To properly understand the best methods of sealing the joints of concrete structures, some of the myths and misconceptions about sealants must be discussed. Laboratory testing and engineering analysis have led to some “rules of thumb” when selecting and applying preformed flexible joint sealants in a compression joint – in this case for a box culvert.

HOW MUCH SEALANT IS NEEDED?

The answer is as varied as the types and sizes of precast 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. The joint tightness must be such thatthe sealant is compressed a minimum of 50% of its original height. However, it is preferred that the joint be as tight as possible.

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 several 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 pressure 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. However, it may require about 12 psi of applied pressure to achieve 85% compression. As the sealant is compressed, it increases in width, thus increasing the area to which the compression force is applied. Eventually, the area reaches a point of balanced pressures with the designed resistance pressure of the sealant and compression will stop. An analogy to describe this is the use of snowshoes 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).

WHY IS SURFACE PREPARATION IMPORTANT?

How well the concrete joint surface is prepared 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 is of poor quality (i.e. porous concrete, excessive bugholes, uneven surfaces, damaged concrete, contaminants, etc.), water can bypass the sealant and potentially compromise the system’s watertightness. This can result in leaks or wetness in or near the joint.

Preformed flexible joint sealants will perform best when applied to a dry, clean, sound, uniform, and well-cured concrete surface that is free of dust, dirt, form release agent, or anything else that may inhibit sealant adhesion or joining the concrete sections together. Hydrostatic resistance of the joint can be improved through surface preparation such as patching the concrete, filling bugholes, cleaning and drying the concrete, priming the concrete, coating the concrete, or using concrete permeability reducing products.

WHERE SHOULD THE SEALANT BE PLACED?

Sealant placement is critical for any application. The best practice is to place sealant where the concrete touches tightest. It is also preferred to fill the annular space as much as possible. The sealant should compress and fill the joint such that the interface at the wall where fluid is anticipated is filled. This may be on the inside of the structure, on the outside, or both. If the seal should stop infiltration and exfiltration, two rows of sealant should be used. Perform a sealant fitment test prior to final structure installation to ensure optimal sealant size and placement. Refer to the sealant technical literature for additional placement details.

Consult with the Concrete Sealants engineering team for specific guidance.