5.2 Concrete Technology

5.2 Concrete Technology

Introduction to Concrete

Concrete is the most ubiquitous man-made construction material in the world, forming the backbone of modern infrastructure. It is a composite material composed of cement, fine aggregate (sand), coarse aggregate, and water, which chemically reacts to form a hardened, stone-like mass. Its versatility, durability, and ability to be cast into complex shapes make it indispensable in civil engineering. This unit covers the constituent materials, fundamental properties, the scientific process of mix design, essential testing methods, and the principles of quality control, all governed by relevant IS (Indian Standards) and NS (Nepal Standards).

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1. Constituent Materials and Their Properties

The quality of concrete is directly governed by the properties of its individual components.

1.1 Cement (The Binder)

1. Types and Selection:

   • OPC (33, 43, 53): For general construction. Higher grade (53) gives higher early strength.

   • PPC (Portland Pozzolana Cement): Better long-term strength, lower heat of hydration, improved sulphate resistance.

   • Others: Rapid Hardening, Sulphate Resisting, Low Heat Cement for specific applications.

2. Key Properties (from IS 4031 tests):

   • Fineness: Affects rate of hydration and water demand.

   • Standard Consistency: Water required for normal hydration.

   • Setting Time: Must allow time for placement and compaction.

   • Compressive Strength: Primary indicator of cement quality (tested at 3, 7, 28 days).

1.2 Aggregates (The Inert Filler)

Aggregates constitute 60-80% of the concrete volume, providing bulk, dimensional stability, and economy.

1. Fine Aggregate (Sand):

   • Size: Particles passing 4.75mm IS sieve.

   • Sources: Natural river sand, crushed stone sand (M-Sand).

   • Properties:

     • Grading (Particle Size Distribution): Governs workability and water demand (as per IS 383). A well-graded sand packs densely.

     • Fineness Modulus (FM): An empirical number indicating coarseness/fineness. Typical range: 2.2 to 3.2.

     • Bulking: Increase in volume of damp sand. Must be accounted for in volume batching.

     • Impurities: Silt, clay, organic matter reduce strength and must be limited (Silt Content Test).

2. Coarse Aggregate:

   • Size: Retained on 4.75mm IS sieve. Commonly 10mm, 20mm, or 40mm nominal size.

   • Properties:

     • Grading: Important for achieving a dense, workable mix.

     • Shape & Texture: Angular aggregates provide better interlocking but require more water. Rounded aggregates improve workability.

     • Strength: Aggregate crushing value, impact value, and abrasion value tests (per IS 2386) assess suitability for wear and tear.

     • Water Absorption: Determines the state of aggregate (dry, saturated surface dry - SSD) for accurate water-cement ratio calculation.

1.3 Water

1. Quality: Must be potable (fit for drinking). pH between 6 and 8.

2. Impurities: Excessive chlorides cause corrosion of reinforcement. Sulphates cause sulphate attack. Organic matter affects setting time.

3. Rule of Thumb: If water is fit for drinking, it is fit for making concrete.

1.4 Admixtures (Optional but Common)

Chemicals added in small quantities (<5% by weight of cement) to modify properties of fresh or hardened concrete.

1. Water-Reducers/Plasticizers: Increase workability without adding water or reduce water for same workability, thereby increasing strength.

2. Superplasticizers (High-Range Water Reducers): Drastically increase workability for flowing concrete or enable very low water-cement ratios for high-strength concrete.

3. Retarders: Delay setting time for hot weather concreting or long transportation.

4. Accelerators: Speed up setting and early strength gain (e.g., calcium chloride, but use with caution due to corrosion risk).

5. Air-Entraining Agents: Introduce microscopic air bubbles to improve frost resistance and workability.

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2. Properties of Concrete

Concrete properties are categorized into Fresh (Plastic) Concrete and Hardened Concrete.

2.1 Properties of Fresh Concrete

1. Workability:

   • Definition: The ease with which concrete can be mixed, placed, compacted, and finished without segregation.

   • Factors Affecting Workability: Water content, aggregate grading/shape, cement content, admixtures, and temperature.

   • Measurement Tests:

     • Slump Test (IS 1199): Most common site test. Measures consistency. Suitable for medium to high workability mixes. $\text{Slump (mm)} = \text{Height of mould} - \text{Height of slumped concrete}$

     • Compaction Factor Test: More precise, for low to medium workability mixes.

     • Flow Test: For high workability/flowing concrete.

2. Segregation:

   • Separation of coarse aggregates from the mortar matrix due to excessive handling or poor mix design. Results in non-uniform concrete.

3. Bleeding:

   • Upward movement of water to the surface after placement, causing a weak, porous layer (laitance) on top and water pockets under aggregates.

2.2 Properties of Hardened Concrete

1. Strength:

   • Compressive Strength: The primary specified property. Defined as the 28-day characteristic compressive strength ($f_{ck}$), e.g., M20, M25, M30 where the number denotes $f_{ck}$ in N/mm². Tested on 150mm cubes or 150x300mm cylinders (IS 516). $f_{ck} = \frac{\text{Load at Failure}}{\text{Cross-sectional Area}}$

   • Tensile Strength: Very low (about 8-12% of compressive strength). Measured indirectly via Split Tensile Test (IS 5816) on cylinders.

   • Flexural Strength (Modulus of Rupture): Important for pavements. Tested by loading a beam (IS 516).

2. Durability:

   • Ability to withstand weathering, chemical attack, abrasion, and other deterioration processes over its design life.

   • Key factors: Permeability (most critical), cement content, water-cement ratio, curing, and exposure conditions.

3. Elastic Properties:

   • Modulus of Elasticity ($E_c$): Stress-strain ratio in the elastic range. Indicates stiffness. Given by $E_c = 5000 \sqrt{f_{ck}}$ N/mm² in IS 456.

   • Poisson's Ratio ($\mu$): Ratio of lateral to axial strain. Typically 0.15-0.2 for concrete.

4. Creep and Shrinkage:

   • Creep: Time-dependent inelastic deformation under sustained load. Causes increased long-term deflections.

   • Shrinkage: Volume reduction due to loss of moisture (drying shrinkage) and chemical processes (autogenous shrinkage). Leads to cracking if restrained.

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3. Concrete Mix Design (IS 10262)

Mix design is the process of determining the proportions of cement, sand, coarse aggregate, and water to produce concrete of specified strength, workability, and durability most economically.

3.1 Fundamental Principles

1. Water-Cement Ratio Law (Abram's Law): For given materials and conditions, the strength of concrete is inversely proportional to the water-cement ratio (w/c). $\text{Strength} \propto \frac{1}{(\text{w/c ratio})}$

   • Lower w/c → Higher strength & durability, but lower workability.

2. Rule of Absolute Volume: The total volume of concrete is equal to the sum of the absolute volumes of cement, water, aggregates, and entrapped air. $V_c + V_w + V_{fa} + V_{ca} + V_{air} = 1 \text{ m}^3$

3.2 Step-by-Step Procedure (As per IS 10262:2019)

1. **Step 1: Determine Target Mean Strength (f_t = f_{ck} + 1.65 \times s $where$s$$ is the standard deviation (based on plant control data, from Table in code).

2. Step 2: Select Water-Cement Ratio:

   • Based on $f_t$ and cement type from empirical relationships in the code.

   • Check against maximum w/c ratio specified for durability from IS 456 (e.g., 0.45 for RCC in mild exposure).

3. Step 3: Select Water Content:

   • From code tables based on workability (slump) and aggregate size. Adjusted for other factors (shape, admixtures).

4. Step 4: Calculate Cement Content: $\text{Cement Content} = \frac{\text{Water Content}}{\text{w/c ratio}}$

   • Check against minimum cement content for durability from IS 456.

5. Step 5: Determine Volume of Coarse Aggregate:

   • From code tables based on w/c ratio and fine aggregate grading zone.

6. Step 6: Calculate Mix Proportions:

   • Use absolute volume method to find masses of Fine and Coarse Aggregates.

7. Step 7: Adjustments for Moisture in Aggregates:

   • Aggregate batched in field is moist.

   • Free Moisture in aggregates gets added to mix water.

   • Absorbed Moisture does not contribute to mix water.

   • Therefore, weight of aggregates to be batched is increased, and weight of water to be added is reduced accordingly.

8. Final Output: Cement : Sand : Coarse Aggregate ratio by weight, and Water content in liters per bag of cement.

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4. Testing of Concrete

4.1 Tests on Fresh Concrete

1. Workability Tests:

   • Slump Test (IS 1199): For routine site control.

   • Compaction Factor Test: More accurate for low workability mixes.

2. Temperature Measurement: Important for hot/cold weather concreting.

4.2 Tests on Hardened Concrete

1. Compressive Strength Test (IS 516):

   • Specimen: 150mm cubes or 150x300mm cylinders.

   • Curing: Standard curing in water at 27±2°C until testing.

   • Test Age: 7 days (for early indication) and 28 days (for acceptance).

   • Loading Rate: 140 kg/cm²/min (approx. 5.2 kN/sec for a 150mm cube).

   • Acceptance Criteria (IS 456):

     • Average strength of 3 consecutive samples ≥ $f_{ck}$.

     • No individual test result < $f_{ck} - 4$ N/mm².

2. Non-Destructive Tests (NDT):

   • Rebound Hammer (Schmidt Hammer): Gives a surface hardness number correlated to compressive strength. Useful for comparative assessment.

   • Ultrasonic Pulse Velocity (UPV): Measures time for a pulse to travel through concrete. Correlates to density, uniformity, and presence of cracks.

   • Core Cutting and Testing: Direct but destructive method to extract and test cores from the actual structure to assess in-situ strength.

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5. Quality Control in Concrete Construction

Quality control ensures the constructed element meets the design intent. It is a continuous process from material sourcing to final curing.

5.1 Stages of Quality Control

1. Pre-Construction:

   • Approval of material sources (cement, aggregates).

   • Approval of mix design based on trial mixes.

   • Calibration of batching plant and testing equipment.

2. During Construction (Batching, Mixing, Transporting, Placing, Compacting):

   • Batching: Ensure accuracy of weight/volume measurement. Weight batching is preferred.

   • Mixing: Ensure uniform mixing. Minimum mixing time as per code (e.g., 2 minutes in drum mixer).

   • Transport & Placement: Prevent segregation and loss of workability.

   • Compaction: Use vibrators effectively to remove entrapped air and achieve full compaction. Over-vibration causes segregation.

3. Post-Construction:

   • Curing: Most critical and often neglected step. Maintain moisture and temperature for sufficient hydration.

     • Methods: Water ponding, sprinkling, wet coverings, curing compounds.

     • Duration: Minimum 7 days for OPC, 10 days for concrete with mineral admixtures, as per IS 456.

   • Testing: Regular sampling and testing of cubes for compressive strength.

   • Inspection: Visual inspection for honeycombing, cracks, surface defects.

5.2 Relevant Codes and Standards

• IS 456:2000: Plain and Reinforced Concrete - Code of Practice (The primary design code).

• IS 10262:2019: Concrete Mix Proportionaling - Guidelines.

• IS 383:2016: Specification for Coarse and Fine Aggregates.

• IS 516:1959 (Reaffirmed 2021): Method of Tests for Strength of Concrete.

• IS 1199:1959 (Reaffirmed 2018): Methods of Sampling and Analysis of Concrete.

• IS 9013:1978: Method of Making, Curing and Determining Compressive Strength of Concrete Test Specimens.

• NBC 109:2020 (Nepal): Concrete Structure.

Conclusion: Concrete technology is a blend of material science, empirical relationships, and rigorous site practice. A deep understanding of how material properties, proportions (mix design), and construction practices (placing, compacting, curing) interact to determine the final performance of concrete is essential for any civil engineer. Adherence to standard codes ensures safety, durability, and economy in concrete construction.