Tuesday, December 13, 2016

Top 100 Interview Question and Answers Concrete

These are general Interview Question and Answers which every one must know before giving interview in construciton department for the job of Civil Engineer, Material Engineer, Lab Technician etc

question 1
The normal pouring temperature of concrete is
  31±2 Degree Centigrade
question 2
Shape of a particle is not important property of aggegates.



question 3
The temperature at which hydration occurs does not affect the rate of heat development.


question 4
Dislicate C2S is not most important compound in cement.

question 5
Water not fit for drinking should be used in making concrete with a Ph value of 6 to 8.


question 6
Sea water does not increase the risk of corosion of reinforcement.

question 7
Curing water should be free from substances that attack hardened concrete.


Quality of water is recommended for mixing of concrete which is fit for drinking.


question 9
Quality of water is not important property in setting of concrete.

question 10
Toughness is the resistance of aggregates to failure by impact.
question 11
Use of aggregates in concrete is not beneficial.<


question 12
Good bond increases strength of concrete.

question 13
The particle size distribution is called grading.

question 14
Natural aggregates are formed by the process of weathering and abrasion.


question 15
Setting time is used to describe the stiffening of cement paste.


question 16
Finess test is only the laboratory test to confirm quality of cement.


question 17
Cement is made from a combination of limestone, clay and gypsum.


question 18
Modulus of elasticity is denoted by E


question 19
Density is not a property of concrete.


question 20
Good concrete has to be satisfactory in its hardened state.

question 21
The heat generated by a large amount of hydrating cement may lead to cracking.


question 22
Describe work ability of concrete.
Answer:Workability is one of the physical parameters of concrete which affects the strength and durability as well as the cost of labor and appearance of the finished product. Concrete is said to be workable when it is easily placed and compacted homogeneously i.e. without bleeding or Segregation. Unworkable concrete needs more work or effort to be compacted in place, also honeycombs &/or pockets may also be visible in finished concrete.
Factors affecting workability:
  1. Water content in the concrete mix
  2. Amount of cement & its Properties
  3. Aggregate Grading (Size Distribution)
  4. Nature of Aggregate Particles (Shape, Surface Texture, Porosity etc.)
  5. Temperature of the concrete mix
  6. Humidity of the environment
  7. Mode of compaction
  8. Method of placement of concrete
  9. Method of transmission of concrete
How To improve the workability of concrete
  1. Increase water/cement ratio
  2. Increase size of aggregate
  3. Use well-rounded and smooth aggregate instead of irregular shape
  4. Increase the mixing time
  5. Increase the mixing temperature
  6. Use non-porous and saturated aggregate
  7. With addition of air-entraining mixtures

question 23
A mix has an aggregate/cement ratio of 8 and a concrete porosity 18 per cent. As suing there is no entrapped air. Calculate the water/cement ratio of a mix given that the specific gravity of aggregate is 2.7 and the degree of hydration is 92 per cent.

question 24
Expalin       a) Explain internal and external vibrators of concrete.                                                      b) What is difference between an additive and an admixture?
Answer No# 24
a)       Internal Vibrator:They are also called immersion, poker or needle vibrators. They essentially consist of a power unit and a long flexible tube at the end of which a vibrating head is attached. Power is provided by electric motor, compressed air or petrol engine. The long tube houses a flexible shaft which rotates an eccentric weight inside the vibrating head. The frequency of the vibrator is about 700 cycles per minute.
The vibrating head is inserted in the concrete. They are most effective as the vibrating head comes into intimate contact with concrete.\
External Vibrator:They are also called form vibrators. They are clamped to the formwork horizontally and vertically at suitable spacing not exceeding 90 cm in either direction. As the work progress they are shifted. They vibrate the concrete from the vibration of the forms and thus much energy is wasted.
They are used only if the use of internal vibrators is not practicable as in the case of thin and congested sections, arches and tunnel lining, etc.

b)      Additive Additives are substance which is added to cement during its manufacturing stage to get new property for cement.

Admixture:Admixtures are additions to a concrete mix that can help control the set time and other aspects of fresh concrete. Common admixtures include accelerating admixtures, retarding admixtures, fly ash, air entraining admixtures, and water-reducing admixtures.
Admixtures are used to obtain following objectives:
1.      to accelerate or retard setting and hardening.
2.      to improve workability.
3.      to increase strength.
4.      to improve durability.
5.      to decrease permeability.
6.    to impart other desired properties.

question 25
Explain       a) What are the effects of hot weather on fresh concrete?                              b) What is the effect of temperature during first 24 hours on 28 day strength of concrete?
Answer: a) Any operation of concreting done at atmospheric temperature above 400 C may be put under hot weather concreting. The effect of hot weather are.


A higher temperature of fresh concrete results in a more rapid hydration and leads to reduced workability/ accelerated setting. This reduces the handling time of concrete.


Concrete mixed, placed and cured at higher temperature normally develops higher early strength than concrete produced and cured at normal temperature but at 28 days or later the strength are generally lower.


Rapid evaporation may cause plastic shrinkage and cracking and subsequent cooling of hardened concrete would introduce tensile stresses.
Question 25

    B) During Concrete poursfirst 24 hours the required temperature of concrete is 33±2 ° c & the curing tank required temperature (where will we put cylinder of concrete) is 23±1.7 ° c. So we must follow these limits. If the temperature exceeds the required limit, then we will not achieve required 28 days strength.

question 26
a) What is shrinkage?      b) Why is the depth of cover to steel specified?

a)    Shrinkage is volumetric change in concrete due to loss of moisture content and lead to crack.
Shrinkage also causes curling/warping which can lead to a variety of slab issues including decreased load-carrying capacity (structural cracking) and joint stability problems such as spalling.

There are mainly two shrinkage

·         Drying shrinkage
·         Plastic shrinkage

The drying shrinkage is happen where harden concrete loss the moisture in hardened but drying shrinkage increase while water is added to much when making of concrete so it loss high percentage water and drying shrinkage increase.

Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic.
These cracks appear mostly on horizontal surfaces.They are usually parallel to each other.

In plastic shrinkage the crack are appears when the water is directly losses because of high temperature.

Question 27
a) Discuss the relation between compression strength and tensile strength of concrete                                         b) Describe the role of aggregate in creep of concrete

a)  The theoretical compressive strength is to be eight times larger than the tensile strenghth. This implies a fixed relation between the two strengths. In fact there is a close relation but not a direct proportionlity the ration fo the two strenghts depends on the general level of strength of the concrete. Generally, the ratio of tensile to compressive is lower the higher the compressive strength. Thus, for example, the tensile strength increases with age at a lower rate than the compressive srength. However, there are several other factors which affect the relation between the two strengths, the main one being the method of testing the concrete in tension, the size of the specimen, the shape and surface texture of coarse aggregate, and the moisture conditin of the concrete.


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