MCQs on Ingredients of Concrete
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Q31: Which of the below property of aggregates is not desirable?
Difficulty: Easy
Solution:
A smooth texture is undesirable in concrete aggregates because it results in a weak mechanical bond between the aggregate surface and the cement paste, thereby reducing the concrete’s strength.
Q32: Which of the following is/are deleterious material in aggregate?
Difficulty: Easy
Solution:
Coal, clay lumps, and soft fragments are all classified as deleterious materials because they degrade the quality, strength, and durability of the concrete.
Q33: The maximum size of aggregate in concrete depends upon :
Difficulty: Medium
Solution:
The maximum permissible size of aggregate is governed by structural dimensions, clear cover, and reinforcement spacing to ensure proper flow and compaction without honeycombing.
Q34: The type of aggregates not suitable for high strength concrete and for pavements subjected to tension is :
Difficulty: Medium
Solution:
Rounded aggregates have a smooth surface that develops a weaker bond with the cement matrix. Due to this poor interlocking and bonding, they are unsuitable for high-strength concrete and tension-prone pavements.
Q35: Using the largest maximum size of aggregate in concrete reduces
Difficulty: Medium
Solution:
Utilizing the largest maximum size of aggregate reduces the total surface area that needs to be coated by cement paste. This leads to a reduction in water requirement and cement content, which subsequently lowers drying shrinkage.
Q36: An aggregate which passes through 25 mm I.S. sieve and is retained on 20 mm sieve, is said to be flaky if its least dimension is less than :
Difficulty: Medium
Solution:
Mean dimension = (25 + 20) / 2 = 22.5 mm. An aggregate is flaky if its least dimension is less than 0.6 times its mean dimension. Flakiness limit = 0.6 x 22.5 = 13.5 mm.
Q37: On a grading curve, the gap grading is represented by :
Difficulty: Easy
Solution:
A horizontal line on a particle size distribution (grading) curve indicates gap grading, meaning there are no particles present within the size range represented by the horizontal section.
Q38: The bulk density of aggregates is generally expressed as :
Difficulty: Easy
Solution:
In laboratory testing (e.g., IS 2386 part 3), bulk density is practically calculated and commonly expressed as kg/litre, which is numerically equivalent to the specific gravity for water.
Q39: If the engineer-in charge approves, the 10 cm cubes may be used for the work test of concrete provided maximum nominal size of aggregate does not exceed :
Difficulty: Medium
Solution:
100 mm (10 cm) cubes can be used for compressive strength testing provided the maximum nominal size of the aggregate does not exceed 20 mm. Note that options are in cm but the standard specifies mm.
Q40: Consider the following statements regarding aggregates : 1. Dry aggregates absorb water from the mixing water and thus affect the workability 2. Aggregates containing surface moisture contribute extra 3. The free moisture content in fine aggregate result in reduction of volume 4. The free moisture content in coarse aggregate result in bulking of volume
Difficulty: Medium
Solution:
Statements 1 and 2 are correct. Dry aggregates absorb mixing water, reducing workability, while surface moisture adds to the total water content. Statements 3 and 4 are incorrect because bulking (volume increase) happens in fine aggregate (sand), not in coarse aggregates.
Q41: A flaky aggregate is said to be elongated if its length is
Difficulty: Medium
Solution:
An aggregate particle is strictly termed elongated if its length is greater than 1.8 times the mean sieve size. For a particle that is already flaky, its length is generally compared as roughly twice (or >1.8) the mean dimension.
Q42: For the construction of cement concrete floor, the maximum permissible size of aggregates is
Difficulty: Easy
Solution:
For the construction of relatively thin sections like cement concrete floors, the maximum permissible size of coarse aggregates is typically restricted to 10 mm.
Q43: What should be done to ensure constant moisture content in aggregates?
Difficulty: Medium
Solution:
To maintain a constant and uniform moisture content, aggregate piles should be spread over a large area, kept under 1.50 m in height, and left to drain naturally for at least 24 hours before use.
Q44: An excess of flaky particles in concrete aggregates :
Difficulty: Medium
Solution:
Flaky aggregates possess a high surface-area-to-volume ratio. This decreases the mix’s workability, necessitates more water and sand to maintain the desired consistency, and subsequently impairs the concrete’s overall durability.
Q45: The type of aggregates of same nominal size, which contains less voids when compacted are :
Difficulty: Easy
Solution:
Rounded or spherical aggregates pack closely together and contain the minimum percentage of voids (around 32-33%) when compacted, compared to angular or irregular aggregates.