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RE: Mat Slab Control Joints

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It would be pretty hard to put effective control joints in something this
thick and as you surmised probably won't achieve much.  If you are concerned
about shrinkage effects on the quality of the finished floor, I would
specify a very low shrinkage concrete with some form of shrinkage
compensating admixture, Eclipse Floor (or Eclipse) is a product put out by
Grace (www.graceconstruction.com) that they claim will only add about $20/yd
and will substantially improve the shrinkage characteristics of the
concrete.  If you couple that with a high quality, well-graded aggregate
such as limestone and control the w/(C+P) ratio to something less than 0.45
you should be able to substantially reduce the cracking.

Properly preparing the sub-base to reduce friction between the slab and the
soil is also a must.  Try to avoid any thickened edges or pads under heavily
loaded columns as they restrain the shrinkage dramatically and will create
cracking.  

You might also want to look at:

1) www.fsv.cvut.cz/~kristek.  This is a web site that will calculate the
shrinkage characteristics of a given mix and slab geometry.  

2)  ACI Concrete International January 2001 "Gatorade Floor: Quenching
Thirst, Joints, Cracks and Curl" an article about superflat warehouse
floors.

This is all dependant on the client and what sort of finished product they
want and what they are going to use it for.  They may not want to go to the
expense of fancy admixtures in which case specifying a total 35day shrinkage
in the realm of 0.035%-0.05%, a low w/(c+p) ratio (less than 0.45) and
quality, well-graded aggregates is the way to go.  Although you will
probably upset the ready mix supplier with these restrictions.


Thank you,


Nicholas Blackburn, PE
mailto:nblackburn(--nospam--at)fdgoak.com


-----Original Message-----
From: GEORGE SIMMON [mailto:georgesimmon(--nospam--at)hotmail.com]
Sent: Sunday, February 25, 2001 6:48 AM
To: seaint(--nospam--at)seaint.org
Cc: georgesimmon(--nospam--at)hotmail.com
Subject: 


This is a pre-engineered metal building (typical 25 ft column spacing) with 
2-1/2 ft thick mat foundation which will also serve as finish floor.  Would 
you provide control joints? Do not want to cut reinforcement which will make

mat less effective.  Just cutting concrete will not necessarily provide 
weakened planes at those locations.  Any thoughts.  Thanks.
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