emperature gradient through a pit wall

huangyhg

temperature gradient through a pit wall
i am working on the design of an interior concrete heat treating pit about 25'x 40'x 10' deep.  my question has to do with the fact that the temperature inside the pit will be around 150 deg. f while the soil around the outside of the pit will be around 60 deg. f. there will be a 90 deg temperature gradient through the walls and the bottom slab.  i computed the thermal expansion through the 10" pit wall and it seems negligible (.01"), but i'm wondering if the steel rebar will "sink up" the heat and create a problem.  should i be looking at thicker wall for protection or rebar in each face cover the difference in expansion?  the computed expansion in the 40' length of the pit is .313" which is less than i would have expected.
any thoughts on this would be appreciated.
     yes, the steel will transfer the heat much quicker than the aggregate matrix.  however, the rates of thermal expansion of concrete and of steel are very similar and any stresses induced due to the differences in expansion can easily be handled by the inherent strength of the composite material.  the rate of temperature increase should be held to a modest change per time, i.e. for fresh cast concrete in a pre-cast factory the rate of temperature change was limited to ten degrees fahrenheit per hour with an upper limit of 160 degrees. this produced crack free product with f'c of 5000 psi after ten hours steam cure.  a layer of exterior insulation would contain the heat in the mass of concrete and slow the gain of the soil for lesser heat production requirements.  
imho, as a former mechanical engineer, the concrete, steel and even the surrounding soil will reach thermal equilibrium after a period of time (assuming the 150 degree temperature is maintained). will probably take at least a couple of days to reach steady state. i doubt if fact that the steel conducts heat better will be a problem.
any issues will most likely surface if the inside of the pit is routinely heated and cooled causing the interior face to expand/contract at a different rate than the exterior face. plenty of rebar should manage that.
at electric generating stations in-ground reinforced concrete sumps, containing "hot" liquids are common. i'm not aware of any problems. and, where i like, this is with the exterior face of sump being continuously "water cooled" by a high water table.