Construction Material Note

CONCRETE TECHNOLOGY

1. Introduction

Concrete is a heterogeneous, multiphase material composed of a binding material called cement, along with aggregate particles. In simple terms:

1.1 Use of Concrete in Structures and Types of Concrete

Concrete is widely used in construction due to the following reasons:

Strength: It can bear high compressive forces.
Moldability: It can be shaped into desired forms when fresh.
Durability: It resists water, corrosion, and weathering.
Cost-Effectiveness: It is relatively cheap.
Availability: Materials like cement, aggregates, and water are locally available.
Workability: It is easy to work with and durable.

1.2 Different Types of Concrete

Concrete can be classified based on design, purpose, and binding materials.

Based on Design

Type	Description
Plain Cement Concrete (PCC)	No reinforcement, strong in compression, weak in tension.
Reinforced Cement Concrete (RCC)	Contains reinforcement, strong in both compression and tension.
Pre-stressed Cement Concrete	Stresses are applied before use, strong in both compression and tension.

Based on Purpose

Type	Description
Fibre Reinforced Concrete	Uses fibrous structures for reinforcement.
Lightweight Concrete	Uses lightweight aggregates to reduce overall weight.
Non-cracking Concrete	Used in structures prone to cracking.
Fire-resistant Concrete	Used in structures exposed to high temperatures.
Aerated Concrete	Contains aluminum as a mixture.
Chemical-resistant Concrete	Resists chemical attacks.
Heavyweight Concrete	Uses heavy aggregates for specific applications.
Vacuum Concrete	De-aired to increase strength and durability.

Based on Binding Material

Type	Description
Cement Concrete	Uses cement as the binding material.
Lime Concrete	Uses lime as the binding material.

1.3 Concrete Materials: Role of Different Materials

Concrete is made up of aggregates, cement, water, and admixtures.

Aggregates

Aggregates are chemically inert materials that occupy 70-80% of the concrete volume, significantly impacting its properties. They are classified based on weight and source.

Type	Description
Lightweight Aggregates	Used to reduce the weight of concrete.
Normal Weight Aggregates	Commonly used, further classified as natural or artificial.
Heavyweight Aggregates	Used for specific applications requiring high density.

Properties of Aggregates

Property	Description
Shape	Rounded (best for economy), irregular, angular, flaky (poor concrete if in excess).
Size	Important for workability and strength.
Texture	Smooth (less bonding) vs. rough (high bonding strength).
Bulk Density	Indicates how densely aggregates are packed.
Specific Gravity	Important for heavy and lightweight aggregates.
Bulking of Sand	Increase in sand volume due to moisture content.

Chemical Properties

Aggregates may react with alkali in cement, causing deterioration. Preventive measures include using low-alkali cement, minimizing void space, controlling moisture and temperature, choosing inert aggregates, and using admixtures like pozzolanas.

Mechanical Properties

Property	Description
Crushing Strength	Measures the compressive force an aggregate can bear.
Impact Value (Toughness)	Indicates the ability to resist repeated loads.
Abrasion Value	Resistance to wearing effects, tested by methods like Los Angeles test, Dorry Abrasion test, and Deval Attrition test.

Thermal Properties

Specific Heat and Thermal Conductivity: Important in mass concreting for temperature control and insulation properties.

Coefficient of Expansion: Interacts with the thermal expansion of cement paste in set concrete.

1.4 Grading of Aggregates

Grading ensures that a sample of aggregates contains all standard fractions in the required proportion, minimizing voids. Good grading results in durable and workable concrete.

Sieve Analysis

Sieve analysis divides a sample of aggregate into various fractions, each consisting of particles of the same size. This process helps achieve a well-graded aggregate sample.

Sieving the sample through a series of sieves arranged in order of size, with the largest sieve on top.
Material retained on each sieve represents the aggregate fraction coarser than the sieve below and finer than the sieve above.

1.5 Fineness Modulus of Aggregate

Fineness modulus is the summation of cumulative percentages of particles retained on standard sieves, divided by 100. It represents the average size of particles in the sample.

Fineness Modulus Value	Description
High Value	Indicates the presence of coarser particles.
Low Value	Indicates the presence of finer materials.

1.6 Alkali-Aggregate Reaction and Its Prevention

Mechanism of Concrete Deterioration by Alkali-Aggregate Reaction

Mixed water turns into a strong caustic solution.
This caustic solution attacks silica from the gel, causing a rapid reaction.
Continuous water supply and favorable temperature lead to more silica formation, resulting in cracking patterns.
These cracks cause strength and elasticity loss.
Dissolved carbon dioxide converts Ca(OH)₂ to CaCO₃, causing high volume change and further deterioration.

Preventive Measures

Measure	Description
Low Alkali Cement	Use cement with low alkali content.
Minimize Void Space	Ensure minimal void space in the concrete mix.
Control Moisture and Temperature	Maintain controlled moisture conditions and temperature.
Use Inert Aggregates	Choose aggregates that are chemically inert.
Use Admixtures	Incorporate admixtures like pozzolanas if necessary.

1.7 Cement: Manufacturing, Composition, and Properties

Types of Cement
Hydraulic Cement: Stable under water.
Non-Hydraulic Cement: Not water-resistant (e.g., Limestone).

1.71 Portland Cement

Portland cement is a hydraulic cement that sets and hardens under water. It’s made from calcium silicate and gypsum.

1.72 Manufacturing Process

Crushing and Proportioning: Limestone is crushed.
Raw Milling and Blending: Particles are ground and blended.
Pyro-Processing: Materials are heated in rotary kilns.
Burning and Cooling: Clinker is formed and cooled.
Cement Milling, Storage, and Packing: Gypsum is added to regulate setting time.

1.73 Composition of Portland Cement

Component	Chemical Formula	Common Name
Calcium Oxide	CaO	Lime
Silica	SiO₂	Silica
Alumina	Al₂O₃	Alumina
Iron Oxide	Fe₂O₃	Iron Oxide
Sulfate	SO₃	Sulfate
Water	H₂O	Water

1.74 Hydration Reactions

Compound	Reaction	Strength	Rate	Heat Liberation
C₃S	2C₃S + 6H → C₃S₂H₃ + 3CH	High	Moderate	High
C₂S	2C₂S + 4H → C₃S₂H₃ + CH	Low initially, high later	Slow	Low
C₃A	C₃A + 3CSH₂ + 26H → C₆AS₃H₃₂	Low	Fast	Very High
C₄AF	C₄AF + 10H + 2CH → C₄AFH₁₂	-	-	-

1.8 Physical Properties of Cement

Fineness: Affects hydration rate and durability.
Soundness: Ability to maintain volume after setting.
Consistency: Checked using Vicat plunger apparatus.
Setting Time: Initial set (time to lose plasticity) and final set (time to harden).
False Set and Flash Set: False set can be restored by mixing; flash set cannot.
Compressive Strength: Measured using 50 mm cubes.
Bulk Density: Varies from 830 kg/m³ to 1650 kg/m³.

1.9 Special Types of Cement

Type	Description	Uses
Rapid Hardening Cement	Gains strength quickly	Prefabricated construction, road repairs, cold weather
Sulphate Resisting Cement	Low C₃A and C₄AF	Marine conditions, sewage treatment works
Low Heat Cement	Emits low heat	Mass concreting of dams, bridges
Portland Pozzolana Cement	Contains 25% pozzolanic material	Hydraulic structures
Air Entraining Cement	Contains air-entraining agents	Snowy places to protect from freezing and thawing
Coloured Cement	Contains 5-10% colored pigment	Decoration
Masonry Cement	Used as mortar in brickwork	-
Antibacterial Cement	Contains antibacterial agents	Public pools
Waterproof Cement	Used in underground tanks and wet areas	-

1.10 Use of Water in Concrete

Water is used for:
Mixing concrete
Curing concrete
Cleaning aggregates
Increasing workability (carefully to avoid segregation)
Water used in concrete should have the following features:
1. Free from organic matter.
2. pH value between 6 and 8.
3. Drinking water may not always be suitable.
4. Free from carbonates and bicarbonates as they affect setting time.

1.11 Admixtures in Concrete

Admixtures are materials added to concrete before or during mixing to impart specific properties. They help improve workability, durability, and strength, and reduce permeability, segregation, and bleeding.

Types of Admixtures

I. Chemical Admixtures

Accelerators: Speed up the setting time.
Retarders: Slow down the setting time.
Plasticizers: Increase workability.
Super Plasticizers: Highly improve workability and strength.
Waterproofing Admixtures: Make concrete water-resistant.
Coloring Admixtures: Add color for decorative purposes.

II. Mineral Admixtures

Rice Husk Ash: High in silica, improves strength.
Surkhi: Made from powdered bricks, used for waterproofing.
Metakaoline: Competes with silica fume, enhances strength.
Silica Fume: Improves strength and durability, used in high-performance concrete.
Common Admixtures and Their Uses

Admixture Type	Description	Uses
Plasticizers	Improve workability without adding water	Mass concreting
Super Plasticizers	Advanced plasticizers for high strength	Self-leveling, high-strength concrete
Accelerating Plasticizers	Speed up strength development	Cold weather concreting
Retarding Plasticizers	Slow down setting time	Long transportation, hot weather concreting
Waterproofing Admixtures	Make concrete water-resistant	Hydraulic structures
Coloring Admixtures	Add color to concrete	Decorative purposes

Uses of Mineral Admixtures
1. Increase water tightness.
2. Decrease heat of hydration.
3. Reduce thermal shrinkage.
4. Improve workability and durability.
5. Minimize alkali-aggregate reaction.
6. Reduce segregation and bleeding.

1.12 Concrete vs. Steel as Structural Materials

Feature	Concrete	Steel
Temperature Resistance	High	Lower
Maintenance	Low	Higher
Permeability	Low	Higher
Durability	High	Moderate
Recyclability	Moderate	High
Toxicity	Higher	Lower

1.13 Mechanical Properties of Aggregates

Property	Description
Crushing Strength	Measures compressive force aggregates can withstand.
Impact Value (Toughness)	Ability to resist repeated loads.
Abrasion Value	Resistance to wearing effects.

1.14 Stress-Strain Behavior of Concrete

Concrete exhibits a non-linear stress-strain relationship due to microcracks. The largest cracks appear at ultimate stress, leading to strength and elasticity loss.

Non-Linear Behavior of Concrete

Concrete’s behavior is non-linear due to the formation of microcracks. These microcracks form because of:

Differential movement between the aggregate and cement phases.
Internal stresses within the cement paste.
Changes in temperature and moisture.

2. MAIN COMPOUNDS OF CEMENT AND THEIR ROLES IN STRENGTH DEVELOPMENT

2.1 The main compounds in cement that contribute to strength development are:

Compound	Role in Strength Development
Tricalcium Silicate (C₃S)	Reacts quickly, providing early strength and generating heat.
Dicalcium Silicate (C₂S)	Reacts slowly, contributing to long-term strength.
Tricalcium Aluminate (C₃A)	Reacts quickly, causing initial stiffening (flash set).
Tetracalcium Aluminoferrite (C₄AF)	Hydrates rapidly, releasing heat but does not significantly contribute to strength.

2.2 Grades of Cement

Ordinary Portland Cement (OPC) is classified into grades based on its compressive strength:

Grade	Use
33 Grade	General construction like plastering and finishing.
43 Grade	Structural works and precast items.
53 Grade	High-strength applications like multistory buildings.

2.3 Structure of Concrete

Concrete is a multiphase and heterogeneous material, consisting of:

Macrostructure: Visible mix of cement paste and aggregates.
Microstructure: Microscopic view of the macrostructure.

Concrete has three phases:

Aggregate Phase: Includes particles of varying shapes and sizes.
Binding Medium Phase: Incoherent mass of hydrated cement paste.
Transition Phase: Interfacial region between coarse aggregates and hydrated cement paste.

2.4 Hydrated Cement Paste (HCP)

Hydration of cement is a gradual process, forming different products that contribute to the properties of concrete:

Phase	Description
Calcium Silicate Hydrate (C-S-H)	Poorly crystalline, fibrous mass that forms the main strength-giving component.
Calcium Hydroxide (Ca(OH)₂)	Hexagonal crystals that maintain the pH and prevent corrosion but can react with sulfates.
Calcium Sulpho-Aluminate	Forms hexagonal crystals, contributing to the volume of hydrated paste.
Unhydrated Clinker Grains	Dense particles that depend on the size of cement particles and degree of hydration.

2.5 Voids in Hydrated Cement Paste

Interlayer Spacing of C-S-H: Very small voids that can cause shrinkage when water is removed.
Capillary Voids: Larger voids that affect the strength and durability of concrete. Spaces without solid components of HCP.

Influence: Water-cement ratio affects the size and amount of these voids, impacting concrete strength.

2.51 Types:

Large Voids (>50 nm): Affect concrete strength.

Small Voids (<50 nm): Cause shrinkage.

2.52 Air Voids: Formed when air gets trapped in fresh concrete.

Types:

Entrained Air Voids: Spherical, 50-200 micrometers in size.
Entrapped Air Voids: Larger and irregular, negatively affecting strength.

2.53 Water in HCP

Capillary Water: Present in voids larger than 2.5 nm, affecting strength and shrinkage.
Absorbed Water: Held on the surface of solids, causing shrinkage.
Interlayer Water: Associated with C-S-H structure, lost when dried, causing shrinkage.

2.54 Stability, Durability, and Strength of HCP

Stability: HCP is dimensionally unstable, held by weak van der Waals forces.
Strength: Porosity affects strength; higher porosity means lower strength.
Durability: Affected by permeability and environmental exposure. Acidic environments can deteriorate HCP.

2.55 Transition Zone in Concrete

The transition zone is a critical phase in concrete, often considered the strength-limiting phase. It is characterized by poor-quality paste and the development of microcracks due to shrinkage and temperature variations.

Significance:

Concrete is brittle in tension and tough in compression.
This phase is weak and prone to permeability with increased water-cement ratio.

Structure:

No proper structure; more porous near coarse aggregates.
Higher water-cement ratio around larger aggregates.

Strength:

Micro cracks develop due to shrinkage and temperature changes.
Water films around large aggregates contribute to crack formation under tension.

2.56 Effects of Transition Zone on Concrete Properties

Strength Limitation: Transition zone is the weakest phase.
Micro cracks: Develop due to shrinkage and temperature variations.
Hydration Strength: Decreases with larger coarse aggregate size and higher water-cement ratio.
Behavior: Concrete is brittle in tension and tough in compression.

2.57 Workability of Concrete

Workability describes the ease with which concrete can be mixed, transported, and placed. It is influenced by the water content and consistency of the mix.

2.571 Importance: Ensures uniform composition and ease of handling.

2.572 Tests:

Slump Test: Measures the consistency of fresh concrete using a slump cone.

2.573 Slump Test for Workability

The slump test measures the consistency of fresh concrete. Here’s how to perform it:

Mix Ingredients: Thoroughly mix the dry ingredients until a uniform color is achieved, then add the required amount of water.
Fill the Mould: Place the mixed concrete into the slump cone mould in four layers, each about one-fourth of the mould’s height.
Compact Each Layer: Compact each layer 25 times with a tamping rod.
Strike Off the Top: Level the top surface with a trowel or tamping rod.
Remove the Mould: Lift the mould vertically and measure the subsidence (slump) of the concrete in millimeters.

2.574 Types of Slump:

1 True Slump: General reduction in height without breaking up.

2 Shear Slump: Indicates lack of cohesion, not suitable for placement.

3 Collapse Slump: Indicates a very wet mix.

2.575 Compaction Test

The compaction test measures the workability of concrete by determining the compaction factor.

Procedure:

1 Fill Upper Hopper: Place the concrete sample in the upper hopper and level it.

2 Transfer to Lower Hopper: Open the trap door to let the concrete fall into the lower hopper.

3 Transfer to Cylinder: Open the trap door of the lower hopper to let the concrete fall into the cylinder.

4 Weigh the Cylinder: Weigh the cylinder with partially compacted concrete ().

5 Fully Compact the Concrete: Refill the cylinder with the same concrete in layers, compacting each layer fully.

6 Weigh Again: Weigh the cylinder with fully compacted concrete ().

7 Compaction Factor:

2.576 Vebe Test

The Vebe test measures the remolding ability of concrete under vibration.

Procedure:

1 Fill Slump Cone: Place a slump cone in the center of a cylindrical container and fill it with concrete.

2 Remove Cone: Remove the slump cone and place a clear plastic disk on top.

3 Vibrate: Start the Vebe table and measure the time for the concrete to remold to the shape of the container.

2.577 Flow Test

The flow test measures the spread of concrete to assess its workability.

Procedure:

1 Fill Mould: Place a frustum cone mould on a flow table and fill it with concrete in two layers, compacting each layer.

2 Lift Mould: Lift the mould and jolt the table 15 times.

3 Measure Spread: Measure the average diameter of the spread concrete.

2.578 Factors Affecting Workability

1 Water Content: More water increases workability.

2 Mix Proportions: Higher aggregate/cement ratio decreases workability.

3 Aggregate Size: Larger aggregates increase workability.

4 Aggregate Shape: Rounded aggregates provide higher workability.

5 Surface Texture: Smooth aggregates improve workability.

6 Grading of Aggregates: Better grading increases workability.

7 Admixtures: Improve workability (e.g., air-entraining agents, super-plasticizers).

8 Cement Content: Higher cement content increases workability.

9 Temperature and Time: Higher temperatures and longer times decrease workability.

2.6 Water-Cement Ratio

The water-cement (W/C) ratio is the ratio of the weight or volume of water to cement in a concrete mix. It affects the strength and durability of concrete.

Example:

If 20 kg of water is used per 50 kg bag of cement, the W/C ratio is 0.4 by weight.

Abram’s Law:

( S ): Strength of concrete

( A ): Empirical constant

( B ): Constant depending on cement properties

( x ): Water-cement ratio by volume

2.7 Nominal Mix

Mix concrete generally refers to the proportions of cement, sand, and coarse aggregate in a mix. These proportions can be either adopted from standards or designed rationally. A concrete mix with adopted proportions is called a nominal mix. However, there is no guarantee that a nominal mix will achieve the desired strength or be economical. Design mix concrete is preferred over nominal mix concrete. Nominal mix may be used for grades of M20 or lower if design mix concrete cannot be used for some reason.

2.71 Proportions for Nominal Mix Concrete

Grade of Concrete	Total Quantity of Dry Aggregates by Mass per 50 kg of Cement (Kg)	Max Proportion of Fine Aggregate to Coarse Aggregate (by Mass)	Quantity of Water per 50 kg of Cement (Max)
M5	800	Generally 1:2 but subject to an upper limit of 1:1 and a lower limit of 1:2	60
M7.5	625		45
M10	450		34
M15	330		32
M20	250		30

2.72 Nominal Mix Proportions

Grade of Concrete	Nominal Mix Proportion (Cement: Sand: Aggregate)
M5	1:5:10
M7.5	1:4:8
M10	1:3:6
M15	1:2:4
M20	1:1.5:3

Note: The proportions shown in the table are by weight.

2.73 Probabilistic Concept in Mix Design

The strength of concrete depends on the quality of materials and construction methods used. Variability in these factors causes variation in strength. Major sources of variability include:

Variation in the quality of constituent materials.

1 Variation in mix proportions due to the batching process.

2 Variation in the quality of batching and mixing equipment.

3 Quality of supervision and workmanship.

4 Variation due to sampling and testing of concrete specimens.

These variations reduce the strength of concrete. To mitigate this, we must understand the statistical concepts underlying such variations and provide suitable safeguards. Before construction, several sample test cubes are prepared and their strengths are measured. The test results are plotted as a histogram, which approximates a normal distribution curve.

2.74 Normal Distribution Curve

The normal distribution curve is symmetrical about its mean and is specified by two parameters: mean strength and standard deviation.

Characteristic Strength: The value of the strength below which not more than 5% of the test results are expected to fall.

Target Mean Strength: Considering the inherent variability of concrete strength, the mix is designed to have a target mean strength greater than the characteristic strength by a suitable margin.

2.75 Concrete Mix Design by DOE, ACI, and IS Methods

Mix design is the process of selecting suitable ingredients of concrete and determining their relative proportions to produce concrete of certain minimum strength and durability as economically as possible.

Information Required for Mix Design

1 Grade designation

2 Type of cement

3 Maximum nominal size of aggregate

4 Minimum cement content

5 Maximum water-cement ratio

6 Workability

7 Exposure conditions

8 Maximum temperature of concrete at the time of placing

9 Method of placing

10 Degree of supervision

2.76 DOE Method of Concrete Mix Design

I. Calculate the Target Mean Strength: Where (S ) is the standard deviation (3.5 MPa for M10-M15, 4 MPa for M20-M25, 5 MPa for M30-M50).

II. Calculate the Water-Cement Ratio:

This is done in two steps:

1 Determine the water-cement ratio from the required strength.

2 Check the water-cement ratio from durability considerations and adopt the lower value.

3 Select the Water Content:

4 Depending on the type and maximum size of aggregate, select the water content to achieve the specified slump or Vee-Bee time.

III. Calculate the Cement Content:

Check the minimum cement content from durability considerations and adopt the minimum value.

2.77 Simplified Concrete Mix Design Guide

1. Aggregate Content

Total Aggregate Content: Calculate by subtracting the cement and water content from the wet density of concrete.

Wet Density: Refer to the appropriate figure for values based on water content and relative density of combined aggregate.

Assumed Specific Gravity:

Crushed Aggregates: 2.7
Uncrushed Aggregates: 2.6
1. 2. Proportions of Fine and Coarse Aggregates

Factors: Water-cement ratio, maximum size of aggregate, workability level, and grading zone of fine aggregate.

Assumptions:

Mean Strength: 30 MPa
Cement Type: Ordinary Portland Cement (OPC) with uncrushed coarse aggregate
Compressive Strength: 42 MPa (from standard tables)
Water-Cement Ratio: 0.60 (considering durability)
Slump: 75 mm
Water Content: 195 kg/m³ (from standard tables)

3. Calculations

Cement Content: Derived from the water content and water-cement ratio.
Wet Density: Determined based on specific gravity and free water content.
Total Aggregate Content: Calculated by subtracting the cement and water content from the wet density.

4. Fine and Coarse Aggregate Proportions

Fine Aggregate (F.A.): Typically a percentage of the total aggregate content.
Coarse Aggregate (C.A.): The remaining portion after accounting for fine aggregate.

5. Estimated Quantities

1. Cement

2. Fine Aggregate

3. Coarse Aggregate

4. Water

6. Additional Considerations

1. Durability: Ensure the water-cement ratio meets durability requirements.

2. Workability: Adjust slump and water content based on specific project needs.

2.8 Segregation and Bleeding

Process by which well-mixed fresh concrete becomes non-uniform.

2.81 Types of Segregation

Internal Segregation: Coarse or heavy aggregates settle at the bottom, while lighter or finer ones rise to the top during compaction.
External Segregation: Caused by external forces such as improper handling, compaction errors, or inadequate cement cohesiveness.

2.82 Factors Affecting Segregation

1 Large Aggregate Size: Aggregates over 25 mm tend to segregate more.

2 High Quantity of Large Particles: Leads to uneven distribution.

3 High Specific Gravity of Coarse Aggregate: Compared to fine aggregate.

4 Low Fine Fraction in Sand: Insufficient fines can cause segregation.

5 Low Cement Content: Reduces cohesiveness.

6 Unfavorable Aggregate Shape: Irregular shapes can cause segregation.

7 Moisture Content: Both too dry and too wet concrete can lead to segregation.

2.83 Causes of Segregation

1 Poorly graded aggregate and excessive water content.

2 Badly proportioned mix.

3 Insufficiently mixed concrete.

4 High height of dropping.

5 Badly designed mixer.

6 Concreting under water.

7 Concreting in heavily reinforced concrete members.

2.84 Effects of Segregation

1 Decreases the strength of concrete.

2 Results in a non-homogeneous mass.

3 Causes rock pockets, sand streaks, and porous layers in hardened concrete.

4 Excess mortar rises to the surface, causing plastic shrinkage cracks.

2.85 Prevention of Segregation

1 Correctly proportioning the mix.

2 Proper handling, transporting, placing, compacting, and finishing.

3 Remixing if segregation is observed.

4 Use of workability agents.

2.86 Bleeding in Concrete

Bleeding: The appearance of water on the surface of concrete after it has consolidated but before it has set. This is a special form of segregation where mixing water, being the lightest component, rises to the surface.

2.87 Causes of Bleeding

1 Lack of fines.

2 Excessive water content in the mix.

3 Poorly graded aggregate.

2.88 Effects of Bleeding

1 Loss of homogeneity.

2 Increased permeability in concrete.

3 Reduced bond between aggregate and cement paste, decreasing the strength of concrete.

4 Reduced bond between reinforcement and concrete.

5 Formation of “Laitance” (cement paste on the surface), which reduces wearing quality.

2.89 Key Factors and Effects of Segregation and Bleeding

Aspect	Segregation	Bleeding
Types	Internal, External	N/A
Factors	Aggregate size, particle quantity, specific gravity, fine fraction, cement content, aggregate shape, moisture content	Lack of fines, excessive water, poorly graded aggregate
Causes	Poor grading, excessive water, bad mix, insufficient mixing, high drop height, bad mixer, underwater concreting, heavily reinforced members	Lack of fines, excessive water, poorly graded aggregate
Effects	Decreased strength, non-homogeneous mass, rock pockets, sand streaks, porous layers, plastic shrinkage cracks	Loss of homogeneity, increased permeability, reduced bond strength, laitance formation
Prevention	Proper mix proportioning, handling, transporting, placing, compacting, finishing, remixing, workability agents	Proper mix design, adequate fines, controlled water content

2.810 Quality Control in Concrete Construction

Key Areas of Quality Control

Mixing:

1 Use mechanical mixers with water measuring devices.

2 Ensure uniform distribution of materials.

3 Check workability frequently.

Handling:

1 Transport concrete using appropriate methods to avoid segregation.

2 Avoid bad handling methods that promote segregation.

Placing:

1 Deposit concrete as close to its final position as possible.

2 Avoid rehandling and ensure proper compaction before initial setting.

Compaction:

1 Use mechanical vibrators.

2 Avoid over-vibration and under-vibration.

3 Ensure thorough compaction around reinforcement and formwork.

Curing:

1 Prevent moisture loss and control the temperature of concrete.

2 Use methods like wet gunny bags, hessian cloth, and frequent water addition.

3 Concrete in Extreme Temperatures

2.9 Hot Weather Concreting

Effects:

1 Accelerated setting.

2 Reduced strength due to evaporation.

3 Increased tendency to crack.

4 Rapid evaporation during curing.

Precautions:

1 Control the temperature of concrete ingredients.

2 Use shade, cold water, and proper mix design.

3 Maintain low temperatures during production and delivery.

4 Protect and cure concrete properly.

5 Cold Weather Concreting

Effects:

1 Delayed setting.

2 Freezing of concrete at early stages.

3 Stresses due to temperature differentials.

Precautions:

1 Control the temperature of aggregates.

2 Use methods to keep concrete temperature above the permissible minimum.

2.10 Controlling Concrete Temperature:

1. Heating Aggregates and Mixing Water: Maintain the temperature of ingredients as high as practicable by heating aggregates and mixing water.
Insulating Formwork:

Heat Conservation: Use insulating formwork covers to conserve heat generated during cement hydration, maintaining concrete temperature above desirable limits for the first 3 to 7 days, even in lower ambient temperatures.

Proportioning of Concrete Ingredients:

Cement Quantity: Increase the quantity of ordinary Portland cement, rapid hardening Portland cement, or use accelerating admixtures to achieve required strength faster.

Air Entraining Agents: Recommended for use in cold weather to improve workability and durability.

Placement, Protection, and Curing:

Surface Preparation: Remove all ice, snow, and frost before placing concrete. Ensure the surface and steel are sufficiently warm.

Temperature Control: For conditions below -1°C, the concrete should be mixed at 15.5°C and placed at 10°C. Water curing is not necessary during freezing or near-freezing conditions.

Delayed Removal of Formwork:

Extended Duration: Due to slower strength gain in cold weather, formwork and props should

Aspect	Description
Heating Aggregates	Maintain high temperature by heating aggregates and mixing water.
Insulating Formwork	Use covers to conserve heat during hydration for up to 7 days.
Cement Quantity	Increase cement or use rapid hardening cement/accelerating admixtures.
Air Entraining Agents	Recommended for cold weather to improve concrete properties.
Surface Preparation	Remove ice, snow, and frost; ensure surfaces are warm before placing concrete.
Temperature Control	Mix at 15.5°C and place at 10°C for conditions below -1°C.
Formwork Removal	Keep formwork and props in place longer due to slower strength gain.

3. PROPERTIES OF HARDENED CONCRETE

3.1 Deformation of Hardened Concrete

Concrete deforms due to various strains, which can lead to cracking. Understanding these strains is crucial for calculating deformation and deflection in structural members.

Types of Strains:

Type	Description
Elastic Strains	Instantaneous deformations when an external stress is first applied.
Shrinkage Strains	Deformations due to moisture loss or cooling of concrete.
Creep Strain	Time-dependent deformation under prolonged stress.
Thermal Strain	Deformation due to temperature changes.

3.2 Stress-Strain Relationship:

1 The stress-strain curve for hardened cement paste is almost linear.

2 Aggregates are more rigid than cement paste, leading to lower strain under the same stress.

3 Concrete does not return to its original length upon unloading due to creep and micro-cracking, resulting in residual strain (hysteresis loop).

3.3 Micro-Cracking:

1. Micro-cracks form at the interface between aggregate particles and cement paste due to differential movement.

2. Cracks also form within the cement paste due to temperature and moisture changes and load application.

3.4 Stages of Concrete Behavior:

Stage	Description
25-30% Ultimate Strength	Random cracking in the transition zone around large aggregates.
50% Ultimate Strength	Stable crack growth from the transition zone into the paste; micro-cracks develop.
75% Ultimate Strength	Paste and bond cracks join to form major cracks; major cracks grow while smaller cracks close.
Ultimate Load	Major cracks link up vertically, leading to failure.

3.5 Modulus of Elasticity

The modulus of elasticity measures the stiffness or resistance to deformation of concrete. It depends on the rate of stress application and is valid only for the elastic portion of the stress-strain curve.

Types of Modulus of Elasticity:

Type	Description
Initial Tangent Modulus	Slope of a line tangent to the stress-strain curve at the origin.
Tangent Modulus	Slope of a line tangent to the stress-strain curve at any point.
Secant Modulus	Slope of a line from the origin to a point on the curve corresponding to 40% of the failure stress.
Chord Modulus	Slope of a line between two points on the stress-strain curve.

Dynamic Modulus of Elasticity:

Represents the elastic behavior of concrete more accurately than the static modulus due to the phenomenon of creep. Determined by subjecting the concrete member to longitudinal vibration at its natural frequency.

Formula: Where:

= Dynamic modulus of elasticity

= Constant

= Resonant frequency

= Length of specimen

3.6 Shrinkage

Shrinkage is the reduction in the volume of freshly hardened concrete due to exposure to ambient temperature and humidity. It causes volumetric strain, which is three times the linear strain.

3.61 Types of Shrinkage:

1. Plastic Shrinkage: Occurs due to water evaporation from freshly placed concrete while the cement paste is still plastic.

2. Drying Shrinkage: Happens due to water evaporation from hardened concrete exposed to air.

3. Autogenous Shrinkage: Results from self-desiccation during the hydration of concrete with a low water-cement ratio.

4. Thermal Shrinkage: Caused by a significant drop in temperature.

5. Carbonation Shrinkage: Occurs when carbon dioxide in the atmosphere reacts with calcium hydroxide in the concrete, forming calcium carbonate.

3.62 Factors Influencing Shrinkage:

1 Cement Paste and Aggregate Content: Shrinkage is induced by the cement paste but restrained by the aggregate.

2 Type of Aggregate: Lightweight aggregates lead to higher shrinkage.

3 Water/Cement Ratio: Higher ratios result in larger shrinkage.

4 Relative Humidity: Lower humidity increases shrinkage.

5 Time: Shrinkage occurs over long periods, with drying shrinkage happening early and carbonation shrinkage over longer periods.

6 Size and Shape of Concrete Member: Shrinkage is affected by the volume-to-surface ratio.

3.7 Creep

Creep is the increase in strain under a sustained constant stress, considering other time-dependent stresses like shrinkage and swelling.

3.71 Factors Affecting Creep:

1 Water/Cement Ratio: Higher ratios increase creep.

2 Aggregate Stiffness: Stiffer aggregates reduce creep.

3 Aggregate Fraction: Higher aggregate content reduces creep.

4 Theoretical Thickness: Higher thickness reduces creep and shrinkage.

5 Temperature: Higher temperatures increase both the rate and ultimate creep.

6 Humidity: Higher humidity reduces creep.

7 Age of Concrete at Loading: Older concrete at the time of loading creeps less.

3.8 Key Factors and Effects of Shrinkage and Creep

Aspect	Shrinkage	Creep
Types	Plastic, Drying, Autogenous, Thermal, Carbonation	N/A
Factors	Cement paste content, aggregate type, water/cement ratio, humidity, time, size and shape	Water/cement ratio, aggregate stiffness, aggregate fraction, theoretical thickness, temperature, humidity, age at loading
Effects	Volumetric strain, cracking, reduced durability	Increased strain, reduced strength, residual strain after unloading

3.9 Fatigue, Impact, and Dynamic Loading

Fatigue: Caused by repeated application of stress to the concrete, leading to failure by fracture under cyclic stress.

3.91 Types of Fatigue Failure:

1 Static Failure: Occurs under a sustained or slowly increasing load, known as static fatigue or creep rupture failure.

2 Simple Fatigue Failure: Occurs under cyclic or repeated loading.

3.92 Effect of Porosity, Water/Cement Ratio, and Aggregate Size

Porosity:

1 Higher porosity decreases the strength and increases the permeability of concrete.

2 Factors affecting porosity include water/cement ratio, degree of hydration, air content, consolidation, mineral admixtures, and aggregate type.

Water/Cement Ratio:

The ratio of water to cement affects the strength and workability of concrete.

Lower ratios increase strength, especially when concrete is vibrated.

Aggregate Size:

1 Larger aggregates result in lower strength due to less surface area for gel bonds and increased heterogeneity.

2 Smaller aggregates provide more uniform load distribution and higher strength.

4. Testing of Concrete and Quality Control

4.1 Various Strengths of Concrete

Tensile Strength:

1 Concrete is weak in tension and develops cracks under tensile forces.

2 Tensile strength is negligible compared to compressive strength.

3 Formula:

Compressive Strength:

1 Concrete has high compressive strength, which is crucial for most structural designs.

2 Compressive strength is easy to measure and relates to many properties like elasticity, impermeability, and resistance to wear and fire.

Shear Strength:

1 Shear stress arises from a combination of tensile and compressive stresses or torsion.

2 Shear strength is about 12% of compressive strength.

3 Resisted by uncracked concrete, aggregate interlocking, and shear across longitudinal steel in reinforced concrete.

Bond Strength:

1 Resistance to slip of steel reinforcement bars embedded in concrete.

2 Higher grade concrete and deformed bars increase bond strength.

3 Bond strength is due to adhesion, frictional resistance, and mechanical resistance.

4.2 Compressive Strength Tests

Cube Test:

1 Specimens are cast in 150 mm x 150 mm x 150 mm molds.

2 Cured for 28 days and tested at various intervals.

3 Compressive strength is calculated as the force at failure divided by the area.

Cylinder Test:

1 Specimens are cast in cylindrical molds (150 mm diameter, 300 mm height).

2 Cured for 28 days and tested at various intervals.

3 Compressive strength is calculated similarly to the cube test.

4.3 Tensile Strength Tests

Flexure Test:

1 Measures tensile strength under bending.

2 Maximum tensile stress in the bottom fiber of the test beam is known as the modulus of rupture.

Splitting Test (Brazilian Test):

1 A concrete cylinder is placed horizontally between platens of a testing machine.

2 Load is increased until failure occurs by splitting.

3 Tensile splitting strength is calculated using the maximum load, length, and diameter of the specimen.

Variability of Concrete Strength

Sources of Variability:

1 Quality of constituent materials.

2 Mix proportions due to batching process.

3 Quality of batching and mixing equipment.

4 Supervision and workmanship.

5 Sampling and testing of concrete specimens.

Acceptance Criteria:

1 For compressive strength: Mean strength from four consecutive tests must meet specified limits.

2 For flexural strength: Mean strength from four consecutive tests must exceed specified characteristic strength by at least 0.3 N/mm².

4.4 Non-Destructive Tests of Concrete

Objectives:

1 Determine the strength of concrete.

2 Detect cracks, delamination, or voids.

3 Measure thickness and permeability.

4 Assess corrosion of reinforcement.

5 Check carbonation or chloride penetration.

6 Identify aggregate alkali reaction.

Strength Type	Description	Importance
Tensile	Weak in tension, develops cracks, negligible compared to compressive strength	Necessary to determine for structural integrity
Compressive	High compressive strength, easy to measure, relates to many properties	Crucial for structural design and performance
Shear	Arises from tensile and compressive stresses or torsion, about 12% of compressive strength	Important for resisting combined stresses and torsion
Bond	Resistance to slip of steel reinforcement, increased by higher grade concrete and deformed bars	Essential for the durability and strength of reinforced concrete structures

4.5 Common Non-Destructive Tests:

Schmidt Rebound Hammer:

1 Measures surface hardness and provides an estimate of concrete strength.

2 Advantages: Easy, cheap, robust, and requires minimal skill.

3 Disadvantages: Not directly related to strength, measures only surface strength, and data interpretation can be difficult.

Ultrasonic Pulse Velocity Method:

1 Measures the travel time of ultrasonic waves through concrete to assess quality.

2 Advantages: Can determine modulus of elasticity and check concrete quality.

3 Disadvantages: Requires specialized equipment and interpretation of results.

5. Concrete Durability

5.1 Durability is the ability of concrete to last a long time without significant deterioration. It involves resisting weathering, chemical attack, and abrasion while maintaining desired engineering properties.

5.2 Effect of Water and Permeability on Concrete Durability

Permeability:

1 Permeability is the ease with which liquids or gases can travel through concrete.

2 It affects the watertightness of structures and resistance to chemical attack.

3 Concrete is porous, allowing moisture movement by flow, diffusion, or absorption.

4 Permeability is governed by the porosity of the cement phase and the water-cement ratio.

Reducing Permeability:

1 Use a mix with a low water-cement ratio.

2 Use well-graded aggregates for dense concrete.

3 Ensure longer curing durations.

5.3 Water as an Agent of Deterioration:

Water enters permeable pores and cavities, causing chemical decomposition and reducing durability.

5.4 Physical and Chemical Causes of Concrete Deterioration

Physical Causes:

1 Abrasion: Wear on pavements and industrial floors by vehicular traffic.

2 Erosion: Wear by abrasive action of fluids containing solid particles.

3 Cavitation: Loss of mass due to vapor bubble formation and collapse in rapidly flowing water.

Chemical Causes:
Sulfate Attack:

1 Sulfates in soil, groundwater, seawater, and industrial processes attack concrete.

2 Damage starts at edges and corners, leading to cracking and spalling.

3 Factors: Sulfate concentration, water table level, groundwater flow, and soil porosity.

Attack by Sea Water:

1 Sea water contains sulfates but does not generally cause expansion.

2 Concrete between tide marks is severely attacked due to alternating wetting and drying.

Alkali-Aggregate Reaction (AAR):

1 Chemical reaction between alkali in cement and reactive siliceous materials in aggregate.

2 Factors: Alkali content, admixtures, aggregate reactivity, and environmental conditions.

Acid Attack:

1 Acids in the atmosphere or industrial environments dissolve and weaken concrete.

2 Specifications: Cement content (320-400 kg/m³), water-cement ratio (0.4-0.5), surface protection, and sacrificial layers.

Carbonation

5.1 Process:

1 CO₂ from the air penetrates concrete and reacts with calcium hydroxide to form calcium carbonate, causing shrinkage.

2 Carbonation reduces the alkalinity of pore water, leading to corrosion of steel reinforcement.

5.2 Factors Influencing Carbonation:

1 Higher CO₂ concentration increases carbonation.

2 More carbonation occurs in urban areas and tunnels.

3 The pH value of pore water in hardened concrete is between 12.5 to 13.5, which prevents steel corrosion.

5.3 Rate of Carbonation:

1 Relative Humidity: High rate at 50-70% relative humidity.

2 Grade of Concrete: Slower in stronger concrete.

3 Permeability: Higher in more permeable concrete.

4 Protection: Higher in unprotected concrete.

5 Depth of Cover: Higher in concrete with less cover.

6 Time: Increases with time.

5.4 Corrosion of Steel in Concrete

5.41 Causes:

1 Carbonation: Reduces alkalinity, leading to corrosion.

2 Electro-Chemical Action: Occurs when dissimilar metals are in contact in the presence of moisture and oxygen.

5.42 Electrochemical Process:

Anodic Reaction:

Cathodic Reaction:

☺Formation of rust increases the volume of steel, causing cracking and spalling.

5.43 Preventive Measures:

1 Provide adequate cover to reinforcing bars.

2 Limit chloride content in cement.

3 Use galvanized steel reinforcement.

4 Apply protective coatings to concrete.

5 Avoid using sea water for curing.

6 Use slag cement or Portland pozzolana cement.

5.44 Key Factors and Effects of Concrete Deterioration

Aspect	Physical Causes	Chemical Causes
Types	Abrasion, Erosion, Cavitation	Sulfate Attack, Sea Water Attack, Alkali-Aggregate Reaction, Acid Attack, Carbonation
Factors	Traffic, fluid abrasion, vapor bubble formation	Sulfate concentration, water table level, groundwater flow, soil porosity, CO₂ concentration
Effects	Surface wear, cracking, mass loss	Cracking, spalling, expansion, strength loss, corrosion of reinforcement
Prevention	Use dense concrete, proper curing, surface protection	Use sulfate-resistant cement, control alkali content, use protective coatings, sacrificial layers

6. INTRODUCTION TO MASONRY STRUCTURES

6.1 Masonry involves building structures by laying individual masonry units such as bricks, concrete blocks, or stones. These units are typically bound together with cement, lime, or mud mortar. Masonry construction is known for its durability, aesthetic appeal, and cost-effectiveness, although it is labor-intensive.

6.2 Uses of Masonry Structures

1 Structural and Non-Structural Purposes: Masonry is used for walls, columns, arches, and domes.

2 Infill Walls: Used in structural buildings.

3 Arches and Retaining Walls: Common in dams and coffer dams.

4 Roofing and Cladding: Provides durable and aesthetically pleasing finishes.

6.3 Advantages of Masonry Structures

1 Versatility: Can perform various functions.

2 High Compressive Strength: Ideal for structures under compression.

3 Flexibility in Layout: Can be constructed without large capital expenditure.

4 Durability: Requires little maintenance and provides thermal and acoustic insulation.

5 Local Materials: Made from locally available materials.

6 Fire and Weather Resistance: Offers good resistance to fire and adverse weather conditions.

6.4 Disadvantages of Masonry Structures

1 Low Tensile Strength: Not suitable for elements subjected to bending.

2 Weather Degradation: Extreme weather can cause surface degradation.

3 Heavy Weight: Requires strong foundations.

4 Susceptibility to Frost and Chemical Attack: Can cause spalling in brickwork.

6.5 Construction Technology

6.51 Definition

1 Closer: A portion of a brick cut lengthwise.

2 Queen Closer: Half of a brick cut lengthwise.

3 King Closer: A triangular portion of a brick cut between the center of one end and the center of one side.

4 Bat: A portion of a brick cut across the width.

5 Quoin: The external corner of a wall.

6 Quoin Header: A corner header in the face of a wall.

7 Quoin Stretcher: A corner stretcher in the face of a wall.

6.52 Types of Bonds

English Bond:

1 Consists of alternate courses of headers and stretchers.

2 Vertical joints in the header courses align, and vertical joints in the stretcher courses align.

3 Queen closers are placed after the first header in each heading course to break vertical joints.

Flemish Bond:

1 Each course consists of alternate headers and stretchers.

2 Single Flemish Bond: Combines Flemish bond on the face with English bond on the backing.

3 Double Flemish Bond: Same appearance on both front and back elevations, with alternate headers and stretchers in each course.

Rat-Trap Bond:

1 Bricks are laid on edge with shiner and rowlock visible, creating an internal cavity.

2 Saves materials and provides thermal comfort.

3 Suitable for load-bearing and partition walls.

6.6 Hollow Blocks and Compressed Earth Blocks

I. Hollow Blocks

1 Defined as bricks with greater than 25% void areas.

2 Provide structural components and brick finish without additional materials.

3 Reduce weight and improve thermal insulation.

II. Concrete Blocks

1 Made from cast concrete with hollow centers to reduce weight.

2 Common for load-bearing walls and infill walls in framed structures.

3 Advantages: Locally available materials, different shapes and sizes, durable, good thermal and sound insulation, environment-friendly.

III. Compressed Earth Blocks

1 Made by compacting earth mixed with stabilizers like cement or lime.

2 Advantages: Better strength and durability, quality control, locally available materials, good thermal and sound insulation.

6.7 Masonry as Infill Wall

Infill Walls: Confined by reinforced concrete or steel frames.

Contribution to Structural Performance: Increases stiffness and alters behavior from frame action to shear action, improving performance during earthquakes.

6.8 Reinforced and Unreinforced Masonry

Reinforced Masonry: Incorporates steel reinforcement to resist tensile, compressive, and shear stresses.

Types:

1 Reinforced hollow unit masonry.

2 Reinforced grouted cavity masonry.

3 Reinforced pocket type wall.

7. DESIGN OF MASONRY WALLS FOR GRAVITY

7.1 Introduction to Codal Provisions

Masonry constructions are essential components of load-bearing structures. They support loads and provide various functions such as dividing space, thermal and acoustic insulation, fire resistance, and weather protection. Historically, masonry design was based on thumb rules, leading to uneconomical wall thicknesses beyond three stories. Since the 1950s, theoretical and experimental research has led to the development of codes of practice. In Nepal, the Nepal National Building Code (NBC) 2024 is followed for masonry construction design1.

7.2 Key Definitions:

Wall: A vertical member whose width exceeds four times its thickness.
Column: A vertical member with a height-to-length ratio greater than that of a wall, resulting in less permissible stress under vertical loading.

7.3 Codal Provisions (NBC 2024):

1. Effective Height: The height of a wall or column considered for calculating slenderness ratio, depending on the degree of restraint imposed by floors and beams framing into the wall or column.

7.4 Design Considerations

Load Calculations:

1 Effective Thickness: Actual thickness of the masonry unit plus mortar thickness.

2 Weight of Slab: Calculated based on the area and bearing width.

3 Self-Weight of Wall: Up to the plinth level.

4 Live Load from Roof: Based on the area and load intensity.

Stress Calculations:

1 Total Weight: Sum of the weights of the slab, wall, and live load.

2 Total Stress: Load on the wall divided by the area.

Slenderness Ratio:
1. Effective Height: Calculated based on the height and degree of restraint.
2. Effective Thickness: Actual thickness of the wall.
3. Allowable Stress:
  1. Basic Compressive Stress (): Determined from tables based on the crushing strength of masonry units and mortar type.
  2. Allowable Stress (): Calculated using factors such as area reduction, stress reduction, and shape modification.

Aspect	Description	Importance
Effective Height	Height considered for slenderness ratio, depends on restraint by floors and beams	Crucial for stability and load-bearing capacity
Load Calculations	Includes weight of slab, self-weight of wall, and live load from roof	Determines the total load on the wall
Stress Calculations	Total weight divided by the area	Ensures the wall can safely bear the load
Slenderness Ratio	Ratio of effective height to effective thickness	Affects the stability and design of the wall
Allowable Stress	Based on basic compressive stress and modification factors	Ensures the wall material can withstand the applied loads

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