Materials
Sealants are typically designed to resist hydrostatic forces at a precast concrete joint to provide a watertight seal. The American Society of Testing and Materials (ASTM) specification C990 defines the required composition and performance testing requirements for preformed butyl sealants. Sealants that meet this standard will provide a reasonable level of assurance for performance in hydrostatic conditions. Butyl rubber content and hydrocarbon content are synonymous terms.

Shape
A traditional misconception of preformed sealants is that wider is better. That is a myth. Preformed butyl rubber sealants work best at higher rates of compression. As the sealant is compressed, the resistance force for further compression increases as a function of the percentage of compression. Time and temperature, as well as the sealant composition, will affect this rate. For instance, the force (psi) to compress the sealant the initial 10% to 50% may range from 2psi to 4psi. But as the sealant reaches 75% compression, the resistance force is about 8psi, and at 85% compression, this reaches a force of about 12psi. As the sealant is compressed, the sealing gasket increases in width as it becomes thinner. This width increases the surface area of the applied force. As the area reaches a point of equilibrium with the designed resistance force of the sealant, compression will stop. The analogy best used is the use of snow shoes. As the area of the applied force is increased, the force per square inch is reduced, thus enabling a person to walk on top of the snow.

Size / Volume
How much sealant is needed to seal the joint? The answer is as varied as the types and sizes of concrete castings being produced. There is no magic answer. Too little sealant may cause the joint to leak; too much sealant will be difficult to compress. The size of sealant required is determined by the volume needed to fill enough of the void to eliminate the possibility that a hydrostatic force will “push” the sealant. A minimum gasket width in the joint of 2″-3″ is common, but a wider band is preferred. A maximum joint gap of 3/8″ is recommended, but it is preferred that the joint be as tight as possible. A rule of thumb for finding the width after compression is to use this formula: (TF/P)/100, where TF=Total force applied in pounds, P=the length of the entire perimeter in feet. Example: 15,000 pounds of force is applied to a rectangular casting 10′ x 5′. The perimeter is 30 feet. Calculate: (15,000/30)/100=5.Therefore, the sealant will compress to a nominal width of about 5″. For box culverts, the net force must be determined by subtracting the force required to move the concrete section from the total force.

Placement
Sealant placement is critical in providing the best, watertight joint. Best practice is to place sealant where the concrete touches tightest. It is also preferred to fill the annular space. On a box culvert, the best practice is to place some of the sealant on the bell in the angled surface along the bottom, and 6″ above the center on each side. Continue placing the sealant on the spigot end of the mating piece on the angled surface along the top, and 6″ below the center on each side.

Adhesion
The adhesion of the sealant to the concrete affects the hydrostatic resistance. Water can pass by sealant through the concrete by absorption if the joint surface is not properly prepared. This will result in wetness in or near the joint, opposite of the side of the joint where water is present. Sealant will adhere to a clean, dense, well-cured concrete surface. To improve adhesion, use one of ConSeal’s primers to create a surface that the sealant will adhere to.