Aggregates are materials such as sand, gravel, crushed stone and recycled concrete that are mixed with cement and water to form concrete. There are various types of aggregates classified based on grain size, density, geographical origin and shape. Fine aggregates are smaller than 4.75mm while coarse aggregates are larger. Aggregates provide properties like volume, stability and resistance to wear or erosion in concrete. Admixtures are added to concrete to improve properties during casting, setting or service and include chemicals to improve workability or minerals to reduce water requirements.

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BUILDING MATERIAL
SCIENCE-3
ASSIGNMENT -2
AGGREGATES AND ADMIXTURES
AGGREGATES
● WHAT ARE AGGREGATES?
● Aggregate​, in building and construction, material used for mixing with cement,
bitumen, lime, gypsum, or other adhesive to form concrete or mortar.

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● The ​aggregate​ gives volume, stability, resistance to wear or erosion, and other
desired physical properties to the finished product​.
ORIGIN OF AGGREGATES
Fine aggregates are obtained from a variety of sources. The sources of aggregate are invariably
close to their demand locality; It is difficult to transport the large quantity of aggregate (in
tonnes) and there will be high cost of transportation. They can be sourced from pits, riverbanks
and beds, the seabed, gravelly or sandy terraces, beaches and dunes. The other deposits that
provide granular materials can be processed with minimal extra effort or cost. Sand and gravel,
which are unconsolidated sedimentary materials, are important sources of natural aggregate.
The occurrence of high quality natural sands and gravels with in economic distance of major
urban areas may be critical for viable concrete construction in those areas.
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MANUFACTURING PROCESS
● The first stage of aggregate processing involves quarrying where a large deposit
of desirable aggregate is identified and extracted from the ground. Hardened
substances are either blasted or cut into smaller manageable-sized rubble and
transported by truck or conveyor belts to the processing area.
● At the processing site, the aggregates are fed through a crusher. In most cases,
a plant will have a primary crusher, which will reduce the size of the quarried
rubble down to chunks about six-inches in circumference.
● The product from the primary crusher is then screened for larger usable
products. The remaining material is transported by a conveyor belt to a second
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V​ibratory feeder for secondary crushing and screening which is reducing and
separating the aggregate into some more usable products.
● Finally, after tertiary crushing, the ​Aggregate Screen​ — with single, double, and
triple deck configurations — allows for the quick identification and separation of
variously sized aggregate for use in concrete and asphalt.
CLASSIFICATION OF AGGREGATES
❖ Classification of aggregates based on: Grain Size
❖ Classification of aggregates based on: Density
❖ Classification of aggregates based on: Geographical Origin
❖ Classification of aggregates based on: Shape
● Classification of aggregates based on: Grain Size
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If you separate aggregates by size, there are two overriding categories:
● Fine
● Coarse
The size of ​fine aggregates​ is defined as 4.75mm or smaller. That is, aggregates
which can be passed through a number 4 sieve, with a mesh size of 4.75mm. Fine
aggregates include things such as sand, silt and clay. Crushed stone and crushed
gravel might also fall under this category.
Typically, fine aggregates are used to improve workability of a concrete mix.
Coarse aggregates​ measure above the 4.75mm limit. These are more likely to be
natural stone or gravel that has not been crushed or processed. These aggregates will
reduce the amount of water needed for a concrete mix, which may also reduce
workability but improve its innate strength.
● Classification of aggregates based on: Density
There are three weight-based variations of aggregates:
● Lightweight
● Standard
● High density
Different density aggregates will have much different applications.​ Lightweight and
ultra lightweight aggregates​ are more porous than their heavier counterparts, so
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they can be put to great use in green roof construction, for example. They are also used
in mixes for concrete blocks and pavements, as well as insulation and fireproofing.
High density aggregates​ are used to form heavyweight concrete. They are used for
when high strength, durable concrete structures are required – building foundations or
pipework ballasting, for example.
● Classification of aggregates based on:
Geographical Origin
Another way to classify aggregates is by their origin. You can do this with two groups:
● Natural
● Processed
​Aggregates taken from ​natural ​sources, such as riverbeds, quarries and
mines. Sand, gravel, stone and rock are the most common, and these can be
fine or coarse.
Artificial aggregates​ or ‘by-product’ aggregates, they are commonly taken
from industrial or engineering waste, then treated to form construction
aggregates for high quality concrete. Common processed aggregates include
industrial slag, as well as burnt clay. Processed aggregates are used for both
lightweight and high-density concrete mixes.
● Classification of aggregates based on: Shape
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Shape is one of the most effective ways of differentiating aggregates. The shape of your
chosen aggregates will have a significant effect on the workability of your concrete.
Aggregates purchased in batches from a reputable supplier can be consistent in shape,
if required, but you can also mix aggregate shapes if you need to.
The different shapes of aggregates are:
● Rounded​ ​– ​Natural aggregates smoothed by weathering, erosion and
attrition. Rocks, stone, sand and gravel found in riverbeds are your most
common rounded aggregates. Rounded aggregates are the main factor
behind workability.
● Irregular –​ ​These are also shaped by attrition, but are not fully rounded.
These consist of small stones and gravel, and offer reduced workability to
rounded aggregates​.
● Angular –​ ​Used for higher strength concrete, angular aggregates come in
the form of crushed rock and stone. Workability is low, but this can be
offset by filling voids with rounded or smaller aggregates.
● Flaky –​ Defined as aggregates that are thin in comparison to length and
width. Increases surface area in a concrete mix.
● Elongated – ​Also adds more surface area to a mix – meaning more
cement paste is needed. Elongated aggregates are longer than they are
thick or wide.
● Flaky and elongated – ​A mix of the previous two – and the least
efficient form of aggregate with regards to workability.
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PROPERTIES OF AGGREGATES
But there are properties or characteristics of aggregate which influence the properties of
resulting concrete mix. These are as follows-
1. Composition
2. Size & Shape
3. Surface Texture
4. Specific Gravity
5. Bulk Density
6. Voids
7. Porosity & Absorption
8. Bulking of Sand
9. Fineness Modulus of Aggregate
10.Surface Index of Aggregate
11.Deleterious Material
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12.Crushing Value of Aggregate
13.Impact Value of Aggregate
14.Abrasion Value of Aggregate
Maximum size of aggregate should be less than-
● One-fourth of the minimum dimension of the concrete member.
● One-fifth of the minimum dimension of the reinforced concrete member.
● The minimum clear spacing between reinforced bars or 5 mm less than the minimum
cover between the reinforced bars and form, whichever is smaller for heavily reinforced
concrete members such as the ribs of the main bars.
Depending upon the amount of moisture content in aggregates, it can exist in any
of the 4 conditions-
● Very dry aggregate ( having no moisture)
● Dry aggregate (contain some moisture in its pores)
● Saturated surface dry aggregate (pores completely filled with moisture but no moisture
on surface)
● Moist or wet aggregates (pores are filled with moisture and also having moisture on
surface)
Aggregates should not contain any harmful material in such a quantity so as to
affect the strength and durability of the concrete. Such harmful materials are
called deleterious materials. Deleterious materials may cause one of the following
effects-
● To interfere hydration of cement
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● To prevent development of proper bond
● To reduce strength and durability
● To modify setting times
Deleterious materials generally found in aggregates, may be grouped as under
● Organic impurities
● Clay , silt & dust
● Salt contamination
APPLICATIONS OF AGGREGATES
● They are added to concrete mixes to keep cement costs down and to reduce the
shrinkage and cracking that is often found in concrete structures.
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● They are used to line railroad tracks, helping carry the load of trains and
providing better drainage of water away from the rails.
● They fill in voids around the foundations of homes and to protect underground
pipes that are located beneath homes and other structures.
● They are used in the manufacture of some brands of cat litter.
● They are included in roofing shingles to make them durable enough to stand up
to the weather.
● They are used to build roads, with about 30,000 tons of aggregates needed to
build one mile of a four-lane highway.
● They are incorporated in landscaping designs to create drainage for plant life and
to boost the aesthetic appeal of homes and businesses.
● They are used when creating bricks for use in construction, helping to make them
stronger.
● Crushed limestone is used at power plants to reduce the high levels of sulfur
dioxide emissions.
● They are used to provide the minerals that are essential for creating cosmetics,
soaps, fertilizers, glues and many other everyday products.
ADMIXTURES
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● Admixtures are artificial or natural materials added to the concrete besides
cement, water and aggregate to improve certain ​properties ​of concrete during
casting or setting or service stage.
● To improve the properties of the concrete required. It can be divided in 2 groups
that is:
1. Chemical
2. Mineral
● Major functions and advantages of using
admixtures:
● To improve workability of ​fresh concrete
● To improve ​durability ​by entrainment of air
● To reduce the water required
● To accelerate setting & hardening & thus to produce high early strength
● To aid curing
● To impart water repellent / waterproofing property
● To cause dispersion of the cement particles when mixed with water
● To retard setting
● To improve wear resistance (hardness)
● To offset / reduce ​shrinkage ​during setting & hardening
● To cause expansion of concrete and automatic prestressing of steel.
● To aerate mortar / concrete to produce a light-weight product
● To impart colour to concrete
● To offset or reduce some chemical reaction
● To reduce bleeding
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● To reduce the evolution of heat
● TYPES OF CHEMICAL ADMIXTURES
Additives are used in concrete mixes to enhance its properties. Depending on the
specific function or action on the concrete property, it can be classified into following:
● Accelerator
● Water reducing ​Admixture
● Superplasticizer
● Air Entraining
● Retarding
● Corrosion ​Inhibitors
● Alkali-Aggregate Reaction Inhibiting
● Shrinkage Reducing
Accelerator
● ​Added to increase the rate of hydration of concrete mix which then
lead to the increase in the rate of development of strength and
● ​greater heat evolution.
● Shorten the setting time.
● ​More rapid gain of strength & rapid setting.
● ​Disadvantages is possible cracking due to heat evolution & possibility
of corrosion of embedded reinforcement.
● Water reducing ​Admixture​/Plasticizers
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● ​Used to reduce the amount of water necessary to produce a
concrete of a given consistency
● ​To increase the slump for a given water content
● ​To obtain specified strength at lower cement content
● ​Increases workability with faster gain of strength
● ​Increase the slump, reduce water content, save cement
● ​Disadvantages, it has the risk of corrosion.
​Superplastizers
● ​ ​By adding to a hydraulic binder, gives very high workability and
allows a large decrease in water content for a given workability
● ​Allows the particles to be more workable where it enable working
with low w/c ratio
● ​Enhances hydration process, increases strength
● ​Eliminate concrete segregation & allow good dispersion of cement
particles in water,accelerating the rate of hydration
● ​Uniform distribution of cement particles is partly responsible for the
highly early strength in concrete made with superplasticizer
● ​Advantages of using Superplasticizers are
a)​ ​Decreased time to place and finish
b)​ ​Accelerated curing period
c)​ ​Early removal of formwork
● ​Excessive dosage may render concrete too fluid, causing severe
segregation.
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Secondary case study
Admixtures-
Crushed stone, for construction.
rs 675/ton
ECA aggregates
rs 600/litre
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Expanded clay aggregate natural brown aggregates hydroclay.
rs690/ bag
Whitish limestone chips, packaging types.
rs5.3/ kilogram
Admixtures-
Perma concrete admixtures, packaging
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rs78/kilogram
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