GPR is very effective in detecting and locating filler joists in concrete slabs. Unlike some other methods, GPR can differentiate between dense beam reinforcement and filler joists.

In this case, we were asked to undertake a non-intrusive filler joist location survey and mark up the position of detected filler joists in situ.

An on-site mark-up survey is often required to verify construction details or locate the filler joists so they can be avoided when planning penetrations through the floor slab.

Filler joist construction

Filler joist construction is an early form of reinforced concrete floor slab construction, most common between 1880 and 1920. The reinforcement usually comprised embedded iron or steel joists, although earlier examples can have wrought iron sections. The steel joists span in one direction and are approximately 1 m apart; the floor slab is in-filled with unreinforced concrete, often containing clinker or broken brick.

Clinker can be problematic when locating the filler joists due to its metallic content. This attenuates the GPR signal and limits penetration depth. No clinker was present on this site, and we were able to locate and mark up the position of the filler joists as requested. If clinker concrete is present, it is still sometimes possible to locate the filler joists due to the strong reflection from the joists in the GPR data. The amount of clinker and the depth of the filler joists are major factors in whether the filler joists can be located.

GPR radargram from a filler joist location survey showing detected filler joists.
GPR radargram showing filler joists detected in a concrete floor slab