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CONCRETE TECHNOLOGYwww.cpi-worldwide.com CPI %u2013 Concrete Plant International %u2013 6 | 2025 29as per the guidelines [13, 14]. The permeability coefficients for each mix were calculated using Darcy%u2019s law, and the OPI values were computed from the negative logarithms of the average of the Darcy coefficients (OPI = -log K).At 28 days , the TPT was conducted on the concrete panels on site according to [5] and [15]. Three determinations were taken on the panel faces exposed to the sun. The tests were taken at the same points that would be drilled for the OPI core samples. At 56 days, another three determinations were taken on the concrete panels to observe the effect of age on the air permeability. Between 28 and 56 days the panels were kept in a semi-controlled environment in the laboratory without any exposure to direct sunlight (average temperature of 22%u00b0C and 60% relative humidity).Results and discussionOxygen Permeability Index testFigure 4 shows the OPI results at 28 and 56 days. Note, the higher the OPI values the superior the durability performance. The OPI results ranged between 9.89 and 10.35, with values higher than the minimum required OPI specification (9.65 %u2013 for moderate to severe carbonation environments) by the South African National Roads Agency (SANRAL) [18]. Table 1: Summary of mix compositions.Description Mix compositionMix 1 Mix 2 Mix 3 Mix 4 Mix 5Concrete grade (MPa)40 40 30 30 30CEM I (kg/m%u00b3) 355 0 285 0 210CEM II/A-L (kg/m%u00b3) 0 355 0 285 0Fly ash (kg/m%u00b3) 0 0 0 0 90Coarse aggregate (19 mm quartzitic sandstone) (kg/m%u00b3)1100 1100 1080 1080 1080Dune sand (kg/m%u00b3) 515 515 570 570 615Crusher sand (kg/m%u00b3)220 220 245 245 215Superplasticizer (% binder)0,60 0,60 0,65 0,65 0,60Water (l/m%u00b3) 175 175 175 175 175w/b 0,50 0,50 0,60 0,60 0,60