background to reinforced concrete design rc buildings

Civil Engineering, Faculty of Engineering & Physical Science, University of Surrey

PG Cert/PG Diploma/MSc in Civil, Bridge & Structural Engineering Page ENGM041.1.1

Unit 1. Background to reinforced concrete design, RC buildings,

materials, how concrete structures behave and innovations in concrete.

(Study time allocation – 20 hours)

Introduction

The purpose of this package is explain the principles and aims of the design of reinforced

concrete structures and to give some background to the design of these buildings.

Although much of the theory used in RC design is fundamental, adhering to the laws of

mechanics, ultimately the design must conform to the requirements of a code of practice.

In this course it is to BS EN 1992-1-1 Eurocode 2: Design of Concrete Structures

published by the British Standards Institution, and which is henceforth referred to as

EC2, Eurocode 2 or simply the Code. Eurocode 2 is a Europe wide code which enables

designers across Europe to practice in any country which adopts the code. There is

however, one proviso, the design of buildings in a specific country requires the National Annex for that country to be applied in the design. The National Annex for the UK for

example contains all the nationally determined parameters which refer specifically to the

UK due to our unique circumstances.

What is reinforced concrete.

The following schematic diagram summarises the primary components of conventional

reinforced concrete.

Cement

powder

Water

Cement

paste

Sand

Mortar

Gravel Concrete

Reinforcing

rods

Reinforced concrete



Examples of reinforced concrete structures.

High rise reinforced concrete frame

Arch bridge in reinforced concrete



Simple domestic dwelling. Pre-cast concrete beam.

The construction of reinforced concrete structures.

Reinforced concrete comprises concrete and reinforcing bars acting as a composite

material.

Reinforcing cages. Reinforcement in beams and columns consists essentially of straight

longitudinal bars enclosed by links or stirrups which together form a cage of

reinforcement.

Longitudinal bars

Links

Reinforcement cage (Section)

Reinforced

concrete beam



Reinforcement cage for a beam under construction

Reinforcement cage for a column being craned into position



Reinforcement cage being fixed in position in the formwork

Reinforcement cage.

Formwork



Concrete column after formwork has been removed.

Starter bars for next lit of concrete



The same building in a more advanced state

Typical two-way spanning floor slab reinforcement.

Formwork for casting slab

supported on the floor below

Column reinforcement cage attached

to starter bars ready for next lift.



Concentration of reinforcement at a column head in flat slab

construction

Concrete being poured and compacted on a ribbed slab.

Concrete being

delivered by pump.

Concrete vibration using

a poker vibrator.



Changes to RC design Philosophy.

The philosophy behind design has altered since reinforced concrete was first used as a

structural material. Initially the design was for strength. Structures were made strong

enough to sustain the loads they were to be subject to. The effects of serviceability then

needed to be considered. For example, structures were designed to have limited

deflections so users felt comfortable when using them. Aspects such as fire loading or

the effect of blasts on reinforced concrete had an influence on strength requiring

designers to also consider these. At present designers are being asked to consider all

these aspects as well as to consider the whole life of the structure. In effect designers

need to design economically, ensure the users are happy when occupying the building,

ensure unforeseen circumstances are catered for and then to design for the disposal of the

building in an economic manner when its design life is over. Further, they are required to

design such that the embodied energy is minimised with respect to the whole life running

costs of the building.

Basic principal of reinforced concrete design. Concrete is strong in compression and weak in tension. To overcome the weak tensile

behaviour we strengthen the concrete where it is likely to go into tension with steel

reinforcing bars.

Practical issue.

In reality a cage of reinforcement is provided along the entire length of the beam. Bars

are bigger in the tension zones to carry this load but needed elsewhere to enable links to

be fixed. Links resist shear (covered later) as well as holding the longitudinal

reinforcement in place. The diagram below demonstrates the reinforcement needed

Tension Tension

Tension

Compression Compression

Compression



Lap to ensure continuity

of reinforcement

Main tension reinforcement

Continuity reinforcement

Links to resist shear and / or hold

continuity reinforcement in position



Reinforcement – good practice.

1. Well designed reinforcement allows concrete to be placed easily

2. Well designed reinforcement should be easy to fix.

3. Reinforcement helps the concrete to resists externally applied loads. It should

have adequate cover to ensure it is protected, it acts compositely with the concrete

to achieve the required strength and to afford adequate fire protection.

4. Well designed and manufactured reinforcement will form a strong rigid cage

which can be easily placed.

Load deformation properties of reinforced concrete elements.

To enable the behaviour of the composite to be appreciated, the behaviour of a simply

supported beam will be examined. Consider a simply supported reinforced concrete I

beam. The figure below indicates the support and loading on the beam.

Support Support

Load



Stage 1. Load is applied to the concrete and the beam deflects slightly. The concrete and

steel behave as one until a load is achieved when the concrete in tension at the bottom of

the beam cracks. If the beam were not reinforced it would fail at this stage. The

photograph below indicates the beam in this stage. Failure of the reinforced concrete

beam has not occurred but the width of the cracks are very important and must be

controlled in order to satisfy the serviceability (appearance) of the beam and to ensure the

reinforcement is protected. The figure below is an idealised description of this

behaviour.

Deflection

Lo

ad

Cracking load



Stage 2. The load on the beam is increased steadily. The cracks along the lower soffit of

the beam lengthen upwards and increase in number. The tension effects of the concrete

are now negligible but concrete in the top part of the beam which is under compression

and the reinforcement which is in tension are behaving nearly elastically until the strains

(deformations) reach a critical value. The precise location of this critical value will be

difficult to fix as judgement will be needed to determine when linear behaviour ceases.

This stage of a beam’s behaviour generally extends beyond the normal serviceability

conditions expected of a beam in use and the ultimate capacity is said to be achieved

when the ‘critical’ strains are reached. The figure below illustrates this behaviour.

Ran

ge

of

serv

ice

load

ing

Deflection

Lo

ad

Cracking load



Stage 3. Increasing the load further will cause the steel in the tension zone to yield and

become plastic at the position of maximum bending moment in the beam. Alternatively,

the concrete in the top of the beam may develop longitudinal cracks and crush or in some

instances a combination of both may occur. The photograph and figure below illustrate

this.

Ran

ge

of

serv

ice

load

ing

Deflection

Lo

ad

Cracking load

Post yield loading

Plastic failure



Stage 4. When plastic failure has occurred, the beam will continue to deform with no

further load until it fails either by tensile failure of the reinforcement or through concrete

crushing. The photograph and figure below illustrate this behaviour.

Post

yie

ld l

oad

ing

Pla

stic

fai

lure

Ran

ge

of

serv

ice

load

ing

Deflection

Load

Cracking load

Failure

Ultimate load



Over and under reinforced concrete beams.

The above example helps one to understand the concept of over and under reinforced

beams. Under reinforced beams reach the limit of their service loading capability and fail

when the reinforcement yields. They have been designed so the concrete in compression

is stronger than the tensile strength of the steel at this load. Failure is not catastrophic as

once it is reached, the steel yields until it fails. This may occur over a considerable

period of time and in many instances when the yield point of the steel is reached, strain

hardening of the reinforcement occurs so there may be a temporary strengthening of the

beam.

With over reinforced beams failure occurs when the concrete in compression crushes.

The beam in this instance is designed so the reinforcement is stronger than the concrete.

Unfortunately concrete is a brittle material and failure is instantaneous so beams which

fail in this manner fail catastrophically and without warning.

Crack formation in concrete beams.

A concrete element in tension cracks at right angles to the load. If an element is under

compression, cracks are parallel to the load. The Figure illustrates this.

Tension crack

Compression cracks



Identifying crack formation in reinforced concrete beams.

The Figure below shows the principal compressive and tensile stresses in a simply

supported reinforced concrete

beam.

Compressive stresses take the form of an arch while the tensile stresses are shaped as a

catenary. In accordance with the rule that tensile cracks occur at right angles to the

stresses and compressive cracks in a direction, parallel to stresses, tensile and

compressive cracks are shown on the figure in black.

• At mid span in the bottom where the beam is in tension, the cracks are at right angles to the span. These are termed flexural cracks.

• Nearer the supports the cracks are inclined towards the centre of the span due to

the relationship between the principal stresses at this location . These cracks are

termed the flexural-shear cracks.

• At the top of the beam, where the beam is in compression, cracks are parallel to

the top face of the beam and are compression cracks

• Near the supports the tensile stresses, aided by the orthogonal compressive

stresses, produce diagonal cracks at right angles to the tensile stresses. These are

termed web shear cracks.

Uniformly distributed load

Principal compressive stresses

Principal tensile stresses

Web

shear

cracks

Flexural cracks Flexural

shear cracks

Compression cracks



A bridge beam tested under flexure is shown.

This close up shows the compression cracks at the top of the beam and flexural and

flexural-shear cracks.

Principal compressive stresses

Principal tensile stresses

Web

shear

cracks

Flexural cracks

Flexural

shear cracks

Compression cracks



Principles of reinforced concrete design.

Aims of Design.

The aim of Reinforced Concrete design is to ensure that :-

1. with an appropriate degree of reliability, the structure will sustain all loads and

external influences likely to occur during construction, and subsequent

occupation. It should also have adequate durability to keep maintenance costs at a

minimum. (ultimate limit state and durability)

2. with acceptable probability, the structure will remain fit for the use for which it is

required – keeping in mind the intended life of the structure and its costs.

(serviceability limit state).

3. the structure will remain ft for the use for which it is required and will not be

damaged by events such as explosions, impact or accidents “to an extent

disproportionate to the original cause”. (robustness).

These requirements can be achieved by :-

1. making a suitable choice of materials,

2. paying proper attention to design and detailing and

3. specifying control procedures for all stages of design and construction.

Future requirements for designers are likely to include :-

1. The need to design for demolition as well as construction.

2. The need to consider sustainability issues. i.e. it will be incumbent on the

designer to consider the initial cost of the building or structure, the cost of heating

and cooling the building during occupation and techniques to minimise energy

usage during the life of the structure and the cost of demolition, all at the concept

stage.

Limit states.

Definition. A limit state is defined as states beyond which the structure no longer

satisfies the performance requirements of the design and are classified as :-

1. Ultimate Limit state. (ULS).

These are associated with collapse or other forms of structural damage likely to

endanger life. They include:

• Loss of equilibrium of the structure or any part of it considered as a rigid

body.

• Failure by



o Excessive deformation

o Transformation of the structure or any part of it into a mechanism.

o Rupture

o Loss of stability of the structure or any part of it, including supports and

foundations.

o Fatigue or other time dependant effects.

The ultimate limit state should ensure structures are designed so that adequate means

exist to transmit the design ultimate dead, imposed and wind loads safely from the

highest support to the foundations. Robustness within a structure is required to prevent

disproportionate failure or collapse and as such may be considered an ultimate limit state.

Robustness.

Structures should include in the design a degree of robustness. For example, they should

not be unreasonably susceptible to the effects of accidents or explosions.

An important gas explosion in the1960’s illustrates the importance of the ULS. It

occurred on the 20th storey of a block of flats in London called Ronan Point. Instead of

merely blowing out the windows and causing local damage to the flat where the

explosion occurred, the walls were dislodged and since the floor above could not act as a

cantilever and so support itself, it collapsed onto the floor below so overloading it and

this in turn collapsed onto the floor below causing progressive failure downwards. In

addition, the floors above collapsed as some of their support had been removed and they

were not sufficiently “robust” to survive.

Ronan Point – Progressive collapse



To be robust, the layout of the structure should be such that damage to small areas or

failure of a single element will not lead to progressive collapse. The concept is, however,

wider than this and design should ensure when local high loads are sustained by part of a

structure, the whole does not fail. For example in the Ronan Point building the floor

above should have sustained all existing loads above it without collapse. Ties would

have prevented the walls from being dislodged. The best way of designing for this is to

link the structure together with ties. In addition, when designing, the removal of certain

elements from the structure must not result in the overall collapse of the building or

significant parts of the building. The collapse at Ronan Point was a significant catalyst in

causing the Building Regulations to be altered.

The figure below indicates a method of tying a structure together.

Examples of ULS. Collapse, Overturning, Buckling

External column

and wall ties

anchored or tied

horizontally into the

structure at each

floor and roof level.

Internal ties at two directions approximately

at right angles, continuous throughout their

length and anchored to the peripheral ties at

each end unless continuing as horizontal

ties in beams and walls

Continuous peripheral

ties at each floor and

roof level

Continuous

vertical ties from

foundation to roof

Robustness details



2. Serviceability Limit state (SLS)

These are concerned with the functioning of the structure under normal use, the

comfort of the people using the structure, the appearance of the structure and damage

to finishes or non structural members. They include :-

o deformations affecting the appearance, user’s comfort or effective use

of the structure.

o vibrations limiting the effective use of the structure or affecting user’s

comfort.

o cracking of the concrete affecting adversely the appearance, durability

or water tightness of the structure.

Serviceability limit states are just as important as ultimate limit states and are often the

controlling factor in design. For example it is deflection limits, not strength that control

how most slabs will span and indeed the span / depth ratio of many beams, governs the

design, not ULS.

The following effects can all influence how a structure performs often affecting the

serviceability. In most cases, however, appropriate design loads, well specified material

properties, slenderness limits and other simple rules allow for them.

• Creep

• Sway

• Shrinkage

• Settlement

• Temperature

• Cyclic loading.

Serviceability criteria govern the deflection and deformation of a structure and in this

respect, should not adversely affect its efficiency or appearance. Deflections should be

compatible with the movements acceptable by the component parts. eg. Finishes,

services, partitions, glazing and cladding.

For tall slender structures, the effect of lateral deflections should be considered.

However, the accelerations associated with the deflections may be more critical as these

are what the occupants will notice.

Cracking will always occur in reinforced concrete. Generally this is not a sign of failure

but the widths of these cracks must be limited, to ensure water and other deleterious

substances do not penetrate the concrete and cause corrosion of the reinforcement but

also as they must not be visible to the general public from say a meter or so away. Rules

exist in the codes which enable the widths of the cracks to be controlled through

including secondary reinforcement.

SLS. Deformation, Cracking, Vibration.



Durability of concrete.

Concrete durability is largely affected by how the structure performs under service loads.

Consider, for example, crack widths which are directly related to reinforcement

corrosion. The consequences of durability, though, can have repercussions on the

ultimate limit state if deterioration progresses significantly.

It is important to ensure all concrete elements are sufficiently durable. For a concrete

element to be durable it must be designed and constructed to protect the embedded

reinforcement from corrosion and generally to perform in a satisfactory manner in the

environment it is placed in and for the design life of the structure. Durability mainly

affects the reinforcement embedded within the concrete, but there are instances when the

concrete itself is affected and degrades. For example, certain types of ground sulphates

(or even pollutants with particular sulphates in them) can cause expansive reactions in

concrete significantly weakening it. Further, certain alkalis which are sometimes found

in the aggregate used to manufacture concrete have similar effects. Fortunately, both

these problems are rare in this region.

The concrete environment.

The environment in which concrete finds itself refers to any chemical or physical

reactions to which the structure as a whole, individual elements of the structure, or the

concrete itself is exposed and which may result in effects not included in the loading

conditions considered at the structural design stage. Inadequate attention to the durability

at the design and construction stages may subsequently lead to considerable expenditure

on inspection, maintenance and repair. Consequently, durability has gained in

importance in all codes over the years.

Corrosion of Reinforcement.

Steel reinforcement embedded in concrete is surrounded by a highly alkaline pore

solution with a pH value in excess of 12.5. Such an alkaline environment causes the steel

to be passivated, i.e. a highly impermeable oxide layer forms on the surface of the steel

which protects it from corrosion. Corrosion of steel reinforcement is likely to occur

when loss of passivity takes place, which is usually due to carbonation or chloride

ingress.

Carbonation ingress.

Acidic gases like carbon dioxide combine with water to form weak solutions of carbonic

acid which is washed over the surface of the concrete usually as a result of rain. This

acid reacts with parts of the concrete to de-passivate it and turn it acidic. Over time the

acidic front penetrates deeper into the concrete and if it reaches the reinforcement can

result in corrosion if moisture and oxygen are present. The penetration of the carbon



front has been observed to be proportional to (time)0.5

. The Figure below indicates early

and later stages of carbonation.

Chloride attack.

Chlorides diffuse into the concrete from the surface, the concentration decreasing with

depth. When chlorides reach the steel surface in sufficient concentration, the passive

layer is broken down and the protective alkaline environment is degraded so corrosion

can occur if water and oxygen are present.

In the past calcium chloride was used as an accelerating admixture. It caused the

concrete to gain strength more rapidly so that high rise buildings could be built more

quickly as the supporting formwork could be removed quickly. One serious side ffect of

calcium chloride was that if included in sufficient quantities and if oxygen and water

were present, reinforcement corrosion commenced. It is now banned but many buildings

still exist with calcium chloride included.

Chloride concentration

Concrete

cover

Chloride ingress into concrete

Exposed surface

Chloride

profile. Ingress

from the

surface

Chloride profile.

Chlorides included

at mixing

Exposed surface Exposed surface

Carbonation

zone

Carbonation – early stages

Reinforcement safe

Carbonation – later stages

reinforcement vulnerable to corrosion

Carbonation of concrete



Reinforcement corrosion, cover, concrete strength.

The chart shown below attempts to relate the age for corrosion activity to start, concrete

cover and concrete grade to carbonation and chloride attack.

1. Chlorides are a more sever risk than carbonation

2. For a given age for corrosion activation, a series of concrete strengths and covers

will work. For example stronger concrete and less cover has the same effect as

weaker concrete and more cover. This is an over simplification of the situation as

cover affects the effective depth (a parameter used in the design of concrete

elements) and hence the overall size of elements.

Increasing

concrete

strength

C25

C30

C35

C40

C50

10 20 30 40 50 60 70 80 90 100

Minimum cover (mm)

Ag

e o

f co

rrosi

on

act

ivat

ion

(y

ears

)

10

20

30

40

50

60

70

80

90

100

C45

C40

C35

C30

C25 Carbonation

C45

Chlorides

Effect of Carbonation and chlorides on

concrete corrosion



Alkali aggregate reaction (AAR).

Three forms of this reaction occur

1. Alkali-silica reaction (ASR). This is the most common form

2. Alkali-silicate.

3. Alkali- carbonate.

This reaction is an expansive reaction that can cause cracking and disruption of the

concrete. AAR is a reaction within the concrete unlike most durability problems which

are associated with reinforcement deterioration.

For AAR to occur all three of the following conditions must be met.

1. Sufficient moisture within the concrete

2. The concrete must have a high alkali content – alkali sources can be internal

arising from cement, water, chemical admixtures, and some aggregates or external

through exposure to sea water.

3. The aggregate must contain an akali reactive constituent. Some aggregates

containing particular varieties of silica are susceptible to attack from alkalis.

Alkali aggregate reaction is relatively rare in the UK and where past experience with

particular cement / aggregate combinations indicates no tendency to the reaction,

precautions need not be taken. If, however, unfamiliar materials are being used, there

may be a risk and additional testing or access to the national database on aggregates

should be made.

To minimize risk of AAR occurring :-

1. Limit the alkali content of the mix.

2. Use a cement with a low effective alkali content.

3. Change the aggregate to one which is known to be low risk.

4. Limit the degree of saturation or moisture content of the concrete when hardened

by using for example, impermeable membranes.

Sulphate attack.

Normally concrete is only at risk from sulphates if it is buried as it is the sulphates

present in ground water which cause degradation to the concrete. So foundations and

retaining walls for example are at risk. In addition, in areas with high atmospheric

pollution airborne suphates can cause degradation to the concrete if they are washed over

the concrete by rain over a long period of time.

Sulphates are, however, present in most cements and in some aggregates and excessive

amounts of water-soluble sulphates from these sources can be deleterious to concrete.

Sulphate attack in concrete is expansive and again affects the concrete itself.



The water soluble sulphate content (SO3) of the concrete mix should not exceed 4% by

mass of the cement in the mix. The sulphate content is calculated as the total obtained

from the various constituents of the mix.

Sulphate attack can often be offset by using sulphate resisting cement. Even using this

cement which has the ingredients likely to react with sulphates removed from it, is not

100% reliable. In association with sulphate resisting cements, good compaction and

quality control is necessary. Binary cements – those which contain pfa or ggbs are more

sulphate resistant than conventional Portland cements.

Durability and design.

Designing for strength is relatively straight forward but designers need to be aware of the

long term requirements of building and consider durability. To assess the degree of

durability required designers should consider at least the following :-

• Intended use of the structure. Designers need to view a nuclear power station

differently from a garden path. Consider a situation where many “heavy metal

rock” fans are dancing on a balcony in a purpose built venue and a car parking

garage. Different needs exist in the different situations and design will be

influenced by these.

• Required performance criteria. Using the nuclear power station and the garden

path example again. Clearly very high performance criteria are consistently

needed in the former as a failure will be catastrophic whilst in the garden path

failure will be unlikely to affect anyone’s life.

• Expected environmental conditions. Concrete protection will vary, depending on

the environment. A sea wall exposed to splash will need greater protection than

say the internal beam in a department store.

• Composition, properties and performance of materials. Durability is affected by

the aggregate and cement type and in some instances by the water used in the

concrete. These factors need to be considered at the design stage.

• Shape of members and structural detailing. Designers have the ability to

influence the architectural details to some extent. Good detailing is essential to

reduce maintenance costs.

• Quality of workmanship and level of supervision. This is obvious but sometimes

difficult to implement. The construction phase is very pressured and quality

control is important. A well built structure will always be more durable than a

poorly built one.

• Particular protective measures. Designers can reduce degradation of reinforced

concrete by including targeted protective measures. For example, waterproof

membranes can be included to prevent groundwater from saturating concrete.

Good detailing can prevent concrete from being periodically wetted.

• Likely maintenance during the intended life. Clients will want the best of both

worlds. Low build and zero maintenance costs. There is always a cost

implication in the long term if construction costs are cut.



Cover as a means of influencing durability.

Cover to the reinforcement is generally regarded as the most important line of defence

against reinforcement corrosion and is defined as the distance between the outer surface

of the reinforcement and the nearest concrete surface. It is always the least distance.

Concrete cover is required for three main reasons. Firstly to ensure the reinforcement is

not affected by deleterious materials from the environment. Second to ensure the encased

reinforcement is fully bonded, and thirdly to provide resistance to fire.

The required cover is determined using Table 4.1 (Exposure classes - durability) and

Table 4.2 (Bond requirements) of part 1 of the code. In addition, Tables NA.2 and NA.3

from the national annex provide guidance on durability requirements and Tables 5.2 –

5.11 from part 2 of the code cover the requirements for fire. These tables are given in

unit 3.

.

The following clauses from EC2 give some indication of the durability requirements. In

this area in particular, ensure the NA is also consulted. An amendment to the NA in

December 2009 changed the durability criteria.

Co

ver

Cover

Main reinforcing bars

Stirrup



4.1 Durability requirements

4.1.0 Notation (See also 1.6 and 1.7)

dg - Largest nominal maximum aggregate size

∆h - Tolerance on cover to reinforcement (difference between minimum and nominal

cover)

Φ - Diameter of a reinforcing bar, diameter of a tendon or of a prestressing duct

Φn - Equivalent diameter of a bundle of reinforcing bars.

4.1.1 General

P(1) The requirement of an adequately durable structure is met if, throughout its required

life, a structure fulfils its function with respect to serviceability, strength and stability

without significant loss of utility or excessive unforeseen maintenance.

P(2) To provide the required overall durability, as defined in P(1) above, the intended use

of the structure shall be established, together with the load specifications to be

considered. The required life of the structure and the maintenance programme shall also

be considered, in assessing the level of protection required.

P(3) Durability may be affected both by direct actions and also by consequential indirect

effects inherent in the performance of the structure (e.g. deformations, cracking, water

absorption, etc). The possible significance of both direct and indirect effects shall be

considered.

(4) For most buildings, the general provisions in this Code will ensure a satisfactory life.

However, the required level of performance — and its duration — should be considered

consciously, at an early stage in the design. Modifications to the recommended measures

may be required in certain circumstances, e.g. for temporary or monumental structures, or

for structures subjected to extreme or unusual actions (either direct or indirect effects —

see P(3) above).

4.1.2 Actions

4.1.2.1 General

P(1) Actions shall be assessed in accordance with the definitions given in 2.2.2 and based

on values given in appropriate international or national codes. In special cases, it may be

necessary to consider modification of these values to meet particular durability

requirements.

4.1.2.2 Environmental conditions

P(1) Environment, in this context, means those chemical and physical actions, to which

the structure as a whole, the individual elements, and the concrete itself is exposed, and

which results in effects not included in the loading conditions considered in structural

design.

(2) For the design of normal buildings, environmental conditions should be classified in

accordance with Table 4.1, to establish the overall level of protection required in

accordance with the provisions of ENV 206.

(3) In addition, it may be necessary to consider certain forms of aggressive or indirect

action individually (see 4.1.2.3, 4.1.2.4, 4.1.2.5).



4.1.2.3 Chemical attack

P(1) The effects of chemical attack shall be considered in design.

P(2) Consideration shall be given to the effects of chemical attack both on the concrete

and any embedded metal.

(3) Chemical attack may arise from:

— the use of the building (storage of liquids, etc);

— an aggressive environment (see Table 4.1 and ENV 206, Clause 6.2);

— contact with gases or solutions of many chemicals, but usually from exposure to acidic

solutions or to solutions of sulphate salts (see ENV 206, Table 3 and ISO 9690);

— chlorides contained in the concrete (see 5.5 in ENV 206 for the permitted maxima);

— reactions between the materials in the concrete (e.g. alkali-aggregate reaction, see 5.7

in ENV 206 and National Standards).

— (4) For most buildings, adverse chemical reactions can be avoided by adopting an

appropriate material specification, e.g. the provisions in ENV 206, to achieve a dense

impermeable concrete with appropriate mix ingredients and properties (see Table 3, ENV

206). In addition, adequate cover is required to protect the reinforcement (see 4.1.3.3).

4.1.2.4 Physical attack

P(1) The effects of physical attack shall be considered in design.

(2) Physical attack can occur because of:—

— abrasion (see 7.3.1.4 in ENV 206);

— freeze-thaw action (see ENV 206, Table 3);

— water penetration (see Table 3 and 7.3.1.5 in ENV 206).

(3) For most buildings, physical attack can be resisted through an appropriate material

specification, e.g. the provisions of ENV 206, combined with an appropriate limitation of

cracking under the relevant load combination (see 4.4.2).

4.1.2.5 Consequential indirect effects

P(1) Deformation of the structure as a whole, of individual structural elements or non-

load bearing elements (e.g. due to imposed loads, temperature, creep, shrinkage, micro-

cracking, etc.) can lead to consequential indirect effects, and these shall be considered in

design.

(2) For most buildings, the influence of indirect effects can be accommodated by

complying with general requirements, given elsewhere in this Code, for durability,

cracking, deformation, detailing, — and for strength, stability and robustness of the

structure as a whole. Additionally, consideration may have to be given to the

following:—

— minimising deformation and cracking due to time-dependent factors (e.g. early-age

movement, creep, shrinkage, etc) — see 3.1;

— minimising restraints due to deformation (e.g. by the provision of bearings or joints,

while ensuring that these do not permit the ingress of aggressive agents);

— if restraints are present, ensuring that any significant effects are taken into account in

design.



4.1.3 Design

4.1.3.1 General

P(1) Early in the design process, the effects, and possible significance, of the actions in

4.1.2 shall be considered in relation to the durability requirement in 4.1.1.

(2) For most buildings, reference should be made to the design criteria in 4.1.3.2 and to

the requirements for concrete cover to reinforcement in 4.1.3.3 and to the general

material and construction factors in 4.1.4 and 4.1.5.

(3) Other factors to be considered in design and detailing, in order to achieve the required

level of performance, should include the following:—

— the adoption of a structural form which will minimise the uptake of water or exposure

to moisture.

— the size, shape and design details of exposed elements or structures should be such as

to promote good drainage and to avoid run off or standing pools of water. Care should be

taken to minimise any cracks that may collect or transfer water. In the presence of cracks

crossing a complete section and likely to transport water containing chlorides, additional

protective measures (coated bars, coatings, etc.) may be necessary.

— attention, in design and detailing, to the different aspects of indirect effects (see

4.1.2.5);

— for most components in buildings, resistance to reinforcement corrosion is provided

by having an adequate cover of low-permeability, good-quality concrete (see 4.1.3.3 and

ENV 206). For the more severe conditions of exposure (see Table 4.1), consideration

may need to be given to protective barriers either to the concrete surface or to the

reinforcement.

4.1.3.2 Design criteria

P(1) To produce a durable concrete, the requirements primarily of chapters 2 and 3 of this

document, shall be met, together with those of ENV 206 — while considering local

conditions, materials and practices.

P(2) For reinforced concrete corrosion protection to reinforcement shall be provided by

compliance with the requirements contained in the following clauses:

4.4.1 stress conditions

4.4.2 cracking

4.4.3 deformation

4.1 (and ENV 206) general durability requirements

4.1.3.3 concrete cover

P(3) For prestressed concrete, in addition to the requirements in P(1) and P(2) above, the

prestressing steel shall be protected from all aggressive actions.

(4) For exposure classes 1–4, prestressed sections should be checked for cracking in

accordance with 4.4.2.1(7) and 4.4.2.2(5) – (8).



4.1.3.3 Concrete cover

P(1) The concrete cover is the distance between the outer surface of the reinforcement

(including links and stirrups) and the nearest concrete surface.

P(2) A minimum concrete cover shall be provided in order to ensure:

— the safe transmission of bond forces;

— that spalling will not occur;

— an adequate fire resistance;

— the protection of the steel against corrosion (see P(3) below and ENV 206).

P(3) The protection of reinforcement against corrosion depends upon the continuing

presence of a surrounding alkaline environment provided by an adequate thickness of

good quality, well-cured concrete. The thickness of cover required depends both upon

the exposure conditions and on the concrete quality.

P(4) The minimum concrete cover required for the criterion in P(3) above shall first be

determined. This shall be increased by an allowance (∆h) for tolerances, which is

dependent on the type and size of structural element, the type of construction, standards

of workmanship and quality control, and detailing practice. The result is the required

nominal cover which shall be specified on the drawings.

P(5) To transmit bond forces safely, and to ensure adequate compaction, the concrete

cover, to the bar or tendon being considered, should never be less than:

— Φ or Φn

— or (Φ + 5 mm) or (Φn + 5 mm) if dg > 32 mm

where:

Φ - is the diameter of the bar, diameter of a tendon or of the duct (post-tensioning)

Φn - is the equivalent diameter for a bundle

dg - is the largest nominal maximum aggregate size.

Reference should also be made to 5.4 in ENV 206.

P(6) The minimum concrete cover to all reinforcement including links and stirrups should

not be less than the appropriate values given in Table 4.2, for the relevant exposure class

defined in Table 4.1.

P(7) Where surface reinforcement is used (see 5.4.2.4), the cover should either comply

with (6) above, or special protective measures should be taken (e.g. protective coatings).

P(8) The allowance (∆h) for tolerances will usually be in the range |0 mm ≤ ∆h ≤ 5 mm|,

for precast elements, if production control can guarantee these values and if this is

verified by quality control. The allowance will be in the range |5 mm ≤ ∆h ≤ 10 mm| for

insitu reinforced concrete construction. Additional rules for construction and

workmanship (including tolerances) are given in chapter 6 of the code.

P(9) For concrete cast against uneven surfaces, the minimum covers given in Table 4.2

should generally be increased by larger tolerances. For example, for concrete cast directly

against the earth, the minimum cover should be greater than |75 mm|; for concrete cast

against prepared ground (including blinding) the minimum cover should be greater than

|40 mm| surfaces having design features, such as ribbed finishes or exposed aggregate,

also require increased cover.

P(10) The required minimum covers given in Table 4.2, as modified to allow for

tolerances, may be insufficient for fire protection. Particular requirements for fire

resistance are given in separate documents.



P(11) For pre-tensioned members, the minimum cover should not be less than 2Φ, where

Φ is the diameter of a tendon. Where ribbed wires are used, the minimum cover should

not be less than 3 Φ.

P(12) For post-tensioned members, the minimum cover is to the duct. The cover should

be not less than the diameter of the duct. For rectangular ducts, the cover should be not

less than the lesser dimension of the duct cross-section nor half the greater dimension.

4.1.4 Materials

P(1) Materials shall comply with the requirements of appropriate international or national

standards. The choice of materials shall be made, taking account of the environmental

conditions including any aggressive actions. These should be considered in conjunction

with other factors such as design and detailing, standards of workmanship and

construction, and intended maintenance regimes — to produce the required level of

performance for the structure throughout its service life.

(2) For concrete the requirements should generally be in accordance with ENV 206.

These requirements relate to the constituents and composition of the mix and to the

processes involved in mixing, transporting, placing, compacting and curing the concrete

in the structure.

(3) For reinforcement, the requirements of 3.2 apply.

(4) For prestressing steel, the requirements of 3.3 apply.

(5) For anchorage devices, the requirements of 3.4 apply. For exposure classes 2 – 5, any

anchorage or fixing device which is not fully embedded in the concrete may have to be

protected against corrosion by special measures.

(6) Other materials may be used, provided that full account is taken of their effects on

design requirements and that there are satisfactory data on their suitability and quality.

4.1.5 Construction

P(1) The standard of workmanship on site shall be such as to ensure that the required

durability of the structure will be obtained. The combination of materials and procedures

used in making, placing and curing the concrete shall be such as to achieve satisfactory

resistance to aggressive media for both concrete and steel.

P(2) During construction, adequate measures shall be taken, by means of supervision and

quality control, to ensure that the required properties of the materials and standards of

workmanship are achieved.

(3) The requirements for workmanship are given in chapter 6 of the code and in ENV

206.



Fire design.

The structural design for fire is covered in BS EN 1992 – 1 – 2 and its associated

National Annex. The behaviour of the structure exposed to fire is assessed by either :

• Modelled fire exposure or

• Nominal fire exposure, which uses a standard model for which tabulated data for

fire resistance is provided.

This course only covers the second point, not fire testing.

Building Regulation requirements regarding internal fire spread fall into three categories

:-

Resistance to load bearing elements, R (stability)

Resistance to fire penetration, E (integrity)

Resistance to transfer of excessive heat, I (insulation).

In England and Wales the Building regulations specify the period of fire resistance which

the structure and its elements must provide. This ranges from 30 – 240 minutes,

depending on the use of the structure, to allow occupants to evacuate the building and to

enable fire fighters to deal with the blaze in safety. Standard fire resistances are

expressed by their category identifiers and the number of minutes. For example, - R120,

EI60 (non-loadbearing), REI240, etc.

The resistance of reinforced concrete to fire depends on

• The concrete cover to the reinforcement.

• The member thickness

• The type and quality of all materials and workmanship.

Concrete covers that satisfy bond and durability requirements may not give adequate

resistance to fire.

The fire resistance of columns and walls will be based on the estimated capacity of the

element when burning and the load it sustains in the fire. As a simplification and to avoid

an extra analysis, design actions for fire design may be related to the ULS values by a

reduction factor, ηfi, as follows

Ed,fi = ηfi Ed

Where, E in the code represents the effect of an action.

So for example, with axial loads,

Nd,fi = ηfi Nd



Where, actionsofncombinatioULS

actionsofncombinatioSLSfi =η

For example 1,1,

1,

kQkG

kfik

fiQG

QG

γγ

ψη

+

+=

And 1,21,1 ψψψ orfi =

Further details on the terms used in the above equations are given in Unit 3.

As a further simplification, ηfi = 0.7 may be used (generally conservative).



The economic and environmental case for concrete framed buildings.

Which material?

Frame cost should not dictate the choice of frame material. Rather it should be just one of

a number of issues that should be considered when making the choice. Only then can one

be confident that the best and optimum structural solution is achieved. The following

checklist should assist designers and cost consultants to achieve the best value solution.

Frame costs

The recent rises in reinforcement and steel prices have increased frame costs but the

difference between the costs of steel and concrete frames still remains insignificant.

Steel v Concrete: the impact of recent price rises.

A 50% increase in European steel prices during 2004 left many in the construction

industry reviewing design solutions that have a heavy reliance on steel. A study by

leading construction economists Franklin + Andrews* examining the impact of the steel

price rises has found that the whole project costs for concrete framed buildings are

marginally less than for steel framed buildings. Since 2004, steel costs dropped but

Chinese demand is recreating the picture seen in 2004.

Costs are for the 2nd quarter of 2004.

Concrete Steel

3-storey £5,107,845 £5,190,067

7-storey £10,796,986 £10,962,115

* Economic Bulletin Volume 7. 2 , July 2004, Franklin + Andrews

Foundation costs.

Foundations typically represent approximately 3% of whole project initial cost. For the

heavier reinforced concrete solution, the foundations will be more expensive, but this still

only represents a small proportion of total cost and can be offset by using post-tensioned

slabs typically 15% lighter.

Cladding costs.

The thinner the overall structural and services zone, the less the cladding costs. Given

that cladding can represent up to 25% of the construction cost it is worth minimising the

cladding area. The minimum floor-to-floor height is almost always achieved with a

concrete flat slab and separate services zone.



Partitions

Sealing and fire stopping at partition heads is simplest with flat soffits. Significant

savings of up to 10% of the partitions package can be made compared to the equivalent

dry lining package abutting a profiled soffit with downstands. This can represent up to

4% of the frame cost.

Air tightness

Part L of the Building Regulations now require pre-completion pressure testing. Failing

these tests means a time consuming process of inspecting joints and interfaces and then

resealing them where necessary. Concrete edge details are simpler to seal and have less

risk of failure. Some contractors have switched to concrete frames on this criterion alone.

Services co-ordination/ installation/adaptability

The soffit of a concrete flat slab provides a zone for services distribution free of any

downstand beams. This reduces coordination effort for the design team and therefore the

risk of errors. It permits flexibility in design and allows co-ordination effort to be focused

elsewhere. Services installation is simplest below a flat soffit. This permits maximum off

site fabrication of services, higher quality of work and quicker installation.

These advantages should be reflected in cost and value calculations. Indeed, M&E

contractors quote an additional cost of horizontal services distribution below a profiled

slab of up to 15%. Flat soffits also allow greater future adaptability. New layouts and

cellular arrangements plus different service requirements are straightforward.

Fire protection

For concrete structures fire protection is generally not needed as the material has inherent

fire resistance of up to 4 hours. This removes the time, cost and separate trade required

for fire protection.

Acoustics

To meet the more stringent Part E of the Building Regulations, additional finishings to

walls and floors are often required. The inherent mass of concrete means these additional

finishings are minimised or even eliminated.

Vibration

The inherent mass of concrete means that concrete floors generally meet vibration criteria

at no extra cost without any extra stiffening. For more stringent criteria, such as for

laboratories or hospital operating theatres, the additional cost to meet vibration criteria is

small compared to other structural materials.



Exposed soffit

A concrete structure has a high thermal mass. By exposing the soffits this can be utilised

through fabric energy storage (FES) to reduce initial plant costs and ongoing operational

costs. Furthermore, the cost of suspended ceilings can be reduced or eliminated.

Programme

Concrete frames have short lead-in times and with modern framework systems floor-to-

floor construction periods are reduced. Most CONSTRUCT members quote

500m2/week/crane on reasonably large and simple flat slab projects and more where

Hybrid Concrete Construction can be used.

For example, where precast columns are used in conjunction with post-tensioning, one-

week cycle times are possible. However, more important is whole project programme.

Concrete provides a safe working platform and semi-internal conditions so that services

installation and follow-on trades can commence early in the programme. And concrete

has the flexibility to accommodate design changes later in the process.

Net lettable area

The difference in net lettable area provided by different solutions for a building can be of

significant value. Whilst concrete structures may have larger columns, finishing is not

necessarily required and typically columns below 0.25m2 are not deducted from net

lettable area. Reduction in column size can be achieved by the use of high strength

concrete.

Concrete structures have reduced floor-to-floor heights, hence fewer steps between floors

and less plan area. Alongside these, RC shear walls are narrower than braced steel

frames. Therefore, the stair/stability core area is minimised freeing up more net lettable

area.

Whole life value

Concrete's range of inherent benefits, fabric energy storage, fire resistance and sound

insulation means that concrete buildings tend to have lower operating costs and lower

maintenance requirements. This is an important consideration for owner-occupiers and

PFI consortia.



Hybrid construction

What is Hybrid Construction

Hybrid concrete construction marries together the advantages of precast and insitu

concrete construction with significant benefits. For example, the adoption of a hybrid

concrete frame instead of a composite steel frame on a shell-and core office project in

central London resulted in construction savings of 29 percent and a 13 percent increase in

net lettable floor area.

Utilising Hybrid Concrete construction.

The UK has been slow to realise the benefits of hybrid concrete construction (HCC),

despite the widely appreciated construction benefits. Until recently one barrier to its use

was the lack of guidance but this has now been addressed by The Concrete Centre's 'Best

Practice Guidance for Hybrid Concrete Construction'.

Further, reports such as ‘Accelerating Change’ from the Strategic Forum for Construction

and ‘Rethinking Construction’ by Egan, have focused attention on the need for the UK

construction industry to move on from its inherent conservatism, modernise and increase

efficiency.



In terms of costs, insitu reinforced concrete is commonly viewed as being the most

economic framing option while precast concrete promotes speed and factory quality.

Combining the two as a hybrid frame results in even greater construction speed, quality

and overall economy. Traditional formwork typically accounts for up to 40 percent of an

insitu frame costs. These costs can be significantly reduced by increasing the use of

precast concrete which has no on-site formwork requirement. This also reduces the

duration of operations critical to the overall construction programme. Precasting is not

constrained by site progress or conditions and can continue independently of on-site

operations. Some HCC techniques can remove the need for follow-on trades such as

ceilings and finishes further improving the programme. HCC also encourages speed of

construction by promoting increased buildability.

Concrete produces robust, and adaptable buildings that are inherently fire resistant,

vibration free and quiet. Exposure of the hybrid concrete frame can be used to exploit

concrete's inherent thermal properties to form naturally ventilated, low-energy buildings.

The finish and shape of the exposed units can also assist with even distribution of lighting

levels and the reduction of noise levels. Long spans can be easily achieved using large

units or by pre-stressing or post-tensioning.

Tunnel form of construction

Tunnel form is a modern method of construction, which simplifies the construction

process by enabling a smooth and fast operation that can result in frame costs being

reduced by 15 per cent and frame programme time savings of 25 per cent. During the

tunnel form construction process, a structural tunnel is created by pouring concrete into

steel formwork to make the floor and walls of a cellular structure. Each 24 hours, the

formwork is moved so that another tunnel can be formed. When a storey has been

completed, the process is repeated on the next floor. A strong, monolithic structure is thus

constructed that can reach 40 or more storeys in height. The use of high strength concrete

ensures fast construction.

Tunnel form construction is used in 40% of all residential construction in Belgium and

Holland. It has been used on the largest demonstration project for the Housing Forum,



the Millennium Plus development in Hackney, London and is currently being used for the

construction of a number of hotels and student residences throughout the UK.

Using concrete results in a robust construction, with excellent sound insulation and

reduced heating costs as major advantages.

Tunnel form is a fast-track method of construction that is well suited to repetitive cellular

projects such as hotels, apartment blocks and student accommodation.

With tunnel form, the structural engineer designs the one-way spanning slabs and wall.

The innovation is with the formwork system. As long as the architect has chosen or is

prepared to work within the constraints of regular wall alignments, tunnel form is an

excellent structural solution.



Self assessment questions.

1. Why do we form reinforcement cages?

2. Discuss why we need cover to reinforcement from :

a. A mechanical design point of view.

b. A durability point of view

3. When a concrete cube is crushed, vertical cracks indicate the commencement of

failure. What sort of cracks are these? How does the cube actually fail?

4. Describe the process a reinforced concrete beam undergoes from first load until

collapse if :

a. It is under reinforced

b. It is over reinforced.

5. Discuss the benefits of a concrete framed building over the whole life of the

structure from sourcing materials to disposing of the structure. Consider

maintenance, embodied energy, running costs and demolition.

6. Define ultimate & serviceability limit states for reinforced concrete.

7. Why are structures designed to be robust? Discuss this in the context of Ronan

Point and the World Trade Centre collapses.

8. Modern codes recommend a series of ties in and around a building to improve

robustness. Why should this help? How and where should ties be positioned.

9. Modern codes require designers to consider durability. How can designers ensure

chloride ingress and carbonation effects do not reduce design life.

10. Consider similar reinforced concrete buildings being built in

a. North Scotland.

b. Lusaka, Zambia.

c. Riyadh in Saudi Arabia.

Make recommendations on how designers should approach each of the bullet

points below at the three locations. The bullet points are from durability and

design, earlier in Unit 1; repeated below. • Intended use of the structure. Designers need to view a nuclear power station differently from a garden path.

Consider a situation where many heavy metal fans are dancing on a balcony in a purpose built venue and a

car parking garage. Different needs exist in the different situations and design will be influenced by these.

• Required performance criteria. Using the nuclear power station and the garden path example again.. Clearly

very high performance criterial are consistently needed in the former as a failure will be catastrophic whilst in

the garden path failure will be unlikely to affect anyone’s life.

• Expected environmental conditions. Concrete protection will vary, depending on the environment. A sea

wall exposed to splash will need greater protection than say the internal beam in a department store.

• Composition, properties and performance of materials. Durability is affected by the aggregate and cement

type and in some instances by the water used in the concrete. These factors need to be considered at the

design stage.

• Shape of members and structural detailing. Designers have the ability to influence the architectural details to

some extent. Good detailing is essential to reduce maintenance costs.

• Quality of workmanship and level of supervision. This is obvious but sometimes difficult to implement. The

construction phase is very pressured and quality control is important. A well built structure will always be

more durable than a poorly built one.

• Particular protective measures. Designers can reduce degradation of reinforced concrete by including

targeted protective measures. For example, waterproof membranes can be included to prevent groundwater

from saturating concrete. Good detailing can prevent concrete from being periodically wetted.

• Likely maintenance during the intended life. Clients will want the best of both worlds. Low build and zero

maintenance costs. There ia always a cost implication in the long term if construction costs are cut.

11. What four factors need to be considered to determine cover to reinforcement. For

each factor outline a process to enable the cover required to be found.



Learning outcomes.

At the end of this unit you should be conversant with :

• The basic constituents of reinforced concrete.

• The fundamental aims of design and how various participants react to these.

• The behaviour of reinforced concrete beams from initial load to collapse.

• The different behaviours of over and under reinforced beams.

• Ultimate and Serviceability limit states.

• Robustness requirements.

• Preventing chloride and carbonation ingress from corroding reinforcement.

• The damage sulphate attack can inflict on concrete and how to abrogate this.

• Cover requirements for reinforcement in concrete with respect to environmental

factors, fire resistance, bonding reinforcement into concrete and other deviations.

• The relative benefits of steel and reinforced concrete frames.

• Hybrid and tunnel forms of construction.

background to reinforced concrete design rc buildings

Documents

concrete design

formwork civil engineering

position civil engineering

lit of concrete civil

design of concrete structures

concrete column

concrete frame arch

cage of reinforcement