mobile deconstruction

[david s]

During the deconstruction process I cut the outer oven in two to show construction. When I built this oven around 9 years ago the safer (exonerated as a carcinogen) blanket price was prohibitive, so I used not to use it preferring to only use vermicrete. The blanket shown is rather thin (old type) that I already had.The inner one piece casting has survived surprisingly well, with a few minor cracks. The outer shell also survived surprisingly well considering I made it as thin as I dared to save weight. It is only 10 mm thick with chicken wire reinforcing.

The second pic shows the decorative arch with 8 mm rebar reinforcement. Notice that its expansion caused cracks in the surrounding concrete. This explains why heavy steel rebar is a poor choice in oven design and why the recommended reinforcing is stainless steel needles. In normal structural steel construction the change in temperature is so slow that the steel and concrete will be much the same temperature and therefore much the same expansion. With an oven the temperature ramps up so quickly the highly conductive steel expands faster than the less conductive surrounding refractory causing problems. There was no sign of significant rusting although temperature will accelerate corrosion so stainless is a preferred material. The tiny diameter of the needles creates much increased surface area allowing them to dissipate their heat to the surrounding refractory. Lesson learned, I no longer use heavy rebar as reinforcing. The mix I made for the cast decorative arch was not a strong one as I was pinching weight. I used vermiculite/perlite for around 2/3 of the aggregate in the mix resulting in a far lighter but weaker cast. This may also have some bearing on the resulting cracks although I feel the first reason is more likely the culprit.

[david s]

I've recently rebuilt my mobile oven because it was full of cracks after nine years of service and abuse, although reluctantly because it still performed well and had become an old friend. A high priority for the original build was weight because it's designed to roll on and off the trailer and this ultimately led to some of the problems that later appeared.

The supporting slab was made of Hebel Power Panel (4mm internally cast steel reinforcing) that cracked badly, although I'm still unsure whether it was from heat or road speed bumps and corrugations. I used this material in the belief that it would provide both sufficient strength and insulation whilst making a considerable contribution to weight reduction. Hebei or AAC is around 1/3 the weight of equivalent standard reinforced concrete and I believe around a third the strength. Consequently I wouldn't use it again in the rebuild.I also had a 1" layer of vermicrete between the Hebel and the one piece cast floor to take the sting out of the floor in an effort to protect the Hebel from higher temps. I also used a one piece cast dome and a one piece cast floor. I was concerned about multi pieces rattling to bits having read that brick mobile ovens tend to do that. The lesson here is that a one piece floor cracks. Because of the uneven heat and therefore uneven expansion, you can expect any large cast pieces to crack. Not that it really matters because it still works adequately, however I now cast the floors in two pieces with a tongue and groove join.