Concrete Materials & Mix Proportioning Flashcards
7 cards from real ACI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Concrete Materials & Mix Proportioning flashcards as text
What is the primary function of calcium chloride when used as a concrete admixture?
Answer: Accelerate setting and early strength gain
Calcium chloride is a common accelerating admixture that speeds hydration reactions, promoting faster set times and early strength gain.
Reactive aggregates containing silica can cause a damaging long-term expansion known as:
Answer: Alkali-silica reaction (ASR)
Alkali-silica reaction (ASR) occurs when reactive silica in aggregates reacts with alkalis in cement paste, forming a gel that absorbs water and expands, cracking the concrete.
According to ACI 211.1, the required average compressive strength (f'cr) used for mix design is higher than the specified strength (f'c) to account for:
Answer: Statistical variability in production and testing
f'cr includes a margin above f'c based on the standard deviation of test results, ensuring the specified strength is met statistically even with normal production variability.
What does the term 'unit weight' (density) of concrete typically refer to in mix proportioning?
Answer: Mass of concrete per unit volume, used to verify mix proportions
Unit weight (density) is the mass of fresh concrete per unit volume (lb/ft³ or kg/m³) and is used to check that mix proportions account for all materials correctly.
Which of the following best describes the role of retarding admixtures (ASTM C494 Type B) in concrete?
Answer: They delay the initial set, extending workability time
Retarding admixtures slow cement hydration, extending the time before initial set — especially useful in hot weather or long-haul transportation.
In concrete mix design, 'entrapped air' differs from 'entrained air' in that entrapped air:
Answer: Is random, large, non-spherical voids that reduce strength
Entrapped air consists of random, large, irregular voids that occur naturally during mixing and reduce strength, unlike the uniform microscopic bubbles of intentionally entrained air.
When the water-cement ratio is held constant, increasing the cement content in a mix will primarily:
Answer: Increase paste volume and potentially improve workability
At a constant w/c ratio, adding more cement requires proportionally more water, increasing total paste volume which generally improves workability and cohesion.