**3. Laboratory Tests**

#### *3.1. Materials*

The tested samples and elements presented in this research study were built using clay brick, class 25. The dimensions of the bricks were 25 × <sup>12</sup> × 6.5 cm3 (the same as in the real-life stair flight construction). For the preparation of joints, lime–cement mortar, class M5, was used. For the preparation of masonry mortar, a factory-made, dry, lime–cement mortar class M5 mixture was used (Quick-mix TWM-M5; Sievert, Strzelin, Poland). The mixture consisted of a cement binder, slaked lime, quartz fillers and refining additives. The thickness of the joint was about 1 cm. To determine the properties of the materials used, initial tests in accordance with current standards [108–111] were conducted. The results of the tests are presented in Table 2.

### *3.2. Laboratory Models of Stair Flights*

Laboratory tests were conducted on two models of the stair flights—without stair treads (M1) and with stair treads (M2). The models were made in a 1:1 scale based on the measurements of the analyzed construction, with a width of a single 25 cm brick (Figure 9).

**Figure 9.** Laboratory models of stair flights (dimensions in mm): model M1—without treads (**a**); model M2—with treads (**b**). P1–P3—force application points; R—force from the arch registered with a force gauge; u—vertical displacement in the middle of the arch span.


**Table 2.** Properties of used materials.

The dimensions of the flight were taken from the real-life structure and were noted as follows:


The stair flight construction was supported on a self-made test bench frame. During the masonry works, a wooden structure with expanded polystyrene was used as a centering for the arch. The support was removed while the arch structure was drying, so that the structure could be pressed down naturally. Due to indoor conditions, the bricks were soaked in water before construction, then wetted with water daily during the mortar curing processes. In order to take measurements, a force gauge was placed in the upper corner of the flight to transfer the vertical force.

Measurements were conducted with inductive sensors (for vertical displacements) and two force gauges connected to an MGC Plus HBM Hottinger Bridge. In addition, the bridge was connected to the ARAMIS optical three-dimensional displacement and strain measurement system. The model was painted on the back side with a white–black pattern to enable image correlation. First, the model was tested in the elastic range with a concentrated force applied at three different locations, P1, P2 and P3. At the force application locations, horizontal surfaces were prepared with quick-setting mortar with a strength, after 24 h, of at least 25 MPa. The assumed limit loads at points P1 and P3 were up to 1.5 kN, while the limit for point P2 was noted when the first crack was registered, after which the test was stopped. The loads at P1, P2 and P3 were applied separately to the tested specimen. Then, on the basis of the M1 model, the M2 model was created by adding treads over the staircase (Figure 3). In order to create the geometry of the steps, it was necessary to precisely cut the bricks at the angles correlating with the arch.

The model with treads (M2) was tested similarly to the model without treads (M1). Points P1, P2 and P3 were determined at the same locations as in the case of the M1 model. Assumed force limits for points P1 and P3 were up to 6 kN. At the middle point, P2, the structure was loaded up to failure. The loads were applied separately to the specimen.
