CONCRETE WORKS IN HIGHWAY ENGINEERING
Er. S. Karthigeyan, Deputy Director – III, HRS
Er.P.Elango, Assistant Director, Concrete Lab, HRS
Introduction
Concrete has been used as a construction material in largest quantity for several decades
due to its excellent engineering properties and also due to the economy of this material. Except
for cement all the other ingredients can be used from the locally available resources.
When properly prepared its strength is almost equal to the strength of naturally
occurring hard stone.
The properties of concrete making materials influence the properties of concrete both in
the fresh state as well as in the hardened state. Hence proper care should be taken in selecting
the concrete making materials.
Ingredients of Concrete
The basic ingredients of concrete are cement, fine aggregate (sand), coarse aggregate
and water. Sometimes admixture is also used in order to alter one or more of the specific
properties of concrete such as increasing the workability, reducing the water cement ratio, etc.
Water
7- 15 %
Coarse
Aggregate
40 – 50 %
Cement
14 - 21 %
Fine
Aggregate
20 – 30 %
1
I. Cement
According to the Strength, cement is classified in to 3 grades namely
(1) 33 Grade.
(2) 43 Grade.
(3) 53 Grade.
33 Grade means strength of 1:3 cement mortar cubes at the end of 28 days is between
33 to 43 N/mm2 (33 Mpa).
43 Grade means strength of mortar cube at the end of 28 days is between 43 and 53 N/
mm2 (43 MPa).
53 Grade means strength of cement mortar cube at the end of 28 days of cement mortar
cube is above 53 N/ mm2 (53 MPa).
Tests on cement
The following are the five basic tests for cement:
1. Consistency test.
2. Initial setting time and final setting time.
3. Soundness test.
4. Fineness test.
5. Compressive strength.
1.
Consistency Test
Figure 1: Vicat apparatus with Plunger
2
It is necessary to determine for any cement, the water content which will produce a
paste of standard consistency. This is determined using Vicat's apparatus. For a standard
penetration of 10 mm. diameter plunger under its own weight the water content required is
determined. For a penetration of about 5-7 mm from the bottom of the mould the amount of
water used gives the standard consistency.
2. Initial setting time and final setting time
This is the term used to describe the stiffening of cement paste and it refers to a change
from fluid to a rigid state. It is customary to talk about initial setting which is basically the
beginning of the stiffening and final setting is marked by the disappearance of plasticity. The
setting process should not too early, because of freshly mixed concrete should remain in plastic
condition for a sufficient period to permit satisfactory compaction and finishing after
transporting and placing of concrete. On the other hand too long a setting process is also
undesirable because this will cause a delay in strength development after finishing.
For finding out the initial setting time, the plunger in the Vicat apparatus is replaced by
1 mm diameter needle. The water to be added to the cement is 0.85 times to the standards
consistency. The time when the penetration of the needle is 5 mm from the bottom plate from
the time of addition of water is initial setting time, which shall not be less than 30 minutes.
The needle is replaced by annular arrangement. Initially when the annular arrangement
is getting applied on the surface of the cement paste, a circle and dot is seen on top of the
cement paste in the mould. But later on the circle disappears and only the dot is seen on the
surface and at that stage final setting is said to have occurred. The final setting time is from the
time of water is added to the cement till the appearance of dot only, which shall not be more
than 600 minutes (10 hours).
3
Figure 2: Needle for final Setting time
3. Soundness Test
Unsoundness is the harmful property that occurs when the hardened cement paste
develops and undue expansion that is manifested by cracking of the mass. The usual cause of
unsoundness is the presence of over burned free or uncombined lime and excess quantities of
crystalline magnesia. This may cause disintegration of the hardened paste or concrete.
The apparatus used is Le- Chatlier Apparatus. The water required for this test is 0.78
times of consistency of cement. After filling the Le-Chatlier mould with the cement paste, it
has to be kept in water for 24 hours. Then initial reading between the two needles to be
measured. This arrangement is kept in boiling water for 3 hours. Then the arrangement should
be removed from boiling water and the final reading between the 2 needles to be measured.
The difference between the final and initial reading is the expansion, which shall not be
more than 10 mm.
Figure 3: Le – Chatlier Apparatus
4
4. Fineness Test
The term fineness refers to average size of the cement particles. A higher fineness
means a more finely ground cement. The significance of fineness lies in the fact it affects
several technically important properties of cement and concrete. For instance, the higher the
fineness the higher the strengths are developed at earlier ages. Also finer cement bleeds less,
contributes better workability and less expansion.
The apparatus used for this test is Blaine's Air Permeability Apparatus. After
calibrating the permeability cell by mercury displacement method, the weight of the sample
required for this test shall be determined. Using the standard cement of known porosity, the
time taken for the air to push the liquid in the U tube from two levels shall be measured (Ts).
The same weight of the test sample is taken and the test is repeated (T). From the formula
given below this specific surface of the cement can be determined.
Ss T
S=
Ts
Where,
S
- Specific surface area in sq.cm per gram of test sample.
Ss
- Specific surface area in sq.cm per gram of standard sample used
in calibration of Apparatus.(3020 sq cm/gm)
T
- Measured time interval in second for test sample
Ts
- Measured time interval in seconds for standard sample.
The requirement for fineness is not less than 225 sq m /kg.
5
Figure 4: Blaine Air Permeability apparatus
5. Compressive Strength
The Strength developing ability of a cement is its most sought after property because
concrete is a construction material. The hardening of the cement paste is the result of the
cement hydration, the subsequent development of bonds in the hydration process and gradual
reduction of the internal porosity.
The compressive strength of cement is determined by preparing cement mortar cubes
using cement and standard sand (Ennore sand) in the ratio of 1 : 3, ie. 200 gms of cement and
600 gms of standard sand. (l/3 in each size of sand). Water to be calculated from the formula,
P/4+3% (Percentage for total mortar of 800 gms), where P is the consistency of cement
Requirements as per Indian Standards:
Table 1: Compressive Strength of cement
33 Grade
43 Grade
53 Grade
IS 269 - 2015
IS 8112 - 2013
IS 12269 - 2013
3 Days
16 MPa
23 MPa
27 MPa
7 Days
22 MPa
33 MPa
37 MPa
28 Days
33 Mpa
43 MPa
53 MPa
Age
6
Figure 5: Compression testing machine
II. Aggregates
Coarse Aggregates
Aggregates out of which 90% retained on IS: 4.75 mm sieve are termed as coarse
aggregates. Generally HBG (Hard Blue Granite) is used as coarse aggregate.
Fine Aggregates
Aggregates out of which 90% is passing through IS: 4.75mm sieve are termed as fine
aggregates. Sand is used as fine aggregate.
According to grading, aggregates are classified into graded aggregate and single size
aggregate. A graded aggregate is one which comprises different fractions of all size ranges.
A single size aggregate is one, which comprises particles falling essentially within a
narrow limit of size fractions.
Fine aggregate should comply with the requirements of any grading zone given in IS:
383 - 2016.
Sieve analysis is done to determine the particle size distribution of the fine and coarse
aggregates (IS: 2386, Part-I). The sample is either prepared by quartering or by using a sample
divider. On completion of sieving the materials retained on each sieve shall be weighed.
7
As per IS: 383 the grading for sand is divided into four zones,
zone-I to zone-IV.
Generally Zones-I to III are preferred and aggregate conforming to Grading zone-IV should
not be used in RCC, unless tests have been made to ascertain the suitability of proposed mix
proportions.
Table 2: Fine Aggregate Grading Requirement
Sieve Size
Zone – I
Zone – II
Zone – III
Zone – IV
10 mm
100
100
100
100
4.75 mm
90 – 100
90 – 100
90 – 100
95 – 100
2.36 mm
60 – 95
75 – 100
85 – 100
95 – 100
1.18 mm
30 – 70
55 – 90
75 – 100
90 – 100
600 micron
15 – 34
35 – 39
60 – 79
80 – 100
300 micron
5 – 20
8 – 30
12 – 40
15 – 50
150 micron
0 – 10
0 – 10
0 – 10
0 – 15
Size of Coarse Aggregate
The nominal maximum size of coarse aggregate should be as large as possible within
the limits specified but in no case greater than one-fourth of the minimum thickness of the
member. For example, if the thickness of c.c. pavement is 100 mm the maximum size of
aggregate shall be 25 mm.
For heavily reinforced members the maximum size of the aggregate should usually be
restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less
than the minimum cover to the reinforcement, whichever is smaller.
The grading of coarse aggregate shall be within the limits given in IS: 383-2016.
Table 3: For 40 mm Nominal Size of aggregate
IS Sieve Designation
Percentage Passing
80 mm
100
40 mm
90 - 100
20 mm
30 – 70
10 mm
10 – 35
8
0-5
4.75 mm
Table 4: For 20 mm Nominal Size of aggregate
IS Sieve Designation
Percentage Passing
40 mm
100
20 mm
90 - 100
10 mm
25 – 55
4.75 mm
0 – 10
Table 5: For 12.5 mm Nominal Size of aggregate
IS Sieve Designation
Percentage Passing
20 mm
100
12.5 mm
90 – 100
10 mm
40 – 85
4.75 mm
0 – 10
Tests on Coarse Aggregate
1. Aggregate Crushing Value: - IS: 2386 (Part-IV)-1963
This test gives a relative measure of the resistance of the aggregate to crushing under
gradually, applied compressive load.
The sample taken shall be 12.5 mm passing and 10 mm retained. The standard cylinder
is filled in three layers with 25 times tamping for each layer. The sample in the cylinder is
weighed. In the compression testing machine the load is applied at the rate of 4 T per minute
and on reaching a maximum load of 40 T the aggregates is removed and sieved in IS: 2.36 mm
sieve. The fraction passing is weighed and it should not be more than 45% for surfaces, other
than wearing surface and not more than 30% for wearing surfaces.
2. Aggregate Impact Value: - IS: 2386 (Part-IV) -1963
It gives a relative measure of the resistance of the aggregate to sudden shock or impact.
Aggregate passing through IS: 12.5 mm and retained on 10 mm sieve is taken. The mould of
9
the impact testing machine is filled in 3 layers with 25 times tamping for each layer. The test
sample is then subjected to 15 blows with the standard hammer. The aggregate is removed and
sieved through IS: 2.36 mm sieve. The maximum passing shall not be more than 45% for
surfaces other than wearing surfaces and not more than 30 % for wearing surfaces.
3. Los Angeles Abrasion Value: - IS: 2386 (Part-IV) 1963
Abrasion is an important consideration especially for concretes exposed to wearing
actions.
In the Los Angeles Abrasion testing machine the sample is placed and rotated at a speed
of 20 to 33 rev/min with a charge of steel balls (48 mm dia and weighing about 390 to 448 g)
for 500 revolutions. The sample weighing 5 kg is taken in the following manner:Table 6: Clause 5.3.2
Passing
Retaining
40 mm
25 mm
20 mm
12.5 mm
25 mm
20 mm
12.5 mm
10 mm
Sample to be taken in
grams
A
B
40 mm
20 mm
1250
1250
1250
2500
1250
2500
The sample is taken out and sieved through IS: l.7 mm sieve and the amount passing
shall not be more than 30% for wearing surfaces and not more than 50% for surfaces other than
wearing surfaces.
4. Flakiness Index
About 5 kg of the sample is taken and passed through the Flakiness Index Gauge meant
for testing the Flakiness Index. As per MORTH specifications the amount passing through it
should not be more than 35%.
5. Soundness
In this test the aggregates subjected alternatively to immersion in sulphate solution and
to drying. This causes disruption of the particles due to the pressure generated by the
formation of salt crystals. The degree of unsoundness is expressed by the reduction in the
particle size after a specified number of cycles. Other tests include subjecting it to freezing
and thawing.
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However the conditions of all these tests do not really represent those when the
aggregate is influenced by the presence of the surrounding cement paste. Only a service
record can satisfactorily prove the durability of any aggregate.
III. WATER
The quality of the water used for mixing is very important because it may interfere with
the setting time of cement, adversely affect the strength of concrete or may lead to corrosion
of reinforcement.
Most of the specifications say that the quality of water should be of that water which is
fit for drinking. But there is no absolute criterion of portability of water.
As a rule any water with a PH (Degree of acidity) of 6.0 to 8.0 and which does not taste
saline is suitable for use.
Test on Water
Compare setting time of cement and strength of mortar cubes using the water in
question with corresponding results obtained using good or distilled water. If there is no
appreciable difference between the behavior of mortar cubes made using distilled water and
the water in question, then the water can be safely used. A tolerance of 10% for variations in
strength is allowed. If need be, water to be tested for chlorides and sulphates content as per
IS: 3025.
IV. Steel Reinforcement for Concrete Structures
Two types of steel are used in Concrete structures
1. Mild Steel as per IS: 432-Part- l -1982
2. HYSD (High yield strength deformed bars) as per IS: 1786 - 2008.
HYSD bars satisfy the requirements for diameter and percentage elongation as per
Clause-6, 7 of IS - 1786.
Generally Fe 415 is used which has a yield strength of 415 N./mm2
Selection of Test Specimens:
For checking nominal mass, tensile strength, bend test and rebound test, test specimen
of sufficient length shall be cut from each size of the finished bar / wire at random at a
frequency not less than specified in Table-1,2,3,4 0f IS:1786 -2008.
The yield strength of mild steel is only half of that of HYSD bars, hence HYSD bars are
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predominantly used.
It is mandatory to test the steel used for reinforcement. Generally Fe 415 is available in
market but Fe 500 and Fe 550 are also available, to be specially ordered which come with a
single and double star respectively for every metre.
The standard length of the rods purchased from reputed firms like TISCO, ISCO, SAIL
come in 12 m (40') lengths whereas local rolling mills produce in lengths of 11 m (36') only.
The nominal diameter of the bar is to be determined by weighing
1 m cut length of
the bar and finding out the equivalent diameter of a circular bar of same length and not by
using calipers. As per Table-1900-3 of MORTH Specifications for Roads and Bridges,
anticorrosive treatment has to be given for steel reinforcement to be used in coastal areas (15
km from coast line). Adequate extra cover should be given to reinforcement for protection
against corrosion. Cover may be provided as per Clause 25.4 of IS: 456 Pre stressing steel wire
should conform to IS: 1785 (Parts-1 and 11)-1983.
Table 7: Properties of Steel bars
Properties
Fe 415
Fe 415 D
Fe 500
Fe 500 D
0.2 % Proof Stress Minimum (Mpa)
415
415
500
500
Tensile Strength Minimum (Mpa)
485
500
545
565
Elongation Precentage Minimum (Mpa)
14.5
18
12
16
Admixtures
Admixtures are materials, added to Concrete, to modify the properties; either in the
fresh state immediately after mixing or after the mix has hardened. Admixtures have become
one of the essential components of Concrete in recent years. Admixtures are broadly classified
as Chemical Admixtures and Mineral Admixtures.
Chemical Admixtures:
Chemical Admixtures are organic or inorganic compounds in the form of liquid used for
one or more of the following functions:

Accelerate or retard the initial setting time of fresh concrete mix

Increase workability without increasing water content of concrete
12

Decrease the water content at the same workability

Reduce the rate of Slump loss

Improve pumpability of the mix

Reduce segregation of constituents of concrete

Inhibit corrosion of embedded steel reinforcement
Based on the usage in concrete, chemical admixtures are classified as :

Accelerators

Retarders

Plasticizers

Super plasticizers
Accelerators:
Accelerating admixtures are added to concrete either to increase the rate of early
strength development or to shorten the time of setting, or both. Chemical composition of
accelerators includes some of inorganic compounds such as soluble chlorides, carbonates and
silicates.
Retarders:
Retarders are admixtures which delay the setting time of concrete. Retarders are useful
for concreting in hot weather, when normal setting time is shortened by the high temperature,
and in preventing the formation of cold joints between successive layers in mass concrete. A
retarder dosage of 0.05 % by weight of cement leads to 4 hours retardation.
Plasticizers:
Plasticizer generally reduces the required water content of a concrete mixture for a
given slump. These admixtures disperse the cement particles in concrete and make more
efficient use of cement. This increases strength or allows the cement content to be reduced
while maintaining the same strength.
Superplasticizers:
Superplasticizers or High range water reducers are most widely used chemical
admixtures in concrete. They allow water reduction in the range of 15 % to 20 % in concrete
mix. They also help in producing high workable concrete and retarding the same for longer
time without affecting the setting time and strength gain process of the concrete.
13
Figure 6: Superplasticizers
Mineral Admixtures for Concrete:
Fly ash
Fly ash is a by-product obtained during the combustion of pulverized coal in thermal
power plants. The main constituent of fly ash is silica. Non crystalline forms of silica, alumina
and iron present in the in fly ash are principally responsible for the pozzolanic reaction with
calcium hydroxide which results from the hydration of Portland cement. Fly ash increase longterm strength, improves durability, decrease permeability and reduce alkali-aggregate
expansion of hardened concrete. Properties of cement blended with Fly ash @ 25% by mass
have already been tested and has been recommended for adoption by Bureau of Indian
Standards.
Ground Granulated Blast furnace Slag (GGBS):
GGBS is produced as a by-product during the manufacture of iron in a blast furnace.
Calcium oxide and Silicon dioxide are the major constituents of GGBS. Concrete with GGBS
gains strength more slowly, tending to have lower strength at early stages and higher strength
at later stages.
Silica Fume:
Silica fume is a very fine non-crystalline powder obtained as a by-product from silicon
metal production industries. Silicon dioxide is the reactive material in Silica Fume. Adding
Silica fume brings millions of very small particles to concrete mixture. Silica fume fill in the
spaces between cement grains similar to fine aggregates fills in the spaces between coarse
aggregates. Even if Silica fume does not react chemically, the micro filler effect would bring
significant improvements in the nature of concrete.
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V. Concrete
Concrete is a versatile materials which is widely used for construction works. As
already described, the ingredients of concrete are cement, sand as fine aggregate and metal as
coarse aggregate mixed uniformly by addition of water. The hardness of concrete is depending
upon hydration process, which starts as soon as water is added to the mixture of cement, sand
and metal. Concrete mixes are produced to have the desired properties in the fresh and
hardened states as the situation demands.
Influences of Materials and mix proportions
Aggregates occupy nearly 70 to 75% of the total volume of concrete. The total surface
area of the aggregate is to be minimized to the extent possible by the proper choice of size,
shape and proportion of fine and coarse aggregate. Different size of fractions is to be so chosen
as to minimize the voids content. To mobilize such mixture water is needed for lubricating
effects. The requirements of workability are such that there should be enough cement paste to
surround the aggregate particles as well as to fill the voids in the aggregates. The water content
of the mix is the primary factor governing the workability of fresh concrete. The workability
increases with the water content.
For the same volume of aggregates in the concrete, use of coarse aggregates of larger
size gives higher workability because of reduction in the total specific surface area. Use of
flaky and elongated aggregates will result in low workability primarily because of increase in
particle interference.
The use of fine sand with corresponding increase in specific surface area increases the
water demand.
Table 1700-7, MORTH (Fifth Revision)
Maximum Nominal Size of
Components
Coarse Aggregates(mm)
1. RCC Well Curb The size (maximum nominal) of coarse
20
aggregates for concrete to be used in various components
2. RCC / PCC Well Steining
40
3. Well Cap or Pile Cap, solid type
40
Piers and Abutments
15
4. RCC work in girders, slabs, wearing coat, kerb, approach
20
slab, hollow piers, abutments, pier / abutments caps,
piles etc.,
5. PSC Work
20
6. Any other items
As specified by Engineers.
Workability
The concrete should have workability such that it can be placed in toe formwork and
compacted with minimum effort without causing segregation or bleeding. The choice of
workability depends upon the size of the section and the concentration of reinforcement. The
aim should be to have the minimum possible workability consistent with satisfactory placing
and compaction of concrete. It should be remembered that insufficient workability resulting in
incomplete compaction, will severely affect the strength, durability and surface finish of
concrete.
Compressive Strength
The compressive strength of hardened concrete is considered to be the most important
property and can be measured on standard size cube,
i.e., 150 mm x 150 mm x 150 mm. It can be taken as an index of overall quality of
concrete.
Among the materials and mix variables, water/cement ratio is the most important
parameter governing Compressive strength. Besides water/cement ratio, the following factors
also affect the compressive strength of concrete:

The characteristics of cement

Characteristics and proportions of aggregates.

Degree of compaction

Efficiency of Curing.

Age at the time of testing for mix proportion and placing.
Water/Cement Ratio
Most of the desirable properties of hardened concrete depend primarily upon the quality
of the cement paste. Hence the first step in proportioning mix design should be the selection of
the appropriate water/cement ratio.
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Within the normal range of strengths, compressive strength is nearly inversely related to
the water / cement ratio.
Some of the advantages of reducing the water/ cement ratio
are as follows:1. Increased compressive strength / flexural strength.
2. Increased denseness.
3. Reduced porosity.
4. Increased water tightness.
5. Lower absorption.
6. Increased resistance to weathering.
7. Better bond between successive layers.
8. Better bond between concrete and reinforcement.
9. Less volume change from wetting and drying.
Aggregate-Cement Ratio
As long as the workability is maintained at a satisfactory level, the compressive strength of
concrete had been found to increase with the increase in aggregate cement ratio.
Cement Content
Generally, the cement content itself would not have a direct role on the strength of
concrete. If cement content is required to increase the workability of concrete for a given
water/cement ratio, then the compressive strength may increase with the richness of the mix.
However, for a particular water / cement ratio, there would be an optimum cement content
resulting 28 days compressive strength being the highest. Increasing the cement content above
the optimum value may not increase the strength.
Effect of Age of Testing
Concrete is generally tested for its compressive strength at the age of 28 days. Because
of continuing hydration, the later age strength would generally be higher than at 28 days,
however, the exact increase will depend upon the type of cement, mix composition and the
extent of curing. The mix proportions themselves influence the rate of gain of strength, in that
concrete with lower water/cement ratio tends to attain high initial strength and therefore
further gain in strength at later ages is proportionately smaller than with high water / cement
ratio.
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Effect of placing, compaction and curing
The concrete should be placed in its final position in the formwork as early as possible
after the completion of mixing, so that there is no drying out of the mix, and the mix is
workable enough to receive the compaction. Dropping of concrete from great heights may lead
to segregation and entrainment of air bubbles displacement of reinforcement and damage the
already placed concrete. It is preferable to pour the concrete from a height of 1m.
When the fresh concrete is compacted by vibration, the particles are set in motion
reducing inter-particle friction so that concrete is easily placed. Vibration eliminates most air
pockets on the surface of the concrete. The presence of 5% voids in the hardened concrete left
due to incomplete compaction may result in decrease in compressive strength by about 35%.
The hydration of cement can take place only when the capillary Pores remain
saturated. The additional water available from outside is needed to fill the gel-pores which
will otherwise make the capillary empty. The function of curing is to prevent the loss of water
in the concrete from evaporation, normally done by covering with wet gunny bags,
membrane, curing compounds and continuous pounding of water. Concrete will continue to
gain strength with time provided the sufficient moisture is available for hydration of cement
which can be assured by proper moist curing.
Structural Concrete
Structural Concrete is classified based on the grade of concrete as follows
Nominal Mix Concrete:
M15, M20
Standard Concrete:
M15, M20, M25, M30, M35, M40, M45, M50
High Performance Concrete:
M30, M35, M40, M45, M50, M55, M60, M65, M70, M75, M80, M85, M90
Nominal Mix Concrete
Nominal Mix Concrete is made on the basis of nominal mix proportioned by weight of
its main ingredients – cement, coarse aggregate, fine aggregate and water.
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Table 1700-6, MORTH (Fifth Revision)
Proportions for Nominal Mix Concrete
Grade of
Concrete
Total Quality of Dry Aggregate by Mass by 50 kg
Proportions of fine and
of cement to be taken as the sum of individuals
coarse aggregates (By
masses of fine and coarse aggregates (kg)
mass)
Generally 1 : 2
M-15
Subject to upper limit 1 : 1.5
350
and lower limit of
1 : 2.5
Generally 1 : 2
M-20
Subject to upper limit 1 : 1.5
250
and lower limit of
1 : 2.5
Standard Concrete
Standard Concrete is made on the basis of design mix proportioned by weight of its
ingredients, which in addition to cement, aggregates and water, may contain chemical
admixtures to achieve certain target values of various properties in fresh condition,
achievement of which is monitored and controlled during production by suitable tests.
Generally concrete grades up to M50 are included in this type.
High Performance Concrete
High Performance Concrete is similar to standard concrete but contains additional
one or more mineral admixtures providing binding characteristics and partly acting as inert
filler material which increases strength, reduces its porosity and modifies its other properties
in fresh as well as hardened condition. Concrete of grades up to M90 are included in this
type.
Tests for Concrete
i) Compressive Strength
ii) Workability
iii) Non Destructive Tests
Compressive Strength
Concrete cubes of 15 x 15 x 15 cm to be cast in 3 layers, each to be vibrated thoroughly
19
and finished. If it is hand tamping, three layers, each layer to be compacted 35 times and
finished. On surface dry, the specimen to be covered by wet gunny bag. After 24 hours they
are demoulded and cured for 7 days and 28 days and tested. A minimum of 3 Specimen cubes
of one Sample for analysing the test results are required.
The minimum frequency of sampling of concrete of each grade:
Table 1700-9 MORTH (Fifth Revision)
Quantity of Concrete in m3
No. of Samples
1-5
1
6-15
2
16-30
3
31-50
m m3
51 and above
4
4 plus one additional sample for each
additional 50 m3 or part there of
Acceptance Criteria for compressive Strength:
1. Cubes
The concrete shall be taken as having the specified compressive strength when both the
following conditions are met:
a) The mean strength determined from any group of four consecutive non-overlapping
samples exceeds the specified characteristic compressive strength by 3 MPa
b) Strength of any sample is not less than the specified characteristic compressive
strength minus 3 MPa
2. Cores
When the concrete does not satisfy both the conditions given in (1) above,
representative cores shall be extracted from the hardened concrete for compression test
in accordance with the method described in IS: 1199 and tested as described in IS: 516
to establish whether the concrete satisfies the requirement of compressive strength.
Concrete in the member represented by a core test shall be considered acceptable if the
average equivalent cube strength of the cores is equal to at least 85 percent of the cube
strength of the grade of concrete specified for the corresponding age and no individual
core has strength less than 75 percent of the specified strength.
20
Workability of fresh concrete by Slump Test
Standard slump cone size
Standard tamping rod
Top dia = l0 cm
Length = 0.6 m
Bottom dia
Dia
= 20 cm
= 16 mm
Height = 30 cm
Figure 7: Slump Cone
Test Procedure:
Concrete shall be poured in four layers - each layer 25 blows. On removing the cone
slowly, the slumped concrete height has to be measured. The difference between this reading
and the original height of 30 cm is the slump of concrete.
Table 1700-4, MORTH (Fifth Revision)
Sl.No
1.
Type
(a) Structures with exposed inclined surface requiring low
slump concrete to allow proper compaction
(B) Plain Cement Concrete
2.
Slump (mm)
25
25
RCC, structures with widely reinforcement eg. solid
column, piers, abutment footing, well steining
40 -50
RCC structure with fair degree of congestion of
3.
reinforcement eg.Pier and abutment, Caps, Box culvert
well curb, well cap, walls with thickness greater than 300
50 - 75
mm
RCC PSC structures with highly congested reinforcement
4.
eg. Deck Slab Girders, Box Girders, walls with thickness
less than 300 mm
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75 - 125
5.
Under water concreting through tremie eg. Bottom plug,
cast- in-situ Pilling
150 - 200
Non Destructive Tests
Non Destructive tests are used to obtain estimation of the properties of
concrete in the structure. Non Destructive tests provide alternative to core tests for
estimating the strength of concrete in a structure, or can supplement the data
obtained from a limited number of core specimens tested. These methods are based
on measuring a concrete property that bears some relationship to strength. The
accuracy of these methods is determined by the degree of correlation between
strength and quality of the concrete and the parameter measured by the non
destructive tests.
The following Non destructive tests are commonly conducted on concrete
structures:
i)
Ultrasonic Pulse Velocity Test as per IS : 13311 part 1-1992
Figure 8: Ultrasonic Pulse Velocity Test
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Table 8: Concrete Quality Grading
ii)
Pulse Velocity (km/s)
Quality
Above 4.5
Excellent
3.5 to 4.5
Good
3.0 to 3.5
Medium
Below 3.0
Doubtful
Rebound Hammer Test as per IS : 13311 part 2-1992
Figure 9: Position of Rebound Hammer - Vertically Downwards
Figure 10: Position of Rebound Hammer - Vertically Upwards
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Prestressed Concrete:
There are two main types of Prestressed Concrete for structural components of
Concrete Bridges
I) Pre-tensioned Concrete
II) Post-tensioned Concrete
Pretensioned Concrete:
Steel tendons are stressed by Jacks anchored to fixed blocks in the casting yard,
Concrete is then placed in moulds or casting beds around these tendons. When the concrete has
hardened sufficiently, the tendons are released. As they try to return to their original length,
large compressive forces are applied to the concrete.
This process is nearly always carried out in a factory environment and is the usual way
of manufacturing precast Prestressed bridge girders.
Post-tensioned Concrete:
For this type of construction, normally associated with in-situ Concrete, the tensioning
forces are applied to the tendons after the concrete is placed and hardened. Ducts are
incorporated into the formwork and the concrete is placed around them. After the concrete has
hardened, the stressing tendons are threaded through the ducts and are stressed using Jacks. A
special grout is injected into the ducts around the tendons to provide bond and protection from
corrosion. Pos-tensioning is mainly carried out on site although it has been used for special
precast girders.
CONCRETE STRUCTURES
INSPECTION OF BRIDGES
There are about 4,000 major and minor bridges in our State apart from culverts.
All the bridges in a division are to be inspected in detail at least once in a year. Bridges
in hilly areas also to be inspected at least twice a year before and after monsoon. Long span
bridges (individual spans greater than 30 m) are also to be inspected by a senior engineer (S.E)
at least once in a year.
The bridges are to be inspected and maintained as per procedures in IRC-SP:18 and
SP:35. Though detailed procedure is given in the above publication the salient points to be
noted during inspection are:-
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Culverts
1. Check whether the vents are choked with debris, vegetable matter, etc., and clear them.
2. Check for cracks, distresses in body wall and correct them.
3. Check whether the roadway is level on the culvert. If there is a depression suspect
failure of pipes, slab, etc., and take action to rectify it.
4. Observe during monsoons whether flow of water is proper or whether there is
overtopping of the culvert/breach of road adjoining the culvert in which case collect
hydraulic particulars, investigate and redesign the culvert for larger discharge.
Minor and Major Bridges
1. Check the backfill at abutments/approaches and if settlement seen correct it to a level
surface.
2. Check abutment, and wing walls for distress.
3. Check the slope projection works at abutment and replace / repack loose stones.
4. The foundation is to be checked before monsoon for erosion of bed exposing footing,
pile cap, pier cap to a larger extent or damage of concrete, abrasion of concrete.
5. Aprons / Bed protection works to be ensured to avoid erosion of footings underneath
piers and abutments.
6. The abutment and piers are to be checked for cracks, if any, at points just below
bearings.
7.
The deck slab has to be checked for spalling of concrete/delaminating by visual
observation and by tapping with a hammer for dull hollow sound. Spalling is due to
corrosion of rebars and subsequent pressure caused due to swelling of bars.
8. Arch bridges are to be checked for the following:
a. Cracks across the span (which are dangerous and bridge may fail suddenly) in the
rib.
b. Cracks along the span (rib separating into two or more pieces).Though this is not
severe, action may be taken to repair them by providing, a relieving slab between
piers on the top of deck.
c. Cracks between spandrel wall and arch rib (along the arch) which shows
separation of the components of the bridge have to be repaired.
d. Vertical cracks at the skew back portion just above the pier, which indicates
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deflection of the arch rib.
e. For a detailed information on theoretical assessment of carrying capacity of arch
bridges IRC:SP:37 may be referred.
9. The beams of the deck may be inspected for cracks and the cracks may be identified
whether due to shear, bending or otherwise (shrinkage etc.,).
10. The bearings may be inspected for proper functioning. Especially, Neoprene pads may
be inspected for distresses by way of squeezing out of plates in the bearing, tearing of
the polymer or puncture of beam into the bearing or bearing into the concrete.
Also check for cracks at bearing points in beams in which case one may suspect
malfunctioning of bearings.
11. Vegetation present on the structure at drainage spouts are to be removed.
12. All drainage spouts are to be cleaned regularly.
13. The wearing coat of bridges are to be inspected and all the joints
are to be raked,
cleared and joint filler board with joint sealant to be applied. Clogging of joints does
not allow free movement of the slabs and result in cracks.
14. Expansion joints of all bridges are to be checked for proper functioning, cleaned and
filled with bitumen sealants wherever applicable.
15. The deck and beams near expansion joints and drainage spouts are to be checked for
corrosion of bars due to water logging
(clogging of debris).
16. All coastal bridges are to be inspected for corrosion of steel, spalling and
delaminating of concrete.
17. The Designs wing, H.R.S., may be contacted for advice on inspection/remedial
measures.
Repairs and Rehabilitation
A number of procedures for repair and rehabilitation are available starting from patch
repairs with epoxy mortar to extensive guniting.
1. Crack Repair
First identify whether cracks are active or inactive.
Active means the crack is live either expanding and contracting or extending in length.
Inactive cracks are dead cracks which do not vary in size for quite a period of time.
a.
Inactive cracks may be filled with resins or cement grouts (non-shrink type).
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Filling may be done by simply pounding or pressure injection (pressure
grouting).
Cracks of width > 10 mm. may be filled with single size aggregate and then
with non shrink cement grouts.
b.
Active Crack:- Cracks to be cleared properly by water jetting, compressed air,
etc.,
Sealing with a flexible sealant or filling with a resin, providing a chase and
filling it with flexible sealant.
2. Repair of Spalled concrete
Corrosion is the process. of rusting of steel due to action of saline water (chlorides) .
Structures near the coast (upto15 kms from sea or creek) and those on backwaters are prone to
corrosion.
The corrosion product (rust or iron oxide) is larger in volume and creates great pressure
on the concrete (cover concrete) and concrete ultimately cracks. This cover concrete separates,
(called delaminating) and falls from the structure (called spalling of concrete).
Repair:
All loose concrete should be chipped off and the spalled area should be cleaned by
water jetting. Any damaged reinforcement to be replaced with new piece of bar by welding
and placing in position. The reinforcement may be protected by applying zinc rich coating.
Then a bonding agent may be applied, to ensure proper bond between old and new concrete.
The epoxy mortar, either ready-to-use or prepared by adding admixtures to normal cement
mortar may then be applied layer by layer up to a maximum thickness of 1 inch at a time,
(which may be allowed to cure and then next layer applied) Polymer based mortar or concrete
can also be used for repair.
3. Guniting
(Application of concrete by spraying) Guniting is used for large scale repairing of
structures (such as decks of bridges, columns, beams etc.,) which have damaged
extensively.
Here mortar is applied over the area to be required by pressure from a gun. A guniting
machine with compressor is required for the above work. It has to be done by skilled
workmen as loss of mortar (rebound) will be high.
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For large repairs proper temporary supports, frameworks are to be provided.
4. Bridge Wearing coats
When the wearing coat of bridges is extensively damaged, they may be replaced by a
new wearing coat.
The cracks on the wearing coat may be repaired by filling with non-shrink cement
based grouts or by filling with sand mastic (a mix of bitumen and sand).
IMPORTANT INDIAN STANDARDS PERTAINING TO
CONCRETE AND STRUCTURES
I. Cement
1.
IS : 4031-1988
Methods of Physical Tests for Hydraulic Cement (Part - I to 13)
2.
IS : 4032-1985
Methods of Chemical Analysis of Hydraulic Cement
3.
IS : 3535-1986
Methods of Sampling Hydraulic Cement
4.
IS : 269-2015
Specification for 33 Grade ordinary Portland cement
5.
IS : 8112-2013
Specification for 43 Grade ordinary Portland cement
6.
IS : 12269-2013
Specification for 53 Grade ordinary Portland cement
7.
IS : 12330-1988
Specification for Sulphate resisting Portland cement.
II. Aggregates - Metal and Sand (Coarse Aggregate and Fine Aggregate)
1.
2.
IS : 2386 - 1963
(Part - I to 8)
IS: 383 - 2016.
Methods of tests for Aggregates for Concrete.
Specification for coarse and fine aggregates from Natural sources
for concrete.
III. Steel
1.
IS : 1786-2008
2.
IS : 2751-1979
3.
IS : 1139-1966
4.
IS : 432-1982
High Strength deformed steel bars and wires for Concrete
reinforcements.
Code of Practice for welding of mild steel, plain and deformed
Bars for reinforced concrete construction
Hot rolled mild steel, medium tensile steel and high yield strength
steel and deformed bars for concrete reinforcement.
Specification for mild steel and medium tensile steel bars.
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IV. Concrete
1.
IS : 10262-2009
Concrete Mix Proportioning-Guidelines
2.
IS : 456 - 2000
Code of practice for plain and reinforcement concrete.
3.
IS : 1343 - 1980
Code of practice for Prestressed concrete
4.
IS : 516 - 1959
Method of test for strength of Concrete.
5.
IS : 1199 - 1959
Method of sampling and Analysis of fresh concrete.
6.
IS : 9013 -1978
7.
IS : 6509- 1985
Methods of making, curing and determining compressive strength
of accelerated cured concrete test specimen.
Code of practice for installation of joints in concrete pavement.
V. Bricks
1.
IS : 1077 - 1992
Specification for common burnt clay building bricks.
2.
IS : 2180 - 1988
Heavy duty burnt clay building bricks.
3.
IS : 3102 - 1971
Classification of burnt clay solid bricks.
4.
IS : 5454 - 1978
Methods of sampling of clay building bricks.
5.
IS : 3583 - 1988
Specification for paving bricks
6.
IS : 2212 - 1991
Code of practice for brick works.
I.S. Special Publications
Hand Book for Quality Control for construction of Roads and
1.
SP - 11
2.
SP - 23
Hand Book on concrete mixes.
3.
SP - 24
Code of practice for plain and reinforced concrete.
4.
SP - 34
Concrete reinforcement
5.
SP - 35
Guidelines for Inspection and Maintenance of Bridges.
Runways.
I.R.C.
Standard Specification and code of practice for construction of
1.
I.R.C. - 15
2.
I.R.C. - 112
Code of practice for Concrete Road Bridges
3.
I.R.C. -44
Method of concrete mix design for Concrete pavement.
concrete Roads.
29
4.
I.R.C. - 58
Guidelines for the design of ridge pavement for Highways.
5.
I.R.C. - 84
Code of practice for curing of C.C. pavement.
6.
I.R.C. - 77
Tentative guideline for repairing concrete pavement using
Synthetic Resins.
*****
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