PSI - Issue 74

Lucyna Domagała et al. / Procedia Structural Integrity 74 (2025) 17 – 24 Lucyna Domagała / Structural Integrity Procedia 00 (2025) 000–000

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3. Results and discussion The mean tests results of fine aggregate concretes are presented in Table 3. The use of different types of lightweight aggregates in concrete had a visible impact on the differentiation of the properties of the composites.

Table 3. Properties of fine aggregate concretes at 28 days

Concrete designation

Density in natural condition, kg/m 3

Oven dry density, kg/m 3

Moisture content, %

Water absorption, %

Compressive strength, MPa

Flexural strength, MPa

Thermal conductivity coefficient in natural condition, W/(m K)

Thermal conductivity coefficient in oven-dry condition, W/(m K)

S P

600 630 570 750

530 560 520 690

13.0 12.2 10.0

35.8

2.9 7.3 3.3 3.1 7.2

0.9 2.0 0.8 0.9 1.6 3.8

0.10 0.09 0.12 0.16 0.20

0.08 0.07 0.10 0.13 0.17

105.4

G C V R

59.6 33.3 31.8

9.0 9.6 4.3

1170 2070

1070 1980

9.6

22.6

-

-

Their oven dry density ranged from 520 kg/m 3 for concrete with expanded glass (C) to 1070 kg/m 3 for concrete with sintered fly ash (V). Compared to the reference concrete with normal-weight aggregate, the lightweight concretes showed a lower density by 46 to 74%. While the density of the composite itself can be an indicator of its porosity, it does not provide information on the porosity structure, which is so important due to both the physical and mechanical properties of the material. A more reliable indicator of the porosity structure of concrete is its water absorption. Due to significant differences in the porosity structure of the lightweight aggregates used, there is no direct relationship between the density and water absorption of the insulating and structural concretes made. Their water absorption ranged from 31.8% to 105.4%. These values were therefore approximately 3 ÷ 11 times higher in relation to the water absorption shown by the reference concrete. The highest water absorption result obtained for the composite containing perlite aggregate draws particular attention. Due to the very high water absorption of perlite itself (WA 30 min = 130%), incomparably higher than the water absorption of other lightweight aggregates, the water absorption of concrete with this aggregate was also significantly higher in relation to other concretes. Due to the swelling properties of perlite aggregate, its use for insulating and structural concretes operating in conditions of increased humidity should be considered inappropriate. Lightweight concretes with polystyrene granulate, expanded clay and sintered fly ash, despite the varied water absorption of these aggregates (WA 30 min = 0 ÷ 17%), showed comparable, lowest water absorption values. The relatively high water absorption of composites with expanded glass is probably related to the poor adhesion of the cement paste to the smooth surface of this aggregate and its different structure, which makes it impossible to seal its coating with the paste. The compressive strength of lightweight concretes ranged from 2.9 MPa for concrete with polystyrene grains (S) to 7.3 MPa for concrete with perlite aggregate (P). Compared to the reference concrete with normal-weight aggregate, lightweight concretes showed lower compressive strength by 68 to 87%. However, there is no direct relationship between the density of lightweight concretes and their strength. For example, concrete with perlite aggregate of almost twice lower density than concrete with sintered fly ash showed similar compressive strength. The cause of such a state may be explained by the analysis of fractures of individual concrete specimens, presented in Fig. 3. Comparison of the images of fractures of lightweight concretes indicates that composites with polystyrene and expanded glass aggregates showed a lack of proper adhesion of the paste to smooth granulate grains. Meanwhile, concretes with perlite aggregate, expanded clay and sintered fly ash showed very good bonding. As a result, they were able to transfer higher stress in the compression test, and when they were demaged, the aggregate grains did not separate from the paste, but broke.