## 3 Results and Discussion
### Hardened Properties
#### 3.2.3 Freeze-Thaw Resistance  
According to the frost-resistance criteria of the Polish standard PN-B-06250:1988 [26], for concrete to be rated as \"frost resistant\", the weight variation (AG) must not exceed 5%, the loss of compressive strength (AR) must not be higher than 20%, and cracks must not occur during the test. The results of the durability tests (decrease in strength) of the mixes, including their resistance to freeze-thaw cycles for frost resistance of F25 and F100, are shown in the Figure 6. The dashed line indicates an average decrease of compressive strength of 20%. The decrease of strength after 25 and 100 freeze-thaw cycles is represented by plain and crosshatched bars, respectively.  
Figure 5: Water absorption of the tested mixes showing standard deviation.  
Figure 6: Decrease in compressive strength of mixes subjected to either 25 or 100 freeze-thaw cycles.  
An analysis of the results of the frost-resistance test revealed a big difference between the behaviour of concrete with fly ash and with glass powder when subjected to subzero temperatures. The effect of waste-glass powder on the durability of concrete after several freeze-thaw cycles was clearly noticeable. Based on the analysis of test results represented in Figure 6, it can be stated that only mixes with the addition of glass powder satisfied the requirements of either the F25 or F100 class, as per PN-B-06250:1988 [26]. The compressive strength of concrete with the addition of glass powder decreased by 1% to 11.4% on average, depending on the number of freeze-thaw cycles, which was far below the allowed limit of 20%. The weight change did not exceed 0.1% (weight loss after 25 freeze-thaw cycles) and 0.2% (weight increase after 100 freeze-thaw cycles), which was less than the allowed limit of \(\Delta\)G = 5%. Figure 6 shows that the average decrease in compressive strength after 28 days of curing (C_GP_45.0) was smaller than the average decrease in compressive strength after 7 days of curing. In the authors' opinion, considering the uncertainty of the measurements (expanded uncertainty of 3%), this difference was not significant. The results for mixes containing fly ash admixture turned out to be diametrically different. Upon completion of freeze-thaw cycling, significant damage to the surface and edges of the samples was observed. One of every six specimens of fly-ash concrete mixes (C_FA_22.5) was destroyed in the tests (Figure 7a). An average decrease of compressive strength for these concretes exceeded the 20% value allowed by the norm [26] and, depending on the number of cycles applied, fell within a range between 23% and 59%. The weight change did not exceed 0.6% (weight increase after 25 freeze-thaw cycles) and 1.5% (weight increase after 100 freeze-thaw cycles). Therefore, the tested concretes with the addition of fly ash should be considered nonresistant to frost action.  
The greatest reduction in compressive strength (of 28%) was observed for the mix containing 22.5% fly ash. In turn, the concrete containing 22.5% glass powder showed the smallest (1%) change in compressive strength. Furthermore, compressive strength changed more significantly in the case of the concrete mixes subjected to a greater number of freeze-thaw cycles (100). Concrete with 45% fly ash was characterized by the greatest reduction of compressive strength (by 43%) and concrete with 45% of glass powder showed the smallest reduction in compressive strength (of 5%).  
Figure 7: Specimens of concrete: C_FA_22.5 (**a**) and C_GP_22.5 (**b**) after 100 freeze-thaw cycles.  
The results were subjected to a statistical analysis in the Statistica software [28]. Strength calculations showed that the difference between the average compressive strengths (also referred to as the average decrease of compressive strength) between fly ash concrete and concrete containing glass powder was significant only in the case of the decrease of compressive strength in the freeze-thaw resistance test.