What is Concrete Undersealing?

S. Mithra

When concrete on streets, parking lots, or warehouse floors begins to degrade, there are a few methods of repair available. Concrete undersealing is a process by which hot asphalt is pumped underneath the concrete slab so it flows into patches that were hollow and eroded. After concrete undersealing, the concrete is more stable and can be overlaid with a thin layer of asphalt to repair cracks, saving the expense and time of replacing the entire surface.

Undersealing can help prevent cracks in concrete.
Undersealing can help prevent cracks in concrete.

Large slabs of concrete are under constant duress from heavy loads, precipitation, ultraviolet rays from the sun, and the stress of expanding and contracting with hot and freezing weather. Aging concrete develops sinkholes, shows soil seepage at joints, cracks into fissures, or slides from its location. These are indications that the ground underneath the floor or road has eroded. One can rip out the concrete and replace it or make minor surface repairs in the hope that the problems are not major. Concrete undersealing provides a better option.

A man smoothing out a concrete floor.
A man smoothing out a concrete floor.

Concrete undersealing has many advantages in saving an existing slab of concrete. It is much less expensive than replacing a stretch of road or span of ground. It also doesn't disturb the existing street for weeks, thus allowing continued traffic and access to businesses. Also, if an overlay has been planned, an initial treatment of concrete undersealing ensures that the overlay doesn't exacerbate the problem by adding weight to an already unstable structure.

The underseal itself is different from the asphalt used on the top surface of roads. It is a mixture of cement, ash, and water, similar to grout, that is more liquid than ordinary asphalt. Underseal only melts at a very high temperature. It is applied around 400° F (200° C) so that it flows into every nook and cranny beneath the road. It's also engineered with a high melting point so it will not ooze or liquefy even at the most brutal desert temperatures. Concrete undersealing will only fill in crevices around 3-6" (7.6-15.2 cm) large. Anything more cavernous may not be appropriate for concrete undersealing.

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