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17. 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.
18. 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.
19. 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.
20. 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.
21. 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.
22. 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.
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