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Understanding Aggregate Characteristics in Concrete Engineering

Learn about aggregate types, sizes, soundness, gradation, moisture levels, and quality control in concrete construction engineering technology. Understand how aggregates impact fresh and hardened concrete performance. Discover the importance of aggregate size, shape, texture, soundness, and gradation for optimal concrete mixtures.

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Understanding Aggregate Characteristics in Concrete Engineering

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  1. Prepared by: Marcia C. Belcher Construction Engineering Technology

  2. Introduction to Aggregates • Aggregates generally occupy 65- 80% of a concrete’s volume • The characteristics impact performance of fresh and hardened concrete

  3. Aggregate Size • Two categories: • Fine (sand) = passing #4 sieve • Coarse (stone) = retained on #4 sieve or larger • Maximum Aggregate Size: • 1/5 minimum dimension of non-reinforced structural member. • 3/4 clear spacing between bars or between reinforcing bars and forms • 1/3 depth of slabs on grade

  4. Aggregate Size • Higher maximum aggregate size lowers paste requirements, increases strength and reduces w/c ratio • Excessively large aggregates reduce strength due to reduced surface area for bonding.

  5. Aggregate:•Shape and Surface Texture • Ideal aggregates: • spherical or cubical • round shape, fine porous surface • reduced particle interaction (friction) • results in good workability and good surface area for bonding • natural sands are good examples of this • Substandard aggregates: • angular • elongated • flaky or rough • high particle interaction • requires more cement paste to achieve workability • results in increased cost

  6. Aggregate Soundness • Aggregate is considered unsound when volume changes in the aggregate are induced by weather • Ie: • alternate cycles of wetting and drying • freezing and thawing • Results in concrete deterioration

  7. Characteristics of Quality Aggregate • Should be clean, hard & free of chemicals • Should retain shape, strength & texture • Moderate to low absorption

  8. Good Gradation • Concrete with good gradation will have fewer voids to be filled with cement paste ( economical mix) • Concrete with good gradation will have fewer voids for water to permeate ( durability) • Particle size distribution affects: • Workability • Mix proportioning • Freeze-thaw resistance ( durability)

  9. Moisture In Aggregates • Aggregates have two types of moisture: • Absorbed moisture – retained in pores • Surface moisture – water attached to surface • Aggregates have four moisture states: • Oven dry: all moisture removed • Air dry: internal pores partially full & surface dry • Saturated-surface dry: pores full & surface moisture removed • Wet: pores full and surface film • SSD aggregate does not add or subtract water • Not easily obtained in the field

  10. Moisture States of Aggregates

  11. Absorption • We must determine how much water dry aggregate will consume into its voids • This takes water away from the mix and reduces workability & W/C ratio • We adjust mix proportions for absorption • We want to: • provide aggregates water for absorption • maintain workability of the mix

  12. Quality Control of Aggregates • Past performance records from source • Laboratory freeze-thaw testing • Cyclic freeze/thaw testing of lab beam specimens • Damage measured by reduction in the dynamic modulus of elasticity of specimens

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