**4. Discussion of the Problem**

As already emphasized in the introduction, the literature lacks a comprehensive methodology for controlling the process of making cement floors, which step by step would enforce subsequent control activities. This section only draws attention to those control activities, which, if neglected or omitted, result in serious flooring problems that significantly affect their final quality.

It should be started that the importance of control measures to be taken before flooring process is being started. These activities, included in Stage 1 of the developed methodology, are key to achieving a good final result.

So, currently the important problem is that it is allowed to make floors in buildings without built-in window and door joinery. This promotes the formation of drafts and intense sunlight drying of freshly laid cement mortar [38]. Due consideration is not given to the thermal and humidity conditions that should be provided in the facility, namely the required air and substrate temperature in the range from +10 to +25 ◦C and relative humidity in the range from 65% to 95%. Cement mortar built into the floor is not cared for by moistening it with water. These are the main reasons for the low surface compressive strength of the upper floor zone and large differences in this strength along the thickness, which is very often demonstrated on the basis of samples taken from the floors tested by the ultrasonic method. An example illustrating the above is Figure 5, which presents examples of the authors' results of the course of the longitudinal velocity of the ultrasonic wave *c*<sup>l</sup> along the thickness *h* of the cement mortar, and the course of compressive strength *f* <sup>m</sup> of the cement mortar along the thickness *h* of the cement floor determined on this basis. These tests were performed by a non-destructive ultrasonic method with exponential heads with a frequency of 40 kHz.

Figure 8b indicates the mean compressive strength *f* m1 of 20 MPa, which is required by the designer and declared by the cement mortar manufacturer, and also the average strength *f* m obtained from the tests and shown with a vertical line. Figure 8 shows that within the subsurface zone of the floor up to a depth of about 15 mm from the upper floor surface, the compressive strength *f* <sup>m</sup> was much lower than the strength *f* m1 required by the designer and declared by the manufacturer, which disqualified this floor. The occurrence of such large differences in strength along the floor thickness could not be seen in aggregate segregation, which is known and described in the literature [39–45].

An example is also Table 2, which shows how significantly the strength parameters of floors might differ, for which the control activities preceding its execution were neglected in relation to the requirements contained in Table 1.

The evenness of the upper surface of the concrete base was not checked, including the occurrence of so-called local "humps", especially interfering with the leveling of the polystyrene arrangement, which, combined with the lack of ongoing control of the thickness of the mixture being laid, resulted in obtaining a floor thickness that was not in accordance with the design. Numerous studies of the authors show that the thickness of floors is either smaller or larger in relation to the design, which is illustrated in Figure 9. Too low a thickness promotes cracking of the floor during use, whereas too large a thickness limits the floor's load-bearing capacity.


**Table 2.** Sample results of strength tests carried out for one of the cement floors tested in accordance with [46] (own study based on the data provided [25])**.**

**Figure 8.** An example of the course: (**a**) the longitudinal velocity of the ultrasonic wave *c*<sup>l</sup> along the thickness *h* and (**b**) the compressive strength *f* m of the cement mortar along thickness *h.*

**Figure 9.** The thickness of the floors in comparison with the designed thickness (based on the data presented in [25]).

The problem of controlling the evenness of the insulation foil laying on the surface of a concrete substrate or polystyrene was underestimated. As a result of the corrugation of insulation foil laid under the floor on a foamed polystyrene surface, there were irregular grooved recesses on the bottom surface of the floor. As shown in Figure 10, these grooves locally reduced the thickness of the floor by a few, or even several millimeters, which promoted cracking of the floor during use.

**Figure 10.** Example view: (**a**) the bottom surface of the floor, with grooved recesses constituting the "imprint" of the corrugated foil and (**b**) measuring the depth of the grooved cavities.

In the developed methodology for controlling the process of making cement floors, in Stage 2, very much attention was paid to checking the expansion joints. Lack of such control, or sporadic controls, contribute to the incorrect execution of expansion joints, or their lack and consequent cracking of floors (for example Figure 11). Incorrect circumferential separating of floors from load-bearing walls results in a lack of continuity of expansion joints, floors being too small in width, joints being too shallow or the filling of joints with a material other than the non-absorbent flexible foam used for this purpose, e.g., multilayer cardboard.

**Figure 11.** An exemplary view: (**a**) of a core borehole made in the floor, and also a core sample showing an expansion joint that is cut too shallow and (**b**) cracks on the surface of floors, along with an approximation of the cut core sample documenting the depth of one of the cracks.

Whereas a common disadvantage are expansion joints that are incorrectly cut, or cut at the wrong time, in cement floors. Such errors cause the dimensions of the dilated fields to be too large, the cuts to be too shallow, or the gaps to be cut too late (Figure 11). The developed methodology clearly indicates the necessary scope of control, which should exclude the problem of incorrectly made expansion joints.

In the methodology developed to control the process of making cement floors, in Stage 3, a lot of attention was paid to control strength tests after flooring. Currently, the compressive and flexural strength is being tested. At present, subsurface tensile strength, abrasion resistance and compressive strength of a cement mortar along the thickness are very rarely tested. Lack of these tests results in less durability or loosening of the applied overlay.
