Concrete Technology – Short Notes for GATE and SSC JE exam

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1. Introduction to Concrete

What is Concrete?

Concrete is a composite construction material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens over time. In simple terms, it’s a mixture of cement, aggregates (sand and gravel), and water, which, when mixed, forms a paste that hardens into a stone-like mass.

Historical Development

The use of concrete dates back to ancient times, with early forms used by the Romans (e.g., Roman concrete in the Pantheon). Modern concrete, using Portland cement, was developed in the 19th century and has since become the most widely used construction material globally.

Advantages and Disadvantages of Concrete

Advantages:

Versatility: Can be molded into any shape.
Strength & Durability: High compressive strength and long lifespan.
Fire Resistance: Non-combustible.
Economy: Relatively inexpensive to produce.
Availability: Raw materials are widely available.
Energy Efficiency: Good thermal mass properties.

Disadvantages:

Low Tensile Strength: Weak in tension, requiring reinforcement (steel).
Shrinkage & Creep: Prone to volume changes over time.
Formwork Requirement: Needs formwork for casting, which can be costly.
Weight: Relatively heavy.
Time-Dependent Strength Gain: Gains strength slowly over time.

Types of Concrete

Plain Cement Concrete (PCC): Concrete without any reinforcement. Used for foundations, pavements, etc., where tensile stresses are minimal.
Reinforced Cement Concrete (RCC): Concrete with steel reinforcement bars embedded within it to resist tensile forces. Most common type for structural elements like beams, columns, slabs.
Prestressed Concrete: Concrete in which internal stresses are introduced to counteract the stresses resulting from external loads, improving its performance, especially in tension.

2. Constituents of Concrete

Cement

Cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. The most common type is Portland cement.

Types of Cement

Ordinary Portland Cement (OPC): Most common type, available in 33, 43, and 53 grades (as per IS 269).
Portland Pozzolana Cement (PPC): Contains pozzolanic material (e.g., fly ash, calcined clay) which reacts with calcium hydroxide released during cement hydration, improving durability and reducing heat of hydration.
**Portland Slag Cement (PSC):: Contains granulated blast furnace slag. Offers better durability and reduced heat of hydration.
Rapid Hardening Cement: Gains strength quickly, suitable for prefabrication and cold weather concreting.
Low Heat Cement: Produces less heat during hydration, used in mass concrete structures (dams) to prevent thermal cracking.
Sulphate Resisting Cement: Used in structures exposed to sulphate attack (e.g., foundations in sulphate-rich soils, marine structures).
High Alumina Cement: Rapid hardening, high early strength, and good resistance to chemical attack.
White Cement: Used for architectural purposes due to its white color.
Hydrophobic Cement: Contains water-repellent chemicals, reducing deterioration during storage in damp conditions.

Chemical Composition and Hydration of Cement

The main chemical compounds (Bogues Compounds) in OPC are:

Tricalcium Silicate (): Alite – Responsible for early strength (up to 7 days).
Dicalcium Silicate ( $C_{2} S$ ): Belite – Responsible for later strength (after 7 days).
Tricalcium Aluminate ( $C_{3} A$ ): Celite – Responsible for initial setting and early heat of hydration. Contributes little to strength.
Tetracalcium Aluminoferrite ( $C_{4} A F$ ): Ferrite – Contributes to strength and reduces clinkering temperature.

Hydration of Cement: The chemical reaction between cement and water, leading to the hardening of the cement paste. This process forms calcium silicate hydrate (CSH) gel (the primary binding agent) and calcium hydroxide ( $C a (O H)_{2}$ ).

Properties of Cement

Fineness: Affects rate of hydration and strength development. Measured by specific surface area (Blaine’s air permeability method) or sieve analysis.
Consistency: Amount of water required to form a cement paste of standard consistency. Determined by Vicat apparatus.
Setting Time:
- Initial Setting Time: Time from adding water until the paste starts losing plasticity (minimum 30 minutes for OPC).
- Final Setting Time: Time until the paste has completely lost its plasticity and attained sufficient firmness (maximum 600 minutes for OPC).
- Determined by Vicat apparatus.
Soundness: Ability of hardened cement paste to retain its volume after setting. Unsoundness is due to excess free lime or magnesia, causing expansion and cracking. Measured by Le Chatelier apparatus (for lime) and Autoclave test (for lime and magnesia).
Strength: Compressive strength of cement mortar cubes (1:3 cement:sand ratio) at 3, 7, and 28 days.

Tests on Cement

Field tests (color, presence of lumps, feel)
Laboratory tests: Fineness, Consistency, Initial & Final Setting Time, Soundness, Compressive Strength, Specific Gravity.

GATE & SSC JE Numerical Tips: Cement Properties

Consistency: Normal consistency (P) is typically around 28-35% for OPC.
- Water for initial setting time test = $0.85 P \times weight of cement$ .
- Water for compressive strength test = $(P /4 + 3) \times weight of cement$ .
Setting Time: Remember minimum initial and maximum final setting times for OPC (30 min and 600 min).
Strength: For 33, 43, 53 grade OPC, the 28-day compressive strength of cement mortar cubes should be at least 33, 43, 53 MPa respectively.

Aggregates

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and cement, form concrete. They typically constitute 60-80% of the concrete volume.

Classification of Aggregates

Fine Aggregates: Particles passing through a 4.75 mm IS sieve (e.g., sand).
Coarse Aggregates: Particles retained on a 4.75 mm IS sieve (e.g., gravel, crushed stone).

Properties of Aggregates

Shape: Angular, rounded, flaky, elongated. Affects workability and strength.
Texture: Smooth or rough. Affects bond with cement paste.
Strength: Should be strong enough to resist crushing.
Specific Gravity: Useful for mix design calculations.
Bulk Density: Mass of aggregate per unit volume (including voids).
Water Absorption: Amount of water absorbed by aggregate. Affects effective water-cement ratio.
Durability: Resistance to weathering, abrasion, and chemical attack.

Grading of Aggregates

Sieve Analysis: Determines the particle size distribution of aggregates.
Fineness Modulus (FM): An index number representing the average size of particles in an aggregate. Higher FM means coarser aggregate.
- Formula: $FM = 100 Sum of cumulative percentage retained on standard sieves$
- Standard Sieves for FM (mm): 80, 40, 20, 10, 4.75, 2.36, 1.18, 600 micron, 300 micron, 150 micron.

Deleterious Substances in Aggregates

Impurities like clay lumps, silt, organic impurities, and soft particles can adversely affect concrete strength, durability, and setting time.

Tests on Aggregates

Sieve Analysis
Specific Gravity and Water Absorption
Bulk Density
Crushing Value
Abrasion Value (Los Angeles Abrasion Test)
Impact Value
Soundness Test

GATE & SSC JE Numerical Tips: Aggregate Grading

Fineness Modulus Calculation: Practice calculating FM from given sieve analysis data. This is a common numerical question.
- Short Trick: Remember the standard sieve sizes. For a quick check, higher FM implies coarser aggregate, and lower FM implies finer aggregate.

Water

Water plays a crucial role in concrete, participating in the hydration reaction and providing workability.

Quality of Water for Concrete Mixing and Curing

Potable water (fit for drinking) is generally suitable.
Should be free from harmful amounts of acids, alkalis, oils, salts, sugar, and organic matter.
IS 456 recommends limits for various impurities.

Impurities and their Effects

Chlorides: Can cause corrosion of reinforcement.
Sulphates: Can lead to sulphate attack.
Organic Matter: Can interfere with hydration and reduce strength.
Silt and Suspended Solids: Can reduce bond and strength.

Admixtures

Admixtures are materials added to concrete (other than cement, water, and aggregates) to modify its properties in the fresh or hardened state.

Purpose of Admixtures

Improve workability without increasing water.
Reduce water content for a given workability.
Accelerate or retard setting time.
Improve durability.
Reduce bleeding or segregation.
Enhance strength.

Types of Admixtures

Plasticizers (Water Reducers): Reduce water requirement for a given slump or increase slump for a given water content.
Superplasticizers (High Range Water Reducers): More effective than plasticizers, allowing for very low water-cement ratios and high workability.
Retarders: Delay the setting time of concrete, useful in hot weather or for long hauls.
Accelerators: Speed up the setting and early strength development, useful in cold weather or for early formwork removal.
Air-entraining Agents: Introduce microscopic air bubbles, improving freeze-thaw resistance and workability, but slightly reducing strength.
Mineral Admixtures:
- Fly Ash: A by-product of coal combustion. Acts as a pozzolan, improving long-term strength, durability, and workability.
- Ground Granulated Blast Furnace Slag (GGBFS): A by-product of iron manufacturing. Improves durability, reduces heat of hydration, and enhances long-term strength.
- Silica Fume (Microsilica): A by-product of silicon/ferrosilicon alloy production. Extremely fine pozzolan, significantly improves strength, impermeability, and durability.

3. Properties of Fresh Concrete

Workability

Definition and Factors Affecting Workability

Workability is the ease with which concrete can be mixed, transported, placed, compacted, and finished without segregation or bleeding.
Factors Affecting Workability:

Water-Cement Ratio: Higher W/C ratio increases workability.
Aggregate Properties: Shape, texture, grading, and maximum size of aggregates. Rounded, smooth, well-graded aggregates improve workability.
Admixtures: Plasticizers and superplasticizers increase workability.
Temperature: Higher temperature reduces workability.
Time: Workability decreases with time due to hydration.

Measurement of Workability

Slump Test (IS 1199): Most common test. Measures the slump (reduction in height) of a conical concrete specimen. Suitable for medium to high workability.
- Slump values: Very low (0-25mm), Low (25-50mm), Medium (50-100mm), High (100-150mm).
Compacting Factor Test (IS 1199): Measures the degree of compaction achieved by a standard amount of work. Suitable for low to medium workability.
Vee-Bee Consistometer Test (IS 1199): Measures the time required to transform a concrete frustum into a cylindrical shape under vibration. Suitable for very low to low workability.
Flow Test (IS 1199): Measures the spread of concrete on a flow table after a certain number of drops. Suitable for high workability and self-compacting concrete.

GATE & SSC JE Numerical Tips: Workability Tests

Slump Test: Understand the typical slump ranges for different concrete applications (e.g., roads, beams/slabs, mass concrete). Questions often involve identifying suitable slump for a given application.
Compacting Factor: Remember the relationship between compacting factor and degree of compaction. For example, a compacting factor of 0.95 indicates very good workability.
Vee-Bee Time: Lower Vee-Bee time indicates higher workability.

Segregation and Bleeding

Segregation: Separation of the constituent materials of concrete, leading to a non-uniform mix. Can be due to coarse aggregate separating from mortar or cement paste separating from water.
- Causes: Excessive vibration, dropping concrete from a height, over-mixing, poor mix design (e.g., too much water).
- Effects: Honeycombing, reduced strength, increased permeability.
Bleeding (Water Gain): Tendency of water to rise to the surface of freshly placed concrete due to the settlement of heavier solid particles.
- Causes: High water-cement ratio, excessive fines in aggregates.
- Effects: Formation of a weak, porous layer (laitance) on the surface, reduced bond with reinforcement, increased permeability.
Prevention Methods: Proper mix design, controlled water-cement ratio, correct compaction, use of air-entraining agents or finer cement.

Setting Time

Initial Setting Time: The time elapsed from the moment water is added to the cement until the paste ceases to be fluid and workable. (Min. 30 minutes for OPC)
Final Setting Time: The time elapsed from the moment water is added to the cement until the paste has completely lost its plasticity and has attained sufficient firmness to resist certain pressure. (Max. 600 minutes for OPC)
Factors Affecting Setting Time: Water-cement ratio, fineness of cement, temperature, chemical composition of cement, presence of admixtures.

4. Properties of Hardened Concrete

Strength of Concrete

The most important property of hardened concrete.

Compressive Strength

Factors Affecting Compressive Strength:
- Water-Cement Ratio: Most significant factor. Lower W/C ratio (up to a limit) leads to higher strength.
- Degree of Compaction: Proper compaction removes air voids, increasing strength.
- Age of Concrete: Strength increases with age (most significant gain in first 28 days).
- Type and Quality of Cement: Higher grade cement gives higher strength.
- Type and Quality of Aggregates: Strong, well-graded aggregates contribute to higher strength.
- Curing Conditions: Proper curing (moisture and temperature) is essential for hydration and strength development.
- Temperature: Higher temperatures generally accelerate early strength gain but can reduce long-term strength if not controlled.
Cube vs. Cylinder Strength: Cube strength (tested on 150mm cubes) is generally higher than cylinder strength (tested on 150mm x 300mm cylinders) for the same concrete mix. This is due to the end restraint effect in cubes. (Cylinder strength $\approx 0.8 \times$ Cube strength).

Tensile Strength

Concrete is weak in tension.

Split Tensile Test: Measures the tensile strength by applying a compressive load along the diameter of a concrete cylinder, causing it to split.
Flexural Strength (Modulus of Rupture): Measures the tensile strength in bending using a concrete beam.
Relationship between Compressive and Tensile Strength: Tensile strength is typically 7-15% of the compressive strength. An approximate relationship is $f_{cr} = 0.7 f_{c k}$ (as per IS 456), where $f_{cr}$ is flexural strength and $f_{c k}$ is characteristic compressive strength.

GATE & SSC JE Numerical Tips: Strength Relationships

Cube vs. Cylinder Strength: Remember the approximate relation: $f_{cy l in d er} \approx 0.8 \times f_{c u b e}$ . This is frequently asked in objective questions.
Tensile Strength (Flexural Strength): The formula $f_{cr} = 0.7 f_{c k}$ is crucial for many numerical problems, especially in RCC design.
- Short Trick: For quick calculations, if $f_{c k}$ is a perfect square, it simplifies the calculation (e.g., for M25, $f_{cr} = 0.7 25 = 0.7 \times 5 = 3.5 MPa$ ).

Elasticity and Creep

Modulus of Elasticity ( $E_{c}$ ): A measure of concrete’s stiffness. It’s the ratio of stress to strain. For concrete, it’s not constant and depends on strength and age.
- As per IS 456, short-term static modulus of elasticity $E_{c} = 5000 f_{c k}$ .
Creep of Concrete: The increase in strain (deformation) in concrete over time under sustained stress. It’s a time-dependent deformation.
- Factors Affecting Creep: Magnitude of sustained stress, age of loading, duration of loading, water-cement ratio, aggregate content, ambient temperature and humidity.
- Effects of Creep: Increased deflection in beams, loss of prestress in prestressed concrete, redistribution of stresses.

GATE & SSC JE Numerical Tips: Modulus of Elasticity & Creep

Modulus of Elasticity: The formula $E_{c} = 5000 f_{c k}$ is very important for numerical problems related to concrete stiffness and deflection.
- Short Trick: Be careful with units. $f_{c k}$ is in MPa.
Creep Coefficient ( $θ$ ): Defined as the ratio of ultimate creep strain to elastic strain.
- $θ = Elastic Strain Ultimate Creep Strain$
- Values as per IS 456:
  - 7 days loading: 2.2
  - 28 days loading: 1.6
  - 1 year loading: 1.1
- Numerical Application: Creep strain = Elastic strain $\times θ$ . Total strain = Elastic strain + Creep strain. Questions often involve calculating total strain or creep strain given elastic strain and age of loading.

Shrinkage

Shrinkage is the volume change in concrete due to loss of moisture.

Types of Shrinkage:
- Plastic Shrinkage: Occurs in fresh concrete before setting, due to rapid evaporation of surface water. Causes surface cracks.
- Drying Shrinkage: Occurs in hardened concrete due to loss of moisture to the environment. Long-term phenomenon.
- Autogenous Shrinkage: Occurs due to hydration of cement, independent of moisture exchange with the environment. More significant in high-strength concrete with low W/C ratio.
- Carbonation Shrinkage: Occurs due to reaction of $C O_{2}$ from the atmosphere with $C a (O H)_{2}$ in hardened concrete.
Causes and Effects: Cracking, warping, and stress development.
Minimizing Shrinkage: Proper curing, low water-cement ratio, use of shrinkage-reducing admixtures, good aggregate grading.

Durability of Concrete

Durability is the ability of concrete to resist weathering action, chemical attack, abrasion, or any other process of deterioration.

Factors Affecting Durability:
- Permeability: The ease with which fluids can pass through concrete. Highly permeable concrete is less durable.
- Freeze-Thaw Attack: Damage caused by repeated freezing and thawing of water in concrete pores.
- Chemical Attack: Deterioration due to exposure to aggressive chemicals (e.g., sulphates, acids).
- Carbonation: Reaction of atmospheric $C O_{2}$ with $C a (O H)_{2}$ in concrete, reducing alkalinity and making reinforcement vulnerable to corrosion.
- Chloride Attack: Chlorides penetrate concrete, causing corrosion of steel reinforcement.
- Alkali-Aggregate Reaction (AAR): Reaction between alkalis in cement and certain reactive forms of silica in aggregates, causing expansive gels and cracking.
Measures to Improve Durability: Low water-cement ratio, adequate cement content, proper compaction, adequate cover to reinforcement, proper curing, use of durable aggregates, use of suitable admixtures (e.g., mineral admixtures, air-entraining agents), use of sulphate-resisting cement in aggressive environments.

5. Concrete Mix Design

Objectives of Mix Design

To determine the most economical and practical proportions of concrete constituents to produce concrete that meets specified requirements of:

Strength: Desired characteristic strength.
Workability: Sufficient for placement and compaction.
Durability: Resistance to environmental exposure.

Factors Influencing Mix Design

Characteristic strength required.
Type of cement and aggregate.
Maximum nominal size of aggregate.
Degree of quality control.
Type of exposure conditions.
Workability required.

Methods of Mix Design (IS Method – IS 10262)

The Indian Standard method (IS 10262) provides a detailed procedure for proportioning concrete mixes. It involves several steps:

Target Mean Strength ( $f_{c k}^{'}$ ): Calculated based on characteristic strength ( $f_{c k}$ ) and standard deviation ( $σ$ ) to account for variations: $f_{c k}^{'} = f_{c k} + 1.65 σ$ (for 5% defective).
- Standard Deviation ( $σ$ ) as per IS 456:
  - For M10, M15: 3.5 MPa
  - For M20, M25: 4.0 MPa
  - For M30, M35, M40, M45, M50: 5.0 MPa
Selection of Water-Cement Ratio: Based on target strength and durability requirements (from IS 456 tables).
Selection of Water Content: Based on maximum aggregate size and slump requirement.
Calculation of Cement Content: From water content and W/C ratio. Check minimum cement content for durability (from IS 456).
Proportion of Volume of Coarse and Fine Aggregates: Based on maximum aggregate size and zone of fine aggregate.
Mix Calculations: Determine the weights of each ingredient per cubic meter of concrete.
Trial Mixes and Adjustments: Prepare trial mixes, test properties, and adjust proportions if necessary.

Proportioning of Concrete Mixes

Nominal Mix: Fixed proportions of cement, sand, and aggregate (e.g., 1:2:4, 1:1.5:3). Used for minor concrete works where strength is not critical. Not recommended for RCC.
Standard Mix: Proportions specified by IS codes for certain grades (e.g., M5, M7.5, M10, M15, M20). For M20, the nominal mix is 1:1.5:3, but it’s often designed.
Designed Mix (Performance Based): Proportions determined by laboratory tests to achieve specific strength, workability, and durability requirements. Most common for important structures.

Water-Cement Ratio Law

Abrams’ Water-Cement Ratio Law states that for a given set of materials, the strength of concrete is inversely proportional to the water-cement ratio, provided the mix is workable. Lower W/C ratio generally leads to higher strength and durability.

Target Mean Strength

The average strength to be aimed for during mix design to ensure that the characteristic strength is achieved with a certain probability (usually 95%).

GATE & SSC JE Numerical Tips: Mix Design Basics

Target Mean Strength: Be able to calculate $f_{c k}^{'}$ using the standard deviation values provided in IS 456. This is a common numerical question.
Water-Cement Ratio: Understand the concept of effective water-cement ratio, considering water absorbed by aggregates.
Volume Calculations: Be prepared for problems involving calculating the quantities of materials (cement, sand, aggregate, water) for a given volume of concrete, often using specific gravity and bulk density.
- Short Trick for Dry Volume: For 1 $m^{3}$ of wet concrete, the dry volume of materials (cement, sand, aggregate) is approximately $1.54 m^{3}$ . This factor is useful for quick estimations in nominal mix problems.

6. Special Concretes

High Strength Concrete (HSC)

Concrete with compressive strength greater than 60 MPa. Achieved by low water-cement ratio, use of superplasticizers, silica fume, and high-quality aggregates.

High Performance Concrete (HPC)

Concrete that meets special performance and uniformity requirements that cannot be achieved by conventional concrete. It has enhanced strength, durability, and workability.

Self-Compacting Concrete (SCC)

Concrete that can flow and consolidate under its own weight without the need for external vibration, yet remains cohesive enough to prevent segregation. Achieved by special admixtures and high paste volume.

Lightweight Concrete

Concrete with a density significantly lower than normal concrete (typically $< 1920 kg/m^{3}$ ). Achieved by using lightweight aggregates (e.g., pumice, expanded clay) or by introducing air voids. Used for non-structural elements or where dead load reduction is critical.

Heavyweight Concrete

Concrete with a density significantly higher than normal concrete (typically $> 2400 kg/m^{3}$ ). Achieved by using heavy aggregates (e.g., barytes, magnetite, steel shot). Used for radiation shielding (e.g., nuclear power plants).

No-Fines Concrete

Concrete containing only cement and coarse aggregate, with no fine aggregate. Has high permeability and is used for drainage applications.

Roller Compacted Concrete (RCC)

A stiff, dry concrete mix compacted by vibrating rollers. Used primarily for dams and pavements.

Fibre Reinforced Concrete (FRC)

Concrete containing discrete, randomly oriented fibers (steel, glass, synthetic) to improve its tensile strength, ductility, and crack resistance.

Polymer Concrete

Concrete in which a polymer (e.g., epoxy, polyester) replaces cement as the binder. Offers very high strength, rapid hardening, and excellent chemical resistance.

Geopolymer Concrete

A type of concrete that uses industrial by-products (like fly ash or GGBFS) activated by an alkaline solution (e.g., sodium hydroxide and sodium silicate) instead of Portland cement. It has lower carbon footprint and good durability.

7. Quality Control of Concrete

Importance of Quality Control

Ensures that the concrete produced meets the specified requirements and maintains uniformity throughout the construction. Reduces variability and ensures safety and performance.

Statistical Quality Control

Uses statistical methods to monitor and control the quality of concrete. Involves calculating characteristic strength, standard deviation, and plotting control charts.

Acceptance Criteria

As per IS 456, the concrete is deemed to comply with the specified characteristic compressive strength if:

The mean of the results of four non-overlapping consecutive samples is greater than or equal to $f_{c k} + 0.825 σ$ .
Any individual test result is not less than $f_{c k} - 3 MPa$ .

GATE & SSC JE Numerical Tips: Quality Control & Acceptance

Acceptance Criteria: Understand and apply both conditions for acceptance. Numerical problems might give a series of test results and ask if the concrete is acceptable.
- Short Trick: For the second condition ( $f_{c k} - 3 MPa$ ), quickly check if any individual sample falls below this limit. For the first condition, calculate the mean of the latest four samples and compare it with $f_{c k} + 0.825 σ$ .

8. Testing of Concrete

Tests on Fresh Concrete

Slump Test: (Already discussed in Section 3) Measures workability.
Compacting Factor Test: (Already discussed in Section 3) Measures workability.
Vee-Bee Consistometer Test: (Already discussed in Section 3) Measures workability.
Flow Test: (Already discussed in Section 3) Measures workability.

Tests on Hardened Concrete

Compressive Strength Test: (Already discussed in Section 4) Most common test. Cubes (150mm) or cylinders (150mm x 300mm) are cast and tested at 7 and 28 days.
Split Tensile Strength Test: (Already discussed in Section 4) Measures tensile strength of concrete.
Flexural Strength Test: (Already discussed in Section 4) Measures the modulus of rupture using a concrete beam.
Non-Destructive Tests (NDT): Do not cause damage to the concrete structure.
- Rebound Hammer Test (IS 13311 Part 2): Measures the rebound of a spring-loaded hammer impacting the concrete surface. Provides an indication of surface hardness and strength.
- Ultrasonic Pulse Velocity (UPV) Test (IS 13311 Part 1): Measures the time taken for ultrasonic pulses to travel through concrete. Higher velocity indicates better quality and strength.
Core Cutting Test: Involves drilling out cylindrical core samples from the hardened concrete structure for compressive strength testing in the laboratory. Used when there is doubt about the quality of concrete in a structure.