Concrete Technology

1. Introduction of Concrete
Mr. Kiran R. Patil
Assistant Professor,
Department of Civil Engineering,
D. Y. Patil College of Engineering & Technology,
Kolhapur

2. Aggregates
• The aggregates provide about 70 – 80% of concrete volume. Therefore, they must be
strong, durable, clean and of proper shape.
• They must be well graded. They should not be chemically reactive.
• Aggregates are cheaper than cement and give greater volume stability and durability to
concrete. Basically, the aggregates are used to provide bulk to concrete.
• They also reduce shrinkage. To increase the density of concrete; aggregates of two or
more sizes are used.
• Fine aggregate helps in producing workability and uniformity in concrete mix.
• Classification of Aggregates
• Classification based on Source–
(i) Natural Aggregates
(ii) Artificial Aggregates
• Natural Aggregates: Natural aggregates are obtained from natural deposits of sand or
from quarries. These aggregates may be igneous, sedimentary or metamorphic.
Aggregates obtained from igneous rocks are hard, trough and dense. Examples of these
rocks are Granite, Basalt, and Trap.
• Artificial Aggregates: Broken bricks and blast furnace slag are used as artificial
aggregates. The aggregates obtained from good quality bricks having crushing strength
3.5 N/mm2 are suitable for mass concrete but not suitable for R.C.C.

3. • Classification based on Size –
(i) Fine Aggregate: passes from 4.75mm IS sieve
(ii) Coarse Aggregate: Retain on 4.75mm IS sieve
• Classification based on Shape –
(i) Rounded Aggregates : Fully water warn
(ii) Angular Aggregates: Having well defined edges
(iii) Flaky and Elongated Aggregates: Thickness is small relative to the width or length
• Classification based on Unit Weight –
(i) Normal weight aggregates: Sp. gravity- 2.5-2.7, Density: 2300-2600kg/m3
(ii) Heavy weight aggregates: Sp. gravity- 2.8-2.9, Density: 2800-2900kg/m3
(iii) Light weight aggregates: Density: 1200 kg/m3

4. Physical Properties of Aggregates:
1. Size:
• Aggregates are divided into two categories based on size-Coarse aggregate (C.A.) and
Fine aggregate (F.A.). The aggregate bigger than 4.75 mm size is called coarse
aggregate and aggregate smaller than 4.75 mm is called fine aggregate.
• Coarse Aggregate –
• The graded coarse aggregate is described by its nominal size 40 mm, 20 mm, 16 mm,
12.5 mm, 10 mm, etc.
• A graded aggregate of nominal size 20 mm means an aggregate most of which passes
the 20 mm I.S. sieve.
• Fine Aggregate –
• The fine aggregate may be natural sand or crushed sand obtained by crushing the
boulders. I.S.383 -1970 has divided the fine aggregate into four grading zones
depending upon the particle size distribution.
• The grading zone becomes progressively finer from grading Zone I to Zone IV. Zone I
aggregate should not be used for R.C.C.

5. 2. Shape:
• Aggregates are classified into three types based on shape – Rounded aggregates,
Angular aggregates, Flaky and Elongated aggregates.
• Rounded Aggregate: The aggregate with rounded particles (river gravels) has
minimum voids. It gives minimum ratio of surface area to the volume, thus requiring
minimum cement paste to make good concrete. These aggregate makes concrete more
workable. The only disadvantage is that the interlocking between the particles is less
and hence development of the bond is poor, making it unsuitable for high strength
concrete and for road pavements.
• Angular Aggregate: The aggregate with sharp edges and angular particle has higher
voids. It requires more cement paste to make workable concrete of high strength than
that required by rounded aggregate. The interlocking between the particles is excellent
and it provides a good bond. These aggregate are suitable for high strength concrete and
for road pavements.
• Flaky and Elongated Aggregate: A particle is said to be flaky when its thickness
(least dimension) is less than 0.6 times the mean dimension. The mean dimension of an
aggregate is the average of the sieve sizes through which the particle passes and
retained, respectively.
• e.g. 20 mm sized aggregate is that which passes through 20 mm sieve and is retained
on 16 mm sieve. Its mean dimension is
20+16
2
= 18 mm and 0.6 𝑥 18 = 10.8 𝑚𝑚
• Thus, the aggregate particles having the minimum dimension less than 10.8 mm is
considered as flaky.

6. • A particle is said to be elongated when its longest dimension is greater than 1.8 times
the mean sieve size to which the particle belongs.
• e.g. For a 20 mm size, an aggregate particle with the longest dimension greater than
18 𝑥 1.8 = 32.4 𝑚𝑚 would be considered elongated.
• Flaky and elongated aggregates are not desirable.
• They reduce the workability and tend to be oriented in a plane during compaction. The
hardened concrete may result in stratified structure.
3. Texture:
• The surface texture is a measure of smoothness or roughness of the aggregate. The
surface texture may be glassy, smooth, granular, rough, crystalline, porous or
honeycombed.
• The bond depends upon the surface texture of the aggregate. An aggregate with rough
texture is preferred to one with a smooth texture.
• As surface smoothness increases, contact area decreases, hence a highly polished
particle will have less bonding area with the matrix than a rough particle of the same
volume.
• A smooth particle, however, will require a thinner layer of paste to lubricate its
movements with respect to other aggregate particles. It will, therefore, permit denser
packing for equal workability and hence, will require lower paste content than rough
particles.
• It has been also shown by experiments that rough textured aggregate develops higher
bond strength in tension than smooth textured aggregate.

7. 4. Strength:
Generally three tests are conducted to determine the strength of aggregate –
(i) Aggregate Crushing Value:
The crushing strength is more important. I.S. 383 – 1970 prescribes a maximum crushing
value as 45% for the aggregate used for concrete other than for wearing surfaces and 35% for
concrete for wearing surfaces such as roads, pavements and runways.
(ii) Aggregate Impact Value:
Toughness is the resistance of the aggregate to failure by impact. Toughness is measured by
Impact Value Test. According to I.S 383 – 1970, the impact value should not exceed 45% for
the aggregate used for concrete other than for wearing surfaces and 35% for concrete for
wearing surfaces.
(iii) Aggregate Abrasion Value:
Hardness of the aggregate is its resistance to wear. It is obtained in terms of Aggregate
Abrasion Value by using Los Angeles machine. Satisfactory aggregate should have an
abrasion value not more than 30% for the aggregate used for wearing surfaces and 50% for
aggregate used for non-wearing surfaces.
5. Specific Gravity:
The specific gravity of an aggregate is defined as the ratio of mass of solid sample to the mass
of an equal volume of water at the same temperature.
• The main use of specific gravity is to design a concrete mix, to calculate the yield of
concrete for a given proportion and to calculate voids ratio for a given aggregate.

8. • Specific gravity of normal weight aggregate varies from 2.5 to 2.9. It is more than 2.9 for
heavy weight aggregate.
6. Bulk Density:
• The bulk density of an aggregate is defined as the mass of material in a given volume. It is
expressed in kg/litre. The bulk density of an aggregate depends upon particle size, shape,
grading and moisture content.
• Higher bulk density for a coarse aggregate indicates that there are lesser voids to be filled
by sand and cement. Bulk density is used for converting proportions by weight into the
proportions by volume.
7. Voids Ratio:
The empty spaces between the aggregate particles are called voids. It is the difference
between the gross volume of aggregate mass and the volume occupied by the particle alone.
• Voids Ratio =1 -
𝑏𝑢𝑙𝑘 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑔𝑟𝑎𝑣𝑖𝑡𝑦
• Percentage voids =
specific gravity−bulk density
specific gravity
x 100
8. Soundness:
• Soundness is the ability of aggregate to resist volume changes due to changes in physical
conditions. The physical conditions that affect the soundness of aggregate are freezing,
thawing, variation in temperature, alternate wetting and drying, etc.
• Aggregate which undergo more than the specified amount of volume change is said to be
unsound aggregate.

9. • The soundness test consists of alternate immersion of aggregate sample in a saturated
solution of sodium sulphate or magnesium sulphate and oven-drying it under specified
conditions. Loss in weight is measured for specified number of cycles.
• The average loss of weight after 10 cycles should not exceed 12% when tested with
sodium sulphate and 18% when tested with magnesium sulphate.
9. Cleanliness:
• The aggregate should be free from impurities which are likely to interfere with hydration
process, prevention of effective bond between the aggregate and paste.
• Excessive silt and clay may result in excessive shrinkage and increased permeability. It
also reduces bond between concrete and steel.
• As per IS: 383 – 1970, the total percentage of such impurities should not be more than 5%.
10. Bulking of Fine Aggregate:

10. • The increase in the volume of a given mass of fine aggregate caused by the presence of
water is called bulking.
• Free moisture forms a film around each sand particle. This film exerts surface tension
which keeps the neighboring particles away from it. Thus, there will not be point contact
between the particles. This causes increase in the volume.
• The bulking increases gradually with moisture content up to a certain point and then begin
to decrease with further addition of water as the films are broken. When the sand is fully
saturated with water, the bulking is practically nil.
• For ordinary sands, the bulking varies from 15 to 30 %. Due to the bulking, sand shows
unrealistic volume. If the sand is measured by volume and no allowance is made for
bulking, the mix will be richer than the specified because for given mass, most sand
occupies a considerably larger volume than the same mass of the dry sand. This results in
a mix undersupplied in sand (under sanded mix) increasing the chances of segregation and
honeycombing of concrete. Thus, it is necessary to increase the measured volume of sand
by the percentage of bulking.
• e.g. For a bulking of 20 %, if no allowance is made for bulking, a nominal mix 1:2:4 will
become 1:1.6:4
11. Maximum Size of Aggregate
• Use of the largest possible size aggregate reduces the cement and water requirements.
This is due to the fact that workability of concrete decreases with the maximum size of
the aggregate. Use of larger size aggregate is beneficial in mass concrete due to the lesser
cement consumption.

11. • This will also reduce the heat of hydration and shrinkage cracks. Due to smaller surface area
of the large size aggregate, the water-cement ratio can be decreased which increases the
strength.
• In practice, the size of aggregate is limited by –
1. Thickness of section
2. Spacing of reinforcement
3. Clear cover
4. Mixing, handling and placing methods
• For strengths up to 20 N/mm², aggregates up to 20 mm may be used.
• According to IS: 456 – 2000, the maximum nominal size of coarse aggregate:
1. should not be greater than ¼ th the minimum thickness of the member
2. should be 5 mm less than the minimum clear distance between the main bars
3. 5 mm less than the minimum cover to the reinforcement whichever is smaller.
12. Fineness Modulus (F.M.):
• F.M. is a numerical index of coarseness or fineness of the material. It gives some idea of the
mean size of the particles present in the entire body of the aggregate.
• F.M. is obtained by adding the cumulative percentages of aggregate retained on each of the
standard sieves ranging from 80 mm to 150 micron and dividing this sum by an arbitrary
number 100. The larger the F.M., the coarser is the material.
• The sieves that are used for the sieve analysis of the aggregate for concrete as per IS: 2386
(Part I) – 1963 , are 80 mm, 40 mm, 20 mm, 10 mm, 4.75 mm, 2.36 mm. 1.18 mm, 600
micron, 300 micron and 150 micron.

12. • The F.M. can be regarded as an average size of a sieve on which the material is retained
and the sieves are counted from the finest.
• e.g. A F.M. of 5 indicates that the 5th sieve ( 2.36 mm) is the average size
• F.M. for fine aggregate varies between 2 and 3.5.
• F.M. for coarse aggregate varies between 5.5 and 8
• The object of finding F.M. is to grade the given aggregate for the most economical mix for
the required strength and workability with minimum quantity of cement.
• The higher F.M. will give a harsh mix and lower F.M. will give an uneconomical mix. The
F.M. is also important for measuring the slight variations in the aggregate from the same
source.
• Example: Sieve analysis of Fine Aggregate
• Weight of sample taken = 500 gm

14. • Grading of Aggregates
• The particle size distribution of an aggregate as determined by sieve analysis is
called grading of aggregates. If all particles of an aggregate are of uniform size, the
compacted mass will contain more voids; whereas an aggregate comprising of
particles of various sizes will give a mass containing lesser voids. The particle size
distribution of an aggregate should be such that smaller particles fill the voids
between the larger particles.
• The proper grading of an aggregate produces dense concrete and need less quantity
of fine aggregate and cement paste. Thus, it is essential that coarse and fine
aggregate should be well graded to produce quality concrete.
• Grading Curve:
• The curve showing cumulative percentage of material passing the sieves representing
Y – axis with the sieve sizes on X – axis is called the Grading Curve. The grading
curve indicates whether the grading of a given sample conforms to the specified or is
too coarse or too fine or deficient in a particular size.

16. • Alkali-Aggregate Reaction (AAR)
• The alkali-aggregate reaction is the reaction between the reactive silica in the aggregates
and alkalies in cement.
• The reactive silica occurs in trap, chert, silicious limestone, rhyolite, andesite, etc. The
reaction starts with attack on the reactive silica in the aggregate by alkaline hydroxides in
cement.
• The hydroxides are derived from the alkalies (Na2O and K2O) in the cement. As a result
of this reaction, an alkali silicate gel is formed. Swelling of this gel may cause cracking
and disruption of cement paste.
❖ The factors promoting AAR are,
➢ Reactive type of aggregates
➢ High alkali content in cement
➢ Availability of water
➢ Alternate wetting and drying
➢ Temperature in the range of 10°C to 38°C
❖ Control of AAR: The rate of AAR can be controlled by,
➢ Selecting non-reactive aggregate
➢ Using low-alkali cement
➢ Controlling moisture and temperature
➢ Adding admixtures like Pozzolana