## 3 Results and Discussion
### Scaling under Freezing and Thawing
#### 3.3.3 Influence of the Efficiency Factor  
The results from the salt frost scaling test presented in Figure 18 show that Mix 6, produced with an efficiency factor of 1.0, presents a better performance against salt-frost scaling than Mix 3, with the \(k\)-factor of 0.6 recommended by SS 13 70 03 [5]. In fact, while specimens of Mix 3 present an accumulated scaled material higher than the acceptable limit of 1.0 kg/m\({}^{2}\) at the end of the test, the mean value of the scaled material in the specimens of Mix 6 is under 0.5 kg/m\({}^{2}\) after 112 freeze/thaw cycles, which reveals, therefore, good frost resistance. Theoretically, it was expected that Mix 6 would display a lower salt scaling resistance, since an increase in the water/cement ratio usually results in a more porous concrete, thus more able to absorb water, resulting in a higher degree of saturation. Consequently, the hydraulic and/or osmotic pressures inside the paste will increase, resulting in a higher chance of damage due to freeze/thaw cycles [4]. In this case, the pore structure of each of the concrete qualities may have played a more important role in the salt-frost resistance of the mixes than the water/binder ratios and simple porosity of the concrete mixes. The air void parameters shall, therefore, be analysed. The air content of Mix 6 measured according to SS-EN 12350-7 [8] was the targeted 4.5%. The results of the Air Void Analyser, however, reveal a slightly higher air content for Mix 6 (with 6% for batch number 1) than for Mix 3 (6.2% and 5.5% for batches 1 and 2, respectively). The specific surface measured for Mix 6 was 23.7 mm\({}^{-1}\), which is higher than the 19.4 and 20.2 mm\({}^{-1}\) obtained for Mix 3, and closer to the 25 mm\({}^{-1}\) recommended by the AVA manufacturer. Moreover, the spacing factor obtained for Mix 6 was 0.18 mm, which within the limits recommended by the literature for satisfactory salt-frost resistance of a concrete quality (\(<\)0.20 mm), unlike the spacing factor determined for Mix 3, of 0.22 mm. Considering the results obtained by the AVA for Mixes 3 and 6, it seems that a slight increase in the air content (from 5.85% to 6%) may result in relevant differences regarding the air void structure of concrete, namely an increase in the specific surface and a decrease of the spacing factor. Another possible reason why Mix 6 revealed a better resistance to frost scaling is that Mix 6 (with an effective water-binder  
Figure 18: Mean values of the accumulated mass of scaled material per area of freezing surface after 28, 56 and 112 cycles for concrete with 50% GGBS and different efficiency factors. ratio of 0.45) contains more capillary pores with larger sizes than Mix 3 (with an effective water-binder ratio of 0.39). These large capillary pores allow an easy transport of the unfrozen water to the empty air pores. Further investigation should be carried out to determine the capillary suction of the concrete mixes. This would allow assessing whether the worse results of Mix 3, when compared to Mix 6, were actually related to a significant difference in the capillary pore systems of the hardened concrete with GGBS, which may need even smaller spacing factor for adequate transferring the unfrozen water to the air pores. Nevertheless, the results show that it is possible to produce salt-frost resistant concrete with 50% of slag replacement using efficiency factors larger than the recommended by the standards, at least as high as 1.0. According to these results, it also seems that ensuring an adequate air pore system affects more significantly the performance of concrete under freeze/thaw cycles than the water/binder ratio and the compressive strength of concrete. This fact raises, once again, the question of the adequacy of the use of efficiency factors. In fact, the \(k\)-factors recommended in the literature and standards were determined solely based on the compressive strength development of a concrete mix with additions, when compared with ordinary Portland cement concrete, by adjusting the water/binder ratio.