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  • 1.  External Temperatures on Foundation Rebar

    Posted 03-03-2020 03:25 PM

    I am working on a project where there are external "high" temperatures on a foundation due to equipment operation and I have the following questions:
    When designing foundation rebar for external temperatures due to external environment (in my case 70 degrees C), is it common for the thermal temperature to govern the rebar design (causing additional reinforcement rebar))?

    Since the external heat is above the foundation, would this only affect the top rebar? If so, to what depth?

    What would be the effect on the foundation if no extra rebars are added and what would be best mitigation method in case the foundation is affected?

    Thank you,

     Rocio Rodriguez

    Rocio Rodriguez P.E., M.ASCE
    Structural Engineer
    Siemens Energy Inc.
    Orlando FL

  • 2.  RE: External Temperatures on Foundation Rebar

    Posted 03-04-2020 08:57 AM
    ​It's been a while since I had to deal with high temperatures on foundations (first 7 years out of school dealing with steel mill equipment....), but I'll give it a shot:

    - extreme high temperatures - I don't recall the threshold - can deteriorate the concrete itself regardless of the amount of rebar.  I don't suspect you're near that temperature.  Think building fires.

    - spalling of the concrete surface down to the layer of rebar, though,  can be an issue at high temperatures that are below the threshold.  I believe that differential temperatures with in the concrete contribute to this - for instance where the surface temperature rises fairly quickly and the internal core of the concrete stays cool.  I don't think that's as much of an issue when temperatures rise more slowly and the entire mass of concrete can slowly ramp up to the high temperature at a uniform rate

    - Concrete foundations in high-temperature uses can also be damaged by thermal expansion and contraction of the equipment it supports.  The equipment is typically made of steel or some other metal and expansion can be quite rapid.  The thermal rise and expansion of the equipment can be much greater than that of the foundation, particularly if the foundation is a large mat in contact with the ground (a heat sink or cooling source).  That differential expansion can lead to high stresses on anchor bolts or on individual piers supporting the equipment - particularly those at the far ends of the equipment. For this situation you can see if the equipment manufacturer can allow slotted holes and slide bearings in the baseplates  to allow for expansion and contraction relative to the foundation.  In lieu of that, make sure any anchor bolts have adequate edge distance and preferably also some confining reinforcing bars.  If the foundation will be restraining expansion consider extra reinforcing to accommodate the additional loads due to thermal expansion being restrained.

    Honestly (thinking about it now) 70C isn't really that high (158 F) compared to some of the exposures we'd see in steel mills.  I wouldn't be too concerned about it - pavement can get up to 130 to 140 F on a very hot sunny.

    Greg Thein

    Greg Thein, PE
    Cleveland, OH

  • 3.  RE: External Temperatures on Foundation Rebar

    Posted 03-05-2020 07:41 AM

    Can you provide more information ? What is the shape of foundation / arrangement and sizes ? 70 degrees C is not much. Is it a sustained temperature in the equipment or just a case of design condition (occasional) ?

    On the side note: Concrete pavements in middle east / any other warmer regions of the world get exposed to ambient temperatures in the range of 55 degrees C​ or so and expansion / contraction stresses managed by suitably engineered joints without the need for extra reinforcing. I have designed some pits containing hot liquids (up to 200 degrees C) wherein we provided insulating material along with brick lining to bring the concrete surface temperature in range of 55 degrees C.

    Rushikesh Trivedi, C. Eng., MASCE
    New Delhi, India

    Rushikesh Trivedi C.Eng,M.ASCE

  • 4.  RE: External Temperatures on Foundation Rebar

    Posted 03-12-2020 11:00 AM

    The mat foundation is 5 feet thick with a localize heat source (insulated 6 inch pipes with heat of about 140 F/60C) not placed directly on the foundation. The pipe is about 3 inches from the top of the foundations, local ambient temperature is about 70F/20C.

    Would the temperature gradient be enough to affect the rebar or concrete design of this foundation?

    Is there any ACI code/section that would be specific for this?

    Rocio Rodriguez P.E., M.ASCE
    Structural Engineer
    Orlando FL

  • 5.  RE: External Temperatures on Foundation Rebar

    Posted 03-16-2020 01:10 PM

    In such a case I think it won't affect the foundation because 1) the pipe is insulated & 2) pipe is not supported directly on mat. And also, temperature mentioned is not very high !

    You can utilize simple text book formulae to calculate the temperature drop at surface of concrete using thermal conductivity of insulation and 6" layer of ambient air.

    Rushikesh Trivedi
    P. Eng. (India), C. Eng, MASCE
    Lead Engineer
    Fluor India