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INTRODUCTION TO REINFORCED CONCRETE

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INTRODUCTION TO REINFORCED CONCRETE
1.1 INTRODUCTION
The word ‘concrete’ comes from the Latin word concretus
(meaning compact or condensed), the perfect passive
participle of concrescere, from con (together) and crescere
(to grow). This name was chosen perhaps due to the fact
that this material grows together, due to the process of
hydration, from a visco-elastic, moldable liquid into a hard,
rigid, solid rock-like substance. The Romans fi rst invented
what is today known as hydraulic cement-based concrete or
simply concrete. They built numerous concrete structures,
including the 43.3 m diameter concrete dome, the Pantheon,
in Rome, which is now over 2000 years old but is still in
use and remains the world’s largest non-reinforced concrete
dome (see case study in Chapter 2 for more details about the
Pantheon).
Concrete is used in nearly every type of construction.
Traditionally, concrete has been primarily composed of
cement, water, and aggregates (aggregates include both coarse
and fi ne aggregates). Although aggregates make up the bulk
of the mix, it is the hardened cement paste that binds the
aggregates together and contributes to the strength of concrete,
with the aggregates serving largely as low-cost fi llers (though
their strength also is important).
Concrete is not a homogeneous material, and its strength
and structural properties may vary greatly depending upon its
ingredients and method of manufacture. However, concrete
is normally treated in design as a homogeneous material.
Steel reinforcements are often included to increase the tensile
strength of concrete; such concrete is called reinforced cement
concrete (RCC) or simply reinforced concrete (RC).
As of 2006, about 7.5 billion cubic metres of concrete
were produced each year—this equals about one cubic metre
per year for every person on the earth (see Table 1.1). The
National Ready Mixed Concrete Association (NRMCA)
estimates that ready-mixed concrete production in 2005 was
about 349 million cubic metres in the USA alone, which is
estimated to have about 6000 ready-mixed concrete plants.
TABLE 1.1 Annual consumption of major structural materials in the world
Material Unit Weight
(kg/m3)
Million
Tonnes
Tonnes/Person
Structural steel 7850 1244 0.18
Cement 1440 3400 0.48
Concrete 2400 18,000 2.4 (990 litres)
Timber 700 277 0.04
Drinking
water+
1000 5132 0.73 (730 litres)
Notes: The estimated world population as of August 2012 is 7.031 billion.
+Assumed as two litres/day/person
Concrete technology has advanced considerably since its
discovery by the Romans. Now, concrete is truly an engineered
material, with a number of ingredients, which include a host of
mineral and chemical admixtures. These ingredients should be
precisely determined, properly mixed, carefully placed, vibrated
(not required in self-compacting concretes), and properly cured
so that the desired properties are obtained; they should also be
inspected at regular intervals and maintained adequately until
their intended life. Even the cement currently being used has
undergone a number of changes. A variety of concretes is also
being used, some tailored for their intended use and many with
improved properties. Few specialized concretes have compressive
strength and ductility matching that of steel. Even though this is
a book on RC design, it is important for the designers to know
about the nature and properties of the materials they are going
to specify for the structures designed by them. As concrete
technology has grown in parallel with concrete design, it is
impossible to describe all the ingredients, their chemistry, the
different kinds of concretes, and their properties in this chapter.
Hence, only a brief introduction is given about them, and
interested readers should consult a book on concrete technology
(many references are given at the end) for further details.
1
1
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2 Design of Reinforced Concrete Structures
1.1.1 Brief History
Many researchers believe that the fi rst use of a truly
cementitious binding agent (as opposed to the ordinary lime
commonly used in ancient mortars) occurred in southern Italy
around second century BC. Volcanic ash (called pozzuolana,
found near Pozzouli, by the Bay of Naples) was a key
ingredient in the Roman cement used during the days of the
Roman empire. Roman concrete bears little resemblance to
modern Portland cement concrete. It was never put into a
mould or formwork in a plastic state and made to harden, as
is being done today. Instead, Roman concrete was constructed
in layers by packing mortar by hand in and around stones of
various sizes. The Pantheon, constructed in AD 126, is one of
the structural marvels of all times (Shaeffer 1992).
During the Middle Ages, the use of concrete declined,
although isolated instances of its use have been documented
and some examples have survived. Concrete was more
extensively used again during the Renaissance (14th–17th
centuries) in structures like bridge piers. Pozzolanic materials
were added to the lime, as done by the Romans, to increase its
hydraulic properties (Reed, et al. 2008).
In the eighteenth century, with the advent of new technical
innovations, a greater interest was shown in concrete. In 1756,
John Smeaton, a British Engineer, rediscovered hydraulic
cement through repeated testing of mortar in both fresh and
salt water. Smeaton’s work was followed by Joseph Aspdin, a
bricklayer and mason in Leeds, England, who, in 1824, patented
the fi rst ‘Portland’ cement, so named since it resembled the
stone quarried on the Isle of Portland off the British coast (Reed,
et al. 2008). Aspdin was the fi rst to use high temperatures to
heat alumina and silica materials, so that cement was formed.
It is interesting to note that cement is still made in this way. I.K.
Brunel was the fi rst to use Portland cement in an engineering
application in 1828; it was used to fi ll a breach in the Thames
Tunnel. During 1959–67, Portland cement was used in the
construction of the London sewer system.
The small rowboats built by Jean-Louis Lambot in the early
1850s are cited as the fi rst successful use of reinforcements
in concrete. During 1850–1880, a French builder, Francois
Coignet, built several large houses of concrete in England and
France (Reed, et al. 2008). Joseph Monier of France, who is
considered to be the fi rst builder of RC, built RC reservoirs in
1872. In 1861, Monier published a small book, Das System
Monier, in which he presented the applications of RC. During
1871–75, William E. Ward built the fi rst landmark building in
RC in Port Chester, NY, USA. In 1892, François Hennebique
of France patented a system of steel-reinforced beams,
slabs, and columns, which was used in the construction of
various structures built in England between 1897 and 1919.
In Hennebique’s system, steel reinforcement was placed
correctly in the tension zone of the concrete; this was backed
by a theoretical understanding of the tensile and compressive
forces, which was developed by Cottançin in France in 1892
(Reed, et al. 2008).
CASE STUDY
The Ingalls Building
The Ingalls Building, built in 1903 in Cincinnati, Ohio, is the
world’s fi rst RC skyscraper. This 15-storey building was designed
by the Cincinnati architectural fi rm Elzner & Anderson and
engineer Henry N. Hooper. Prior to 1902, the tallest RC structure
in the world was only six storeys high. Since concrete possesses
very low tensile strength, many at that time believed that a concrete
tower as tall as the Ingalls Building would collapse under wind
loads or even its own weight. When the building was completed
and the supports removed, one reporter allegedly stayed awake
through the night in order to be the fi rst to report on the building’s
failure.
Hooper designed a monolithic concrete box of 200 mm walls,
with the fl oors, roof, beams, columns, and stairs all made of
concrete. Columns measured 760 mm by 860 mm for the fi rst 10
oors and 300 mm2 for the rest. It was completed in eight months,
and the fi nished building measured 15 m by 30 m at its base and
was 64 m tall.
Still in use, the building was designated a National Historic
Civil Engineering Landmark in 1974 by the American Society of
Civil Engineers; in 1975, it was added to the American National
Register of Historic Places.
15-storey Ingalls Building in Cincinnati, Ohio
(Source: http://en.wikipedia.org/wiki/Ingalls_Building)
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 3
Earnest L. Ransome patented a reinforcing system using
twisted rods in 1884; he also built the fi rst RC framed building
in Pennsylvania, USA, in 1903. In 1889, the fi rst concrete
reinforced bridge was built. The Ingalls building, which is
the fi rst concrete skyscraper, was built in 1904 using the
Ransome system and is still in use.
By the 1900s, concrete was generally used in conjunction
with some form of reinforcement, and steel began to
replace wrought iron as the predominant tensile material.
A signifi cant advance in the development of RC was the pre-
stressing of steel reinforcing, which was developed by Eugène
Freyssinet, in the 1920s, but the technique was not widely used
until the 1940s. Victoria skyscraper in Montreal, constructed
in 1964, with a height of 190 m and utilizing 41 MPa concrete
in the columns, paved way for high-strength concretes (HSCs)
(Shaeffer 1992).
In 1908, Prof. Mörsch and Bach of the University of Stuttgart
conducted a large number of tests to study the behaviour of
RC elements. Prof. Mörsch’s work can be considered to be the
starting point of modern theory of RC design. Thaddeus Hyatt,
an American, was probably the fi rst to correctly analyse the
stresses in an RC beam and in 1877 published a small book. In
1895, A. Considére of France tested RC beams and columns
and in 1897 published the book Experimental Researches on
Reinforced Concrete. Several early studies of RC members
were based on ultimate strength theories, for example,
exure theory of Thullie in 1897 and the parabolic stress
distribution theory of Ritter in 1899. However, the straight
line (elastic) theory of Coignet and Tedesco, developed in
1900, was accepted universally because of its simplicity.
The ultimate strength design was adopted as an alternative
to the working stress method only in 1956–57. Ecole des
Ponts et Chaussees in France offered the fi rst teaching course
in RC design in 1897. The fi rst British code was published
in 1906 and the fi rst US code in 1916. The fi rst Indian code
was published in 1953 and revised in 1957, 1964, 1978,
and 2000.
1.1.2 Advantages and Disadvantages of Concrete
Reinforced concrete has been used in a variety of applications,
such as buildings, bridges, roads and pavements, dams,
retaining walls, tunnels, arches, domes, shells, tanks, pipes,
chimneys, cooling towers, and poles, because of the following
advantages:
Moulded to any shape It can be poured and moulded into
any shape varying from simple slabs, beams, and columns to
complicated shells and domes, by using formwork. Thus, it
allows the designer to combine the architectural and structural
functions. This also gives freedom to the designer to select
any size or shape, unlike steel sections where the designer is
constrained by the standard manufactured member sizes.
Availability of materials The materials required for concrete
(sand, gravel, and water) are often locally available and are
relatively inexpensive. Only small amounts of cement (about
14% by weight) and reinforcing steel (about 2–4% by volume)
are required for the production of RC, which may have to be
shipped from other parts of the country. Moreover, reinforcing
steel can be transported to most construction sites more easily
than structural steel sections. Hence, RC is the material of
choice in remote areas.
Low maintenance Concrete members require less mainte-
nance compared to structural steel or timber members.
Water and fi re resistance RC offers great resistance to the
actions of fi re and water. A concrete member having suffi cient
cover can have one to three hours of fi re resistance rating
without any special fi re proofi ng material. It has to be noted
that steel and wood need to be fi reproofed to obtain similar
rating—steel members are often enclosed by concrete for
re resistance. If constructed and cured properly, concrete
surfaces could provide better resistance to water than
steel sections, which require expensive corrosion-resistant
coatings.
Good rigidity RC members are very rigid. Due to the
greater stiffness and mass, vibrations are seldom a problem in
concrete structures.
Compressive strength Concrete has considerable compres-
sive strength compared to most other materials.
Economical It is economical, especially for footings,
basement walls, and slabs.
Low-skilled labour Comparatively lower grade of skilled
labour is required for the fabrication, erection, and construction
of concrete structures than for steel or wooden structures.
In order to use concrete effi ciently, the designer should
also know the weakness of the material. The disadvantages of
concrete include the following:
Low tensile strength Concrete has a very low tensile
strength, which is about one-tenth of its compressive strength
and, hence, cracks when subjected to tensile stresses.
Reinforcements are, therefore, often provided in the tension
zones to carry tensile forces and to limit crack widths. If
proper care is not taken in the design and detailing and also
during construction, wide cracks may occur, which will
subsequently lead to the corrosion of reinforcement bars
(which are also termed as rebars in the USA) and even failure of
structures.
Requires forms and shoring Cast in situ concrete con-
struction involves the following three stages of construction,
which are not required in steel or wooden structures:
(a) Construction of formwork over which concrete will be
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4 Design of Reinforced Concrete Structures
poured—the formwork holds the concrete in place until
it hardens suffi ciently, (b) removal of these forms, and (c)
propping or shoring of new concrete members until they gain
suffi cient strength to support themselves. Each of these stages
involves labour and material and will add to the total cost of
the structure. The formwork may be expensive and may be in
the range of one-third the total cost of an RC structure. Hence,
it is important for the designer to make efforts to reduce the
formwork cost, by reusing or reducing formwork.
Relatively low strength Concrete has relatively low strength
per unit weight or volume. (The compressive strength of
normal concrete is about 5–10% steel, and its unit density is
about 31% steel; see Table 1.2.) Hence, larger members may
be required compared to structural steel. This aspect may be
important for tall buildings or long-span structures.
TABLE 1.2 Physical properties of major structural materials
Item Mild Steel Concrete1
M20 Grade
Wood
Unit mass,
kg/m3
7850 (100)32400 (31)3290–900
(4–11)3
Maximum stress in
MPa
Compression
Tension
Shear
250 (100)
250 (100)
144 (100)
20 (8)
3.13 (1.3)
2.8 (1.9)
5.2–232 (2–9)
2.5–13.8 (1–5)
0.6–2.6 (0.4–1.8)
Young’s modulus,
MPa
2 × 105 (100) 22,360 (11) 4600–18,000
(2–9)
Coeffi cient of
linear thermal
expansion, °C ×
106
12 10–14 4.5
Poisson’s ratio 0.3 0.2 0.2
Notes:
1 Characteristic compressive strength of 150 mm cubes at 28 days
2 Parallel to grain
3 The values in brackets are relative percentage values as compared to steel.
Time-dependent volume changes Concrete undergoes
drying shrinkage and, if restrained, will result in cracking or
defl ection. Moreover, defl ections will tend to increase with
time due to creep of the concrete under sustained loads (the
defl ection may possibly double, especially in cantilevers).
It has to be noted that both concrete and steel undergo
approximately the same amount of thermal expansion or
contraction; see Table 1.2.
Variable properties The properties of concrete may widely
vary due to variation in its proportioning, mixing, pacing,
and curing. Since cast in situ concrete is site-controlled, its
quality may not be uniform when compared to materials such
as structural steel and laminated wood, which are produced in
the factory.
CO2 emission Cement, commonly composed of calcium
silicates, is produced by heating limestone and other
ingredients to about 1480 °C by burning fossil fuels, and it
accounts for about 5–7 per cent of CO2 emissions globally.
Production of one ton of cement results in the emission of
approximately one ton of CO2. Hence, the designer should
specify cements containing cementitious and waste materials
such as fl y ash and slags, wherever possible. Use of fl y ash and
other such materials not only reduces CO2 emissions but also
results in economy as well as improvement of properties such
as reduction in heat of hydration, enhancement of strength
and/or workability, and durability of concrete (Neville 2012;
Subramanian 2007; Subramanian 2012).
1.2 CONCRETE-MAKING MATERIALS
As already mentioned, the present-day concrete is made up
of cement, coarse and fi ne aggregates, water, and a host of
mineral and chemical admixtures. When mixed with water,
the cement becomes adhesive and capable of bonding the
aggregates into a hard mass, called concrete. These ingredients
are briefl y discussed in the following sections.
1.2.1 Cement—Portland Cement and Other Cements
The use of naturally occurring limestone will result in
natural cement (hydraulic lime), whereas carefully controlled
computerized mixing of components can be used to make
manufactured cements (Portland cement). Portland cements
are also referred to as hydraulic cements, as they not only
harden by reacting with water but also form a water-resistant
product. The raw materials used for the manufacture of
cement consist of limestone, chalk, seashells, shale, clay,
slate, silica sand, alumina and iron ore; lime (calcium) and
silica constitute about 85 per cent of the mass.
The process of manufacture of cement consists of grinding
the raw materials fi nely, mixing them thoroughly in certain
proportions, and then heating them to about 1480°C in
huge cylindrical steel rotary kilns 3.7–10 m in diameter and
50–150 m long and lined with special fi rebrick. (The rotary
kilns are inclined from the horizontal by about 3° and rotate
on its longitudinal axis at a slow and constant speed of about
1–4 revolutions/minute.) The heated materials sinter and
partially fuse to form nodular shaped and marble- to fi st-sized
material called clinker. (It has to be noted that at a temperature
range of 600–900°C, calcination takes place, which results
in the release of environmentally harmful CO2). The clinker
is cooled (the strength properties of cement are considerably
infl uenced by the cooling rate of clinker) and ground into fi ne
powder after mixing with 3–5 per cent gypsum (gypsum is
added to regulate the setting time of the concrete) to form
Portland cement. (In modern plants, the heated air from the
coolers is returned to the kilns, to save fuel and to increase
the burning effi ciency). It is then loaded into bulk carriers or
packaged into bags; in India, typically 50 kg bags are used.
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 5
Two different processes, known as dry and wet, are used in the
manufacture of Portland cement, depending on whether the mixing
and grinding of raw materials is done in dry or wet conditions.
In addition, a semi-dry process is also sometimes employed in
which the raw materials are ground dry, mixed with water, and
then burnt in the kilns. Most modern cement factories use either
a dry or a semi-dry process. The schematic representation of the
dry process of cement manufacture is shown in Fig. 1.1.
Portland Cement
Portland cement (often referred to as ordinary Portland cement
or OPC) is the most common type of cement in general use
around the world. The different types of cements covered by
the Indian and US standards and their chemical compounds are
shown in Table 1.3. Cement production in India consists mainly
of the following three types (see Fig. 1.2): OPC 39 per cent,
Portland pozzolana cement (PPC) 52 per cent, and Portland
slag cement (PSC) 8 per cent. All other varieties put together
comprise only 1 per cent of the total production (Mullick 2007).
TABLE 1.3 Types of Portland cements
India/UK USA
(ASTM)
Typical Compounds3
OPC (IS 269,
IS 8112 and
IS 12269)
Type I1C3S 55%, C2S 19%, C3A 10%,
C4AF 7%, MgO 2.8%, SO3 2.9%,
Ignition loss 1.0%, and free CaO 1.0%
(C3A < 15%)
Type II1C3S 51%, C2S 24%, C3A 6%, C4AF
11%, MgO 2.9%, SO3 2.5%, Ignition
loss 1.0%, and free CaO 1.0%
(C3A < 8%)
Rapid
hardening
Portland
cement (IS
8041:1990)
Type III1C3S 57%, C2S 19%, C3A 10%,
C4AF 7%, MgO 3.0%, SO3 3.1%,
Ignition loss 0.9%, and free CaO
1.3%
Its seven day compressive strength is
almost equal to Types I and II 28 day
compressive strengths.
Low heat
Portland
cement (IS
12600:1989)
Type IV C3S 28%, C2S 49%, C3A 4%, C4AF
12%, MgO 1.8%, SO3 1.9%, Ignition
loss 0.9%, and free CaO 0.8% (C3A <
7% and C3S < 35%)
Sulphate
resisting
Portland
cement (IS
12330:1988)
Type V C3S 38%, C2S 43%, C3A 4%, C4AF 9%,
MgO 1.9%, SO3 1.8%, Ignition loss
0.9%, and free CaO 0.8% [C3A < 5%
and (C4AF) + 2(C3A) < 25%]
PSC (IS
455:1989, IS
12089:1987)
Type IS Made by grinding granulated high-
quality slag with Portland cement clinker
(Continued)
1
2
3
4
5
6
7
8
9
10
Quarrying raw materials
such as clay, shale, and
limestone
Crushing of
raw materials
Pre-homogenization
and grinding in raw mill
Pre-heating in cyclones by
hot gases from the kiln and
pre-calcinator
Pre-calcinator
Production of clinker in rotary kiln
Cooling and storing of clinker
Blending with gypsum
Grinding of cement
Storing in cement silos
and packaging
Electro static precipitator
Kiln flame
FIG. 1.1 Dry process of cement manufacture (a) Schematic representation (b) View of MCL Cement plant, Thangskai, Meghalaya
Sources: www.cement.org/basics/images/fl ashtour.html and http://en.wikipedia.org/wiki/File:Cement_Plant_MCL.jpg (adapted)
Others, 1%Slag cement, 8%
PPC, 52%
OPC, 39%
FIG. 1.2 Production trend of different varieties of cement in India
Source: Mullick 2007
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6 Design of Reinforced Concrete Structures
TABLE 1.3 (Continued)
India/UK USA
(ASTM)
Typical Compounds3
PPC [IS
1489-
Part 1:1991
(fl y ash
based), IS
1489-Part
2:1991
(calcined
clay based)]
Type IP A blended cement made by inter-
grinding Portland cement and pozzolanic
materials without burning
Ternary
blended
cement
Type
IT(SX)
(PY)2
A blended cement made by inter-
grinding Portland cement, slag, and
pozzolana without burning
Notes:
1
Types Ia, IIa, and IIIa have the same composition as Types I, II, and III, but
have an air-entraining agent ground into the mix.
2
The letters X and Y stand for the percentage of supplementary cementitious
material (SCM) included in the blended cement, and S and P are the types
of SCMs, where S is for slag and P for pozzolan. For example, Type IT(S25)
(P15) contains 25 per cent slag and 15 per cent pozzolans.
3 See Table 1.5 for explanation of these compounds.
There are other types, such as high alumina cement (IS
6452:1989), super sulphated cement (IS 6909:1990),
hydrophobic Portland cement (IS 8043: 1991), white
cement (IS 8042:1989), concrete sleeper grade cement
(IRS-T 40:1985), expanding cements, and masonry cement
(IS 3466:1988), which are used only in some special
situations. (Refer to Mehta and Monteiro (2006) and Shetty
(2005) for details regarding these cements.) Geopolymer
cements are inorganic hydraulic cements that are based
on the polymerization of minerals (see Section 4.4.7 of
Chapter 4).
Ordinary Portland cement is the most important cement
and is often used, though the current trend is to use PPC
(see Fig. 1.2). Most of the discussions to follow in this
chapter pertain to this type of cement. The Bureau of Indian
Standards (BIS) has classifi ed OPC into the following three
grades:
1. 33 grade OPC, IS 269:1989
2. 43 grade OPC, IS 8112:1989
3. 53 grade OPC, IS 12269:1987
The number in the grade indicates the compressive strength
of the cement in N/mm2 at 28 days. The 33 grade cement is
suitable for producing concrete up to M25. Both 43 grade and
53 grade cement are suitable for producing higher grades of
concrete. The important physical properties of the three grades
of OPC and other types of cements are compared in Table 1.4.
The chemical composition of OPC is given in Table 1.5 and
Fig. 1.3.
Approximately 95 per cent of cement particles are smaller
than 45 micrometres, with the average particle being around
15 micrometres. The overall particle size distribution of
cement is called neness. Fineness affects the heat released
and the rate of hydration; greater fi neness causes greater
early strength (especially during the fi rst seven days) and
more rapid generation of heat. Soundness refers to the ability
of the cement paste to retain its volume after setting and is
related to the presence of excessive amounts of free lime or
TABLE 1.4 Physical properties of various types of cements
S. No. Type of
Cement
IS Code Fineness
m2/kg
(min.)
Setting Time in
Minutes
Soundness Compressive Strength in MPa
Initial
(min.)
Final
(max.)
Le Chatelier
(max.) (mm)
Auto Clave, for
MgO, (max.) (%)
3 days 7 days 28 days
1. OPC 33 269:1989 225 30 600 10 0.8 16 22 33
2. OPC 43 8112:1989 225 30 600 10 0.8 23 33 43
3. OPC 53 12269:1987 225 30 600 10 0.8 27 37 53
4. PPC (fl y
ash-based)
1489:1991
(Part 1)
300 30 600 10 0.8 16 22 33
5. PSC (slag) 455:2002 225 30 600 10 0.8 16 22 33
6. SRC 12330:1988 225 30 600 10 0.8 10 16 33
TABLE 1.5 Chemical composition of OPC (Bogue’s Compounds)
S. No. Compound Cement
Chemist
Notation
(CCN)*
Typical
Composition
as %
Mineral
Phase
1. Tricalcium silicate
3(CaO)·SiO2
C3S 45–65 Alite
2. Dicalcium silicate
2(CaO)·SiO2
C2S 15–30 Belite
3. Tricalcium aluminate
3(CaO)·Al2O3
C3A 5–10 Aluminate
4. Tetracalcium
alumino ferrite
4(CaO)·Al2O3·Fe2O3
C4AF 5–15 Ferrite
5. Gypsum CaSO4·2 H2O 2–10
*Cement chemists use the following shorthand notation:
C = CaO, S = SiO2, A = Al2O3, F = Fe2O3, M = MgO,
H = H2O, N = Na2O, K = K2O,
S
= SO3.
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 7
magnesia in the cement. Consistency indicates the degree of
density or stiffness of cement. Initial setting of cement is that
stage when the paste starts to lose its plasticity. Final setting
is the stage when the paste completely loses its plasticity and
attains suffi cient strength and hardness. The specifi c gravity
of Portland cement is approximately 3.15.
As seen in Table 1.5 and Fig. 1.3, there are four major
compounds in cement and these are known as tricalcium silicate
(C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A),
and tetracalcium alumino ferrite (C4AF). Their composition
varies from cement to cement and plant to plant. (The levels
of the four clinker minerals can be estimated using a method
of calculation fi rst proposed by Bogue in 1929 or by the X-ray
diffraction analysis, which gives the exact measurement.) In
addition, to these compounds, there are other minor compounds
such as MgO, Na2O, K2O, SO3, fl uorine, chloride, and trace
metals, which are present in small quantities (Moir 2003). Of
these K2O and Na2O are called alkalis and are found to react
with some aggregates, resulting in alkali–silica reaction (ASR),
which causes disintegration of concrete at a later date.
The silicates C3S and C2S are the most important compounds
and are mainly responsible for the strength of the cement paste.
They constitute the bulk of the composition. C3A and C4AF do
not contribute much to the strength, but in the manufacturing
process they facilitate the combination of lime and silica and
act as a fl ux. The role of the different compounds on different
properties of cement is shown in Table 1.6.
Portland Pozzolana Cement
As mentioned already, the Romans and Greeks were aware that
the addition of volcanic ash results in better performance of
concrete. The name pozzolan is now frequently used to describe
a range of materials both natural and artifi cial. [A pozzolan
may be defi ned as a siliceous or siliceous and aluminous
material, which in itself possesses little or no cementitious
value. However, in fi nely divided form and in the presence of
water, it reacts chemically with calcium hydroxide released
by the hydration of Portland cement, at ordinary temperature,
to form calcium silicate hydrate and other cementitious
compounds possessing cementitious properties (Mehta 1987)].
Fly ash, ground granulated blast furnace slag (GGBS), silica
fume, and natural pozzolans, such as calcined shale, calcined
clay or metakaolin, are used in conjunction with Portland
cement to improve the properties of the hardened concrete.
The latest amendment (No. 3) to IS 1489 requires that PPC be
manufactured by the inter-grinding of OPC clinker with 15–35
per cent of pozzolanic material. The generally used pozzolanic
materials in India are fl y ash (IS 1489-Part 1) or calcined clay
(IS 1489-Part 1). Mixtures using three cementitious materials,
called ternary mixtures, are becoming common, but no Indian
specifi cation regarding this has been developed yet. UltraTech
PPC, Suraksha, Jaypee Cement (PPC) are some of the brand
names of PPC in India. As of now, in India, PPC is considered
equivalent to 33 grade OPC.
PPC offers the following advantages:
1. Economical than OPC as the costly clinker is replaced by
cheaper pozzolanic material
2. Converts soluble calcium hydroxide into insoluble cementitious
products, thus improving permeability and durability
3. Consumes calcium hydroxide and does not produce as
much calcium hydroxide as OPC
4. Improves pore size distribution and reduces micro-cracks at
the transition zone due to the presence of fi ner particles than
OPC
5. Reduces heat of hydration and thermal cracking
6. Has high degree of cohesion and workability in concrete
and mortar
The main disadvantage is that the rate of development of
strength is initially slightly slower than OPC. In addition, its
CaCO
3
(Limestone)
Fe
2
O
3
(Iron oxide)
SiO
2
(Silica sand)
2SiO
2
Al
2
O
3
(Clay, shale)
3CaO
SiO
2
2CaO • SiO
2
3CaO • Al
2
O
3
4CaO • Al
2
O
3
Fe
2
O
3
CaO • SO
3
2H
2
O
Kiln
Gypsum + Clinker
Finished
cement
Interground
1450°C
FIG. 1.3 Chemical compounds of cement
TABLE 1.6 Role of different compounds on properties of cement
Characteristic Different Compounds in Cement
C3SC
2SC
3AC
4AF
Setting Quick Slow Rapid
Hydration Rapid Slow Rapid
Heat
liberation
(Cal/g) 7 days
Higher Lower Higher Higher
Early strength High up to
14 days
Low up to
14 days
Not much
beyond 1
day
Insignifi cant
Later strength Moderate
at later
stage
High at
later stage
after 14
days
––
© Oxford University Press 2013. All rights reserved.
8 Design of Reinforced Concrete Structures
effect of reducing the alkalinity may reduce the resistance
to corrosion of steel reinforcement. However, as PPC
signifi cantly lowers the permeability, the risk of corrosion is
reduced. The setting time is slightly longer.
Portland Slag Cement
Blast furnace slag is a non-metallic product consisting essentially
of silicates and alumino-silicates of calcium developed in a
molten condition simultaneously with iron in a blast furnace.
GGBS is obtained by rapidly cooling the molten slag, which is
at a temperature of about 1500°C, by quenching in water or air
to form a glassy sand-like granulated material. Every year about
nine million tons of blast furnace slag is produced in India.
The GGBS should conform to IS I2089:1987. PSC is obtained
either by intimate inter-grinding of a mixture of Portland
cement clinker and granulated slag with the addition of gypsum
or calcium sulphate or by an intimate and uniform blending of
Portland cement and fi nely ground granulated slag. Amendment
No. 4 of IS 455 requires that the slag constituent not be less than
25 per cent or more than 70 per cent of the PSC. It has to be
noted that PSC has physical properties similar to those of OPC.
The following are some advantages of PSC:
1. Utilization of slag cement in concrete not only lessens the
burden on landfi lls; it also conserves a virgin manufactured
product (OPC) and decreases the embodied energy
required to produce the cementitious materials in concrete.
Embodied energy can be reduced by 390–886 million
Joules with 50 per cent slag cement substitution. This is a
30–48 per cent reduction in the embodied energy per cubic
metre of concrete (http://www.slagcement.org).
2. By using a 50 per cent slag cement substitution less CO2
is emitted (amounting to about 98 to 222 kg per cubic
metre of concrete, a 42–46% reduction in greenhouse gas
emissions) (http://www.slagcement.org).
3. Using slag cement to replace a portion of Portland cement
in a concrete mixture is a useful method to make concrete
better and more consistent. PSC has a lighter colour,
better concrete workability, easier fi nishability, higher
compressive and fl exural strength, lower permeability,
improved resistance to aggressive chemicals, and more
plastic and hardened consistency.
4. The lighter colour of slag cement concrete also helps
reduce the heat island effect in large metropolitan areas.
5. It has low heat of hydration and is relatively better
resistant to soils and water containing excessive amounts
of sulphates and is hence used for marine works, retaining
walls, and foundations.
Both PPC and PSC will give more strength than OPC at the
end of 12 months. UltraTech Premium, Super Steel (Madras
Cement), and S 53 (L&T) are some of the brand names of
PSC available in India.
Storage of Cement
Cement is very fi nely ground and readily absorbs moisture;
hence, care should be taken to ensure that the cement bags are
not in contact with moisture. They should be stored in airtight
and watertight sheds and used in such a way that the bags that
come in fi rst are the fi rst to go out. Cement stored for a long
time tends to lose its strength (loss of strength ranges from
5–10% in three months to 30–40% in one year). It is better to
use the cement within 90 days of its production. In case it is
used at a later date, it should be tested before use.
Tests on Cement
The usual tests carried out for cement are for chemical and
physical requirements. They are given in IS 4031 (different
parts) and IS 4032. Most of these tests are conducted at a
laboratory (Neville 2012).
Fineness is measured by the Blaine air permeability test,
which indirectly measures the surface area of the cement
particles per unit mass (m2/kg), or by actual sieving (IS 4031-
Part 1:1996 and Part 2:1999). Most cement standards have
a minimum limit on fi neness (in the range 225–500 m2/kg).
Soundness of cement is determined by Le-Chatelier and
autoclave tests, as per IS 4031-Part 3:1988. Consistency is
measured by Vicat apparatus, as per IS 4031-Part 4:1988. The
paste is said to be of standard consistency when the penetration
of plunger, attached to the Vicat apparatus, is 33–35 mm. The
initial and fi nal setting times of cement are measured using
the Vicat apparatus with different penetrating attachments, as
per IS 4031-Part 5:1988. It has to be noted that the setting
time decreases with increase in temperature; the setting time
of cement can be increased by adding some admixtures. The
compressive strength of cement is the most important of all
the properties. It is found using a cement–sand mortar (ratio
of cement to sand is 1:3) cube of size 70.6 mm, as per IS 4031-
Part 6:1988. The compressive strength is taken as the average
of strengths of three cubes. The heat of hydration is tested
in accordance with IS 4031-Part 9:1988 using vacuum fl ask
methods or by conduction calorimetry.
A web-based computer software called Virtual Cement
and Concrete Testing Laboratory (eVCCTL) has been
developed by scientists at the National Institute of Standards
and Technology (NIST), USA, which can be used to explore
the properties of cement paste and concrete materials. This
software may be found at http://www.nist.gov/el/building_
materials/evcctl.cfm.
1.2.2 Aggregates
The fi ne and coarse aggregates occupy about 60–75 per cent
of the concrete volume (70–85% by mass) and hence strongly
infl uence the properties of fresh as well as hardened concrete,
its mixture proportions, and the economy. Aggregates
used in concrete should comply with the requirement of
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 9
S. No. Factors Infl uence on Concrete Property
6. Gradation or particle size
distribution
Water demand (strength),
cohesion, bleeding, and
segregation
7. Maximum size of aggregate Strength and water demand
8. Presence of deleterious
materials such as dust, clay,
silt, or mud
Water demand (strength),
cohesion, bond, and durability
TABLE 1.8 Properties of aggregates
Aggregate Property Aggregate Property
Specifi c Gravity Minimum
Voids (%)
Gravel 2.67 River sand
Granite 2.80 Fine 43
Sand 2.65 Coarse 35
Basalt 2.85 Mixed and moist 38
Bottom ash 1.57 Mixed and dry 30
Bulk density (kg/l)
Broken granite 1.68 Broken stone,
graded
Broken stone 1.60 Maximum
size: 25 mm
46
Stone screening 1.45 Maximum
size: 50 mm
45
IS 383:1970. Aggregates are commonly classifi ed into fi ne
and coarse aggregates. Fine aggregates generally consist
of natural sand or crushed stone with particle size smaller
than about 5 mm (materials passing through 4.75 mm IS
sieve). Coarse aggregates consist of one or a combination of
gravels or crushed stone with particle size larger than 5 mm
(usually between 10 mm and 40 mm). Aggregates can also be
classifi ed in two more ways. Depending on the source, they
could be either naturally occurring
(gravel, pebbles, sand, etc.) or syn-
thetically manufactured (bloated clay
aggregates, sintered fl y ash aggregate,
etc.). Moreover, depending on the
bulk density, aggregates can either be
normal weight (1520–1680 kg/m3),
lightweight (less than 1220 kg/m3),
or heavyweight (more than 2000 kg/
m3). The normal weight aggregates—
sand, gravel, crushed rock (e.g.,
granite, basalt, and sand stone),
and blast furnace slag—are used
to produce normal weight concrete
with a density of 2200–2400 kg/m3.
Aggregates such as expanded shale,
clay, slate, slag, pumice, perlite,
vermiculite, and diatomite are used
to produce structural lightweight
concrete (SLWC) with density
ranging from about 1350 kg/m3 to
1850 kg/m3. Heavyweight aggregates
consist of hematite, steel, or iron and
are used in special applications such
as providing radiation shielding and
abrasion resistance (ACI 301M:10
2010, ACI Committee E-701 2007).
The factors of aggregates that may directly or indirectly
infl uence the properties of concrete are given in Table 1.7
(Ambuja technical booklets 5:1996, 125:2007). Only normal
weight aggregates are discussed here and should confi rm to IS
383:1970. The coarse aggregates form the main matrix of the
concrete and hence provide strength to the concrete, whereas
the fi ne aggregates form the fi ller matrix and hence reduce the
porosity of concrete. Some properties of aggregates are shown
in Table 1.8.
TABLE 1.7 Factors of aggregates that may affect properties of concrete
S. No. Factors Infl uence on Concrete Property
1. Specifi c gravity/Porosity Strength/Absorption of water
2. Crushing strength Strength
3. Chemical stability Durability
4. Surface texture Bond grip
5. Shape (see Fig. 1.4) Water demand (strength)
Sieve
size
Sieve
size
Rounded
40
mm
80
mm
40
mm
20
mm
10
mm
Fine
20
mm
10
mm
4.8
mm
Irregular Crushed Agg.
size
2.4 1.2 0.6 0.3 0.15 2.4 1.2 0.6 0.3 0.15
FIG. 1.4 Different shapes and sizes of aggregates
Source: Ambuja technical booklet 125:2007
(Continued)
© Oxford University Press 2013. All rights reserved.
10 Design of Reinforced Concrete Structures
TABLE 1.8 (Continued)
Aggregate Property Aggregate Property
Specifi c Gravity Minimum
Voids (%)
Beach or river
Shingle
1.60 Maximum
size: 63 mm
41
River sand Stone
screening
48
Fine 1.44 Fineness
Modulus
Medium 1.52 Sand 2.70
Coarse 1.60 Bottom ash 2.08
In several countries including India, natural course aggregates
and river sand are scarce; at the same time, the waste from
the demolition of buildings is escalating. The amount of
construction waste in India alone is estimated to be around
12–14.7 million tons per annum (Rao, et al. 2011). In such
places, recycled coarse aggregates (RCA) could be used
profi tably. More details about RCA and their use in concrete
may be found in the works of Dhir and Paine (2010), Rao, et
al. (2011), and Subramanian (2012). In general, mechanical
properties such as compressive strength, split and tensile
strengths, and modulus of elasticity are reduced with
increasing percentage of RCA. It is suggested that 25 per
cent of RCA may be used in concrete, as it will not affect the
properties signifi cantly (Rao, et al. 2011). Other substitutes
for coarse aggregate include incinerator bottom ash aggregate
and sintered fl y ash pellets. Recycled glass aggregates,
bottom ash from thermal power plants, and quarry dust have
signifi cant potential for use as fi ne aggregates in concrete
(Dhir and Paine 2010; Mullick 2012). Clause 5.3.1 of IS
456 stipulates that such aggregates should not contain more
than 0.5 per cent of sulphates as SO3 and should not absorb
more than 10 per cent of their own mass of water. Before
using these materials, it is better to study their effect on the
properties of concrete. For example, manufactured sand,
often referred to as M-sand, from crushed gravel or rock is
cubical in shape and results in increased water demand of the
concrete mix.
Aggregates must be clean, hard, strong, and durable; they
should be free from coatings of clay, absorbed chemicals, and
other fi ne materials that could affect the hydration and bond
of the cement paste. Aggregates are usually washed to remove
impurities and graded at the site or plant. Grading or particle
size distribution of aggregates is a major factor determining
the workability, segregation, bleeding, placing, and fi nishing
characteristics of concrete. The grading of fi ne aggregates
has been found to infl uence the properties of green (fresh)
concrete more than those of coarse aggregates. The grading
requirements recommended by the Indian and US standards
for fi ne aggregates is given in Table 1.9. Combined gradation
of fi ne and coarse aggregate may result in better control of
workability, pumpability, shrinkage, and other properties of
concrete (Kosmatka, et al. 2011). In general, aggregates that
do not have a large defi ciency or excess of any size and give
a smooth grading curve will produce the most satisfactory
results (Kosmatka, et al. 2011). Coarse and fi ne aggregates
should be batched separately.
TABLE 1.9 Grading requirements for fi ne aggregates
IS Sieve
Designation
Percentage Passing by Weight for
Grading Zone
ASTM
Standard
C 33
I II III IV
10 mm 100 100 100 100 100
4.75 mm 90–100 90–100 90–100 95–100 95–100
2.36 mm 60–95 75–100 85–100 95–100 80–100
1.18 mm 30–70 55–90 75–100 90–100 50–85
600 µm 15–34 35–59 60–79 80–100 25–60
300 µm 5–20 8–30 12–40 15–50 5–30
150 µm 0–10 0–10 0–10 0–15 0–10
The neness modulus (FM) of either fi ne or coarse aggregate
is calculated by adding the cumulative percentages by
mass retained on each of the series of sieves and dividing
the sum by 100. The higher the FM, the coarser will be the
aggregate. The maximum size of coarse aggregate should not
be greater than the following: one-fourth of the maximum
size of member, 5 mm less than the maximum clear distance
between the main bars, or 5 mm less than the minimum cover
of the reinforcement. For RCC works, 20 mm aggregates
are preferred. In thin concrete members with closely spaced
reinforcement or small cover and in HSC, Clause 5.3.3 of IS
456 allows the use of 10 mm nominal maximum size. Rounded
aggregates are preferable to angular or fl aky aggregates, as
they require minimum cement paste for bond and demand less
water. Flaky and elongated aggregates are also susceptible to
segregation and low strength.
It should be noted that the amount of water added to make
concrete must be adjusted for the moisture conditions of the
aggregates to accurately meet the water requirement of the mix
design. Various testing methods for aggregates to concrete are
described in IS 2386-Parts 1 to 8:1963.
1.2.3 Water
Water plays an important role in the workability, strength, and
durability of concrete. Too much water reduces the concrete
strength, whereas too little will make the concrete unworkable.
The water used for mixing and curing should be clean and free
from injurious amounts of oils, acids, alkalis, salts, sugars, or
organic materials, which may affect the concrete or steel. As per
Clause 5.4 of IS 456, potable water is considered satisfactory
for mixing as well as curing concrete; otherwise, the water to
be used should be tested as per IS 3025-Parts 1 to 32 (1984
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 11
to 1988). In general, sea water should not be used for mixing
or curing concrete. The permissible limits for impurities as per
Clause 5.4 of IS 456 are given in Table 1.10. The pH value of
water used for mixing should be greater than six.
TABLE 1.10 Permissible limits for impurities in mixing water
Impurity Maximum Permissible Limit
IS 456 (mg/l) ASTM C 94
(ppm)
Organic 200
Inorganic 3000
Sulphates
(such as SO3)
400 3000
Chlorides
(such as Cl)
2000 (for plain concrete
work)
500 (for RCC)
10001
Suspended matter 2000 50,000
Alkalis
(such as Na2O +
0.658K2O)
– 600
Note:
1 Prestressed concrete or concrete in bridge decks 500 ppm (ppm and mg/l are
approximately equal)
In general, the amount of water required to be added for
cement hydration is less compared to that required for
workability. For complete hydration of Portland cement, only
about 36–42 per cent water (this is represented by the water/
cement or water/cementitious ratio, usually denoted by w/c
ratio or w/cm ratio), that is, w/c of 0.36–0.42, is needed. If
a w/c ratio greater than about 0.36 is used, the excess water,
which is not required for cement hydration, will remain in the
capillary pores or may evaporate in due course. This process
leads to drying shrinkage (drying shrinkage is destructive as it
leads to micro-cracking and may eventually weaken concrete).
Similarly, when a w/c ratio of less than about 0.36 is used,
some cement will remain unhydrated. The space initially
taken up by water in a cementitious mixture will be partially
or completely replaced over time by the hydration products.
If a w/c ratio of more than 0.36 is used, then porosity in the
hardened material will remain, even after complete hydration.
This is called capillary porosity and will lead to corrosion of
reinforcement.
1.2.4 Admixtures
It is interesting to note that the Romans were the fi rst to
use admixtures in concrete in the form of blood, milk, and
lard (pig fat). Present-day admixtures may be classifi ed as
chemical and mineral admixtures.
Chemical Admixtures
Chemical admixtures are materials in the form of powder or
uids that are added to the concrete immediately before or
during mixing in order to improve the properties of concrete.
They should comply with the requirements of IS 9103:1999.
Admixtures are used for several purposes, such as to increase
owability or pumpability of fresh concrete, obtain high
strength through lowering of w/c ratio, retard or accelerate time
of initial setting, increase freeze–thaw resistance, and inhibit
corrosion (Krishnamurthy 1997). Normal admixture dosage
is about 2–5 per cent by mass of cement. The effectiveness
of an admixture depends upon factors such as type, brand,
and amount of cementing materials; water content; aggregate
shape, gradation, and proportions; mixing time; slump; and
temperature of the concrete (Kosmatka, et al. 2011).
The common types of admixtures are as follows (Rixom
and Mailvaganan 1999; Aïtcin, et al. 1994; Kosmatka, et al.
2011):
1. Accelerators enhance the rate of hydration of the concrete
and, hence, result in higher early strength of concrete and
early removal of formwork. Typical materials used are
calcium chloride, triethenolamine, sodium thiocyanate,
calcium formate, calcium nitrite, and calcium thiosulphate.
Typical commercial products are Mc-Schnell OC and Mc-
Schnell SDS. Typical dosage is 2–3 per cent by weight of
cement. As the use of chlorides causes corrosion in steel
reinforcing, they are not used now.
2. Retarders slow down the initial rate of hydration of cement
and are used more frequently than accelerators. They are often
combined with other types of admixtures like water reducers.
Typical retarders are sugars, hydroxides of zinc and lead,
calcium, and tartaric acid. Typical dosage is 0.05 per cent to
0.10 per cent by weight of cement. Commonly used retarders
are lignosulphonic acids and hydroxylated carboxylic acids,
which act as water-reducing and water-retarding admixtures;
they delay the initial setting time by three to four hours when
used at normal ambient temperatures.
3. Water-reducing admixtures are used to reduce the
quantity of mixing water required to produce concrete.
Water-reducing admixtures are available as ordinary
water-reducing admixtures (WRA) and high-range
water-reducing admixtures (HRWRA). WRA enable
up to 15 per cent water reduction, whereas HRWRA
enable up to 30 per cent. Popularly, the former are called
plasticizers and the latter superplasticizers. In modern
day concreting, the distinction seems to be disappearing.
Compounds used in India as superplasticizers include
sulphonated naphthalene formaldehyde condensates
(SNF), sulphonated melamine formaldehyde condensates
(SMF), and modifi ed lignosulphonates (MLS). Some new
generation superplasticizers include acrylic polymer based
(AP) superplasticizers, copolymers of carboxylic acid with
acrylic ether (CAE), polycarboxylate ethers (PCs), and
multi-polycarboxylate ethers (MCEs). The naphthalene
© Oxford University Press 2013. All rights reserved.
12 Design of Reinforced Concrete Structures
and melamine types of superplasticizers or HRWRA are
typically used in the range 0.7–2.5 per cent by weight of
cement and give water reductions of 16–30 per cent. The
PCs are more powerful and are used at 0.3–1.0 per cent
by weight of cement to typically give 20 per cent to over
40 per cent water reduction. Use of superplasticizers with
reduced water content and w/c ratio can produce concretes
with (a) high workability (in fresh concretes), with increased
slump, allowing them to be placed more easily, with less
consolidating effort, (b) high compressive strengths, (c)
increased early strength gain, (d) reduced chloride ion
penetration, and (e) high durability. It has to be noted that it is
important to consider the compatibility of superplasticizers
with certain cements (Jayasree, et al. 2011; Mullick 2008).
4. Air entraining admixtures are used to entrain tiny air bubbles
in the concrete, which will reduce damage during freeze–
thaw cycles, thereby increasing the concrete’s durability.
Furthermore, the workability of fresh concrete is improved
signifi cantly, and segregation and bleeding are reduced or
eliminated. However, entrained air entails a trade off with
strength, as each 1 per cent of air may result in 5 per cent
decrease in compressive strength. The materials used in such
admixtures include salts of wood resins, some synthetic
detergents, salts of petroleum acids, fatty and resinous acids
and their salts, and salts of sulphonated hydrocarbons.
5. Corrosion inhibitors are used to minimize the corrosion of
steel and steel bars in concrete.
The other chemical admixtures include foaming agents (to
produce lightweight foamed concrete with low density),
alkali–aggregate reactivity inhibitors, bonding admixtures
(to increase bond strength), colouring admixtures, shrinkage
reducers, and pumping aids. It is important to test all chemical
admixtures adequately for their desired performance. It is also
desirable to prepare trial mixes of concrete with chemical
admixtures and test their performance before using them in
any large construction activity (see also Clause 5.5 of IS 456).
They should not be used in excess of the prescribed dosages,
as they may be detrimental to the concrete.
Mineral Admixtures
Mineral admixtures are inorganic materials that also have
pozzolanic properties. These very fi ne-grained materials are
added to the concrete mix to improve the properties of concrete
(mineral admixtures) or as a replacement for Portland cement
(blended cements). Pozzolanic materials react with the calcium
hydroxide (lime) released during the hydration process of
cement to form an additional C-S-H gel. This can reduce
the size of the pores of crystalline hydration products, make
the microstructure of concrete more uniform, and improve the
impermeability and durability of concrete. These improvements
can lead to an increase in strength and service life of concrete.
Some of the mineral admixtures are briefl y described here:
1. Fly ash is a by-product of coal-fi red thermal power plants. In
India, more than 120 million tons of fl y ash is produced every
year, the disposal of which poses a serious environmental
problem. Any coal-based thermal power station may produce
four kinds of ash: fl y ash, bottom ash, pond ash, and mound
ash. The quality of fl y ash to be used in concrete is governed
by IS 3812 (Parts 1 and 2):2003, which groups all these types
of ash as pulverized fuel ash (PFA). PFA is available in two
grades: Grade I and grade II (Class F—siliceous fl y ash and
Class C—calcareous fl y ash, as per ASTM). Both these grades
can be used as admixtures. Up to 35 per cent replacement of
cement by fl y ash is permitted by the Indian codes. Fly ash is
extracted from fl ue gases through electrostatic precipitator in
dry form. It is a fi ne material and possesses good pozzolanic
properties. The properties of fl y ash depend on the type of
coal burnt. The lower the loss on ignition (LOI), the better
will be the fl y ash. The fi neness of individual fl y ash particles
range from 1 micron to 1 mm in size. The specifi c gravity of
y ash varies over a wide range of 1.9 to 2.55. For a majority
of site-mixed concrete, fl y ash-based blended cement is the
best option. Fly ash particles are generally spherical in shape
and reduce the water requirement for a given slump. The
use of fl y ash will also result in reduced heat of hydration,
bleeding, and drying shrinkage.
2. Ground granulated blast furnace slag is a by-product
of steel production and has been used as a cementitious
material since the eighteenth century. It is currently inter-
ground with Portland cement to form blended cement,
thus partially replacing Portland cement. It reduces the
temperature in mass concrete, permeability, and expansion
due to alkali–aggregate reaction and improves sulphate
resistance. See Section 1.2.1 for more details on PSC.
3. Silica fume is also referred to as micosilica or condensed
silica fume. It is a by-product of the production of silicon
and ferrosilicon alloys. Silica fume used in concrete should
conform to IS 15388:2003; as per Clause 5.2.1.2 of IS 456,
its proportion is 5–10 per cent of cement content of a mix.
Silica fume is similar to fl y ash, with spherical shape, but
has an average particle size of about 0.1 micron, that is, it
is 100 times smaller than an average cement particle. This
results in a higher surface to volume ratio and a much faster
pozzolanic reaction. Silica fume addition benefi ts concrete
in two ways: (a) The minute particles physically decrease
the void space in the cement matrix—this phenomenon is
known as packing. (b) Silica fume is an extremely reactive
pozzolan; it increases the compressive strength and improves
the durability of concrete. Silica fume for use in concrete
is available in wet or dry form. It is usually added during
concrete production at a concrete plant. However, it generally
requires the use of superplasticizers for workability.
4. Rice husk ash (RHA) is produced by burning rice husk
in controlled temperature, without causing environmental
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 13
pollution. (India produces about 125 million tons of
paddy and 30 million tons of rice husk.) It exhibits high
pozzolanic characteristics and its use in concrete results
in high strength and impermeability. Water demand and
drying shrinkage should be studied before using rice husk.
5. High-reactivity Metakaolin (HRM) is obtained by calci-
nation of pure or refi ned kaolinitic clay at a temperature
between 650 °C and 850 °C followed by grinding to achieve
a fi neness of 700–900 m2/kg. The strength and durability of
concrete produced with the use of HRM is similar to that
produced with silica fume. Whereas silica fume is usually
dark grey or black in colour, HRM is usually bright white
in colour, making it the preferred choice for architectural
concrete, where appearance is important.
More details about mineral admixtures may be found in the
works of Bapat (2012) and Ramachandran (1995).
1.3 PROPORTIONING OF CONCRETE MIXES
Concrete mix design is the process of proportioning
various ingredients such as cement, cementitious materials,
aggregates, water, and admixtures optimally in order to
produce a concrete at minimal cost and will have specifi ed
properties of workability and homogeneity in the green state
and strength and durability in the hardened state (SP 23:1982).
Earlier mix design procedures such as minimum voids
method, Fuller’s maximum density method, Talbot–Richart
method, and fi neness modulus method are based on the principles
of minimum voids and maximum density (Krishna Raju 2002).
The modern mix design methods include the Road Note No.
4 method, the ACI (American Concrete Institute) method, the
USBR (United States Bureau of Reclamation) method, the
Bolomeya model, the British mix design method, and the BIS
method (Krishna Raju 2002; Nataraja and Reddy 2007). All
these methods are mostly based on empirical relations, charts,
graphs, and tables developed through extensive experiments
using locally available materials. Although the older BIS code
(IS 10262:1982) differed from the ACI method (ACI 211.1,
1991) in some aspects, the present BIS code (IS 10262:2009)
is in line with the ACI code method (Nataraja and Das 2010).
In all these mix proportioning methods, the ingredients are
proportioned by weight per unit volume of concrete.
The main objective of any concrete mix proportioning
method is to make a concrete that has the following features:
1. Satisfi es workability requirements in terms of slump for
easy placing and consolidating
2. Meets the strength requirements as measured by the
compressive strength
3. Can be mixed, transported, placed, and compacted as
effi ciently as possible
4. Will be economical to produce
5. Fulfi ls durability requirements to resist the environment in
which the structure is expected to serve
Changes in Procedure for Mix Proportioning in
IS 10262:2009
As per Clause 9.1.1 of IS 456, the minimum grade of concrete
to be used in an RCC should not be less than M20. Moreover,
all concretes above M20 grade for RCC work must be design
mixes. Concrete grades above M60 fall under the category of
HSC and hence should be proportioned using the guidelines
given in specialist literature, such as ACI 211.4-93 and the
work of Krishna Raju (2002) and de Larrard (1999).
The 2009 version of the code does not contain the graph
of w/c ratio versus 28-day compressive strength. Now, the
relationship between w/c ratio and the compressive strength
of concrete needs to be established for the materials actually
used or by using any other available relationship based on
experiments. The maximum w/c ratio given in IS 456:2000
for various environmental conditions may be used as a starting
point. The water content per cubic metre of concrete in the
earlier version of the standard was a constant value for various
nominal maximum sizes of aggregates. However, in the revised
version, the maximum water content per cubic metre of concrete
is suggested. Another major inclusion in the revised standard is
the estimation of volume of coarse aggregate per unit volume
of total aggregate for different zones of fi ne aggregate. As air
content in normal (non-air entrained) concrete will not affect
the mix proportioning signifi cantly, it is not considered in the
revised version; it is also not considered in IS 456:2000.
Data for Mix Proportioning
The following basic data is required for concrete mix
proportioning of a particular grade of concrete:
1. Exposure condition of the structure under consideration
(see Table 3 of IS 456:2000 and Table 4.4 in Chapter 4 of
this book for guidance)
2. Grade designation—The minimum grade of concrete
to be designed for the type of exposure condition under
consideration (see Tables 3 and 5 of IS 456:2000 and Table
4.4 in Chapter 4 and Table 1.11 of this book for guidance)
3. Type of cement (OPC, PPC, PSC, etc.)
TABLE 1.11 Grades of concrete
Group Grade Designation Specifi ed Characteristic
28-day Compressive
Strength of 150 mm cube,
N/mm2
Ordinary
concrete
M10–M20 10–20
Standard
concrete
M25–M60 25–60
High-strength
concrete
M65–M100 65–100
© Oxford University Press 2013. All rights reserved.
14 Design of Reinforced Concrete Structures
4. Minimum and maximum cement content (see Tables 3, 4,
5, and 6 of IS 456:2000 and Tables 4.4 and 4.5 in Chapter
4 of this book for guidance)
5. Type of aggregate (basalt, granite, natural river sand,
crushed stone sand, etc.)
6. Maximum nominal size of aggregate to be used (40 mm,
20 mm, or 12.5 mm)
7. Maximum w/c ratio (see Tables 3 and 5 of IS 456:2000
and Tables 4.4 and 4.5 in Chapter 4 of this book for
guidance)
8. Desired degree of workability (see Table 1.12, which is
based on Clause 7 of IS 456)
9. Use of admixture, its type, and conditions of use
10. Maximum temperature of concrete at the time of placing
11. Method of transporting and placing
12. Early age strength requirements, if required
TABLE 1.12 Workability of concrete
Placing Conditions Degree of Workability Slump, mm
Mud mat, shallow
section, pavement
using pavers
Very low 0.70–0.80
(compacting factor)
Mass concrete;
lightly reinforced
slabs, beams, walls,
columns; strip
footings
Low 25–75
Heavily reinforced
slabs, beams, walls,
columns
Medium 50–100
Slip formwork,
pumped concrete
Medium 75–100
In situ piling, trench
ll
High 100–150
Tremie concrete Very high 150–200
(fl ow test as per
IS 9103:1999)
Note: Internal (needle) vibrators are suitable for most of the placing conditions.
The diameter of the needle should be determined based on the density and
spacing of reinforcements and the thickness of sections. Vibrators are not
required for tremie concrete.
The step-by-step mix proportioning procedure as per IS 10262
is as follows (IS 10262:2009; Nagendra 2010):
Step 1 Calculate the target mean compressive strength
for mix proportioning. The 28-day target mean compressive
strength as per Clause 3.2 of IS 10262 is
+
×
ff
=
s
f
f
f
1
.5
6
(1.1)
where
f
f
f
is the target mean compressive strength at 28 days
(N/mm2), f
ck is the characteristic compressive strength at
28 days (N/mm2), and s is the standard deviation (N/mm2).
Standard deviation should be calculated for each grade of
concrete using at least 30 test strength of samples (taken from
site), when a mix is used for the fi rst time. In case suffi cient
test results are not available, the values of standard deviation
as given in Table 1.13 may be assumed for proportioning the
mix in the fi rst instance. As soon as suffi cient test results are
available, actual standard deviation shall be calculated and
used to proportion the mix properly.
TABLE 1.13 Assumed standard deviation
S. No. Grade of Concrete Assumed Standard Deviation, N/mm2
1. M10 3.5
2. M15
3. M20 4.0
4. M25
5. M30
5.0
6. M35
7. M40
8. M45
9. M50
10. M55
Note: These values correspond to strict site control of storage of cement,
weigh batching of materials, controlled addition of water, and so on. The values
given in this table should be increased by 1 N/mm2 when the aforementioned are
not practised.
Step 2 Select the w/c ratio. The concrete made today has
more than four basic ingredients. We now use both chemical
and mineral admixtures to obtain concretes with improved
properties both in fresh and hardened states. Even the qualities
of both coarse and fi ne aggregates in terms of grading, shape,
size, and texture have improved due to the improvement in
crushing technologies. As all these variables will play a
role, concretes produced with the same w/c ratio may have
different compressive strengths. Therefore, for a given set
of materials, it is preferable to establish the relationship
between the compressive strength and free w/c ratio. If such
a relationship is not available, maximum w/c ratio for various
environmental exposure conditions as given in Table 5 of IS
456 (Table 4.5 in Chapter 4 of this book) may be taken as a
starting point. Any w/c ratio assumed based on the previous
experience for a particular grade of concrete should be
checked against the maximum values permitted from the point
of view of durability, and the lesser of the two values should be
adopted.
Step 3 Select the water content. The quality of water
considered per cubic metre of concrete decides the workability
of the mix. The use of water-reducing chemical admixtures in
the mix helps to achieve increased workability at lower water
contents. The water content given in Table 1.14 (Table 2 of
IS 10262) is the maximum value for a particular nominal
maximum size of (angular) aggregate, which will achieve a
slump in the range of 25 mm to 50 mm. The water content per
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 15
unit volume of concrete can be reduced when increased size
of aggregate or rounded aggregates are used. On the other
hand, the water content per unit volume of concrete has to
be increased when there is increased temperature, cement
content, and fi ne aggregate content.
In the following cases, a reduction in water content is
suggested by IS 10262:
1. For sub-angular aggregates, a reduction of 10 kg
2. For gravel with crushed particles, a reduction of 20 kg
3. For rounded gravel, a reduction of 25 kg
For higher workability (greater than 50 mm slump), the
required water content may be established by trial, an increase
by about 3 per cent for every additional 25 mm slump, or
alternatively by the use of chemical admixtures conforming
to IS 9103:1999.
Use of water reducing admixture Depending on the per-
formance of the admixture (conforming to IS 9103:1999) that
is proposed to be used in the mix, a reduction in the assumed
water content can be made. Water-reducing admixtures will
usually decrease water content by 5–10 per cent and super-
plasticizers decrease water content by 20 per cent and above
at appropriate dosages. As mentioned earlier, the use of PC-
based superplasticizers results in water reduction up to 30–40
per cent.
Step 4 Calculate the content of cementitious material. The
cement and supplementary cementitious material content
per unit volume can be calculated from the free w/c ratio of
Step 2. The total cementitious content so calculated should
be checked against the minimum content for the requirements
of durability and the greater of the two values adopted.
The maximum cement content alone (excluding mineral
admixtures such as fl y ash and GGBS) should not exceed
450 kg/m3 as per Clause 8.2.4.2 of IS 456.
Step 5 Estimate the proportion of coarse aggregate. Table
1.15 (Table 3 of IS 12062) gives the volume of coarse
aggregate for unit volume of total aggregate for different
zones of fi ne aggregate (as per IS 383:1970) for a w/c ratio
of 0.5, which requires to be suitably adjusted for other w/c
ratios. This table is based on ACI 211.1:1991. Aggregates of
essentially the same nominal maximum size, type, and grading
will produce concrete of satisfactory workability when a given
volume of coarse aggregate per unit volume of total aggregate
is used. It can be seen that for equal workability, the volume
of coarse aggregate in a unit volume of concrete is dependent
only on its nominal maximum size and the grading zone of
ne aggregate.
Step 6 Identify the combination of different sizes of coarse
aggregate fractions. Coarse aggregates from stone crushes are
normally available in two sizes, namely 20 mm and 12.5 mm.
Coarse aggregates of different sizes can be suitably combined
to satisfy the gradation requirements (cumulative per cent
passing) of Table 2 in IS 383:1970 for the given nominal
maximum size of aggregate.
Step 7 Estimate the proportion of fi ne aggregate. The
absolute volume of cementitious material, water, and the
chemical admixture is found by dividing their mass by
their respective specifi c gravity, and multiplying by 1/1000.
The volume of all aggregates is obtained by subtracting the
summation of the volumes of these materials from the unit
volume. From this, the total volume of aggregates, the weight
of coarse and fi ne aggregate, is obtained by multiplying their
fraction of volumes (already obtained in Step 5) with the
respective specifi c gravities and then multiplying by 1000.
Step 8 Perform trial mixes. The calculated mix proportions
should always be checked by means of trial batches. The
concrete for trial mixes shall be produced by means of actual
materials and production methods. The trial mixes may be
made by varying the free w/c ratio by ±10 per cent of the
pre-selected value and a suitable mix selected based on the
workability and target compressive strength obtained. Ribbon-
type mixers or pan mixers are to be used to simulate the site
conditions where automatic batching and pan mixers are used
for the production of concrete. After successful laboratory
trials, confi rmatory fi eld trials are also necessary.
The guidelines for mix proportioning for HSC are provided
by ACI 211.4R:93, for concrete with quarry dust by Nataraja,
TABLE 1.14 Maximum water content per cubic metre of concrete for
nominal maximum size of (angular) aggregate
S. No. Nominal Maximum Size
of Aggregate, mm
Maximum Water Content*, kg
1. 10 208
2. 20 186
3. 40 165
Note: These quantities of mixing water are for use in computing cementitious
material contents for trial batches.
*Water content corresponding to saturated surface dry aggregate
TABLE 1.15 Volume of coarse aggregate per unit volume of total
aggregate for different zones of fi ne aggregate
Nominal
Maximum
Size of
Aggregate,
mm
Volume of Coarse Aggregate* Per Unit Volume of Total
Aggregate for Different Zones of Fine Aggregate (for w/c
Ratio = 0.5)
Zone IV Zone III Zone II Zone I
10 0.50 0.48 0.46 0.44
20 0.66 0.64 0.62 0.60
40 0.75 0.73 0.71 0.69
Note: The volume of coarse aggregate per unit volume of total aggregate needs
to be changed at the rate of ±0.01 for every ±0.05 change in w/c ratio.
*Volumes are based on aggregate in saturated surface dry condition.
© Oxford University Press 2013. All rights reserved.
16 Design of Reinforced Concrete Structures
et al. (2001), and for concrete with internal curing by Bentz,
et al. (2005). Rajamane (2004) explains a procedure of mix
proportioning using the provisions of IS 456:2000. Optimal
mixture proportioning for concrete may also be performed
using online tools such as COST (Concrete Optimization
Software Tool) developed by NIST, USA (http://ciks.cbt.nist.
gov/cost/).
1.4 HYDRATION OF CEMENT
When Portland cement is mixed with water, a series of
chemical reactions takes place, which results in the formation
of new compounds and progressive setting, hardening of the
cement paste, and fi nally in the development of strength. The
overall process is referred to as cement hydration. Hydration
involves many different reactions, often occurring at the
same time. When the paste (cement and water) is added to
aggregates (coarse and fi ne), it acts as an adhesive and
binds the aggregates together to form concrete. Most of the
hydration and about 90 per cent strength development take
place within 28 days; however, the hydration and strength
development continues, though more slowly, for a long time
with adequate moisture and temperature (50% of the heat is
liberated between one and three days, 75% in seven days,
and about 90% in six months). Hydration products formed
in hardened cement pastes are more complicated, and the
chemical equations are shown in Table 1.16. More details
of the chemical reactions may be found in the works of
Johansen, et al. (2002), Lea (1971), Powers (1961), and
Taylor (1997).
As shown in Fig. 1.5, tricalcium silicate (C3S) hydrates
and hardens rapidly and is mainly responsible for the initial
set and early strength of concrete. Thus, OPC containing
increased percentage of C3S will have high early strength.
On the other hand, dicalcium silicate (C2S) hydrates and
hardens slowly and contributes to strength increase only after
about seven days. Tricalcium aluminate (C3A) is responsible
for the large amount of heat of hydration during the fi rst few
days of hydration and hardening. It also contributes slightly to
the strength development in the fi rst few days. Cements with
low percentages of C3A are more resistant to soils and waters
containing sulphates. Tetracalcium aluminoferrite (C4AF)
contributes little to strength. The grey colour of cement is
due to C4AF and its hydrates. As mentioned earlier, gypsum
(calcium sulphate dihydrate) is added to cement during fi nal
grinding to regulate the setting time of concrete and reacts
with C3A to form ettringite (calcium trisulphoaluminate or
AFt). In addition to controlling setting and early strength gain,
gypsum also helps control drying shrinkage (Kosmatka, et
al. 2003). Figure 1.5 shows the relative reactivity of cement
compounds. The ‘overall curve’ has a composition of 55 per
cent C3S, 18 per cent C2S, 10 per cent C3A, and 8 per cent
C4AF.
0
0%
20%
40%
60%
80%
100%
20 40 60 80 100
Age, days
Degree of reaction, % by mass
C
3
S
C
2
S
C
3
A
C
4
AF
Overall
FIG. 1.5 Relative reactivity of cement compounds
Source: Reprinted from Tennis, P.D. and H.M. Jennings 2000, ‘A Model for Two
Types of Calcium Silicate Hydrate in the Microstructure of Portland Cement
Pastes’, Cement and Concrete Research, Vol. 30, No. 6, pp. 855–63, with
permission from Elsevier.
Heat of hydration When Portland cement is mixed with
water, heat is liberated as a result of the exothermic chemical
reaction. This heat is called the heat of hydration. The heat
generated by the cement’s hydration raises the temperature
TABLE 1.16 Portland cement compound hydration reactions
Basic Cement Compounds Hydrated Compounds
2(C3S)
Tricalcium
silicate
+11H
Water
= C3S2H8
Calcium
silicate hydrate
(C-S-H)
+3 (CH)
Calcium
hydroxide
2(C2S)
Dicalcium
silicate
+9H
Water
= C3S2H8
Calcium
silicate hydrate
(C-S-H)
+CH
Calcium
hydroxide
C3A
Tricalcium
aluminate
+3(C
S
H2)
Gypsum +26H
Water
= C6A
S
3
H32
Ettringite (AFt)
2(C3A)
Tricalcium
aluminate
+C6A
S
3
H32
Ettringite
(AFt)
+4H
Water
= 3(C4A
S
H12)
Calcium mono-
sulphoaluminate
(AFm)
C3A
Tricalcium
aluminate
+CH
Calcium
hydroxide
+12H
Water
= C4A13H
Tetracalcium
aluminate
hydrate
C4AF
Tetracalcium
aluminoferrite
+10H
Water
+2(CH)
Calcium
hydroxide
= 6CAF12H
Calcium alumino-
ferrite hydrate
S
= SO3 (Sulfur trioxide)
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 17
of concrete; temperature rises of 55°C have been observed
with mixes having high cement content. Such a temperature
rise will result in the cracking of the concrete. As a rule of
thumb, the maximum temperature differential between
the interior and exterior concretes should not exceed 20°C
to avoid crack development. ACI 211.1:91 states that as a
rough guide, hydration of cement will generate a concrete
temperature rise of about 4.7–7.0 °C per 50 kg of cement
per cubic metre of concrete. Usually, the greatest rate of
heat of hydration occurs within the fi rst 24 hours and a large
amount of heat evolves within the fi rst three days. Factors
infl uencing heat development in concrete include the cement
content (cements with higher contents of tricalcium silicate
(C3S) and tricalcium aluminate (C3A) and higher fi neness
have higher rates of heat generation), w/c ratio, placing and
curing temperature, the presence of mineral and chemical
admixtures, and the dimensions of the structural element.
Higher temperatures greatly accelerate the rate of hydration
and the rate of heat liberation at early stages (less than seven
days). Kulkarni (2012) observed that over the years there is a
large increase in the C3S content and fi neness of cement, both
of which speed up the hydration reaction and provide high
early strength and accompanying side effect of higher heat
of hydration (for example, in 1920s, the cement in the USA
contained 21% of C3S and 48% of C2S; now their proportion
is completely reversed and it is 56% of C3S and 17% of C2S).
In view of these changes in the cement characteristics, design
strengths could be achieved with low cement content and
higher w/c ratio.
Mineral admixtures (e.g., fl y ash), can signifi cantly reduce
the rate and amount of heat development. The methods to
minimize the rise in concrete temperature include cooling
the mixing water, using ice as part of the mixing water, using
a moderate-heat Portland cement or moderate- or low-heat
blended cement, using chemical admixtures (water-reducer or
water-retarder), keeping cement contents to a minimum level,
or cooling the aggregate. Moreover, curing with water helps to
control temperature increases and is better than other curing
methods.
1.5 TYPES OF CONCRETE
Depending on where it is mixed, concrete may be classifi ed as
site-mixed concrete or ready-mixed (factory-mixed) concrete
(RMC). Site mixing is not always recommended as the
mixing may not be thorough and the control on the w/c or
w/cm ratio cannot be strictly maintained. Hence, it is used
only in locations where RMC is not readily available. Concrete
without reinforcement is called plain concrete and with
reinforcement is called RCC or RC. Even though concrete
is strong in compression, it is weak in tension and tends to
crack when subjected to tensile forces; reinforcements are
designed to resist these tensile forces and are often provided
in the tension zones. Hence, only RCC is used in structures.
Depending on the strength it may attain in 28 days, concrete
may be designated as ordinary concrete, standard or normal
strength concrete (NSC), HSC, and ultra-high-strength
concrete (UHSC). In IS 456, the grades of concrete are
designed as per Table 1.11. Clause 6.1.1 of IS 456 defi nes
the characteristic strength as the strength of the concrete
below which not more than fi ve per cent of the test results will
fall (refer to Section 4.7.3 and Fig. 4.25 of Chapter 4). The
minimum grade for RC as per IS 456 is M20; it should be noted
that other international codes specify M25 as the minimum
grade. In general, the usual concretes fall in the M20 to M50
range. In normal buildings M20 to M30 concretes are used,
whereas in bridges and prestressed concrete construction,
strengths in the range of M35 to M50 are common. Very high-
strength concretes in the range of M60 to M70 have been
used in columns of tall buildings and are normally supplied
by ready-mix concrete companies (Kumar and Kaushik
2003).
Concrete with enhanced performance characteristics is
called high-performance concrete (HPC). Self-compacting
concrete (SCC) is a type of HPC, in which maximum
compaction is achieved using special admixtures and without
using vibrators. Structural engineers should aim to achieve
HPC through suitable mix proportioning and the use of
chemical and mineral admixtures.
When fi bres are used in concrete, it is called bre-
reinforced concrete (FRC). (Fibres are usually used in
concrete to control cracking due to plastic shrinkage and
drying shrinkage.) High-performance FRCs are called ductile
bre-reinforced cementitious composites (DFRCCs); they
are also called ultra-high-performance concretes (UHPCs)
or engineered cementitious composites (ECCs). Due to the
non-availability of standard aggregates or to reduce the self-
weight, lightweight aggregates may be used; such concretes
are called SLWCs or autoclaved aerated concretes (AACs). A
brief description of these concretes is given in the following
sections.
1.5.1 Ready-mixed Concrete
Ready-mixed concrete is a type of concrete that is manufactured
in a factory or batching plant, based on standardized mix
designs, and then delivered to the work site by truck-mounted
transit mixers. This type of concrete results in more precise
mixtures, with strict quality control, which is diffi cult to follow
on construction sites. Although the concept of RMC was known
in the 1930s, this industry expanded only during the 1960s.
The fi rst RMC plant started operating in Pune, India, in 1987,
but the growth of RMC picked up only after 1997. Most of
the RMC plants are located in seven large cities of India,
© Oxford University Press 2013. All rights reserved.
18 Design of Reinforced Concrete Structures
and they contribute to about 30–60 per cent of total concrete
used in these cities. (Even today, a substantial proportion
of concrete produced in India is volumetrically batched and
site-mixed, involving a large number of unskilled labourers
in various operations.) The fraction of RMC to total concrete
being used is 28.5 per cent. RMC is being used for bridges,
yovers, and large commercial and residential buildings
(Alimchandani 2007).
The RMC plants should be equipped with up-to-date
equipment, such as transit mixer, concrete pump, and concrete
batching plant. RMC is manufactured under computer-
controlled operations and transported and placed at site using
sophisticated equipment and methods. The major disadvantage
of RMC is that since the materials are batched and mixed at
a central plant, travelling time from the plant to the site is
critical over longer distances. It is better to have the ready mix
placed within 90 minutes of batching at the plant. (The average
transit time in Mumbai is four hours during daytime.). Though
modern admixtures can modify that time span, the amount and
type of admixture added to the mix may affect the properties of
concrete.
1.5.2 High-performance Concrete
High-performance concrete may be defi ned as any concrete
that provides enhanced performance characteristics for a
given application. It is diffi cult to provide a unique defi nition
of HPC without considering the performance requirements
of the intended use. ACI has adopted the following broad
defi nition of HPC: ‘A concrete meeting special combinations
of performance and uniformity requirements that cannot
always be achieved routinely by using only conventional
materials and normal mixing, placing, and curing
practices. The requirements may involve enhancements
of characteristics such as easy placement, compaction
without segregation, long-term mechanical properties,
early-age strength, permeability, density, heat of hydration,
toughness, volume stability, and long service life in severe
environments’ (ACI 363 R-10). Table 1.17 lists a few of
these characteristics. Concretes possessing many of these
characteristics often achieve higher strength (HPCs usually
have strengths greater than 50–60 MPa). Therefore, HPCs
will often have high strength, but a HSC need not necessarily
be called HPC (Mullick 2005; Muthukumar and Subramanian
1999).
The HPCs are made with carefully selected high-quality
ingredients and optimized mixture designs (see Table 1.18).
These ingredients are to be batched, mixed, placed,
compacted, and cured with superior quality control to get
the desired characteristics. Typically, such concretes will
have a low water–cementitious materials ratio of 0.22
to 0.40.
TABLE 1.18 Typical HPC mixtures used in some structures
Ingredients
Structure
Two
Union
Square,
Seattle,
1988
Great
Belt Link,
East
Bridge,
Denmark,
1996
Kaiga
Atomic
Project
Unit 2,
India,
1998
Petronas
Tower,
Malaysia,
1999
Urban
Viaduct,
Mumbai,
India,
2002
Water
kg/m3130 130 136 152 148
Portland
cement, kg/m3513 315 400 186 500
Fly ash, kg/m3 40 – 345*
Silica fume,
kg/m343 23 25 35 50
Coarse
aggregates,
kg/m31080 1140 1069 1000
762
(20 mm)
+ 384
(10 mm)
Fine
aggregates,
kg/m3
685 710 827 725 682
Water reducer,
L/m3 1.5 –
Air content % 5.5 2 1.5
Superplasti-
cizer,
L/m3
15.7 5.0 5.82 9.29 8.25
(Continued )
TABLE 1.17 Desired characteristics of HPCs
Property Criteria that may be specifi ed
High strength 70–140 MPa at 28–91 days
High early compressive strength 20–28 MPa at 3–12 hours or 1–3
days
High early fl exural strength 2– 4 MPa at 3–12 hours or 1–3
days
High modulus of elasticity More than 40 GPa
Abrasion resistance 0–1 mm depth of wear
Low permeability 500–2000 Coulombs
Chloride penetration Less than 0.07% Cl at 6 months
Sulphate attack 0.10% or 0.5% maximum
expansion at 6 months for
moderate or severe sulphate
exposures
Low absorption 2–5%
Low diffusion coeffi cient 1000 × 1014 m/s
Resistance to chemical attack No deterioration after 1 year
Low shrinkage Shrinkage strain less than 0.04%
in 90 days
Low creep Less than normal concrete
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 19
TABLE 1.18 (Continued)
Ingredients
Structure
Two
Union
Square,
Seattle,
1988
Great
Belt Link,
East
Bridge,
Denmark,
1996
Kaiga
Atomic
Project
Unit 2,
India,
1998
Petronas
Tower,
Malaysia,
1999
Urban
Viaduct,
Mumbai,
India,
2002
W/cm ratio 0.25 0.34 0.32 0.25–
0.27
0.269
Slump, mm 175 +
25
180–220 130–180
(at
plant)
80–120
(at site)
Strength at 28
days, MPa
119 – 75.9 80 79.6–
81.3
Strength at 91
days, MPa
145 81.4
(180
days)
100
(56
days)
87.2–
87.4
*Mascrete, which is a cement–fl y ash compound in the ratio 20:80
Superplasticizers are usually used to make these concretes fl uid
and workable. It should be noted that without superplasticizers,
the w/cm ratio cannot be reduced below a value of about 0.40.
Typically, 5–15 L/m3 of superplasticizer can effectively replace
45–75 L/m3 of water (Aïtcin and Neville 1993). This drastic
reduction in mixing water reduces the distance between cement
particles, resulting in a much denser cement matrix than NSC.
The optimal particle-packing mixture design approach may be
used to develop a workable and highly durable design mixture
(with cement content less than 300 kg/m3), having compressive
strength of 70–80 MPa (Kumar and Santhanam 2004).
As the crushing process takes place along any potential zones
of weakness within the parent rock, and thus removes them,
smaller particles of coarse aggregates are likely to be stronger
than the large ones. Hence, for strengths in excess of 100 MPa,
the maximum size of aggregates should be limited to 10–12 mm;
for lesser strengths, 20 mm aggregates can be used (Aïtcin
and Neville 1993; Aïtcin, 1998). Strong and clean crushed
aggregates from fi ne-grained rocks, mostly cubic in shape, with
minimal fl aky and elongated shapes are suitable for HPC. In
order to have good packing of the fi ne particles in the mixture,
as the cement content increases, the fi ne aggregates should be
coarsely graded and have fi neness modulus of 2.7–3.0.
As the HPC has very low water content, it is important to
effectively cure HPC as early as possible. Membrane curing
is not suitable for HPC, and hence fogging or wet curing
should be adopted to control plastic and autogenous shrinkage
cracking (see Section 1.7).
HPC has been primarily used in tunnels, bridges, pipes
carrying sewage, offshore structures, tall buildings, chimneys,
and foundations and piles in aggressive environments for its
strength, durability, and high modulus of elasticity. It has also been
used in shotcrete repair, poles, parking garages, and agricultural
applications. It should be noted that in severe fi res, HPC results
in bursting of the cement paste and spalling of concrete. More
information on HPC may be obtained from ACI 363R-10 and IS
9103:1999 codes and the works of Zia, et al. (1991), Zia, et al.
(1993), Aïtcin and Neville (1993), and Aïtcin (1998).
Self-compacting Concrete
Self-compacting concrete, also known as high-workability con-
crete, self-consolidating concrete, or self-levelling concrete, is
a HPC, developed by Prof. Okamura and associates at the Uni-
versity of Tokyo (now Kochi Institute of Technology), Japan,
in 1988 (Okamura and Ouchi 2003). SCC is a highly workable
concrete that can fl ow through densely reinforced and com-
plex structural elements under its own weight and adequately
ll all voids without segregation, excessive bleeding, excessive
air migration, and the need for vibration or other mechanical
consolidation. The highly fl owable nature of SCC is due to
very careful mix proportioning, usually replacing much of the
coarse aggregate with fi nes and cement, and adding chemical
admixtures (EFNARC 2005). SCC may be manufactured at a
site batching plant or in an RMC plant and delivered to site by
a truck mixer. It may then be placed by either pumping or pour-
ing into horizontal or vertical forms. To achieve fl uidity, new
generation superplasticizers based on polycarboxylic esters
(PCE) are used nowadays, as it provides better water reduction
and slower slump loss than traditional superplasticizers. The
stability of a fl uid mix may be achieved either by using high
nes content or by using viscosity-modifying agents (VMA).
Several new tests have evolved for testing the suitability
of SCC (see Fig. 1.6). They essentially involve testing the
(a) fl owability (slump fl ow test), (b) fi lling ability (slump fl ow
test, V-funnel, and Orimet) (It may be noted that in the slump
ow test, the average spread of fl attened concrete is measured
horizontally, unlike the conventional slump test, where vertical
slump is measured.), (c) passing ability (L-box, J-ring, which is a
simpler substitute for U-box), (d) robustness, and (e) segregation
resistance or stability (simple column box test, sieve stability
test). The details of these test methods may be found in the
works of Okamura and Ouchi (2003) and Hwang, et al. (2006).
The SCC has been used in a number of bridges and precast
projects in Japan, Europe, and USA (Ouchi 2003). Recently,
SCC has been used in a fl yover construction in Mumbai,
India (ICJ, August 2009). The various developments in SCC
undertaken in India may be found in ICJ (2004, 2009). An
amendment (No. 3, August 2007) in the form of Annex J was
added to IS 456, which prescribes the following for SCC:
1. Minimum slump fl ow: 600 mm
2. Amount of fi nes (< 0.125 mm) in the range of 400–600
kg/m3, which may be achieved by having sand content
more than 38 per cent and using mineral admixture to the
order of 25–50 per cent by mass of cementitious materials
3. Use of HRWRA and VMA
© Oxford University Press 2013. All rights reserved.
20 Design of Reinforced Concrete Structures
1.5.3 Structural Lightweight Concrete
Some of the early structures from the Roman Empire that still
survive today, including the Pantheon, have elements that were
constructed with lightweight concrete. The use of lightweight
concrete in modern times started when Steven J. Hayde, a
brick-maker from Kansas City, Missouri, developed a rotary
kiln method for expanding clays, shales, and slates in the early
1900s. SLWC is made with lightweight coarse aggregates such
as natural pumice or scoria aggregates and expanded slags;
sintering-grate expanded shale, clay, or fl y ash; and rotary-
kiln expanded shale, clay, or slate (ACI E1-07). The in-place
density (unit weight) of such SLWC will be of the order of
1360–1850 kg/m3, compared to the density of normal weight
concrete of 2240–2400 kg/m3. For structural applications, the
strength of such SLWC should be greater than 20 MPa. The
use of SLWC allows us to reduce the deadweight of concrete
elements, thus resulting in overall economy. In most cases,
the slightly higher cost of SLWC is offset by reductions in
the weight of concrete used. Seismic performance is also
improved because the lateral and horizontal forces acting on
a structure during an earthquake are directly proportional to
the inertia or mass of a structure. Companies like Lafarge
produce varieties of industrial lightweight aggregates;
examples include Aglite™, Haydite™, Leca™, Litex™,
Lytag™, True Lite™, and Vitrex™ (www.escsi.org). As a
result of these advantages, SLWC has been used in a variety
of applications in the past 80 years. The reduced strength of
SLWC is considered in the design of the ACI code by the
factor l.
An effective technique developed to help mitigate and
overcome the issues of autogenous shrinkage and self-
desiccation is internal curing; autogenous shrinkage is defi ned
as a concrete volume change occurring without moisture
transfer to the environment, as a result of the internal chemical
and structural reactions (Holt 2001). Autogenous shrinkage
is accompanied by self-desiccation during hardening of the
concrete, which is characterized by internal drying. Self-
desiccation, or internal drying, is a phenomenon caused by the
chemical reaction of cement with water (Persson and Fagerlund
2002). The reaction leads to a net reduction in the total volume
of water and solid (Persson, et al. 2005). The porosity of
lightweight aggregates provides a source of water for internal
curing, resulting in continued enhancement of the strength
and durability of concrete. However, this does not prevent the
need for external curing. More details about the mix design,
production techniques, properties, and so on may be found in the
ACI 213R-03 manual and the works of Neville (1996), Clarke
(1993), Bentz, et al. 2005, and Chandra and Berntsson (2002).
Autoclaved Aerated Concrete
Autoclaved aerated concrete, also known as autoclaved
cellular concrete (ACC) or autoclaved lightweight concrete
(ALC) with commercial names Siporex, e-crete, and Ytong,
was invented in the mid-1920s by the Swedish architect Johan
Axel Eriksson. It is a lightweight, strong, inorganic, and non-
toxic precast building material that simultaneously provides
strength, insulation, and fi re, mould, and termite resistance.
Though relatively unknown in countries such as the USA,
300
f100
f200
f500
Abrams cone
900
900
Base plate
f200
Fresh SCC sample
Adapter
H
Δh
Smooth bars 3
(or 2) f12 mm; gap
41 (or 59) mm
680mm
280
Obstacle
200
Height
Open the centre
gate
f300
Bar diameter:
16 mm
37.5 mm
100
25
515
75
450
150
225
65
Hinged
trapdoor
(a) (b)
(c)(d) (e)
FIG. 1.6 Tests on self-consolidating concrete (a) Slump fl ow test (b) L-box (c) J-ring (d) V-funnel (e) U-fl ow test
Source: Okamura and Ouchi 2003, reprinted with permission from JCI.
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 21
India, Australia, and China, AAC
now accounts for over 40 per cent of
all construction in the UK and more
than 60 per cent of construction in
Germany.
Autoclaved aerated concrete is
a precast product manufactured by
combining silica (either in the form of
quartz/silica sand or recycled fl y ash),
cement, lime, water, and an expansion
agent—aluminium powder—at the
rate of 0.05–0.08 per cent (it has to
be noted that no coarse aggregates
are used). Aluminium powder
reacts with calcium hydroxide and
water to form numerous hydrogen
bubbles, resulting in the expansion of
concrete to roughly two to fi ve times
its original volume. The hydrogen
subsequently evaporates, leaving a
highly closed-cell aerated concrete.
When the forms are removed from the material, it is solid
but still soft. It is then cut into either blocks or panels and
placed in an autoclave chamber for 12 hours. AAC blocks
(typically 600 mm long, 200 mm high, and 150–300 mm
thick) are stacked one over the other using thin-set mortar,
as opposed to the traditional concrete masonry units (CMU)
construction.
1.5.4 Fibre-reinforced Concrete
Fibres are added to concrete to control cracking caused by
plastic shrinkage and drying shrinkage. The addition of
small closely spaced and uniformly dispersed fi bres will act
as crack arresters and enhance the tensile, fatigue, impact,
and abrasion resistance of concrete. They also reduce the
permeability of concrete. Though the fl exural strength may
increase marginally, fi bres cannot totally replace fl exural steel
reinforcement (the concept of using fi bres as reinforcement is
not new; fi bres have been used as reinforcement since ancient
times, for example, horsehair in mortar and asbestos fi bres in
concrete).
Clause 5.7 (Amendment No. 3) of IS 456:2000 permits
the use of fi bres in concrete for special applications to
enhance its properties. Steel, glass, polypropylene, carbon,
and basalt fi bres have been used successfully; steel fi bres
are the most common (see Fig. 1.7). Steel fi bres may be
crimped, hooked, or fl at. This type of concrete is known
as FRC.
The amount of fi bres added to a concrete mix is expressed
as a percentage of the total volume of the composite (concrete
and fi bres) and termed volume fraction, which is denoted by
Vf and typically ranges from 0.25 per cent to 2.5 per cent (of
which 0.75–1.0 is the most common fraction). The aspect ratio
of a fi bre is the ratio of its length to its diameter. Typical aspect
ratio ranges from 30 to 150. The diameter of steel fi bres may
vary from 0.25 mm to 0.75 mm. Increasing the aspect ratio of
the fi bre usually increases the fl exural strength and toughness
of the matrix. However, fi bres that are too long tend to ‘ball’
in the mix and create workability problems (Subramanian
1976b). To obtain adequate workability, it is necessary to use
superplasticizers. The ultimate tensile strength of steel fi bres
should exceed 350 MPa. More information on FRC may be had
from the works of Parameswaran and Balasubramanian (1993)
and Bentur and Mindess (2007) and ACI 544.1R-96 report.
1.5.5 Ductile Fibre-reinforced Cementitious
Composites
Ductile fi bre-reinforced cementitious composite is a broader
class of materials that has properties and superior perfor-
mance characteristics compared to conventional cementi-
tious materials such as concrete and FRC. DFRCCs have
unique properties including damage reduction, damage tol-
erance, energy absorption, crack distribution, deformation
compatibility, and delamination resistance (delamination
is a mode of failure in composite materials—splitting or
separating a laminate into layers) (Matsumoto and Mihashi
2003). The various subgroups of DFRCC are shown in
Fig. 1.8 and Table 1.19 (Matsumoto and Mihashi 2003). It
should be noted that DFRCC encompasses a group of high-
performance fi bre-reinforced cementitious composites
(HPFRCC). UHPC, also known as ultra-high performance
bre-reinforced concrete (UHPFRC) or reactive powder con-
crete (RPC), developed in France in the late 1990s, is a new class
(a) (c)
(b)
For quick and easy mixing
Flat Wire
Glued fibre-bundles
FIG. 1.7 Fibres used in concrete (a) Different types and shapes of steel fi bres (b) Fine fi brillated
polypropylene fi bres (c) Glass fi bres
Courtesy: Dr V.S. Parameswaran
© Oxford University Press 2013. All rights reserved.
22 Design of Reinforced Concrete Structures
of DFRCCs that have ultra-strength and ultra-performance
characteristics.
DFRCC
HPFRCC
FRCC
Cementitious material
ECC
UHPFRCSIFCON SIMCON
Ductal
FRC, FRM
Concrete,
mortar,
cement
FIG. 1.8 Classifi cation of cementitious materials
Source: Matsumoto and Mihashi 2003, reprinted with permission from JCI
TABLE 1.19 Characteristics of different cementitious materials
Characteristics Cement,
Mortar
Concrete,
FRC
DFRCC HPFRCC
Material
response
Brittle Quasi
brittle
Quasi-
brittle
(tension)
or ductile
(fl exure)
Ductile
Strain softening
or hardening
(see Fig. 1.9)
Strain
softening
Strain
softening
(tension) or
hardening
(fl exure)
Strain
hardening
Cracking
behaviour
(fl exure)*
Localized
cracking
Localized
cracking
Multiple
cracking
Multiple
cracking
Cracking
behaviour
(tension)
Localized
cracking
Localized
cracking
Localized
cracking
Multiple
cracking
*Cracking behaviour in fl exure is dependent on specimen size. This comparison
is based on specimen size of 100 × 100 × 400 mm
Source: Matsumoto and Mihashi 2003, reprinted with permission from JCI
Engineered Cementitious Composites
Engineered cementitious composites are a special type of
HPFRCC that has been micro-structurally tailored based
on micro-mechanics. ECC is systematically engineered to
achieve high ductility under tensile and shear loading. By
employing material design based on micro-mechanics, it can
achieve maximum ductility in excess of three per cent under
uniaxial tensile loading with only two per cent fi bre content
by volume. Experiments have shown that even at the ultimate
load (5% strain), the crack width remains at about 60 µm and
is even lower at strain below one per cent.
As shown in Fig. 1.10, extensive experimental studies
have demonstrated superior seismic response as well as
minimum post-earthquake repair (Fischer and Li 2002).
It should be noted that even at high drift level of 10 per
cent, no spalling of the reinforced ECC was observed; in
contrast, the RCC column lost the concrete cover after bond
splitting and being subjected to heavy spalling. The test
results also illustrated the potential reduction or elimination
of steel stirrups by taking advantage of the shear ductility
of ECC. The tensile ductility in ECC also translates into
shear ductility since the material undergoes diagonal
tensile multiple cracking when subjected to shear (Li, et al.
1994).
(a) (b)
FIG. 1.10 Damage of column at 10% drift after reverse cyclic loading
(a) RCC (b) ECC without stirrups
Source: Fischer and Li 2002, reprinted with permission from ACI
Life cycle cost comparison showed that ECC slab systems
provide 37 per cent cost effi ciency, consume 40 per cent less
total primary energy, and produce 39 per cent less carbon
dioxide compared to conventional RCC systems (Li 2003).
More details about the behaviour and application of ECC may
be found in the study of Li (2003).
Ductile
Tensile stress
Tensile strength
First cracking
strength
First cracking strain Tensile ultimate
strain
Strain
Strain softening
Brittle
Strain hardening
Strain softening
C
B
A
FIG. 1.9 Defi nitions of brittle, ductile, strain softening, and strain
hardening under uniaxial tensile loading
Source: Matsumoto and Mihashi 2003, reprinted with permission from JCI
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 23
Ultra-high-performance Concrete
Ultra-high-performance concrete is a high-strength, high-
stiffness, self-consolidating, and ductile material, formulated
by combining Portland cement, silica fume, quartz fl our, ne
silica sand, high-range water reducer, water, and steel or organic
bres. Originally it was developed by the Laboratoire Central
des Ponts et Chaussées (LCPC), France, containing a mixture
of short and long metal fi bres and known as multi-scale fi bre-
reinforced concrete (Rossi 2001). It has to be noted that there are
no coarse aggregates, and a low w/cm ratio of about 0.2 is used in
UHPC compared to about 0.4–0.5 in NSC. The material provides
compressive strengths of 120–240 MPa, fl exural strengths of
15–50 MPa, and post-cracking tensile strength of 7.0–10.3 MPa
and has modulus of elasticity from 45 GPa to 59 GPa [Ductal®
(Lafarge, France), CoreTUFF® (US Army Corps of Engineers),
BSI®, Densit® (Denmark), and Ceracem® (France and
Switzerland) are some examples of commercial products].
The enhanced strength and durability properties of UHPC are
mainly due to optimized particle gradation that produces a very
tightly packed mix, use of steel fi bres, and extremely low water
to powder ratio (Nematollahi, et al. 2012).
Some of the potential applications of UHPC are in
prestressed girders and precast deck panels in bridges, columns,
piles, claddings, overlays, and noise barriers in highways. The
60 m span Sherbrooke pedestrian bridge, constructed in 1997
at Quebec, Canada, is the world’s fi rst UHPC bridge without
any bar reinforcement. More details of this bridge may be had
from the works of Blais and Couture (1999) and Subramanian
(1999). The 15 m span Shepherds Creek Road Bridge, New
South Wales, Australia, built in 2005 is the world’s fi rst UHPC
bridge for normal highway traffi c. Since then, a number of
bridges and other structures have been built utilizing UHPC
all over the world (see www.fhwa.dot.gov).
The materials for UHPC are usually supplied by the
manufacturers in a three-component premix: powders
(Portland cement, silica fume, quartz fl our, and fi ne silica
sand) pre-blended in bulk bags; superplasticizers; and organic
bres. Care should be exercised during mixing, placing, and
curing. The ductile nature of this material makes concrete to
deform and support fl exural and tensile loads, even after initial
cracking. The use of this material for construction is simplifi ed
by the elimination of reinforcing steel and its ability to be
virtually self-placing. More details about UHPC may be found
in the works of Schmidt, et al. (2004), Fehling, et al. (2008), and
Schmidt, et al. (2012). A comparison of stress–strain curves in
concretes is provided in Fig. 1.11. The infl uence of fi bres and
confi nement on the ductility of RPC should be noted.
Slurry Infi ltrated Fibrous Concrete and Slurry
Infi ltrated Mat Concrete
Slurry infi ltrated fi brous concrete (SIFCON), invented by
Lankard in 1979, is produced by infi ltrating cement slurry
(made of cement and sand in the proportion 1:1, 1:1.5, or 1:2,
with fl y ash and silica fume equal to 10–15% by weight of
cement, w/cm ratio of 0.3–0.4, and superplasticizer equal to
2–5% by weight of cement) into pre-placed steel fi bres (single
plain or deformed fi bres) in a formwork. It has to be noted
that it does not contain any coarse aggregates but has a high
cementitious content. Due to the pre-placement of fi bres, its
bre volume fraction may be as high as 6–20 per cent. The
confi ning effect of numerous fi bres yields high compressive
strengths from 90 MPa to 210 MPa, and the strong fi bre
bridging leads to tensile stain hardening behaviour in some
SIFCONs. Slurry infi ltrated mat concrete (SIMCON) is
similar to SIFCON, but uses pre-placed fi bre mat instead of
steel fi bres. SIFCON and SIMCON are extremely ductile and
hence ideally suitable for seismic retrofi t of structures (Dogan
and Krstulovic-Opara 2003). They also have improved
uniaxial tensile strength, fl exural, shear, impact strengths, and
abrasion resistance (Parameswaran 1996). They are best suited
for the following applications: pavement rehabilitation, safety
vaults, strong rooms, refractory applications, precast concrete
products, bridge decks and overlays, repair and rehabilitation
of structures, especially in seismic zones, military applications,
and concrete mega-structures, such as offshore platforms
and solar towers. More details about SIFCON and SIMCON
may be found in the works of Parameswaran, et al. (1990),
Parameswaran (1996), Lankard (1984), Naaman, et al. (1992),
Sashidhar, et al. (2010, 2011) and Hackman, et al. (1992).
1.5.6 Ferrocement
Ferrocement also known as ferrocrete, invented by Jean Louis
Lambot of France, in 1848, is a composite material like RCC.
In RCC, the reinforcement consists of steel bars placed in
the tension zone, whereas ferrocement is a thin RC made
0
0
50
100
150
200
250
300
350
400
0.005 0.01 0.015 0.02
Longitudinal strain, mm/mm
Confined and
pressed RPC
with fibres
Confined and
pressed RPC
without fibres
Confined
RPC
Confined RPC
without fibres
RPC with fibres
High
performance
concrete
Normal strength concrete
Stress, MPa
with fibres
FIG. 1.11 Comparison of stress–strain curves of NSC, HPC, and
RPC
Source: Blais and Couture 1999
© Oxford University Press 2013. All rights reserved.
24 Design of Reinforced Concrete Structures
of rich cement mortar (cement to sand ratio of 1:3) based
matrix reinforced with closely spaced layers of relatively
small diameter wire mesh, welded mesh, or chicken mesh.
(The diameter of wires range from 4.20 mm to 9.5 mm and
are spaced up to 300 mm apart.) The mesh may be metallic
or synthetic (Naaman 2000). The mortar matrix should
have excellent fl ow characteristics and high durability. The
use of pozzolanic mineral admixtures such as fl y ash (50%
cement replacement with fl y ash is recommended) and use of
superplasticizers will not only permit the use of water–binder
ratio of 0.40–0.45 by mass but will also enhance the durability
of the matrix. A mortar compressive strength of 40–50 MPa is
recommended.
During the 1940s, Pier Luigi Nervi, an Italian engineer,
architect, and contractor, had used ferrocement for the
construction of aircraft hangars, boats and buildings. It has
to be noted that though Nervi used a large number of meshes
in his structures, in many present-day applications, only
two layers of mesh reinforcement are used. Applications
of ferrocement include boats, barges, water tanks, pipes,
biogas digesters, septic tanks, toilet blocks, and monolithic
or prefabricated housing (Subramanian 1976a). Recently,
Spanos, et al. (2012) studied the use of ferrocement panels
as permanent load bearing formwork for one-way and two-
way slabs. Such panels provide economic advantages and the
slabs incorporating them will provide superior serviceability
performance. At the new Sydney Opera House, the sail-shaped
roofs (built of conventional RC) have been covered with tile-
surfaced panels of ferrocement, which serve as waterproofs
for the concrete underneath. More information about the
design and construction of ferrocement may be had from the
study of Naaman (2000) and ACI 549.1R-93 manual.
Polymer concrete Polymer concrete is obtained by
impregnating ordinary concrete with a monomer material
and then polymerizing it by radiation, by heat and catalytic
ingredients, or by a combination of these two techniques.
Depending on the process by which the polymeric materials
are incorporated, they are classifi ed as (a) polymer concrete
(PC), (b) polymer impregnated concrete (PIC), and (c)
polymer modifi ed concrete (PMC). Due to polymerization,
the properties are much enhanced and polymer concrete is
also used to repair damaged concrete structural members
(Subramanian and Gnana Sambanthan 1979).
In addition to these types of concrete, prestressed concrete
is often used in bridges and long-span structures; however, it is
outside the scope of this book. A prestressed concrete member
is one in which internal stresses (compressive in nature) are
introduced, which counteract the tensile stresses resulting
from the given external service level loads. The prestress is
commonly introduced by tensioning the high-strength steel
reinforcement (either by using the pre-tensioning or the
post-tensioning method), which applies pre-compression to
the member. The design of prestressed concrete members
should conform to IS 1343:1980.
1.6 REINFORCING STEEL
As stated earlier, steel reinforcements are provided in RCC to
resist tensile stresses. The quality of steel used in RCC work
is as important as that of concrete. Steel bars used in concrete
should be clean and free from loose mill scales, dust, loose rust
and any oily materials, which will reduce bond. Sand blasting
or similar treatment may be done to get clean reinforcement.
As per Clause 5.6 of IS 456, steel reinforcement used in
concrete may be of the following types (see Table 1.1 of SP
34 (S&T):1987 for the physical and mechanical properties of
these different types of bars):
1. Mild steel and medium tensile steel bars (MS bars)
conforming to IS 432 (Part 1):1982
2. High-yield strength-deformed steel bars (HYSD bars) con-
forming to IS 1786:2008
3. Hard drawn steel wire fabric conforming to IS 1566:1982
4. Structural steel conforming to Grade A of IS 2062:2006
It should be noted that different types of rebars, such as plain
and deformed bars of various grades, say grade Fe 415 and
Fe 500, should not be used side by side, as this may lead to
confusion and error at site. Mild steel bars, which are produced
by hot rolling, are not generally used in RCC as they have
smooth surface and hence their bond strength is less compared
to deformed bars (when they are used they should be hooked
at their ends). They are used only as ties in columns or stirrups
in beams. Mild steel bars have characteristic yield strength
ranging from 240 N/mm2 (grade I) to 350 N/mm2 (medium
tensile steel) and percentage elongation of 20–23 per cent
over a gauge length of 5.65
area
.
Hot rolled high-yield strength-deformed bars (HYSD bars)
were introduced in India in 1967; they completely replaced
mild steel bars except in a few situations where acute bending
was required in bars greater than 30 mm in diameter. They
were produced initially by cold twisting (CTD bars) and later
by heat treatment (TMT bars) and micro-alloying. They were
introduced in India by Tata Steel as Tistrong bars and later as
Tiscon/Torsteel bars. Cold twisted deformed bars (CTD bars
or Torsteel bars) are fi rst made by hot rolling the bars from
high-strength mild steel, with two or three parallel straight ribs
and other indentations on it. After cooling, these bars are cold
twisted by a separate operation, so that the steel is strained
beyond the elastic limit and then released. As the increase in
strength is due to cold-working, this steel should not be normally
welded. In CTD bars, the projections will form a helix around
the bars; if they are over-twisted, the pitch of the helixes will
be too close. Cold twisting introduces residual stresses in steel,
© Oxford University Press 2013. All rights reserved.
Introduction to Reinforced Concrete 25
and as a result, these bars corrode much faster than other bars;
hence, these are not recommended in many advanced countries.
Thermo-mechanically treated reinforcement bars (TMT
Bars) are a class of hot rolled HYSD bars, which are rapidly
cooled to about 450°C by a controlled quenching process using
water when they are leaving the last stand of the rolling mill at
a temperature of about 950°C. This sudden partial quenching,
along with the fi nal cooling, transforms the surface layer of
the bars from austenite to tempered
martensite, with a semi-tempered middle
ring of martensite and bainite and a
ne-grained ferrite–pearlite core (see
Fig. 1.12). TMT bars can be welded as
per IS 9417 using ordinary electrodes
and no extra precautions are required.
Strength, weldability, and ductility are
the main advantages of TMT bars; in
addition, they are also economical.
TMT bars produced by SAIL or Tata
are known as SAIL-TMT or TISCON-
TMT. Bars produced by RINL are called
REBARS. As it is visually diffi cult to
distinguish TMT bars from mild steel
deformed bars, the following procedure
is suggested in IS 1786: A small piece
(about 12 mm long) can be cut and the transverse face lightly
ground fl at on progressively fi ner emery papers up to ‘0’ size.
The sample can be macro-etched with nital (fi ve % nitric acid
in alcohol) at ambient temperature for a few seconds to reveal
a darker annular region corresponding to martensite or bainite
microstructure and a lighter core region.
By micro-alloying with elements such as copper,
phosphorus, and chromium, thermo-mechanically treated
corrosion resistant steel bars (TMT CRS bars) are produced,
which have better corrosion resistance than ordinary TMT bars.
It is better to adopt precautions against corrosion even while
using such bars, as they are not 100% corrosion-resistant.
Though IS 1786 specifi es four grades for these HYSD bars,
namely Fe 415, Fe 500, Fe 550, and Fe 600, and additional
three grades with a suffi x D, denoting that they are ductile,
the availability of Fe 550, Fe 600, Fe 415D, Fe 500D, and Fe
550D grades are limited (the numbers after Fe denoting the
0.2% proof or yield stress, in N/mm2).
The most important characteristic of the reinforcing bar is its
stress–strain curve; the important property is its characteristic
yield strength or 0.2 per cent proof stress as the case