Quality Control
Specific
quality control requirements for the work are indicated throughout the Contract
Documents. The requirements of this section are primarily related to the
performance of the work beyond furnishing of manufactured products.
Quality is constantly checked by PDCA cycle i.e.,
1. Plan
2. Do
3. Check
4. Action
Quality
control involves incorporating the following:
1. Formulation & implementation of
project specific quality plan
2. Maintaining records for incoming
material inspection
3. Maintaining records for tests on
construction materials
Material
list of quality records
1.
Quality Plan
2.
Documents Control Sheet
3.
Internal & statistical quality control document
4.
Mix design of concrete
5. Tests reports of bricks, cement, steel,
concrete & aggregates
At
QC , an Inspection Testing Plan is received at the foremost .It is then the
purchase list of equipments is prepared according to the ITP and the acceptance
of the raw materials is done.The basic raw materials used at site are listed
below
1.
Sand
2.
Cement
3.
Stone Chips
4.
Admixture
5.
Bricks
6.
Adhesive
7.
Waterproofing Components
8.
Water Sealant
SAND:
In quality control, sand is checked for zone1, 2, 3 & 4 of which last two are of basic importance.
Zone
2 Sand: Used for Concreting
Zone
3 Sand: Used for Brick Masonry Work
· Sieve Analysis
Sieve analysis determines the
gradation or distribution of aggregates particular sizes within a given sample.
For zone 3
sampling the percentage
passing of sand through 600 micron sieve should be between 60-79.It is checked
for silt contents.
There are two methods of silt check
1. By Volume Method
2. By Weight Method
By volume method the silt content
should be between 8% and by weight method it should be between 3%. IS code:
383-1970 & IS: 2386-1963.
For zone 4
sampling the percentage
passing of sand through 300 micron sieve should be between 20-65.IS code: 1542-1992
CEMENT :
It
is mainly divided in the following three parts
1.
PPC - Portland Pozzolona Cement
2.
OPC - Ordinary Portland Cement
3.
PSC - Portland Slag Cement
N.B:
Initital setting time of cement should not be less than 30minutes & final
setting time should not be more than 10 hours
Some
of the tests performed in QC for cement are:
1. Consistency Test
2. Compressive Test
3. Initial Setting Time
4. Final Setting Time
COARSE AGGREGATE:
Stone chips are coarse aggregate used in
concrete mix. Sieve analysis is performed in site laboratory for stone chips.
Stone chips are passed through IS sieves of perforations 40 mm ,20 mm, 10 mm,
6mm , etc and graded according to the IS code ref 2386(Part 3) (Procedure) and
383(Actual Criteria).
ADMIXTURE:
IS:
9103-1999 covers the following types of admixtures
1.
Accelerating admixtures
2.
Retarding admixtures
3.
Water reducing admixture
4.
Super plasticizer admixture and
5.
Air entraining admixtures
BRICKS:
The
bricks at site are divided into two main categories namely
1. Traditional or Standard Bricks
2. Modular Bricks
As per CPWD specifications the size of
traditional brick is (230x110x70) mm and the size of modular brick is (190x90x90)
mm.
The strength of traditional brick is 3 N/mm
·
Brick Testing :
1. Size & Shape
2. Sharpness
3. Cold Crushing Strength
4. Water Absorption
5. Soundness
Concrete:
Concrete is the homogeneous mixture of
Sand, Cement, Coarse aggregate and Water. Concrete is used to take load in
compressive zone.
·
Mixing :
All concrete, whether plain or reinforced, ordinary or controlled,
shall be mixed in a standard type of concrete mixer not less than two minutes.
Materials for concrete shall be deposited into the drum while it is in rotation.
IS code reference for mix design: 10262-2009
Properties of Concrete that should be checked at QC are
1. Workability
2. Durability
3. Segregation
4. Bleeding
5. Curing
6. Slump Test
7. Cube test
Workability
The term workability is used to describe the ease of difficulty with
which the concrete is handled, transported and placed between the forms with
minimum loss of homogeneity.
The workability, as a physical property of concrete alone irrespective
of a particular type of construction, can be defined as the amount of useful
internal work, necessary to produce full compaction.
If the concrete mixture is too wet, the coarse aggregates settle at the
bottom of concrete mass and the resulting concrete becomes of non uniform
composition.On the other hand, if the concrete mixture is too dry, it will be
difficult to handle and place it in position. Both these conflicting conditions
should be correlated by proportioning carefully various components of concrete mixture.
The important in correction withworkabilty are as follows:
1) If more water is added to
attain the required degree of workmanship, it results into concrete of low
strength and poor durability.
2) If the strength of
concrete is not be affected ,the degree of workability can be abstained :
a. by slightly changing the
proportions of fine and coarse aggregates, in case the concrete mixture is too
wet ; and
b. By adding a small quantity
of water cement paste in the proportion of original mix, in case the concrete
mixture is too dry.
3) A concrete mixture for one
week may prove to be too stiff or too wet for another work. For instance, the
stiff concrete mixture will be required in case of vibrated concrete work while
wet concrete mixture will be required for this sections containing reinforcing
bars.
4) The workability of
concrete is affected mainly by water content, water cement ratio and
aggregate-cement ratio.
5) The workability of
concrete is also affected by the grading, shape, texture and minimum size of
the coarse aggregates to be used in the mixture
Durability
The
durability of concrete is defined as its ability to resist weathering action,
chemical attack, abrasion or any other process of deteriorations. Durable
concrete will retain its original form, quality and serviceability when exposed
to its environment.
Generally,
constructionindustry needs faster development of strength in concrete so that
the projects can be completed in tirmof before time. The demand is cleared by
high early strength cement, use of very low W/C ratio through the use of
increased cement content and reduced water content.With higher quantity of
cement content, the concrete exhibits greater cracking tendencies because of
increased thermal and drying shrinkage. As the creep coefficient is low in such
concrete, there will not be much scope for relaxation of stresses.Therefore;
high early strength concretes are more prone to cracking than moderate or low
strength concrete.
Field
experience have also corroborated that high early strength concrete are more
crack-prone. According to a recent report, the cracks in pier caps have been
attributed to the use of high cement content in concrete. Contractors
apparently though that a higher than the desired strength would speed up the
construction time, and therefore used high cement content.
Similarly,
report submitted by National Cooperative Highway Research Programme(NCHRP) of
USA during 1995 , based on their survey ,showed that more than,100000 concrete
bridge deck in USA showed full depth transverse cracks even before structures
were less than one month old.The reasons given are that combination of thermal
shrinkage and drying shrinkage caused most of the cracks.It is to be noted that
deck concrete is made of high strength concrete .These concretes have a high
elastic modulus at an early age.Therefore,they developed high stresses for a
given temperature change or amount of dying shrinkage.The most important point
is that such concrete creeps little to relieve the stresses.
Segregation
Segregation
can be defined as the separation of the constituent materials of concrete. A
good concrete is one in which all the ingredients are properly distributed to
make a homo generous mixture.If a sample of concrete exhibits a tendency for
separation of say ,coarse aggregate from the rest of the ingredients then, that
sample is said to be showing the tendency for segregation. Such concrete is
only going to be weak ; lack of homogeneity is also going to induce al
undesirable properties in the hardened concrete.
Bleeding
Bleeding
is sometimes referred as water gain. It is a particular form of segregation, in
which some of the water from the concrete comes out to the surface of the concrete,
being of the lowest specific gravity among all the ingredients of concrete.
Bleeding is predominantly observed in a highly wet mix, badly proportioned and
insufficiently mixed concrete.
Due
to bleeding, water comes up and accumulates at the surface.Sometimes, along
with the water; certain quantity of cement also comes to the surface. When the
surface is worked up with in the towel and floats, the aggregates goes down and
the cement and water come up to the top surface.
Method of Test for Bleeding of Concrete
A
cylindrical container of approximately 0.01meter cube capacity, having an
inside diameter of 250 mm and height 280 mm used. A tamping bar similar to the
one used for slump test is used. A pipette for drawing off free water from the surface,
a graduated jar of 100 centimeter cube capacity is required for test.
A
sample of freshly mixed concrete is obtained. The concrete is filled in 50 millimeter
layer for a depth of 250±3 millimeter (5 layers) and each layer is tamped by
giving stokes, and the top surface is made smooth by toweling.
The
test specimen is weighed and the weight of the concrete is noted. Knowing the
total water content in 1 meter cube of concrete quantity of water in the
cylindrical container is also calculated.
The
cylindrical container is kept in a level surface free from vibration at a
temperature of 27⁰C ± 2⁰C; it is covered with a lid. Water accumulated at the
top is dawn by means of pipette at 10 minutes interval for the first 40 minutes
and at 30 minutes interval subsequently till bleeding ceases. To facilitate
collection of bleeding water the container may be slightly titled. All the
bleeding water collected in a jar.
Curing
The
concrete surfaces are kept wet for a certain period after placing of concrete
so as to promote the hardening of cement. It consists of a control of
temperature and of the moisture movement from and into the concrete. The term
curing of concrete is used to indicate all such procedures and process.
Period
of concrete depend on the type of cement and nature of work. For ordinary
Portland cement, the curing period is about 7 to 14 days. If rapid hardening
cement is used, the curing period can be considerably reduced.
Following
are the purposes of the curing of concrete:
1.
The curing protects the concrete surfaces from sun and wind
2.
The presence of water is essential to cause the chemical action which
accompanies the setting of concrete.
3.
The strength of concrete gradually increases with age, if curing is
efficient.th increase in strength is sudden and rapid in early stages and it
continues slowly for an indefinite period.
4.
By proper curing, the durability and impermeability of concrete are increased
and shrinkage is reduced.
5.
The resistance of concrete to abrasion is considerably increased by proper
curing.
Following
are the basic factors on which the evaporation of water from the concrete
surface depends:
1.
Air temperature
2.
Fresh Concrete temperature
3.
Relative humidity; and
4.
Wind velocity
Slump Test
Slum
test is the most commonly used method of measuring consistency of concrete
which can be employed either in laboratory or at site of work. The apparatus
for conducting the slump test essentially consists of a metallic mould in the
form of a frustum of a cone having the internal dimensions as under:
Bottom Diameter : 20
cm
Top diameter : 10 cm
Height : 30 cm
For
tamping the concrete, a steel tamping rod 16 mm die, 0.6 meter along with
bullet and is used. The mould is placed on a smooth,horizontal,rigid and non –
absorbant surface.The mould is then filled in four layers,each approximates ¼
of the height of the mould.Each layer tamped 25 times by the tamping rod taking
care to distribute the strokes evenly over the cross section.After the top
layer has been rodded,the concrete is stuck off level with a trowel and tamping
rod.The mould is removed from the concrete immediately by raising it slowly and
carefully in a vertical direction.This allows the concrete to subside.Th
subsidence is referred as SLUMP of concrete.The difference in level between the
height of the mould and that of the highest point of the subsided concrete is
measured. The difference in height in mm .is taken as Slump of Concrete.
0 comments: