Civil
Engineering
What are the
steps involved in the concreting process, explain?
The major steps involved in the process of concreting are
as follows:
1. Batching
2. Mixing
3. Transporting and placing of concrete
4. Compacting.
>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.
1. Batching
2. Mixing
3. Transporting and placing of concrete
4. Compacting.
>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.
>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.
>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.
>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.
Describe
briefly the various methods of concrete curing.
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 include:
(a)
Spraying of water: on walls, and columns can be cured by sprinkling water.
(b)
Wet covering of surface: can be cured by using the surface with wet gunny bags
or straw
(c)
Ponding: the horizontal surfaces including the slab and floors can be cured by
stagnating the water.
(d)
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.
(e)
Application of curing compounds: compounds having calcium chloride can be
applied on curing surface. This keeps the surface wet for a very long time.
What
do you understand by “preset” during the installation process of bridge bearings?
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:
>
Length of bearing
> 2 x irreversible movement.
> 2 x reversible movement.
> 2 x irreversible movement.
> 2 x reversible movement.
The
bearing initially is placed right in the middle point of the upper bearing
plate. No directional effect of irreversible movement is considered. But 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 + 2 x reversible movement.
Why
are steel plates inserted inside bearings in elastomeric bearings?
In order to make an elastomeric bearing act/ function as a
soft spring it should be made to allow it to bulge laterally and also 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. But the presence of the metal
plates does not affect the shear stiffness of the bearings.
What
reinforcements are used in the process of prestressing?
The major types of reinforcements used in prestressing are:
> Spalling Reinforcement: The spalling
stresses leads to stress behind the loaded area of the anchor blocks. This
results in the breaking off of 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: These types
of reinforcements are required where several anchorages exist where the
prestressing 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.
In the
design of bridge arguments what considerations should be made to select the orientation
of the wing walls?
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. But on the downside this method is not the most economical. The advantage
of this type of design is that they cause the least amount of disturbance to
the slope embankment.
> Wing walls at an angle to abutments:
This design method is considered to be 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.
In case if
concrete box girder bridges how is the number of cells determined?
When the depth of a box girder bridge exceeds 1/6th or
1/5th of the bridge width then the design recommended is that of a single cell
box girder bridge. But 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.
Under what
circumstances should pot bearings be used instead of elastomeric bearings?
Pot bearings are preferred over elastomeric bearings in
situations where there are chances of high vertical loads in combinations of
very large angle of rotations. Elastomeric 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
maintained in high load and rotation environment. Also the usage of elastomeric
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 elastomeric
bearings in such cases.
Why should
pumping be not used in case of concreting works?
During the pumping operation the pump exerted pressure must
overcome any friction between the pumping pipes and the concrete, also 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 is
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. Also if a proper
selection of the aggregate grading can vastly improve the concrete pump
ability.
Why should
curing not be done by ponding and polythene sheets?
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. Ponding is not
preferred for curing as this method of thermal curing is greatly affected by
cold winds. In addition to that in ponding 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.
But the usage of polythene can be a drawback as it can be easily blown away by
winds and also the water lost by self-desiccation cannot be replenished.
What are the
different types of slump test indications?
Slump tests are performed to empirically measure the
workability 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
>
Shear Slump
>
Collapse Slump
True Slump: This type of slump is characterized by the general drop of the concrete mass evenly without visible signs of deterioration or disintegration.
True Slump: This type of slump is characterized by the general drop of the concrete mass 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 long run 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 to be harsh and lean.
Why is
propping required for long structures once the formwork is removed?
Once the process of concreting is performed the striking of
the formworks 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 formwork. Also the props should not be made to stand long as it can
lead to overstress for the structures.
Explain the
mechanism of cavitations in pipes and drains?
The formation of air bubbles in a fluid due to low pressure
conditions lower than the saturation pressure is known as cavitations. This is
considered to be 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. But since the fluid pressure is
lower than the saturation pressure the dissolved gases get released from the
flowing fluid. These air bubbles suddenly 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.
For what purpose
bedding is used under storm water drains, explain?
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
get supported along pipe lengths in place of sockets. Otherwise it can lead to
uneven stress being induced on the pipes eventually damaging it.
Why are
pull-out tests performed for soil nails?
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%.
Describe
briefly the advantages and disadvantages of using plastic and timber fenders?
The advantages and disadvantages of using plastic fenders
are as follows:
·
Plastic fenders are
low in strength with a relatively high resistance to abrasion.
·
Plastic fenders are
resistant to chemical and biological attacks.
·
Plastic fenders have
moderate energy absorption capacity.
·
The berthing reactions
are also comparatively moderate and depend on the point of contact.
·
Also since they are
made from recycled material they are environmental friendly.
The
advantages and disadvantages of using timber fenders are:
·
Timber fenders are low
in strength and are very susceptible to marine borer attacks and rotting.
·
The energy absorption
capacity is very low.
·
Also generally the
contact pressure between the vessels and the fender are high.
Explain why
concrete barriers have curved surface profiles?
The concrete safety fencings are made to contain vehicles
in their carriageway being travelled so as to reduce the chances of rebounding
into the roads leading to more hazards. In the case of normal fencings upon
vehicle crashes the fencings give away so as to absorb as much energy as
possible henceforth reducing the impact on the vehicles. But in the case of
concrete barriers their purpose is not to absorb energy of vehicles crashing
into the barrier but to retain them. They have a curved design so as to allow
the vehicles that hit them to slightly go up on the barrier but not overturn.
They also prevent the vehicle from again getting back on the road by rebounds.
This helps in vastly reducing the chances of other vehicle hazards.
Why is the
use of granular sub-base in concrete carriageways not preferred, explain?
Some of the reasons why granular sub-base is not preferred
in concrete carriageways:
>
Sub bases are permeable and hence water can seep through them easily. The soil
particles get pumped out through the joints on the application of traffic
loads. This results in the creation of voids underneath the pavement structure.
This leads to the weakening of the concrete surface and it can crack easily
upon intense traffic loads.
>
Instead if lean concrete is used for carriageways it greatly increases the
strength of the roads and the load carrying capacity of the roads is increased.
>
Sub-bases implementation requires a lot of workmanship which can lead to an
un-uniform distribution of the sub-base. This can lead to the cracking of the
carriageway when there is severe traffic loading.
Why are
separation membranes used between concrete pavement slab and sub-base?
The purpose of separation membrane between the concrete pavement
slab and the sub-base are as follows:
>
The separation membrane reduces the frictional forces between the concrete
slabs and the sub-base. The membrane aids the movement of the concrete slab in
reference to the sub-base when changes in the level of the moisture and
temperature occur.
>
It aids in the segregation of sub-base materials from freshly placed concrete.
>
The separation membrane also helps in the reduction of cement and water loss in
the form of immature concrete. Immature concrete greatly affects the strength
of the concrete. It also affects the durability of it.
A good example of separation membranes is polythene sheeting which is commonly used.
A good example of separation membranes is polythene sheeting which is commonly used.
In the roof
of a pumping station explain briefly the components of a waterproofing
system.
The components of a typical waterproofing system on the
roof of a pumping station are as follows:
>
Right above the structural finish level of the roof (concrete) a uniform
thickness screed is applied so as to facilitate the application of the
waterproofing membrane. The surface provide for the membrane should always
possess good cohesion properties and must be thin so as to prevent any
un-uniformity. This thin layer also acts as a layer of thermal insulation.
>
Right above this layer the waterproofing membrane is deployed to secure the
water tightness of the roof.
>
In order to enhance the thermal insulation of the roof an insulation board is
sometime placed right above the waterproof membrane. The insulation board helps
in the maintenance of a stable temperature in both weathers.
During
reclamation how the occurrences of mud waves can can be rectified?
There are several solutions to the rectification of the problem
of mud waves:
> Complete removal of all the disturbed mud:
This method can be considered to be one of the fastest methods. As soon as the
disturbed mud is removed some filling material is used to replace the disturbed
mud. But economically this method can be expensive as compared to others.
> Accelerated consolidation of disturbed mud:
In this method surcharging loads are placed on top of the mud waves. Along with
this band drains are installed to accelerate the consolidation process. This
method is quite slow compared to the other methods.
> Partial removal of the disturbed mud:
This method is the hybrid of the above two methods where the top layer is
removed whereas the lower level is treated with the surcharging process.
In
reclamation works what are the importance of geotextiles and sand?
The primary purposes of geotextiles and sand in reclamation
works are as follows:
> Geotextiles: They are used to separate
the marine mud from the reclamation fill. Also geotextiles are used as
reinforcements in reclamation processes to increase its stability. It is still
debated as to whether the usage of geotextiles is better or is the old
processes followed are better as the performance has not been comparable to the
conventional methods.
> Sand: In reclamation process sand is
used to spread the load of any future public dumps placed on top of it. Sand
also acts as drainage for the excess pore water pressure of band drain
installations.
In block work seawalls what is the
purpose of slip joints?
Joints which are formed from the cope level to the toe
level of seawalls through a complete vertical plane are known as slip joints.
Such joints are designed so as to handle the differential settlements between
the seawalls adjacent panels. In the slip joints the aggregates inside the half-round
channels enables some vertical movements. These vertical movements are induced
by differential settlements. This enables in the interlocking of the adjacent
panels of the seawalls to link the panels in one unit against the earth
pressure (lateral) which is exerted on the seawalls.
For a
washout valve why are two gate valves required in normal practice?
The washout valves are primarily used for normal
maintenance works such as that of water main. This can be like to allow water
to flow out during the cleaning of the water main. The junction at whom a pipe
branches out to a washout point usually a gate valve is installed so that the
two pipelines are separated. The gate valve installed above usually remains
open during normal operation. Another gate valve is installed further
downstream and this remains closed during normal operation of the washout
valve. In case this valve is not installed then the pipe section of the
branched out pipe would remain dry during normal operation and high chances of
damage and leakage can take place. When the downstream valve is installed the
branched out water main contains water under normal operation. With two gate
valves installed a leakage can be detected immediately.
What are the
different approaches in designing the floors of the service reservoirs?
In general there are two main approaches of designing the
reservoir floors to prevent leakage of water due to seasonal and shrinkage
movements:
>
In this approach the movement joints of the reservoir floor panels are such
that the free expansion and contraction of the panels takes place. Every panel
is isolated from the other panels and two panels have a sliding layer between
them to help in sliding.
>
The second method does not provide any room for free movement. With seasonal
and shrinkage movements, some cracks are designed to voluntarily occur on the
floors of the service reservoirs. These tiny cracks are spread throughout the
floor and are simply too minute to cause any leakage or corrosion of the
floors. But the difference also in this method is that the amount of
reinforcement used is much more than the first approach.
