Concrete Technology - Aggregate

1. UNIT-III
INTRODUCTION
Aggregates are the important constituents in concrete. They give body to the
concrete, reduce shrinkage and effect economy, It occupy 70-80% of the volume of
concrete, Cement is the only factory made standard component in concrete other
ingredients namely water and aggregates are natural material and can vary to any
extent in many of their properties. The study of aggregates can best be done under
the following sub-headings
 Classification
 Source
 Size
 Shape
 Texture
 Strength
 Specific gravity and bulk density
 Moisture content
 Cleanliness
 Soundness
 Chemical properties
 Thermal properties
 Durability
 Sieve analysis
 Grading
Classification
 Aggregates can be classified based on the size of the aggregates as coarse
and fine aggregates

2.  Aggregates can be also classified based on the weight of the aggregates as i)
Normal aggregates
ii) Light weight aggregates iii) Heavy weight aggregates
 Normal weight aggregates can be further classified as natural aggregates and
artificial aggregates
 Natural aggregates- sand, gravel, crushed rock such as granite, quartzite,
basalt, sandstone
 Artificial aggregates- Broken bricks, slag, fly ash, bloated clay
Source
 Natural aggregates originated from bed rocks
 Three kinds of rocks namely Igneous, sedimentary and metamorphic rocks
These classification based on the made of formation of rocks
Igneous rocks: The igneous rocks are formed by cooling of molten magma or lava at
the surface of the earth
Sedimentary rocks: They are formed originally below the sea bed and subsequently
lifted up
Metamorphic rocks: They are originally either igneous or sedimentary rocks
which are subsequently metamorphosed due to extreme heat and pressure
Aggregates from igneous rocks
 Most igneous rocks make highly satisfactory concrete aggregates because they
are normally hard, tough and dense
 The igneous rocks have massive structure

3.  They may occur light coloured or dark coloured
 The igneous rocks is one of the widely occurring types of rocks on the rocks on
the faces of the earth bulk of concrete aggregates that are derived from igneous
origin
Aggregates from sedimentary rocks
 Igneous rocks and metamorphic rocks are subjected to wearing agencies such
as sun, rain and wind
 Due to these weathering agencies transport and deposit the particle of rock,
deep beneath the ocean bed where they are cemented together by some of the
cementing materials
Aggregates from metamorphic rocks
 Igneous and sedimentary rocks may be subjected to high temperature and
pressure which causes metamorphic which changes the structure
 Metamorphic rocks shows foliated structure
Size
 The maximum size of aggregates used for plain cement concrete is 80mm
 The maximum size of aggregates used for reinforced cement concrete is
20mm Using the largest possible maximum size will result in:
i. Reduction of the cement content
ii. Reduction in water requirement
iii. Reduction of drying shrinkage

4. The maximum size of the aggregates may be limited by following condition
i. Thickness of section
ii. Spacing of reinforcement
iii. Clear cover
iv. Mixing, handling and placing techniques
Bigger than 4.75mm is considered as coarse aggregates and less 4.75mm is a fine
aggregate
Shape
 The shape of the aggregates is an important characteristic since it affect the
workability of concrete
 The shape of the aggregates depending upon the parent rocks and type of
crusher
Shape of particle
i. Rounded
ii. Irregular or partly rounded
iii. Angular
iv. Flaky
 By using the rounded shape aggregates the cement content is less required and

5. angular shape aggregates more cement is required
 But angular shape aggregates gives more strength compare than rounded shape
 Angularity number is based on the percentage voids in the aggregates after
compaction in a specified manner
Texture
 Particle surface are smooth or rough
 Surface texture depends on hardness, grain size, pore structure, structure of
rock and degree of forces acting on the particle surface
 Hard, dense, fine grained materials will generally have smooth fracture surface
 Surface smoothness increases, contact area decreases, and have less bonding
area and require a thinner layer of paste
Strength
 The strength of the rocks does not exactly represent the strength of the
aggregates in concrete
 Since concrete is an assemblage of individual piece of aggregates bond
together by cementing material
 The strength is dependent also on the bond between the cement paste and
aggregates
 If either the strength of the paste or the bond between the paste and aggregates
is low, a concrete of poor quality
 When the cement paste of good quality and the bond between cement paste
and aggregates is satisfactory, the concrete of good quality

6.  Conclude that while strong aggregates cannot make strong aggregates are an
essential requirements
Bulk density
 The bulk density or unit weight of an aggregate gives valuable information
regarding the shape and grading of aggregates
 The bulk density depends on the particle size and shape of the particle
 The higher the bulk density the lower is the voids content
 Specific gravity
 Average specific gravity of the rocks vary from 2.6 to 2.8
 Specific gravity of aggregates is used for design calculations of concrete
 Specific gravity of aggregates is required to be consideration when deal with
light weight and heavy weight concrete
Moisture content
 Some of the aggregates are porous and absorptive
 Porosity and absorption of aggregates will affect the water/cement ratio
 The water absorption of aggregates is determined by measuring the increase in
weight of an over dry sample when immersed in water for 24 hours
Bulking of aggregates
The bulking increases with the increase in moisture content upto certain limit and
beyond that the further increase in the moisture content result in the decreases in the
volume and at a moisture content representing saturation point

7. Cleanliness
 Fine aggregates obtained from natural source is likely to contain organic
impurities in the form of silt and clay
 Coarse aggregates stacked in the open and unused for long time may contain
moss and mud in the lower level of the stack
 Sand is decayed with vegetable matter, humus, organic matter and other
impurities
 Fine aggregates contains permissible quality of organic impurities or not a
simple test known as colorimetric test is made
Soundness
 Soundness refers to the ability of aggregates to resist excessive change in
volume as a result of changes in physical conditions
 These physical condition that affect the soundness of aggregates are the
freezing the thawing, variation in temperature, alternate wetting and drying
under normal condition and wetting and drying in salt water
Grading
 The particle size distribution of aggregates is called grading
 The grading determine the paste requirement for a workable concrete since the
amount of voids requires needs to be filled by the same amount of cement
paste in a concrete mixture
 To obtain grading curve for aggregates, sieve analysis has to be conducted
 Fine different kinds of size distribution, dense graded, gap graded, uniformly
graded, well graded and open graded

8. UNIT-III
Chemical properties
The chemical properties of aggregates have to do with the molecular structure of the
minerals in the aggregates particles, Reaction with asphalt and cement, There are
several types of substance found in mineral aggregates which may have a negative
effect, Most are rarely significant but varies organic substance may retard hardening,
reduce strength development or cause excessive air entrainment in Portland cement
concrete
Composition
Some aggregates have minerals that are subject to oxidation, hydration and
carbonation
Thermal properties - There are three thermal properties
i)Coefficient of thermal expansion ii)specific heat iii)thermal conductivity
 The coefficient of thermal expansion of aggregates influences the value of such
a coefficient of concrete containing the given aggregates
 The higher the coefficient of aggregates the higher the coefficient of the concrete
 The linear thermal coefficient of expansion of concrete may be taken as 9.9x10-6
per0
c but the range may be from about 5.8x10-6
per0
c to 14x10-6
per0
c
Sieve analysis
 Sieve analysis is conducted to a aggregates, which we call gradation
 The aggregates used for making concrete are normally of the maximum size
80mm,40mm,20mm,10mm,4.75mm, 2.36mm, 60micron, 300micron and
150micron Coarse aggregates and those fractions from 80mm to 4.75mm
 Fine aggregates and those fraction 4.75mm to 150µ

9. UNIT-III
Testing of aggregates
In order to decide the suitability of the aggregate for use in pavement
construction, following tests are carried out:
 Crushing test
 Abrasion test
 Impact test
 Soundness test
 Shape test
 Specific gravity and water absorption test
 Bitumen adhesion test
Crushing test
 One of the model in which pavement material can fail is by crushing under
compressive stress. A test is standardized by IS: 2386 part-IV and used to
determine the crushing strength of aggregates.
 The aggregate crushing value provides a relative measure of resistance to
crushing under gradually applied crushing load.
 The test consists of subjecting the specimen of aggregate in standard mould to
a compression test under standard load conditions.
 Dry aggregates passing through 12.5 mm sieves and retained 10 mm sieves
are filled in a cylindrical measure of 11.5 mm diameter and 18 cm height in
three layers.
 Each layer is tampered 25 times with at standard tamping rod. The test sample
is weighed and placed in the test cylinder in three layers each layer being
tampered again.
 The specimen is subjected to a compressive load of 40 tonnes gradually applied

10. at the rate of 4 tonnes per minute. Then crushed aggregates are then sieved
through 2.36 mm sieve and weight of passing material (W2) is expressed as
percentage of the weight of the total sample (W1) which is the aggregate
crushing value.
 Aggregate crushing value =W1 /W2 x 100
Abrasion test
 Abrasion test is carried out to test the hardness property of aggregates and to
decide whether they are suitable for different pavement construction works.
 Los Angeles abrasion test is a preferred one for carrying out the hardness
property and has been standardized in India (IS:2386 part-IV).
 The principle of Los Angeles abrasion test is to find the percentage wear due to
relative rubbing action between the aggregate and steel balls used as abrasive
charge.
 Los Angeles machine consists of circular drum of internal diameter 700 mm and
length 520 mm mounted on horizontal axis enabling it to be rotated.
 An abrasive charge consisting of cast iron spherical balls of 48 mm diameters
and weight 340-445 g is placed in the cylinder along with the aggregates.
 The number of the abrasive spheres varies according to the grading of the
sample. The quantity of aggregates to be used depends upon the gradation and
usually ranges from 5- 10 kg.
 The cylinder is then locked and rotated at the speed of 30-33 rpm for a total of
500 -1000 revolutions depending upon the gradation of aggregates.
 After specified revolutions, the material is sieved through 1.7 mm sieve and
passed fraction is expressed as percentage total weight of the sample. This
value is called Los Angeles abrasion value.
 A maximum value of 40 percent is allowed for WBM base course in Indian
conditions. For bituminous concrete, a maximum value of 35 is specified
Impact test

11. The aggregate impact test is carried out to evaluate the resistance to impact of
aggregates. Aggregates passing 12.5 mm sieve and retained on 10 mm sieve is filled
in a cylindrical steel cup of internal dia 10.2 mm and depth 5 cm which is attached to a
metal base of impact testing machine.
The material is filled in 3 layers where each layer is tamped for 25 numbers of blows.
Metal hammer of weight 13.5 to 14 Kg is arranged to drop with a free fall of 38.0 cm by
vertical guides and the test specimen is subjected to 15 numbers of blows.
The crushed aggregate is allowed to pass through 2.36 mm IS sieve. And the impact
value is measured as percentage of aggregates passing sieve (W2) to the total weight
of the sample (W1).
Aggregate impact value =W1/W2 x100
Aggregates to be used for wearing course, the impact value shouldn't exceed 30
percent. For bituminous macadam the maximum permissible value is 35 percent. For
Water bound macadam base courses the maximum permissible value defined by IRC
is 40 percent
Soundness
 Soundness test is intended to study the resistance of aggregates to weathering
action, by conducting accelerated weathering test cycles.
 The Porous aggregates subjected to freezing and thawing are likely to
disintegrate prematurely. To ascertain the durability of such aggregates, they
are subjected to an accelerated soundness test as specified in IS:2386 part-V.
 Aggregates of specified size are subjected to cycles of alternate wetting in a
saturated solution of either sodium sulphate or magnesium sulphate for 16 - 18
hours and then dried in oven at 105 -110oC to a constant weight.
 After five cycles, the loss in weight of aggregates is determined by sieving out
all undersized particles and weighing. And the loss in weight should not exceed
12 percent when tested with sodium sulphate and 18 percent with magnesium
sulphate solution.
Shape tests

12.  The particle shape of the aggregate mass is determined by the percentage of
flaky and elongated particles in it.
 Aggregates which are flaky or elongated are detrimental to higher workability
and stability of mixes.
 The flakiness index is defined as the percentage by weight of aggregate
particles whose least dimension is less than 0.6 times their mean size.
 Test procedure had been standardized in India (IS:2386 part-I)
 The elongation index of an aggregate is defined as the percentage by weight of
particles whose great dimension (length) is 1.8 times their mean dimension.
This test is applicable to aggregates larger than 6.3 mm
 This test is also specified in (IS:2386 Part-I). However there are no recognized
limits for the elongation index
Specific gravity and water absorption test
 The specific gravity and water absorption of aggregates are important properties
that are required for the design of concrete and bituminous mixes.
 The specific gravity of a solid is the ratio of its mass to that of an equal volume
of distilled water at a specified temperature. Because the aggregates may
contain water- permeable voids, so two measures of specific gravity of
aggregates are used: apparent specific gravity and bulk specific gravity.
 Water absorption, the difference between the apparent and bulk specific
gravities is nothing but the water permeable voids of the aggregates.
 The specific gravity of aggregates normally used in road construction ranges
from about to 2.9. Water absorption values ranges from 0.1 to about 2.0 percent
for aggregates normally used in road surfacing.
Bitumen adhesion test
 Bitumen adheres well to all normal types of road aggregates provided they are
dry and free from dust.
 Bitumen adheres well to all normal types of road aggregates provided they are

13. dry and free from dust. In the absence of water there is practically no adhesion
problem of bituminous construction.
 Adhesion problem occurs when the aggregate is wet and cold.
 This problem can be dealt with by removing moisture from the aggregate by
drying and increasing the mixing temperature.

14. UNIT-III
Workability of concrete
 It is desirable that freshly mixed concrete be relatively easy to transport, place,
compact and finish without harmful segregation
 A concrete mix satisfying three conditions in said to be workable
 Workability is the most important property of freshly mixed concrete
 There is no single test method that can simultaneously measure all the properties
 Involved in workability
 It is determined to a large extent by measuring the consistency of the mix
Consistency
 Consistency is the fluidity or degree of wetness of concrete
 It is generally dependent on the shear resistance of the mass
 Its major factor in indicating the workability of freshly mixed concrete
Factors affecting concrete workability:
1. Water-Cement ratio
2. Amount and type of Aggregate
3. Amount and type of Cement
4. Weather conditions
5. Temperature
6. Wind
7. Chemical Admixtures
8. Sand to Aggregate ratio
1. Water content or Water Cement Ratio
More the water cement ratio more will be workability of concrete. Since by simply
adding water the inter particle lubrication is increased. High water content results in a
higher fluidity and greater workability. Increased water content also results in

15. bleeding. another effect of increased water content can also be that cement slurry will
escape through joints of formwork.
High water content results in a higher fluidity and greater workability. Increased water
content also results in bleeding. another effect of increased water content can also be
that cement slurry will escape through joints of formwork.
2. Amount and type of Aggregate
Since larger Aggregate sizes have relatively smaller surface areas (for the cement
paste to coat) and since less water means less cement, it is often said that one
should use the largest practicable Aggregate size and the stiffest practical mix. Most
building elements are constructed with a maximum Aggregate size of 3/4" to 1", larger
sizes being prohibited by the closeness of the reinforcing bars.
Because concrete is continuously shrinking for years after it is initially placed, it is
generally accepted that under thermal loading it will never expand to it's originally-
placed volume. More the amount of aggregate less will be workability.
Using smooth and round aggregate increases the workability. Workability reduces if
angular and rough aggregate is used.
Greater size of Aggregate- less water is required to lubricate it, the extra water is
available for workability
Angular aggregates increases flakiness or elongation thus reduces workability. Round
smooth aggregates require less water and less lubricationand gretaer workability in a
given w/c ratio
Porous aggregates require more water compared to non absorbent aggregates for
achieving same degree of workability.

16. 3. Aggregate Cement ratio
More ratio, less workability. Since less cement mean less water, so the paste is stiff.
4. Weather Conditions
1. Temperature: If temperature is high, evaporation increases, thus
workability decreases.
2. Wind: If wind is moving with greater velocity, the rate of evaporation also
increase reduces the amount of water and ultimately reducing workability.
5. Admixtures
Chemical admixtures can be used to increase workability.
Use of air entraining agent produces air bubbles which acts as a sort of ball bearing
between particles and increases mobility, workability and decreases bleeding,
segregation. The uses of fine pozzolanic materials also have better lubricating effect
and more workability.
6. Sand to Aggregate ratio
If the amount of sand is more the workability will reduce because sand has more
surface area and more contact area causing more resistance. The ingredients of
concrete can be proportioned by weight or volume. the goal is to provide the desired
strength and workability at minimum expense. A low water-cement ratio is used to
achieve a stronger concrete. It would seem therefore that by keeping the cement
content high one could use enough for god workability and still have a low w/c ratio.
the problem is that cement is the most costly of the basic ingredients. The dilemma is
easily seen in the graphs below.

17. VOLUME CHANGES IN CONCRETE
 Concrete changes slightly in volume for various reasons, and understanding the
nature of these changes is useful in planning or analyzing concrete work
 Cracks develop because concrete is relatively weak in tension but quite strong
in compression
 Concrete volume changes due to thermal properties of aggregates and concrete
due to alkali/aggregates reaction due to sulphate action etc.,
 Most objectionable defects in concrete is the presence of cracks
 The cracks in floor and pavements is than due to shrinkage
 The term shrinkage is loosely used to describe the various aspects of volume
changes in concrete due to different reasons
Shrinkage can be classified into following reasons
i. Plastic shrinkage
ii. Drying shrinkage
iii. Autogeneous shrinkage
iv. Carbonation shrinkage
Plastic shrinkage
 Loss of water by evaporation from the surface of concrete is to be reasons of
plastic shrinkage
 Loss water results in the reduction of volume
 Water at the surface dries out the surface concrete collapses causing cracks

18. Prevention
 Plastic shrinkage can be reduced mainly by preventing the rapid loss of water
from surface
 This can be done by covering the surface with polyethylene sheeting
immediately on finishing operation
 Monomolecular coating by fog spray that keeps the surface moist and also use
of small quality of aluminium power
Drying shrinkage
 The loss of water contained in hardened concrete does not result in any
appreciable dimension change
 It is the loss of water held in gel pores that causes the change in the volume
 Under drying conditions the gel water is lost progressively over a long time as
long as the concrete is kept in drying conditions
Autogeneous shrinkage
 Where no moisture movement to or from the paste in permitted when
temperature is constant some shrinkage may occur
 The shrinkage of such conservative system is known as a autogeneous
shrinkage
 Autogeneous shrinkage is of minor importance is not applicable in practice to
many situations except that of mass of concrete in the interior of a concrete dam
Carbonation shrinkage
 Carbon dioxide present in the atmosphere reacts in the presence of water with
hydrated cement
 Carbon hydroxide gets converted to calcium carbonate and some other cement

19. compounds are decomposed