Issue 59

O. Rahim et alii, Frattura ed Integrità Strutturale, 59 (2022) 344-358; DOI: 10.3221/IGF-ESIS.59.23

granular slag is slow in the hydration process preventing the formation of hydration products within 28 days that fill the voids. The results obtained are considered suitable compared to the results obtained on high ultra-performance concretes [41] and high performance concretes [39].

HPC control

HPC 10% Slag HPC 20% Slag HPC 30% Slag HPC 50% Slag

12

10,64

10,237

9,335

10

8,456

8,121

8

6

4

2

0

Average tensile strength by bending (MPa)

Concretes

Figure 11: Tensile strength by bending of HPCs at 28 days.

Compressive strength (at 2, 7, 14, 28, and 360 days) The mechanical resistance in the compression test is the main benchmark demonstrating the quality of concrete. The evolution of the compressive strength for different mix designs of HPCs is shown in Fig. 12.

HPC control

HPC 10% Slag HPC 20% Slag HPC 30% Slag HPC 50% Slag

0 10 20 30 40 50 60 70 80 90 100

Average compressive strength (MPa)

2

7

14

28

360

Time (Days)

Figure 12: Compressive strength of HPCs at 2, 7, 14, 28, and 360 days.

In the short term and up to 28 days, the compressive strength decrease when the substitution rate increases. A non significant decrease is relatively for HPCs with 10%, 20%, and 30% of ground granulated blast furnace slag and significant for HPC with 50% as a partial replacement for cement. This is mainly due to the nature of the ground granulated blast furnace slag from the steel plant of El-Hadjar (Annaba, Algeria) which presents in the short term a relatively slow hydraulic reactivity but sufficient to ensure a compressive strength at 28 days greater than 60 MPa required for having an HPC conforming to the standard as it is observed by several researches [39–41]. In this phase, the Blaine fineness of the slag which is higher than that of the cement fills the tiny voids and accentuates the densification. Furthermore, a remarkable increase in compressive strengths is observed for HPC with the addition of slag in partial substitution of the cement after 360 days of curing, which is greater than those of HPC without the addition of ground granulated blast furnace slag. This is due to the chemical reactions between the portlandite and the silica of the slag with the formation of an additional calcium

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