What is an
AGGREGATE?
AGGREGATE
Functions of Aggregates in Concrete
Strength & Durability: Aggregates act as the strong “backbone” of concrete, helping
it resist loads and making it more durable than just cement alone.
Cost-Effective Filler: Aggregates are cheaper than cement, so they fill up space and
reduce the overall cost of the mix.
Reduce Volume Changes: Aggregates help minimize cracking caused by cement
shrinking during hardening or changes in moisture.
FUNCTIONS OF AGGREGATES
3 maın functıons in concrete:
❖ To provide a mass of particles which are suitable
to resist the action of applied loads & show
better durability then cement paste alone.
❖ To provide a relatively cheap filler for the
cementing material.
❖ To reduce volume changes resulting from setting
& hardening process & from moisture changes
during drying.
CHARACTERISTICS OF QUALITY
AGGREGATE
❑ Must be clean & durable & Free from organic
impurities & dust (WHY?)
- It may prevent the cement paste from coating
the aggregate properly.
- Also preventing bonding.
- Also lowering the strength of the concrete.
REQUIREMENT OF GOOD AGGREGATES
It should be chemically inactive.
Chemically Stable:
➢ It should be sufficiently strong.
➢ It should be hard enough.
Strong & Hard
Durable
➢ It should be durable.
➢ It should have rough surface.
Rough Surface &
Proper Shape
➢ It should be of proper shape.
➢ It should have limited pores(porosity). Low
Porosity
DESIRABLE PROPERTIES OF
AGGREGATES
Should not break easily under heavy loads.
 1) Resistance to crushing .Abrasion Resistance: Can resist wearing or
(important for roads).
 2) Resistance to abrasion.grinding
Impact Resistance: Should not shatter when
hit.
 3) Resistance to impact. Good Shape: Round or angular aggregates
are better; avoid flaky or elongated shapes.
 4) Good shape.
Low Water Absorption: Less water
absorption means stronger concrete.
Weather Resistance: Should not crack or
 5) Water absorption.
break under extreme weather (like freezing
 6) Resistance to weathering. or thawing).
Good Bonding: Should stick well with cement
 7) Good adhesion.
for better strength.
Surface Texture: A rough surface is better for
 8) Surface texture.
bonding.
Low Porosity: Less space inside for water or
 9) Porosity.
air.
Surface Chemistry: Should not interfere with
 10) Surface chemistry. cement's bonding process
1. Unit Weight and Voids
The weight of aggregates in a certain volume, and how much empty space (voids) they
have.
2. Specific Gravity
How dense the aggregates are compared to water.
3. Particle Shape and Surface Texture
Affects the strength and workability of concrete.
4. Shrinkage of Aggregates
Aggregates help reduce concrete shrinking when it dries.
5. Absorption and Surface Moisture
Aggregates shouldn’t soak up too much water, as it affects the concrete mix.
6. Resistance to Freezing and Thawing
Should resist breaking due to water freezing and expanding inside.
Unit Weight
(Unit mass or bulk density)
The weight of the aggregate required to
fill a container of a specified unit
volume.
Definition: It is the weight of
aggregates needed to fill a
container of a specific volume,
including both the aggregate
particles and the voids
between them.
• Volume is occupied by both the
aggregates and the voids between the
aggregate particles.
• Depends on size distribution and shape of
particles and how densely the aggregate is
packed
o Loose bulk density
When aggregates are loosely packed.
o Rodded or compact bulk density
When aggregates are densely packed.
Normal-weight concrete… bulk density of aggregate is
approximately 75-110 lb per cubic foot.
Weight
Examples of
Uses for the Concrete
Aggregates Used
vermiculite, ceramic
can be sawed (cut/ sliced) or
nailed (attached/ secured),
also used for its insulating
properties
lightweight
expanded clay, shale or
slate, crushed brick
used primarily for making
lightweight concrete for
structures, also used for its
insulating properties
normal weight
crushed limestone,
sand, river gravel,
crushed recycled
concrete
used for normal concrete
projects
heavyweight
steel or iron shot
(small balls); steel or
iron pellets
used for making high density
concrete for shielding against
nuclear radiation
ultra-lightweight
Expanded Clay:
Made by heating clay to about 1,200°C.
During heating, gases form bubbles inside the
clay, making it expand.
This creates a lightweight material with a
honeycomb structure, useful as a lightweight
aggregate in construction.
Vermiculite has low thermal
conductivity, low moisture
content, acoustic-insulating
properties
EXPANDED CLAY
Expanded clay is a light weight aggregate made by heating
clay to around 1,200 °C. The yielding gases expand the clay
by thousands of small bubbles forming during heating
producing a honeycomb structure.
Shale is a sedimentary rock while slate is a
metamorphic rock formed from shale.
Shale: A sedimentary rock formed from compacted clay
and mud.
Slate: A metamorphic rock created when shale is exposed
to heat and pressure over time, making it harder and more
durable.
Density Classification of Concrete Aggregates
More voids mean more water and mortar are needed in the mix.
Angular aggregates create more voids than rounded ones.Coarse Aggregates: 30–45%
voids.Fine Aggregates: 40–50% voids.Using a mix of different-sized aggregates helps fill
voidsand reduces their total volume.
Voids
• Void content affects mortar requirements in mix design;
water and mortar requirement tend to increase as aggregate
void content increases.
• Void content between aggregate particles increases with
increasing aggregate angularity.
• Void contents range from 30-45% for coarse aggregates to
about 40-50% for fine aggregates.
• Total volume of voids can be reduced by using a collection of
aggregate sizes.
The cement paste requirement for concrete is proportional to the void
content of the combined aggregate.
Definition: Compares the weight of an object (like an aggregate) to the weight of an
equal volume of water.
Specific Gravity (Relative density)
• Ratio of the weight of an object
to the weight of an equal
volume of water (at standard
temperature and pressure).
Specific Gravity Use
Aggregate specific gravity is used in
a number of applications including
Superpave mix design, deleterious
particle identification and
separation, and material property
Use: Helps in mix design (e.g.,
change identification.
Superpave), identifying harmful particles,
and checking material property changes.
Specific Gravity (Relative density)
Bulk Specific Gravity
Bulk Specific Gravity:
Includes the aggregate and its permeable voids.
Compares the total weight (including water inside
the voids) to water.
The ratio of the mass of a unit volume of aggregate, including the
water permeable voids, at a stated temperature to the mass of an
equal volume of gas-free distilled water at the stated temperature.
Apparent Specific Gravity
The ratio of the mass of a unit volume of the impermeable portion
of aggregate (does not include the permeable pores in aggregate)
to the mass of an equal volume of gas-free distilled water at the
stated temperature.
Apparent Specific Gravity:
Includes only the solid, impermeable parts of the aggregate (ignores permeable
pores).
Compares this weight to water.
Rough, angular, or elongated particles need more water to make the mix workable.
Smooth, rounded particles are easier to work with and need less water.
Particle Shape and Surface Texture
• Rough textured, angular, elongated particles require more water to
produce workable concrete than do smooth, rounded, compact aggregates.
Flat & Elongated Particles: Should be limited to 15% of total aggregate weight, as they can
weaken concrete if too many are used.
• Aggregates should be relatively free of flat and elongated particles (limit
to 15% by weight of total aggregate).
• Important for coarse and
crushed fine aggregate
these require an increase in
mixing water and may affect
the strength of the concrete,
if cement water ratio is not
maintained.
Particle Shape
Sphericity is a measure of how spherical (round) an object is. Sphericity is
defined as a function of the ratio of the surface area of the particle to its
volume.
Particles with a high ratio of surface area to volume, increase the water
demand for a given workability of the concrete mix.
Particles with high surface area compared to volume need more water for the same workability, as
more water is needed to coat the particles.
Particle Surface
SMOOTH
ROUGH
The surface texture of aggregate can be either smooth or rough.
A smooth surface can improve workability, yet a rougher surface
generates a stronger bond between the paste and the aggregate
creating a higher strength.
Shrinkage of Aggregates
Large Shrinkage = fine grained Sandstones, Slate, Basalt, Trap Rock
etc.
Sandstones
Low Shrinkage =
Granite
Slate
Basalt
Trap Rock
Quartz, Limestone, Granite, Feldspar
Feldspar
Quartz
Limestone
What happens if
abnormal aggregate
shrinkage occurs?
Problems of Shrinkage
• Excessive cracking
Shrinkage can cause cracks in the concrete.
• Large deflection of reinforced beams and slabs
Reinforced beams and slabs may bend more than expected.
• Some spalling (chipping or crumbling)
Pieces of concrete may chip or crumble.
If shrinkage is more than
0.08%, the aggregate isn't
suitable for concrete.
If more than 0.08 percent shrinkage occurs, the aggregate is
considered undesirable.
Absorption and Surface Moisture
If water content of the
concrete mixture is not kept
constant, the compressive
strength, workability, and
other properties will vary
from batch to batch.
If the water content in the mix isn't consistent, it will affect:
Compressive Strength: Concrete may become weaker.
Workability: The mix may become too stiff or too runny.
Batch Quality: Each batch may have different properties,
leading to inconsistency.
Moisture Conditions of Aggregates
completely dry).
1. Oven dry- all moisture removed i.e. fully absorbent.
2. Air dry- surface moisture removed i.e. dry at the
particle surface but containing some interior moisture.
Surface is dry, but some moisture is inside the particles.
3. Saturated, surface dry (SSD) –surface moisture
removed, all internal pores full. Neither absorbing
water nor contributing water to the concrete mixture.
Surface is dry, but all pores inside are completely filled with water.
4. Wet or moist- pores full containing an excess of
moisture on the surface.
Pores are full, and there’s extra water on the surface.
Moisture Conditions of Aggregates
Absorption and Surface Moisture
• Absorption Capacity (AC) -- maximum amount of water the
aggregate will absorb. The range for most normal-weight
aggregates is 1 - 2%.
Absorption Capacity = (WSSD - WOD) / WOD x 100%
•
Surface Moisture (SM) -- amount of water in excess of SSD
Surface Moisture = (W wet - WSSD) / WSSD x 100%
The moisture content (MC) of aggregate is given by:
Moisture Content = (W stock - WSSD) / WSSD x 100%
Resistance to Freezing and Thawing
•
Important for exterior concrete.
•
Affected by an aggregate's high porosity, absorption,
permeability and pore structure.
• If aggregates or concrete absorbs so much water that when
the water freezes and expands the concrete cannot
accommodate the build up of internal pressure, pop–outs may
occur.
1 According to source or nature of formation.
2 According to size .
3 According to shape.
4 According to unit weight.
Natural Aggregates:
:Found in natural deposits like sand and gravel.Also obtained by cutting rocks from quarries.
Types of Rocks for Aggregates:
Igneous Rocks:Formed from cooled lava or magma.Provide high-quality aggregates that meet
most requirements.
Sedimentary Rocks:Formed from layers of compressed sand, silt, or clay.Often give good-quality
aggregates.
Metamorphic Rocks:Formed when existing rocks change under heat and pressure.
Not suitable for aggregates because they often have thin, weak layers
• Natural Aggregates
• Artificial Aggregates
➢ Main sources of these aggregate are natural
deposits of sand ,and gravel or obtained from
quarries by cutting rocks .
➢ The rocks are generally of three types i.e.
Igneous, Sedimentary and Metamorphic rocks.
➢ The aggregates obtained from Igneous rocks
match the requirements to a great extent.
➢ Also sedimentary rocks provide good quality of
aggregates.
➢ Metamorphic rocks are not used for aggregates
because of formation of thin structure which are
not desirable.
➢
These aggregates are generally obtained from
natural aggregates
by performing certain
chemical reactions , applying force, heating the
natural aggregates etc.
➢ For example -broken bricks, fly ash (Fly ash is one
of the coal combustion products, composed of the fine particles that are
, blast- furnace
driven out of the boiler with the flue gases.)
slag [obtained by extinguishing molten iron slag (Slag is the glass-like byproduct left over after a desired metal has been separated (i.e., smelted) from
its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide) from
a blast furnace in water or steam, to produce a glassy, granular product that is
then dried and ground into a fine powder.)
etc.
Blast- Furnace Slag
According To Size
•
Artificial Aggregates:
Made by modifying natural materials using
processes like chemical reactions, heating,
or applying force.
Examples:
Broken Bricks: Crushed bricks used as
aggregates.
Fly Ash: A fine powder from coal-fired
power plants.
Blast Furnace Slag:
By-product of extracting metal from ore in a
blast furnace.
Slag is cooled quickly with water or steam
to form a glassy, granular material, then
ground into a fine powder.
Size of Aggregates
• Coarse aggregate e.g.
Gravel – 4.76mm or
more (5mm)
• Fine aggregate e.g.
sand – less than
4.76mm
Fine Aggregates
➢ Natural sand, crushed rock, crushed gravel that pass through
5mm Sieve (< 5 mm )
➢ “Sharp” sand has angular
grains – used for concrete.
➢ “Soft” sand has smaller
rounded grains – used
for mortars & renderings.
➢ F.A. content usually 35%
to 45% by mass or volume
of total aggregates.
Coarse Aggregate
• The residue of the 5 mm sieve and
passed through 75 mm sieve ( 5
mm).
• Coarse consists of crushed gravel,
uncrushed gravel and partially
crushed, uncrushed,
crushed gravel. Includes
or partially crushed gravel.
• By natural disintegration of rocks
or by artificial crushing of rocks or
Sourced from natural rock
gravel. disintegration or artificial crushing.
• Typically between 9.5 and 37.5
mm.
Typical sizes: 9.5–37.5 mm.
10 mm: Used for small sections with
• 10mm for small section work with
dense reinforcement.
a lot of close reinforcement. 20mm 20 mm: Common for general
construction work.
for general work.
❖ All-in-aggregates are combine mixture of
fine aggregates and coarse aggregates .
❖ It contains various fractions of fine and
coarse aggregates.
❖ For High quality concrete these type of
aggregates are used.
• Rounded Aggregates
• Irregular Aggregates
• Angular Aggregates
• Flaky and elongated Aggregates
Aggregate Shape
Rounded:
Fully smooth and water-worn (e.g., river gravel).Requires less cement for good concrete.Has
33–35% voids.High workability but weak interlocking, reducing structural strength.
➢ Rounded: Completely water worn & fully shaped by
attrition. (River Gravel)
➢ Irregular: Partly shaped by attrition so it contains
some rounded edges. (Land Gravel)
Partially smooth with some rounded edges (e.g., land gravel).
Aggregate Shape
➢ Angular: Has sharp corners, show little evidence
of wear. (Crushed Stone)
Provides better interlocking and strength.
➢ Flaky: Thickness is relatively small with respect to
two other dimensions.
➢ Elongated: Have lengths considerably larger than
two other dimensions.
L
t
w
➢ In rounded aggregates less amount of cement is
required for preparing concrete of good quality.
➢ There are 33% to 35% voids in rounded aggregates .
➢ Due to smooth and rounded structure there is poor
locking among or between the aggregates and thus
reducing strength of structure but they provide high
workability.
➢ These aggregates vary in surface as compared to
rounded aggregates.
➢
They are not perfectly round . They have 35 to 37%
perfectly round, with 35–37% voids.Require more cement
of voids.Not
paste due to larger surface area.Provide better interlocking
than rounded aggregates, but still not ideal.
➢
Because of more surface area they require more
cemented paste.
➢ The interlocking provided by these aggregate is not
up to the required level but better than rounded
aggregates.
❑ These aggregates have angular structure i.e. having
shape and well defined edges, and are rough particles.
❑ Voids percentage lies in the range of 37 to 40 %.
❑ Due to rough and angular structure the interlocking is
quite good and also required more cement paste for a
workable concrete with high strength.
❑ Crushed stone is an example.
Sharp edges and rough surfaces (e.g., crushed stone).
Voids: 37–40%.Excellent interlocking for high-strength concrete but need more
cement paste for workability.
Flaky And Elongated
Aggregates
➢ The flaky aggregates are those in which the least dimension of
the aggregates is less than 3/5 the times the mean dimensions.
➢ Mean dimensions is the average size of sieves from which the
aggregates passes or on which they are retained.
➢ The percentage of voids are high for these aggregates.
➢ Elongated aggregates can be defined as those aggregates which
has its length or greatest dimension , greater than 9/5 of its
mean dimension .
Surface Texture
SMOOTH
Flaky Aggregates:
Thin aggregates where the smallest
dimension is less than 3/5 of the
average size.
High void content.
ROUGH
Elongated Aggregates:
Long aggregates where the largest
dimension is more than 9/5 of the
average size.
Also have high void content.
Surface Texture
➢ This affects the bond to the cement paste & also
influences the water demand of the mix.
Smooth: Bond b/w cement paste & agg. is weak.
➢
Rough: Bond b/w cement paste & agg. is strong.
➢ Surface texture is not a very important property from
compressive strength point of view but agg. having
rough surface texture perform better stresses.
importance:
Affects the bond with cement paste.
Influences the water demand in the mix.
Types:
Smooth Surface:Weak bond with cement paste.
Rough Surface:Stronger bond with cement paste.
Impact on Strength:Surface texture has less impact on compressive strength,
but rough aggregates perform better under stress.