Aggregates
Chapter 4
Aggregates
• Granular mineral particles used
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Various types of cementing material
Road bases
Backfill
Subbases
Railroad ballast
Fill under floor slabs
Concrete blocks
Water filtration beds
Drainage structures
Riprap
Gabion material
Sources
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Natural sand and gravel deposits
Crushed rock
Slag and mine refuse
Rubble and refuse
Artificial and processed materials
Pulverized concrete and asphalt pavements
Other recycled and waste materials
Sources
• Natural sand and gravel deposits – crushed
rock – make up bulk of aggregates
• Although the use of recycled materials is
growing
• Natural sand and gravel deposits are used for
aggregates
– Consist of sand or gravel soils
Natural sand and gravel deposits
• Used for aggregate
• Have been naturally sorted to eliminate most
silt and clay
• Mine in open pit mines –
• May be crushed further to produce different
sizes
Properties of aggregates
• Three major classes of rock
• Igneous rock
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Original rock
Formed from the cooling of molten material
Coarse grained igneous rocks cool slowly
Fine grained igneous rocks cooled more quickly
• Sedimentary rocks
– Formed from the solidification of chemical or mineral
sediments deposited
• Metamorphic rocks
– Igneous or sedimentary rocks that have been changed due
to intense heat and presure
Best Types for aggregates
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Igneous and metamorphic rocks are very hard
– Excellent aggregates
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Sedimentary rock is softer
– But limestone and dolomite are acceptable for aggregate choices
– Shale is not a good choice because it is made from clay
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Slag is being used as well
– Water cooled slag is a good choice for base courses and concrete
– Air cooled slag is not a good choice because it could contain sulphur
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Pulverized concrete is being used more and more
Asphalt pavements can be recycled as well
– Limite to 30-50% of total for new installs
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Use of other recycled materials is being investigated as well
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Glass
Rubber pellets
Bricks
Building ruble
Aggregate terms and types
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Fine aggregate (sand sizes) between no4 and no . 200 sieve
Course aggregate larger then no. 4
Pit run – aggregate from a sand or gravel pit no processing
Crushed gravel – aggregate from the crushing of bedrock –
all particles are angular not rounded
Screenings – chips and dust or powder that are produced in
the crushing of bedrock
Concrete sand – sand that has been washed
Fines – silt clay or dust smaller then no. 200 – usually not
desirable
Aggregate and sieve sizes table 4-1 page 128
Nominal size
• Size determined by sieves
• Not necessary that 100 percent be within the specified range
– Small amount about 5 to 10% is allowed to be larger or smaller
– Ex. 3/4in (19mm) 90 percent of sample must be smaller than 19 and
100 % must be smaller then next sieve size which is 1in (25mm)
– Ex fine agg. Should pass through a no. 4 sieve (4.75mm) 100 percent
passes through 9.5 (3/8) – 90 passes 4.75
– Ex coarse aggregates – 19 -4.75 should pass 100 through 25 mm 90
passing the 19mm sieve and 10 passing the 4.75 sieve
– Single coarse aggregate is called clear
• Most particles are between the specified maximum size and minimum size
• Ex 19mm – 100 smaller than 25mm 90 smaller than 19 and 10% smaller than
9.5mm
Aggregate Strength
• Use to distribute imposed loads over surface of the soil
• Or as economical filler material
• The ability for aggregate particles to carry loads
without moving is the most important factor
• Loads come from both horizontal and vertical forces
– The load is distributed over a large area
• Crushed material is much better then rounded
– Rough faces increase the friction between particles
– Reduce the risk of sliding between particles
• Densely graded also increase the strength
– Particles are more easily lock together
Properties
• Gradation
– Grain size analysis
– Specifications for highway bases, concrete and asphalt mixes
require a grain size distribution that will provide a dense strong
mix
– Voids between larger particles are filled with medium particels –
the remaining voids are filled with still smaller particles
– Fuller maximum density curves
• P=(d/D).5 where p is the percent passing sieve size d, and D
represents the maximum sieve size (100% passing)
– Revised maximum density relationship used by federal highway
admin
• 4.5 is used in place of .5
The Problem with Fines
• Fines must be limited
• Silt and clay particles (no. 200 sieve) are relatively weak
• In concrete mixes to much cement is need to cover the fines – in addition
they weaken the bond between the cement and those particles
• Fines in highway bases may lead to drainage and frost heaving problems
• Clay is more harmful the silt
– Maximum values for liquid limit and index of plasticity are often specified for
aggregates
– Washed sieve analyses are required when the amount passing 75um is
important (200 sieve)
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Sample is dried and washed
Wash water is poured out over a 200 sieve
Material retained in the sieve is returned to the sample
It is then dried again and dry sieved
Total amount passing 200 sieve is the sum of the amounts lost in washing and passing the
75 um sieve
• Example page 132 – 4-1
Relative density and absorption
• Are important especially in concrete and asphalt
mixtures
– Necessary to measure accurately the volumes
occupied by the aggregate and any water that might
have seeped into the pores in the particles
• Relative density rd=m/(V *pw)
• Other formula exist for dealing with moisture in
the agg. – page 133
• Example 4-2 page 134
• Example 4-3 page 134
Hardness
• Resistance to wear
• Important
– Pavement surfaces – agg. Not get rounded or polished due
to traffic
– Floors – subject to heavy traffic
– Roadbeds – subjected to innumerable cycles of lad
application and removal
• Most common test is the los Angeles abrasion test
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Agg. Is placed in a drum with a number of steel balls
Drum is rotated a specified number of times
Loss of agg. Or amount ground down is measured
Example 4-4
Durability
• Resistance to disintegration due to cycles of wetting
and drying, heating and cooling and especially freezing
and thawing
• Especially dangerous with particles from sedimentary
rocks
• Soundness test is used
– Sample of aggregate is saturated in a solution of
magnesium sulphate or sodium sulphate and then
removed and dried in a oven
– Repeated five cycles
– The percent lost or broken down is calculated
– Example 4-5 page 137
Other properties
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Particle shape and surface texture
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Affect the strength and bond with cementing materials
Resistance to sliding of one particle over another
Flat –thin or long needle shaped particles break more easily
Rough faces allow a higher friction strength
Deleterious substances
– Harmful or injurious materials
– Include weak or low quality particles and coatings that are found on the surface of aggregate
particles
– Include
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Organic coating
Dust
Clay lumps
Shale
Coal particles
Friable particles easyt o crumble
Chert may break up when exposed to freezing and thawing
Badly weathered particles
Soft particles
Lightweight particles
Specifications
• For each of the properties of aggregates
– Specification vary considerably
– Requirements for road base differ from concrete
and asphalt
• Table 4-3 gives astm standard sizes of
aggregates for highway work
Typical specifications for highway base
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Passing 1in 100%
¾ 90 to 100%
3/8 50 to 75%
No. 4 35-55%
No. 16 15-40%
No. 50 5 to 22%
No. 200 2-8%
Abrasion loss up to 40%
Soundness loss up to 18%
Example 4-6 page 140
Sampling and testing
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Csa standard A23.2-1a and astm standard d75
Give methods to follow in sampling aggregates
Page 143 list astm d75 standards
In conducting lab test – its critical that the
sample tested be representative of the
material being used in the field
– Usually a sample splitter is used to obtain the test
sample
Blending Aggregates
• Meet the gradation requirement for asphalt or
concrete it is often necessary to blend two or
more aggregates together
• Charts are available but normally done by trial
and error
• Example 4-8 page 147