1. What are the different type 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.
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.
2. 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.
3. 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.
4. 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.
5. 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%.
6. 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 depends 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.
7. 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.
8. 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.
9. 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 occurs.
> 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 a separation membranes is polythene sheeting which is
commonly used.
10. 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.
11. During reclamation how can the occurrence of mud waves can be rectified?
There are several solution 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.
12. 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 are 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 a drainage for the excess
pore water pressure of band drain installations.
13. 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-rounds 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.
14. 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 junctions at which 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.
15. 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.