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Re: Low alkali cement[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org, structous(--nospam--at)douglasconsultants.ca
- Subject: Re: Low alkali cement
- From: Kevin Below <kbofoz(--nospam--at)gmail.com>
- Date: Mon, 12 Oct 2009 14:53:37 -0400
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.
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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