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Re: Low alkali cement[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Low alkali cement
- From: Kevin Below <kbofoz(--nospam--at)gmail.com>
- Date: Sun, 18 Oct 2009 23:12:42 -0400
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?
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
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?
David Dickey, PE
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