Annex-VIII

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Nov 29, 2013 (3 years and 10 months ago)

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QUALITY ASSURANCE

IN


Civil WORKS




BY



S.N.PANDEY,


CIVIL WORKS MANAGER, BIHAR EDUCATION PROJECT COUNCIL, PATNA






QUALITY CONSTRUCTION


Quality construction increases the lifespan,
stability and serviceability of the structure. It also
combines all the resources into economical output
and reduces unnecessary repairs and
maintenance. To cap all it disseminates the
reputation of the agency.


Quality of Civil Work depends on the use of correct
structural design, specifications and proper
material and workmanship.


Rational design is an effective tool for cost
effective and quality construction. As such the
design should be based on the latest relevant
Indian Standards




SPECIFICATION AND PROPER
MATERIALS :



The quality building material, like Cement, Brick,
Aggregate (Course & fine) should be procured at
competative rate. The community should be
trained about quality of each building material
and how to procure them. The aim is to procure
best possible quality material at competative
rate.




CEMENT CONCRETE

Followings are constituents of concrete :
-


Aggregate


Sand


Cement


Plasticizers for workability.


CEMENT

Followings are the grades of cement being
manufactured:
-


33,43 & 53 MPa.


ISI / ISO marked cement of ACC, Ultra
tech/Ambuja Birla Gold/Lafarge, Concreto etc
to be procured.



CEMENT


Price increases with increase in grades



For good construction 33 & 43 grades are ideal.


Initial setting time is 30 minutes & final setting time is
600 minutes

With the passage of time cement loses strength due to
hydration as follows:
-


Period


-

% loss in strength of cement


Three months



20


Six





30


Twelve months



40


As such fresh cement is to be used




AGGREGATES


Coarse aggregate
-

20mm down, well graded is required for all
the R.C.C. works in buildings.


Sand
-

Sand of I.S. Zone II and F.M. not less than 2.25


Grading of coarse and fine aggregates shall follow the curve of
Road Note 4 which is attached. On the basis of large nos. of
cube tests carried out combination of the aggregates is
recommended for higher strength and quality concrete in
concluding Para of this note. The method of test for
aggregates for concrete is given in IS 5779.


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,

provided

that

the

concrete

can

be

placed

without

difficulty

so

as

to

surround

all

reinforcement

thoroughly

and

fill

the

corners

of

the

form
.



For

heavily

reinforced

concrete

members

as

in

the

case

of

ribs

of

main

beams,

the

nominal

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
.








BRICKS


Bricks as per I.S. 1077 of class 125
preferably. The water absorption by the
brick should be maximum up to 20%.



WATER CEMENT RATIO : BULKAGE IN SAND



I.S. 456 clauses 9.3 and 9.3.1 deals with quantity of water of maximum
amount to be added to per 50 Kg. of cement.


Water present in sand should be determined. The simplest method of
determining the bulking of sand in the field is to fill a graduated jar with
the sand being used in the work upto a mark. Add sufficient water.
Sand will settle down to lower mark. Difference in the levels of sand
gives the rough idea of bulking which has to be compensated by
increase in the amount of sand. The moisture content and bulking of
medium size sand is given below.


Moisture content



Bulkage



0.4%




2%



0.75%




4%



1.45%




8%



3.1%




16%



4%




20%








WATER CENMENT RATIO : BULKAGE
IN SAND


Moisture content



Bulkage



5%




22%



8%




28%



10%




26%



12%




18%



15%




14%



20%




%

There is no bulkage in sand if fully saturated or completely dry.


It is needless to say that very effort be made to make silt, clay,
dust in the aggregates almost nil. The maximum percentage of
fines in aggregate is limited to 3% of coarse and uncrushed fine
aggregate according to I.S. 383




WORK FORCE


The work force consists of skilled & unskilled workers as
under :

(a)
SKILLED : Masons, carpenter, plumber, electrician,
blacksmith, bar benders, etc. if these are very well trained,
this can make lot of dent in quality construction.

(b)
UNSKILLED : Labour force, which assist the skilled
workers, should also be trained in order to achieve quality.
(examples)



QUALITY CONTROL MEASURES


Visual inspection.


Quantitative Quality measures.


Testing of building material like




Sand


Bricks


Concrete Mix


Cement Water ratio


Cement mortar


QUALITY CONTROL OF CEMENT
CONCRETE

1.
Concrete Mix Design


Concrete must be satisfactory in two stages, namely in the plastic
stage and the hardened stage. If the condition of the plastic concrete
is not satisfactory it can not be properly compacted and its
structural value is reduced. Thus, if there are 5% of air voids due to
incomplete compaction, the strength will be reduced by 30% and
10% of air voids will cause a loss of strength of about 60%. The
satisfactory compaction can be obtained only if the concrete is
satisfactorily and sufficiently workable. The property of workability,
therefore become of vital importance from structural point of view.

2. W
orkability of Concrete


A Concrete is said to be workable, if it can be easily mixed, placed,
compacted and finishes at the surface.


A workable concrete should not show any segregation or bleeding.


Segregation occurs when coarse aggregate try to separate out from
the paste and get accumulated at one side. This results in large
voids, less strength and less durability.










QUALITY CONTROL OF CEMENT
CONCRETE

3.
Tests for workability


Slump Test


I.S. 1199 para 5 deals with this test. The slump
test apparatus is very simple and cheap and the test can be
conducted during the progress of work in the filed. I.S. 456
para 7.1 prescribes the values of workability.










QUALITY CONTROL OF CEMENT
CONCRETE

4.
Durabilit
y


The durability of concrete depends on adequate cement content and
low water cement ratio. Cement and water is needed to bind together
the aggregates firmly. Thus, the suitable combination of the
aggregates is key to quality concrete. The proper grading of the
aggregates make the concrete dense. This also governs the quantity
of water required for workability











FACTOR GOVERNING QUALITY OF
CONCRETE


Cement Aggregate ratio


Grading of the aggregates


Volume/ weight batching


Mixing


Compaction


Curing


Water / Cement Ratio


Quality of Sand


Quality of Water







FACTOR GOVERNING QUALITY OF
CONCRETE

Machine Mixing : Sequence


First pour 25% of the total quantity of water in the drum.


Dry coarse and fine aggregates are to be discharged into drum


Full quantity of cement is to be deposited in the drum


Finally balance quantity of water to be poured. 25 to 30
revolutions are needed for mixing with drum rotating @ 15
-
20
revolutions per minute






(I.S. 456 Clauses 6.1.2, 8.2.4.1 & 9.1.2)

Minimum Cement Content, Maximum Water
-
Cement Ratio and Minimum
Grade of Concrete for Different Exposures with Normal Weight Aggregates
of 20 mm Nominal Maximum Size


Reinforced Concrete



Minimum Cement
Content Kg/m
3

Maximum Free
Water
-

Cement
Ratio

Minimum Grade of
Concrete

300

0.55

M 20

(I.S. 456 Clauses 9.3 & 9.3.1)

PROPORTIONS FOR NOMINAL MIX CONCRETE

Grade of
Concrete

Total Quantity of
dry Aggregates by
Mass per 50 kg of
Cement, to be
Taken as the Sum
of the Individual
Masses of Fine
and Coarse
Aggregates, Kg,
Max

Proportion of
Fine
Aggregates to
Coarse
Aggregates
(by Mass)

Quantity of
Water per 50
kg of Cement,
Max

M20

250 kg

i.e. coarse aggregate
-

166.66 Kg

Fine aggregate
-

83.33 kg

Generally 1:2
but subject to
an upper limit
of 1:1
½
and a
lower limit of
1:2½

30

Volume batching per bag of cement i.e. 35
liters


50 kg cement




250 aggregate (maximum)


volume = 0.0347 m
3



Volume
-

0.1389

m
3


i.e. cement

aggregate ratio = ¼ by volume.


M20 (1; 1.5 :3)

i.e. Cement Aggregate ratio = 1:4.5


By volume (per bag cement) 1:1.5:3 = 35:52.5: 105 (all in litres)


Hence batching is to be done by mass of the aggregate.





REINFORCEMENT



The standard trade/ I.S. marks is to be taken into account. In
case standard bars are not available the weight of piece bar is to
be determined by weighing in the shop which should be
compared with the following table to know whether the bar is
standard one.


Diameter of the rod




Weight in Kg/metre



6mm







0.222


8mm







0.395


10mm






0.617


12mm






0.888


16mm






1.578


TIMING OF CONCRETING IN SUMMER
SEASON


In freshly laid slabs, sometimes cracks occur before concrete
has set due to plastic shrinkage. This happens if concrete
surface loses water faster than bleeding action brings it to
top. Quick drying of concrete results in shrinkage and as
concrete is plastic stage can not resist any tension, short
cracks develop in the material. These cracks maybe 5 to 10 cm.
in depth and the is width could be as much as 3 mm.


Rate of evaporation from the surface of the concrete depends
on temperature of concrete, gain of heat from sun’s radiation,
relative humidity of ambient air and velocity of wind playing
over concrete surface. As such in winter days, there is no
problem of timing of concreting. In summer concreting should
be done during early hours of day when aggregates and
mixing water is comparatively cool and sun rays are slanting.
After concreting, top surface of concrete should be covered
with plastic sheets/gunny bags, straw or any other available
materials. This will minimise surface shrinkage cracking in the
concrete.



FORMWORKS


The shape, and surface finish of concrete depends upon the Form Work.


Suitable chamber in the form works shall be provided in horizontal
members of the structures, specially in long span to counteract the effects of
deflection. As a rough guide, it may be 1 to 250 for beams and 1 in 50 for
cantilevers.


The inside surfaces of formwork shall be coasted with soap solutions, raw
linseed oil or any other material, approved by the Engineer in
-
charge so as to
prevent adhesion of concrete to formwork. Release agents shall be applied
strictly in accordance with the instructions and shall not be allowed to come
into contact with any reinforcement. Immediately before concreting, all
formwork shall be thoroughly cleaned.


Propping and centering
-

Props used for centering shall be of steel or timber
post, ballies or any other material, approved by the Engineer
-
in
-
charge.

FORMWORKS


The shape, and surface finish of concrete depends upon the Form Work.


It should be strong to support wet concrete and have smooth surface to
prevent loss of slurry from the concrete.


Thin, deteriorated or non
-
vertical ballies will not be permitted at site.


Timber ballies shall always be vertical and never be in inclined position.


Any scaffolding shall only be on hard ground and not soft soil.


STRIPPING TIME OF FORM
(Ref: IS
-
456, Para 11.3.1)

Sl. No.

Type of Formwork

Minimum period
before striking forms

1.

Vertical formwork to columns,
walls, beams

16
-
24 h

2.

Slabs spanning up to 4.5 m

Slabs spanning over 4.5 m


7 days

14 days

3.

Beams and Arches spanning up
to 6.0 m/ over 6.0 m


14 days/21 days

CONCLUSION : MIX DESIGN


Following mix design of 1:2:4 (Cement :Sand:
Aggregate) gives strength required for M20


Cement Aggregate ratio
-


1:6 (i.e. 1:2:4)


Sand



-

24%


Nominal size 10mm

-

16%


Nominal size 20mm

-

60%


This combination gives a minimum of 100% excess
strength over nominal mix of 1:2:4. i.e. M15


WATER ABSORPTION OF BRICKS


Field Test : The Test specimen shall consist of five whole dry
bricks and shall be selected at random from the stack. The
apparatus shall consist of a balance sensitive of within 0.2 to 0.3
percent of the weight of the specimen.


The test specimen shall be weighed and shall then be
completely immersed in clean water at room temperature and
allowed to remain in this State for a period of 24 hours. The
specimen shall then be taken out wiped with a damp cloth and
then weighed immediately.


Percentage of water absorption


Weight of bricks after 24 hours immersion


________________________________ x 100


Weight of dry bricks



WATER ABSORPTION OF BRICKS


Laboratory Test : The Test specimen shall consist of five
whole selected at random from the lot of bricks obtained.
The apparatus shall consist of a balance sensitive to
within 0.1 percent of the weight of the specimen.


The test specimen shall be dried to constant weight in
ventilated oven at 110
0

to 115
0

. The specimen, shall then
be cooled approximately to room temperature and
weighed.


The dry specimens shall be completely immersed without
preliminary partial immersion, in clean water at 15.5
0

to
30
0

C for 24 hours. Each specimen shall then be removed,
the surface water wiped off with a damp cloth 2
nd

the
specimen weighed. Weighing any one specimen shall be
completed within three minutes after removing the
specimen from the water.


DETERMINATION OF COMPRESSIVE


STRENGTH OF BRICKS


Five bricks shall be immersed in water at 25
0

to 29
0

C for 24
hours. They shall then be removed and allowed to drains at
room temperature for about five minutes and wiped free from
surplus moisture. Their frogs shall be filled with mortar
composed of one part portland cement and one and a half
parts clean coarse sand graded to 3 mm and down. The
bricks shall then be stored under damp sack for 24 hours.
After the expiry of this period, they shall be immersed in
water for seven days.


At the end of seven days, the samples of bricks
shall be taken out, wiped dry and placed with the
flat surfaces horizontal and the mortar filled face
up wards between two
-
three plywood sheets each
approximately 3 mm thick and carefully centered
between the plates of the compression to testing
machine. The load shall be applied axially at the
uniform rate until failure occurs.


Arithmetic mean of the Maximum load at failure


Compressive Strength = Area of bricks


Thank you