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The Hoover Dam and Curing? Answered

The Hoover Dam contains (I read) 2.6 million cubic metres of concrete. Yet it was poured in relatively small blocks in order to expedite the curing process. Why? If curing is a process of hydration, why is this not a local process, independent of the volume of the concrete? I read also that if poured in one fell swoop, the Dam would have taken 125 years to cure. I'm not certain I've fully understood the curing process if it is truly volume dependent.

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Dornfield

2 years ago

Actually as an addendum to my own question, I notice that in Lesson Two on curing, the video of the 'Han Solo' silicone molds labelled 7.5 hours, the three smaller molds are clearly curing before the larger one to the left. So size does matter? What impact (if any) does the rate of curing have on strength?

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mikeasaurusDornfield

Best Answer 2 years ago

Volume does matter. You want concrete to cure slowly, which will yield a stronger product. However, there's a practical limit to the length of time to wait (in your example, 125 years is a long time).

With the Hoover Dam example, imaging pouring the entire structure at once only to find out the ratio was off and the dam wasn't as strong as it needed to be. Pouring in smaller batches ensure quality and ratio consistency, as well as a faster cure time.

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mjdmikeasaurus

Answer 11 months ago

A concern with very large volumes of concrete is temperature. Concrete curing is an exothermic process which produces a considerable amount of heat. Often the heat of hydration is a welcome byproduct, especially in cold weather, but with very large placements the heat produced can be excessive, which (for various reasons) results in poor quality concrete with low strength and low durability. The Hoover Dam would have been far too large a volume of concrete to place in one placement for this reason alone.

Another reason it had to be placed in batches is purely logistic. Delivery of cement to the site was difficult and to place it in one pour would have required the cement to either be present all at once or reliably delivered on a rapid schedule. The dam was in the middle of nowhere back in the day.

Finally, fresh concrete is essentially a liquid. The pressures on the formwork for the dam would have been pretty much impossible to resist had it been placed in one continuous placement. Bridge abutment walls, which are orders of magnitude smaller than the dam, have formwork failures all the time for this reason, when the forms are not built correctly.