Concrete – Water Quality Acceptance Criterion for Mix Water (IS practice) Compare 28-day cube strengths made with available site water vs distilled water; the site-water concrete should achieve at least this percentage of the distilled-water strength to be acceptable.

Introduction / Context:
Quality of mixing water directly affects setting, strength, and durability of concrete. Indian practice allows the use of non-distilled (available) water if it does not reduce compressive strength of standard cubes beyond a small tolerance when compared with cubes made using distilled water. This question tests the acceptance threshold used in field quality control.

Given Data / Assumptions:

• Two sets of at least three 15 cm cubes are prepared identically except for water type.
• One set uses available site water; the other uses distilled water.
• Cubes are cured and tested at 28 days under standard conditions.
• Decision is based on average strengths of the two sets.

Concept / Approach:

The acceptance criterion compares average 28-day strengths. If the cubes made with available water achieve a sufficiently high percentage of the strength of cubes made with distilled water, the water is deemed acceptable for mixing concrete. The commonly adopted benchmark is 90% or more, reflecting a small allowable difference due to impurities that do not materially impair hydration or long-term performance.

Step-by-Step Solution:

Verification / Alternative check:

Field experience shows potable water and many safe non-potable sources typically meet or exceed 90% of the distilled-water strength; markedly lower values signal harmful contaminants (e.g., excessive organics, sugars, acids, or salts) that retard hydration or create porosity.

Why Other Options Are Wrong:

70% and 80% are too lenient and could allow strength loss with serious durability risks. 95% is overly strict and may reject otherwise acceptable water without practical benefit.

Common Pitfalls:

Not controlling other variables (w/c ratio, cement, curing), using too few specimens, or comparing individual cubes rather than averages.

Final Answer:

90%