Effect of flaky particles in aggregates for concrete: What are the consequences of having an excess percentage of flaky particles?
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ADecreases workability of the fresh mix
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BIncreases water and fine sand demand to reach the same slump
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CAdversely affects durability and mechanical performance
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DContents exceeding about 15% are generally considered undesirable
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EAll of the above
Answer
Correct Answer: All of the above
Explanation
Introduction / Context:Particle shape influences packing, surface area, and interlock. Flaky particles (with small thickness relative to other dimensions) harm both fresh and hardened concrete properties when present in excess.
Given Data / Assumptions:
- Standard shape tests classify flaky or elongated particles.
- Concrete intended for structural use with vibration.
Concept / Approach:Flaky particles tend to orient and bridge, creating voids and harsh mixes. They increase specific surface area, raising water and fines demand for a given slump. In hardened concrete, planes of weakness develop along thin particles, reducing strength and durability. Therefore, specifications usually limit flaky/elongated particles; a common rule of thumb is to keep them below about 15% by mass.
Step-by-Step Solution:Relate shape → higher surface area and poor packing → less workable mix.More water/fines needed → increased paste content → potential shrinkage.Hardened performance suffers due to weak planes and higher porosity.Hence limits (≈ 15%) are adopted to control quality.
Verification / Alternative check:Aggregate standards specify shape indices and maximum percentages for flaky and elongated particles.
Why Other Options Are Wrong:Each individual consequence is real; excluding any understates the impact of flaky particles.
Common Pitfalls:Assuming that stronger paste can offset poor aggregate shape; ignoring the increased segregation risk in pumped concrete.
Final Answer:All of the above.