*3.1. Preparation*

The Faculty of Civil Engineering of the University of Technology in Brno houses a research centre equipped with a vacuum chamber with the dimensions of 6.18 m × 4.18 m × 0.44 m. It is connected to an Edwards GXS250/2600 dry screw pump (Edwards Group Ltd., Stockholm, Sweden) with a Roots booster mechanism. The pump has enough suction performance to create a vacuum of 700 mbar in the chamber. An advantage of applying a load using a vacuum is that the load is uniformly applied on the entire surface of the specimen. Depending on the performance of the pump, the system can deliver different levels of loading up to 100 kN/m<sup>2</sup> [25]. The vacuum was created underneath the slab, causing it to deflect downwards and thus introduce strain to bonding systems and tiles.

The floor area of the vacuum chamber was a factor that limited the size of the test specimen. In order to preserve the airtight seal of the chamber (the walls and floor were connected and sealed together), it was not possible to safely remove the formwork of the specimen if it were to be cast inside the chamber. It was thus necessary to design a monolithic slab with lost formwork consisting of 5 reinforced pre-slabs of 1.2 m × 4.05 m × 0.05 m with additional reinforcement and additional concrete layer of 0.15 m on top. The inner surface of the pre-slabs was fitted with 3 strain gauges (Figure 1)—each slab bore 1 gauge (the leftmost and rightmost slabs were not fitted). The additional top layer was made with C 20/25 XC1 concrete with B500B reinforcement, with a concrete cover of 30 mm.

**Figure 1.** Fitting a strain gauge onto the surface of the lost formwork (reinforced concrete pre-slabs).

Figure 2 shows the structure of the reinforcement. Until they became fully integral to the slab, the pre-slabs were temporarily supported for additional stability. The supports consisted of two massive frames from which descended suspending rods. The rods were cut once the concrete had hardened. The result was a concrete member that behaved like a monolithic slab. The test chamber has a floor area 60–80 mm smaller than the dimensions of the slab. The gaps were sealed with extruded polystyrene.

**Figure 2.** Structure and distribution of the reinforcement.

After the concrete slab had hardened (i.e., when it reached the age of 28 days), its top surface was ground down by 3–5 mm to create a smooth and level surface (the maximum deviation per 1 m was 1 mm). After that, the surface was cleaned using a powerful industrial-grade vacuum cleaner. Besides these, the surface received no other treatment before the installation of the large-format tiles. The test slab, which had the nominal dimensions of 6 m × 4 m (6 m × 4.05 m to be precise), was divided into five symmetrical test fields. Each field consisted of a bed of adhesive and a large-format tile of 3 m × 1 m with cutouts (see Figure 3). One of the tiles was used in another experiment, which is not part of this paper. This is why this article only discusses measurements performed on four of the tiles designated A through D. The first adhesive, identified as type I, is a highly deformable cement adhesive with an extended open time (class C2ES2 according to EN 12004-1 [26]), with standard setting and a large portion of lightweight filler. This adhesive is heavily modified by powdered polymer binder at an amount of 12 wt.% of the dry mix. The second adhesive, identified as type II, is a deformable cementitious adhesive with extended open time and reduced slip (class C2TES1 according to [26]). It is a normally-setting adhesive without any lightweight filler, modified by a powdered polymer binder at 3.5 wt.% of the dry mix. In the case of tile A, the composite included an additional layer of adhesive and a separating membrane from a synthetic non-woven fabric. Table 1 shows the composition of the material system in each test field. The tiles were installed when the slab reached 35 days of age.

**Figure 3.** Tile dimensions (1) including the positions of the cutouts, strain gauges (2), acoustic emission (AE) sensors (3) and measurement lines for the ultrasonic pulse velocity (UPV) test (4) with the position of the transmitter (T) and, for illustration, receiver positions indicated at one of the lines (R).


**Table 1.** An overview and composition of the test fields.

The diagram (Figure 3) shows the precise size and position of the cutouts. The cutouts serve to concentrate stress; i.e., to act as a possible source of failure. Figure 4 shows a diagram of the position of the tiles on the slab as well as the mounting of the slab on the vacuum chamber.

**Figure 4.** Diagram showing the mounting of the substrate slab with the tiles on the vacuum chamber (**a**) top view, (**b**) cross-section.
