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11. 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.
12. 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.
13. 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.
14. 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.
15. 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%.
16. 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.
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