How is a bridge built if cement does not dry in water?

Cement is a dry powder that reacts with water, hardening and acquiring resistance. The problem is that the greater the amount of water for the amount of cement (the so-called water/cement ratio), the lower the strength of the concrete. Who says that is the Abrams Law. For this and other reasons (such as salts and organic matter in the water) we cannot let the cement come into contact with the water.

To build a bridge, temporary structures called cofferdams are used, which hold the water.

In the photo, we have a cofferdam made of metal sheets. They are driven into the bottom of the river/sea and the water is pumped out. If the soil is very permeable, a very large amount of water will continue to enter the bottom, and it will be necessary to continue pumping throughout the work.

When you are curing cement, you keep it wet… So it is misinformation that cement must be dry to cure. It is actually a chemical reaction. Water is used to draw away the excess heat, so the heat won’t damage the cement.

However, it is much harder to keep our cement in place if you just let the water around it interact, and most water sources are not very clean. For example, SALT will badly damage cement, so you really want to avoid any contact with salt water.

So the trick for building under water is not to keep the cement absolutely dry. Rather it is to fill it into the forms that hold it in place without it being contaminated, and then protecting the cement from the worst of contamination.

This changes from an almost impossible task trying to keep an air bubble around the cement, to the possible task of trying to keep clean water around the cement. As this means at least you have equalized pressure inside and outside your barrier for the unfiltered water.

If your water containment is not perfect, you can just maintain a positive pressure. Meaning you keep pumping in clean water, so the clean water exits through any holes, instead of dirty water entering through the holes.