As an aggregate, amphibolite grits fulfilling the requirement of frost resistant aggregates were used. Figure 1 shows the sieving curve of the aggregate mix used in concretes. Border curves (green) are recommended from Polish standard PN-B-06265 [44].
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In sum, the analysis of the reported research results indicates that LWAC may perform in a similar way to, or better than, NWAC when it is subject to water exposure under pressure, chloride attack, or freeze-thaw cycles, but under the condition that the used aggregate is characterized by a relatively low level of water absorption and the total water content in fresh concrete, resulting from both the cement paste composition and aggregate initial moisture content, it is considerably limited. Meanwhile, in practice, lightweight aggregates, especially those with high water absorption, are usually used for LWAC in a pre-saturated condition, in order to prevent workability loss and aggregate segregation in fresh concrete.
The coefficient of variation of compressive strength specified on reference specimens was the same, on average (0.05), as for standard specimens (150 150 150 mm) referred to in Table 6. Nevertheless, due to the smaller size cubes (100 100 100 mm) used for the freeze-thaw tests, the dispersion of results for particular concrete series was slightly higher. As a result, the coefficient of variation ranged from 0.03 up to 0.12, irrespective of the initial aggregate moisture. However, in the case of specimens subject to freeze-thaw cycles, the scatter of compressive strength results was significant. The coefficient of variation for concrete series that did not disintegrate before 150 cycles reached up to 0.23 and seemed to be connected to the strength loss. The concrete series revealing higher strength loss due to freeze-thaw cycles also showed a higher dispersion of results. Both of these phenomena can be explained by the more numerous microcracks occurring in these particular series.
There is also a certain impact of the applied aggregate fraction on the freeze-thaw resistance. The application of the weaker 6/12 mm fraction containing fewer crushed particles turned out to be more advantageous in terms of the LWAC freeze-thaw resistance. Therefore, initially stronger concretes made of the 4/8 mm fraction revealed slightly higher strength loss after freeze-thaw cycles in comparison to LWAC with the 6/12 mm fraction.
It should be emphasized that no direct relationship between the strength and freeze-thaw durability was observed. For example, reference specimens of concretes 1M, 2s, and 2M showed very similar compressive strength values: 49.4, 49.5, and 52.5 MPa, respectively. However, they revealed completely different strength losses after freezing and thawing cycles, i.e., 70.0%, 42.8%, and 14.5%, respectively. Similarly, there was no direct relationship, as indicated in [3], between the cement content in LWAC and its freeze-thaw resistance. For example, concrete 2M containing 386 kg/m3 of cement showed less strength loss after 150 cycles (14.5%) than concrete 1D containing 508 kg/m3 of cement (17.6%).
The analysis of the cement paste structure under larger magnification, as well as the EDS analysis of concretes with pre-saturated aggregates, showed that the visible microcracks were mainly connected to ettringite formation (Figure 12). In particular, many areas of an increased ettringite content and assisting microcracks were observed in the interfacial transition zone (ITZ). This observation is in contradiction to the models of the interfacial transition zone presented in [10], where no ITZ was assumed for saturated sintered fly ash aggregate concrete. Meanwhile, in this research, the concretes prepared with aggregates with a lower initial moisture content usually did not reveal a specific ITZ characterized by a higher porosity and a higher number of ettringite formations at all. As a result, concretes with pre-saturated aggregates, when they were subject to strength tests or freeze-thaw cycles, showed a visibly lower durability and a fracture path thorough the bonds between aggregates and cement paste.
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