Concrete Interview Questions and Answers

Concrete is an intimate mixture of binding material, fine aggregate, coarse aggregate and water.

• Binding material (like cement, lime. Polymer)
• Fine aggregate (sand)
• Coarse aggregate (crushed stone, jelly)
• Water
• Admixture (to gain special properties)

Cement – After addition of water, cement hydrates and binds aggregates and the surrounding surfaces like stone and bricks.

Coarse Aggregate – they give mass to the concrete and prevent shrinkage of cement.

Fine Aggregate – it prevents shrinkage of cement. When surrounded by cement it gains mobility, enters the voids in coarse aggregate and binding of ingredients take place. It adds density to concrete, since it fills the voids. Denser the concrete, higher is its strength.

Water – water activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position.

The steps of concreting are:

1. Batching or measurement of materials
2. Mixing
3. Transporting
4. Placing
5. Compacting
6. Curing
7. Finishing

Calculate the areas of steel bars available at site against the area of steel bars on the drawing by equation.

Where:

D₁            :     Diameter of steel bar on the drawing

D₂           :     Diameter of steel available on site

N₁           :     No. of bars required on the drawing

N₂           :     Calculated No. of bars using steel bars available at site.

• Proper proportion of water-cement ratio.
• Aggregate must be well-graded.
• Concrete should be air-entrained.
• Good vibration, etc.
• Use plasticizer.

It is a method of placing concrete underwater. A tube called “tremie” about 10″ – 12″ in diameter, rigid and watertight are held vertically in water during pouring operation to avoid washing-out of concrete mix.

When calcium chloride added to concrete mixtures, it imparts as an accelerating agent. Increasing the rate of hydration, speeding up the setting, and early strength development. (Never use in Hot Weather conditions).

Chloride attack:

• When the lime in cement react with carbon dioxide (CO2) it will produce calcium carbonate.
• The presence of calcium carbonate in water, increases the amount of alkalinity up to 11%, thereby the water will attack the reinforcement which lead to corrosion.
• While rust increases, the volume of the bars also increases leading to produce cracks in concrete.

Sulfate attack:

• When tricalcium aluminate (C3A) in cement react with sulfate due to the presence of moisture, it produces “gel” expansion in concrete and the volume will also increases leading to cracks in concrete.
• Also contribute to disintegration of the aggregates.

Type V or sulfate resisting cement.

Right after the final placement of concrete mix, when the water sheen or mixing water just begun to disappear.

1 liter per 5 sq. m. ; If applied by hand spray.
1 liter per 3 sq. m. ; If mechanically applied.

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Initial setting time of concrete is 30 minutes and final setting time of concrete is 600 minutes (10 hours) as per IS.

• Plasticizer
• Retarder
• Accelerator
• Plasticizer with retarding agent – Type D
• Plasticizer with accelerating agent
• Superplasticizer
• Superplasticizer with retarding agent

To delay the setting time of about 1 – 4 hours over normal setting time and reduces the mixing water by about 3-9 kg/cu.m. and increased the compressive strength by about 10 to 15%. The common chemical used is “Lignosulfic acid”.

Right after the final placement of concrete mix, when the water sheen or mixing water just begun to disappear.

The higher value of lime saturation factor corresponds to a higher content of free lime which makes the concrete vulnerable to sulphate attack and reduces soundness of concrete.

M25 is the grade of concrete mix where Letter M refers to the “mix” and number 25 refers to the “characteristic strength” of 15 cm cube after 28 days of curing which in the present case is equal to 20 N/mm².

For small works, proportions of ingredients of concrete to produce the concrete of required strength are given in table 3 of IS : 456-2000. Concrete produced by using this fixed proportion of ingredients is called nominal mix concrete.

Mass batching given the correct measurement of materials. When volume batching is used, the precautions to be taken are:

• Materials should not be heaped in measuring boxes.
• Bulking of sand should be considered.
• Water cement ratio should be suitable modified.

The follow up of these precautions is usually left to the non-technical labours. Thus, mass batching is superior to the volume batching

It is very important to produce a dense concrete so as to get the concrete with minimum voids. This can be achieved by proper compaction. Compaction is extremely important as 5% of voids can result in the 30% in strength, 10% of voids can result in the loss of 60% of strength and 25% of voids can result in the loss of 90% in strength. Other properties of concrete like durability. Impermeability etc., also greatly depend on the compaction of concrete.

• Hand compaction
• Ramming
• Compaction by vibration
• Compaction by spinning

Below zero-degree temperature, the free water present in the concrete will turn into ice-crystals. As ice occupies greater volume than the same quantity of water in the liquid state, no hardening has taken place will cause disruption of concrete and effect the long-term durability.

Sugar at the rate of 0.05% by weight of cement when mixed is very effective in retarding the setting time adequately and hence improves strength.

Following are the advantages of PSC over OPC:

• PSC possesses fairly high sulphate attack resistance than OPC.
• Reduced heat of hydration
• Refinement of pore structure
• Reduced permeability and increased durability.