By Kayla Hanson, P.E.
Some of precast concrete’s greatest attributes in water and wastewater storage, treatment and conveyance infrastructure are its strength, resilience and watertightness. Each precast structure – from tanks and manhole sections to pipes and culverts – is manufactured to exact specifications, ensuring superior performance.
Of equal importance to a precast concrete system’s functionality and watertightness is the structure’s sealants. Preformed, flexible joint sealants are integral components for creating watertight joints, which not only facilitate the structure’s performance but also help protect the surrounding environment.
ASTM C990, “Standard Specification for Joints for Concrete Pipe, Manholes, and Precast Box Sections Using Preformed Flexible Joint Sealants,” outlines performance and physical property requirements of both butyl- and bitumen-based preformed flexible joint sealants.
FOLLOW BEST PRACTICES
Follow these best practices for properly selected, sized and installed sealants to help ensure resilient, watertight systems that stand the test of time.
Select the proper sealant type for the job
Certain sealant types and chemical compositions are required for different structures and applications. ASTM C990-compliant sealants are composed of at least 50% hydrocarbons. However, structures that hold, convey or potentially come into contact with fuel or oil shall not be sealed with hydrocarbon-based sealants, because fuel and oil can degrade them. Additionally, structures handling potable water must use sealants that are non-toxic. Local and project specifications will outline requirements, and sealant suppliers can provide advice regarding the best sealant for each situation.
“‘Butyl’ and ‘mastic’ are terms that get confused in the industry,” said Pat Mulhall, vice president of sales with Polylok ZABEL.
Mastic is oil-based and applied with a trowel. It can dry out over time and potentially crack. The term “butyl” is sometimes used as a blanket descriptor for preformed flexible joint sealants.
However, preformed flexible joint sealants can be either butyl- or bitumen-based. Butyl- and bitumen-based joint sealants are formed into ropes or strips, are applied from a roll and can maintain their flexible consistency throughout their lifetimes.
Select the proper sealant size and quantity to achieve at least 50% sealant compression
Preformed flexible joint sealants come in a variety of cross-sectional dimensions. The appropriate sealant size and number of sealant ropes required for a particular application depends on the weight of the adjoining precast concrete component and the joint geometry.
The adjoining concrete structure’s weight and the size and number of sealant ropes used will determine how much the sealant ropes compress. Supplier guidelines typically state preformed flexible joint sealants must deform by at least 50%, and greater than 50% deformation is ideal. The more the sealant is compressed, the wider the band of sealant will become.
“A wider band of compressed sealant in the joint is always going to be better than a narrow band of equal height,” said Sam Lines, engineering manager with Concrete Sealants and 30-year veteran of the precast concrete industry.
The sealant’s cross-sectional area will remain the same regardless of its compression. As the sealant band becomes wider, more of the joint will fill with sealant. This, in turn, can provide a better seal.
As far as the possibility of compressing the sealant too much, Lines said it is extremely unlikely.
“I’ve had the question asked: ‘Can you squeeze all the sealant out of the joint?’” Lines said. “The thinnest it usually gets is between 1/16 and 3/16 of an inch. Somewhere in that range, the sealant is going to stop compressing.
“It’s like a spring. As you start to compress it, it doesn’t take a whole lot of force, but as the spring gets tighter and tighter and the coils get closer and closer, the force required for further compression of the spring gets greater and greater.”
Lines also noted that more sealant is not always better.
“Too many pieces next to each other will distribute the load too much and will prevent the proper compression,” Lines said. “It’s like a snowshoe. Using more sealant distributes the weight over a wider area and reduces the amount of compression.”
Sealant sizes, designations and offerings vary by supplier.
“They are available in a wide range of sizes to fit a variety of joint types in treatment tanks, grease interceptors, manholes, box, elliptical or arch pipe foundations, vaults, wet wells and so on,” said
Dan Brundage, Midwest territory manager with Press-Seal Corporation.
Additionally, sealant size designations may not correspond to the actual sealant dimensions. Like dimensional lumber, the actual sealant dimensions generally are smaller than the nominal sealant size.
When in doubt, reach out to a sealant supplier for guidance, specifications and recommendations.
Assess the joint’s integrity and make any necessary repairs
Inspect the joint and surrounding areas for defects. A hairline crack or spall near or in the joint must be repaired properly before proceeding.
Simply replacing spalled concrete in its original location without repairing it can severely compromise the joint and the structure’s watertightness.
Ensure the joint surface is clean, dry and free of debris
Preformed joint sealants exhibit the strongest adhesion when applied to clean, dry surfaces. The presence of moisture, dust or debris can reduce a sealant’s ability to bond with the concrete, similar to how dust or dirt on steel reinforcement hinders the reinforcement’s bond with the concrete cast around it.
Joints should be prepared with a stiff-bristled brush to remove any concrete high points, flashing or debris. Joint surfaces also should be clean to the touch without leaving excessive lime or dust residue.
Assess the need for additional joint surface preparation
Preformed joint sealants provide the most effective seal when applied to smooth, level surfaces. Even minor high or low points on the joint surface could cause small gaps between the concrete and the sealant, providing miniscule yet viable points of ingress or egress in what is intended to be a perfectly sealed joint.
“The condition of the joint is one of the most important things,” Brundage said.
Additionally, ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks,” and ASTM C1613, “Standard Specification for Precast Concrete Grease Interceptor Tanks,” state the maximum gap between the two mating joint surfaces shall not exceed 3/8 inch before the joint sealant is applied. Sealant manufacturers use this maximum allowable gap measurement to properly size sealants and ensure the selected sealant dimensions provide a proper seal.
Depending on the type of precast structure and its casting orientation, the joint surface may be formed or finished. Formed joints, while generally smooth and precise, could contain small, exposed bug holes. Finished joints could have slightly uneven textures compared to formed surfaces. In either case, consider applying a primer, adhesive or coating to the joint surface to fill in slightly lower points and create a smoother application surface for the sealant. Ensure the primer, adhesive or coating is appropriate for the application and that they dry completely before applying the sealant.
Store the sealant at an appropriate temperature and away from dirt and debris
Butyl-based joint sealants generally are intended to remain around 40 degrees F or warmer to ensure compressibility. Some suppliers offer butyl-based joint sealants that can be applied as cold as 0 degrees. Ensure you are using the right sealant type for the temperatures at which you plan to store it. In cold climates, consider specifying ASTM C765, “Standard Test Method for Low
Temperature Flexibility of Preformed Tape Sealants” to provide some indication of the sealant’s flexibility and adhesion at low temperatures.
Bitumen-based joint sealants become firm and incompressible at colder temperatures and significantly softer and stickier at warmer temperatures.
Be careful to keep the sealant in its packaging and with the protective waxed paper or plastic film intact as long as possible to preserve its adhesive properties. Dust, dirt, debris and other contaminants that come in contact with sealant can compromise its ability to bond to concrete surfaces.
Select the sealant location within the joint
The sealant rope should be positioned as close to the joint’s centerline as possible, regardless of the type of joint. This helps ensure the entire sealant rope remains within the joint after the sealant is compressed by the adjoining concrete component.
“Keeping the sealant within the joint is imperative,” Lines said.
Any sealant that is expelled from the joint and bulges out of the side of the structure reduces the cross-sectional sealant area contained within the joint and reduces the robustness of the seal.
Many joints are interlocking – including tongue-and-groove or shiplap – as opposed to slab-style joints, which consist of two flat surfaces bearing against one another. For tongue-and-groove joints, apply sealant to the depression in the center of the groove, as shown in Figure 1 below. For shiplap joints, the sealant may be applied to either the upper joint surface, lower joint surface or both , as shown in Figure 2 below. If one sealant rope is used, the sealant is typically applied to whichever joint surface (upper or lower) is adjacent to a greater hydrostatic load or is deemed more susceptible to water infiltration or exfiltration.
Determine where to begin the sealant rope application and avoid starting at a corner
A sealant rope’s starting location around the structure’s perimeter is critical. For non-circular structures, begin applying the sealant rope to a joint surface on any side of the structure. Never begin applying the sealant in a corner. Starting the sealant rope in a corner could cause the structure’s adjoining component to bear slightly unevenly on the sealant, which could compromise the seal.
Begin applying sealant at least 12 inches away from any corner to help prevent uneven bearing.
Apply the sealant as a continuous rope without stretching the material
Apply preformed flexible joint sealant with the exposed sealant surface facing down, pressing it directly against the prepared joint. Avoid piecing together multiple pieces of sealant. Instead, apply the sealant as a continuous rope around the perimeter of the joint. Piecing together multiple sealant segments is a more labor-intensive process and introduces opportunities for improper seals and sealant discontinuities.
Ensuring the sealant’s entire cross-section remains intact and contained within the joint is crucial to creating a watertight seal.
“Never stretch the sealant,” Mulhall said.
Stretching sealant reduces the rope’s cross-sectional area, which can compromise a seal and create inconsistencies in how an adjoining concrete component bears on the seal.
Also, work to keep the protective waxed paper or plastic film intact while applying sealant to prevent sealant contamination and reduced adhesion to the adjoining concrete surface. The protective paper should be removed just prior to joining the precast sections.
Cut the sealant rope to create a butt joint
Upon completing the sealant perimeter around the structure’s joint, terminate the sealant rope with a clean cut to create a butt joint or a 45-degree mitered joint. This helps ensure a reliable seal and allows the adjoining concrete surface to bear evenly across the entire perimeter of sealant.
The terminating sealant end also can be laid directly adjacent to – and in contact with – the beginning of the sealant rope, along the tail end to extend a few inches past the sealant starting point.
Avoid overlapping the sealant ends, and never leave gaps in the sealant perimeter
Avoid overlapping the sealant ends when completing the sealant perimeter around the structure’s joint. Even if the sealant is properly applied, a tiny – even microscopic – gap likely will remain under the overlapping sealant segment.
Overlapping the sealant also can create high points in the sealant, which could lead to inconsistent sealant compression around the structure.
Never leave a gap in the sealant rope’s perimeter around the structure. It may be possible for the adjoining precast component to compress the two sealant pieces such that they touch, however, that approach is unreliable. It also is unlikely that joining sealant segments will contain a sufficient sealant surface area or that the two sealant ends thoroughly combine into a continuous, homogeneous sealant rope.
Knead the sealant ends together
Whether a butt joint, mitered joint or aligned side by side, the adjoining sealant ends must be carefully kneaded together to create a continuous sealant rope around the structure’s perimeter.
Working the two sealant ends together so there are no gaps and the sealant’s cross-sectional area is consistent helps ensure a reliable seal and consistent sealant compression around the structure’s perimeter.
If the adjoining concrete structure is removed after the structure was sealed, discard the sealant
“Removing the previously sealed adjoining concrete component will cause the sealant to stretch and tear,” Lines said. “It’ll tear itself apart.”
Compromised sealant should be removed and discarded. Do not attempt to piece the sealant rope back together. After removing the original sealant, the joint surface should be reinspected, and new sealant should be applied.
SMALL BUT MIGHTY
Though it may appear to be a seemingly simple material and its placement a straightforward task, properly applying preformed flexible joint sealant requires careful attention to detail.
Appropriate sealant selection, proper joint preparation and mindful sealant placement are imperative to many precast concrete systems’ functionality and watertightness.
Follow preformed flexible joint sealant best practices to provide reliable, resilient seals that create dependable, watertight joints and enable a variety of precast products to work seamlessly together as continuous and efficient systems.
Kayla Hanson, P.E., is NPCA’s director of technical services.