
The holes are where the tubing will enter and exit, and the L shape is the bench. The form is made of one 4×8 sheet of Laminated 3/4″ MDF. It is screwed together, then hot glued, then silicone sealed along all edges and some exposed MDF was taped over as well to create a water tight form that could hold hundreds of pounds of aerated concrete.
The fins under the seating ledge provide support for the concrete weight it will bear.
It has to live through the pour, holding hundreds of pounds of liquid concrete, then after the concrete cures it has to be able to come apart and knock free of the internal mass.
The form had to wait a bit on the order of two 6″ long sections of 4″ diameter double-wall pellet stove vent that fit into the enter/exit holes. I thought I had the right pieces already…then tape on the flex tubing and add in a rebar cage with expanded metal to carry heat through the mass.
Another layer of expanded metal is added to bolster the seating platform (remember, the form is upside down / inverted for pouring.
All connections have to be secured from the outside, and water tight.
Hats off to the Honey-Do Carpenter for the aircrete cannon and aircrete recipe- just add warm water and a bit of shampoo (and my air compressor) for foam too thick to shave with. Then I use a “barber-pole” drill bit to lift the concrete mix from the bottom of the bucket into the foam laid over it, until all the concrete is suspended in the foam. It seems impossible, but it works.
The aircrete mixing station: 17 five gallon buckets will fill the form- at about 1/3 the weight of concrete. So about 300lbs, should have been 275 but the first three buckets were a bit off. Nora is next to the drill with the concrete mixer attached (black), the silver spiral between Nora and the drill is what I used to blend the concrete into the foam.
The main mass of the form is more than half full. I’m using a red dye in the aircrete.
This is all very tiring, and I’m only half convinced it will work- the form could blow out at the sides/bottom, the aircrete might not set, and even if it does the thing is a lot more massive than I had conceived.
4 hours after the pour is finished and not even beginning to cure out.
I wake up with a dread that the form has popped overnight, and the concrete never set, and it all flooded into the pond and killed all the fish and ruined the pump and waterfall. Now, 5 days later, and after 3 nights below freezing wearing a foam cap, it is solid and curing toward a respectable hardness.

I’m letting it cure into next week. There has been some small shrinkage/settling that will need another round of pouring- I may use a fast setting concrete to make a stronger platform for the mass, then I will remove the form. If all that is successful, I’m going to cut it down in size quite a bit. This is designed as the ideal form to have at the ranch that can fit into the back of the pickup, but it is too heavy to move without at least two more strong guys and a bit much for a 500 mile trip in the back of the little pickup. I’ll remove the bench and bevel an angle into the body of the mass, dropping around 100lbs. If all that works out, it will travel up to the ranch sometime this fall for installation and testing.

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