Test: Concrete Mix Design - Civil Engineering (CE) MCQ

Option d can be eliminated easily because we need atleast some compressive strength. The minimum compressive strength required from structural consideration.

Splitting cylindrical test:

• The cylindrical sample is laid horizontal and it is subjected to the compressive load in the UTM machine.
• During this tensile stress is developed in the middle 5/6 portion of the section of the cylinder due to which its failure takes place.
• This test gives a more uniform and accurate result.
• The greatest advantage of this test is the same sample is used to find the compressive strength of the concrete.

The magnitude of the tensile stress is obtained by –

where, P = Applied load, D = Diameter of the cylinder, L = Length of the cylinder

Option d can be eliminated easily because double compaction is not possible. The adequate workability necessary for full compaction with the compacting equipment available.

1:2:4 concrete mix denotes, 1 part of cement, 2 parts of sand, and 4 parts of coarse aggregate in that particular concrete mix.
1 m³ freshly mixed concrete corresponds to 1.54 m³ dry volume of concrete.
Calculation:
Given,
Wet volume of concrete = 150 Cum, Concrete mix = 1:2:4
1 m³ freshly mixed concrete corresponds to 1.54 m³ dry volume of concrete.
150 m³ freshly mixed concrete corresponds = 150 × 1.54 = 231 m³ dry volume of concrete
In a concrete mix of 1:2:4, Total parts = (1 + 2 + 4) = 7
In a total of 7 parts, cement volume is 1 part, i.e. 1/7
Hence, Cement volume for 231 m³ dry concrete = (1/7) × 231 = 33 m³

The water-cement ratio is a critical factor in determining the strength and durability of concrete. The correct water-cement ratio is essential to achieve the desired concrete properties, including durability for specific site conditions. 
The minimum water-cement ratio ensures that there is just enough water to hydrate the cement and allow for workability, without compromising the strength and durability of the concrete. Using the minimum water-cement ratio helps achieve adequate durability for the particular site conditions.

The assumed value of standard deviation for initial calculations as per IS 456 – 2000 are given below in the table:

Maximum cement content to avoid shrinkage cracking due to the temperature cycle in mass concrete. It gives adequate durability for the particular site condition.

The pH value of water for making reinforced cement concrete should be between 6 to 8.5.
Important properties of water for concrete:

• The pH value must not be less than 6.
• The presence of salts of manganese, tin, zinc, copper, and lead causes a reduction in strength.
• The presence of sugar causes retardation in the setting.

Important Points
The permissible limit for solids as per IS 456 of 2000:

IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40.

Compressive Strength Test of concrete:
According to IS: 516 (1959),
(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.

For testing the compressive strength we generally use, 150 mm size cube. So, we have to go for 35 in this particular question. 

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.