×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Temperature gradient through a pit wall

Temperature gradient through a pit wall

Temperature gradient through a pit wall

(OP)
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.

RE: Temperature gradient through a pit wall

     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.  

RE: Temperature gradient through a pit wall

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.

www.SlideRuleEra.net idea

www.VacuumTubeEra.net r2d2

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources