Understanding waterstops: To ensure success, one must specify for performance, install with care

by Katie Daniel | June 30, 2015 10:06 am

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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.

Material basics
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.

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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.

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Thermoplastic and rubber extrusions
To accommodate varying hydrostatic pressure and movement, most thermoplastic and rubber waterstops come in different extruded profiles, widths, and thicknesses. For years, the most widely used waterstops were those anchored by having a dumbbell shape at each end, which provided a ‘cork-in-the-bottle’ seal when the joint opens. However, American Concrete Institute (ACI) 504R, Guide to Joint Sealants for Concrete Structures, reports this seal is ineffective at small joint movements, and at wider movements the waterstop
is placed in considerable tension. To overcome these issues, waterstop manufacturers 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 15.2-m (50-ft) rolls, in widths of 102 to 305 mm (4 to 12 in.) and thicknesses of 5 to 13 mm (3/16 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. With the tear-web broken, the center bulb can open up to the extended width of the joint without stressing the embedded ribbed sections. The tear-web keeps concrete out of the center bulb during concrete placement. Manufacturers recommend using tear-web waterstops where large movement is expected. They should be installed so the tear-web side faces the direction of positive pressure.

While rubber thermoset waterstops have excellent mechanical properties (i.e. high tensile strength and good elongation), they are difficult to field-fabricate as the rubber is vulcanized, meaning it has already taken a ‘set’ (i.e. thermoset) and cannot be heat-welded together like thermoplastic materials.

Waterstop size (i.e. width) is determined by the expected head of water pressure to be encountered at the joint. The general rule of thumb is the larger the size of a waterstop (e.g. widths of 102, 152, 229 mm [4, 6, 9 in.]), the greater the hydrostatic pressure resisted by the waterstop.

However, it is not just size/width affecting performance—profile thickness and ribbing also play important roles with thicker products resisting higher hydrostatic pressures. Waterstop manufacturers can recommend size and type when actual project design conditions are available for review. Further, the manufacturer can provide guidance on the minimum depth of embedment the waterstop should be installed in the concrete for the expected hydrostatic pressure.

Installation failures

The problem with waterstops is their susceptibility to improper installation or damage during the concrete placement. The following list illustrates some of the many potential installation failures for waterstops. dumbbell or ribber center-bulb roll ends overlapped but not welded or spliced together; installed too close to steel reinforcement; dumbbell splices glued together with sealant; not welded; strip waterstop installed with concave gap (void) under it; polyvinyl chloride (PVC) transition glued together—no fabricated part; PVC welded on the edge only and not fully across its profile thickness; poorly consolidated concrete adjacent to waterstop; overheated burnt or charred thermoplastic welds; dumbbell or ribbed center-bulb product not centered in joint; dumbbell not properly tied into reinforcement so it shifted during concrete pour; hole cut in flange of dumbbell to pass the rebar through; overlapping, not butting, hydrophilic strip roll ends; flange of dumbbell cut narrower to fit around reinforcing steel; misaligned ribs or centerbulb at splice; concrete extending on flange not removed from waterstop prior to second pour; and hydrophilic-strip waterstops installed only with fasteners.

Special thermoplastic materials and profiles
For better chemical resistance to fluids, dumbbell and ribbed center-bulb waterstops are produced with thermoplastics and rubbers such as polyethylene and TPV—the latter for primary and secondary containment structures, as well as ozone contactor wastewater structures. TPV waterstops are resistant 
to a wide range of oils, solvents, and industrial chemicals. Unlike PVC, TPV contains no plasticizer to leech out when exposed to chemicals and fuels.

One manufacturer reports TPV can withstand prolonged exposure to low and high temperatures (−100 to 135 C [−150 to 275 F]) without detrimental effects, but becomes very soft around 150 C (300 F), and melts at approximately 200 C (400 F). Therefore, for applications requiring very high heat resistance, metallic waterstops (discussed later in this article) should be specified and installed. For excessively cold climates, ‘arctic-grade’ PVC waterstop is specially formulated to retain its flexibility and physical properties to the range of −45 C (−50 F).

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Waterstops made with thermoplastics are also extruded into other profiles: split-leg, labyrinth, base-seal, and retrofit.

Split-leg
The split-leg profile eliminates having to split the formwork for the waterstop installation, but quality splicing is difficult at intersections and plane transitions. Split-leg allows the ‘split’ side to be placed flat against the formwork for the first concrete pour. After the first pour has cured and the formwork removed, the two halves of the split-leg are glued together and then encapsulated in the second pour.

The split-leg profile should not be used where shape of the forms or location of the reinforcing steel prohibits opening of the split flange. The split cannot open properly and be placed against formwork in a plane transition or fitting configuration. For these reasons, such profiles are not generally used for containment of chemicals.

Labyrinth
Labyrinth-profile waterstops are used primarily in vertical construction joints where little or no movement is expected. Like the split-leg, they can be installed without split forming. With labyrinth, quality splicing is extremely difficult at intersections and changes of direction due to their complex profile shape. For these reasons, labyrinth profiles are also not generally used for containment of chemicals.

Base-seal
Base-seal waterstops have a large flat profile with ribs only on one side. They are used at joints of slab-on-grade and for property line shoring walls where the waterstop is placed to be positioned on the exterior surface of the concrete, rather than embedded within. This design is easy to form and ensures accurate functional placement.

Retrofit
There are also special extrusion shapes designed to provide a fluid-tight seal between existing and new construction. These retrofit waterstops are installed without the labor-intensive and structurally destructive saw-cut-and grouting of the existing concrete to embed half the waterstop. Further, retrofit-profile waterstops can be employed to isolate structural elements such as pilasters, columns, and tank pads. Retrofit profiles are available for both construction and expansion joints.

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Hydrophilic waterstops
Unlike PVC dumbbell waterstops that function by passively blocking water migration, hydrophilic strip waterstops react with water and swell to from 
a positive watertight seal in the concrete joint. Commercially available for several decades, hydrophilic waterstops have a long, proven track record. A hydrophilic waterstop is effective in providing protection against water infiltration through concrete construction joints, including joints under high hydrostatic pressure and intermittent hydrostatic conditions.

Hydrophilic waterstops are produced in rolls or strips. The profile is usually a small rectangle with 20 x 25 mm (¾ x 1 in.) and 20 x 10 mm (3/4 x 3/8 in.) being the two prominent sizes. Hydrophilic strip systems, both bentonite and rubber, are used mainly in construction joints—most are not intended for use in expansion joints. These systems are simple to install, and do not have to be secured in place before the first concrete pour; further, sections do not need to be heat-welded together. The material is adhered directly to the cured surface of the first concrete pour in preparation for the second concrete pour to form the joint.

The strips are adhered to cured concrete using an adhesive or primer. This adhesion is important so the waterstop is not displaced during the concrete pour. Alternately, a covering strip of steel mesh fitted over the strip can be nailed 300 mm (12 in.) on-center (o.c.) to secure the waterstop.

Hydrophilic-strip waterstops should not be installed with only mechanical fasteners (i.e. nails only without steel mesh covering) placed along the product length. When installed only with fasteners, a thin layer of concrete paste can extrude under the waterstop during vibrating and can cause it to lose most of, if not all, its effectiveness. Even worse, concrete placement can tear the waterstop off the fasteners and cause this portion to be displaced into the concrete, out of the plane of the joint. Therefore, one must always specify and install these strip waterstops adhered in place or with the steel mesh cover system.

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Applications for hydrophilic waterstops are not limited to cast-in-place concrete construction joints. Due to their flexibility and conformability, hydrophilic waterstops can be easily installed around pipe penetrations, I-beams, concrete pilings, and irregular-shaped surfaces. Hydrophilic waterstops are also used to seal new concrete placed against existing concrete. Additionally, they are lightweight and flexible, and do not require any special order prefabricated Ts, Ls, or crosses. The installation efficiency of this type of waterstop makes them
a favorite amongst contractors.

For hydrophilic waterstops, manufacturers typically require a minimum 50 to 75 mm (2 to 3 in.) of concrete coverage depth, depending on the profile size and material type. These waterstops must be installed in strict accordance with the manufacturer’s minimum concrete coverage guidelines to eliminate the potential of concrete spalling due to insufficient concrete coverage. They need to be installed without overlapping adjoining strip ends; the strip ends must be tightly abutted to form a continuous system.

Since these products expand in the presence of water, they should not be prematurely wetted. This requires the second concrete placement take place promptly after the waterstop placement; otherwise, the waterstop might expand if exposed to rain. The ‘free water’ in a concrete mix is not enough to physically swell a hydrophilic waterstop. After the concrete has cured, a sufficient external source of water that migrates into the joint and comes in contact with the waterstop is required for it to actually swell. If no external water source comes in contact with the hydrophilic waterstop, it simply remains dormant.

Not all hydrophilic waterstops use bentonite clay as their hydrophilic agent. Hydrophilic rubber waterstops expand upon contact with water and maintain a solid material integrity that will not deteriorate due to uncontrolled expansion. However, since they are produced with different types of hydrophilic agents, they may have different expansive properties (in terms of rate and volume) when exposed to water. Also, some are produced with portions of the profile made of non-swelling rubber—this means the entire profile does not activate and swell.

This class of hydrophilic waterstop has the same limitations as less-expensive bentonite-based waterstop products—namely, the inability to swell in fluids other than water, and the lack of expansion joint functionality.

Special Conditions: Shotcrete

With pneumatically applied concrete (i.e. shotcrete) foundation wall construction, it is advisable to install a waterstop in all construction joints and whenever possible all lift joints. Typically, the preferred waterstop type is a strip hydrophilic class waterstop, in lieu of a PVC dumbbell, due to the application of the shotcrete. With ribbed center-bulb waterstop profiles, it is difficult to spray and properly consolidate the shotcrete behind the wide waterstop flange. The consolidation can also be affected by any movement (vibration) of the waterstop flange by the force of the shotcrete spray. Today, many waterproofing membrane manufacturers include their own branded waterstops as part of their overall waterproofing system. Therefore, for waterproofing warranty eligibility and issuance, the manufacturers require the use of their waterstops.

In-situ performance of hydrophilics
Hydrophilic waterstops are effective in providing protection against water infiltration through concrete construction joints, including those under high-pressure and intermittent hydrostatic conditions. Due to their product design, hydrophilic waterstops exhibit a high degree of expansion when hydrated in a non-confined, free-swell condition, such as in 
a bowl of water.

In a non-confined condition, the product may exhibit some fracturing as it approaches maximum expansion. Although this may appear to be a negative aspect of the material, the fracturing is an indicator of positive performance for in-situ concrete joint conditions. It means the waterstop compound expands and easily conforms to the interior surfaces surrounding it within a concrete joint, no matter how irregular.

Further, the plasticity and expansion properties provide it the ability to extrude into cracks and advance around angle transitions that may be present in the joint. Bentonite compounded strips have been observed expanding through narrow cracks to fill larger void areas beyond the poorly consolidated concrete interface with the waterstop strip.

This dynamic sealing ability is possible because of the expansive properties; the same capabilities would be impossible with a high-tensile-strength material such as a hydrophilic rubber waterstop. It has been observed through lab testing that hydrophilic rubber waterstops have limited expansion into cracks, gaps, and joints that surround them due to their more rigid material properties. The hardness of the rubber may allow the waterstop to expand only a swallow convex amount across the opening of the crack and not extrude into its depth.

The maximum expansion a hydrophilic material exhibits in non-confined, free-swell conditions will not materialize in properly consolidated concrete. In-situ, hydrophilic waterstops only have to expand slightly to form the positive seal against the concrete interface around it. This limited swell affords the product a ‘reserve swell potential’ to seal poorly consolidated concrete or for concrete cracking that may occur later during building settlement or seismic activity.

Once hydrated, hydrophilic waterstops rarely undergo wet-dry cycling. When confined in concrete and below-grade, the material maintains its hydrated equilibrium and does not shrink during typical water-table fluctuations or intermittent water conditions. Interestingly, bentonite can be hydrated and dried an infinite number of times without losing its original, natural swelling capacity. Similarity it can 
be frozen and thawed repeatedly without losing swell performance. Finally, even if in-situ drying were to occur (an extremely remote possibility), bentonite has been proven to rehydrate to its original level 
of performance.

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Non-expanding mastic strip waterstops
Mastic-strip waterstops are not susceptible to prehydration expansion like bentonite or hydrophilic rubber materials, but their reliance on adhesion to the concrete might prevent complete sealing of the joint if some areas are not bonded or joined together properly.

Like hydrophilic waterstops, most non-expanding mastic waterstops are produced in rolls or strips. The profile is usually a small rectangle shape, with
20 x 25 mm (¾ x 1 in.) being the most common for butyl-rubber compounds and dimensions of 
38 x 12 mm (1 ½ x ½ in.) for asphaltic-based strips.

Mastic waterstops are typically a strip of tacky, asphaltic, or butyl rubber based compound. They are designed to stick to a primed surface of a cured concrete cold joint. The strip is adhered at a minimum embedment depth in accordance with the manufacturer’s guidelines and then a second concrete pour is cast that encapsulates the remaining three sides. The heat from the hydrating concrete causes the product to become even tackier, thereby sealing the joint by acting as an internal, adhered sealant.

Adhesion is important. Should the waterstop be displaced during the concrete pour, it can easily lose most, if not all, its water barrier performance. Mastic waterstops are designed for construction joints only—they should not be specified for use in expansion joints.

Mastic waterstop hydrostatic resistance performance is very limited as the only barrier to migrating fluids is the adhesion to the concrete of the low profile strip. Mastic waterstops are the lowest-performing of any commercially available waterstop and are priced accordingly, being the lowest in cost.

Injection-hose waterstops
Injection-hose waterstops are usually permeable or perforated hose installed during new construction with a series of injection ports and valves. These ports are placed exposed at the interior surface of the concrete for later access. Should a leak occur after construction, a grout pump can be used to inject a resin (usually polyurethane) to seal the joint and fill any adjacent voids in the concrete.

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Manufacturers purport some products to be ‘re-injectable,’ but this is frequently not the case, due to cured resin surrounding the hose or contractors not adhering to the manufacturer’s guidelines (i.e. chiefly not cleaning out the hoses properly after the initial resin injection).

Each injection hose system uses permeable hose that is installed at construction joints, in short lengths (typically less than 9 m [30 ft]). The hose core is surrounded by a woven mesh outer layer so resin injected under pressure can pass out of the tube into the joint, but cement fines and aggregates cannot enter the hose and clog it. The hose is secured to the first concrete pour after it sets with clamps mechanically fastened (typically 305 mm [12 in.] on center).

Adjacent perforated hoses are placed overlapped by 150 to 300 mm (6 to 12 in.) at each segment end, with a short non-perforated tube attached in-line that exits the concrete as the injection port. After the structure is built, polyurethane resin is injected into the hose to seal the joint and fill any cracks or voids in the joint area. They can also be used to seal construction joints between new and existing structures. Typical applications include secondary containment, basements, and tunnels.

Metallic waterstops
Metallic waterstops can obstruct the passage of corrosive fluids, even at severely elevated temperatures. Therefore, metallic waterstops are used primarily for severe chemical and high-temperature environments that can degrade other materials. A variety of metals, grades, and gauges are available.

Stainless steel offers broad-spectrum corrosion resistance, and is virtually unaffected by the effects of ozone, making it an ideal choice for ozone contactor structures used in modern water treatment plants. Stainless steel waterstops are available in many standard shapes and sizes, including profiles for new and retrofit. All change-of-direction fabrications should be pre-manufactured, leaving only straight butt-welding in the field.

Metallic waterstops are probably the most difficult to install, as split-forming is always necessary by means of tungsten-inert-gas (TIG) or metal-inert-gas (MIG) welding. All straight-run material should be welded edge-to-edge (no overlapping).

The stiffness of metal waterstops can lead to cracking of adjacent concrete. Thereby, rigid metal waterstops should be installed with adequate concrete coverage.

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Multiple system installation
Waterstop systems are relatively low 
in cost, so having a secondary product installed can be a wise and inexpensive investment. This leads to the question, ‘If the concrete joint has the appropriate width clearance, why not use one waterstop as the primary barrier and another as a secondary barrier as an insurance policy?’ If the primary system fails in any way due to material or installation defect, the second system is there to ensure fluid-tight integrity at the concrete joint.

To clarify, when a ribbed center-bulb or hydrophilic waterstop is properly installed, there is really no need for a secondary waterstop system. Regardless of polymer or manufacturer, these waterstop products really only leak from poor installation procedures and a lack of quality assurance.

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A typical belt-and-suspenders approach would be to have a ribbed center-bulb waterstop on the positive-pressure side of the joint and a hydrophilic or mastic strip-applied waterstop several inches away from it on the negative pressure side of the joint. An alternative secondary waterstop would be an injection tube system placed on the low pressure side.

The benefits of redundancy in installed waterstop systems is great and the cost is low, especially when amortized over an extended life of the concrete structure in which they are installed.

On projects, hydrophilic waterstops can also be used around pipe and mechanical penetrations, while PVC hydrophobic waterstops can be specified for the concrete construction joints.

This author has seen critical infrastructure subway projects with three waterstop systems specified and installed in the construction joints of the thick concrete walls. The installation included a PVC ribbed center bulb to the positive pressure side, a bentonite hydrophilic strip in the center, and an injectable hose waterstop installed closest to the interior face of the concrete wall.

Conclusion
Contractors have perhaps the most important role with regards to waterstops—the installation. These systems should be treated as strategic building envelope barrier material for the long-term success of a project. For any waterstop to be effective, proper design, installation, and concreting practices must be followed. Foremost, one must select a product size and profile suitable for the expected joint movement, hydrostatic head, and chemical resistance required.

Stacy Byrd, GRP, LEED AP, is the national products manager at CETCO, a manufacturer of waterproofing, green roof systems, composite drainage, and waterstops. He has a quarter-century of technical waterproofing design application and practical field experience with below-grade foundations, plaza-decks, tunnels, and vegetated roofs on commercial, institutional, civic, and government projects. Byrd is an active member with ASTM International Committee D08 on Roofing and Waterproofing and a Green Roof Professional (GRP). He can be reached at stacy.byrd@cetco.com[13].

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