*2.1. Object of Investigations*

Integrated GPR and ultrasonic inspection were conducted in St. Nicholas' Church in Gda ´nsk, Poland (Google Maps coordinates in a WGS84 system: 54.352206 N, 18.651510 E). This historical object from the 14th century is the only church in the city, which survived World War II without destruction. However, due to the long process of the settlement of the pillars, some damages appeared and progressed in the structural elements. Recently, the process of damage has accelerated, resulting in the serious cracking of vaults, displacement of arches and deformation of the floor. The poor condition of the church led to its closure in November 2018 and undertaking of repair works.

The conducted floor inspection reported in this paper was the first stage of investigations directed to assess the technical state of the church. NDT tests were performed on the floor of two aisles, south and north, as shown in Figure 1. In both aisles, the top floor layer was made of stone tiles measuring approximately 43 cm × 43 cm. In the central part of each aisle, tombstones were laid. The cross-section layers of the floor were identified in a few trial pits. One of the pits located in the north aisle is shown in Figure 2. Based on a visual inspection of the cross-section visible in the trial pit, particular layers in the floor were identified (Figure 3). It was found that the stone tiles laid on the cement mortar layer had a thickness of approximately 2–6 cm. Below was a layer of sand and ground backfill. It was also identified that at some edges, an air gap with a thickness of approximately 1–2 cm was visible below the cement mortar.

**Figure 1.** Photograph of the floor in St. Nicholas' Church in Gda ´nsk, Poland (Google Maps coordinates in a WGS84 system: 54.352206 N, 18.651510 E): (**a**) south aisle; (**b**) north aisle.

**Figure 2.** Photographs of the trial pit in the floor: (**a**) edge 1; (**b**) edge 2; (**c**) edge 3; (**d**) edge 4.

**Figure 3.** Schematic sketch of floor layers in the trial pit: (**a**) appropriate cross-section (without air gap); (**b**) cross-section with damage (air gap).

Additional investigations were conducted at the laboratory, on two circular concrete slabs. The slabs had a diameter of 49 cm and a thickness of 10 cm. One of the slabs had a table tennis ball with a diameter of 40 mm embedded in its center, at the half height of the slab. During the tests, the slabs were stacked on top of each other and gradually moved apart to simulate air gaps of varying thickness: 1, 4, 20 and 50 mm (see Figure 4).

**Figure 4.** Circular concrete slabs with air gaps of different thickness: (**a**) 1 mm; (**b**) 4 mm; (**c**) 20 mm; (**d**) 50 mm; and (**e**) upper plate after cutting.
