Issue 61

F. A. H. Saleh et alii, Frattura ed Integrità Strutturale, 61(2022) 89-107; DOI: 10.3221/IGF-ESIS.61.06

A more significant decrease in this strength was recorded for SCSC SR compared to SCSC PR with the same content rubber. Drops in strengths of 0.1, 11.2, 18.6 and 36.11%, respectively, were recorded for SCSC GR for the same substitution rates rubber, calculated with respect the strengths of the SCSC reference concrete. The changes in flexural strength values obtained in the present experimental work converged with those values reported by Si et al. [40]. The latter found that the modification of concrete compositions by substitution of rubber played a key role in the reduction of the three-point bending tensile strength. In turn, Albano et al. [41] noticed that coarse rubber aggregates showed a negative effect on flexural strength compared to compressive strength. The powder rubber developed lower strength drops than coarse rubber aggregates [38]. In the cases studied herein, and for low rubber content, the flexural strengths were identical. However, for high substitution rates of 20%, a difference ( ∆ f tk ) was clearly marked between the tensile strengths of the two (SCSC GR and SCSC PR) concretes and amounted to 1.6 MPa. The strength loss was compensated between SCSC PR and SCSC SR. This was illustrated by a strength gain of almost 2 MPa. All flexural strength values were higher than VC and lower than SCSC. The improvement of the flexural strength of the SCSC GR concretes was mainly due to the sharp irregular form of the grains, which fitted perfectly in the cement matrix. This feature can expand without tearing off in the case of the application of tensile stresses, as it was already confirmed by Segre et al. [42] in their study on the influence of crack propagation and failure in rubber-based concrete. Hardened state: Compressive strength The results obtained from the compressive strength tests after 7, 28 and 365 days are illustrated in Fig. 8. Generally, all the compressive strengths of the SCSCs belonged to the same strength class. At 7 days, the strengths varied between 30 MPa and 40 MPa and at 28 days, between 35 MPa and 50 MPa. However, they increased from 45 MPa to 65 MPa, after one year of water curing. All the compressive strengths values were always lower than those of SCSC and higher than VC ones. The values of the ratios between the compressive strength ( f c ) of each concrete at time ( t ) expressed in days and those of the concrete at 28 days ( f ck ) are shown in Fig. 9 for the different rubber contents. At the age of 7 days, the results showed a reduction in compressive strength with increasing rubber content in the case of powdered concrete and sand. However, in the case of gravel rubber concrete, this ratio remained constant.

Relative of fc/fck at 7 days Relative of fc/fck at 365 days

Concretes references

SCSC PR

SCSC SR

SCSC GR

1,4

1,3

1,2

1,1

0,8 Relative strength f c / f ck 0,9 1,0

Strength at 28 days

0,7

SCSC PR20

SCSC PR15

SCSC PR10

SCSC

SCSC SR5

SCSC SR10

SCSC SR15

SCSC

VC SCSC SCSC PR5

SCSC GR10

SCSC GR5

SCSC GR15

SR20

GR20

Concretes compositions

Figure 9: Relatively compressive strengths fc/fck at 7 and 365 days of concrete compositions - VC, SCSC, SCSC GR, SCSC PR and SCSC SR. For the one-year age, the results showed a clear increase in relative strengths vs. rubber contents introduced into the concrete. However, it should be noted that the reduction in compressive strength of RSCSC was only marked in the case of rubber powder (RP) and sand rubber (SR). However, this trend was reversed in the case of gravel rubber (GR) which confirmed the role and effect of the rubber grain size in the elaborated concretes. It seems that gravel rubber was suitable

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