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Dr. Pasindu Weerasinghe
Department of Civil Engineering
University of Moratuwa
CE 2122
Design of Concrete Structures I
Introduction
Subject Details
2
• 3 Credit compulsory subject
• 2 hrs Lectures per week – Wednesday 10.15 am - 12.15 pm
• 2 hrs Lab classes/Tutorials per week – Wednesday 1.15 pm - 3.15 pm
Lectures and Practicals
Assessments
Continuous Assessments (40% of final mark)
Design Report and Laboratory Practicals
Final Examination (60% of final marks) – 3 hrs
Learning Outcomes
3
• Propose alternative solutions for a proposed building so that preliminary designs
could be conducted for the selection of optimum solutions
• Perform structural modelling and analysis for low/medium rise buildings while
verifying the results of analysis to complete the structural designs
• Articulate through the design standards to conduct detailed design calculations for
different components of reinforced concrete low/medium rise buildings
• Apply standard methods of production of detailed drawings to communicate the
final outcome of structural design
Supporting Material
4
National Annex
to Eurocode
References
5
Bhatt, P., MacGinley,T. J. and Choo, B. S. (2013).
Reinforced concrete design to Eurocodes Design
Theory and Examples (4th ed.). CRC Press,
Taylor and Francis Group.
Reynolds, C. E. and Steedman, J. C.
(2007). Reinforced concrete designer’s
Handbook (11th ed.). London: E &F N
Spon, Taylor & Francis Group.
Manual for the design of reinforced
concrete building structures to EC2.
Published for the Institution of
Structural Engineers UK.
Dias, W. P. S. and Sivakumar, K.
Graded examples in Reinforced
concrete design to Eurocode 2
Introduction
6
• Concrete is the most consumed material after water, with three tonnes per
year used for every person in the world.
• Twice as much concrete is used in construction as all other building materials
combined
‘Concretus’
To Grow Together Concrete
Brief History of Concrete
7
• Used by ancient Romans 2000+
years ago
• Knowledge of concrete lost in
Middle Ages
• 1824 – Artificial cement is
invented/patented in England;
called “Portland cement”. This
paved the way to modern concrete
Typical RC Structures
8
Residential buildings
Bridges
Dams
Foundations Highrise buildings
9
Why Concrete ?
10
• Excellent durability/ Excellent
resistance to water
• Mouldability
• Economical and readily available
• Less energy input compared with
steel
• Utilization of Industrial by-products
• High-temperature resistance
Why Reinforced Concrete ?
11
Compression
Steel reinforcement provide the required tensile strength
Stress – Strain Curve
12
• Concrete has high compressive strength
but no useful tensile strength
• Steel has high tensile strength, and steel
is compatible with concrete
Plain Concrete Reinforcing Steel
Stress – Strain Behaviour – Reinforced Concrete
13
Strain
Stress
Plain
Concrete
Reinforced
Concrete
Brittle Failure Ductile Failure
Brittle Failure Ductile Failure
Reinforcing Steel
14
Put steel bars (“rebars”) in the areas of
concrete where tensile stresses will develop
Reinforced Concrete
Steel
• Types of steel
• Stress-strain curve
• Ductility
• Reinforcement Cage
Steel as reinforcement
16
• Steel bars are popular as reinforcement for concrete elements
Two types of reinforcement
(based on the surface profile)
Plain Bars Deformed Bars
Steel as reinforcement
17
Plain Bars
Deformed Bars
• Contain longitudinal and transverse ribs rolled into the
surfaces (sometimes without longitudinal ribs)
• Ribs are in the shape of a spiral, chevron or crescent
• This profile can effectively increase the bonding between
steel bars and concrete
• Comes in diameters of 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28,
32, 36, 40 and 50 mm
• Smooth, even surfaces for bonding with concrete,
hence less bonding strength compared to
deformed bars
• Comes in diameters of 6, 8, 10, 12, 14, 16, 18, 20
and 22 mm
 If the diameter < 6 mm, called steel wires
Stress-Strain Behaviour of Steel
18
hot-rolled low-carbon steel and hot-rolled low-alloy steel high-carbon steel
Design strength
based on Yield
Stress
Yield plateau
Necking
Ultimate
strength
Design strength based
on 0.2% proof stress
Design Stress-Strain Curve for Steel
19
𝒇𝒚𝒅 =
𝒇𝒚𝒌
𝜸𝒎
Design Strength (𝑓𝑦𝑑) =
𝑓𝑦𝑘
𝛾𝑚
=
𝑓𝑦𝑘
1.15
= 0.87𝑓𝑦𝑘
Strain at yield point
Youngs modulus of steel 𝐸𝑠 = 200,000 𝑀𝑃𝑎
=
𝑓𝑦𝑑
𝐸𝑠
=
0.87𝑓𝑦𝑘
200000
= 4.35 × 10−6
𝑓𝑦𝑘
Ductility
• Ductility of steel reinforcement depends on the strain at maximum load (Ɛuk), the ratio
between the maximum and the yield strength 𝑓𝑡
𝑓𝑦 𝑘
20
Ductility classes defined in Annex C of EN 1992-1-1
Reinforcement Cage
21
• Reinforcements in structural members can be strapped
or welded into reinforcement cages before being
placed in forms.
• Secures the relative position of the reinforcement
and helps to improve the bonding between two
materials
• Hooks at ends – to avoid plain bars slipping in
concrete under tension (Hook ends are not
necessary for plain bars under compression)
• The ribs of deformed bars allow the bars to form a
better bond with concrete. Therefore hooks are
unnecessary at the ends.
• Welded steel cages and wire fabrics are well
bonded with concrete, installing hooks at their
ends is not necessary.
Concrete
• Strength
• Stress-strain curve
• Elastic modulus and Poisson’s
ratio
• Creep and Shrinkage
• Admixtures
Concrete is a Composite Material
23
[Modify properties]
Fine aggregate -
Coarse aggregate –
Gravel or crushed
stone
Binder
The function of each component
24
• Pieces of inert hard material (gravel or crushed rocks
for example) are used to give the concrete its basic
strength. The are called coarse aggregates
• Smaller pieces of gravel, crushed rock or sand is added
to fill the gaps between the larger pieces. The are
called fine aggregates
• A paste formed from cement and water is mixed in the
aggregates which hardens over a period of time,
binding it together and forming an extremely durable
solid mass
Concrete is a multi-phase material
25
Macroscopic Level
Two Phases
Aggregate Phase
Hardened Cement
Paste Phase (HCP)
Microscopic Level
Interfacial
Transition Zone
(ITZ)
(10 to 50 𝜇m)
Three Phases
The physical
process of
hydration
Higher w/c cement ratios result in greater distances
between the cement grains and therefore a greater
volume of pores for a given degree of hydration. This
makes the HCP more permeable through which
aggressive chemicals can penetrate
Interfacial Transition Zone
26
Micro hardness distribution
Transition
Zone
Weak Link
1. Size of voids are larger
2. The size and
concentration of calcium
hydroxide and ettringite
are larger
3. More cracks
Greater
influence on
the mechanical
behaviour
Interfacial Transition Zone
27
• Aggregate and cement paste –linear up to failure
• Concrete stress-strain response- in between
aggregate and cement paste
• Concrete does not have a linear behaviour up to
failure
Influence of the Transition Zone
Hydration of cement
28
• The cement clinker consists of four compounds which combine with water to produce the hydration
products which in time form the HCP
• Tricalcium Silicate (3CaO.SiO2 – C3S) - Contributes to the early strength
• Dicalcium Silicate (2CaO.SiO2 – C2S) - Contributes to the eventual strength of the concrete
• Tricalcium Aluminate (3CaO.Al2O3 – C3A) - First to react with water and needs to be controlled to
avoid flash setting
• Tetracalcium Alumnoferrite (4CaO. Al2O3.Fe2O3 – C4AF) – Plays a relatively minor role in the hydration
process. Its primary significance is in the context of its reaction with sulfates and its potential impact
on durability in sulfate-rich environments
Strength of Concrete
W/C Ratio
Compaction
Age
Quality of Cement
Aggregate/Cement
Ratio Quality of
aggregate Max.
aggregate size
Method of curing
Main
Factors
Secondary
Factors
29
Stress-Strain Behaviour of Concrete (In Compression)
30
𝜺
Design compressive
strength
=
𝜶𝒄𝒄𝒇𝒄𝒌
𝜸𝒎
=
0.85𝑓𝑐𝑘
1.5
= 0.567𝑓𝑐𝑘
𝜺𝟎 = 𝟎. 𝟎𝟎𝟑𝟓
Cylinder strength,
Cubic strength and
Mean strength
Tensile Strength of Concrete
31
• Varies between 8% and 15% of its
compressive strength
• Typically neglected in design
Reasons for low tensile strength;
• Concrete is filled with fine cracks
• The cracks affect negligibly when concrete is subjected to
compression loads (the loads cause the cracks to close
and permit compression transfer)
• The micro cracks badly affect on tensile load transfer.
Hence the tensile strength is normally neglected in design
calculations
Elastic Modulus of Concrete
32
• The modulus of elasticity is a variable because of the
non linear relationship of stress–strain behaviour of
concrete under axial compression
𝐸𝑐𝑚 = 1.25𝐸𝑑 − 19 𝑘𝑁/𝑚𝑚2
Poisson’s Ratio of Concrete
33
• When a concrete cylinder is subjected to compressive
loads, it not only shortens in length but also expands
laterally.
• The ratio of this lateral expansion to the longitudinal
shortening is referred to as Poisson’s ratio.
• Poisson’s ratio
• about 0.11 for the higher-strength concretes
• about 0.21 for the weaker-grade concretes
• average value is about 0.16.
Creep in Concrete
34
• Creep is the increase in strain with time due to a sustained load
Factors affecting creep
• Stress magnitude
• Material characteristics
• Composition of concrete
• Age of concrete at the time of loading
• Environmental conditions
• Water cement ratio (inversely
proportional)
• Mechanical properties of aggregate
• Fabrication method and curing condition
• Dimensions of the member
• Arrangement of reinforcements
Shrinkage, Swelling and Thermal Expansion of
Concrete
35
• Shrinkage - the decrease in the volume of a concrete member when it loses
moisture by evaporation
 Drying shrinkage : expiration of moisture from the concrete to the surrounding air
 Autogenous Shrinkage: The macroscopic volume reduction of cementitious materials when
cement hydrates, after initial setting. Significant in high strength concrete and inversely
proportional to the water cement ratio.
• Swelling - the volume increases through water absorption
• Linear thermal expansion coefficient of concrete is related to its composition and
aggregate property. The value of concrete (1.0–1.5) × 10−5 is close to that of (1.2 ×
10−5) steel.
Admixtures
36
• Water reducing admixtures (Superplasticisers) improve the workability without increasing the
water demand
• Air entraining admixtures reduce bleeding and segregation. They improve consistency and
cohesiveness. However, they reduce strength
• Accelerating admixtures - Accelerate its early strength development
• Retarding admixtures - Used to slow the setting of the concrete and to retard temperature
increases
Water reducing
admixtures
Reinforced
Concrete
• Compatibility
• Advantages
• Disadvantages
Reinforced Concrete
38
• Major shortcoming of concrete is its lack of
tensile strength.
• Reinforcing bars have tensile strengths equal
to approximately 100 times that of the usual
concrete used.
• The advantages of each material seem to
compensate for the disadvantages of the
other
• The two materials bond together very well
there is little chance of slippage between two
materials.
The process of concreting
39
Formwork
and
falsework
Reinforcement Concreting Surface
finishing
Curing of concrete
Removal of formwork
Compatibility of steel and concrete
40
• Reinforcing bars are subject to corrosion. The concrete surrounding
reinforcements provides good protection.
• The strength of steel degrades when it exposes to fire. But enclosing the
reinforcing steel in concrete produces very satisfactory fire ratings.
• Coefficients of thermal expansions of concrete (in the range
between 1.0 − 1.5 × 10−5 0𝐶−1
) and reinforcements (1.2 × 10−5 0𝐶−1
)
are quite close. Hence reinforced concrete works well for temperature
changes.
Reinforced Concrete - Advantages
41
• Considerable compressive strength per unit cost
• High resistance to the fire (During fires of average intensity, members with a
satisfactory cover of concrete over the reinforcing bars suffer only surface damage
without failure)
• High resistance to water (the best structural material available for situations
where water is present).
• A low-maintenance material, with long service life
• The strength increases over a very long period, due to the lengthy process of the
solidification of the cement paste.
• Ability to be cast into an extraordinary variety of shapes
• A lower grade of skilled labour is required
• High rigidity
• Energy efficient and ability to consume waste
Reinforced Concrete - Disadvantages
42
• Forms are required to hold the concrete in place and false work or shoring to
keep the forms in place
• The low strength per unit of weight of concrete (large dead weight) - Lightweight
aggregates can be used to reduce concrete weight, but the cost of the concrete
increases
• The properties of concrete vary widely with the variations in proportioning and
mixing.
• The placing and curing of concrete should be carefully controlled
• Shrinkage and creep problems
• Low toughness
43
How could I decide whether to use reinforced concrete as the
structural material in a structure ?
What are the material properties
of concrete and steel ?
What are the loads acting on a particular
structure or a structural element ?
How can I design a structural
element to resist that load ?
What are the dimensions of
the structural element ?
How much reinforcing steel do I
need to add ?
What are the failure modes
that need to be prevented ?
What is the design philosophy ?
44
Thank You

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introduction to design of concrete structures

  • 1. . Dr. Pasindu Weerasinghe Department of Civil Engineering University of Moratuwa CE 2122 Design of Concrete Structures I Introduction
  • 2. Subject Details 2 • 3 Credit compulsory subject • 2 hrs Lectures per week – Wednesday 10.15 am - 12.15 pm • 2 hrs Lab classes/Tutorials per week – Wednesday 1.15 pm - 3.15 pm Lectures and Practicals Assessments Continuous Assessments (40% of final mark) Design Report and Laboratory Practicals Final Examination (60% of final marks) – 3 hrs
  • 3. Learning Outcomes 3 • Propose alternative solutions for a proposed building so that preliminary designs could be conducted for the selection of optimum solutions • Perform structural modelling and analysis for low/medium rise buildings while verifying the results of analysis to complete the structural designs • Articulate through the design standards to conduct detailed design calculations for different components of reinforced concrete low/medium rise buildings • Apply standard methods of production of detailed drawings to communicate the final outcome of structural design
  • 5. References 5 Bhatt, P., MacGinley,T. J. and Choo, B. S. (2013). Reinforced concrete design to Eurocodes Design Theory and Examples (4th ed.). CRC Press, Taylor and Francis Group. Reynolds, C. E. and Steedman, J. C. (2007). Reinforced concrete designer’s Handbook (11th ed.). London: E &F N Spon, Taylor & Francis Group. Manual for the design of reinforced concrete building structures to EC2. Published for the Institution of Structural Engineers UK. Dias, W. P. S. and Sivakumar, K. Graded examples in Reinforced concrete design to Eurocode 2
  • 6. Introduction 6 • Concrete is the most consumed material after water, with three tonnes per year used for every person in the world. • Twice as much concrete is used in construction as all other building materials combined ‘Concretus’ To Grow Together Concrete
  • 7. Brief History of Concrete 7 • Used by ancient Romans 2000+ years ago • Knowledge of concrete lost in Middle Ages • 1824 – Artificial cement is invented/patented in England; called “Portland cement”. This paved the way to modern concrete
  • 8. Typical RC Structures 8 Residential buildings Bridges Dams Foundations Highrise buildings
  • 9. 9
  • 10. Why Concrete ? 10 • Excellent durability/ Excellent resistance to water • Mouldability • Economical and readily available • Less energy input compared with steel • Utilization of Industrial by-products • High-temperature resistance
  • 11. Why Reinforced Concrete ? 11 Compression Steel reinforcement provide the required tensile strength
  • 12. Stress – Strain Curve 12 • Concrete has high compressive strength but no useful tensile strength • Steel has high tensile strength, and steel is compatible with concrete Plain Concrete Reinforcing Steel
  • 13. Stress – Strain Behaviour – Reinforced Concrete 13 Strain Stress Plain Concrete Reinforced Concrete Brittle Failure Ductile Failure Brittle Failure Ductile Failure
  • 14. Reinforcing Steel 14 Put steel bars (“rebars”) in the areas of concrete where tensile stresses will develop Reinforced Concrete
  • 15. Steel • Types of steel • Stress-strain curve • Ductility • Reinforcement Cage
  • 16. Steel as reinforcement 16 • Steel bars are popular as reinforcement for concrete elements Two types of reinforcement (based on the surface profile) Plain Bars Deformed Bars
  • 17. Steel as reinforcement 17 Plain Bars Deformed Bars • Contain longitudinal and transverse ribs rolled into the surfaces (sometimes without longitudinal ribs) • Ribs are in the shape of a spiral, chevron or crescent • This profile can effectively increase the bonding between steel bars and concrete • Comes in diameters of 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40 and 50 mm • Smooth, even surfaces for bonding with concrete, hence less bonding strength compared to deformed bars • Comes in diameters of 6, 8, 10, 12, 14, 16, 18, 20 and 22 mm  If the diameter < 6 mm, called steel wires
  • 18. Stress-Strain Behaviour of Steel 18 hot-rolled low-carbon steel and hot-rolled low-alloy steel high-carbon steel Design strength based on Yield Stress Yield plateau Necking Ultimate strength Design strength based on 0.2% proof stress
  • 19. Design Stress-Strain Curve for Steel 19 𝒇𝒚𝒅 = 𝒇𝒚𝒌 𝜸𝒎 Design Strength (𝑓𝑦𝑑) = 𝑓𝑦𝑘 𝛾𝑚 = 𝑓𝑦𝑘 1.15 = 0.87𝑓𝑦𝑘 Strain at yield point Youngs modulus of steel 𝐸𝑠 = 200,000 𝑀𝑃𝑎 = 𝑓𝑦𝑑 𝐸𝑠 = 0.87𝑓𝑦𝑘 200000 = 4.35 × 10−6 𝑓𝑦𝑘
  • 20. Ductility • Ductility of steel reinforcement depends on the strain at maximum load (Ɛuk), the ratio between the maximum and the yield strength 𝑓𝑡 𝑓𝑦 𝑘 20 Ductility classes defined in Annex C of EN 1992-1-1
  • 21. Reinforcement Cage 21 • Reinforcements in structural members can be strapped or welded into reinforcement cages before being placed in forms. • Secures the relative position of the reinforcement and helps to improve the bonding between two materials • Hooks at ends – to avoid plain bars slipping in concrete under tension (Hook ends are not necessary for plain bars under compression) • The ribs of deformed bars allow the bars to form a better bond with concrete. Therefore hooks are unnecessary at the ends. • Welded steel cages and wire fabrics are well bonded with concrete, installing hooks at their ends is not necessary.
  • 22. Concrete • Strength • Stress-strain curve • Elastic modulus and Poisson’s ratio • Creep and Shrinkage • Admixtures
  • 23. Concrete is a Composite Material 23 [Modify properties] Fine aggregate - Coarse aggregate – Gravel or crushed stone Binder
  • 24. The function of each component 24 • Pieces of inert hard material (gravel or crushed rocks for example) are used to give the concrete its basic strength. The are called coarse aggregates • Smaller pieces of gravel, crushed rock or sand is added to fill the gaps between the larger pieces. The are called fine aggregates • A paste formed from cement and water is mixed in the aggregates which hardens over a period of time, binding it together and forming an extremely durable solid mass
  • 25. Concrete is a multi-phase material 25 Macroscopic Level Two Phases Aggregate Phase Hardened Cement Paste Phase (HCP) Microscopic Level Interfacial Transition Zone (ITZ) (10 to 50 𝜇m) Three Phases The physical process of hydration Higher w/c cement ratios result in greater distances between the cement grains and therefore a greater volume of pores for a given degree of hydration. This makes the HCP more permeable through which aggressive chemicals can penetrate
  • 26. Interfacial Transition Zone 26 Micro hardness distribution Transition Zone Weak Link 1. Size of voids are larger 2. The size and concentration of calcium hydroxide and ettringite are larger 3. More cracks Greater influence on the mechanical behaviour
  • 27. Interfacial Transition Zone 27 • Aggregate and cement paste –linear up to failure • Concrete stress-strain response- in between aggregate and cement paste • Concrete does not have a linear behaviour up to failure Influence of the Transition Zone
  • 28. Hydration of cement 28 • The cement clinker consists of four compounds which combine with water to produce the hydration products which in time form the HCP • Tricalcium Silicate (3CaO.SiO2 – C3S) - Contributes to the early strength • Dicalcium Silicate (2CaO.SiO2 – C2S) - Contributes to the eventual strength of the concrete • Tricalcium Aluminate (3CaO.Al2O3 – C3A) - First to react with water and needs to be controlled to avoid flash setting • Tetracalcium Alumnoferrite (4CaO. Al2O3.Fe2O3 – C4AF) – Plays a relatively minor role in the hydration process. Its primary significance is in the context of its reaction with sulfates and its potential impact on durability in sulfate-rich environments
  • 29. Strength of Concrete W/C Ratio Compaction Age Quality of Cement Aggregate/Cement Ratio Quality of aggregate Max. aggregate size Method of curing Main Factors Secondary Factors 29
  • 30. Stress-Strain Behaviour of Concrete (In Compression) 30 𝜺 Design compressive strength = 𝜶𝒄𝒄𝒇𝒄𝒌 𝜸𝒎 = 0.85𝑓𝑐𝑘 1.5 = 0.567𝑓𝑐𝑘 𝜺𝟎 = 𝟎. 𝟎𝟎𝟑𝟓 Cylinder strength, Cubic strength and Mean strength
  • 31. Tensile Strength of Concrete 31 • Varies between 8% and 15% of its compressive strength • Typically neglected in design Reasons for low tensile strength; • Concrete is filled with fine cracks • The cracks affect negligibly when concrete is subjected to compression loads (the loads cause the cracks to close and permit compression transfer) • The micro cracks badly affect on tensile load transfer. Hence the tensile strength is normally neglected in design calculations
  • 32. Elastic Modulus of Concrete 32 • The modulus of elasticity is a variable because of the non linear relationship of stress–strain behaviour of concrete under axial compression 𝐸𝑐𝑚 = 1.25𝐸𝑑 − 19 𝑘𝑁/𝑚𝑚2
  • 33. Poisson’s Ratio of Concrete 33 • When a concrete cylinder is subjected to compressive loads, it not only shortens in length but also expands laterally. • The ratio of this lateral expansion to the longitudinal shortening is referred to as Poisson’s ratio. • Poisson’s ratio • about 0.11 for the higher-strength concretes • about 0.21 for the weaker-grade concretes • average value is about 0.16.
  • 34. Creep in Concrete 34 • Creep is the increase in strain with time due to a sustained load Factors affecting creep • Stress magnitude • Material characteristics • Composition of concrete • Age of concrete at the time of loading • Environmental conditions • Water cement ratio (inversely proportional) • Mechanical properties of aggregate • Fabrication method and curing condition • Dimensions of the member • Arrangement of reinforcements
  • 35. Shrinkage, Swelling and Thermal Expansion of Concrete 35 • Shrinkage - the decrease in the volume of a concrete member when it loses moisture by evaporation  Drying shrinkage : expiration of moisture from the concrete to the surrounding air  Autogenous Shrinkage: The macroscopic volume reduction of cementitious materials when cement hydrates, after initial setting. Significant in high strength concrete and inversely proportional to the water cement ratio. • Swelling - the volume increases through water absorption • Linear thermal expansion coefficient of concrete is related to its composition and aggregate property. The value of concrete (1.0–1.5) × 10−5 is close to that of (1.2 × 10−5) steel.
  • 36. Admixtures 36 • Water reducing admixtures (Superplasticisers) improve the workability without increasing the water demand • Air entraining admixtures reduce bleeding and segregation. They improve consistency and cohesiveness. However, they reduce strength • Accelerating admixtures - Accelerate its early strength development • Retarding admixtures - Used to slow the setting of the concrete and to retard temperature increases Water reducing admixtures
  • 38. Reinforced Concrete 38 • Major shortcoming of concrete is its lack of tensile strength. • Reinforcing bars have tensile strengths equal to approximately 100 times that of the usual concrete used. • The advantages of each material seem to compensate for the disadvantages of the other • The two materials bond together very well there is little chance of slippage between two materials.
  • 39. The process of concreting 39 Formwork and falsework Reinforcement Concreting Surface finishing Curing of concrete Removal of formwork
  • 40. Compatibility of steel and concrete 40 • Reinforcing bars are subject to corrosion. The concrete surrounding reinforcements provides good protection. • The strength of steel degrades when it exposes to fire. But enclosing the reinforcing steel in concrete produces very satisfactory fire ratings. • Coefficients of thermal expansions of concrete (in the range between 1.0 − 1.5 × 10−5 0𝐶−1 ) and reinforcements (1.2 × 10−5 0𝐶−1 ) are quite close. Hence reinforced concrete works well for temperature changes.
  • 41. Reinforced Concrete - Advantages 41 • Considerable compressive strength per unit cost • High resistance to the fire (During fires of average intensity, members with a satisfactory cover of concrete over the reinforcing bars suffer only surface damage without failure) • High resistance to water (the best structural material available for situations where water is present). • A low-maintenance material, with long service life • The strength increases over a very long period, due to the lengthy process of the solidification of the cement paste. • Ability to be cast into an extraordinary variety of shapes • A lower grade of skilled labour is required • High rigidity • Energy efficient and ability to consume waste
  • 42. Reinforced Concrete - Disadvantages 42 • Forms are required to hold the concrete in place and false work or shoring to keep the forms in place • The low strength per unit of weight of concrete (large dead weight) - Lightweight aggregates can be used to reduce concrete weight, but the cost of the concrete increases • The properties of concrete vary widely with the variations in proportioning and mixing. • The placing and curing of concrete should be carefully controlled • Shrinkage and creep problems • Low toughness
  • 43. 43 How could I decide whether to use reinforced concrete as the structural material in a structure ? What are the material properties of concrete and steel ? What are the loads acting on a particular structure or a structural element ? How can I design a structural element to resist that load ? What are the dimensions of the structural element ? How much reinforcing steel do I need to add ? What are the failure modes that need to be prevented ? What is the design philosophy ?