A concrete mix design calls for a 28-day compressive strength of 4,000 psi. What is the typical water-cement ratio for this strength?
Correct Answer
C) 0.45
For 4,000 psi concrete, a water-cement ratio of approximately 0.45 is typical. Lower ratios produce higher strength, while higher ratios reduce strength due to increased porosity.
Why This Is the Correct Answer
A water-cement ratio of 0.45 is the standard for achieving 4,000 psi compressive strength in concrete. This ratio provides the optimal balance between workability and strength development. The relationship between water-cement ratio and strength is well-established in concrete technology, where lower ratios create denser, stronger concrete while higher ratios increase porosity and reduce strength.
Why the Other Options Are Wrong
Option A: 0.55
A water-cement ratio of 0.65 is much too high for 4,000 psi concrete and would produce very weak concrete, typically under 2,500 psi. This high water content creates excessive porosity and poor durability characteristics.
Option B: 0.65
A water-cement ratio of 0.35 is too low for 4,000 psi concrete and would typically produce much higher strength concrete (6,000+ psi). This low ratio would also create workability issues, making the concrete difficult to place and finish properly without special admixtures.
Option D: 0.35
A water-cement ratio of 0.55 is too high for 4,000 psi concrete and would typically result in lower strength concrete around 3,000 psi or less. The excess water creates increased porosity in the hardened concrete, significantly reducing compressive strength.
Memory Technique
Use '4-45' - 4,000 psi pairs with 0.45 water-cement ratio. Think 'Four-Forty-Five' as a memorable phrase linking the strength to the ratio.
Reference Hint
ACI 318 Building Code Requirements for Structural Concrete, Chapter 19 (Concrete Quality) or ACI 211 Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete
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