Up north in Lansing, Iowa, the Iowa and Wisconsin DOTs have been working hard to construct the new Lansing bridge crossing the Mississippi River.
Last week the last pier on the Wisconsin side was poured, but due to how late in the year this was poured there were some expressing concerns about the bridge’s structural integrity online primarily from pouring the concrete when it’s this cold out.
It is true, that extreme temperatures in either direction can cause complications when pouring concrete, to explain why, let’s first look at what concrete is.
Concrete is made up of cement, fine and coarse aggregate, such as rocks and sand, and water.
When water is mixed into the dry concrete mixture a chemical reaction takes place with the cement. Creating a strong crystal-like structure, locking the aggregates in place, and creating a hardened material.
The air temperature around the curing concrete can determine how long it takes to cure, but when it comes to the cold, there are a few extra steps to take when pouring.
Because it consists of water as the key ingredient in the mixing of concrete, It takes time for that water to react with the cement and form those nice tight crystals that make the concrete hard. So if the water would freeze, it would not be available to react with the chemicals and the concrete to make it hard, so you’d end up with a very weak product that would likely fall apart.” Says Clayton Burke, Project manager of the IA-9/WI-82 Mississippi River Bridge
On the flip side, when it’s overly hot out, the combined heat from the air and the curing concrete can accelerate the rate water evaporates, which will cause the concrete to cure faster and be less workable, as well as cause cracking and creating a weaker product.
Whereas in the winter time, the curing process is much slower, giving crews more time to work with the concrete, as long as they keep it at a consistent temperature until it’s set
To do so, a few extra steps are taken such as ensuring the water used is heated before arriving on site, ensuring the forms are heated before pouring, and using blankets around the curing concrete.
The chemical reaction that’s taking place to cure the concrete can give off a large amount of heat, so in some cases, a cooling element can be involved to keep the temperature stable.
When we place concrete in such a large element, we really care about the internal temperature and the external temperature of the concrete, and we don’t want that to be too different. And the internal temperature got up to about 120 degrees. So in this case, we used cooling tubes, even though it’s freezing outside, we’re cooling the inside of the concrete because it produces so much heat.” Said Burke.
That being said, in many cases working with concrete in the winter can be an advantage, it just comes with a few extra steps to ensure a proper cure.
Something that many experts take into consideration along with the multitude of environmental factors that go into the construction of large projects.
If you’d like to keep up to date with the project you can follow along on their Facebook Page or their website.