Classification of Aggregates
Mechanical properties of aggregates
Objectives:


To explain the following classifications of aggregates:
o
Size classification
o
Petrographic classification
o
Shape and texture classification
To explain the following mechanical properties of aggregates:
o
Bond
o
Strength
o
Toughness
o
Hardness
CLASSIFICATION OF AGGREGATE
1. Size Classification
Based on the size of the particles of aggregates, following classification is made:

Fine aggregate: aggregate particles smaller than 5 mm (3/16 in.) or No.4 ASTM
sieve size

Coarse aggregate: aggregate particles equal to or larger than 5 mm (3/16 in.) or
No.4 ASTM sieve size
2. Petrographic (i.e. Geological) Classification

From the petrological standpoint, aggregates can be classified into several groups
of rocks having common characteristics, as classified by BS 812: Part 1, and
presented in the following table:

Petrographic examination of aggregates is useful for the following purposes:
o Assessing quality of aggregate
o Comparing a new aggregate with an aggregate which quality is already
known
o Detecting the adverse properties of aggregate such as the presence of some
unstable forms of silica
Table 3.1: Rock type classification of natural aggregates according to BS 812: Part1
: 1975
Basalt Group
Andesite
Basalt
Basic porphyrites
Diabase
Dolerites of all kinds
including theralite
and teschenite
Epidiorite
Lamprophyre
Quartz-dolerite
Spilite
Granite Group
Gneiss
Granite
Granodiorite
Granulite
Pegnatite
Quartz-diorite
Syenite
Limestone Group
Dolomite
Limestone
Marble
Schist Group
Phyllite
Schist
Slate I
All severely sheared
rocks
Flint Group
Chert
Flint
Gritstone Group
(including fragmental
volcanic rocks)
Arkose
Greywacke
Grit
Sandstone
Tuff
Porphyry Group
Aplite
Dacite
Felsite
Granophyre
Keratophyre
Microgranite
Porphyry
Quartz-porphyrite
Rhyolite
Trachyte
Gabbro Group
Basic diorite
Basic gneiss
Gabbro
Hornblende-rock
Norite
Peridotite
Picrite
Serpentinite
Hornfels Group
Contact-altered rocks
of all kinds except
marble
Quartzite Group
Ganister
Quartzitic sandstones
Re-crystallized
quartzite
3. Shape and Surface Texture Classification
A broad classification of aggregates on the basis of shape is presented in the
following table, as given by BS 812: Part 1:
Table 3.2: Particle shape classification of aggregates according to BS 8U: Part 1:
1975; with examples –
Classification
Rounded
Irregular
Flaky
Angular
Elongated
Flaky and elongated
Description
Fully water-worn or
completely shaped by
attraction
Naturally irregular, or partly
shaped by attrition and
having rounded edges
Material of which the
thickness is small relative to
the other two dimensions
Possessing well-defined
edges formed at the
intersection of roughly
planar faces
Material, usually angular, in
which the other two
dimensions
Material having the length
considerably larger than the
width, and the width
considerably larger than the
thickness
Examples
River or seashore gravel;
desert, seashore and wind
blown sand
Other gravels; land or dug
flint
Laminated rock
Crushed rocks of all types;
talus; crushed slag
A shape classification sometimes used in USA is as follows:





Well rounded: no original face left
Rounded: faces almost gone
Subrounded: considerable wear, faces reduced in area
Subangular: some wear but faces untouched
Angular: little evidence of wear
-
-
Following are the terms related to the shape of aggregate:
i)
ii)
iii)
iv)
v)
vi)
vii)
Angularity number
Sphericity
Mean size
Elongated particle
Flaky particle
Elongation index
Flakiness index
(i) Angularity number

Rounded gravel particles possess lesser voids (mostly 33%, i.e. 67% solids, by
volume) as compared to the angular particles

Angularity number measures the percentage of voids in angular particles in excess
of that in the rounded gravel particles

Angularity number
= % of solid volume in a vessel filled with aggregate in a standard manner - 67
(i.e. % volume of solids of the rounded gravel)

The higher the angularity number, the more angular the aggregate.

The range of angularity number for practical aggregates is between 0 and 11
(ii) Sphericitv

Sphericity is defined as a function of the ratio of the surface area of the particle to
its volume.

Sphericity is influenced by the following:
o
o
bedding and cleavage (i.e. split) of the parent rock
types of crushing equipment

Higher sphericity lowers the workability of the concrete. Higher sphericity also
adversely affects the durability of concrete.

Elongated and flaky particles are found to have higher degree of sphericity and
therefore their presence in excess of 10 to 15% of the mass of coarse aggregate is
generally considered undesirable
Typical shapes of aggregates.
(iii) Mean size

The mean size of an aggregate particle is defined as the arithmetic mean of the
sieve size on which the particle is just retained and the sieve size through which
the particle just passes.
(iv) Flaky particle

An aggregate particle is said to be flaky if its thickness (least dimension) is less
than 0.6 times its mean size
(v) Elongated particle

An aggregate particle is said to be elongated if its length (largest dimension) is
more than 1.8 times its mean size
(vi) Flakiness index

Flakiness index is the mass of flaky particles, expressed as a percentage of the
mass of aggregate sample

As per the BS 882:1992, the flakiness index of the combined coarse aggregate
should be less than 50 for uncrushed gravel and should be less than 40 for crushed
rock or crushed gravel
(vii) Elongation index

Elongation index is the mass of elongated particles, expressed as a percentage of
the mass of aggregate sample
The classification of the surface texture is based on the degree to which the particle
surfaces are polished or dull, smooth or rough
The classification of aggregates based on their surface texture, as given by BS 812:
Part 1, is presented in the following table:
Table 3.3: Surface texture classification of aggregates according to BS 812: Part 1:
1975 with examples
Group
_______
Surface Texture
________________
Characteristics
__________________
Examples
_______________
1
Glassy
Conchoidal fracture
Black flint,
vitreous slag
2
Smooth
Water-worn, or smooth
due to fracture of
laminated or fine-grained
rock
Gravels, chert,
slate, marble,
some rhyolites
3
Granular
Fracture showing more
or less uniform rounded
grains
Sandstone, oolite
4
Rough
Rough fracture of fine
or medium-grained rock
containing no easily
visible crystalline
constituents
Basalt, felsite,
porphyry,
limestone
5
Crystalline
Containing easily visible
Granite, gabbro,
6
Honeycombed
crystalline constituents
gneiss
With visible pores and
cavities
Brick, pumice,
foamed slag,
clinker, expanded
clay
Effect of the shape and surface texture of aggregate:

The shape and surface texture of aggregate, especially of fine aggregate, have a
strong influence on the water requirement of the mix

More water is required when there is a greater void content of the loosely-packed
aggregate

Generally, flakiness and shape of the coarse aggregate have an appreciable effect
on the workability of concrete

The workability decreases with an increase in the angularity number
MECHANICAL PROPERTIES OF AGGREGATES
1. Bond
The bond between the surface of the aggregate particles and cement matrix is a decisive
factor for the strength of concrete

Both the shape and surface texture of aggregate influence considerably the bond
and therefore the strength of concrete

A rougher texture results in a greater adhesion or bond between the particles and
the cement matrix

The larger surface area of a more angular aggregate provides a greater bond.
However, workability is reduced.

Softer, porous and mineralogical heterogeneous particles, allowing penetration by
the paste, possess a better bond than those textures which do not permit the paste
penetration

There is no test for determining the quality of bond. However, as a thumb rule, the
bond is said to be good if a crushed concrete specimen contain some aggregate
particles broken right through
2. Strength
The compressive strength of aggregate is generally more than that of the concrete
Following indirect tests are conducted to determine the strength of aggregate:
(i)
Crushing strength test on prepared rock samples
(ii)
Aggregate crushing value (ACV) test on aggregate particles
(i) Crushing strength test on a prepared rock sample

Tests on prepared rock samples are rarely used in practice

Average value of crushing strength of a rock sample is about 200 MPa (30,000
psi)

However, excellent rock for aggregate may have crushing strength of 80 MPa
(12,000 psi)
(ii) Aggregate crushing value (ACV) test on aggregate

ACV test (prescribed by BS 812: Part 110) provides useful information pertaining
to the strength of aggregate

The aggregate to be tested should pass a 1/2in. sieve and should be retained on
3
/8in. sieve

In this test, the aggregate sample is crushed using a compression testing machine
through which a pressure of 3200 psi is applied for a period of 10 min

After releasing the load, the crushed sample is sieved through 2.36 mm (No.8
ASTM) sieve

The ACV is taken as the percentage of powder passing through 2.36 mm sieve
(expressed as a fraction of the total weight of the sample)

A higher ACV is an indication of a lower compressive strength of aggregate

For ACVs greater than 25 to 30, this test is insensitive to the variation in strength
of weaker aggregates

For this reason, ten percent fines value test is used
3. Toughness

Toughness can be defined as the resistance of aggregate to failure by impact

Toughness of aggregate is determined by aggregate impact value (AIV) test,
which is similar to the ACV test with only difference that the load applied is
impact

The impact is provided by a standard hammer falling 15 times under its own
weight upon the aggregate in a cylindrical container

Following are the recommended AIVs:
o 25% when the aggregate is to be used in heavy duty concrete floor
finishes
o 30% when the aggregate is to be used in concrete pavement wearing
surfaces
o
45% when to be used in other concrete
4. Hardness

Hardness, or resistance to wear, is an important property of concrete used in
roads and floor surfaces subjected to heavy traffic

Hardness is expressed in terms of aggregate abrasion value of the bulk
aggregate, determined using the Los Angeles test

More will be the abrasion value, less will be the aggregate hardness