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RE: Low alkali cement

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Kevin,
I have had bad reactive aggregate experiences with interior slabs on ground.  Exterior slabs on ground will have water and water vapor serving as a catalyst from below and above.  Interior slabs on ground will have water serving as a catalyst from the atmosphere, from wash downs, and from holidays in the vapor barrier (if applicable).  It does not take that much water or water vapor to form the reactive paste in the concrete matrix. 
 
Many engineers have been dismissive of interior concrete with reactive aggregate.  I am not.  I require mitigation for any application with reactive aggregate.  Experience has indicted to me that water from these other sources and the relative reactivity of the aggregates are enough to trigger and maintain the formation of the reactive paste unless mitigated in some manner. 
 
There are many cost effective methods of mitigation of reactive aggregate.  It does not cost that much to mitigated it.  It costs a lot to try to repair it later. 
 
I thought I could skate with not having too many years until I retire.  But I observed 18 month old concrete with the classic map cracking in an interior protected space and on a membrane.  It does not get more protected than that. 
 
Testing confirmed that we had reactive paste in only 18 months.  (The following is read with a loud voice)  I then waded in the water and became an ASR believer.  We need to CAST OUT the evils of reactive paste!  SAVE yourselves from the wickedness of cracked concrete, lawyers, and the law suits that they carry.  Embrace the healing mitigation of F ash, lithium, or imported aggregate.  .... do I hear an AMEN!     

Regards, Harold Sprague


 

Date: Sun, 18 Oct 2009 23:12:42 -0400
Subject: Re: Low alkali cement
From: kbofoz(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org

Harold,
I should have mentioned that the comments on slabs on ground apply only to interior slabs.  (I forgot to mention that, because 99.9% of my sogs are interior.)
Does that concur with your experience?

On Mon, Oct 12, 2009 at 3:38 PM, Harold Sprague <spraguehope(--nospam--at)hotmail.com> wrote:
Kevin,
That is a very good observation and cautionary tale.  I have had major problems with reactive aggregate on a project that can not be discussed in an open forum.  A non AAR mix is preferable to control costs.  Give the contractors the latitude to either use different aggregate or mitigating the mix.  Fly ash mixes and lithiums are commonly used and effective. 
 
The newer testing procedures only take about 16 days.  If your testing lab has experience, they will develop several possible mixes at one time.  They will need at least 28 days for the confirmation cylinders for a regular mix design submittal anyway.  The time for testing for the reactive aggregate should not be an issue. 
 
I would suggest that you also get a petrographic analysis of the aggregate.  This gives some additional information, but costs a bit more.  ASR is relatively easy to predict an mitigate.  ACR is much more problematic. 
 
I am a bit curious about being dismissive about slabs on ground.  That is where it is generally the worst.  Paving in particular drives the necessity for mitigation.  The source for the water can be from wash downs for from the ground.  Even atmospheric water vapor can be a problem.  Texas, New Mexico and California have all had serious problems with reactive aggregate in relatively arid regions. 
 
A good resource for reactive aggregate in building construction is Dr. Doug Hooton of the University of Toronto. 

Regards, Harold Sprague


 

Date: Mon, 12 Oct 2009 14:53:37 -0400
Subject: Re: Low alkali cement
From: kbofoz(--nospam--at)gmail.com
To: seaint(--nospam--at)seaint.org; structous(--nospam--at)douglasconsultants.ca


I'm far behind in my messages, but this is an interesting subject, and important here in Quebec, where there are AAR problems (RAG in French).

In reaction to this reaction, I started by specifying non-AAR aggregates, to be demonstrated by the concrete supplier.  All the goof local suppliers have done the appropriate testing on their aggregates, so there is no extra cost for the testing.
However, this generally meant that only granitic aggregates could be used, at a higher cost.
Discussions with our tech lab led to our specifying a non-AAR mix, as opposed to a non-AAR aggregate.  This generally costs no extra.  Again, testing is required, but has usually already been done and documented adequately.
They also pointed out that only concrete that is in contact with the earth or frequently wet is subject to the problem, except that research has shown that there have never been problems with a slab-on-ground.
So now we specify that mixes must be non-AAR for footings, foundation walls, and exterior concrete.
However, as discussed in that extraordinary book The Black Swan (which all engineers should read), absence of proof is not proof of absence.  So one day maybe we will discover a SOG that also has the problem, but for the moment, we do not require non-AAR mixes for SOGs.

On Thu, Feb 19, 2009 at 5:30 PM, Dickey, David <David.Dickey(--nospam--at)masonandhanger.com> wrote:
When specifying the type of portland cement to be used in a concrete mix, do you specify low alkali cement? 
 
Do you specify it on all of your projects?  Or none or your projects? 
 
Is there data available that shows areas of the country with siliceous aggregates that could contribute to alkali-silica reaction? 
 
How much more does low alkali cement cost?
 
Is the testing that can be specified to determine the alkali-aggregate reactivity, ASTM C1260, something that is routinely done by the concrete supplier that can be requested, or would the testing be a significant additional cost to the project?
 
Thanks,
 
David Dickey, PE
Lexington, KY



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