Concrete MCQ Quiz - Objective Question with Answer for Concrete - Download Free PDF

Explanation:

High temperature and humidity variations:

1. Concrete durability is negatively impacted by significant fluctuations in temperature and humidity.

2. These variations can cause thermal expansion and contraction, leading to cracks and weakening of the concrete.

3. Additionally, moisture changes can lead to cycles of wetting and drying, which can result in deterioration due to processes like freeze-thaw damage.

 Additional Information Durability of Concrete

1. Resistance to Weathering:

   • Concrete must resist weathering effects like rain, wind, and temperature changes, which can cause surface wear and deterioration over time.

2. Corrosion Resistance:

   • Reinforced concrete must resist corrosion of steel reinforcement. This is influenced by the quality of concrete, proper mix proportions, and adequate cover over the reinforcement.

3. Water Resistance:

   • Concrete should have low permeability to prevent water penetration, which can cause freezing and thawing damage, or allow harmful chemicals (like chlorides or sulfates) to attack the concrete.

4. Abrasion Resistance:

   • Concrete exposed to high wear, such as in pavements or industrial floors, should be resistant to abrasion to maintain surface integrity.

5. Shrinkage and Cracking:

   • Proper curing and adequate mix design prevent shrinkage and cracking, ensuring the long-term durability of concrete structures.

6. Exposure Conditions:

   • Concrete must be designed based on the exposure conditions (e.g., exposure to chemicals, freeze-thaw conditions, or seawater), ensuring the mixture is adapted to withstand the specific environmental conditions.

7. Temperature Fluctuations:

   • Concrete exposed to large temperature variations can experience expansion and contraction, which might lead to cracking if not properly designed or cured.

8. Cement Quality:

   • The type and quality of cement used in the mix affect the durability. Higher-grade cements are better suited for environments with harsh conditions.

9. Proper Curing:

   • Curing ensures that concrete gains sufficient strength and durability by maintaining proper moisture levels during the early stages of setting.

10. Chemical Attack Resistance:

    • Concrete must resist the effects of chemical attacks like those from acidic soils, sea water, or industrial effluents that can cause degradation.

11. Design and Maintenance:

    • Proper design, including reinforcement details and quality control during construction, ensures long-lasting concrete durability. Regular inspection and maintenance also enhance the service life of concrete structures.

Explanation:

Adding more fine aggregate (sand) to Ready Mix Concrete (RMC) affects its properties in several ways:

1. Increases workability: Fine aggregates play a significant role in enhancing the workability of concrete. A higher proportion of fine aggregates can improve the flowability and ease of mixing, placing, and finishing the concrete.

2. Compressive strength: While increasing fine aggregates might slightly reduce the compressive strength (as the finer particles can reduce the interlocking strength of aggregates), this is not a direct improvement. The effect on compressive strength depends on the overall mix proportions and the quality of materials used.

3. Resistance to freezing and thawing: The addition of fine aggregates does not directly enhance the resistance to freezing and thawing, which is typically related to the air content and water-cement ratio.

4. Setting time: More fine aggregate might increase the setting time slightly, but this is generally not as significant as the other factors like cement content and admixtures.

 Additional InformationReady Mix Concrete (RMC) is a type of concrete that is produced in a batch plant according to a set mix design and delivered to the construction site in a ready-to-use condition.

Definition and Characteristics:

• Pre-mixed concrete: RMC is mixed in a plant and transported to the site in a ready-to-pour condition, unlike traditional concrete, which is mixed on-site.

• Consistency and Quality: Since RMC is produced in a controlled environment, it offers better quality, consistency, and uniformity than site-mixed concrete.

Components of RMC:

• Cement: The binding agent that holds the aggregates together.

• Aggregates: Fine aggregates (sand) and coarse aggregates (gravel or crushed stone).

• Water: For hydration of cement.

• Admixtures: Chemical admixtures can be added to modify the properties of the concrete (e.g., accelerating or retarding set time, enhancing durability, etc.).

Advantages of RMC:

• Quality Control: The mix proportions are precisely controlled and tested for each batch, ensuring better quality and consistency.

• Labor Efficiency: Reduces labor costs at the site as mixing and production are done off-site.

• Time-Saving: RMC can be delivered on-site in a ready-to-use condition, reducing the time required for mixing and waiting for concrete to set.

• No Need for On-Site Storage: Materials like cement and aggregates are stored at the plant, saving space on construction sites.

• Faster Construction: It speeds up the construction process because of the ready availability of the material and reduction in delays.

Types of RMC:

• Transit Mix RMC: Mixed in the truck as it is transported to the site.

• Central Mix RMC: Mixed completely in a plant, then transported to the site.

• Shrink-Mixed RMC: Mixed partially at the plant and completed on the way to the site.

Explanation:

According to IS 10262:2009, the maximum water content for concrete is influenced by the nominal maximum size of the coarse aggregate and the slump range.

1. Nominal maximum size of coarse aggregate: For a 20 mm nominal maximum size of angular coarse aggregate, the IS 10262 provides specific guidelines for the mixing water content.

2. Slump range of 25 to 50 mm: This indicates a medium workability requirement, which is typical for normal-strength concrete.

3. From IS 10262:2009, for 20 mm nominal maximum size angular coarse aggregate and a slump of 25-50 mm, the maximum mixing water content is 186 kg/m³.

 Additional Information

1. Water-cement ratio: IS 10262 specifies the water-cement ratio based on the desired strength and workability. The mix should be designed to achieve the required strength while ensuring adequate workability and durability.

2. Importance of water content: Proper water content is critical to the hydration process of the cement, which affects the strength and durability of the concrete. Too much water can lead to segregation and reduced strength, while too little water can result in poor workability.

Explanation:

The typical mix ratio for M20 grade RCC (Reinforced Cement Concrete) is: 1 : 1.5 : 3

In this mix, the proportions are:

• 1 part cement

• 1.5 parts sand (fine aggregate)

• 3 parts aggregate (coarse aggregate)

M20 grade concrete is designed to have a compressive strength of 20 MPa (megapascals) at 28 days of curing. This ratio is commonly used for medium-strength concrete in construction.

Additional Information

M20 Grade Concrete:

1. It is a medium-strength concrete mix suitable for general construction purposes such as foundations, beams, slabs, and pavements.

2. The number "20" in M20 refers to the compressive strength of the concrete, which is 20 MPa (megapascals) after 28 days of curing.

Mix Proportions:

1. The mix ratio of 1:1.5:3 (cement: sand: coarse aggregate) is commonly used for M20 grade concrete.

2. This is a nominal mix, often used when the exact properties of materials are unknown or when specific mix design is not needed.

Water-Cement Ratio:

1. The water-cement ratio plays a crucial role in the strength and durability of concrete.

2. For M20 grade, the water-cement ratio is usually around 0.5 to 0.6 to achieve optimal strength and workability.

Strength Consideration:

1. Concrete's strength increases over time with curing, but it is typically tested at 28 days to determine whether it has achieved the desired compressive strength.

Testing:

1. M20 concrete is often tested using a cube compression test, where a 150 mm x 150 mm x 150 mm concrete cube is tested after 28 days to check if it has achieved the expected strength.

Applications:

1. M20 grade concrete is used in elements such as foundations, slabs, beams, footings, and other structural elements where moderate strength is required.

Explanation:

In polymer-modified concrete (PMC), the polymer component is added to improve the properties of conventional concrete.

The key roles of the polymer include:

1. Reduce water permeability: The polymer creates a dense and cohesive bond between the particles of the concrete, which significantly reduces the passage of water and other fluids.

2. Increase flexibility: The polymer enhances the elastic properties of concrete, making it more flexible and less brittle, which improves its ability to resist cracking under stress or deformation.

Additional Information

1. Increase setting time significantly: Polymers generally do not increase setting time significantly; in fact, some may accelerate the curing process.

2. Reduce strength and durability: The use of polymers generally improves strength, durability, and other performance characteristics of concrete.

3. Increase brittleness: Polymers increase flexibility and reduce brittleness, not the other way around.

As per table 5 of IS 456 : 2000 

Minimum cement content, Maximum water-cement ratio, and Minimum grade of concrete for different exposures with normal weight of aggregates of 20 mm Nominal 

Explanation:

From the above table, It is observed that as the compaction factor increases slump increases.

Slump test is the most general test that is used for the measurement of workability of concrete. It helps in understanding the internal work done by concrete for self-compaction.

Explanation:

Non-tilting drum mixer 

• A reversing drum mixer (also commonly called a non-tilting mixer) is a type of concrete mixer that produces concrete in single batches.
• The entire drum rotates around its axis as materials are loaded through a charge chute at one end of the drum and exit through a discharge chute at the opposite end of the drum.

Tilting drum mixer 

• Tilting drum mixer means the drum will discharge concrete by tilting downwards.
• It is a rapid discharge process and is used for larger projects.
• Rapid means it delivers concrete by gravity that is tilting the drum downwards because of this the concrete mix obtained will not be subjected to segregation.
• Low workable concrete containing large-sized aggregates greater than 7.5cm is mixed efficiently with this tilting type mixer.

Mixing efficiency depends on some of the factors as follows:

• Shape of the drum
• Angle of the drum
• Size of blades
• Angle of blades

Concept:

Theoretical strength of concrete is = 240 x3

x → gel-space ratio = 0.59

Calculation:

Theoretical strength = 240 × (0.59)3

Explanation:

Schmidt’s Rebound Hammer

The rebound hammer is also called as Schmidt hammer.

The rebound hammer test is a non-destructive testing method of concrete that provides a convenient and rapid indication of the compressive strength of the concrete. 

The Schmidt hammer provides an inexpensive, simple and quick

Note:

The results are affected by factors such as smoothness of surface, size and shape of the specimen, moisture condition of the concrete, type of cement and coarse aggregate, and extent of carbonation of surface.

The rebound hammer is a surface hardness tester for which an empirical correlation has been established between compressive strength and rebound number.

But the thickness of concrete cannot be estimated by this method.

Other non-destructive methods of testing are:

1. Ultrasonic pulse velocity test

It is mainly used to measure the time of travel of ultrasonic pulse passing through the concrete and hence concrete quality.

2. Pull out test

This technique can thus measure quantitatively the in-situ strength of concrete when proper correlations have been made.

3. Penetration method

This test is developed to measure the chloride permeability of in-place concrete non-destructively.

4. Radioactive methods

It can be used to detect the location of reinforcement, measure density and to check whether honeycombing has occurred in structural concrete units.

Explanation

As per clause 5.4 of IS 456 : 2000

Potable water is considered satisfactory for mixing Concrete and the permissible limits for solids is shown in table below:

Explanation:

Lime Concrete:

• Lime concrete is a composite mixture of hydraulic lime(or slaked lime) as binding material, sand as fine aggregate, and gravel as coarse aggregate in appropriate proportions.

Cement Concrete:  

• Cement concrete is a composite mixture of cement as binding material, sand as fine aggregate, and gravel as coarse aggregate in appropriate proportions

Difference between lime concrete and cement concrete:

• ​Lime hardens much more slowly than cement-containing mortars, making it much more workable.
• Lime is also less brittle and less prone to cracking, and any cracked areas can absorb carbon dioxide and mend over time.
• The cement hardens very quickly but may be too strong for some applications, e.g., working with old bricks.

Explanation:

Super-plasticizers

They are admixtures that work on surfactant property, in which they disperse and deflocculate cement particles thus making concrete flowing, pourable, and easily placed.

Examples: Sulfonated Melamine formaldehyde resin, sulfonated naphthalene-formaldehyde resin, Mixtures of saccharates, and acid amides.

Note:

(Super-water reducers) - 15 to 30 % water reduction.

Purpose:

1. To increase the workability of concrete without any change in the composition of the mix.

2. To reduce the water content of mixing water, to reduce the water/cement ratio resulting in the increase of strength and durability of concrete.

3. To reduce cement and water contents in the concrete to reduce the cost of production of concrete. The reduction in cement and water contents reduces the creep, shrinkage, and heat of hydration.

4. To slow the setting rate of the concrete while retaining the flowing properties of a concrete mixture and remove air bubbles.

Explanation:

(i) Aerated concrete is made by introducing air or gas into a slurry composed of Portland cement or lime and finely crushed siliceous filler so that when the mix sets and hardens, a uniformly cellular structure is formed.

(ii) Aerated concrete can be manufactured by:

• By the formation of gas by chemical reaction within the mass during liquid or plastic state.
• By mixing the preformed stable foam with the slurry.
• By using finely powdered metal (usually aluminium powder) with the slurry and made to react with the calcium hydroxide liberated during the hydration process, to give out large quantity of hydrogen gas.

Explanation

As per clause 5.4 of IS 456 : 2000

Potable water is considered satisfactory for mixing Concrete and the permissible limits for solids is shown in table below: