system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we construct things in water? Things like bridges or roads that need support pillars that go into the water. If they're made from concrete, how does the concrete ever dry if it's underwater? If it's in the ocean, does the very bottom of the pillar touch the seafloor?
gpt
Concrete curing is a chemical reaction that actually requires water to complete, although excess water in the mixture can yield a weaker cured concrete. Special concrete mixtures are used which are more resistant to chlorides (when pouring in seawater), and care is taken not to agitate an underwater concrete pour too much so that the mix doesn't thin out. You don't need concrete to dry to cure it. In fact, keeping a pour damp throughout the curing process will generally lead to a more complete cure, because the process generates heat that can evaporate water away from the surface, leading to slightly weaker concrete at the surface. Underwater you don't have that problem, though you do need to take care to keep the concrete and the surrounding water at its surface from being agitated and intermingling too much.

Bridge pillars and the like are often constructed using caissons, which is like a waterproof shoring form that extends to the riverbed / seabed, and then has all the water pumped out so that dry construction techniques can be used.