Concrete_and_Fire Irish Concrete

8/7/2019 Concrete_and_Fire Irish Concrete

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Concrete

and FireUSING CONCRETE TO ACHIEVE SAFE, EFFICIENTBUILDINGS AND STRUCTURES


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CONTENTS

1 Introduction

2 Protecting people and property:

t e ro e o re sa ety stan ar s

4 What happens to concrete in a re

Material and structural performance

8 Putting it all together: re engineering

10 Continuous improvement: the role

of research and development

12 Summary

13 Further reading

e at BRE.Photo ca tions to o hereh o er

Front cover, main image:

The Royal College of Obstetricians and Gynaecologists, Regents Park

Architects: Nicholas Hare Architects LLP

Photographer: Martin Charles


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In re, concrete performs well as an engineered structure and as a

material in its own right: this publication explains how.

It is vitally important that we create buildings and structures that protect both

people and property as effectively and as efciently as possible. Annual statistics on

deaths caused by res in the home and elsewhere make for unpleasant reading and

sadly it is often through these events that we learn more about re safety design.

A mass of national and international legislation exists to protect us from the hazards of re and

it is being updated continuously as a result of research and development. However, this can prove

unwieldy for all but technical re specialists straightforward information and guidance is required

for busy professionals, whether they be architects, speciers, insurers or from Government bodies. This

publication is aimed at precisely these people who need a summary of the importanceof re safety design and the role which concrete can play in preventing the spread of re and

protecting lives. Buildings and structures are covered and reference is made to tunnels and other more

extreme situations where concrete is used.

Concrete is specied in building and civil engineering projects for several reasons, sometimes cost,

sometimes speed of construction or architectural appearance, but one of concretes major inherent

benets is its performance in re, which may be overlooked in the race to consider all the factors

affecting design decisions.

In most cases, concrete does not require any additional protection because of its built-in resistance

to re. It is non-combustible (i.e. it does not burn) and has a slow rate of heat transfer, which makes

it a highly effective barrier to the spread of re. Design of concrete structures for re is related to the

strength, continuity of reinforcement and adequate detailing of connections.

Ongoing research has included work on bre reinforcement and is supporting the move from

prescriptive design codes to a performance based approach founded on calculation methods and

advanced computer models. These demonstrate how concrete structures can be designed robustly for

the most severe re conditions.

Readers should note that this is not a comprehensive guide on re safety design; rather it is an overview

of the key issues as they relate to concrete. A list of relevant codes and standards is given on page 13 for

those seeking further technical information.


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PROTECTING PEOPLE ANDPROPERTY: THE ROLE OF FIRE

SAFETY STANDARDSWe are all aware of the damage that re can cause in terms of loss of life, homes and

livelihoods and it is always disturbing to hear of such events.

A study of 16 industrialised nations (13 in Europe plus the USA, Canada and Japan)

oun t at, in a typica year, t e num er o peope ki e y res was 1 to 2 per

100,000 inhabitants and the total cost of re damage amounted to 0.2% to 0.3%

of GNP. In the USA specically, statistics collected by the National Fire Protection

Association or t e year 2000 s owe t at more t an 4,000 eat s, over 100,000

injuries and more than $10bn of property damage were caused by re. UK statistics

suggest that of the half a million res per annum attended by reghters, about one

third occur in occupied buildings and these result in around 600 fatalities (almost all of

which happen in dwellings). The loss of business resulting from res in commercial and

ofce buildings runs into millions of pounds each year.

T e extent o suc amage wi epen on a num er o actors suc as ui ing esign

and use, structural performance, re extinguishing devices and evacuation procedures.

This is by no means an exhaustive list, but it shows how re safety is a formidable

su ject an requires c ear an e ective egis ation, an co es an gui ance on esign,

construction and occupancy. The aim of design for re safety is to ensure that buildings

and structures are capable of protecting both people and property against the hazards

o res. A t oug re sa ety stan ar s are written wit t is express purpose, it is

understandably the safety of people that assumes the greater importance. Appropriate

design and choice of materials is crucial in ensuring re safe construction.

Codes and regulations on re safety are updated continually, usually as a result ofresearc an eve opment. However, rat er more ramatic revisions may take p ace

in response to disasters or catastrophic events, which can require immediate and/or

signicant actions to prevent reoccurrence. This dates back as far as the Great Fire of

Lon on in 1666 w en re swept t roug t e streets o overcrow e , tim er rame

housing causing death and devastation. This resulted in fundamental changes to

patterns of development in the city thereafter. In fact, this is when the orientation of

t e oors, joists an eams was c ange rom running para e to a row o terrace

houses (and so providing a continuous wooden connection between adjacent houses),

to running from front to back.

More recent y, researc o owing t e Wor Tra e Centre isaster in 2001 is resu ting

in revisions to re sa ety aspects o ui ing co es re ating to ta ui ings, an in

particular the need for robust design. Progress in structural design and materials

tec no ogy as ma e structures easier to construct, more ura e an more e cient in

t e use o materia s. T e question remains w et er structures s ou a so e esigne

to survive extreme events. Concrete is very durable under most conditions and,

wit correct esign an etai ing, can make a vita contri ution to a ui ing s

in erent ro ustness.

It is vital that

buildings andstructures are

capable of

protecting people

and property

against the hazards

of re: concrete has

a major role to

play in this.


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T e Pentagon Bui ing per ormance report, pro uce

as part of this research, states that the structural

resilience imparted by the design and construction

o t is concrete ui ing provi e resistance to

progressive collapse. It also states that it is important

to integrate such features as continuity, redundancy,energy a sor ing capacity an reserve strengt , w ic

is vital for high occupancy buildings.

In practice, standard testing methods are used to

etermine t e re per ormance o materia s an

building or structural elements. These replicate the

conditions of typical res, either on a smaller scale

(e.g. in a specially built oven/furnace) or in a full-scale

test (i.e. on a part or whole mock-up of a building).

Strict contro s on t e temperatures an rate o rise in

temperature are published for three specic scenarios,

based on the conditions experienced in real res.

Stan ar re scenarios or ui ings

(ISO 834 or BS 476)

Offshore and petrochemical res (hydrocarbon test

developed by Mobil)

Tunnel res (RWS, Netherlands and RABT, Germany)

Each of these has a different (idealised) temperature-

time curve appropriate to t e con itions as s own

in the graph below. Notice that the temperature in a

ui ing re rises muc more s ow y an peaks at a

ower temperature t an, or examp e, a y rocar on

re (from burning vehicles) because there is less

ombustible material present.

Standard re curves for three scenarios: tunnels, hydrocarbons and buildings.

EXPERIENCE OF FIRES

The Concrete Society investigated a large number and

variety of re damaged concrete structures within the UK.

Part of this investigation detailed information gathered

on the performance, assessment and repair of over 100

structures, including dwellings, ofces, warehouses, factories

and car parks, of both single and multi-storey construction.

The forms of construction examined included at, trough

and wafe oors, and associated beams and columns, of

in-situ and precast construction and both reinforced and

prestressed concrete.

Examination of the items for damage and repair

showed that:

1. Most of the structures were repaired. Of those that

were not, many could have been but were demolished

or reasons other than the damage sustained.

2. Almost without exception, the structures performed

well during and after the re.


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WHAT HAPPENS TOCONCRETE IN A FIRE

FIRES

Fires require t ree components:

Fuel

Oxygen

Heat source

Fires are cause y acci ent, energy sources or natura means, ut t e majority o res

in buildings are caused by human error (e.g. discarded cigarettes). Once a re starts

and the contents and/or materials in a building are burning, then the re spreads via

ra iation, convection or con uction wit ames reac ing temperatures o etween

600C and 1200C. Harm is caused by a combination of the effects of smoke and

gases, which are emitted from burning materials, and the effects of ames and highair temperatures.

CHANGES TO CONCRETE IN A FIRE

Concrete does not burn it cannot be set on re like other materials in a building

and it does not emit any toxic fumes when affected by re. It will also not produce

smoke or rip mo ten partic es, un ike some p astics an meta s, so it oes not a to

the re load.

For these reasons concrete is said to have a high degree of re resistance and, in the

majority of applications, concrete can be described as virtually reproof. This excellent

performance is due in the main to concretes constituent materials (i.e. cement andaggregates) which, when chemically combined within concrete, form a material that

is essentially inert and, importantly for re safety design, has a relatively poor thermal

conductivity. It is this slow rate of heat transfer (conductivity) that enables concrete

to act as an effective re shield not only between adjacent spaces, but also to protect

itself from re damage.

T e rate o increase o temperature t roug t e cross section o a concrete e ement

is re ative y s ow an so interna zones o not reac t e same ig temperatures as a

surface exposed to ames. A standard ISO 834/BS 476 re test on 160 mm wide x

300 mm eep concrete eams as s own t at, a ter one our o exposure on t ree

sides, while a temperature of 600C is reached at 16 mm from the surface, this value

halves to just 300C at 42 mm from the surface a temperature gradient of 300

egrees in a out an inc o concrete!

Even a ter a pro onge perio , t e interna temperature o concrete remains re ative y

low; this enables it to retain structural capacity and re shielding properties as a

separating element.

Concrete does

not burn, producesmoke or emit

toxic vapours.

It is an effective

protection against

the spread of re

due to its slow rate

of heat transfer.

Diagrammatic representation of standard compartment re.

A standard compartment re

A: Oxygen rawn in to fee reB: Smoke plume rising

C: If the ames reach the ceiling they will spread out and increase the heat radiation

downward

D: Smoke ayer forming e ow cei ing an escen ing

E: Heat ra iate ownwar onto surface contents


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Temperature (C) What happens

1000

900 Air temperatures in res rarely exceed this level, but ame

temperatures can rise to 1200C an eyon .

800

700

600 Above this temperature, concrete is not functioning at its full structural

apacity.

550-600 Cement-based materials experience considerable creep and lose their

oa earing capacity.

400

00 Strength loss starts, but in reality only the rst few centimetres of

oncrete exposed to a re will get any hotter than this, and internally

t e temperature is we e ow t is.

50-420 Some spalling may take place, with pieces of concrete breaking away

rom t e sur ace.

Dousing a re test at BRE.

AFTER THE FIRE

Inspection of re-affected structures is based on

a visua c eck an comparison wit simi ar cases.

Any concrete exposed to temperatures above 300C

is removed and replaced. Below this temperature,

concrete can e repaire y increasing t e overa

dimensions to take the design load. Often all that is

required is a simple clean up. Speed of repair is an

important actor in minimising t e oss o usiness

after a major re.

Repair is preferable to demolition and reinstatement

or cost reasons. The Concrete Societys report

Assessment an repair o re amage concrete

structures* gives a comprehensive account of

this issue.

In rea ity, t e e aviour o concrete in re can e rat er

omplex and will very much depend on a number of

actors including mix design, imposed loads and

structura esign, w ic are out ine in t e next section.

* See page 13

Concrete provides protection against heat from re.

W en concrete is expose to t e ig temperatures

of a re, a number of physical and chemical changes

can take place. These changes are shown in the chart

e ow, w ic reates temperature eve s wit in t e

concrete (not the ame temperatures) to some

indicative changes in its properties.


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MATERIAL AND STRUCTURALPERFORMANCE IN FIREThere are two key components to concretes successful performance in re; rst its

asic properties as a ui ing materia an secon y, its unctiona ity in a structure.

We know that concrete is non-combustible and that it has a slow rate of heat transfer.

These benets applied via appropriate mix design and adequate structural detailing

mean t at, in t e vast majority o structures, concrete can e use wit out any

additional re protection.

CONCRETE AS A MATERIALBuilding materials can be classied in terms of their reaction to re and their resistance

to re, w ic wi etermine respective y w et er a materia can e use an w en

a itiona re protection nee s to e app ie to it. EN 13501-1 c assies materia s into

seven grades (A1, A2, B, C, D, E and F). The highest possible designation is A1 (non-

combustible materials) and in 1996 the European Commission compiled a binding list

o approve materia s or t is c assication, w ic inc u es concrete an its minera

constituents.

Concrete fu s t e requirements of c ass A1 ecause it is effective y non-

combustible (i.e. does not ignite at the temperatures which normally occur in re).

CONCRETE IN STRUCTURESFor concrete structures in general, good design for re safety means creating a robust

structure with adequate continuity of reinforcement and alternative load paths. But

ow oes t is overa aim trans ate to in ivi ua e ements? Co es suc as BS 8110

and Eurocode 2 are based on the premise that such elements require a measure of re

resistance appropriate to their location, function, load, level of reinforcement and, of

course, size an s ape.

Fire resistance is the ability of a particular structural element (rather than a generic

building material) to full its designed function for a period of time in the event of a

re. The function will depend on the elements position and role within the structure,

i.e. whether it has any re protecting/separation role, and the time component relates

to t e time e apse e ore one o t ree re imit states e ow is reac e ; t ese

appear in UK an European re sa ety co es.

Concrete is a

non-combustiblematerial. It can also

be used effectively

as a loadbearing,

separating and re-

shielding structural

element in a

single operation.

Dense, simply supported, solid reinforced concrete slab:

Fire resistance

period

30minutes

60minutes

90minutes

120minutes

180minutes

240minutes

Thickness (mm) 75 95 110 125 150 170Cover (mm) 15 20 25 35 45 55

Lightweight, simply supported, solid reinforced concrete slab:

Fire resistance

period

30minutes

60minutes

90minutes

120minutes

180minutes

240minutes

Thickness (mm) 70 90 105 115 135 150

Cover (mm) 15 15 20 25 35 45

Dense, concrete column: fully exposed

Fire resistance

period

30minutes

60minutes

90minutes

120minutes

180minutes

240minutes

Thickness (mm) 150 200 50 300 400 450

Cover (mm) 20 25 30 35 35 35

Lightweight concrete column: fully exposed

Fire resistance

period

30minutes

60minutes

90minutes

120minutes

180minutes

240minutes

Thickness (mm) 150 160 00 40 320 360

Cover (mm) 20 20 25 35 35 35A: The structure should retain its loadbearing capacity.

B: The structure should protect people from harmful smoke and gases.

C: The structure should shield people from heat.

Tabulated values from BS 8110.


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Relationship between applied load ratio and re resistance for reinforced

concrete columns.

T e eat ow generate y re in concrete structures

produces differential temperatures, moisture levels and

pore pressures. These changes affect concretes ability

to per orm at t e t ree imit states. As a structure

must be designed to prevent failure by exceeding the

relevant re limit states, the following changes muste avoi e :

Loss o en ing, s ear or compression strengt in

he concrete.

Loss of bond strength between the concrete and the

rein orcement.

Therefore, for any element there are two key

measurements to consi er:

1. Overa imensions, suc t at t e temperature o

he concrete throughout the section does not reach

ritical levels.

2. Average concrete cover, such that the temperature

t e rein orcement oes not reac critica eve s(500C for steel reinforcing and 350C for pre-

stressing tendons).

Accepted values for these dimensions have changed

over time as a result of research and development,

testing an o servation o re-a ecte concrete

structures, with data for design becoming more

accurate by providing additional information on:

The effects of continuity

Pre-stresse concrete

Lig tweig t concrete

Choice of aggregate

Depth of cover

Tabulated values have been compiled, which have on the

whole been extremely useful because of their inherent

simp icity. An in icative set o minimum imensions is

shown on page 6 for both dense concrete and lightweight

concrete elements. Readers should refer to BS 8110 for a

compre ensive set o ata ta es.

It is important to note that Eurocode 2 adopts a

i erent approac to re sa ety esign, w ic is muc

more exible than prescriptive data tables. It is based

on the concept of load ratio, which is the relation of

t e oa app ie at t e re imit state to t e capacity

of the element at ambient temperature.

Rea ers s ou re er to t e Euroco e ocumentation

for further details. See www.eurocode2.info for

information.

NEW FIRE STUDYA recent report as een prepare at t e request

of The Concrete Centre and the British Cement

Association to investigate the background to the

met o s or esta is ing t e re resistance o

concrete structures specied in the relevant parts of

the UK concrete Code BS 8110. The work focused on

t e origina researc an test resu ts un erpinning t etabulated data in BS 8110, which have been revisited

in order to assess the relevance of the approach to

mo ern orms o concrete construction.

T is stu y is important in t at it rings toget er in

one ocument a o y o in ormation covering test

resu ts an researc carrie out over a num er o

years. There was a danger that much of the important

work in support of the development of codes and

stan ar s wou e ost. Hence a stu y was carrie out

to collate and assess all relevant information to ensure

that the important lessons from the past are recordedn to e p ene t e strategy or a new generation o

odes and standards.

T e investigation s owe t at t e experimenta resu ts

used as the basis for developing the tabulated data in

BS 8110 support t e provisions o t e Co e in re ation

to assume perio s o re resistance. In many cases

the provisions are very conservative as they are based

n t e assumption t at structura e ements are u y

stresse at t e re imit state.

T e gures e ow are taken rom t e new re report.

The rst is a comparison between the performance

rom re tests and the assumed performance from

t e Co e or restraine oor s a s, ase pure y on

the depth of cover. The requirements are based on the

ailure of those specimens where spalling took place.

Spa ing as t ere ore een taken into account in t e

evelopment of tabulated data.

Comparison between measured (buff) and assumed (orange) periods of re

resistance based on depth of cover.


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PUTTING IT ALL TOGETHER:FIRE ENGINEERINGMoving on from the physical characteristics of concrete as a material and its

per ormance in re it is important to p ace t is in ormation in t e context o w o e

building design. For any building or structure, regardless of its complexity, design for

re safety should address the following:

Passive measures: these contain the re and prevent structural collapse, e.g. re

resisting oors an wa s.

Active measures: t ese contro t e re itse an /or aci itate t e evacuation o

occupants, e.g. automatic re alarms, sprinkler systems, smoke control fans and

smoke vents.

Passive measures are t e mainstay o re sa ety esign; in erent y re-resistant

materials such as concrete are vital. An over-reliance on active measures such as

sprinklers is not advisable water supplies may be subject to even greater demand in

t e uture an may not a ways e avaia e.

Goo practice in esign or re sa ety incorporates t ese aspects an more in w at is

termed re engineering for large, complex structures that warrant additional design

effort. This approach is based on rst principles and thus develops a specic solution

or a specic esign, w ic aims to e t e most e cient an e ective.

Although prescribed data (such as dimensions for thickness and cover) may be used,

the aim of re engineered structures is to move away from the traditional methods

an create a re strategy e icate to t e project in an , ase or examp e on t e

buildings design, how it will be used, fuel load and the probability of a re occurring.

For this reason, computer software is used to perform the probabilistic analysis of the

e aviour o ot re an peop e.

Fire engineering consi ers t e pro em o re sa ety in a o istic way, giving ue

attention to issues such as property protection where appropriate. An emphasis on

rst principles means that the nal structure can be designed as efciently as possible

ecause a actors re ating to re sa ety ave een taken into consi eration at an ear y

stage in the design process. This upfront approach ensures that undue pressure is not

put upon the structural engineer to design in additional re protection at the last

moment, w ic can e a cost y an time consuming usiness.

Taking a responsi e, rea istic approac to re engineering ca cu ations is critica . T e

design of structures should not only meet requirements, but exceed these to allow for

t e panic actor peop e eaving ui ings in an emergency o not necessari y e ave

accor ing to t e strict ogic o a re engineering ca cu ation.

Good decision making early on in the procurement and design process can make all

the difference and concrete is an excellent choice due to its inherent re resistance

properties, w ic can e urt er rene in etai e esign. Concrete is a so extreme y

compatible with a re engineering approach because the full effect of its structural

continuity can be utilised, potentially making it structurally much more efcient.

From a whole building standpoint, concrete can satisfy the four principal objectives of

re sa ety in a num er o ways some examp es are given opposite re ating to recent

European legislative requirements.

CONCRETE IN EXTREME APPLICATIONS

Concrete is the most important construction material used today. Vast volumes of it have

een an are sti eing use to ui structures, many o w ic are erecte to protect

life, the environment and property. Concrete is versatile and adaptable, and the structures

it creates can be designed to afford the desired protection.

Diagram of temperature gradients within a column at time of

maximum air temperature during a re (from BRE report).

The excellent

re protectingqualities of

concrete place it

rmly ahead of

competitors when

it comes to re

engineering.


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Tunnels

Tunnel res tend to reach very high temperatures due

to urning ue an ve ic es, reporte y up to 1350C,

but usually around 1000 - 1200C. Peak temperatures

are reached more quickly than in buildings mainly

ecause o t e caoric potentia o y rocar ons

contained in petrol and diesel fuel.

Major inci ents, suc as t ose in t e C anne Tunne

(1996), Mont Blanc (1999) and St Gotthard (2001),

have publicised the devastating consequences of tunnel

res an t e various s ortcomings o t e construction

materials and structural solutions involved.

T e use o concrete or roa sur aces in tunne s is

helpful, not only because it can provide part of the

structural design, but also because it does not burn

an t ere ore oes not a to t e re oa wit in t e

tunnel. From 2001, all new road tunnels in Austria

over one kilometre in length were required to use

a concrete pavement. Concrete is o ten use as a

tunnel lining on its own or with a thermal barrier.

Much research effort has gone into developing these

ining materia s to minimise t e e ects o spa ing

from concrete surfaces when exposed to severe res.

Protective structures

Concrete is probably the most versatile material in

t e wor wit w ic to ui protective structures or

defence, research or commercial purposes. It can be

moulded into almost any shape and designed to the

strengt necessary to wit stan pre icte impose

dynamic or static stress. Where radiation shields are

necessary normal weight concrete is considered to

e an exce ent materia or construction ecause it

attenuates gamma and neutron radiation. Concrete is

used, for example, in pressure and containment vessels

or nuc ear reactors an or partic e acce erators suc

as cyclotrons. The addition of heavier aggregates, such

as haematite, makes concrete even more effective at

preventing gamma ray penetration. T is per ormance

characteristic of concrete applies not only to

protective shields but also to the storage of radioactive

waste an structures in w ic isotopes are an e .

Blast protection

Structures that are specically meant to afford

protection against asts inc u e missi e si os,

xplosive stores, facilities where explosives are handled

nd tested, factories where explosive conditions can

rise, an mi itary an civi e ence s e ters. Concrete

is the building material of choice for such structures,

whether for an underground structure or one within a

norma ui ing .

In addition, there is growing awareness of the

vunera i ity o ui ings to externa attack t e

UK Secure and Sustainable Buildings Bill is likely to

propose changes to building design to improve blast

protection, particu ar y or Government properties.

Precast concrete cladding panels used on the MI5

Headquarters in London prevented the building

su ering signicant amage a ter a rocket attack a

ew years ago.

Liquid fuel storage

Concrete storage tanks for oil and other ammable

iqui s are seen a over t e wor . Due to concrete s

xce ent re resistance compare wit some ot er

materials, the tanks can be built nearer to one another

n any re is ess ike y to sprea to a jacent tanks.

Objective Requirement Use of concrete

To ensure stability of the

oa earing construction e ements

over a specic period of time

Elements should be made of non-combustible

materia an ave a ig re resistance.

Concrete as a material is inert and non-

combustible (class A1); most of its strength is

retained in a typical re due its low thermal

conductivity.

To limit the generation and spread

of re and smoke

Walls and ceilings should be made of non-

combustible material; re separating walls should

e non-com usti e an ave a ig re resistance.

In addition to the above, adequately designed

connections using concrete are less vulnerable to

re an make fu use of its structura continuity.

To assist the evacuation of

occupants an ensure t e safety of

rescue teams

Escape routes should be made of non-combustible

materia an ave a ig re resistance, w ic can

e use wit out anger for a onger perio .

Concrete cores are extremely robust and can

provi e very ig eve s of resistance. S ipforming

or jumpforming are particu ar y effective met o s

of construction.

To faci itate t e intervention of

rescue parties (re ghters)

Loa earing e ements s ou ave a ig re

resistance to enable effective re ghting; there

should be no burning droplets.

In a ition to t e a ove statements, in most res,

concrete will not produce any molten material.

Copyright: Daylight.com


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CONTINUOUS IMPROVEMENT:THE ROLE OF RESEARCH

AND DEVELOPMENTWe know that concrete fares well in re both as a material and as a structural element,

but what can be done to improve its performance even further? There are three main

aspects o concrete s e aviour in re t at ave warrante specic researc an

development, to improve:

1. Understanding of the physical and chemical changes that occur.

2. Predicting the effects of these changes.

3. Proposing strategies to prevent any deterioration that could compromise structural

integrity (loss of strength) and/or re resistance (loss of protection via spalling).

Systematic researc into t e e ects o re on concrete ui ings ates ack to t e ear y

1900s, wit scientists ooking at ot t e e aviour o concrete as a materia an t eintegrity of concrete structures. Franois Hennebique, one of the pioneers of reinforced

concrete, carrie out a u sca e test in Paris as ear y as 1920 at a reg ters congress.

From 1936 to 1946 a series o tests was carrie out at t e Fire Researc Station in

Borehamwood; these formed the basis for modern design codes for concrete structures

suc as CP 110, w ic ater ecame BS 8110. Furt er in ormation on t e major c anges

to re esign co es in t e UK is covere in t e compre ensive BRE stu y Fire sa ety o

concrete structures: Background to BS 8110 re design*. This report explains how research

an eve opment as in orme co e eve opment an ow newer, per ormance- ase

approac es are etter equippe to aci itate t e e cient esign o ro ust concrete

structures.

A full scale re test was carried out on the concrete test building at BRE Cardington

in Septem er 2001. T e resu ts rom t e test were summarise in t e BRE pu icationConstructing the Future issue 16. The article said, The test demonstrated excellent

performance by a building designed to the limits of Eurocode 2. The report also went on

to state, T e ui ing satise t e per ormance criteria o oa earing, insu ation an

integrity when subjected to a natural re and imposed loads. The oor has continued to

support the loads without any post re remedial action being carried out.

MOVING FROM PRESCRIPTIVE TO

PERFORMANCE-BASED DESIGN

One of the most signicant changes in re safety design for structures has been the

move away rom prescriptive, ta u ate va ues or in ivi ua e ements, w ic arebased on research tests and observations of re-affected structures. Such data can be

inherently conservative when translated into generic tables, because it assumes that

e ements act in iso ation an are u y stresse , w ereas we know t at t e e ements in

concrete structures act quite differently as part of a whole.

Individual elements that conform to a particular rating (as tested on a specimen in a

standard re) can be expected to have a much better re performance when acting as

part of a structure. In fact, the use of prescriptive, target re resistance ratings such as

those found in BS 8110 has been found to be rather limiting in practice, particularly in re

engineered structures. Elements are classied in strict time periods (e.g. 30, 60, 90 or 120

minutes) and the delineation between aggregates is based simply on lightweight or dense

concrete, which does not reect the range of concretes commonly used today.

For these reasons, performance-based structural analysis (using computers) has cometo t e ore wit mo e ing tec niques now capa e o simu ating structura con itions

that are very difcult to study even in a full-scale re test, such as cooling patterns

after collapse. The development of such software has encompassed thermal analysis

Almost 100 years

of dedicatedresearch on

concretes

inherent strengths

in re has resulted

in a culture

of continuous

improvement.

* See page 13


13/16

11

(e.g. for separating walls), structural analysis (e.g.

for loadbearing oors) and hydral analysis (i.e. to

predict moisture movement and spalling). Computer

programs capa e o per orming a t ree types o

analysis (thermohydromechanical analysis) were rst

developed in the 1970s and have been rened byEuropean researc ers in t e UK an Ita y, particu ar y

in response to tunnel res.

Since the 1990s, the performance-based approach has

permeate into nationa ui ing co es in countries

such as Sweden, Norway, Australia and New Zealand

this is a cost effective and highly adaptable approach

to esign. Euroco e 2 is ase on suc an approac

to re safety design and by considering minimum

dimensions in terms of load ratios for individual

e ements, it is in erent y more exi e an we

founded in its methodology.

USE OF FIBRES TO

PREVENT SPALLING

Researc as a so ocuse on t e use o res in

oncrete as a means of preventing the worst cases of

spalling (where pieces of concrete fall away from the

sur ace o a structura e ement w en it is expose to

high temperatures). It is caused mainly by a build up

f pressure beneath the concrete surface (in pores) so

racks eve op para e to t e sur ace. W en t e sum

f such stresses exceed that which the concrete can

ccommodate, there is a sudden release of energy and

portion o t e eate area reaks away.

T e worst examp es can occur, or examp e, in tunne

res, where burning vehicles, fuel and loads have

aused major incidents. Here the concrete tunnel

ining may reac a very ig temperature an concreteparticles may spall, sometimes explosively, away from

the surface.

Hig per ormance concretes, w ic are o ten use or

tunne s an ri ges, can e particu ary vu nera e to

spalling because they are very dense. These concretes

re c aracterise y ow permea i ity an so pore

pressure can ui up easi y. One option is to cover

the surface of the structural concrete with a thermal

arrier, ut a more e cient so ution is to incorporate

po ypropy ene res wit in t e mix. By me ting at

160C, these bres provide channels for moisture

movement wit in t e concrete, t us increasing

permea i ity an re ucing t e risk o spa ing .

The use of bres is a proven technique and research is

ontinuing to optimise performance. Computer software

is now avai a e t at pre icts spa ing y mo e ing

the build up of pore pressures in concrete during re.

These models give a far more accurate picture of what

is a very comp ex p enomenon, ena ing esigners to

produce more efcient concrete structures.

Polypropylene bres provide protection against spalling.


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12

Concrete is a

versatile materialand when

appropriately

designed is

inherently re

proof due to its

non ammability

and thermal

insulation

properties.

SUMMARYFire sa ety is a key consi eration in t e esign an use o ui ings an structures;

extensive legislation and design codes are in place to protect people and property

from the hazards of re. The continuous development of these codes has ensured thatongoing researc an eve opment work is incorporate in current practices, uring

design, construction and occupancy.

Extensive research on the performance of concrete in re means that we have an

exce ent un erstan ing o t e e aviour o concrete ot in a structure an as a

materia in its own rig t. T is asic science wi provi e t e essentia in ormation to

support the move from prescribed tabulated values for re resistance to computer

simu ation an per ormance ase re engineering.

W i e prescriptive ata wi continue to ave a ro e to p ay, new stan ar s suc as

Eurocode 2 will incorporate greater degrees of exibility to the sizing of concrete

elements for re safety. This means designers will have more scope for efcient design

o concrete structures t at meet everyone s nee s.

BENEFITS OF USING CONCRETE

Concrete is non-combustible (i.e. it does not burn).

Concrete is inherently re resistant (i.e. it does not support the spread of re).

Concrete has a slow rate of heat transfer (making it an effective re shield).

Concrete oes not pro uce any smoke, toxic gases or emissions in a re situation.

Concrete does not contribute to the re load of a building.

Under typical re conditions, concrete retains most of its strength.

Polypropylene bres can be used to prevent spalling.

Ski u mix esign urt er renes concretes in erent per ormance.

For the vast majority of applications, concrete does not require any additional,

ost y reproong measures.

Fire amage to concrete is typica y minima , requiring on y a minor c ean up.

Concrete has been given the highest possible material classication for

its re resistance.

Connections designed using concrete are more robust in a re situation.

Bespoke concrete mixes can be designed to cater for extreme re loads.


15/16

13

FURTHER READINGBuilding Research Establishment. Fire safety of concrete structures: background to BS 8110 re design.

Building Research Establishment/Fire Research Station, Garston, Watford, UK. 2004. 41 pp.

Cem ureau/BIBM/ERMCO. Improving re sa ety in tunne s: t e concrete pavement so ution.

Cembureau. Brussels, Belgium. April 2004. 8 pp.

Concrete Rein orcing Stee Institute. Fire resistance o rein orce concrete ui ings.

Engineering Data Report Number 52. CRSI. Schaumburg, Illinois, USA. 2003. 6 pp.

Concrete Society.Assessment and repair of re damaged concrete structures.

Tec nica Report TR33. Concrete Society. Cam er ey, Berks ire, UK. 1990. 77 pp.

Federal Emergency Management Agency. World Trade Center building performance stud y.

FEMA Region II, New York, USA. May 2002.

Horvath. S. Fire safety and concrete: re resistance and architectural design.

Cimbeton (Centre for Information on Cement and its Uses). Paris, France. 13 pp.

K oury, G. E ect o re on concrete an concrete structures. Procee ings o Structura Engineering Materia s Journa .

John Wiley & Sons, Chichester, UK. 2000. Vol.2. pp.429-447.

Kruger, J.E. an Lunt, B.G. Protection a or e y concrete. Division o Bui ing Tec no ogy, CSIR, Sout A rica.

Lunt, B.G. Civil defence planning and the structural engineer.

Neck, U. Comprehensive re protection with precast concrete elements the future situation in Europe.

In BIBM 17t Internationa Congress o Precast Concrete In ustry: Congress Procee ings. Turkis Precast Concrete

Association, Ankara, 2002. (CD)

Sto ar , P. an A ra ams, J. ire rom rst princip es: a esign gui e to ui ing re sa ety(2nd edition).

E & FN Spon, Lon on, UK. 1995. 178 pp.

RELEVANT CODES AND STANDARDSEuroco e 2 Part 1.2. Design o concrete structures; Genera ru es structura re esign.

BSI, London. 2002. 102 pp.

BS 8110 Structural concrete. Use of concrete,Parts 1, 2 and 3.

EN 13501-1, Fire c assi cation o construction pro ucts an ui ing e ements. Part 1: C assi cation using test ata

from reactions to re tests. 2002. 44 pp.

ACKNOWLEDGEMENTSThe Concrete Centre wishes to acknowledge assistance from BCA and CEMBUREAU.

PICTURE CREDITSFront cover Martin Charles; Design and Materials

Insi e ront cover BRE

Page 1 Traditional Housing Bureau

Page 2 Trevor Jones

Page 5 Rastra; BRE

Page 9 CEMBUREAU; Roger Eastell

Page 11 Christeyns UK/Oscrete

Page 12 Britis Precast

Outside back cover Trevor Jones


16/16

CPD presentations on concrete and re are available on request.Please call 0700 4 500 500 or 0700 4 CONCRETE, or [email protected]

Ref. TCC/05/01ISBN 1-904818-11-0

First pu is e 2004

The Concrete Centre 2004

T e Concrete Centre, Riversi e House,

4 Meadows Business Park, Station Approach, Blackwater, Camberley, Surrey GU17 9AB

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