What proportion of strength must questionable water samples achieve compared to potable water samples to indicate compliance?

To indicate compliance, questionable water samples must achieve a strength proportion of at least 90% when compared to potable water samples. This benchmark is critical in ensuring that the water quality is adequate for use in concrete mixing, as the effectiveness of the water significantly impacts the overall strength and durability of the concrete.

When mixing concrete, the quality of water used is essential because impurities or excessive contaminants can affect hydration and the bonding of the cement particles. By setting the requirement at 90%, regulatory standards ensure a sufficient margin for safety and performance, allowing for some variability in water quality while still maintaining structural integrity. This standard helps to uphold industry practices that prioritize the longevity and safety of concrete structures.

The other options represent lower thresholds that do not meet the necessary compliance level for ensuring that questionable water samples are reliable substitutes for potable water, potentially leading to compromised concrete performance.