BMC Unit 6 aggregates

Unit 6 Aggregates
Prepared by:K.SAHITYA,
Asst prof.
Civil engineering dept.
BVC college,odalarevu
UNIT. VI: AGGEGATES
Classification of aggregate – Coarse and fine aggregates- particle shape
and texture – Bond and Strength of aggregate – Specific gravity – Bulk
Density, porosity and absorption – Moisture content of Aggregate-
Bulking of sand – Sieve analysis.
Aggregates
A combination of different sizes and shapes normally of stones. Maximum size is 75 mm.
 As an underlying material for foundations and pavements
 As an ingredients in Portland cement concrete and asphalt concrete
Classificationofaggregates
Basedon size:
 classified into 2 categories:
• fine aggregates - those aggregates which pass through 4.75 mm sieve or
aggregates with size less than 5 mm.
• Coarse aggregates – those aggregates Passing through 75 mm sieve and entirely
retained on 4.75 mm sieve OR those aggregates with size greater than 5 mm
Basedon source or method of manufacture:
 classified into 2 categories:
• Natural aggregate/uncrushed aggregate- Those from the river beds, river sand and
ex-mines. Normally rounded in shape and have smooth surface texture.
• Manufactured aggregate / crushed aggregate –those obtained by mechanically
crushing rocks, boulders, or cobbles. Normally angular in shape and have rough
surface texture

Basedon Density:
 Based on specific gravity or density measured in bulk, aggregate is divided into 3 types:
• Lightweight aggregate
• Normal-weight aggregate
• Heavyweight aggregate
 Lightweight fine aggregate;
 Lightweight fine aggregate is any aggregate with bulk density less than 1120kg/m3 and
lightweight coarse aggregate is any aggregate with bulk density less than 880kg/m3.
 They are commonly used as ingredients in the manufacture of lightweight concrete, for
making lightweight masonry blocks (to improved their thermal and insulating properties
and nailing characteristic), and lightweight floor and roof slabs.
 2 types of lightweight aggregate:
 Natural lightweight aggregates (eg: palm oil shell, rice husk, etc)
 Manufactured (also called synthetic) lightweight aggregates
 Normal weight aggregates
 Crushed stone, gravel and ordinary sand are examples of normal weight aggregate.
 They are commonly used in manufacture of normal weight concrete, asphalt concrete and
roadway sub-base.
 The average values of sp.gr. For sand and gravel are 2.6 and 2.65 respectively. Bulk
density of normal weight aggregate is around 1520 to 1680kg/m3.
 Heavy weight aggregates
 Those aggregate with high density and is used primarily in the manufacture of
heavyweight concrete, employed for protection against nuclear radiation and as bomb
shelter.
 The unit weight of heavyweight concrete varies from 2400kg/m3 with sp.gr range from
4.0 to 4.6.(eg: mineral ores and barite)
Physicalproperties of aggregates
 Particle shape and texture
 Bond and Strength of aggregate
 Specific gravity

 BulkDensity,
 Porosity and absorption
 Moisture content of Aggregate
Particle shape and texture
 Rounded Vs. Angular aggregates: The roundness of the aggregate will affect its
packing properties as well as the interlock obtained between aggregates. Angular
aggregates result in better packing and interlock. On the other hand, rounder aggregates
typically need less water for the same workability. Roundness may be defined by the
sphericity of aggregate, or, from the other viewpoint, as the angularity of the aggregate.
Because of weathering, river gravel (or sand) gives rounded aggregate, while angular
aggregates are obtained using crushed stone.
 Flakiness or elongation of the aggregate can result in anisotropic packing, poor
compaction, and lowered concrete strengths. As discussed earlier, flakiness and
elongation could result from the geological nature of the aggregate (aggregates from
rocks showing directional properties would tend to be flaky and elongated).
 Texture of the aggregate is rough or smooth, and dictates the strength of the paste-
aggregate bond. Rougher aggregates show better bond with paste, but also cause an
increase in the water demand. Weathered aggregates are smooth, while crushed
aggregates are rough.
Bond and Strength of aggregate
 The strength of aggregate tested independently is generally always higher than that of
concrete. However, aggregate in concrete is stronger than the concrete for conventional
concrete, while the aggregate strength is lower than concrete strength for high strength
concrete, where the cementitious matrix is extremely strong.
 Aggregate strength depends on its parent rock composition, texture and structure.
 In practice, majority of normal aggregates are considerably stronger than concrete
 A good average value of crushing strength of aggregates is 200N/mm2
 Generally strength is depends upon resistance to crushing(crushing strength value)
toughness(impact value),hardness(abrasion value)
The following are the Strength tests
S.no property Test to be conducted Value
founded
Desired value
1 Crushing
strength
Aggregates crushing
strength
Crushing
strength
value
A good average value of
crushing strength of
aggregates is 200N/mm2
2 hardness Los Angeles abrasion test Abrasion
value
A satisfactory aggregate
should have an abrassion
value of not more than
30% for aggregates used
for wearing surfaces and
50% for aggregates used

for non wearing surfaces
3 toughness Aggregates Impact value
test
Impact
value
The aggregate impact
value shall not exceed
45% by weight for
aggregate used for
concrete other than those
used for wearing surfaces
and 30% for concrete for
wearing surfaces
Specific gravity and BulkDensity
 Aggregates are porous, and posses a number of voids, some of which are penetrable and
some impenetrable. This nature of aggregate makes it difficult to describe one value for
its specific gravity. Based on the test methods used, three types of specific gravity are
calculated – Bulk, Saturated surface dry, and Apparent. Determination of the true specific
gravity of the aggregate is not possible (unless it is crushed to the smallest possible
dimension) because the true volume of the aggregate can never be determined (as some
pores are totally inaccessible).
 Fine aggregate is susceptible to increases in volume due to the presence of moisture. This
phenomenon is called bulking. Bulking can lead to problems during volume batching of
the aggregates and causes harsh mixes with compaction problems. Crushed sand is more
susceptible to this problem compared to natural sand
 Specific gravity of aggregates must be 2.4 to 3.0 for any construction
 Bulk density of aggregates is the mass of aggregates required to fill the container of a unit
volume after aggregates are batched based on volume.
 Lightweight fine aggregate is any aggregate with bulk density less than 1120kg/m3 and
lightweight coarse aggregate is any aggregate with bulk density less than 880kg/m3.
 The average values of sp.gr. For sand and gravel are 2.6 and 2.65 respectively. Bulk
density of normal weight aggregate is around 1520 to 1680kg/m3.
 The unit weight of heavyweight concrete varies from 2400kg/m3 with sp.gr range from
4.0 to 4.6.(eg: mineral ores and barite
,Porosity and absorption:
 Aggregate normally have pores of various sizes.
 Aggregates will absorb water when it is dry but normally release water in the concrete
mix when it is wet.
 The amount of water and its rate of permeation depends on the size and volume of
aggregate

 Since the aggregate comprises 75% of the concrete volume, it is essential to note that
porosity of an aggregate contribute to the overall porosity of concrete.
 The internal pore characteristics are very important properties of aggregates. The size, the
number, and the continuity of the pores through an aggregate particle may affect the
strength of the aggregate, abrasion resistance, surface texture, specific gravity, bonding
capabilities, and resistance to freezing and thawing action.
 Absorption relates to the particle's ability to take in a liquid. Porosity is a ratio of the
volume of the pores to the total volume of the particle. Permeability refers to the
particle's ability to allow liquids to pass through. If the rock pores are not connected, a
rock may have high porosity and low permeability
 Absorption capacity (AC or absorption) represents the maximum amount of
water the aggregate can absorb. It is calculated from the difference in weight
between the SSD and OD states, expressed as a percentage of the OD weight:
AC = (WSSD - WOD) / (WOD) x 100%
WSSD and WOD represent the weight of the aggregate sample in the SSD and
OD states
 The absorption capacity is used in mix proportioning calculations and can be
used to convert from SSD to OD system or vice versa. Most normal weight
aggregates have absorption capacities in the range of 1 to 2%. Abnormally high
absorption capacities indicate high-porosity aggregates, which may have
potential durability problems.
Moisture content of Aggregate:
 Since aggregates contain some porosity, water can be absorbed into the body of the
particles or retained on the surface of the particle as a film of moisture. The following
four moisture states are defined:
o Oven-dry (OD): All moisture is removed from the aggregate by heating in an
oven at 105 C to constant weight (overnight heating usually is sufficient). All
pores are empty.
o Air-dry (AD): All moisture removed from surface, but internal pores partially full.
o Saturated-surface-dry (SSD): All pores filled with water, but no film of water on
the surface.
o Wet: All pores completely filled with water with a film on the surface.
 Of these four states, only OD and SSD states correspond to specific moisture contents,
and either of these states can be used as reference states for calculating moisture contents.
The AD and wet states represent the variable moisture contents that will exist in
stockpiled aggregates.

 Moisture content test is used to determine the water content of a materials by drying a
sample to constant mass at a specified temperature. The water content of a given soil is
defined as the ratio, expressed as a percentage, of the mass of the pore water to the mass
of the solid material (or "solids").
Sand
 Sand is a naturally occurring granular material composed of finely
divided rock and mineral particles.
 the most common constituent of sand is silica (silicon dioxide, or SiO2), usually in the
form of quartz.
Bulking of sand
 The increase in the volume of sand due to the presence of moisture is known as bulking
of sand. This is due to the fact that moisture forms a film of water around the sand
particles and this results in an increase in the volume of sand. The extent of bulking
depends on the grading of sand. The finer the material the more will be the increase in
volume for the given moisture content.
 For a moisture content of 5–8 per cent, the increase in volume may be about 20–40 per
cent depending upon the gradation of sand. When the moisture content is further
increased, the sand particles pack near each other and the amount of bulking is decreased.
Hence, dry sand and the sand completely flooded with water have practically the same
volume.

Sieve analysis or grading of aggregates
Coarse and fine aggregates to be used for making concrete should be well graded.
Gradation means the particle size distribution of aggregates. Test for grading of aggregates is
carried out using the sieve analysis method,by this sieve analysis we can find fineness modulus
and we can classify the different types sands based on grain size distribution
Fineness modulus of sand
Different construction works require different standards of sand for construction.
• Brick Works: finest modulus of fine sand should be 1.2 to 1.5 and silt contents should not
be more than 4%.
• Plastering Works: finest modulus of fine sand should not be more than 1.5 and silt
contents should not be more than 4%.
• Concreting Works: coarse sand should be used with finest modulus 2.5 to 3.5 and silt
contents should not be more than 4%.
Classification of sand Basedon the grain size distribution

Fine sand: The sand passing through a sieve with clear openings of 1.5875 mm is known as
fine sand. Fine sand is mainly used for plastering. .
Coarse sand: The sand passing through a sieve with clear openings of 3.175 mm is known as
coarse sand. It is generally used for masonry work.
Gravelly sand: The sand passing through a sieve with clear openings of 7.62 mm is known as
gravelly sand. It is generally used for concrete work.

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