Concrete MCQ Quiz in मराठी - Objective Question with Answer for Concrete - मोफत PDF डाउनलोड करा

Explanation:

Abram's law:

According to Abram's law for complete (or full) compaction, and at a certain age and normal temperature, the strength of concrete can be taken to be inversely proportional to the water-cement ratio.

In concrete design mixing with a low water/cement ratio, increases the compressive strength of concrete.

Size of Aggregates: 

Larger size aggregates have less surface area, so it decreases the water demand and increases the workability at a given water-cement ratio.

CONCEPT:

Non-destructive Tests:

Non-destructive tests are used to ascertain the quality of hardened concrete (strength, durability, elastic properties), generally following test are characterized as non-destructive test are:

1. Schmidt Rebound hammer test

2. Ultrasonic Pulse velocity test

3. Penetration method

4. Pull out Test method

5. Radioactive and nuclear test method

Destructive Test:

In the case of destructive tests, the concrete specimens (cube, cylinder, etc) are loaded till destruction in the laboratory, and strength properties are determined from the tests. The following test are characterized as destructive test are:

1. Compressive strength

2. Tensile strength

• Splitting tensile test
• Modulus of rupture test

3. Bond strength 

Steam curing is advantageous where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold weather.

Curing of concrete by steam under pressure:

1. Increases the rate of gain of compressive strength of concrete
2. Reduces the shear strength of concrete
3. Increases the speed of chemical reaction

Steam curing substantially reduces the time for attaining strength for any concrete.

For pre-casting, the time for concrete to attain strength is considered as idling time. If this time is minimized, we can have more precast elements produced in less time using the same set of moulds.

Hence steam curing is mostly adopted for pre-cast structures.

Explanation:

The 7 days compressive strength is approximately 0.65 times the  28 days compressive strength of cement concrete.

Strength of concrete is generally tested after 28 days as concrete cube strength because concrete gains strength with time after casting. It takes much time for concrete to gain 100 % strength and the time for the same is still unknown. The rate of gain of concrete compressive strength is higher during the first 28 days of casting and then it slows down.

The below table shows the concrete compressive strength with age:

Explanation:

The slump test is the most commonly used method of measuring the consistency of concrete which can be employed either in the laboratory or at the field.

Slump value is the amount (depth) by which the concrete settles down after removing the slump cone.

The table below reveals the type if vibrator required for compact for different values of slumps.

Concept: -

Compressive Strength Test of concrete:

According to IS: 516 (1959),

For cylindrical specimens:

• The number of strokes per layer should not be less 30

For cubical specimen

Case 1)

• The concrete of 150×150 mm size is compacted with 35 strokes per layer

Case2) 

• Concrete of 100×100 mm size is compacted with 25 strokes per layer.

 Additional Information

• Test specimens should be stored at a temperature of 27 ± 3 ℃ at 90% humidity for 24 ± 1/2 hours from the time of addition of water. After this time, specimens are removed from mould and placed in water and taken out just before the test.
• After 28 days or 7 days of curing, the specimen is placed in a compression testing machine which applies a gradual load of 14 MPa per minute.
• The average of three values is taken as the compressive strength of concrete, provided that individual variation is not more than ± 15% of the average.
• 7 days compressive strength = 2/3 of 28 days compressive strength.
• Cube strength = 1.25 cylindrical strength.

Concept:

Measurement of air content in fresh concrete:

Mainly there are three methods of measuring the air contents of fresh concrete is as follows:

1. Gravimetric Method
2. Volumetric Method
3. Pressure Method​

Gravimetric Method:

• This method is simple and does not require any special equipment, as such it is the first method used for calculating the air contents in concrete.
• The method principally comprises the determination of den­sity of fresh concrete compacted by a standard method.
• This density is then compared with the theoretical density of air-free concrete calculated from mix proportions and specific gravities of the constituent materials of the concrete.

​Volumetric Method:

• This method aims at measuring the volume of air in the sample of fresh concrete directly.
• The sample of a known volume of concrete is taken and all air is removed from it by any method and then the amount of water required to restore the original volume is determined.
• This method is tedious and time-consuming.

​Pressure Method:

• This is the most popular and best-suited method for site use.
• It is based on the relation between the volume of air and the applied pressure (at a constant temperature) given by Boyle’s law.
• This meter is directly graduated for the percentage of air and propor­tions or properties of materials need not be known.
• This meter is not suitable for use with porous aggre­gates and needs calibration if used at high altitudes.​

Explanation:

Blaine air permeability method:

• It is used for determining the fineness of Portland Cement measures the specific surface area of fine materials in square centimeters per gram of test sample.

Explanation:

The process of adding a finely ground, fully hydrated Portland cement to the fresh concrete in order to make high-strength concrete is called seeding.

There are some more special methods of making high strength(performance) concrete and are given below

1. Revibration
2. High speed slurry mixing;
3. Use of admixtures
4. Inhibition of cracks
5. Sulphur impregnation;
6. Use of cementitious aggregates

More is the compacting factor more will be the workability and more will be slump value.