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Prepared by: Marcia C. Belcher Construction Engineering Technology

Prepared by: Marcia C. Belcher Construction Engineering Technology. Introduction to Aggregates. Aggregates generally occupy 65- 80% of a concrete’s volume The characteristics impact performance of fresh and hardened concrete. Aggregate Size. Two categories: Fine (sand) = passing #4 sieve

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Prepared by: Marcia C. Belcher Construction Engineering Technology

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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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