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Re: w/c ratio after concrete set

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David Handy wrote:

. > If the contractor asks for more water, the driver just turns the valve and
. > the damage is done. You would have to park yourself beside the truck for
. > every pour. 

Even if you parked yourself beside the truck, what could you do.  The driver 
delivers the concrete to the buyer, the contractor.  The contractor tells the 
driver to add water.  Is the driver going to say, "No?"  Not on your life!  
You can't give the driver directions as he/she is not under your control.  
The only thing that the driver (and you) can do is record the added water.

The thing that concerns me more than the adding of water is the mixing (or 
lack thereof) that is done after the water is added.  Early in my career when 
I was inspecting a pour (placement, if you will) I walked to the counter to 
get the counter reading and noticed that rubber or leather finger on the drum 
was missing.  I called this to the attention of the job superintendent and 
later I got a call from the concrete supplier --- they had only two trucks 
that had working counters.  I have never seen the driver get the counter 
number to make sure that they had 100 revolutions at mixing speed after 
adding water.  I have never seen a delivery ticket that had the counter 
number on the ticket when cement was introduced into water.  Some of the 
clumps that I have seen come out of a truck would indicate that maybe they 
had about 10 revolutions at mixing speed and the rest at agitating speed.  If 
I wasn't at the chute when the clumps came out, and pointed to them, I would 
guess that they would have gone in the forms.

. > We are trying create a Master Spec that will ensure that we get what we 
. > ask for with minimal chances for error. A slab was poured with 10 times 
. > the amount of retarder. The concensus of everybody I have talked to is 
. > that for flatwork we should avoid retarder all together as it can lead to 
. > problems like this even when the dosage is a little closer to the right
. > amount. It was not in our spec, but now is noted that it shall not be 
. > used.

Don't you require that the design mix be provided to you prior to the 
concrete delivery?  Don't you provide your inspectors or observers with the 
various weights/volumes of materials that would be in 5-, 6-, 8-, 10-yard 
trucks so that they can quickly compare what is on the delivery ticket and 
what proportions should be in the mix?

Without the use of retarders, concrete would not be placed between June 1st 
and October 1st here in the southwest.  It would flash set and plastic 
shrinkage cracks would develop before the finishers could get on it.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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