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: •
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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 •
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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
o
o
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
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 Tuff 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 Quartz-diorite
Gabbro Group Basic diorite Basic gneiss Gabbro Hornblende-rock Norite Peridotite Picrite Serpentinite
Hornfels Group
Contact-altered rocks of all kinds except marble Pegnatite
Porphyry Group Quartzite Group Aplite Ganister Dacite Quartzitic sandstones Felsite Re-crystallized Granophyre quartzite Keratophyre Microgranite Porphyry Quartz-porphyrite Rhyolite Trachyte
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 Examples Fully water-worn or River or seashore gravel; completely shaped by desert, seashore and wind attraction blown sand Naturally irregular, or partly Other gravels; land or dug shaped by attrition and flint having rounded edges Material of which the Laminated rock thickness is small relative to the other two dimensions Possessing well-defined Crushed rocks of all types; edges formed at the talus; crushed slag 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
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
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Angularity number measures the percentage of voids in angular particles in excess of that in the rounded gravel particles
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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)
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The higher the angularity number, the more angular the aggregate.
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The range of angularity number for practical aggregates is between 0 and 11
(ii) Sphericitv •
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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
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Higher sphericity lowers the workability of the concrete. Higher sphericity also adversely affects the durability of concrete.
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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
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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
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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
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More water is required when there is a greater void content of the loosely-packed aggregate
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Generally, flakiness and shape of the coarse aggregate have an appreciable effect on the workability of concrete
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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
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A rougher texture results in a greater adhesion or bond between the particles and the cement matrix
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The larger surface area of a more angular aggregate provides a greater bond. However, workability is reduced.
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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
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Average value of crushing strength of a rock sample is about 200 MPa (30,000 psi)
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However, excellent rock for aggregate may have crushing strength of 80 MPa (12,000 psi)
(ii) Aggregate crushing value (ACV) test on aggregate •
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•
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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
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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)
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A higher ACV is an indication of a lower compressive strength of aggregate
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For ACVs greater than 25 to 30, this test is insensitive to the variation in strength of weaker aggregates
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For this reason, ten percent fines value test is used
3. Toughness •
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•
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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
o
o
4. Hardness
25% when the aggregate is to be used in heavy duty concrete floor finishes 30% when the aggregate is to be used in concrete pavement wearing surfaces 45% when to be used in other concrete
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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
