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RE: More Concrete

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Farzin,
Tom gave you some excellent advice.  I would offer some more considerations. 
 
1.  Have the admixture technical reps buy into the process.
 
2.  Have a preconstruction meeting with all of the stake holders attending (concrete producer, admixture manufacturer, pumper, finishing contractor, GC, testing lab, etc.) to buy into the process.  If you use an overhead pump, the concrete you get at the maximum reach will not be the same concrete as they raise the boom to its maximum height (close to the truck).  For placing, I prefer 1. Tailgating if you can; 2. Mini-conveyor; 3.  Bucket;  4.  Georgia buggy; 5. Horizontal line pump (slick line); 6. Overhead pump; in that order.  The method of placement selected will have an impact on the mix.  Even the screed that you use will have an impact.  I prefer the Copperhead laser screed.  I prefer the polycarboxolate high range water reducers, and I prefer a uniformly graded aggregate (Jim Shilstone lesson).  The polycarb high range mix is a bit different to finish.  It is not hard, but there will not be the bleed water to which most finishers are accustomed. 
 
3.  Do a test pour with all of the entities and replicate the field conditions. 
 
4.  Plan for the worst (break downs, traffic tie ups, etc.). 

Regards, Harold Sprague


 

To: seaint(--nospam--at)seaint.org
Subject: Re: More Concrete
From: Tom.Hunt(--nospam--at)fluor.com
Date: Thu, 16 Dec 2010 09:53:11 -0800

Farzin,

This is unfortunately common practice in certain regions of the U.S.  When it works (or they think it works) everyone is happy however when it goes bad all hell breaks loose and lawsuits abound.  The biggest issue is maintaining contractual responsibilities and maintaining quality control.  If the concrete turns out bad, who do you blame.  If you add anything to the truck at the jobsite then it should be by a representative of the batch plant and detailed records need to be kept.

If left to the construction contractor at site then they are likely to give some low paid kid a bucket with a black line drawn on it and he crawls up the truck and dumps it in.  To add a plasticizer properly it should be sprayed in with a long tube handle otherwise a lot of hit will just be coating the upper truck vanes and not get into the concrete.  Another issue is that many times the construction contractor just does not know when to stop.  They may keep adding water and plasticizer until they have a watery goo.

Typically I reject this operation however if you get lots of pressure from above then make sure you have lots of quality control,  have a batch plant representative at the job site, and keep detailed records of everything added to the trucks.  Also make sure the truck do not sit too long which is another trick the construction contractors like to pull.  Per code requirements they have to pour the concrete within 90 minutes of adding the first water.  If no one is watching, they will keep the trucks longer and just keep adding water and plasticizer which may not show up in the slump test.

Thomas Hunt, S.E.
Fluor




From:        fsrahbar(--nospam--at)aol.com
To:        seaint(--nospam--at)seaint.org
Date:        12/16/2010 09:14 AM
Subject:        More Concrete




List:
 
At one of our projects, they are planning to pour 500 yards of concrete in one day (5000 psi). This is for a slab and they need to be able to work the concrete after pour.
 
The contractor has suggested this: 1. The trucks will leave the plant with only a portion of the required water. 2. They will add the rest of the water at the site, but not more than the required mix design. 3. They will then add the plasticizer to the mix to get the slump around 7” or 8”.
 
It probably takes about 45 min. for the trucks to get to the site.
 
They will have deputies at the plant and the site to monitor all the work.
 
Any thought or advice?
 
Thank you,

Farzin S. Rahbar, SE

Vice President
David C. Weiss Structural Engineer & Associates, Inc.
(818) 227-8040 Ex. 13 Fax: (818) 227-8041



-----Original Message-----
From: Jason Christensen <jason(--nospam--at)wcaeng.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Sent: Thu, Dec 16, 2010 8:44 am
Subject: Concrete quality

I have a break press machine coming in from overseas, we are to design the foundation for it.  The drawings for the press recommend a concrete quality of “BS 25”.
 
Does anyone have knowledge of what the  BS 25 means?  A US equivalent?
 
Jason
 
=
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