WITH WHICH STANDARDS DO SEALING MATERIALS NEED TO COMPLY?

Generally, ASTM or AASHTO standards are specified for most joint sealing material. These standards define the composition and performance requirements of different products, including preformed flexible joint sealant, external joint wrap, and gun-grade material.

ASTM C990, “Standard Specification for Joints for Concrete Pipe, Manholes, and Precast Box Sections Using Preformed Flexible Joint Sealants.”
ASTM C877, “Standard Specification for External Sealant Bands for Concrete Pipe, Manholes, and Precast Box Sections.”
ASTM C920, “Standard Specification for Elastomeric Joint Sealants.”

In some cases, such as hydrocarbon or chemical resistance, a special sealant material that does not have an applicable ASTM standard must be used.

HOW MUCH SEALANT IS NEEDED?

The answer is as varied as the types and sizes of concrete box culvert sections produced. A misconception is that more joint sealant is better. However, too much sealant – or too little sealant – can result in leaky joints. The amount of sealant required is based on the structure’s dimensions and joint details, and the volume of sealant needed to sufficiently fill the joint gap.

Ideally, sealant coverage is preferred throughout as much of the wall thickness and joint width as possible. A minimum compressed sealant width in the joint of about 2”-3” is common, but a greater width of coverage is preferred. A maximum dry fit joint gap of ½” is recommended, and it is preferred that the joint be as tight as possible.

Preformed flexible joint sealants work best when they are compressed more. Generally, at least 50% sealant compression is required.

Preformed flexible joint sealants are manufactured in a variety of heights, widths, cross-sectional areas, and roll lengths, so there may be more than one way to achieve the required sealant volume. Additionally, the required volume of sealant may be achieved by using two or even three smaller strips of sealant rather than one larger strip of sealant. Consideration should also be given to the height of the sealant vs. the dry fit joint gap and the ability to compress the sealant adequately.

WHAT SHAPE OF SEALANT IS NECESSARY?

As sealant compresses, it retains its cross-sectional area. That is, as sealant compresses and gets thinner, the compressed sealant becomes wider proportionally. As a result, the cross-sectional area of the compressed sealant will be roughly equivalent to the non-compressed sealant.

As the sealant is compressed, the force required to achieve further sealant compression increases as a function of the percentage of compression achieved thus far. Time, temperature, force, and the sealant composition affect the compression rate. For instance, the pressure needed to compress the sealant the initial 10% to 50% may range from 2 psi to 4 psi. As the sealant is compressed, the sealant increases in width and the surface area to which the compression force is applied increases. Eventually, the area reaches a point of equilibrium with the designed resistance force of the sealant and compression will stop. An analogy to describe this is the use of snow shoes to distribute a person’s weight to allow them to walk across snow without sinking. The person’s weight remains the same (as does the force applied to the sealant), however the area across which the person’s weight is applied is increased by the use of snow shoes (and the sealant area to which the compression force is applied is increased as the sealant compresses), which reduces the pressure applied to the snow (and similarly reduces the pressure applied to the sealant, in psi).

WHAT IS SEALANT ADHESION AND WHY DOES IT MATTER?

How well the sealant adheres to the concrete affects the joint’s hydrostatic resistance. Water finds the path of least resistance, and if the concrete joint surface is not properly prepared or the concrete has a particularly porous surface, water can absorb into the concrete, bypass the sealing materials, and potentially compromise the system’s watertightness. This can result in wetness in or near the joint. Sealant will adhere best to a dry, clean, dense, and well-cured concrete surface that is free of dust, dirt, form release agent, and other substances that inhibit bonding. Use a ConSeal primer on the concrete surface before applying sealant to improve the sealant’s adhesion to concrete. A properly primed concrete surface will provide improved sealant and coating performance when compare to an unprimed concrete surface.