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Re: High slump mix design

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Without seeing how congested your rebar really is it is hard to say.  However, it is always good practice to use the largest size aggregate that is practical.  I would guess that 3/8 inch is probably not actually required and you may in fact be able to use 1/2 or 3/4 inch.  Look at your rebar clear spacings very carefully.

Unless you have a very experienced construction crew, trying to deal with 9.5 inch slump concrete can be real dangerous.  From a practical standpoint you will need to use a high end superplasticizer (now called High Range Water Reducing Agent [HRWRA]).  Consult with the plasticizer representative (Master Builders, Sika, etc.) for the dosage details and the practical slump range.  A 7 inch slump (+/-  one inch) is probably more standard.  I also would not attempt this without some trial mixes or a batch plant AND construction crew that is experienced with this type of concrete.  To help with the shrinkage cracking you may also want to specify a W/C ratio of 0.40 or less.

Also note that if your columns or walls are rather large you need to be careful with your formwork as you will see higher hydrostatic loads at the bottom of your forms.

Thomas Hunt, S.E.
ABS Consulting

"Michael Bryson" <mbryson(--nospam--at)>

09/22/2003 12:43 PM

Please respond to

High slump mix design

Slump = 9.5”
3/8” pea gravel
This is for heavily congested with rebar and architecturally exposed columns and walls. I have already told the architect that 3/8” max aggregate will cause extra shrinkage cracking.
Is there a higher potential for honeycombing with a 1” max aggregate and 6” slump?