Introduction to use of aggregates in concrete. Various Properties of aggregates and their effect on fresh and hardened concrete. Flakiness and Elongation Index of Concrete have also been discussed. Various tests performed for suitable usage of concrete in Civil engineering projects have also been discussed.

2. MILITARY COLLEGE OF ENGINEERING
RISALPUR
CE 308 – PRC I - LECTURE 3
AGGREGATES

3. AGGREGATE
 According to ASTM 25 and D8: Aggregate is the granular
material, such as sand, gravel, crushed stone, crushed blast-furnace
slag, or construction and demolition waste that is used with a
cementing medium to produce either concrete or mortar.
 Coarse aggregate - particles larger than 4.75 mm (No. 4 sieve)
 Fine aggregate - particles smaller than 4.75 mm but larger than
75 μm (No. 200 sieve)

4.  Aggregates may be divided into two categories depending on the
source :
Natural aggregate
Artificial aggregate
 Natural aggregates are usually derived from natural sources and
may have been naturally reduced to size (e.g. gravel or shingle) or
may have to be reduced by crushing.
 The most widely used artificial aggregates are clean broken bricks
and air-cooled fresh blast-furnace-slag.
DEPENDING ON SOURCE

5. DEPENDING ON SOURCE

6. DEPENDING ON SOURCE

7. SPECIFIC GRAVITY OF ROCKS

8. ON SPECIFIC GRAVITY

9. ON BULK DENSITY
Bulk density is the weight in a given volume.

10. ON BULK DENSITY

11. Aggregates
Since approximately three quarters of the volume of concrete is
occupied by aggregate, its quality is of considerable importance
The aggregate properties greatly affect the durability and
structural performance of concrete
Aggregate was originally viewed as inert, inexpensive material
disperse throughout the cement paste so as to produce large
volume of concrete
In fact aggregates are not truly inert, because its physical,
thermal and sometimes chemical properties influence the
properties of concrete
From eco point of view it is advantageous to have as many
aggregates and less cement, but the cost benefit has to be
balanced against the desired properties of its fresh and hardened
state

12. Aggregates
Natural aggregates are formed by the process of weathering or
artificial cutting of a larger parent mass
 Thus, many properties of the aggregate depends upon the
parent rock, e.g. chemical and mineral composition,
petrographic classification (Petrography is a branch of
petrology that focuses on detailed descriptions of rocks. ),
specific gravity, hardness, strength, pore structure, color etc.
In addition, there are other properties of aggregates which are
absent in the parent rock, particle shape and size, surface texture
and absorption. All these properties may have a considerable
impact on the quality of fresh and hardened concrete

13. Size classification
Concrete is made with aggregate particles covering a range of
size up to a maximum which usually lies between 10 mm and 50
mm (20 mm as typical).
The particle size distribution is called grading
Low grade concrete may be made with aggregates from
deposits containing a whole range of sizes, ranging from the
largest to the smallest, known as all-in or pit-run aggregates
The alternative, very much more common is to obtain the
aggregate in two separate lots, the main division being at a size
of 5 mm or no. 4 ASTM sieve. This divides fine aggregates from
coarse aggregate
It should be noted that the term aggregate is sometime used to
mean coarse aggregate, a practice which is not correct

14. Petrographic classification
Petrology is the branch of geology that studies the origin,
composition, distribution and structure of rocks.
From petrographic standpoint aggregates can be divided into
several groups of rocks having common characteristics as
classified by BS 812 , part 1, 1975.
The group classification does not imply the suitability of any
aggregate for making of concrete, unsuitable materials may be
found in any group, although some groups tend to have better
record than others.
Silica minerals, carbonate minerals, sulphate minerals etc.

15. Particle shape and texture
In addition to petrological character of aggregate, its
external characteristics are of importance, in particular the
particle shape and texture
The shape of three-dimensional objects is rather difficult
to describe, therefore, it is convenient to define certain
geometric characteristics of such bodies

18. Particle shape and texture
In case of crushed aggregates the particle shape depends on
the nature of parent material and on the type of crusher
Roundness measures the relative sharpness of the edges and
corners of a particle
Roundness is controlled largely by the strength and abrasion
(the process of scarping and wearing) resistance of the parent
rock
Since the degree of packing of particles of one size depends
on their shape, the angularity of aggregates can be estimated
from the proportion of voids in a sample compacted in a simple
way
BS 812: part 1: 1975 defines the Angularity number

19. Void ratio VS Round Aggregates

20. Particle shape and texture
Angularity number can be taken as 67 minus the percentage
of solid volume in the vessel filled with aggregate in a
standard manner
The number 67 in the expression for angularity represents
the solid volume of the most rounded gravel, so that the
angularity number measures the percentage of voids in excess
of that of the round gravel (i.e. 33 %). The higher the number
the more angular the aggregates, the range for practical
aggregates being 0 to 11.
A particle is said to be flaky if its thickness (least
dimension) is less than 0.6 times the mean sieve size of the
size fraction to which it belongs.
The mean size is defined as the arithmetic mean of the sieve
size on which the particle is just retained and the sieve size
through which it passes

21. Particle shape and texture
The mass of the flaky particles, expressed as the
percentage of the mass of the sample, is called the flakiness
index. Elongation index is similarly defined.
Some particles are both flaky and elongated and are ,
therefore, counted in both categories.
Sea aggregates may contain shells whose content needs to
be controlled because they are brittle and they also reduce
the workability of the mix. The shell content is determined
by weighing hand-picked shells and shell fragments from a
sample of aggregates greater than 5mm.
The classification of surface texture is based on the degree
to which the particle surfaces are polished or dull, smooth or
rough. Surface textures are glassy, smooth, granular (more
or less round), rough, crystalline and honeycomb (with
visible voids).

22. Mechanical properties
Bond
Both the shape and surface texture of aggregates influence
considerably the strength of concrete, especially for high
strength concrete; flexural strength is more affected than the
compressive strength.
A rougher texture results in a greater adhesion or bond
between the particles and the cement matrix.
Likewise the larger surface area of a more angular
aggregate provides a greater bond.
Softer and heterogeneous particles result in a better bond.
The determination of quality of bond is rather difficult and
no accepted test exists.
Generally, a crushed concrete specimen should contain
some aggregate particles broken right through, in addition to
the more numerous ones separated from the paste matrix.

23. Mechanical properties
Strength
It is not easy to determine the crushing strength of
aggregate itself.
A few weak particles can be tolerated; after all, voids can
be viewed as aggregate particles of zero strength.
The required information about the aggregate particles has
to be obtained from indirect tests; crushing strength of
prepared rock samples, crushing value of aggregate, and
performance of aggregate in concrete.
The latter simply means either previous experience with
the given aggregate or a trial use of aggregate in a mix
known to have a certain strength with previously proven
aggregates.

24. Mechanical properties
The material to be tested should pass a 14 mm sieve and
be retained on a 10 mm sieve.
When, however, this size is not available, particles of other
sizes may be used, but those larger than the standard will in
general give a higher crushing value, and the smaller ones a
lower value than would be obtained with the same rock of
standard size.
The sample should be dried in oven at 100 to 110 degree
Centigrade for four hours and then placed in a cylindrical
mould and tamped in a prescribed manner.

25. Mechanical properties
Toughness
Toughness can be defined as the resistance of the
aggregate to failure by impact, and it is usual to determine
the aggregate impact value of bulk aggregate.
The details of the test are available in BS 812: part
112:1990
It can be used as a replacement test for strength of
aggregate.

26. EFFECT OF AGGREGATE
PROPERTIES ON FRESH AND
HARDENED CONCRETE

27. STRENGTH
The strength of concrete cannot exceed the strength of the
aggregate.
In practice, concrete strength is likely to be much less than
the strength of the aggregate, because stress concentrations
are generated at the aggregate- cement paste interface
(Interfacial Transition Zone, ITZ) when stress is applied on
concrete.
Weak aggregates may break down during mixing, handling
and compaction.

28. STRENGTH

29. POROSITY AND PERMEABILITY
 The internal pore characteristics are very important properties of
aggregates. The size, the number, and the continuity of the pores
through an aggregate particle may affect the strength of the
aggregate, abrasion resistance, surface texture, specific gravity,
bonding capabilities, and resistance to freezing and thawing action.
Porosity is a ratio of the volume of the pores to the total volume
of the particle.
 Permeability refers to the particle's ability to allow liquids to pass
through. If the rock pores are not connected, a rock may have high
porosity and low permeability. An aggregate can be very porous
but very less permeable.

30. ABSORPTION AND MOISTURE
CONTENT
 Absorption relates to the particle's ability to take in a liquid.
Moisture Content is defined as the water in excess of the
saturated and surface-dry conditions.
 Thus, the total water content of a moist aggregate is equal to the
sum of absorption and moisture content.
Aggregate exposed to rain collects a considerable amount of
moisture on the surface of the particles, and, except at the surface
of the stockpile, keeps this moisture over long periods.
This is particularly true of fine aggregate, and the moisture
content must be allowed for in the calculation of batch quantities
and of the total water requirement of the mix during mix design.

32. ABSORPTION

33. BULKING OF SAND
 In the case of sand, there is another effect of the presence of
moisture i.e. bulking, which is an increase in the volume of a given
mass of sand caused by the films of water pushing the sand
particles apart.
 Bulking of sand affects the proportioning of materials by
volume.
 In volume batching, bulking results in a smaller mass of sand
occupying the fixed volume of the measuring box.

34. SOUNDNESS
 A sound aggregate is able to resist stresses induced by
environmental or climatic conditions.
 Some aggregates degrade or disintegrate under the action of
cycles of freezing and thawing or salt weathering.
 Susceptible aggregates are usually micro porous, that is, they
have high absorption arising from a large proportion of fine
pores.
 Freezing or salt crystallization can induce considerable
stresses in fine pores that may lead to degradation.

35. SOUNDNESS TEST
 The Soundness Test determines
the resistance of aggregates to
disintegrate when exposed to
solutions of Sodium Sulphate and
Magnesium Sulphate.

36. IMPURITIES
Impurity Effect on fresh concrete Effect on hardened concrete
Chlorides Accelerate the set Damp patches and efflorescence
Sulfates Interference with hydration Cracking and spalling
Shells Reduce workability Damage to surface finish
Acid soluble
material in sand
None
Reduced skid resistance of
pavement
quality concrete
Alkali reactive
silica
None Risk of alkali–silica reaction
Swelling clays Increased water demand Reduced strength
Reactive iron
pyrites
Possible reduced yield Surface staining
Mica Increased water demand Reduced strength
Organic matter Possible retardation Possible reduced strength
Coal and lignite Possible retardation Surface staining, pop-outs
Soluble lead or
zinc
Possible retardation Possible reduced strength

37. REACTIVE AGGREGATES

38. REACTIVE AGGREGATES

39. CONCRETE STRENGTH

40. OTHER REQUIREMENTS