Preventing Water Ingress with Waterstops
To ensure that below-grade structures remain free from groundwater intrusion, designers will instruct contractors to install waterstops in addition to waterproofing membranes. This systematic approach provides maximum protection by combining two proven technologies. Membranes are designed to prevent water intrusion through walls and floors. Waterstops prevent water from traveling into the structure by closing the seams created by construction joints.
Preventing Water Ingress with Waterstops
Construction joints exist on all buildings, and manufacturers offer a wide range of products that can be used to close these gaps. But when seams and joints occur below grade contractors need selection and installation processes that are spot on. White Cap specialists can advise you on the proper type of waterstop and supporting waterproofing products such as sealants, caulking, and membranes that will create this systematic approach.
Waterstops act like dams restricting water movement along the joints where structural elements contact each other. Waterstops can be installed horizontally and vertically, and are placed just before the concrete is poured or when elements are set in place.
How Waterstops Work
Waterstops are also used on above-grade structures as well. Large structures often require multiple concrete placements creating cold joints. Some modular building techniques, such as precast construction, require contractors to treat the joints where modular elements are connected. Waterstops seal the seams in structures that act like vessels, such as wastewater treatment plants, retaining walls, and pools.
Another possible application is to install waterstop on expansion joints. Expansion joints separate or isolate abutting concrete structures (walls, slabs, footings columns) protecting them from compressive stresses that may develop due to thermal expansion, settlement, creep, live load deflections, drying shrinkage or crush. These movements can be both lateral and transverse. Typical applications include bridge abutments, pool copings, parking structures, and contact points between slabs and buildings.
Waterstops are an important waterproofing tool in below grade applications. When construction is completed, ground water levels will return to pre-existing conditions. As the ground water fills voids in the soil and near the new building, its presence creates hydrostatic pressure. Often this natural force is strong enough to squeeze water into any untreated joints that exist between the footing and foundation or between underground structural elements. When left untreated during construction, trying to stop this inflow after the structure is completed, especially in footings and foundations, is very difficult and costly. Examples of areas susceptible to water inflow from hydrostatic pressure include foundations, tunnels, footings, and underground parking areas.
Contractors should consult with site engineers to select a waterstop profile and dimension that matches the expected head of hydrostatic pressure the joint will encounter. A general rule of thumb is the larger the size of a waterstop (e.g., width), the greater the hydrostatic pressure it will resist. Your White Cap expert can help you in this selection process.
When Movement is ExpectedSpecial waterstops are used on joints where engineers or contractors have the expectation of some subtle movement either during construction or in a structure’s as-built service life.
The most common uses of waterstops during construction are centered on activities around concrete placement. Contractors install waterstops at construction joints and cold joints. These planes of weakness occur when there is a planned delay in the concreting operations.
There are several causes of movement at construction joints. The most common is drying and shrinkage of fresh concrete.
Temperature changes between placements have also been reported to cause slight movement. Following the placement period, soil settlement, concrete creep, and live-load deflections may cause the construction joint to separate.
When movement is expected along joints after construction, engineers specify expansion or contraction details. There are a wide array of waterstop profiles for these designs. The profiles allow engineers to match the profile to the magnitude of the movement differential and the movement’s orientation. Waterstop profiles can be designed to match the joint’s expected lateral, transverse, and joint movements. Each profile has its own characteristic response to how it will remain in place should there be any movement at the joint.
- Ribbed with Centerbulb waterstops are the most versatile and best sealing waterstop profile. These products come in rolls, and enable workers to position them flat on the host structure. The waterstop’s ribs help restrict movement during placement. As the new piece is applied to the waterstop, the center bulb flattens from the additional weight. The centerbulb’s size is matched to the anticipated force and to any lateral, transverse, and shear movement.
- Base Seal with Tear Web waterstop profiles accommodate large lateral, transverse, and shear movements. When joint movement occurs, the tear web ruptures and allows the U-bulb to deform without placing the remaining waterstop material in tension. 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 to an extended width to reduce stresses on the embedded ribbed sections. Contractors should install these waterstops so the tear-web side faces outward toward the direction of positive water pressure.
- Dumbbell with Centerbulb waterstop profiles accommodate lateral, transverse, and shear movement. You can match the size of the centerbulbs to the movement, increasing its size with the anticipated increase of movement. For better sealing results, opt for profiles that are ribbed.
When the Joint is Designed Not to MoveWaterstops are also used on projects where no movement at the construction joint is expected.
Labyrinth waterstop profiles are used primarily in vertical construction joints where little or no movement is expected. Labyrinth shapes create a keyed joint and do not require split bulkheads. Workers attach the labyrinth waterstops to the formwork prior to the first placement. Following the formwork removal, the embedded waterstop is then enclosed in the second pour, sealing the joint.
When construction joints are secured using continuous steel reinforcement to prevent movement, contractors have several options.
- PVC waterstops are one option. Flat Ribbed profiles are preferred for non-moving joints and provide the best sealing characteristics. Labyrinth profiles are used in vertical joints.
- The second option is to use hydrophilic waterstops. These strips are adhered directly to the exposed joint surface of the first concrete pour just before forming and second concrete pour to form the joint area. The waterstop contains material that reacts with water and swells to fill the joint’s gap. This action forms a positive watertight seal in the concrete joint.
The American Concrete Institute (ACI) recommends against the use of hydrophilic waterstops for hazardous fluids such as fuels, acids, and process chemicals. The waterstops may not swell as intended in fluids other than water.
Hydrophilic waterstops are also available as molded plugs and rings. These units are inserted around formwork spacers, center pens and tie rods to create a watertight seal.
Special Waterstop ConsiderationsFor additions to below grade structures, retrofit waterstops seal joints where new construction meets an existing structure and can be suitable for moving joints. These “tee” shaped systems include stainless steel batten bars and fasteners that workers anchor to the existing structure at the joint’s location. The waterstop is then embedded in the newly placed concrete.
Material selection is a key consideration when purchasing waterstops. When construction joints are in conditions that may have chemical exposures, you should opt for waterstops made with metal or a non-corrosive material.
Workmanship CountsInstalling waterstops requires not only good materials, but an installer’s attention to detail. The process starts when workers carefully inspect waterstop placement areas to be sure they are clear from dirt and debris.
Prior to installation, contractors should consult the manufacturer’s installation documents as each type of material requires special considerations.
Contractors often opt for a split-leg waterstop profile when design professionals anticipate movement. These profiles are often made with PVC, neoprene rubber or thermoplastic rubber. Position the ‘split’ side of the waterstop against the formwork for the first concrete pour. The waterstops should be tied down to the reinforcing steel every 12 inches on center to restrict movement during placement.
It is important that the center of waterstop bulb is positioned at the exact mid-point of the construction joint. This will ensure that the waterstop acts as an expansion joint during any structural movement.
Following curing and formwork removal, the contractor glues the remaining half of the waterstop to the embedded piece. The second leg is then encapsulated in the second pour. This profile works well when reinforcement is not present. And because there could be a chance of a tear in the gluing process, these profiles are not generally used for chemical containment.
On some projects, heat welding of the material laps is required to achieve a proper fusion of the material.
White Cap offers a wide range of products that can help fill in any gaps that can occur during installation and placement.