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
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
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
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.
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 ?