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How to Fill Expansion Joints

By Andrew Latham ; Updated September 21, 2017
Concrete slabs require well-designed expansion joints in order to avoid excessive cracking.

The purpose of an expansion joint is to allow members to expand and contract without affecting the integrity of the structure. This is especially important in structures which must absorb the dimensional changes caused by variations in heat, as in concrete slabs, or by the motion of movable components, such as a pipeline or a duct. Expansion joints are a vital design element required to manage the appearance of cracks in buildings and walkways. These must be carefully designed and constructed for them to work correctly. For instance, the maximum spacing of the expansion joints of a concrete slab must be somewhere between 24 and 36 times the thickness of the slab.

Clean the expansion gap thoroughly. Sweep the gap to remove any sand, pebbles or dirt. Clean the gap further with a cloth and detergent so as to remove any oily substances that could prevent the filler from adhering to the concrete.

Dry the expansion joint. Either wait until the gap air-dries naturally, or if time is an issue, wipe the joint with a clean rag.

Insert a backer rod into the expansion joint. A backer rod is a flexible baton made of fiberboard. This will keep the filler from seeping under the slab, which would increase the wear and tear on the joint filler.

Pour the sealing material over the backer rod. The type of filler you choose will depend on the location and purpose of the expansion joint. If the expansion join is in an area that is likely to get damp, you may require a moisture-cured caulk. Alternatively, if part of a building, your specifications may require the filler to be fire-resistant.

Smooth the surface of the joint and remove excess filler with a putty knife. Allow the filler to harden for as long as the manufacturer recommends. Clean the joint again with fresh water to remove any remaining residue.


Things You Will Need

  • Brush
  • Cloth
  • Detergent
  • Filler
  • Putty knife

About the Author


Andrew Latham has worked as a professional copywriter since 2005 and is the owner of LanguageVox, a Spanish and English language services provider. His work has been published in "Property News" and on the San Francisco Chronicle's website, SFGate. Latham holds a Bachelor of Science in English and a diploma in linguistics from Open University.