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Concrete Mix Design Flashcards

7 cards from real ACI Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. In a concrete mix, 'paste volume' refers to the combined volume of:

    Answer: Cement, water, and entrained air

    Paste is the binding matrix composed of cement, water, and entrained air that coats and bonds aggregate particles together.

  2. When field test records show a standard deviation of 400 psi for concrete with f'c = 4,000 psi, the required average strength (f'cr) using f'cr = f'c + 1.34s is:

    Answer: 4,536 psi

    f'cr = 4,000 + 1.34 × 400 = 4,000 + 536 = 4,536 psi; this formula applies when adequate field test records exist.

  3. What is the primary benefit of air entrainment in concrete exposed to freezing and thawing cycles?

    Answer: It creates small voids that relieve hydraulic pressure from freezing pore water

    Entrained air bubbles act as pressure-relief chambers, allowing expanding ice to move into voids rather than fracturing the cement paste.

  4. How does increasing the maximum aggregate size generally affect the water requirement for a given slump?

    Answer: Water requirement decreases

    Larger aggregates have less total surface area per unit volume, reducing the water needed to coat particle surfaces and achieve the target workability.

  5. Moisture corrections to batch weights are required whenever aggregate moisture content is:

    Answer: Above or below the SSD condition

    Aggregate above SSD contributes free water to the mix while aggregate below SSD absorbs water from it—both conditions require correcting the batch water quantity.

  6. What is the approximate specific gravity of normal-weight fine aggregate (natural sand) used in absolute volume calculations?

    Answer: 2.65

    Natural sand has a typical specific gravity of approximately 2.65, reflecting its primary mineral composition of quartz.

  7. If a trial batch produces a lower slump than specified, the correct adjustment per ACI 211.1 principles is to:

    Answer: Increase both water and cement proportionally to maintain the w/c ratio, or use a water-reducing admixture

    Adding water alone raises the w/c ratio and reduces strength; the proper fix is to add water and cement in proportion to keep w/c constant, or to use a water-reducing admixture.