Mix design practice — estimating mixing water for ordinary concrete In conventional civil engineering practice for ordinary (workable) concrete, an empirical rule of thumb is often used to estimate the quantity of mixing water as a percentage of constituent weights. Which of the following combinations best represents the approximate water required for preparing ordinary concrete?

Difficulty: Easy

Correct Answer: 5% by weight of aggregates plus 20% of weight of cement

Explanation:


Introduction / Context:
On many sites, especially for nominal mixes and ordinary workability ranges, quick empirical rules are used to plan batching water when detailed mix designs are not yet established. This question checks familiarity with one such traditional thumb rule that relates mixing water to percentages of aggregate and cement weights for ordinary concrete workability.


Given Data / Assumptions:

  • Ordinary concrete for general structural work.
  • Normal aggregate moisture corrections and absorption are handled separately.
  • No special admixtures or high-performance requirements are assumed.


Concept / Approach:
Workability is chiefly governed by water–cement ratio and paste volume. A practical on-site approximation is to take mixing water as a small percentage of the total aggregate weight plus a modest percentage of the cement weight. Among common values, about 5% of aggregate weight and about 20% of cement weight yields water quantities consistent with typical target water–cement ratios around 0.45–0.55 for many nominal mixes, before fine-tuning for slump and temperature.


Step-by-Step Solution:

Consider a representative case: aggregates ≈ 1800 kg/m³, cement ≈ 300 kg/m³.Compute water by option (a): 0.051800 + 0.20300 = 90 + 60 = 150 kg.Implied w/c ≈ 150/300 = 0.50, a common target for ordinary concrete prior to adjustments.Cross-check other options: they give unrealistically low or high water for ordinary workability.


Verification / Alternative check:
This estimate is a planning aid. Final water content must be adjusted using slump/compaction-factor targets, aggregate moisture/absorption, and environmental conditions to achieve the specified water–cement ratio and performance.


Why Other Options Are Wrong:

  • (b) 10% + 10% usually underestimates water for common slumps.
  • (c) 5% + 30% often overshoots, risking high w/c unless cement is low.
  • (d) 30% of aggregates is unrealistically large and would flood the mix.
  • (e) Not applicable because (a) is a recognized practical approximation.


Common Pitfalls:
Using thumb rules without correcting for aggregate moisture or desired slump; forgetting that the governing parameter is the water–cement ratio, not absolute water alone.


Final Answer:
5% by weight of aggregates plus 20% of weight of cement

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