Concrete Technology interview questions

Concrete is a composite construction material composed of:

• Cement: The binding agent that hardens and binds the other components together.
• Aggregates: Fine (sand) and coarse (gravel or crushed stone) materials that provide bulk and strength.
• Water: Initiates the chemical reaction (hydration) that causes the cement to harden.
• Admixtures: Chemical additives that modify the properties of concrete (e.g., accelerators, retarders, superplasticizers).

The hydration process is a series of chemical reactions between cement and water. When water is added to cement, the major compounds in cement (C3S, C2S, C3A, and C4AF) react to form calcium silicate hydrate (C-S-H) and calcium hydroxide (CH), which contribute to the hardening and strength of concrete. The process can be divided into five stages:

1. Initial Mixing: Wetting of cement particles.
2. Dormant Period: Concrete remains workable.
3. Setting: Rapid formation of C-S-H and initial hardening.
4. Hardening: Continuous formation of C-S-H, leading to strength gain.
5. Long-term Curing: Slow reactions that continue to increase strength over months and years.

Factors affecting workability include:

• Water-Cement Ratio: Higher ratios increase workability but may decrease strength.
• Aggregate Properties: Size, shape, and grading affect how the aggregates interlock and the overall workability.
• Admixtures: Superplasticizers and other admixtures can enhance workability without increasing water content.
• Mix Proportions: The ratio of fine to coarse aggregates and the overall mix design.
• Temperature: Higher temperatures can reduce workability by accelerating the setting process.
• Mixing Time: Adequate mixing ensures uniform distribution of materials.

Curing is vital for maintaining moisture and temperature conditions to allow complete hydration, ensuring that concrete achieves its desired properties. Proper curing increases strength, durability, and resistance to cracking. Common curing methods include:

• Water Curing: Continuous wetting using methods such as ponding, spraying, or wet coverings.
• Membrane Curing: Applying curing compounds that form a moisture-retaining film on the surface.
• Steam Curing: Using steam to accelerate strength gain, especially in precast elements.
• Covering: Using plastic sheets or burlap to retain moisture.

Mix design is the process of selecting suitable ingredients for concrete and determining their proportions to achieve desired properties. Factors considered include:

• Strength Requirements: Specified compressive strength.
• Durability Requirements: Exposure conditions (e.g., weather, chemical exposure).
• Workability: Ease of placement and compaction.
• Economy: Cost-effectiveness of the mix.
• Aggregate Characteristics: Size, shape, and grading.
• Water-Cement Ratio: Balancing strength and workability.

Different types of concrete include:

• Plain Concrete: Without reinforcement, used for pavements and minor structures.
• Reinforced Concrete (RC): Contains steel reinforcement for added tensile strength.
• Prestressed Concrete: Pre-tensioned or post-tensioned steel tendons to counteract tensile forces.
• Lightweight Concrete: Uses lightweight aggregates to reduce density.
• High-Strength Concrete: Achieves compressive strength greater than 40 MPa.
• Self-Compacting Concrete (SCC): Highly flowable, does not require vibration.
• Fiber-Reinforced Concrete: Contains fibrous materials to improve toughness.

The compressive strength of concrete is tested using:

• Concrete Cube Test: Standard cubes (usually 150mm x 150mm x 150mm) are cured and then crushed in a compression testing machine at specified ages (7, 28 days).
• Concrete Cylinder Test: Cylindrical specimens (usually 150mm diameter and 300mm height) are tested similarly.

Results are expressed in megapascals (MPa) or pounds per square inch (psi).

Segregation:

• Causes: Excessive water content, improper mixing, dropping from a height, over-vibration.
• Prevention: Proper mix design, adequate mixing, minimizing free fall height, and controlled vibration.

Bleeding:

• Causes: High water-cement ratio, poor mix design, insufficient fines.
• Prevention: Using the correct water-cement ratio, proper mix design, and using admixtures to reduce bleeding.

Aggregate grading affects the workability, strength, and durability of concrete:

• Well-Graded Aggregates: Provide better packing, fewer voids, and require less cement paste, resulting in stronger and more durable concrete.
• Poorly Graded Aggregates: Increase void content, require more cement paste, and can lead to reduced workability and higher costs.

The water-cement ratio (w/c ratio) is the ratio of the weight of water to the weight of cement in a concrete mix. It is crucial for:

• Strength Development: Lower ratios lead to higher strength and durability.
• Workability: Higher ratios increase workability but can reduce strength.
• Durability: Lower ratios reduce porosity, making concrete more resistant to weathering and chemical attacks.