## 4 Data Analysis and Results
### Validity and Applicability of the BTS Classification for Evaluating the Stone Durability  
The intensity of the BTS changes after the deterioration process is among the most common criteria for evaluating the durability of a stone. In some previous studies, researchers have used the rate of loss (RL) to investigate the intensity of BTS changes [18; 103; 104]. The RL is calculated through Equation (6):  
\[\text{RL}\;(\%)\;=\;\frac{\text{BTS}_{\text{Initial}}-\text{BTS}_{\text{ Deteriorated}}}{\text{BTS}_{\text{Initial}}}\times 100 \tag{6}\]  
where BTS\({}_{\text{Initial}}\) and BTS\({}_{\text{Deteriorated}}\) are Brazilian tensile strength of the stone sample in initial conditions (i.e., before deterioration process) and after the end of the deterioration process, respectively. Data published in the previous studies was used for the validity and applicability of the BTS classification for evaluating the durability of stones. Given that the focus of the present study is on the building stones with igneous origin, data related to this class of stones were collected from the previous studies. Based on a literature review, the BTS data of igneous building stones exposed to deterioration processes reported by [PERSON] et al. [5], [PERSON] et al. [12], [PERSON] et al. [13], and [PERSON] et al. [14] were collected. The information extracted from the studies of these researchers are presented in Table 6.  
Figure 6: Schematic illustration of tensile stress induced by ice crystallization during freezing–thawing in pore space of the porous stone (after [PERSON] et al. [102]). Using values of BTS in initial conditions and after the deterioration process, the RL of BTS for stones investigated in the previous studies were calculated using Equation (6). It can be seen from Table 6 that the range of RL values varies from 3.3 to 43.2%. The difference in RL can be attributed to difference in stone type in terms of mineralogical composition and texture, physico-mechanical characteristics of the stone (e.g., porosity, water absorption, BTS), type and cycle number of deterioration process, status of the test specimen (i.e., dry or saturation), etc. In order to investigate the validity and applicability of the BTS classification in evaluating the durability of stones against deterioration process, data of BTS in initial conditions and RL were plotted on the scheme of BTS classification. It can be seen from Figure 7 that there is an acceptable trend between the values of the BTS in initial conditions and RL. A stone with higher BTS in initial conditions experienced a lower RL due to the adverse effects of the deterioration process. In addition, based on the BTS value in initial conditions, a stone can fall into the different BTS classes. According to Figure 7, a stone with a better BTS class showed a lower RL value after the deterioration process. In other words, a shift from BTS class I to V will result in a more durable stone against deterioration processes including freezing-thawing, salt crystallization, heating-cooling, and wetting-drying. Thus, BTS classification can be a quick and low-cost indicator for a preliminary evaluation of durability of igneous building stone before its selection for use in a building. This can lead to the selection of the most durable stone for use in a building exposed to harsh climatic conditions with the possibility of deterioration processes. As a result, the stone can be more resistant to deterioration processes during its service life in a building. Analysis of the data of previous studies on the changes in the BTS of igneous stones during deterioration processes using the BTS classification revealed the validity and applicability of the BTS classification for evaluating the stone's durability. However, it should be noted that the BTS classification is only applicable to stones similar to the igneous stone types analyzed in the present study (Table 3) with the UCS and BTS values in the range of 1.18-375.20 and 0.00-30.30 MPa, respectively (Table 2).