In
its fresh state, concrete should,
- Be
composed of accurately batched proportion
- Have
the required W/C ratio
- Be
well mixed
- Not
segregate during transportation and placing
- Be
fully compacted
- Be
provided with specified finish
- Quality
of cement
- Type
of aggregate
- The
grading of aggregate
- Mixing
method and mixing time of concrete
- Method
of transportation
- Curing
These
quantities are depended on the material selected the proportion in which they
are mixed, the method of mixing, placing, transportation and compaction.
MATERIALS OF CONCRETE
Cement
Cement
is a material with adhesive and cohesive properties which make it capable of
bonding mineral fragments. There are many types of cements, ordinary Portland
cement, rapid hardening Portland cement, low heat Portland cement, sulfate
resisting cement, white cement and pigments. Generally ordinary Portland cement
is widely used.
Aggregates
Aggregate
is commonly considered inert filler, which accounts for 60 to 80 percent of the
volume and 70 to 85 percent of the weight of concrete. Although aggregates are
most commonly known to be inert filler in concrete, the different properties of
aggregates have a great impact on the strength, durability, workability, and
economy of concrete.
Aggregates
are classified in to two types, coarse and fine aggregates. Aggregates smaller
than 5mm are known as fine aggregates while lager than 5mm are known as coarse
aggregates. Sand is used as fine aggregates and crushed metal is used as coarse
aggregates widely in Sri Lanka. Usually river sand is used as fine aggregates.
Generally ¾" metal is used as coarse aggregate for normal concrete mix. 1
½" metal used for mass concrete. Metal used for concrete is to be free
from clay or any other foreign matters. Because otherwise it can be affect to
the bonding.
Water
All
water using for mixing concrete mortar grout shall be perfectly fresh, clean
and free from acid, salt and all impurities, which may adversely effected the
setting, hardening and durability of the concrete. This requirement is usually
satisfied by using water which is suitable for drinking .Water is used to
effect the chemical action in cement, which is known a hydration. Therefore the
water/cement ratio is a most important factor when concrete qualities are
concern because it affects the setting, Harding and strength of cement.
Normally water /cement ratios are in the range of 0.4 to 0.6 Water also acts as
a lubricant for the cement and the aggregates in the compaction of the
concrete.
Admixtures
An
admixture is a chemical product that is added to concrete mix during mixing or
during an additional mixing operation prior to the placing of concrete, for the
purpose of achieving a specific modification. There are water-reducing,
retarding, accelerating admixtures.
Their
function is primarily to accelerate the early strength development of concrete
that is hardening although they may also coincidentally accelerate the setting
of concrete.
A
delay in the setting of the cement paste can be achieved by the addition of a
retarding admixture.
MIXING OF CONCRETE
Concrete
must be thoroughly mixed until a uniform color is attained. In our site we used
ready mixed concrete except in some instances where a small quantity was
required.
Hand
mixing
In
small jobs of concrete was mixed by hand. The ingredients were first assembled
on a water tight-mixing platform. First of all in hand mixing work, the
required amount of sand was spread out on the flat surface. The proper number
of bags of cement was spread out on top, and the two materials were turned
together until a uniform color is obtained. The required volume of coarse
aggregate wetted, measured, and spread in a layer on top of the cement and sand
mixer. All the materials were again mixed and depression was formed in the
center of the heap.
Finally,
the required amount of water was added and mixed until the all materials are
thoroughly mixed.
Ready
mixed
Instead
of mixing in site, concrete is delivered from a central plant (called batching
plant), it is referred to as ready mixed or pre mixed concrete. This type of
concrete is extensively used as it offers numerous advantages in comparison
with other methods.
- Close
quality control of batching which reduces the variability of the desired
properties of hardened concrete
- Use
on congested sites where there is little space for a mixing plant and aggregate
stockpiles.
- Use
agitator trucks to ensure care in transportation, thus preventing segregation
and mentally work ability.
BATCHING PLANT
Process
in a batching plant
For Grade 25, 0.75m3 -------------> 208 – Sand (Unit – Bucket)
248
– Metal
88 – Cement
Capacity of silo = 3 bulk (33,000kg)
PLACING AND COMPACTION OF CONCRETE
The
concrete should be placed in its final position rapidly so that it is not too
stiff to work. Water should not be added after concrete has left the mixture. A
heap of concrete, which will have to be moved to some other part of the form,
should not be allowed to accumulate in one place. Normally concrete was placed
in even layers and each of layers was compacted before the next layer was
placed. Each layer was placed before the previous layer has got set. But for
the slab it was placed as single layer.
Compaction
of concrete is the process adopted to expel the entrapped air from the
concrete. If the air is not removed fully the concrete loses its strength
considerably. Compaction can be manual or mechanical compaction. The internal
vibrator or poker was used in our site, which is the most common type of
vibrator. This is a vibrating tube at the end of a flexible drive. Pokers vary
in size, usually from 25mm to 75mm in diameter. A poker vibrator should not be dragged
through the concrete, nor used to help heaps of concrete to spread out. It
should place vertically in the concrete, held in position until air bubbles
cease to come to the surface, then slowly with drawn so that concrete can flow
into the space previously occupied by the poker. This should be repeated at
about 0.5m centers. The concrete should be placed in layers never more than
600mm thick, and the vibrator should be lowered at least 100mm in to layer
beneath
The
method of placing concrete is very important and needs proper planning and
careful study if the structure to be concreted is to be durable, strong and
having a good appearance. All formwork should be check clean and oiled before
concrete is placed on it. Place the concrete as soon as possible in uniform
layers placing height should be less than 4ft.
The
progress of compacting concrete by vibration consists essentially of the
elimination of entrapped air and forcing the particles in to a closer
configuration.
Using
hammer
This
was done in places where vibrator can’t insert such as column concrete. This
concrete was compacted by ramping. Pushing up and down a rammer on the concrete
did ramping. Wide wood piece was used to make the rammer.
Using
a Steel Rod
Steel
rode uses when compacting at the thinner areas where vibrators or rammers can’t
be inserting. This method is very effective and efficient in small concreting
like manhole construction.
Best
and quickest method was vibration. Of several types of vibrators, this is
perhaps the most common one. It is consists of porker, housing eccentric shaft
driven through a flexible drive from a motor. The poker is immersed in concrete
and thus applies approximately harmonic forces to it; hence, the alternative
names of Poker vibrator or immersion vibrator.
The
correct method of insetting Poker to the concrete as follows; Compaction
process consolidates fresh concrete with in the formwork and around
reinforcement. Compaction is necessary to remove entrapped air which is present
in concrete after it is mixed.
