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Re: Large block of concrete

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Thank you, Thomas.  Can you, please, elaborate on control joints and sequenced construction?
We tend to think that large concrete foundations are constructed successfully, mainly because nothing goes terribly wrong within six months after construction. The state of concrete is never monitored although it is known that interior reaction continues for years. The complaint about vibration of floor around the foundation or cracks in the foundation is heard after some years of construction and by then, no one cares to think about the original design or the construction specification or constructionn methods. 
Mixing of concrete grades is, generally, unwelcomed. 4000 psi concrete has nearly become the minimum standard. Nowadays, projects are run by newbies who have practically, no technical expertise and these project engineers strictly go by by the company standards they have sworn alegiance to. They have the apprehension that a wrong concrete may be used at the wrong place, if different grades of concrete are specified. In many cases inspection is poor and specification is compromised at the site. Schedule and cost pressures are brought to bear on textbook concepts on sound engineering practice. This is reality and there is a need to come up with ways to produce a good product, taking into account things that could go wrong. 

THunt(--nospam--at) wrote:


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)>

07/19/2003 05:58 AM

Please respond to

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.  

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