Classification of Aggregate Based on Size and Shape

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:
    • Assessing quality of aggregate
    • Comparing a new aggregate with an aggregate which quality is already known
    • Detecting the adverse properties of aggregate such as the presence of some unstable forms of silica

Rock type classification of natural aggregates according to BS 812: Part1 : 1975

Basalt Group                        Flint Group            Gabbro Group

Andesite                                                        Chert                               Basic diorite

Basalt                                                               Flint                                 Basic gneiss

Basic porphyrites                                                                                  Gabbro

Diabase                                                                                                       Hornblende-rock

Dolerites of all kinds                                                                           Norite

including theralite                                                                                Peridotite

and teschenite                                                                                         Picrite

Epidiorite                                                                                                   Serpentinite

Lamprophyre

Quartz-dolerite

Spilite

Granite Group                                     Gritstone Group                                  Hornfels Group  

                                                                     (including fragmental

                                                                                      volcanic rocks)

Gneiss                                                          Arkose                                                     Contact-altered rocks

Granite                                                        Greywacke                                                of all kinds except

Granodiorite                                            Grit                                                                       marble

Granulite                                                   Sandstone

Pegnatite                                                   Tuff

Quartz-diorite

Syenite

 

Limestone Group                                  Porphyry Group               Quartzite Group

Dolomite                                                     Aplite                                       Ganister

Limestone                                                   Dacite                                     Quartzitic sandstones

Marble                                                         Felsite                                      Re-crystallized

Granophyre                                             quartzite

Keratophyre

Microgranite

Porphyry

Quartz-porphyrite

Rhyolite

Trachyte

Schist Group

Phyllite

Schist

Slate I

All severely sheared

rocks

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: 

Particle shape classification of aggregates according to BS 8U: Part 1: 1975; with examples –

 

ClassificationDescriptionExamples
RoundedFully water-worn or completely shaped by attractionRiver or seashore gravel; desert, seashore and wind blown sand
IrregularNaturally irregular, or partly shaped by attrition and having rounded edgesOther gravels; land or dug flint
FlakyMaterial of which the thickness is small relative to the other two dimensionsLaminated rock
AngularPossessing well-defined edges formed at the intersection of roughly planar facesCrushed rocks of all types; talus; crushed slag
ElongatedMaterial, usually angular, in which the other two dimensions
Flaky and elongatedMaterial having the length considerably larger than the width, and the width considerably larger than the thickness

 

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:

  1. i) Angularity number
  2. ii) Sphericity
  • iii) Mean size
  1. iv) Elongated particle
  2. v) Flaky particle
  3. vi) Elongation index
  • vii) 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) Sphericity

  • 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:
    •  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

 

GroupSurface Texture          CharacteristicsExamples
1GlassyConchoidal fractureBlack flint, vitreous slag

vitreous slag                                       vitreous s

2SmoothWater-worn, or smooth due to fracture of laminated or fine-grained rockGravels, chert, slate, marble, some rhyolites
3GranularFracture showing more or less uniform rounded grainsSandstone, oolite

 

4RoughRough fracture of fine or medium-grained rock containing no easily visible crystalline constituentsBasalt, felsite, porphyry, limestone

 

 

5CrystallineContaining easily visible crystalline constituentsGranite, gabbro, gneiss

 

6HoneycombedWith visible pores and cavitiesBrick, 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

 

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