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                                       Chapter - 2

2.1    Definition
       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.   Classification
       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:-
        Coarse Aggregate (CA). Aggregate predominantly retained on
               the sieve No. 4 (4.75-mm) is called coarse aggregate.
        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].
        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.
        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.     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:-
      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.
                                       8 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
      specified. 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. 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. 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:-     The smallest sieve opening through which the entire amount of
      aggregate is required to pass is called the maximum size.     The smallest sieve opening through which the entire amount of
      aggregate is permitted to pass is called the nominal maximum size.     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.     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
particle.      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.      Some other definitions also exist that do not necessarily
correlate.      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
                     attrition.                                           gravel
                     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.      A similar classification used in United States is as given below
[Neville p.113]:-

                  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      Angularity of aggregate is estimated from the proportions of
voids in a sample compacted in a prescribed way.      Details of measuring flakiness, elongation and                               angularity are
given in ASTM D 4791- 99 and BS-812 and discussed in chapter-xxxx of
this document.
                                                 10     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,
          2.     Smooth
                               laminated or fine-grained rock             marble, some rhyolites
                               Fracture showing more or less uniform
          3.     Granular                                                 Sandstone, oolite
                               rounded grains
                               Rough fracture of fine or medium-grained
                                                                          Basalt, felsites,
          4.     Rough         rock containing no easily visible
                                                                          porphyry, limestone
                               crystalline constituents
                               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.     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].
                                          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.      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.      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
      mixture.      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.      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      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
                        particle size.
                  (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].     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
                        [Neville. p.128]:-
                                                                 Bulk Density
                             Void Ratio  1 
                                                            s  unit mass of water ASTM C 29-97 describes detail procedures for determining
      density and void ratio of aggregate.
2.4.6 Porosity and Absorption Type and volume of internal pores determines the rate of
      absorption of water, total quantity absorbed and its permeability/
      retaintivity. Pores       may       vary     in     size,     shape      and   type   such   as
      continuous/through, on the surface, wholly within solid etc. 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
      cement paste.
                                     13 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. 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     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
      particles.     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.     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.     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
      the paste.     Most stockpil
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