Effect of High Temperature on Concrete Structures 4

[steve1]

Can anyone supply a reference for the effect of heat on concrete structures?

We are engineering a reinforced concrete foundation for a bottom supported boiler. The boiler manufacturer has indicated to us the the temperature at the base of the boiler will be 400 deg F. The boiler sits on a steel skid (supplied by the vendor) which we plan to support with a concrete slab on grade inside a powerhouse.

We are having a debate in the office concerning what, if any, effects this will have on our proposed mat. At least one of the senior engineers has reported that he has never considered temperature in any of his previous designs. Somewhere along the line I seem to remember that there is a relationship between temperature and strength.

Any references will be appreciated.

 

[twinnell]

Heat definitely has an effect on concrete. The boiler could heat up the moisture in the concrete enough to cause it to expand and spawl the concrete. From experience, never place a fire barrel on a bridge deck. I was young…..had to replace 5’x5’x4 inch thick portion of a 9 inch thick deck. Maybe 400 degrees isn’t high enough to get that reaction. I don’t know of any references off the top of my head but I will ask around.

 

[Bagman2524]

Other than expansion (6E-6 "/"/deg F) 400 def F doesn't sound to me like a detremential temperature. I'm not sure what kind of long term effect would be caused by constant exposure to this temperature though.

 

[Bagman2524]

A few references that may help:
ACI 209R Prediction of Creep, Shrinkage and Temperature Effects in concrete structures
ACI SP-39 Behavior of concrete under temp extremes

The stuff I'm reading indicates that you have to design for the stresses caused by the concrete expansion (especially if the concrete is restrained, including any temp gradiant, but other than that the 400 deg shouldn't be a problem for the concrete.

 

[twinnell]

I found this article but didn't see any references.

http://www.findarticles.com/p/articles/mi_m0NSX/is_12_49/ai_n8590878

 

[eric1037]

It seems to me that there would be issues with differing expansion/contraction properties between the steel reinforcement and the concrete, possibly leading to cracking.

 

[twinnell]

My initial response wasn't that the concrete would expand, it is the moisture in the concrete will boil and cause enough pressure to spawl the concrete. Boyscouts 101, never throw rocks in a fire...they will explode due to water pressure.

 

[dik]

The temperature isn't high by concrete standards. The problem is the sustained heat and the long term desication of the concrete gel.

 

[SlideRuleEra]

See the out-of-print Portland Cement Association document "Effect Of Long Exposure Of Concrete To High Temperatures" on this page of my website

http://www.slideruleera.net/PCA-Page.html

http://www.SlideRuleEra.net

 

[LittleWheels]

How mature will the concrete be before the boiler is fired up? Will it go through a lot of hot/cold cycles (probably not)?

There are several methods to achieving a temp-resistant concrete. I suggest you chat to some helpful additive suppliers or concrete manufacturers.

 

[steve1]

The responses, including SlideRuleEra's excellant post, indicate that long term exposure to temperatures under 500 deg F should not be a problem.

We are in the process of obtaining additional info from the ACI, and I will speak with our concrete supplier.

Thanks to all who responded to this post.

 

[rtmote]

Design temperature on the boiler surface is a far cry from the the contact temperature for the concrete. With a steel base plate as an interface, the competition between the conducted temperature and the ambient convective cooling will degrade the temperature significantly. The grouting underside the baseplate will also be effective in insulating the concrete surface.

 

[LittleWheels]

OK, 400 degrees F isn't actually very hot (200 degrees C). I should have converted to 'proper' measurements before answering. Don't know how Americans cope with Fahrenheit, kips and similar wierd beasties

 

[mitchelon]

I am failing to understand why this much heat is being transferred to the foundation or outside of the boiler. Boilers are designed to heat, typically, water and create steam. This has to be the most inefficient boiler in the world if you are loosing all that heat to the outside. Please explain.

 

[Ron]

Steve1...in order for the concrete strength to be significantly affected and for the reversion of the chemical/molecular bonding that takes place during hydration to occur, the temperature must be taken to about 800C. Other than the obvious thermal expansion issues and the cracking potential, the temperatures your concrete will experience will not likely be detrimental.

I would use a coarse aggregate that has a very low absorption (granite, river rock, etc.) so that moisture issues are reduced.

If you want to be conservative, you could use refractory concrete in the upper 6 inches or so.

 

[josecastjr]

It seems to me that if there is sufficient clearance between the bottom of the boiler and the slab (skid height), you could pour a lightweight concrete topping on a galvanized pan that would provide sufficient thermal insulation for the slab.

 

[jt12]

I believe ACI 349 has some concrete temperature provisions. Code isn't applicable for your application, but conservatively you could design to those provisions.