Chapter - 2
CLASSIFICATION AND PROPERTIES OF COARSE
Aggregate is a granular material such as sand, gravel, crushed stone or iron
blast-furnace slag, used with a cementing medium to form hydraulic-cement concrete or
mortar [ASTM C 125].
2.2.1 Classification Based on Particle Size Distribution. Aggregate particle
size varies over a large range i.e. from a few micrometers to tens of millimeters
(say 50 mm or 2 inches). The particle size distribution of aggregate is called
“Grading” or “Gradation”. For the purpose of use in concrete, aggregate is divided
in two size groups as under:-
22.214.171.124. Coarse Aggregate (CA). Aggregate predominantly retained on
the sieve No. 4 (4.75-mm) is called coarse aggregate.
126.96.36.199. Fine Aggregate (FA). Aggregate completely passing the 3/8
inches (9.5-mm) sieve and almost entirely passing the sieve No.4 (4.75-
mm) and predominantly retained on the sieve No.200 (75-μm); OR that
portion of an aggregate passing the sieve No.4 (4.75-mm) and retained on
the sieve No. 200 (75-μm) [ASTM C 125].
2.2.2 Classification Based on Source/Origin. Aggregates may be broadly
classified as natural or artificial, both with respect to source and to method of
preparation [ACI Education Bulletin E1-07].
188.8.131.52. Natural Aggregate. These aggregates are generally
obtained from natural sources, deposited as products of weathering
through the action of wind or water. However, the most commonly used
type is produced from quarries by cutting or blasting the rocks, which are
then reduced to size by crushing and screening.
184.108.40.206. Artificial Aggregate. Also known as manufactured aggregate,
are produced from a range of natural and/or artificial substances through
an industrial process involving chemical reactions and fusion etc. These
are often known by a variety of trade names but are best classified on the
basis of raw material used or by the name of their intended purpose, e.g.
heavy or lightweight aggregate. Vermiculite and Perlite are the example of
lightweight aggregates produced from the heating of naturally existing
vermiculite and perlite (a glassy volcanic rock). These aggregates are
specified in ASTM C 332-99. Other lightweight aggregates are produced
from industrial byproducts such as fly ash and blast furnace slag. The
requirements of lightweight aggregate are spelled out in ASTM C 330-02a.
Artificial aggregate falls beyond the scope of this document.
220.127.116.11. Recycled Objects. The most common materials used as
recycle coarse aggregate are the wastes of old concrete pavements and
structures and brick bats. Extensive research has gone in both the
materials and some of the major conclusions are:-
(1) Recycled Concrete Waste. New concrete prepared with crushed
old concrete as coarse and fine aggregate, has been found to have
the following characteristics [ACI Education Bulletin E1-07 and
Shuaib et. al]:-
(a) Low specific gravity (2.2 to 2.5 in SSD condition).
(b) High absorption (2 to 6 %).
(c) Loss of slump and workability.
(d) Decreased unit weight due to low specific gravity.
(e) Decrease in compressive, flexural, splitting tensile strengths
and modulus of elasticity.
(f) Higher bond strength.
(g) Relationships between various mechanical properties of
virgin aggregate concrete are not applicable concrete made
with recycled to aggregate.
(2) Brick Bat. Broken bricks, a waste product of rejected overburnt
or damaged bricks found in brick kilns and construction sites can be
used as coarse aggregate in areas deficient of such resources or
for the purpose of producing light weight concrete [Khan, 1987].
Important characteristics of this ingredient and the concrete
produced thereof are:-
(a) Low specific gravity (Maximum 2.0).
(b) Low bulk density (approx 70 pcf).
(c) High water absorption (above 10 %).
(d) Low crushing strength (2900-4350 psi).
(e) Reduced unit weight of concrete (105-135 pcf).
(f) Different relationship between mechanical properties than
virgin aggregate concrete.
2.2.3 Classification Based on Geological Origin. From petrological
perspective, the natural aggregate, whether crushed or reduced to size naturally,
can be divided into several groups of their geological modes of formation, i.e.
Sedimentary, Igneous or Metamorphic. Each group is further divided into
categories having common petrological characteristics. ASTM C 294 provides
brief descriptions of some commonly occurring or more important natural and
artificial materials, of which mineral aggregates are composed.
Mineralogical classification is of help in recognizing properties of
aggregate, but does not by itself make an aggregate suitable or unsuitable for
use in concrete [ACI Education Bulletin E1-07]. The acceptance of an aggregate
for use in concrete on a particular job or in meeting a particular specification
should be based upon specific information obtained from tests used to measure
the aggregate’s quality or, more importantly, its service record, or both. ASTM
classification of rocks and minerals is discussed in chapter xxxx.
2.3. Properties of Coarse Aggregate
For convenience, the properties of coarse aggregate can be classified into two
major groups as Physical/Mechanical Properties & Chemical/ Mineralogical
Properties. This classification will be followed in describing the properties of aggregate,
their effects upon concrete properties and the tests conducted to determine these
2.4. Physical/Mechanical Properties
2.4.1 Size and Gradation. An aggregate particle must first be specified by its
size followed by other characteristics such as shape, texture, mineralogy etc.
However, since an aggregate sample may contain more than one size, the
quantity of each size, by mass or sometimes even volume, need be known.
Hence the distribution of particles into fractions according to the particle size in a
sample is called gradation. Gradation is determined using sieve analysis test as
described in ASTM C 136-05 or BS 812: section 103.1.Various terminologies
related to gradation are as under:-
18.104.22.168. Well-graded. It refers to the aggregate in which the particle size
distribution is such that voids created by large size particles are filled by
the next smaller size and so on. In other words, the gradation which gives
the maximum density is well-graded.
22.214.171.124. Uniformly graded. Material that contains most of the particles in
a very narrow size range. In essence, all the particles are the same size.
The gradation curve is steep and only occupies the narrow size range
126.96.36.199. Open graded. Material having only a small percentage of
aggregate particles in the small range. This results in more air voids
because there are not enough small particles to fill in the voids between
the larger particles. The curve is near vertical in the mid-size range and
flat or near-zero in the small-size range.
188.8.131.52. Gap graded. A sample with only a small percentage of
aggregate particles in the mid-size range. The curve is flat in the mid-size
range. Some PCC mix designs use gap graded aggregate to provide a
more economical mix since less sand can be used for a given workability.
184.108.40.206. Fineness Modulus. To characterize the overall coarseness or
fineness of aggregate and to express these by a single number, the idea
of fineness modulus is used. It is defined as the accumulative percentage
retained on specified sieves (No.100, 50, 30, 16, 8, 4, 3/8", 3/4", 1.5", 3",
and 6"), divided by 100.
2.4.2 Maximum Size and Nominal Maximum Size. ASTM C-125-00 defines
the two terms as following:-
220.127.116.11. The smallest sieve opening through which the entire amount of
aggregate is required to pass is called the maximum size.
18.104.22.168. The smallest sieve opening through which the entire amount of
aggregate is permitted to pass is called the nominal maximum size.
22.214.171.124. Aggregate meeting the specification limits shown in the following
would have a maximum size of 1-1/2 in. (37.5 mm) and a nominal
maximum size of 1 in. (25.0 mm).
Sieve size Percent passing
1-1/2 in. (37.5 mm) 100
1 in. (25.0 mm) 95 to 100
1/2 in. (12.5 mm) 25 to 60
No.4 (4.75 mm) 0 to 10
No.8 (2.36 mm) 0 to 5
2.4.3 Particle Shape & Surface Texture.
126.96.36.199. Shape of an aggregate particle is difficult to describe. It is
related to three different characteristics: sphericity, form, and roundness
[Galloway, 1994]. Sphericity is a measure of how nearly equal are the
three principal axes or dimensions of a particle. Form is the measure of
the relation between the three dimensions of a particle based on their
ratios. Roundness describes the relative sharpness called angularity of the
188.8.131.52. Two more parameters have been defined in order to describe
the shape of aggregates better: the elongation and the flakiness. If the
principal dimensions of a particle is too large compared to the other two,
the particle is said to be elongated. Similarly, if two of the dimensions are
too large compared to the third one, the particle is flaky.
184.108.40.206. Some other definitions also exist that do not necessarily
220.127.116.11. A broad classification of shape by BS-12, Part 1 provides a
convenient way of describing the shape of a coarse aggregate particle.
Classification Description Examples
Fully water-worn or completely shaped by River or seashore
Naturally irregular, or partly shaped by attrition Other gravels; land or
and having rounded edges. dug flint.
Material of which the thickness is small Laminated rock.
compared to the other two dimensions.
Possessing well-defined edges formed at the Crushed rock of all
Angular intersection of roughly planar surfaces. types; talus crushed
Material, usually angular, in which the length is
Elongated considerably larger than the other two
Material having the length considerably larger
Flaky & Elongated than the width, and the width considerably
larger than the thickness.
Table 2.1: Broad Classification of Aggregate Shape.
18.104.22.168. A similar classification used in United States is as given below
Well-rounded No original faces left
Rounded Faces almost gone
Sub rounded Considerable wear, faces reduced in area
Sub angular Some wear but faces untouched
Angular Little evidence of wear
22.214.171.124. Angularity of aggregate is estimated from the proportions of
voids in a sample compacted in a prescribed way.
126.96.36.199. Details of measuring flakiness, elongation and angularity are
given in ASTM D 4791- 99 and BS-812 and discussed in chapter-xxxx of
188.8.131.52. The classification of surface texture is based on the degree to
which the particle faces are polished or dull, smooth or rough. There is no
well established method of measuring surface roughness, however, BS-
812 Part 1 s (Table 1.3) provides some characteristics based on which
the surface texture of aggregate particles can be classified.
Group Characteristics Example
1. Glassy Conchoidal fracture Black flint, vitreous slag
Water-worn or smooth due to fracture of Gravel, chert, slate,
laminated or fine-grained rock marble, some rhyolites
Fracture showing more or less uniform
3. Granular Sandstone, oolite
Rough fracture of fine or medium-grained
4. Rough rock containing no easily visible
Containing easily visible crystalline
5. Crystalline Granite, gabbro, gneiss
Brick, pumice, foamed
6. With visible pores and cavities slag, clinker, expanded
Table 2.2: Characteristics of Aggregate based on Surface Texture.
184.108.40.206. Figure 1.1 taken from ref. Quiroga et. al provides two
comparable charts for the visual assessment of particle shape.
Figure 2.1: Aid for visual classification and assessment of aggregate
2.4.4 Specific Gravity (Relative Density). Since aggregate contains pores
which may be both permeable and impermeable, the specific gravity should
accordingly be defined [Neville. p.125, 126].
220.127.116.11. Absolute Specific Gravity. It refers to volume of solid material
excluding all pores, and defined as the ratio of mass of solid, referred to
vacuum, to the mass of an equal volume of gas-free distilled water, both
taken at stated temperature.
18.104.22.168. Apparent Specific Gravity. When the volume of capillary pores
is excluded, whereas the impermeable pores remain included in the
volume of solid, the specific gravity thus obtained is called Apparent
Specific Gravity. Therefore, it is defined as the ratio of mass of oven dried
aggregate (at 100 to 110 C° for 24 hours) to the mass of water occupying a
volume equal to that of the solid including the impermeable pores.
22.214.171.124. Saturated and Surface Dry (SSD) Condition. If the mass of
the aggregate is taken in saturated but surface dry condition, the
aggregate is neither absorbent nor does it contribute water to the concrete
126.96.36.199. Range of Specific Gravity. Majority of natural aggregates have
a specific gravity ranging between 2.6 and 2.7. Specific gravities of some
common rocks are given in the table below [Load Research Laboratory] :-
Rock Group Avg. Sp. Gr Range of Sp. Grs
Basalt 2.80 2.6 – 3.0
Flint 2.54 2.4 – 2.6
Granite 2.69 2.6 – 3.0
Gritstone 2.69 2.6 – 2.9
Hornfels 2.82 2.7 – 3.0
Limestone 2.66 2.5 – 2.8
Porphyry 2.73 2.6 – 2.9
Quartzite 2.62 2.6 – 2.7
Table 2.3: Specific gravities of some common rocks.
188.8.131.52. Test for Density, Relative Density (Specific Gravity), Absorption
of Coarse Aggregate are described by ASTM C 127-04.
2.4.5 Bulk Density and Void Ratio
184.108.40.206. Bulk Density
(1) Since it is physically not possible to determine the density
of solid particles of an aggregate sample, the idea of bulk
density is used.
(2) It is defined as the mass of aggregate that would fill a
container of known volume. Bulk density is used to convert
quantities by mass to quantities by volume.
(3) Properties that affect the bulk density of an aggregate
include grading, specific gravity, surface texture, shape,
and angularity of particles. Aggregates having neither a
deficiency nor an excess of any one size usually have a
higher bulk density than those with a preponderance of one
(4) Higher specific gravity of the particles results in higher bulk
density for a particular grading, and smooth rounded
aggregates generally have a higher bulk density than
rough angular particles of the same mineralogical
composition and grading.
(5) The rodded bulk density of aggregates used for normal
weight concrete generally ranges from 75 to 110 lb/ft 3
(1200 to 1760 kg/m3) [ACI Education Bulletin E1-07].
220.127.116.11. Void Ratio
(1) Void contents range from about 30% to 45% for coarse
aggregates to about 40% to 50% for fine aggregate
[Kosmatka et. al p.87].
(2) Angularity increases void content while larger sizes of well-
graded aggregate and improved grading decreases void
(3) Knowing the apparent specific gravity of the SSD condition,
the void ratio can be calculated from the expression
Void Ratio 1
s unit mass of water
18.104.22.168. ASTM C 29-97 describes detail procedures for determining
density and void ratio of aggregate.
2.4.6 Porosity and Absorption
22.214.171.124. Type and volume of internal pores determines the rate of
absorption of water, total quantity absorbed and its permeability/
126.96.36.199. Pores may vary in size, shape and type such as
continuous/through, on the surface, wholly within solid etc.
188.8.131.52. Large pores can be seen under a microscope or even with naked
eye, however, smallest of aggregate pores are larger than the gel pores of
184.108.40.206. When all the pores in the aggregate are full, it is said to be
saturated-surface-dry or SSD. If it is allowed to lose some water by placing
in air, then this condition is air-dry (AD). When aggregate is dried in an
oven such that no moisture content is left, the aggregate is said to be
oven-dry (OD) or bone-dry.
220.127.116.11. Values of porosity of some common rocks are as given below
[Neville. p.125, 126]:-
(1) Gritstone 0.0 – 48.0
(2) Quartzite 1.9 – 15.1
(3) Limestone 0.0 – 37.5
(4) Granite 0.4 – 3.80
2.4.7 Moisture Content
18.104.22.168. All aggregates contain some moisture based on the porosity of
the particles and the moisture condition of the storage area. Aggregate
exposed to rain and humidity collects moisture on the surface of the
22.214.171.124. The moisture content can range from less than one percent in
gravel up to 40 percent in very porous sandstone and expanded shale.
However, most rocks used as coarse aggregate, rarely contains more than
1% of surface moisture.
126.96.36.199. Aggregate can be found in four different moisture states that
include wet or damp, SSD, AD and OD [ACI Education Bulletin E1-07]. Of
these four states, only OD and SSD correspond to a specific moisture
state and can be used as reference states for calculating moisture content.
188.8.131.52. Aggregate will either add or subtract water to the paste,
depending upon its own state of moisture content. Hence, in concrete mix
design, SSD condition is used as reference because that is an equilibrium
condition at which the aggregates will neither absorb nor release water to
184.108.40.206. Most stockpil