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Heading: 1.1Use of Set retarder to reduce plastic shrinkage
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Greetings All

I am working on a project in the Arctic. The structural designer of the ice shield for the river piers has required the use of a set retarder in the pier concrete in order to reduce shrinkage. This concept came as a bit of a surprise to me, so I thought maybe someone out there has a reference I can review.

The concrete is 35 MPa with Type GU cement and 5 to 8% air. We are using a polycarboxylate high range to improve cement efficiency and help deal with higher proportion of passing 80 µm fines in the aggregates(less than 1% passing 2 µm clay sizes in the washed sand). After more than 20 years of suggestions, this job is the first time we have seen a local ready mix concrete producer actually wash aggregates north of 60°.

Because no one wanted to check out the aggregate source for AAR before the tender was awarded, only accelerated mortar bar testing is available. Of couse, this test determined that aggregates were highly reactive and it was too late to check for false positives with the concrete prism test. Therefore, concrete is produced with 30% Class F fly ash (CaO ~ 6%, 80% plus Si, Fe oxide).

The big picture issue is that the designer of the formwork wants to minimize liquid head and the contractor wants to get concrete placed before winter (which could be very, very soon).

So, assuming that evaporation is not an issue with a tall form, does the use of a set retarder reduce plastic shrinkage? How long should set be delayed? One or two hours would aid in avoiding cold joints, full height set delay would affect form prssures. Can this technique actually reduce the stress on the Nelson studs embedded used to anchor the steel ice shield protecting the bridge piers? Or is the designer "too smart by half"?

Thanks for any pointers



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JD (Dave) Robson


Questioning what I'm hearing.

In another article where the daytime tempretures were to hot to pour, it spoke of pouring the concrete at nite--while adding ice to the mix. Of course they also spoke of a continous pour and problems arising from the method. However, leaving aside the continous pour and it's problems of a bumpy surface, the fact that they poured at nite and added ice suggests that too much heat was not a good thing, so I ask, if the daytime heat with the heat generated by the chemical reaction in the concrete is too much, does that cause the concrete to set alot faster?

If it does, then won't the cold waters in Alaska act as a retarder?
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Jim Ryan
 
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