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Part 1
Part 2
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Part 5
Part 6
1.What are the steps
involved in the concreting process, explain?
2. Describe briefly the
various methods of concrete curing.
3. What do you
understand by “preset” during the installation process of bridge bearings?
4. Why are steel plates
inserted inside bearings in elastomeric bearings?
5. What reinforcements
are used in the process of prestressing?
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.
> 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 includes:
(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.
The bearing initially is placed right in the middle point of the upper bearing
plate. No directional effects 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 a 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: This type 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.
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