Posted on January 7, 2021
One area of construction that can be easily overlooked is waterproofing. This is true in the precast world as well as in other sectors of construction, probably because waterproofing details are easily missed and tend to not cause issues with the building until a few months or years have passed. Water is a powerful force and if it gets into the wrong places within a building, it can be very destructive. Wells Concrete employs a sealants division to prevent that from happening and to prolong the lifespan of the structures they build.
Precast concrete will inherently contain joints due to the nature of the construction – it is not poured in one continuous casting but into each individual unit that makes up the structure. A good example of an application which commonly requires joint sealants is precast concrete wall panels. Panels are arranged side by side to form a continuous wall, with sealant and foam backer rod being used to fill the vertical joints between the panels (the horizontal joints below the panels will typically get grouted since they are a structural joint). Sealant is used both to form a waterproof barrier and to allow for each wall panel to expand and contract with thermal changes and structural loading cycles. Generally, a backer rod and outward-facing sealant bead will be installed on either side of the wall. A detail of this configuration is shown below.
There are two different types of sealant that are most common when sealing between concrete, urethanes, and silicones. Urethanes generally are cheaper and paintable but can have more limited movement capabilities, and silicones are generally more expensive but will tend to more joint movement and may be more durable depending on the climate, however silicones are not paintable and so the color of silicone sealant must be carefully matched to the surroundings. In cases where continuous insulation is desired as a requirement for building codes or LEED standards, insulation can be inserted into the joint between the two sealant joints.
Joint sealants are also commonly applied to precast structures such as parking garages. A garage that has been waterproofed effectively can last decades, whereas a garage that has not been waterproofed can in some cases begin seeing damage within several years. Garages often specify joint sealants at exterior wall panels, interior shear walls and lite walls, fire rated stair and elevator cores, and at saw cut floor joints. Garage floors are often constructed with double-tee beams spanning between lite walls or inverted tees and the exterior wall, with a topping slab poured over the double tees to “tie” the structure together. PCI standards specify that at locations where double tee beam joints exist below the topping slab, a joint should be saw-cut into the topping slab and filled with sealant. This detail can get overlooked while building a garage and can lead to cracking of the topping slab and require extra maintenance or shorten the lifespan of the garage.
Garages and other structures can also benefit from the use of waterproofing products or traffic coatings to waterproof large surfaces. Traffic coatings are often used when a garage contains inhabited space below horizontal surfaces, as these coatings act as a “roof” for the space below while also being able to be driven on by vehicles. Traffic coatings often consist of multiple layers of a urethane or epoxy installed over the concrete substrate, with sand or quartz distributed into the coating for texture and added durability. Waterproofing or dampproofing products are often used at the building’s foundation or at locations where inhabited space exists below grade to prevent moisture from seeping into the foundation structure or the inhabited space.
There are many other products and techniques used to create a waterproof structure. Waterproofing is an essential component of precast construction and is one of the details that is important in constructing a durable, high performing building. Good communication between architect, general contractor, and subcontractor is important in executing these details and delivering a high-quality product to the client.