concrete technology for MSC ALL university in nepal

1. 1.1 Use of concrete in structures and types of concrete
 High compressive strength
 Economical
 Simplicity
 Durability
 Fire proof
 Availability of concrete ingredients easily
 Easy handling and moulding of concrete in to any shape
 Consideration of energy and resource conservation.
 Monolithic character
 Excellent water resistance
 Ease of construction
 Concrete is fire resistant
 Concrete gives a longer service life.
 Concrete has multiple design possibilities.
 Concrete can be used to achieve optimum environmental performance.
 As it is recyclable, it is possible to use it for addition.
 High-performance concrete is used to build bridges.

Type of concrete
Concrete are classified in to various types as
1) Based on binding material
a) Cement concrete: cement is used as bonding material
b) Lime concrete: lime is used as bonding material.
2) Based on design
a) Plain cement concrete
 Provided is no reinforcement
 Bear high compressive force.
 Bear very low tension force due to absence or reinforcement.
b) Reinforcement cement concrete
 Provided is steel reinforcement.
 Bear both compressive force and tension force.
c) Pre stressed cement concrete
d)
1. Normal Strength Concrete
2. Plain or Ordinary Concrete
3. Reinforced Concrete
4. Prestressed Concrete
5. Precast Concrete
6. Light – Weight Concrete
7. High-Density Concrete
8. Air EntrainedConcrete
9. Ready Mix Concrete
10. Polymer Concrete
1. Polymer concrete
2. Polymer cement concrete
3. Polymer impregnatedconcrete
11. High-Strength Concrete
12. High-Performance Concrete
13. Self – Consolidated Concrete

2. 14. Shotcrete Concrete
15. Pervious Concrete
16. Vacuum Concrete
17. Pumped Concrete
18. Stamped Concrete
19. Limecrete
20. Asphalt Concrete
21. Roller Compacted Concrete
22. Rapid Strength Concrete
23. Glass Concrete
Properties anduses of different types of concrete mentionedabove are explained briefly:
1. Normal Strength Concrete
 The concrete that is obtained by mixing the basic ingredients cement, water and aggregate will give us normal strength
concrete.
 Strength of this type of concrete will vary from 10 MPa to 40MPa.
 Normal strength concrete has an initial setting time of 30 to 90 minutes that is dependent on the cement properties and the
weather conditions of the construction site.
2. Plain cement Concrete
 Provided is no reinforcement
 Bear high compressive force.
 Bear very low tension force due to absence of reinforcement
 main constituents are the cement, aggregates, and water.
 Most commonly used mix design is 1:2:4 which is the normal mix design.
 density of the plain concrete will vary between 2200 and 2500 Kg/meter cube.
 The compressive strength is 200 to 500 kg/cm2
.
 used in the construction of the pavements and the buildings
3. Reinforced cement Concrete
 Provided is steel reinforcement.
 Bear both compressive force and tension force.
4. Prestressed Concrete
 Compressive stress is artificially induced before its actual use.
 Strong enough to bear both compressive force and tensile force.
 These are used in the application of bridges, heavy loaded structures, and roof with longer spans.
5. Precast Concrete
Various structural elements can be made and cast in the factory as per the specifications and bought to the site at the time of assembly.
Such concrete units are called as the precast concrete.

3. The examples of precast concrete units are concrete blocks, the staircase units, precast walls and poles, concrete lintels and many other
elements. These units have the advantage of acquiring speedy construction as only assemblage is necessary. As the manufacturingis
done at site, quality is assured. The only precaution taken is for their transportation.
Also Read: Precast Concrete Construction – Process & Advantages
6. Lightweight Concrete.
 It is prepared from lower weight aggregate.
 It is used to decrease composite or gross weight of structure.
 Used for the construction of the long span bridge decks, construction of the building blocks.
 density of LWC is 1440 to 1840 kg/m³’.
7. High-Density Concrete
 density of HDC is 3000 to 4000 kg/m3
can be called as the heavyweight concrete.
 The crushed rocks are used as the coarse aggregates.
 most commonly used in the construction of atomic power plants and for similar projects.
8. Air Entrained Concrete
 Entrained for an amount of 3 to 6% of the concrete.
9. Ready Mix Concrete
 The concrete that mix and bathed in a central mixing plant is called as ready-mix concrete.

4. 10. Polymer Concrete
 When comparedwith the conventional concrete, in polymer concrete the aggregates will be bound with the polymer instead of
cement.
 Reduction of volume of voids in the aggregate.
 Aggregates are graded and mixed accordingly to achieve minimum voids hence maximum density.
This type of concrete has differentcategories:
 Polymer ImpregnatedConcrete
 Polymer cement concrete
 Partially Impregnated
11. High-Strength Concrete
 strength of concrete greater than 40MPa
 increased strength is achieved by decreasing the water-cement ratio even lower than 0.35.
12. High-Performance Concrete
 Strength gain in early age
 Easy placement of the concrete
 Permeability and density factors
 Heat of hydration
 Long life and durability
 Toughness and life term mechanical properties
 Environmental concerns
13. Self – Consolidated Concrete
 No vibration
 Mix has a higher workability.
 Slump value will be between 650 and 750.
14. Shotcrete Concrete
Here the concrete type differs in the way it is applied on the area to be cast. The concrete is shot into the frame or the pr eparedstructural
formwork with the help of a nozzle. As the shootingis carried out in a higher air pressure, the placing and the compaction process will be
occurring at the same time.
Also Read: What is Guniting? Procedure, Applications and Advantages of Guniting
15. Pervious Concrete
 Pervious or permeable concrete are concrete that are designed such a way that it allows the water to pass through it.
 These types of concrete will have 15 to 20% voids of the volume of the concrete when t hey are designed.
16. Vacuum Concrete.

5.  Due to dewatering through vacuum, both workability and high strength are achieved simultaneously.
 Reduction in water-cement ratio may increase the compressive strength by 10 to 50% and lowers the permeability.
 It enhances the wear resistance of concrete surface.
 to reduced shrinkage.
 The formwork can be removedearly and surface can be put to use early.
17. Pumped Concrete
 One of the main property of the concrete used in large mega construction especially for the high-rise construction is the
conveyance of the concrete to heights. Hence one such property of concrete to easily pump will result in the design of
pumpable concrete.
 The concrete that is used for pumping must be of adequate workability so that it is easily conveyed through the pipe. The pipe
used will be rigid or a flexible hose that will discharge the concrete to the desired area.
 The concrete used must be fluid in nature with enough fine material as well as water to fill up the voids. The more the finer
material used, greater will be control achieved on the mix. The grading of the coarse aggregate used must be continuous in
nature.
Also Read: What is Pumped Concrete?Types of Concrete Pumps and Selection
18. Stamped Concrete
Stamped concrete is an architectural concrete where realistic patterns similar to natural stones, granites, and tiles can be obtained by
placing impression of professional stamping pads. These stamping is carried out on the concrete when it is in its plastic condition.
Different coloringstains and texture work will finally give a finish that is very similar to costlier natural stones. A high aesthetic look can
be obtainedfrom a stamped finish economically. This is used in the construction of driveways, interior floors, and patios.
Also Read: What is Stamped Concrete? Features, Methods and Procedures of StampingConcrete
19. Limecrete
 This is a concrete type in which the cement is replaced by lime. The main application of this product is in floors, domes as
well as vaults. These unlike cements have many environmental and health benefits. These products are renewable and easily
cleaned.
20. Asphalt Concrete
 Asphalt concrete is a composite material, mixture of aggregates and asphalts commonly used to surface roads, parking lots,
airports, as well as the core of embankment dams. Asphalt concrete is also called as asphalt, blacktop or pavement in North
America, and tarmac or bitumen macadam or rolled asphalt in the United Kingdom and the Republic of Ireland.
21. Roller Compacted Concrete
 These are concrete that is placed and compacted with the help of earth moving equipment like heavy rollers. This concrete is
mainly employed in excavation and filling needs.
 These concretes have cement content in lesser amount and filled for the area necessary. After compaction, these concretes
provide high density and finally cures into a strong monolithic block.

6. 22.Rapid Strength Concrete
 As the name implies these concretes will acquire strength with few hours after its manufacture. Hence the formwork removal
is made easy and hence the building construction is covered fastly. These have a wide spread application in the road repairs as
they can be reused after few hours.
23. Glass Concrete
 The recycled glass can be used as aggregates in concrete. Thus, we get a concrete of modern times, the glass concrete. This
concrete will increase the aesthetic appeal of the concrete. Theyalso provide long-term strength and better thermal insulation
also.
2. Mix Design of Concrete
 The process of selecting suitable ingredients of concrete and determining their proportions
is referred to as mix design. The objective is producing a concrete of the required, strength,
durability, and workability as economically as possible. The proportioning of ingredient of n
crete is governed by the required performance of concrete in two states, namely the plastic a
nd the hardened states.
 If the plastic concrete is not workable, it cannot be properly placed and compacted. The pro
perty of workability therefore becomes important.
 The compressive strength of hardened concrete which is generally considered to be an index
of its other properties, depends upon many factors like quality and quantity of cement, wat
er and aggregates; batching and mixing; placing, compaction and curing.
 From technical point of view the rich mixes may lead to high shrinkage and cracking in the s
tructural concreteand evolutionof high heat of hydration in mass concretewhich may cause
cracking.
 The cost of concrete is made up of the cost of materials, plant and labour. The variations in t
he cost of materials arise from the fact that the cement is several times costlier than the aggr
egate. Thus the aim is to produce as lean a mix as possible. The actual cost of concrete isrelat
ed to the cost of materials required for producing a minimum mean strength called characte
ristic strength that is specified by the designer of the structure.
REQUIREMENTS OF CONCRETE MIX DESIGN
The requirements which form the basis of selection and proportioning of mix ingredient
s are:
 The minimum compressive strength required from structural consideration
 The adequate workability necessary for full compaction with the compacting equipment
available.
 Maximum water‐ cement ratio and/or maximum cement content to give adequate durabi
lity for the particular site conditions
 Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass
concrete.
 Minimum density for gravity dams and similar structures.
Types of Mixes
1. Nominal Mixes In the past the specifications for concrete prescribed the proportions of
cement, fine and coarse aggregates. These mixes of fixed cement‐ aggregate ratio which
ensures adequate strength are termed as nominal mixes. These offer simplicity and und
er normal circumstances, have a margin of strength above that specified. However, due t
o the variability of mix ingredients the nominal concrete for a given workability varies w
idely in strength.
2. Standard mixes.
The nominal mixes of fixed cement‐ aggregate ratio (by volume) vary widely in strength

7. and may result in under‐ or over‐ rich mixes.
For this reason, the minimum compressive strength has been included in many specifica
tions. These mixes are termed as standard mixes.
IS 456‐ 2000 has designated the concrete mixes into a number of grades as M10, M15, M
20, M25, M30, M35 and M40. In this designation the letter M refers to the mix and the n
umber to the specified 28 day cube strength of mix in N/mm2. The mixes of grades M10,
M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.
5:3) and (1:1:2) respectively.
3. Designed Mixes In these mixes the performance of the concrete is specified by the desi
gner but the mix proportions are determined by the producer of concrete, except that the
minimum cement content can be laid down. This is most rational approach to the selecti
on of mix proportions with specific materials in mind possessing more or less unique cha
racteristics. The approach results in the production of concrete with the appropriate pro
perties most economically. However, the designed mix does not serve as a guide since thi
s does not guarantee the correct mix proportions for the prescribed performance.
For the concrete with undemanding performance nominal or standard mixes (prescribed
in the codes by quantities of dry ingredients per cubic meter and by slump) may be used
only for very small jobs, when the 28‐ day strength of concrete does not exceed 30 N/mm
2
FACTORS AFFECTING THE CHOICE OF MIX PROPORTIONS
The various factors affecting the mix design are:
1. Compressive strength
2. Workability
3. Durability
4. Maximum nominal size of aggregate
5. Grading and type of aggregate
6. Quality Control
1. Compressive strength
It is one of the most important properties of concrete and influences many other
describable properties of the hardened concrete. The mean compressive strength
required at a specific age, usually 28 days, determines the nominal water-cement ratio of
the mix. The other factor affecting the strength of concrete at a given age and cured at a
prescribed temperature is the degree of compaction. According to Abraham’s law the
strength of fully compacted concrete is inversely proportional to the water-cement ratio.
2. Workability
The degree of workability required depends on three factors. These are the size of the
section to be concreted, the amount of reinforcement, and the method of compaction to be
used. For the narrow and complicated section with numerous corners or inaccessible
parts, the concrete must have a high workability so that full compaction can be achieved
with a reasonable amount of effort. This also applies to the embedded steel sections. The
desired workability depends on the compacting equipment available at the site. 3.
Durability The durability of concrete is its resistance to the aggressive environmental
conditions. High strength concrete is generally more durable than low strength concrete.
In the situations when the high strength is not necessary but the conditions of exposure
are such that high durability is vital, the durability requirement will determine the
water-cement ratio to be used.
4. Maximum nominal size of aggregate
In general, larger the maximum size of aggregate, smaller is the cement requirement for
a particular water-cement ratio, because the workability of concrete increases with
increase in maximum size of the aggregate. However, the compressive strength tends to
increase with the decrease in size of aggregate. IS 456:2000 and IS 1343:1980

8. recommend that the nominal size of the aggregate should be as large as possible.
5. Grading and type of aggregate
The grading of aggregate influences the mix proportions for a specified workability and
water-cement ratio. Coarser the grading leaner will be mix which can be used. Very lean
mix is not desirable since it does not contain enough finer material to make the concrete
cohesive. The type of aggregate influences strongly the aggregate-cement ratio for the
desired workability and stipulated water cement ratio. An important feature of a
satisfactory aggregate is the uniformity of the grading which can be achieved by mixing
different size fractions.
6. Quality Control
The degree of control can be estimated statistically by the variations in test results. The
variation in strength results from the variations in the properties of the mix ingredients
and lack of control of accuracy in batching, mixing, placing, curing and testing. The
lower the difference between the mean and minimum strengths of the mix lower will be
the cement content required. The factor controlling this difference is termed as quality
control.
 MIX PROPORTION DESIGNATIONS: The common method of expressing the pr
oportions of ingredients of a concrete mix is in the terms of parts or ratios of cem
ent, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 mean
s that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains
one part of cement, two parts of fine aggregate and four parts of coarse aggregat
e. The proportions are either by volume or by mass. The water‐ cement ratio is u
sually expressed in mass.
 FACTORS TO BE CONSIDERED FOR MIX DESIGN
 The grade designation giving the characteristic strength requirement of concrete.
 The type of cement influences the rate of development of compressive strength of
concrete.
 Maximum nominal size of aggregates to be used in concrete may be as large as po
ssible within the limits prescribed by IS 456:2000.
 The cement content is to be limited from shrinkage, cracking and creep.
 The workability of concrete for satisfactory placing and compaction is related to t
he size and shape of section, quantity and spacing of reinforcement and techniqu
e used for transportation, placing and compaction.
DATA REQUIRED FOR MIX PROPORTIONING BASED ON IS 10262‐ 2009:
 Grade designation
 Type of cement
 Maximum nominal size of aggregate( MNSA)
 Minimum cement content
 Max. water cement ratio
 Workability
 Exposure conditions
 Max temperature of concrete at the time of placing
 Method of transporting and placing
 Early age strength requirement if required
 Type of aggregate
 Max. cement content
 Whether admixture is used or not and the type of admixture.
MIX DESIGN METHODS
1. BIS method
2. ACI method
3. DOE method

9. 4. IS method
1. BIS method
The Bureau of Indian Standards recommended a set of procedure for design of co
ncrete mix (IS 102622009). This method can be applied for both medium and hig
h strength concrete.
Procedure
1. Collection of data: The data required for mix proportioning such as Concrete grad
e, type of cement, Aggregates, maximum size of aggregates, properties of cement
& aggregates etc. shall be collected
1. Determine the mean target strength ft from the specified characteristic compre
ssive strength at 28day fck and the level of quality control ‘S’
ft = fck + t S
ft = fck + 1.65 S,
where S is the standard deviation
fck = Characteristic compressive strength after 28days
t = tolerance factor
S = Standard deviation obtained from table 39 Of SP23
Standard deviation
Table (Assumed Standard Deviation as Per IS 456 Of 2000)
Grade of
concrete
Assumed st
andard dev
iation
(N/mm2)
M10 3.5
M15
M20 4
M25
M30 5
M35
M40
M45
M50
Table: Values of tolerance factor‘t’ (IS: 10262‐ 1982)
Accepted proporti
on of low results
T
1 in 5 0.84
1 in 10 1.28
1 in 15 1.50
1 in 20 1.65
1 in 40 1.96
1 in 100 1.33
2. Selection of water Cement Ratio:
Obtain the free water cement ratio corresponding to the targeted mean stre
ngth from fig. 46 page 119 of SP23 (figure below) .The water cement ratio so chos
en is checked against the limiting water cement ratio for the requirements of dur
ability given in table 5 page 20 of IS 456 and adopt the lower of the two values.
3. Estimation of entrapped air:

10. Estimate the amount of entrapped air for maximum nominal size of the aggregat
e is selected from the table 41 page 113 of SP23
4. Select the water content:
Select the water content for the required workability and maximum size of aggre
gates (for aggregates
in saturated surface dry condition) from table 42 & 43 of SP23
5. Determine the percentage of fine aggregate in total aggregate by absolute volume
from table for the concrete using crushed coarse aggregate.
6. Adjust the values of water content and percentage of sand as provided in the tabl
e for any difference in workability, water cement ratio, grading of fine aggregate
and for rounded aggregate the values are given in table 44
7. Determination of Cement Content:
Calculate the cement content from the water‐ cement ratio and the final w
ater content as arrived after adjustment (cement by mass = water content/water
cement ratio). Check the cement against the minimum cement content from the r
equirements of the durability from IS 456, and greater
of the two values is adopted.
8. Determination of Coarse and Fine aggregate:
From the quantities of water and cement per unit volume of concrete a
nd the percentage of sand already determined in steps 6 and 7 above, calculate th
e content of coarse and fine aggregates per unit volume of concrete from the follo
wing relations:
9. Determine the concrete mix proportions for the first trial mix.
10. Prepare the concrete using the calculated proportions and cast three cubes of 15
0 mm size and test them wet after 28‐ days moist curing and check for the streng
th.
11. Prepare trial mixes with suitable adjustments till the final mix proportions are a
rrived at.
ACIMETHOD (AMERICAN CONCRETEINSTITUTE METHOD)
 This method of proportioning was first published in 1944 by ACI committee.
 It is simple and effective.
 Can have design of air entrained and non‐air entrained concrete.
 Can have design with any type or shape of aggregate.
ACI method is based on the fact that for a given maximum size of well‐ shaped aggregat
es, the water – content (kg/m3) determines the workability of mix.
Procedure:
1) Data collection.
(a) Fineness modulus of selected F.A
(b) Unit weight of dry rodded course aggregate.
(c) Sp. gravity of cement
(d)Sp. gravity of Coarse and fine aggregates in SSD condition
(e) Absorption characteristics of course and fine aggregates.
2)Estimation of average or mean design strength from the minimum strength either by
using standard deviation ‘S’ or by using coefficient of variation.
i.e. ft= Fck +1.65S
3)Selection of water cement ratio. Find the water cement ratio from the strength point of
view from table 115 of ACI code. Find also the water cement ratio from durability point
of view. Adopt lower value of strength consideration
and durability consideration.(use ASI code).

11. 4)Estimation of entrapped air. Determine the entrapped air content from table 119 of AC
I code.
5)Selection of water content. Select the water content for required workability and maxi
mum size of aggregates from table 118 of ACI code. It can also computed by dividing the
total water content by w/c ratio.
6)Cement content. It is computed by dividing the total water content by water cement ra
tio.
7)Bulk volume of dry rodded aggregates. From table 114 the bulk volume of dry rodded a
ggregates (C.A) per unit volume of concrete is selected, for the maximum size of course a
ggregates and fineness modulus of fine aggregates.
8)The weight of course aggregates per unit volume of concrete is calculated by multiplyin
g the bulk volume with bulk density.
9)Solid volume of course aggregate/m3 of concrete is calculated by knowing the specific g
ravity of
Course aggregates. Similarly the solid volume of cement, water and volume of air is calc
ulated in 1 m3 of concrete.
10)Solid volume of sand is computed by subtracting from the total volume of concrete vol
ume of
cement, course aggregates water and air entrapped. Weight of fine aggregate is calculate
d by multiplying the solid volume of fine aggregates by its specific gravity.
Limitation of ACI method
 This method is suitable for normal and heavy weight concrete in the workability rang
e of 25 to 100 mm slump.
 It is recommended for well‐ shaped aggregates
3. DOE method
4. IS method
PROPERTIES OF HARDENED CONCRETE
Hardened concrete is the concrete that is in a solid state and has developed certai
n strength. Reaction continues with time and produced hard, strong and durable solid
material.
1. Strength of concrete
2. Elastic properties of concrete
3. Creep
4. Shrinkage
5. Thermal properties
6. Fatigue
. Strength of concrete
Strength of concrete is commonly considered as its most valuable property, although i
n many practical cases, other characteristics such as durability and permeability may
be more important. Strength usually gives an overall picture of the quality of concret
e because strength is directly related to the structure of the hydrated cement paste. St
rength of concrete could be defined as the ultimate load that causes failure (or is its re
sistance to rupture) and its units are N/mm2 or MPa.
Fracture and failure of Concrete Concrete specimens subjected to any state of stress c
an support loads up to 40‐ 60% of ultimate load without any apparent signs of distres
s. As the load is increased above this level, soft but distinct noises of internal disrupti
on can be heard until, at about 70‐ 90% of ultimate load, small fissures or cracks appe
ar on the surface. At ultimate load and beyond, the specimens are increasingly disrup
ted and eventually fractured into a large number of separate pieces
Types of Concrete Strength
(a)Compressive strength The concrete is primarily used to exploit its compressive stre

12. sses. The compressive ength of concrete
is defined as the strength of 28 days old specimens tested under uniaxial compressive
load. Cubes, cylinders and prisms are the three types of compression test specimens u
sed to determine the compressive strength on testing machines. The cubes are usually
of 100 mm or 150 mm side, the cylinders are 150 mm diameter by 300 mm height. Th
e specimens are cast, cured and tested as per standards prescribed for such tests. Wh
en cylinders are used, they have to be suitably capped before the test, an operation wh
ich is not required when other types of specimen are tested. The compressive strength
s given by different specimens for the same concrete mix are different. The cylinders a
nd prisms of a ratio of height or length to the lateral dimension give strength of about
75‐ 85 % of cube strength of normal strength of concrete. The effect of height/lateral
dimension ratio of specimen on compressive strength is given in fig. below
(fc) cylinder = (0.85‐ 0.80)(fc) cube
Comparison between Cube and Cylinder Strength
It is difficult to say whether cube test gives more realistic strength properties of concr
ete or cylinder gives a better picture about the strength of concrete. However, it can b
e said that the cylinder is less affected by the end restrains caused by platens and hen
ce it seems to give more uniform results than cube. Therefore, the use of cylinder is be
coming more popular, particularly in the research laboratories.
Cylinders are cast and tested in the same position, whereas cubes are cast in one dire
ction and tested from the other direction. In actual structures in the field, the casting
and loading is similar to that of the cylinder and not like the cube. As such, cylinder si
mulates the condition of the actual structural member in the field in respect of directi
on of load.
The points in favor of the cube specimen are that the shape of the cube resembles the
shape of the structural members often met with on the ground. The cube does not req
uire capping, whereas cylinder requires capping. The capping material used in case cy
linder may influence to some extent the strength of the cylinder.
(b)Tensile strength
The tensile strengthof concrete ismuchlowerthanthe compressive strength, largely becauseof the eas
e withwhichcrack can propagate undertensile loads. The tensile strengthof concrete ismeasured inthr
ee ways:Directtension, Splittingtension (Indirectmethod) andflexuraltension.
The methods usedtodetermine the tensilestrengthof concrete canbe broadly classified asdirectandi
ndirectmethod. The directmethods sufferfromanumberof difficulties relatedtoholdingthe(c)
Flexural strength
Concrete as we know is relatively strong in compression and weak in tension. In reinforced conc
rete members, little dependence is placed on the tensile strength of concrete since steel reinforci
ng bars are provided to resist all tensile forces. However, tensile stresses are likely to develop in
concrete due to drying shrinkage, rusting of steel reinforcement, temperature gradients and ma
ny other reasons. Therefore, the knowledge of tensile strength of concrete is of importance.
The determination of flexural tensile strength is essential to estimate the load at which the conc
rete members may crack. As it is difficult to determine the tensile strength of concrete by condu
cting a direct tension test, it is computed by flexure testing. The flexure tensile strength at failu
re or the modulus of rupture is thus determined and used when necessary. Its knowledge is usef
ul in the design of pavement slabs and airfield runway as flexural tension is critical in these cas
es. The modulus of rupture is determined by testing standard test specimens of 150mm X 150m
m X 700 mm over a span of 600 mm or 100 mm X 100mm X 500 mm over a span of 400 mm, und

13. er symmetrical two‐ point
loading. The flexural strength of the specimen is expressed as the modulus of rupture fb which i
f ‘a’ equals the distance between the line of fracture and the nearer support, measured on the ce
nter line of the tensile side of the specimen, in cm, is calculated to the nearest 0.05 MPa as follo
ws: When ‘a’ is greater than 20.0 cm for 15.0 cm specimen or greater than 13.3 cm for a 10.0 cm
specimen, fb=pl/bd^2
Or when ‘a’ is less than 20.0 cm but greater than 17.0 cm for 15.0 specimen, or less than 13.3
Fb=3pl/bd^2
cm but greater than 11.0 cm for a 10.0 cm specimen 3 Where,
b = measured width in cm of the specimen,
d = measured depth in cm of the specimen at the point of failure,
l = length in cm of the span on which the specimen was supported, and
p = maximum load in kg applied to the specimen
If ‘a’ is less than 17.0 cm for a 15.0 cm specimen, or less than 11.0 cm for a 10.0 cm specimen, th
e results of the test be discarded.
The results are affected by the size of the specimens; casting, and moisture conditions; manner o
f loading; rate of loading, etc.
RelationbetweenCompressive strengthand tensile strength
It isseenthat strengthof concrete incompression andtension(bothdirecttensionandflexural
tension) are closely related, butthe relationship isnotof the type of directproportionality. The ration
of the twostrengths depends ongeneral levelof strengthof concrete. Inotherwords, forhigher
compressive strengthconcrete shows highertensilestrength, butthe rate of increase of tensile
strengthisof decreasingorder. Tensile strengthof concrete isproportional tothe square rootof the co
mpressive strength. The proportionality constantdepends onmany factors, suchas the concrete strengt
h and the testmethodusedtodetermine the tensilestrength
The following relations can be used as a rule of thumb:
Direct tensile strength, = 0.35 √fc
Split tensile strength, = 0.50√fc
Flexural tensile strength, = 0.64 √fc
Where fc is compressive strength in N/mm2
(d)Impact strength
The impact strength of concrete is important when it is subjected to sudden load or re
peated impacted load. Impact strength is of importance in driving concrete piles, in fo
undations for machines exerting impulsive loading, and also when accidental impact i
s possible, e.g. when handling precast concrete members. Runway concrete pavements
are also subjected to repeated impacts due to landing and takeoff of aircraft. Angular
and surface rough aggregates (broken granites) exhibit better impact strength.
There is no unique relation between impact strength and strengths of concrete. Howe
ver, some researchers have found that impact strength is related to the compressive st
rength. It has been suggested that the impact strength varies from 0.50 to 0.75 of com

14. pressive cube strength
(e)Bond Strength
We can consider the bond strength from two different angles; one is the bond strength
between paste and steel reinforcement and the other is the bond strength between pa
ste and aggregate. Firstly, let us consider the bond strength between paste and steel r
einforcement. Bond strength between paste and steel reinforcement is of considerable
importance. A perfect bond, existing between concrete and steel reinforcement is one o
f the fundamental assumptions of reinforced concrete. Bond strength arises primarily
from the friction and adhesion between concrete and steel. The roughness of the steel
surface is also one of the factors affecting bond strength. The bond strength of concret
e is a function of compressive strength and is approximately proportional to the compr
essive strength upto about 20 MPa. For higher strength, increase in bond strength be
comes progressively smaller.
The bond strength, is also a function of specific surface of gel. Cement which consists
of a higher percentage of C2S will give higher specific surface of gel, thereby giving hi
gher bond strength. On the other hand, concrete containing more C3S or the concrete
cured at higher temperatue results in smaller specific surface of gel which gives a low
er bond strength. It has been already pointed out that high pressure steam cured conc
rete produces gel whose specific surface is about1/20 of the specific
surface of the gel produced by normal curing. Therefore, bond strength of high pressu
re steam cured concrete is correspondingly lower.
Factors Affecting Strength of concrete
For a given cement and acceptable aggregates, the strength that may be developed by
workable,
properly placed mixture of cement, aggregate and water (under the same mixing, curi
ng and testing conditions) is influenced by:
(a) Ratio of cement to mixing water;
(b) Ratio of cement to aggregate;
(c) Grading, surface texture, shape, strength and stiffness of aggregate particles;
(d)Maximum size of aggregate.
In the above it can be further inferred that water/cement ratio primarily affects the st
rength, whereas other factors indirectly affect the strength of concrete by affecting the
water/cement ratio.
Water/Cement Ratio Strength of concrete primarily depends upon the strength of cem
ent paste. This has been discussed earlier. The strength of paste increases with cemen
t content and decreases with air and water content. In 1918 Abrams presented his cla
ssic law in the form:
S=A/B^x
Where
x = water/cement ratio by volume and for 28 days results the constants A and B are
14,000lbs/sq. in. and 7 respectively .
Abrams water/cement ratio law states that the strength of concrete is only
dependent upon water/cement ratio provided the mix is workable.
The relation between the water/cement ratio and strength of concrete is shown in Fig
ure below. It can be seen that lower water/cement ratio could be used when the concre
te is vibrated to achieve higher strength, whereas comparatively higher water/cement
ratio is required .when concrete is hand compacted. In both cases when the water/ce
ment ratio is below the practical limit the strength of the concrete falls rapidly due to
introduction of air voids.
Gel/Space Ratio

15.  Since concrete is a brittle material, its porosity primarily governs its strength. The com
pressive strength is found to be severely decreasing with increase in porosity.
 The porosity of concrete which governs the strength of concrete is affected by the gel/sp
ace ration in concrete.
 The gel/space ratio is the ratio of solid products of hydration to space available for these hydrat
ion products.
 A higher gel/space ratio reduces the porosity and therefore increases the strength of concrete.
 The gel/space ratio, which governs the porosity of concrete affecting its strength, is affected
by the water/cement ration of concrete.
 A higher water/cement ratio decreases the gel/space ratio increasing the porosity thereby decreasin
g the strength of concrete.
Aggregate/Cement Ratio
 The aggregate/cement ratio, is only a secondary factor in the strength of concrete but it
has been found that, for a constant water/cement ratio, a leaner mix leads to a higher st
rength
 Some water may be absorbed by the aggregate. A larger amount of aggregate absorbs a
greater quantity of water, the effective water/cement ratio being thus reduced
 A higher aggregate content would lead to lower shrinkage and lower bleeding and there
fore to less damage to the bond between the aggregate and cement paste.
As a result in a leaner mix, the voids form a smaller fraction of the total volume of concr
ete and it is these voids that have an adverse effect on strength
Effect of Maximum size of Aggregate
Larger the aggregate lower is the total surface area and therefore, the lower is the requ
irement of water for the given workability.
The use of larger size aggregate did not contribute to higher strength as expected from t
he theoretical considerations due to the following reasons
 The larger maximum size aggregate gives lower surface area for developments of gel bo
nds which is responsible for the lower strength of the concrete
 Secondly bigger aggregate size causes a more heterogeneity in the concrete which will p
revent the uniform distribution of load when stressed
 When large size aggregate is used, due to internal bleeding, the transition zone will bec
ome
much weaker due to the development of micro cracks which result in lower compressive
strength
 For w/c ratio below 0.4, the strength seems to reduce by 38 percent when higher size
aggregates are used
 With an increase in the w/c ratio to 0.5, influence the aggregate falls off, presumably be
cause strength of the hydrated paste itself becomes paramount
 At a w/c ratio of 0.65 no difference in the strength of concretes was made with increase
of aggregate size.
Age of Concrete
 With an increase in age, the degree of hydration generally increases the gel/space ratio
so that strength increases.
 Increase in the strength of concrete (at same w/c ratio) with increase in early age (from
1 to 28 days) of concrete
Age of Concrete 3
days
7
days
14
days
28 day 90 day 180
day
360
day
Strength 0f conc. 40% 65% 90% 100% 115% 120% 130%
Factors influencing strength of concrete in testing can be classified into
i. Factors depending on testing method
(a) Size of test specimen

16. (b)Size of specimen in relation to the size of aggregate
(c) Support conditions of specimen
(d) Moisture conditions of the specimen
(e) Type of loading adopted
(f) Rate of loading of specimen
(g)Type of testing machine
(h)The assumptions made in the analysis relating stress to failure load
ii.Factors independent of type of test
(a)The type of cement and age, type of aggregate and admixture
(b)Degree of compaction
(c)Concrete mix proportions i.e. cement content, aggregate cement ratio, amount of air v
oids and water ‐ cement ratio
(d)Type of curing and temperature of curing
(e)Nature of loading to which the specimen is subjected i.e. static, sustained, dynamic et
(f) Type of stress situation that may exist, viz. uniaxial, biaxial and tri axial
ELASTIC PROPERTIES OF CONCRETE:
 Concrete is generally considered as brittle. However it deforms under load‐ exhibiting
elastic behavior before cracking.
 Elasticity can be defined as that strain appears and disappears immediately on applicat
ion and removal of stress
Determination of Modulus of Elasticity
 Modulus of elasticity of concrete is determined by subjecting a cube or cylinder specime
n to uniaxial compression and measuring the deformations by means of dial gauges
 Dial gauge reading divided by gauge length will give the strain and load applied divided
by the area of c/s will give stress
 A series of readings are taken and stress‐ strain relationship is established
 Modulus of elasticity can also be determined by subjecting a concrete beam to bending and
then using the deflection formulae and substituting the other parameters
 The modulus of elasticity so found out from actual loading is called static modulus of elasticity
 Concrete does not behave as an elastic material even under a short term loading Stress‐ strain g
raph of concrete is not very much curved, up to about 10+15 % of the ultimate
strength of concrete and hence the results are very accurate
 For higher stresses, the stress‐ strain relations will be greatly curved and results are inaccu
rate
Fig. shows the typical stress strain behaviour of cement paste‐ aggregate and concete
 Stress‐ strain relationship of aggregates shows a fairly good straight line. Similarly ce
ment paste alone also shows a good straight line. But the stress‐ strain graph of concret
e which is a combination of aggregate and cement paste together shows a curved shape.
This is due to the development of micro cracks at the interface between cement paste a
nd aggregate. The stress stain behavior of concrete depends on a no. of variables such a
s:
 The properties of the materials with which the concrete is made.
 The loading parameters such as the rate of loading
 The age of testing.
The variation of the stress‐ strain curve for concrete can principally be attributed to t
he cracking of the cement paste. The cement paste as well as the aggregates exhibit line
ar stress‐ strain properties. Internal cracking develops through a number of stages dep
ending on the level of applied stress. The crack starts at the aggregate paste interface k
nown as the transition zone (TZ). And spread in to the cement mortar phase, as the leve
l of stress increases. When the ultimate load is reached, the cracks get interconnected a
nd contribute to the attainment of failure stress, leading to ultimate collapse.

17. Thus the following stages of cracking behavior are clearly witnessed.
Stage I: Even before the load is applied, micro‐ cracks exist in the interface (transition
zone). The number and width of cracks increase in the level of stress. The severity of cr
acking depends on the bleeding characteristics, the strength of the TZ, and the curing h
istory. At low load (about 30% of the ultimate load) the TZ remains stable.
Stage II: Above 30% and up to about 60% of the ultimate stress, the level of stress incra
ses. At 5060% of the ultimate stress, cracks begin to appear in the mortar matrix.
Stage III: The cracks start becoming unstable at about 75% of the ultimate stress. As th
e strain rate increases, the cracks in the mortar phase also increase.
Stage IV: Above 75% of the ultimate stress, the cracks in the matrix join up with the cra
cks in the TZ and failure become imminent. Upon reaching the ultimate load, the stress
decreases with increase in strain. Thus concrete has an almost linear ascending strain
‐ softening characteristic up to ultimate strain and a falling stress‐ strain characteristi
c as shown in fig above
Though the stress‐ strain characteristics of concrete are nonlinear from the beginning, t
he initial tangent OD to the stress‐ strain curve is regarded as the initial tangent modu
lus. The modulus varies with the load level, as well as with the rate of loading. At any l
oad level at T, other than O, the modulusis estimated based on secant line TT’ at the cor
responding load level. Thus there is a need to properly define the modulus, accounting f
or these variations.
Static modulus
The modulus of elasticity of concrete is generally related to compressive strength. The r
elationship depends on the aggregate type, mix proportion, rate of loading, curing condi
tions, and method of measurement. The modulus of elasticity under static loading condi
tions is generally known as its static modulus. The value of the static modulus E for con
crete is determined on the basis of the uniaxial stress‐ strain curve obtained from a sta
ndard test cylinder (15 cm diameter, 30 cm height). The nonlinearity of the stress‐ strai
n curve leads to the following three definitions of static modulus.
A.Tangent modulus evaluated as the slope of the tangent to the stress‐ strain curve at a
ny point. Thusthe initial tangent modulus refers to the shape of the tangent at zero stre
ss, shown as line OD in fig
B.Modulus evaluated as the slope of the line drawn from the origin to a point on the str
ess‐ strain curve corresponding to 45% of the ultimate stress, shown as line OT in fig.
 C.Chord modulus evaluated as the slope of the line drawn between any two points on th
e stressstrain curve or as defined by a prescribed standard. Static chord modulus of elas
ticity is defined as the ratio of the difference of the stress at 40% of the ultimate strengt
h and the stress at 50 millionths (50X10‐ 6) of the strain to the difference in strain corr
esponding to the stress at 40% of the ultimate strength and 50 millionths strain (50X10
‐ 6). IS 456 ‐ 2000 gives the following empirical formula for static modulus:
Ec= 5000√fck
Where EC=static modulus of elasticity in MPa
Fck=characteristic compressive strength of concrete in MP
Dynamic modulus
The dynamic modulus of concrete is relatively more complex. It corresponds to
a very small instantaneous strain due to suddenly applied stress. Thus, it is approximat
ely equal to the initial tangent modulus but is considerably larger than the static secant
modulus. For low, medium, and high strength concrete, the dynamic modulus is genera
lly 40%, 30% and 20%, respectively, higher than the static modulusThe modulus of elasi
ty of concrete would be a property for the case when the material is treated as elastic. B
y considering the stress‐ strain curve of the first cycle the modulus could be defined as i
nitial tangent modulus, secant modulus, tangent modulus or chord modulus as shown i

18. n fig. In the laboratory determination of the modulus of elasticity of concrete, a cylinder
is loaded and un loaded for three or
four cycles, the stress‐ strain curve is plotted after residual strain has become almost n
egligible and the average slope of stress‐ strain curve is taken.
The above modulus of elasticity is sometimes termed the static (secant) modulus of elas
ticity in comparison with dynamic modulus of elasticity obtained by vibration tests of co
ncrete prisms or cylinders. The latter is approximately equal to the initial tangent mod
ulus and hence greater than the static or secant modulus.
Factors affecting the modulus of elasticity of concrete
Since concrete is a multiphase solid, no direct relationship can exist between its density
and modulus as in single‐ phase solids such as metals. The modulus is influenced by de
nsity, porosity, mix proportion, moduli of elasticity of the ingredients, and the characteri
stics of transition zone. These parameters determine the elastic behaviour of concrete.
1.Effect of aggregates: Dense aggregate leads to a high value of E for concrete. A large p
roportion of coarse aggregate leads to a high value of E as well. A very large value of E o
f the aggregate will lead to an elastic mismatch among the aggregate, mortar, and crack
s in the transition zone
Description E (105 MPa)
Granite 1.4
Sandstone 0.2‐ 0.5
Expanded shale 0.07‐ 0.21
Hydrated cement paste .07
concrete 0.1‐ 0.2
2.Effect of hydrated cement paste: The elastic modulus of cement paste is determined b
y its porosity. The water to cement ratio (w/c), air content, admixture dosage, and degre
e of cement hydration control the porosity of cement paste. The modulus of elasticity of
concrete can be represented based on the following simple equation
EC = Ea g + EP (1‐ g)
Where EC is the modulus of elasticity of concrete, Ea is the modulus of elasticity of aggr
egate, EP is the modulus of elasticity of cement paste, g is the volume fraction of the ag
gregate and 1‐ g is the volume fraction of the cement paste. EP is low due to the poor de
nsity of the transition zone.
3.Effect of transition zone: The existing cracks in the transition zone and the orientatio
n of C‐ H crystals as well as existing void space make the transition zone weak. This ca
uses the elastic modulus to drop gradually with increasing loads.
4. Mix proportion:‐ Richer mix shows higher modulus of elasticity than lean mix
5. Age of concrete:‐ Modulus of elasticity of concrete increases with the age of concrete
6.Wetness of concrete: ‐ Wet concrete will show higher modulus of elasticity than dry c
oncrete. Wet concrete being saturated with water experiences less strain for a given str
ess and hence gives higher modulus of elasticity
7.Temperature: ‐ Relation between the modulus of elasticity and strength is not much
effected by temperature up to about 230ºC. Steam cured concrete shows a slightly lower
modulus than water cured concrete of the same strength.
CREEP
 It is defined as the time‐ dependent part of the strain resulting from stress
 Creep can be defined as the increase in strain under sustained constant stress. The gra
dual increase in strain, without increase in stress, is due to creep
 All materials undergo creep under some conditions of loading to greater or smaller exte
nt
 Creep of concrete approximately linear function of stress up to 30 to 40 % of its streng

19.  Creep takes place only under stress
 Under sustained stress with time, the gel, the absorbed water layer, the water held in t
he gel pores etc. undergoes creep.
Mechanism of creep:
 Three basically different mechanism of deformation are possible
1.Compressive stress normal to contact layer
 In this mechanism, the liquid is compressed and squeezed out laterally. This is due to t
he reduction of intercrystalline space
 The squeezing away of liquid against strong frictional forces causes the irrecoverable
changes in the cement gel
2. Visco Elastic Elongation mechanism
 It is faster than the above mechanism
 Complete recovery may be expected long after uploading
 This is due to tensile stresses normal to the contact layer.
3.Shear stress mechanism
 This is due to the shear stresses parallel to the contact layer
 Stress results in water layers
All these mechanism are governed by movement or migration of the various types of wa
ter held
 Under sustained stress, the cement paste continue undergo deformation of creep
 This creep deformation at the discontinuities causes growth of micro cracks
Factors affecting creep
1.Influence of aggregate
 Aggregates undergoes very little creep
 The stronger aggregate the more is the restraining effect and hence the less is the magn
itude of creep
 If the aggregates have higher modulus of elasticity, then the creep is less
 Grading, texture, shape & size of aggregates also affect the creep
 Light weight aggregates shows higher creep than normal concrete
2.Mix proportion
 The amount of cement paste and its quality is one of the most important factors influen
cing creep
 A poorer paste structure undergoes higher creep i.e. the creep increases with increase i
n water cement ratio
3.Cement Paste Content
 A 1% increase in cement pasre by volume will result in approximately a 5% increase in
creeo. This is applicable for concretes with a cement paste volume of 28% to 40%
 The cement paste volume is influenced by the aggregate content of the mix, Greater the
aggregate content, lower the cement paste conctent
4. Influence of Age
 Increase in age causes less creep
 Moisture content of the concrete being different at different age, also influences
the magnitude of creep
Effects of Creep
The magnitude of creep is dependent on many factors like time and level of stress
 In reinforced concrete beams, creep increases the deflection with time and may be a crit
ical consideration in design
 In reinforced concrete columns, creep property of concrete is useful. It cannot
deform independent of steel reinforcement. There will be gradual transfer of stress from
concrete to steel. The extra load in the steel is required to be shared by concrete and thi
s situation results in employment &development of full strength of steel concrete

20.  In eccentrically loaded columns, creep increases the deflection and can load to buckling
 In case of statically indeterminate structures, bean junction etc., creep may relieve the
stress concentration induced by shrinkage, temperature or movement of support
 Creep also reduces the internal stresses due to non‐ uniform load or restrained shrinka
ge
 In mass concrete structures such as dams, creep may be a cause of cracking in the inter
ior of dams
 Creep may also causes the volume change
SHRINKAGE
 Shrinkage is an inherent property of concrete
 Volume change is one of the most detrimental properties of concrete, which affect the lo
ngterm strength and durability
 Shrinkage of concrete can be defined as the volume changes in concrete due to loss of m
oisture at different stages due to different reasons
Classification of shrinkage:‐
a. Plastic shrinkage
b. Drying shrinkage
c. Autogeneous shrinkage
d.Carbonation shrinkage
(a)Plastic shrinkage:
Plastic shrinkage is due to the following reasons:
i.Loss of water by evaporation from the surface of concrete
ii.Loss of water by the absorption by aggregates
 The aggregate position or the reinforcement comes in the way of subsidence due to whic
h cracks may appear at the surface or internally around the aggregate
 In case of floors and pavements where the surface area exposed to drying is large as compare
d to depth when this hot surface exposed to hot sun and drying wind, the surface of conc
rete dries very fast which results in plastic shrinkage
 If the water cement ratio is high, large quantity of water bleeds and accumulates at the surface
. When this water at the surface dries out, the surface concrete collapses causing cracks
Remedies to prevent plastic shrinkage:
 Plastic shrinkage can be reduced by preventing the rapid loss of water from the surface.
This can be done by covering the surface with polythene sheeting
 Re‐ vibration of concrete reduces the plastic shrinkage
 Use of small quantity of Al powder reduces plastic shrinkage
 To control the plastic shrinkage expansive cement can be used
(b)Drying shrinkage
 The shrinkage that takes place after the concrete has set and hardened is called drying
shrinkage
 Most of the shrinkage takes place in the first few months
 Withdrawal of water from concrete stored in un saturated air voids causes drying shrin
kage
 A part of this shrinkage is recovered on immersion of concrete in water
 Shrinkage is more, if the gel pores are finer
 The rate of shrinkage decreases with time
(c)Autogeneous shrinkage
 No moisture movement to or from the cement paste is permitted, when temperature is c
onstant, some shrinkage may occur. Such shrinkage is called Autogeneous shrinkage
 It is of the order about 100X10‐ 6
(d)Carbonation shrinkage:
 The CO2 present in atmosphere reacts with the hydrated cement minerals, in the prese

21. nce of moisture, converting or carbonating Ca(OH)2 to CaCO3
 At the same time, some cement compounds are decomposed
 Shrinkage due to carbonation occurs mainly at intermediate humidities
 Carbonation also results in increased strength and reduced permeability
Factors affecting shrinkage of concrete:
i.w/c ratio: Shrinkage increases with the increase in w/c ratio value
ii. Cement content: Shrinkage rate increases with the increase in cement content
iii. Humidity: Shrinkage rate increases with the decrease in humidity
iv.Type of aggregate: Increase in maximum size of aggregate decreases the shrinkage. G
rading and shape has little effect on shrinkage
v. Type of cement: Rapid hardening cement shrinks more than the other types
vi.Admixtures: Shrinkage increases with the addition of CaCl2 and reduces with lime r
eplacement
vii. Type and shape of specimen
SEGREGATION
It is defined as the separation of constituent materials of concrete. A good concrete is on
e in which all the ingredients are properly distributed to make a homogeneous mixture.
Segregation may be of three types.
1. The course of aggregate separating out or settling down from the rest of the matrix
2.The paste or matrix separating away from course of aggregates
3.Water separating out from the rest of material being a material of lowest specific grav
ity.
Segregation is often seen occurring
 Due to improper proportioning of ingredients and improper mixing
 When concrete is dropped from heights
 When concrete is discharged from badly designed mixer with worn out blades.
 Conveyance of concrete by conveyor belts, wheel barrows, long distant hauls by dum
per etc.
 Excess vibration of too wet concrete
 Working too much with trowel, float or rammer
Segregation can be minimized by
 Restricting quantity of water
 Careful handling, transporting placing and compacting
 Adding air entraining admixture during mixing
 Restricting the height of pour
 BLEEDING
The water gain in concrete structures is referred as Bleeding. The process of rise of
water along with cement particles to the surface of freshly laid concrete is known as
bleeding. The particles of fine sand and cement are carried by the rising water to th
e surface forming a scum layer on hardening. This happens when there is excessive
quantity of water on the mix or when there is excessive compaction.
Bleeding is a particular type of segregation.
 Bleeding can be reduced
 By proper proportioning
 Use of finely divided pozzolanic materials reduces the bleeding by creating a longer
path
 Use of air entraining agents
 Use of fine cement with low alkali content
 Rich concrete mixes are less susceptible to bleeding than lean mixes.
Laitance:
Sometimes during bleeding along with water certain quantity of cement comes up a

22. nd accumulates at the surface. This formation of cement paste at the surface is calld
Laitance. Formation of laitance decreases the wearing quality of slab and pavemen
t surfaces. The laitance formed produces dust in summer and mud in winter. So whi
le concreting the laitance shall be removed before next lift is poured.
Test for Bleeding of concrete:
The test consists of determination of relative quantity of mixing water that will blee
d from a sample of freshly mixed concrete.
 A cylindrical container of approximately 0.01 m3 capacity having an inside diameter
of 250 mm and inside height of 280 mm is used.
 For tamping the concrete a tamping rod 60 cm long, 16 cm dia and with bullet end i
s used
 A pipette for drawing of free water from the surface and a graduated jar of 100 ml c
apacity for measuring quantity of water are needed for the test.
 Fresh concrete is filled in 50 mm layers for a depth of 250 mm
 The test specimen is weighed and knowing the water content for 1m3 of concrete qu
antity of water in the cylindrical container is found
 Cover the container with a lid. Water accumulated at the top is drawn by means of a
pipette at ten minutes interval for the first 40 minutes and their after at 30 minute
s intervals till the bleeding ceases. Weigh this water
5 Durability of Concrete
5.1 Durabilityconcept
Durability isthe ability of concrete toresistweatheringaction, chemical attack, abrasion oranyother pr
ocessof deterioration while maintainingitsdesired engineeringproperties. Differentconcretes
require differentdegrees of durability dependingonthe exposure environmentandthe properties
desired. Concrete ingredients, theirproportioning, interactions between them, placingandcuring
practices, andthe service environmentdetermine the ultimate durability andlife of the concrete. For
many conditions of exposure of concrete structures bothstrengthanddurability have tobe considered
especially atthe design stage.
A durable concrete isone thatperforms satisfactorily underanticipatedexposure conditions duringits
service life span. The materialsandmix proportions usedshould be suchasto maintainitsintegrity
and, if applicable, toprotectembedded metal fromcorrosion. One of the maincharacteristics
influencingthe durability of concrete isitspermeability tothe ingress of water, oxygen, carbon
dioxide, chloride, sulphate andotherpotentially deleterious substances.
Most of the durability problems inthe concrete canbe attributedtothe volume change inconcrete.
Volume change inconcrete iscausedbymany factors. The entire hydration process isnothingbutan
internal volume change, the effectof heatof hydration, the pozzolanicaction, the sulphate attack, the
carbonation, the moisture movement, all type of shrinkages, the effectof chlorides, corrosion of steel
reinforcementandhostof otheraspectscome underthe preview of volume change inthe concrete
whichresults incracks. It is the crack thatpromotes permeability andthusitbecomes apart of cyclic
action, till suchtime thatconcrete deteriorates, degrades, disrupts andeventually fails.
NOTE: Durabilityof concrete maybe definedasthe abilityof concrete toresist:
− weatheringaction,

23. − chemical attack,and –
abrasionwhile maintainingitsdesiredengineeringproperties.
• Differentconcretesrequire differentdegreesof durabilitydependingonthe exposureenvironment
and propertiesdesired.−For example,concreteexposedtotidal seawaterwillhave different
requirementsthananindoorconcrete floor
SIGNIFICANCEOFDURABILITY:
i. A durable concrete has long term ability to resist wear & tear to resist chemical atta
ck and to
resist polluted atmosphere and so negligible loss on repair & maintenance.
ii. Durability increases the increased life of concrete structures.
iii. A durable concrete surface does not require special care and time to time repair and
maintenance.
FACTORS TO BE PROPERLY CONTROLLED TO ACHIEVE DURABILITY:
i.The structural design
ii.Study of environment in which the structure is constructed. Temperature humidit
y and chemical conditions to be examined.
iii.Selection of material for concrete and good mix design.
iv.Concrete specification such as max water cement ratio max cement content type o
f cement and grade of concrete.
v. Quality of concrete cover.
vi.Workability and cohesiveness of concrete mix
vii.Batching mixing transporting placing compacting and most important curing. Ca
re should be taken to avoid segregation
viii. Maintenance and usage in service life
Deterioration in concrete
can take place basically due to porosity.
Concrete has porosity in several types
 Capillary pores
 Entrapped air
 Honey combs
 Cracks
PERMEABILITY
When we talk about durability of concrete, generally we start discussion from the permeability of
concrete, as it has much wider and direct repercussion on durability than that of W/C ratio. For e
xample, micro‐ cracks at transition zone is a consideration for permeability whereas W/C ratio m
ay not get involved directly. It may be mentioned that micro‐ cracks in the initial stage are so sm
all that they may not increase the permeability. But propagation of micro‐ cracks with time due t
o drying shrinkage, thermal shrinkage and externally applied load will increase the permeability
of the system.
Permeability of Cement Paste
 The extent and size of capillary cavities depend on the W/C ratio. It is one of the main fact

24. ors contributing to the permeability of paste. At lower W/C ratio, not only the extent of ca
pillary cavities is less but the diameter is also small. The capillary cavities resulting at low
W/C ratio, will get filled up within a few days by the hydration products of cement. Only
unduly large cavities resulting from higher W/C ratio (say more than 0.7) will not get fille
d up by the products of hydration, and will remain as unsegmented cavities, which is respo
nsible for the permeability of paste
 Permeability of Concrete
Theoretically, the introduction of aggregate of low permeability into cement paste, it is ex
pected to reduce the permeability of the system. Compared to neat cement paste, concrete
with the same W/C ratio and degree of maturity, should give a lower coefficient of permea
bility. But in practice, it is seen from test data, introduction of larger size of aggregates inc
rease the permeability considerably. Permeability of concrete is often referred as the root c
ause for lack of durability. But it can be seen that volume change that takes place in an oth
erwise impervious concrete due to heat of hydration or internal manifestation can crack th
e concrete affecting durability. Microcracks in transition zone even in initially impermeabl
e concrete, can start the cycle of deterioration process in concrete. Therefore, these three f
actors, one follows the other two, like day follows the night, are responsible for affecting d
urability of concrete and concrete structures
The factors governing permeability:
 The Quality of constituent materials
 The quality of pore structure which is based on the water cement ratio admixtures used an
d degree of hydration.
 The quality of interfacial transition zone.
 The degree of compaction
 The adequacy of curing
Chapter -4 testing of concrete
 Strength of concrete is commonly considered its most valuable property, although in
many practical cases, other characteristics, such as durability and permeability may in
fact be more important.
 Strength usually gives an overall picture of the quality of concrete because strength is
directly related to the structure of the hydrated cement paste.
 Strength of concrete could be defined as the ultimate load that causes failure (or is its
resistance to rupture) and its units are force units divided by area (N/mm2 )
 Characteristic strength - Compressive, Tensile and Flexure strength
 Modulus of Elasticity
 Creep and shrinkage of concrete