In a structure that holds water, you need something flexible to inject into cracks or joints. These types of structures are usually above ground and exposed to changing temperatures. As concrete heats up during the day it expands; joints and cracks squeeze together. As it cools, the concrete shrinks and joints and cracks open up. Thus, you need an injection foam that has flexibility to handle the movement. You also need to do the injection work in the early hours of the day. Some contractors have injected cracks in the afternoon; stopping leaks, only to find them leaking again in the morning.
For these scenarios, we recommend the Spetec PUR GT500 or Spetec PUR F400. The GT500 is hydrophilic and absorbs some water during the reaction. In a tight crack or joint it's great and has minimal shrinkage if it dries out. It has lower expansion, maybe 4-6x by volume, so for wide joints it may take more product and will likely require oakum to retain the resin in the joint. It does not have an adjustable set time.
The F400 is a flexible hydrophobic. It combines higher expansion properties with flexibility. It also has a catalyst which allows you to adjust the set time. The F400 is excellent for crack injection, joint injection (especially larger joints), and infrastructure injection as well. It does not shrink because it's hydrophobic and unaffected if no moisture is present.
Now let’s talk a little about hydrostatic pressure. Most injection work is done less than 40' below the water level and head pressure is less than 20 psi. Considering we recommend injecting with pumps capable of 3000+ psi, getting the product in the crack is not the problem. The challenge is keeping the polyurethane in the crack or joint long enough for it to set up. That's where experience comes into play. The contractor must know how to adjust the set time, how to drill the injection holes properly, how to use oakum, and how long to inject on a single packer/port.
We have not seen failures on dams or locks where pressure blew out the polyurethane. Dams across the globe have been injected this way. Where we do see failures (continued leaks, not blowouts) is when the wrong material is selected, or the injections are done during the hotter hours of the day. In fact, some water authorities only inject during the cold months of the year.