Construction Engineering Technology

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26 Νοε 2013 (πριν από 4 χρόνια και 7 μήνες)

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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

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

1/3 depth of slabs on grade

Aggregate Size

Higher maximum aggregate size lowers paste
requirements, increases strength and
reduces w/c ratio


large aggregates reduce strength
due to reduced surface area for bonding.


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:



flaky or rough

high particle interaction

requires more cement paste to achieve workability

results in increased cost


Shape and Surface Texture

Aggregate Soundness

Aggregate is considered unsound when
volume changes in the aggregate are
induced by weather


alternate cycles of wetting and drying

freezing and thawing

Results in concrete deterioration

Characteristics of Quality Aggregate

Should be clean, hard & free of chemicals

Should retain shape, strength & texture

Moderate to low absorption

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 (


Particle size distribution affects:


Mix proportioning

thaw resistance (


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

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

Moisture States of Aggregates


We must determine how much water

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

Quality Control of Aggregates

Past performance records from source

Laboratory freeze
thaw testing

Cyclic freeze/thaw testing of lab beam

Damage measured by reduction in the
dynamic modulus of elasticity of specimens