1. What
are the steps involved in the concreting process, explain?
Answer:
o Batching: The process of measurement of the
different materials for the making of concrete is known as batching. Batching
is usually done in two ways: volume batching and weight batching. In case of volume,
batching the measurement is done in the form of volume whereas in the case of
weight batching it is done by the weight.
o Mixing: In order to create good concrete the
mixing of the materials should be first done in dry condition and after it-wet
condition. The two general methods of mixing are hand mixing and machine
mixing.
o Transportation
and placing of concrete: Once
the concrete mixture is created, it must be transported to its final location.
The concrete is placed on form works and should always be dropped on its final
location as closely as possible.
o Compaction
of concrete: When
concrete is placed, it can have air bubbles entrapped in it, which can lead to
the reduction of the strength by 30%. In order to reduce the air bubbles the
process of compaction is performed. Compaction is generally performed in two
ways: by hand or by the use of vibrators.
2.
Describe briefly the various methods of concrete curing?
Answer:
Curing is the process of maintaining the
moisture and temperature conditions for freshly deployed concrete. This is done
for small duration of time to allow the hardening of concrete.
The
methods that are involved in saving the shrinkage of the concrete includes:
o Spraying
of water: on
walls, and columns can be cured by sprinkling water.
o Wet
covering of surface: can
be cured by using the surface with wet gunny bags or straw
o Ponding: the horizontal surfaces including
the slab and floors can be cured by stagnating the water.
o Steam
curing: Of
pre-fabricated concrete units, steam can be cured by passing it over the units
that are under closed chambers. It allows faster curing process and results in
faster recovery.
o Application
of curing compounds: compounds
having calcium chloride can be applied on curing surface. This keeps the
surface wet for a very long time.
3. What
do you understand by “preset” during the installation process
of bridge bearings?
Answer:
During the installation of bridge bearings,
the size of the upper plates is reduced to save the material costs. This
process is known as preset.
Generally,
the upper bearing plate comprises of the following components:
o Length
of bearing
o 2 x
irreversible movement.
o Two
x reversible movement.
The bearing initially is placed right in
the middle point of the upper bearing plate. No directional effects of
irreversible movement is considered. However, since the irreversible movement
usually takes place in one direction only the displaced direction is placed
away from the midpoint. In such cases, the length of the upper plate is equal
to the length of the length of the bearing + irreversible movement + two x
reversible movement.
4. Why
are steel plates inserted inside bearings in elastomeric bearings?
Answer:
In order to make an elastomeric bearing
act/ function as a soft spring it should be made to allow it to bulge laterally
and the stiffness compression can be increased by simply increasing the
limiting amount of the lateral bulging. In many cases in order to increase the
compression stiffness of the bearing, the usage of metal plates is made. Once
steel plates are included in the bearings the freedom of the bulge is restricted
dramatically, also the deflection of the bearing is reduced as compared to a
bearing without the presence of steel plates. The tensile stresses of the
bearings are induced into the steel plates. However, the presence of the metal
plates does not affect the shear stiffness of the bearings.
5. What
reinforcements are used in the process of restressing?
Answer:
The
major types of reinforcements used in restressing are:
· Spalling Reinforcement: The spalling stresses leads to
stress behind the loaded area of the anchor blocks. This results in the
breaking off the surface concrete. The most likely causes of such types of
stresses are Poisson`s effects strain interoperability or by the stress
trajectory shapes.
· Equilibrium reinforcements: This type of reinforcements are
required where several anchorages exist where the pre stressing loads are
applied in a sequential manner.
· Bursting Reinforcements: These kinds of stresses occur in
cases where the stress trajectories are concave towards the line of action of
load. In order to reduce such stresses reinforcements in the form of bursting
is required.
6.
In the design of bridge arguments what considerations should be made to select
the orientation of the wing walls.
Answer:
Some
of the most common arrangements of wing walls in cases of bridge arguments are
as follows:
· Wing walls parallel to abutments: This method is considered to take
least amount of time to build and is simple as well. However, on the downside
this method is not the most economical. The advantage of this type of design
being that they cause the least amount of disturbance to the slope embankment.
· Wing walls at an angle to abutments: This design method is consider the
most economical in terms of material cost.
· Wing walls perpendicular to abutments: The characteristic of this design is
it provides an alignment continuous with the bridge decks lending a support to
the parapets.
7. in
case if concrete box girder bridges how the number of cells is determined.
Answer:
When the depth of a box girder bridge
exceed 1/6th or 1/5th of the bridge width then the design recommended is that
of a single cell box girder bridge. However, in case the depth of the bridge is
lower than 1/6th of the bridge width then a twin-cell or in some cases multiple
cell is the preferred choice. One should also note that even in the cases of
wider bridges where there depths are comparatively low the number of cells
should be minimized. This is so as there is noticeably not much improvement in
the transverse load distribution when the number of cells of the box girder is
higher than three or more.
8.
Under what circumstances should pot bearings be used instead of Elastoplast bearings?
Answer:
Pot bearings are preferred over Elastoplast bearings in situations where there are chances of high vertical
loads in combinations of very large angle of rotations. Elastoplast bearings always
require a large bearing surface so that a compression is maintained between the
contact surfaces in between the piers and the bearings. This is not possible to
maintain in high load and rotation environment. In addition, the usage of Elastoplast bearings leads to the uneven distribution of stress on the piers.
This results in some highly induced stresses to be targeted at the piers
henceforth damaging them. Due to the above reasons, pot bearings are preferred
over Elastoplast bearings in such cases.
9. Why
should pumping be not used in case of concreting works?
Answer:
During the pumping operation, the pump-exerted
pressure must overcome any friction between the pumping pipes and the concrete
the weight of the concrete and the pressure head when the concrete is placed
above the pumps. Since only water is pump able, all the pressure generated is
by the water that is present in the concrete. The major problem due to pumping
are segregation effects and bleeding. In order to rectify and reduce these
effects, generally the proportion of the cement is increased in order to
increase the cohesion, which leads to the reduction of segregation and
bleeding. In addition, if a proper selection of the aggregate grading can
vastly improve the concrete pump ability.
10.
Why should curing not be done by pounding and polythene sheets?
Answer:
The primary purpose of curing is to reduce
the heat loss of concrete that is freshly placed to the atmosphere and in order
to reduce the temperature gradient across the cross-section of the concrete. Bonding is not preferred for curing as this method of thermal curing is greatly
affected by cold winds.
In addition to that in bonding large
amounts of water is used and has to be disposed off from the construction
sites. Polythene sheets are used on the basis that it creates an airtight environment
around the concrete surface henceforth reducing the chances of evaporation over
fresh concrete surfaces. However, the usage of polythene can be a drawback as
it can be easily blown away by winds and the water lost by self-desiccation
cannot be replenished.
11.
What Are The Different Type Of Slump Test Indications?
Answer:
Slump tests are performed to empirically
measure the work ability of fresh concrete. It is used to measure the
consistency of the concrete. In general, there are three different types of
slumps that occur in slump tests.
They
are as follows:
· True Slump: This type of slump is characterized by the
general drop of the concrete very evenly without visible signs of deterioration
or disintegration.
· Shear Slump: It indicates that the concrete mix is deficient
in cohesion. This type of slump leads to segregation and bleeding. Henceforth
in the end effecting the durability of the concrete.
· Collapse Slump: This type of slump is indicates that the mix of
concrete is simply too wet. The mix is considered harsh and lean.
12.
Why is propping required for long structures once the form work is removed?
Answer:
Once the process of concreting is performed,
the striking of the form works should be done as soon as possible as delay in
this process can lead to the discoloration of the concrete structures. In case
of long structures, particularly long span structures once the structures have
attained enough strength to support themselves it is essential to provide them
with propping as creep deflection can take place, which can greatly reduce the
integrity of the structure.
Due to the above-mentioned reasons,
propping should be done after the removal of form work. In addition, the props
should not be made to stand long as it can lead to over stress for the
structures.
13.
Explain the mechanism of cavitation in pipes and drains?
Answer:
The formation of air bubbles in a fluid
due to low-pressure conditions lower than the saturation pressure is known as cavitation.
This is considered a high potential damage condition where the strength and
durability of the pipes can be greatly reduced. Cavitation works on the
principle of Bernoulli's Equation.
When fluids are at high velocities the
pressure head of fluids reduce accordingly. However, since the fluid pressure
is lower than the saturation pressure the dissolved gases are released from the
flowing fluid. These air bubbles collapse on entering a region of high
pressure. This leads to the damage of the pipelines as a high level of dynamic
pressure is created.
14.
For what purpose bedding is used under storm water drains, explain?
Answer:
Beddings
are primarily made up of granular or concrete materials. They are primarily
used for the following purposes:
- They are used to provide a more uniform support for the under pipes so that the bending moment longitudinally can be reduced greatly.
- In order to enable the pipes to get more load-supporting strength.
- They are also used to act as a platform to achieve a more correct alignment and level pre and post construction.
- In case of pipes, which contain spigot and socket joints, it enables pipes to be supported along pipe lengths in place of sockets. Otherwise, it can lead to uneven stress being induced on the pipes eventually damaging it.
15.
Why are pullout tests performed for soil nails?
Answer:
Pull
out tests are performed for primarily the following reasons:
- In order to detect and the verification of the bond, strength among the soil and the grout adopted during the design of soil nails. This is considered to be as the primary objective of performing pull out tests for soil nails.
- For the detection of any slippage or occurrence of creeps.
- To detect the elastic and deformations (plastic) of any of the test nails employed. Observations are made during the loading and unloading cycles of the soil nails repetitively.
- To achieve the perfect balance the test nails should always be loaded so that the ultimate soil/grout mixture with an upper limit of 80%.
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