CCIL Aggregate Gradation and Properties 5 — Questions and Answers
Question 1: What is the significance of the coefficient of uniformity (Cu) in aggregate gradation analysis?
- It measures the ratio of D60 to D10 particle sizes to indicate how well-graded a material is (Correct answer)
- It represents the percentage of uniform-shaped particles in the aggregate
- It is the ratio of fines content to coarse content by mass
- It measures the consistency of aggregate specific gravity across particle sizes
Correct answer: It measures the ratio of D60 to D10 particle sizes to indicate how well-graded a material is
The coefficient of uniformity (Cu = D60/D10) quantifies the range of particle sizes; values greater than 4 for gravels or 6 for sands indicate well-graded material.
Question 2: In the context of aggregate testing, what is 'moisture content' as determined by ASTM C566?
- The absorbed moisture within aggregate pores below the surface
- The total evaporable moisture in aggregate expressed as a percentage of oven-dry mass (Correct answer)
- The free surface moisture on aggregate particles only
- The moisture required to bring aggregate to SSD condition
Correct answer: The total evaporable moisture in aggregate expressed as a percentage of oven-dry mass
ASTM C566 determines total evaporable moisture content — all moisture that can be driven off by oven drying — expressed as a percentage of the oven-dry aggregate mass.
Question 3: What is the primary concern with using aggregate that has excessive amounts of material finer than the No. 200 sieve (passing 75 µm)?
- It increases aggregate specific gravity
- It can coat aggregate particles and reduce bond strength with cement paste (Correct answer)
- It improves the workability of concrete significantly
- It increases the Los Angeles abrasion resistance
Correct answer: It can coat aggregate particles and reduce bond strength with cement paste
Excessive fines passing the No. 200 sieve, particularly clay and silt, coat aggregate surfaces and interfere with cement paste bonding, reducing strength and durability of concrete.
Question 4: What does the term 'open-graded aggregate' refer to?
- Aggregate with a wide range of particle sizes for maximum density
- Aggregate consisting predominantly of one particle size with minimal fines, creating high void content (Correct answer)
- Aggregate with a high percentage of crushed faces
- Aggregate with particles larger than 75 mm
Correct answer: Aggregate consisting predominantly of one particle size with minimal fines, creating high void content
Open-graded aggregate has most particles concentrated in a narrow size range with very few fines, resulting in high void content that allows free drainage of water.
Question 5: In aggregate quality assessment, what does a high value of 'methylene blue value' (MBV) indicate?
- High resistance to abrasion in the aggregate
- Presence of a large amount of harmful clay minerals or organic matter in the fine fraction (Correct answer)
- High absorption capacity of coarse aggregate
- Low voids content in compacted aggregate
Correct answer: Presence of a large amount of harmful clay minerals or organic matter in the fine fraction
A high methylene blue value indicates that the fine fraction of aggregate contains significant amounts of harmful swelling clay minerals (such as smectite) or organic matter that can damage concrete.
Question 6: Which factor most influences the choice between using dry or wet sieve analysis for a given aggregate sample?
- The maximum aggregate size of the sample
- Whether the aggregate contains significant fines or clay coatings (Correct answer)
- The specific gravity of the aggregate
- The laboratory temperature at time of testing
Correct answer: Whether the aggregate contains significant fines or clay coatings
Wet sieve analysis is required when aggregate contains significant clay coatings or fines that would blind sieves or give inaccurate dry results; otherwise dry sieving may be sufficient.
Question 7: What is the relationship between aggregate particle shape (angularity) and the internal friction angle of a compacted aggregate layer?
- Rounded particles produce a higher internal friction angle than angular particles
- Angular particles produce a higher internal friction angle due to increased particle interlock (Correct answer)
- Particle shape has no significant effect on internal friction angle
- Flat particles produce the highest friction angle regardless of angularity
Correct answer: Angular particles produce a higher internal friction angle due to increased particle interlock
Angular aggregate particles interlock more effectively under load than rounded particles, resulting in a higher internal friction angle and greater shear strength in compacted layers.
CCIL Aggregate Gradation and Properties 5
What is the significance of the coefficient of uniformity (Cu) in aggregate gradation analysis?