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Re: Large block of concrete

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Rajendran,

        I was involved in one such foundation.  In my case it was 11 x 11 x 11 metres (as in 1331 cubic metres of concrete).  I was concerned about the heat problem so I retained a specialist consultant.  One of his recommendations included insulating the formwork and the top of the foundation plus leaving this insulation in place as long as possible but not less than 14 days.  No cracks or other problems have ever been reported to me.

Regards,

H. Daryl Richardson.

thunt(--nospam--at)absconsulting.com wrote:

 
Rajendran,

Large foundations for refinery and power projects are successfully constructed all the time and generally do not need extensive changes to the concrete procedures.  Most of the recommendations are similar to high ambient temperature precautions.  Here are a few of the basics:

*  Use as little as possible amounts of cement.  Typically 3000 psi concrete is more than adequate and you may be able to get a way with 2500 psi.  The large size of these type of foundations are based primarily on mass for vibration control so stress levels are normally very low.

*  Use ASTM C150 Type II cement and make sure you specify the optional requirement for "low heat of hydration".

*  Use as large as practical aggregate.  Nothing smaller than 1 1/2 inch nominal maximum size and if available in your area use 2 inch diameter.

*  Specify 14 days of wet curing.

*  Use a nominal amount of rebar throughout the interior of the mass and not just on the outside surfaces.

*  Consider control joints and sequenced pouring.

*  In extreme cases consider chilled water or ice shavings in the concrete mix design.

I am not sure I am ready to make this a recommendation but I have seen designs using Novomesh which is a blend of steel and synthetic fibers which theoretically helps with reducing both the plastic and shrinkage cracks (or at least spreading them out so they don't look as bad).  This may also help reduce crack propagation from machine vibrations.

Thomas Hunt, S.E.
ABS Consulting
 
 
 
 
 

Padmanabhan Rajendran <rakamaka(--nospam--at)yahoo.com>

07/19/2003 05:58 AM

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Large block of concrete

 

Harold, the concrete guru:

In the refinery and pipeline industries, often a large concrete block is provided to support equipment such as a pump or a compressor skid.

Let us say that one such block is 16' wide X 50' long X 8' deep. I believe that the heat of hydration will be pretty large and the thermal gradient may take years to get dissipated.  Thermal gradient would cause microscopic cracks throughout the concrete. I have not seen any special requirement called out in a typical concrete specification or in the notes on the drawing. What are your thoughts? Should low heat cement be used? If ues, what are the implications? Is there any other strategy to lower thermal gradient? Staged construction may not be acceptable because these clients are typically in a great hurry!

I have come across thousands of dollars spent in repairing old concrete block foundations for compressors. Papers that have been published by agencies recommending and/or performing such repairs point out that the concrete damage arose from the machine vibration. I wonder if the problem was initiated by the microscopic cracks from thermal gradients and susequently, machine vibration took advantage of the weakened concrete.

Thanks.

Rajendran
 
 


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