The hydrophilic seal strip ensures the waterproof qualities of construction joints in buildings and as well as in the ground, in contact with water, whether pressurised or not. It is used in concrete construction joints by acting as a barrier to water seeping through. This waterproof performance is achieved by the swelling of the strip cast in the concrete when coming into contact with water. The swelling against the contact faces surrounding the strip produces a pressure between the strip and its enclosure that prevents the water seeping through the joint. The strip swelling is reversible. In other words, the strip expands in contact with water. In the absence of water, it dries out and regains its original shape.