Aggregates are granular materials like sand, gravel, or crushed stone used with water and cement to make concrete. They come in two sizes: fine aggregates smaller than 5 mm and coarse aggregates larger than 5 mm. Aggregates provide strength, reduce cracking, and lower the cost of concrete. They are selected based on being hard, durable, and free of organic materials or other substances that could weaken the concrete. Aggregates are classified by size, manufacturing method, and density. Physical tests are conducted to determine properties like strength, hardness, porosity, and grading.

1. Aggregate
Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with
water and Portland cement, are an essential ingredient in concrete. A combination of different
sizes and shapes normally stone. Maximum size is 75 mm. Aggregates are divide into two
distinct categories i.e. fine and coarse.
 Fine aggregates: Aggregate particles that pass through a 4.75 mm sieve or aggregates
with size less than 5 mm.
 Course aggregates: Coarse-grained aggregates passing through 75 mm sieve and will
not pass through a sieve with 4.75 mm or aggregates with size more than 5 mm.
Purpose & Uses of Aggregates
In concrete, an aggregate is use for its economy factor, to reduce any cracks and most
importantly to provide strength to the structure.
1. As an underlying material for foundations and pavements
2. Aggregates are used as the base, subbase, and/or surface of roads in several forms
3. In roads and railway ballast, it is use to help distribute the load and assist in ground water
running off the road.
4. Increases the volume of concrete, thus reduces the cost. Aggregates account for 60-75%
of the volume of concrete and 79-85% weight of PCC
5. Provide dimensional stability
6. Influence hardness, abrasion resistance, elastic modulus and other properties of concrete
to make it more durable, strong and cheaper.
7. Other uses include fills, backfills, and drainage and filtration applications.
Selection of Aggregates
 Aggregates shall be hard, durable, clean, and free from adherent coatings and organic
matter and shall not contain appreciable amount of clay.
 Aggregates shall not contain harmful impurities such as iron pyrites, alkalis, salts, coal,
mica, shale or other materials, which will affect hardening and attack reinforcement.
Classification of Aggregates
 Based on Size,
1. Fine aggregate: Aggregate particles that pass through a 4.75 mm sieve or aggregates
with size less than 5 mm.
2. Course aggregate: Coarse-grained aggregates passing through 75 mm sieve and will
not pass through a sieve with 4.75 mm or aggregates with size more than 5 mm.
 Based on method of manufacture,
1. Natural aggregate/uncrushed aggregate: Those from the riverbeds, river sand and ex-
mines. Normally rounded in shape and have smooth surface texture.
2. Manufactured aggregate / crushed aggregate: Those obtained by mechanically
crushing rocks, boulders, or cobbles. Normally angular in shape and have rough
surface texture.
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2.  Based on Density,
1. Lightweight 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. Types are Natural and Synthetic.
2. Normal-weight aggregate: Crushed stone, gravel and ordinary sand are examples of
normal weight aggregate. They are commonly use 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.
3. Heavyweight aggregate: Those aggregate with high density and is use 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)
Physical Properties of Aggregates
 Strength
In practice, majority of normal aggregates are considerably stronger than concrete. A
good average value of crushing strength of aggregates is 200N/mm2.
 Hardness
 Hardness of aggregates is the ability of the aggregate to withstand wear or load or
applied pressure.
 This hardness is depending on the type of parent rock. The test that can obtain the
hardness is the abrasion test.
 A satisfactory aggregate should have an abrasion value of not more than 30% for aggregates
used for wearing surfaces and 50% for aggregates used for non-wearing surfaces.
 Toughness
 Is the resistance of aggregate to failure by impact? This can be determined by
aggregate impact test.
 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.
 Durability
 Durability is ability of the aggregate to withstand external or internal damaging attack
or in other words the soundness of aggregate. This can be obtained by carrying out the
Soundness test.
 Porosity
 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.
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3. Tests on aggregates
a.Texture and shapes test
i.e. test for shapes of aggregates:
1.
2.
3.
4.
5.
6.
round shape –usually natural aggregates
irregular shape- a combination of different shapes
angular shape- usually of crushed stone
flaky shape- where the thickness is less than its length and
width
elongated- usually angular where its length is larger than its
width and thickness
flaky and elongated- its length is larger than its width and its
width is larger than its thickness.
In terms of surface texture, the aggregates may have a smooth
texture, or coarse/rough texture or fissures or porous.
For production of concrete the aggregates which have angular
shape and coarse texture are recommended to have high bond
strength.
•
•
•
•
Strength tests
Aggregates crushing strength
Los Angeles abrasion test
Aggregates Impact value test
Ten percent fine test
Tests on aggregates (Cont’d)
Physical properties:
•
•
Specific gravity: usually 2.6 or 2.7
Water absorption test: aggregates absorb
water because of their porosity.
If all the pores are filled with water the
aggregates are said to be saturated and
surface dry.
If all the water inside the pores are removed
by drying, the aggregates are said to have
maximum dry weight.
Tests on aggregates (Cont’d)
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.
Grading of aggregates:

4. Sieve analysis test equipment
Fineness Modulus (FM)
• The results of aggregate sieve analysis is expressed by a
number called Fineness Modulus.
• Obtained by adding the sum of the cumulative percentages by
mass of a sample aggregate retained on each of a specified
series of sieves and dividing the sum by 100.
• The following limits may be taken as guidance:
• Fine sand : Fineness Modulus : 2.2 - 2.6
• Medium sand : F.M. : 2.6 - 2.9
• Coarse sand : F.M. : 2.9 - 3.2
A sand having a fineness modulus more than 3.2 will be unsuitable
for making satisfactory concrete.
Sieve Analysis of Fine Aggregate
Sieve
No.
Weight
Retained
(gm)
Percent
Retained
Cumulative
Percent Retained
FM
4 1.80 0.36 0.36
2.74
8 20.50 4.10 4.46
16 107.31 21.46 25.92
30 168.37 33.67 59.60
50 129.19 25.84 85.43
100 62.46 12.49 97.93
Pan 10.36 - -
Total 500.0 - 273.70

7. To calculate moisture content of Aggregates
Example:
A sample of damp aggregate weighing 2.35 kg is
dried by hair dryer until it just reaches the free-
running(saturated surface dry) condition. It is
then found to weigh 2.24 kg. After drying in the
oven at 110C to constant mass, it is found to
weigh 2.15 kg. Based on dry mass, calculate:
•free-water content
•the total water content
Solution:
a) Free-water content in aggregate is defined as the water content
when the aggregate is in saturated surface dry conditions.
i.e. initial weight = 2.35 kg
weight at SSD = 2.24 kg
free water content = 0.11 kg
% of free-water content based on dry mass
= 0.11/2.15 x 100 = 5.1%
b) Total water content = free-water content
+
water absorbed by aggregates
= 0.11 + (2.24 – 2.15)
= 0.2 kg
OR 2.35 – 2.15 = 0.2 kg
(i.e overall weight - wt of max. dry aggregates)
% of total water content based on dry mass,
= 0.2/2.15 x 100
= 9.3%