Bulk density, specific gravity, and absorption — effects on concrete Which statements about aggregates and their influence on concrete are correct?
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AFine aggregate bulk density is usually about 10% higher than that of coarse aggregate of similar composition
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BAggregate specific gravity is important for determining moisture content and corrections
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CAggregate absorption and porosity influence concrete properties
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DHighly absorptive aggregates can considerably reduce workability
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EAll of the above
Answer
Correct Answer: All of the above
Explanation
Introduction / Context:Aggregates dominate concrete volume and control fresh and hardened behaviors. Their physical properties—bulk density, specific gravity, absorption, and porosity—directly influence mix proportioning, moisture corrections, and performance. Understanding these relationships is central to producing consistent, durable concrete.
Given Data / Assumptions:
- Similar rock types for fine and coarse fractions.
- Normal structural concrete mix design procedures.
- Standard test methods for density, absorption, and moisture.
Concept / Approach:Fine aggregate often packs with fewer large voids than coarse aggregate, giving a slightly higher bulk density for the same rock type. Specific gravity supports conversion between mass- and volume-based proportions and is essential when calculating absorbed vs. surface moisture. Higher absorption and porosity increase the amount of water drawn from the paste, raising effective water demand and reducing workability if not compensated. Consequently, highly absorptive aggregates must be brought to SSD (saturated surface-dry) condition or water corrections applied during batching to maintain the intended water–cement ratio and slump.
Step-by-Step Solution:
Recognize bulk density trends: fine aggregate may show ~10% higher bulk density than coarse of the same rock due to packing.Use specific gravity to compute mass–volume relationships and moisture corrections.Account for absorption/porosity: higher absorption lowers apparent workability if extra water is not provided.Conclude all listed statements are valid and interrelated.Verification / Alternative check:Trial mixes show slump recovery when water corrections or SSD aggregates are used with porous sands, confirming the direct link between absorption and workability.
Why Other Options Are Wrong:
- Selecting individual statements ignores the holistic impact of aggregate properties on concrete.
Common Pitfalls:Ignoring daily moisture variation in sand; not reweighing moisture corrections; assuming identical bulk density across aggregate shipments.
Final Answer:All of the above