A waterstop installed in concrete joints is an important component of the overall waterproofing design. Use of a waterstop with a waterproofing membrane is considered a belt-and-suspenders approach to provide a dry basement. This is a common practice in waterproofing design because if for some reason water is able to circumvent the membrane, the waterstop is in position to obstruct the water in the most likely place the water leak can occur – through the concrete cold joint. Beyond the concrete cold joint, waterstops are unable to prevent water ingress through cracks that develop in the concrete due to building settlement, load deflection, or concrete shrinkage —waterproofing membrane systems, are available for these crack issues.
WHAT IS A WATERSTOP
Many construction professionals generally think of waterstop as being a dumbbell profile extrusion of PVC thermoplastic, four to nine inches wide, installed in a concrete joint. These dumbbell products have been the most widely used waterstop for decades. Today, there is a wide variety of materials—metal, bentonite, asphaltic, and hydrophilic rubber—with differing compositions, sizes and profiles, used as waterstops in concrete structures. Waterstop is not an elastomeric sealant or coating adhered to the concrete surface at a joint.
It is important the waterstop be manufactured with quality raw materials, without defects. Many material types and profiles are available for different applications and joint conditions. 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 (i.e. cold joints), while some can also be used with expansion joints (See story on p. 19). Selected waterstops must accommodate the expected lateral, transverse, and shear joint movements, as well as, the expected hydrostatic pressure.
STANDARD TYPES OF WATERSTOPS
Waterstops are manufactured in various types, shapes (profiles), sizes, and material compositions. Standard types of waterstops include PVC thermoplastic extrusions, hydrophilic bentonite and rubber strips, and injection hose systems.
PVC Thermoplastic Extrusions
To accommodate varying hydro-static pressure and movement, most PVC waterstops come in different extruded profiles, widths, and thicknesses. For several decades, the most widely used waterstops were those having a dumbbell shape. Today, waterstop manufacturers have developed profiles with multiple raised ribs to provide improved anchoring and sealing performance.
Both ribbed and dumbbell waterstops are available with flat-web or bulbed centers. They typically are available in 50-ft. rolls, in widths of 4 in. to 12 in., and material thicknesses of 3/16 in. to ½ in. Flat-web waterstops are recommended for use in construction and contraction joints where little or no movement is expected. Since the center bulb flexes to accommodate both shear and transverse movements, these waterstops can be used in expansion, contraction, or construction joints. The center bulbs come in various sizes to accommodate differing amounts of joint movement, with larger-diameter center bulbs suitable for greater joint movements.
Some ribbed waterstops have a center bulb with a thin tear-web on one side that ruptures upon joint expansion. The tear-web keeps concrete out of the center bulb during concrete placement. With the tear-web broken, the center bulb can open up to an extended width to reduce stresses on the embedded ribbed sections. Manufacturers recommend using tear-web waterstops where large movement is expected. They should be installed so the tear-web side faces outward toward the direction of positive water pressure.
The width of the waterstop profile should be selected based on the expected head of water pressure the joint will encounter. The general rule of thumb is the larger the size of a waterstop (e.g. width), the greater the hydrostatic pressure resisted by the waterstop.
However, it is not just size that affects performance; profile material thickness and ribbing also play important roles with resisting higher hydrostatic pressures. Waterstop manufacturers can recommend size and type when actual project design conditions are available for review.
Read more: Sealing Cold Joints with Waterstops