A Waterstop, also known as a water bar or hydrophilic seal, is a critical, preformed impermeable barrier embedded within and across the joints of concrete structures to prevent the passage of fluids, primarily water. Its fundamental purpose is to create a permanent, durable, and flexible seal within construction joints, contraction joints, expansion joints, and cold joints, thereby ensuring the long-term watertight integrity and structural durability of below-grade and water-retaining constructions. Acting as a physical barrier against hydrostatic pressure, waterstops block the path of water, chemicals, and contaminants that could otherwise seep through the natural micro-cracks and porous pathways in concrete, preventing leakage, corrosion of reinforcement steel, and potential structural damage. 　　Manufactured from a variety of flexible, durable, and impermeable materials, common types include PVC Waterstops (polyvinyl chloride), prized for their cost-effectiveness, chemical resistance, and versatility in standard applications; Rubber Waterstops (such as EPDM or neoprene), which offer superior elasticity, temperature tolerance, and movement accommodation; and Bentonite Waterstops, a hydrophilic variety that swells upon contact with water to form a tight, self-sealing gel. Metallic and co-polymer versions are also available for specialized needs. These materials are engineered into specific profiles—most commonly ribbed-center bulb (dumbbell), center bulb with fins, or flat—designed to anchor securely within the concrete and provide an extended, labyrinthine path for water, significantly enhancing the seal's effectiveness. 　　The primary applications for waterstops are vast and essential within infrastructure and construction. They are indispensable in: Water and Wastewater Treatment Plants (reservoirs, tanks, clarifiers), Civil and Transportation Infrastructure (tunnels, subway systems, underground parking garages, bridge abutments), Marine Structures (ports, locks, dams), Commercial Foundations (basement walls, elevator pits, raft slabs), and any structure where below-grade waterproofing is paramount. The key benefits of installing a proper waterstop system are profound: it provides a continuous, monolithic barrier independent of external membranes; accommodates joint movement from settlement, thermal expansion, and seismic activity; resists high hydrostatic pressure; protects structural integrity by preventing corrosion and deterioration; and significantly reduces long-term maintenance costs and liability associated with water infiltration. 　　Selecting the correct waterstop involves critical analysis of the joint type (moving or static), expected hydrostatic pressure, chemical exposure (e.g., in sewage or industrial settings), temperature ranges, and the required movement accommodation capacity. Proper installation is equally crucial, requiring meticulous placement, secure splicing (typically by heat welding for PVC or vulcanizing for rubber), and careful concrete placement to avoid displacement. As a fundamental component of modern concrete waterproofing systems, the waterstop is not merely an accessory but an engineered necessity, safeguarding the longevity, safety, and functionality of critical concrete structures against the persistent challenge of water migration.