Polyurethane chemical grouts fall into two categories when it comes to how they relate with water: hydrophilic and hydrophobic.
Hydrophilic grouts are ideal for areas that are constantly wet, because they bond better to wet concrete. They are water loving grouts which means they thrive in moist environments. They perform very well when they are subjected to water 24 hours a day.
However, if that water goes away, hydrophilic grouts are not ideal because they could shrink and crack when the area dries out. Now you take that chance it will leak all over again on the next rain event. We call this a wet/dry situation. What does wet/dry situation mean? This means that water can be present in a crack or void after a hard rain and then a few days later that water is no longer present in the crack and void. The water table drops or the weather dries out and now you have a crack or void with no moisture present. In situations like these, you want to use a hydrophobic grout.
This leads me to a call I received a few months back. The owner of a storage building in Virginia was upset because he had hired a grouter to come to his storage facility and fix leaking cracks inside the storage bins. While the leaks remained sealed initially, they eventually started leaking again after a few rainy day/sunny day cycles. They were very worried that water was going to leak and damage the items that customers had in the storage units. It turned out the contractor had used a hydrophilic grout in this wet/dry situation, which was not the best choice for the reasons explained above.
The solution was simple. The owner went with my grouter, who used a hydrophobic grout to permanently seal the leaks. Knowledge is power, folks!