by Stacy Byrd, GRP, LEED AP
Joints formed between adjacent concrete pours and where mechanical elements penetrate the concrete are the most likely points of water ingress into below-grade concrete structures. To prevent this from occurring, waterstops are commonly specified and installed at every joint in the concrete below-grade.
A waterstop installed in concrete joints is an important component of an overall waterproofing design to protect below-grade portions of a concrete structure. These products’ use in construction joints (i.e. ‘cold joints’) is a good design practice for building foundations, with or without a positive-side waterproofing membrane. In other words, the waterstop can be a belt-and-suspenders approach to provide a dry structure for the occupants and owner.
Below-grade structures present conditions making it very likely water, which is present under intermittent or constant hydrostatic pressure, can infiltrate through the concrete joints. Therefore, waterstops are used as part of the overall waterproofing protection on a variety of concrete structures including:
- basement walls and slabs;
- subway, vehicle, and pedestrian tunnels;
- parking structures;
- water and sewage treatment facilities; and
- canals, locks, and dams.
When most construction professionals think of a ‘waterstop,’ they generally refer to a dumbbell or ribbed profile extrusion of thermoplastic or rubber material, 102 to 305 mm (4 to 12 in.) wide, installed in a concrete joint. Since the 1950s, the most widely used waterstop is polyvinyl chloride (PVC). Strong and flexible, these products have been used due to ease of welding and inherent resistance to groundwater and common waste water treatment chemicals. There are now myriad metal, plastic, asphaltic, and hydrophilic materials, with differing compositions and profiles, utilized to stop water ingress through joints in concrete structures.
A waterstop is a material embedded in the concrete, with the singular purpose to obstruct the passage of water through the joint. In other words, it is not an elastomeric sealant adhered to the exposed surface of a joint. Beyond the joint, waterstops cannot prevent migration of water vapor or capillary moisture potential through a concrete slab to protect the flooring system (e.g. hardwood or tile) from adhesion failure or deterioration. Similarly, waterstops are unable to prevent water ingress through cracks that develop in the concrete due to building settlement or live load deflection—waterproofing membrane systems, vapor retarders, and other construction products are available to the specifier and contractor for these issues.
It is important the waterstop be manufactured with quality raw materials, without defects. Many material types and profiles are available for different applications and conditions so the specifier must choose waterstops, preferably with manufacture consultation, appropriate for all the joint conditions.
The three general concrete joint types are:
- construction (i.e. cold) joint—intentional interfaces between adjacent concrete placements created to facilitate construction;
- expansion (i.e. isolation) joint—separates adjacent concrete pours (e.g. walls, slabs, footings, and columns) protecting them from compressive stresses developed by thermal expansion, seismic events, or live load deflection; and
- contraction (i.e. control) joints—intentional grooves to create a weakened plane to control the location of cracks resulting from shrinkage of concrete while curing.
To choose a waterstop suitable for the project’s joint-sealing needs, it is important to know the various product types and material compositions. Most types are manufactured exclusively for use in cast-in-place concrete construction joints, while some varieties can also be used with expansion joints. Selected waterstops must accommodate the expected lateral, transverse, and shear joint movements, as well as, the expected hydrostatic pressure.
If used for primary or secondary containment structures, the waterstop must be resistant to the fluids or chemicals contained within the structure. Manufactured in various types, shapes (profiles), and sizes, they come in diverse material compositions such as:
- thermoplastic extrusions such as PVC, high-density polyethylene (HDPE), and thermoplastic vulcanizate (TPV);
- thermoset rubber extrusions;
- hydrophilic bentonite and rubber strips;
- asphaltic and butyl rubber non-swelling strips;
- metallic extrusions; and
- injection hose systems.