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RE: Concrete aggregate

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Blake,

I guess I really don't think I understand the question.  The primary reason
you try to use as large an aggregate as possible is that it reduces the
amount of cement required.  Consider that each piece of aggregate requires
cement paste all over its surface area to bind it into the matrix.  If you
use many small pieces as opposed to many large pieces, surface area is
increased and you need more paste. 

The paste is composed primarily of cement.  The cement is what creates heat
in the hydration process (thermal gradient cracking), causes shrinkage
(cracking), is what degrades in certain chemical exposures, and is
expensive.  The goal is to minimize the amount of cement.

The maximum size of aggregate is predicated on congestion and the actual
application.  Dam structures often use 6" maximum aggregate, while heavily
reinforced beams and columns might require a 3/8" maximum aggregate.  It is
also wise to use a well graded aggregate to minimize the amount of cement
paste required.

There are exceptions, but this is the general idea.

Each benefit can be quantified.  You will not see huge changes in economics,
but the performance changes are much more marked. Hydration heats can be
measured, shrinkage can be measured, chloride ingress can be measured, and
porosity can be measured.

Harold Sprague, P.E.
The Neenan Company
harold.sprague(--nospam--at)neenan.com


-----Original Message-----
From: Blake Haley [mailto:blakeh(--nospam--at)weainc.com]
Sent: Wednesday, October 21, 1998 8:41 AM
To: seaint(--nospam--at)seaint.org
Subject: Concrete aggregate


What is the basic diffence between using different size aggregates for
concrete mixes?  And how can this be quantified?  Let's say that we are
talking about 1-1/2", 3/4", and 3/8" chip mix designs.

Thanks,
Blake Haley P.E.
White Engineering Associates, Inc.
Oklahoma City, OK