Set No.19 MCQ's from 36 to 100 @civilconstructions.pdf

🔹Barrage -

A barrage is a type of low-head, diversion dam which consists of a number of large gates that can be opened or closed to control the amount of water passing through.

This allows the structure to regulate and stabilize river water elevation upstream for use in irrigation and other systems.

🔹Purpose -

1. To maintain pond level.

2. To raise the water level during low supplies. In case of higher floods, shutters are dropped down and overflow takes place while in case of gated weir, gates are raised during floods.

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Steps Involved In Concreting
Process
Concrete is a construction material made from a
mixture of cement, aggregate (sand or gravel), water
and sometimes admixtures in required proportions.
Following are the major steps which involved in
concreting process:
Steps In Concreting:
Batching:
The process of measuring different concrete
materials such as cement, coarse aggregate, sand,
water for the making of concrete is known as
batching. Batching can be done in two different
ways.
1. Volume Batching
2. Weight Batching.
In volume batching the measurements of concrete
materials are taken by volume & On the other hand
the measurements are taken by weight in weight
batching.
Mixing:
In this process, all the materials are thoroughly
mixed in required proportions until the paste shows
uniform color and consistency. Hand mixing and
machine mixing are the two different methods of
mixing.
Note: To achieve optimum quality the materials
should be mixed first in dry condition and then in
wet condition.
Transportation:
When the mixing is done properly the freshly made
concrete is then transported to the construction
site, this process is known as transportation. After
that, the concrete is correctly placed on the
formworks. Concrete can be transported to the site
location in two ways
1. Manual Transportation.
2. Mechanical Transportation.
Compaction:
Compaction is the process in which the air bubbles
are eliminated from the freshly placed concrete. It is
required to increase the ultimate strength of
concrete by enhancing the bond with reinforcement.
Curing:
Curing is the process in which the concrete keeps its
moisture for a certain time period to complete the
hydration process. Curing should be done properly to
increase the strength of concrete.
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🔹 Methods of Curing -

1. Curing by spraying water -

Spraying of water continuously on the concrete surface provides an efficient curing. It is mostly used for curing floor slabs. The concrete should be allowed to set sufficiently before sprinkling is started. The spray can be obtained from a perforated plastic box. On small jobs sprinkling of water may be done by hand. Vertical and sloping surfaces can be kept continuously wet by sprinkling water on top surfaces and allowing it to run down between the forms and the concrete. For this method of curing the water requirement is higher.

2. Curing using wet Gunny bags -

This is a widely used method of curing, particularly for structural concrete. Thus exposed surface of concrete is prevented from drying out by covering it with hessian, canvas or empty cement bags. The covering over vertical and sloping surfaces should be secured properly. These are periodically wetted. The interval of wetting will depend upon the rate of evaporation of water. It should be ensured that the surface of concrete is not allowed to dry even for a short time during the curing period. Special arrangements for keeping the surface wet must be made at nights and on holidays.

3. Curing by Ponding Method -

This is the best method of curing. It is suitable for curing horizontal surfaces such as floors, roof slabs, road and air field pavements. The horizontal top surfaces of beams can also be ponded. After placing the concrete, its exposed surface is first covered with moist hessian or canvas. After 24 hours, these covers are removed and small ponds of clay or sand are built across and along the pavements. The area is thus divided into a number of rectangles. The water is filled between the ponds. The filling of water in these ponds is done twice or thrice a day, depending upon the atmospheric conditions. Though this method is very efficient, the water requirement is very heavy. Ponds easily break and water flows out. After curing it is difficult to clean the clay.

4. Steam Curing -

Steam curing and hot water curing is sometimes adopted. With these methods of curing, the strength development of concrete is very rapid.

These methods can best be used in pre cast concrete work. In steam curing the temperature of steam should be restricted to a maximum of 750C as in the absence of proper humidity (about 90%) the concrete may dry too soon. In case of hot water curing, temperature may be raised to any limit, ay 1000C.

At this temperature, the development of strength is about 70% of 28 days strength after 4 to 5 hours. In both cases, the temperature should be fully controlled to avoid non-uniformity. The concrete should be prevented from rapid drying and cooling which would form cracks.

5. Curing using chemicals -

Curing compounds are chemical products usually sprayed directly over the concrete surface and allowing it to dry. The compound forms an impermeable membrane that retards the loss of moisture from the concrete.

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📌 Bleeding & Segregation in Concrete -

🔹Bleeding -

Bleeding is a form of segregation in which water present in the concrete mix is pushed upwards due to the settlement of cement and aggregate. The specific gravity of water is low, due to this water tends to move upwards. Bleeding ordinarily occurs in the wet mix of concrete.

The Prime factor for bleeding in concrete is the high dosage of Water cement ratio. Higher water-cement ratio weakens concrete and leads to excessive bleeding.

The bleeding in concrete is not harmful if the rate of evaporation of water is equal to the rate of bleeding. Normal bleeding is quite good, and it enhances the workability of concrete.

When the concrete is fully plastic, bleeding may not cause much harm. However, concrete is still in the plastic stage later it is subsidized and compacted.

🔹Segregation -

Segregation is the “Separation of constituent materials in concrete.” In concrete technology, segregation is of three types:-

1. Separation of Coarse aggregate from the concrete mixture,

2. Separation of Cement pastes from the concrete during its plastic stage.

3. Separation of water from the concrete mix (Bleeding in concrete)

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🔹Important concept -

TMT bars -TMT stands for "Thermo Mechanically Treated".
Generally we use TMT bars for the construction purposes where we need high strength and high corrosion resistance as compared to normal steel.

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