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An international review of the fire resistance of lightweight concrete
The International Journal of Lightweight Concrete, Volume 2, Number 2
An international review of the fire resistance of lightweight concrete J. C. M. Forrest*
June 1980
INTRODUCTION Since the First International Congress on Lightweight Concrete held in London just 1 2 years ago in May 1 968, there have been extensive advances in the use of lightweight concrete for building structures. Among the many examples of published works are some concerning the tallest buildings in Europe, the United States and Australia where lightweight structural concrete has been the only economical structural material to meet the design's objective. Lightweight concrete is generally chosen for its lower dead load than normal, or dense, concretes. Nevertheless, in building structures, its intrinsic thermal properties of high insulation result in buildings with a higher fire resistance than those constructed from normal concretes. It is no coincidence that the increasing use of all concretes, normal or lightweight, for good fire resistance properties has proceeded apace with the introduction of more stringent fire regulations for buildings, as the deficiencies in older constructions are revealed under fire attack.
FIP/CEB Reports The last few years have seen good progress in the publication of national and international reports covering the fire resistance of concretes, both for normal and lightweight aggregates. In 1 9 7 5 the 'FIP/CEB Recommendations for the Design of Reinforced and Prestressed Concrete Structural Members for Fire Resistance' was published and followed, in May 1 978 by the 'FIP/CEB Report on Methods of Assessment of the Fire Resistance of Concrete Structural Members'. These two reports were prepared under the direction of the FIP Commission for Fire Resistance and represent current international recommendations.
*J. c. M. Forrest is a Chartered Civil and Structural Engineer in private practice in the United Kingdom with Consulting Engineers, Kenchington Little and Partners. He has for the last fifteen years taken a special interest in the use of lightweight concretes and the fire resistance of concrete structures. His practice has contributed to the development of technology in these two special aspects of concrete and particularly in the work of the Concrete Society. tn the past John Forrest has been Chairman of the Lightweight Concrete Committee of the Concrete Society and is currently the Chairman of the Society's Fire Resistance Committee. He was a member of the Group of Engineers who provided the two reports in 1975 and 1978 on the Fire Resistance of Concrete Structures, both issued jointly by the Concrete Society and the Institution of Structural Engineers. He is also a member of the CEB/FIB Commission on Fire Resistance. © The Concrete Society, 1980 0142-0968/80/02220081/$02.00
U K ' G u i d e l i n e s ' In the United Kingdom a group of fire specialists brought together by the Institution of Structural Engineers and the Concrete Society produced a r e p o r t - - ' T h e Fire Resistance of Concrete Structures' in 1975 and this was followed, in May 1978, by a further publication of the Institution of Structural Engineers, jointly with the Concrete Society, of a more comprehensive report 'Design and Detailing of Concrete Structures for Fire Resistance'. This latter report, referred to as 'The Guidelines', is our first national definitive guide on the design of fire resistance of concrete structures and closely follows the philosophy and recommendations incorporated in the FIP/CEB international reports.
CEB/FIP Manual In 1977 'CEB/FIP Manual of Lightweight Aggregate Concrete Design and Technology' was published. It adopted the RILEM recommendation LC2 for the functional classification for lightweight concretes as set out in Table 1. The great majority of structures designed in concrete using lightweight aggregates will have concrete classified as I per this manual, and consequently this paper is confined to the use of such classification of LAC in structures. It is necessary to realise that there are few countries with more than one lightweight aggregate used for structural purposes under classification I. In the United Kingdom sintered PFA is used, in the USA, expanded
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An international review of the fire resistance of /ightweight concrete
Table 1 Functionalclassificationof lightweight concretes-RILEM recommendationLC2
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3. Empirical relationships based on fire tests which enable (i)
Class
Type of lightweight concrete Oven dry density (kg/m 3) Compressive
I
II
III
Structural
Structural and insulating Not stipulated > 3.5
Insulating
Not stipulated > 0.5
< 0.75
< 0.30
< 2000 > 15
strength (N/mm z) Coefficient of thermal conductivity (W/mK)
--
shale and slate is in common use and in Australia a natural material called scoria is used. With such variations in aggregate types and properties, an international review such as this can only generalise and not attempt to be conclusive.
THE DESIGN OF FIRE RESISTANCE OF L I G H T W E I G H T CONCRETES From an international viewpoint the design for fire resistance of all concretes is now contained in two reports. 'FIP/CEB Guide to Good Practice--Recommendations for the design of reinforced and prestressed concrete structural members for fire resistance' (1975). 'FIP/CEB report on methods of assessment of the fire resistance of concrete structural members' (1978). No separate international guidance is available on the fire resistance of lightweight concretes as these are treated in parallel with normal or dense concretes in both reports. During the 12 years since the first International Congress the technology of fire resistance based on fire resistance test data and tabulated data for minimum section sizes and concrete cover has not changed. What has occurred is a large extension of international test programmes enabling much greater certainty to be put to the figures in the tabulated data. Quite recently empirical relationships have been established for the calculation of fire resistance, both on compartments of buildings and elements of structure, and these techniques are now emerging in national guidelines such as those recently published in the United Kingdom.
Options open to the designer The options currently open for the determination of the fire resistance of any concrete constructions are: 1. Fire resistance test data from the laboratory. 2. Tabulated data on minimum section sizes and concrete covers to reinforcement compiled from programmes of fire testing similar components.
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The direct extrapolation or interpolation of the test data. (fi) The estimation of reinforcement temperature with different arrangements of concrete cover and aggregates. (/i/) The estimation of the effect of alteration in test load, boundary and support conditions or in material properties. 4. Computational techniques based on fire tests and other research studies which enable a greater understanding of the behaviour of a structural component or a complete structure exposed to fire and which can help in the estimation of one or more of the following factors: (i) Fire severity from the building and its contents. (fi) Fire severity as related to the ISO/R.834 fire resistance tests. (iii) Heat transfer into the affected sections. (/v) Alteration in the material properties. (v) Structural analysis of the component or structure. (vi) Residual strength after heating.
Use of International Reports The FIP/CEB report on methods of assessment covers the present international recommendations for design. Reference back to the FIP/CEB recommendations is made, but the designer is given greater scope by the new information on such factors as continuity within the structure, thermal restraint and the Appendices dealing with methods of calculation. For lightweight aggregate concretes both FIP/CEB reports are very conservative in their approach. Because of the large variation in the properties of lightweight aggregates used internationally, neither report can be specific without relating the recommendations to a particular aggregate. Consequently, the advantages of higher fire resistance arising from the better insulation afforded by concretes made with lightweight aggregates are only marginal. For example Tables 2 and 3 are extracts of Tables 4.5 and 4.6 respectively from the 'FiP/CEB Report on Methods of Assessment for the fire resistance of normal and lightweight concrete beams'. The footnote to Table 4.6 is qualified by a statement in the text as follows: The values given in the tables apply to normal concrete with a moisture content of 2 to 3% by weight of concrete and made with siliceous aggregates. If carbonaceous aggregates are used the minimum dimension either of the cross-section or the value of the axis distance may be reduced by between 5 and 10%. With lightweight concrete the reduction may be 20% with density of 1.2din ~. but 10% when the density is 1.85tim 3, with linear interpolation for intermediate values. Generally in walls and columns the reduction is only recommended with respect to the minimum dimension.
Design from fire testing For practical design purposes of the fire resistance of lightweight concretes such tables as in the 'FIP/CEB Method of Assessment' can only be a guide and consequently the designer will need to turn to the aggregate manufacturer in order to obtain the data required for his design.
An international review of the fire resistance of lightweight concrete
Table 2.1
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Reinforced and prestressed concrete beams.
Dense aggregate c o n c r e t e Beam width (b) in mm and corresponding minimum axis distance (a) in mm
Fire resistance
b a
80 25
120 15
160 10
200 10
80
b a
120 40
160 35
200 30
300 25
100
b a
150 55
200 45
280 40
400 35
100
b a
200 65
240 55
300 50
500 45
120
b a
240 80
300 70
400 65
600 60
140
b b
280 90
350 80
500 75
700 70
160
F30
F60
F90 F120
Minimum web thickness (ram)
F180 F240
ast = a + 10 m m
/ a
-T b
ast = a
Table 2.2 Reinforced and prestressed concrete beams Lightweight aggregate concrete*
Beam width (b) in m m and corresponding minimum axis distance (a) in mm
Fire resistance
80 20
120 15
160 10
200 10
80
100 40
160 30
200 25
300 20
80
a
120 55
200 40
280 35
400 30
80
F120
b a
160 65
240 50
300 40
500 35
100
F180
b a
190 80
300 65
400 55
600 50
115
F240
b
225 90
350 75
500 65
700 55
130
F30
F60
F90
b
Minimum web thicknes (nun)
a
b a
b
a
ast = a + l O m m
~ - ~ 0.2 bw
bmio
=
(4 3bw)
', ast = a
* Note: For lightweight concrete with a density of about 1.2 t/m 3 , the requirements may be reduced to the values given in the table. For higher densities (which in all probability will be required in practice), dimensions should be increased.
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An international review of the fire resistance of lightweight concrete
published in December 1972 and Figure 1 illustrates the results from these tests.
A series of such tests have been carried out in the United States by the Fire Research Section of the Research and Development Division of the Portland Cement Association. In their Research and Development Bulletin--'Fire Resistance of Lightweight Insulating Concretes', Messrs. Gustaferro, Abrams and Litvin described a programme of tests covering lightweight concretes with densities in the region of 5 0 0 - 1 6 0 0 kg/m 3 (30-100pcf). This bulletin gives in chart form the relationships between slab thickness and fire endurance based on 250°F temperature rise of the unexposed surface--the insulation criteria--for lightweight concretes of various densities, Where manufacturers of lightweight aggregates of similar properties have formed commercial consortia it is economical for fire resistance tests on concrete constructions to be carried out on their behalf by a national testing agency. In the USA Structural Fire Testing for lightweight aggregate concrete floor slabs has been undertaken by the Underwriters Laboratories for the Expanded Shale Clay and Slate Institute. These tests were based on the insulation property of the concrete under international test recommendations. The programme of tests is described in information Sheet No. 16 of the Expanded Shale Clay and Slate Institute
Influence of restraint and continuity afforded by the structure The United States was also among the first
to introduce into its construction standards a distinction in the fire resistance of restrained as against unrestrained structural elements. As a result of the tests at the Portland Cement Association Laboratories a report-'Fire Tests of Concrete Members: An Improved Method for Estimating Restraint Forces' (ASTM STP 464) was published in 1970. This report showed that the magnitude of thermal thrust for a given expansion varied directly with the 'heated perimeter' and the concrete modulus of elasticity. Appendix B of the 'FIP/CEB Methods of Assessment' gives a set of nomograms relating to thermal thrust, expansion, and heated perimeter for normal and lightweight concrete members that are reinforced or pre-stressed. One other structural property under fire conditions, now internationally recognised in building codes and regulations, is the influence of continuity. Both the FIP/CEB Recommendations and the 'Methods of Assessment' give clear guidance on the enhanced fire resistance available from favourable continuity, but
250
10
!
9
t .s
8
i
175
7 6
150
f
L
4
125 .
100
f
THICKNESS IN INCHES
j
3
2"5
200
J
~
J
THICKNESS IN M M 75
f
I
S0
15
2 FIRE RESISTANCE
3 IN
4
HOURS
Figure 1 Fire resistance of expanded shale clay and slate structural concrete floor slabs (by Underwriters Laboratories for ESCSI)
84
~
6
7
8
9
25 10
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An international review of the fire resistance of lightweight concrete
stipulate necessary conditions for the additional reinforcement required in beams. For continuous solid slabs, restrictions on a higher fire resistance for two way spanning are imposed and continuous hollow slab with combustible material in voids can only be classified by fire tests.
Design in the United Kingdom In the United Kingdom the fire resistance of concrete structures is controlled by the Building Regulations, except in the Inner London Boroughs where the 'London Building Acts' apply. In the Regulations current form schedule 8 specifies minimum dimensions for such elements as columns, walls, beams and slabs. This suffices for a large majority of constructions but as the schedule is mandatory problems arise where the constructions are not covered by the Schedule. The data contained in publications such as codes of practice and technical publications by aggregate manufacturers are a help. Also facilities do exist within the regulations for designers to submit alternative proposals for fire resistance design, based either on actual fire tests on the proposed construction or on a method of assessment of fire resistance based on first principles and/or rational design procedure. In the latter case the recently published 'Guidelines' have been of great assistance to the design profession. The contents of the 'Guidelines' enlarge substantially the past accepted practice of fire resistance classification in the United Kingdom. Prior to the report the designer either complied with Schedule 8 of the Building Regulations; made reference to relevant previous fire testing, or special fire tests were made on the proposed construction. This last course was time consuming as well as expensive and consequently fire testing happened very infrequently and only when a lengthy design time permitted a suitable test specimen to be designed, tested and results promulgated for use in the proposed construction. With the joint report a new era of fire resistance design arrived and the designer now has three practical options: 1. 2. 3.
Complywith Schedule 8 ofthe Building regulations. Make interpretations from preceding fire testing or carry out a new fire test and interpret accordingly. Calculate the required fire resistance from the recommendations of the 'Guidelines'.
In respect of option 3 the recommendations in the 'Guidelines' closely follow the work of A. Gustaferro and others in the Portland Cement Association of the USA in the development of a method of rational design for fire resistance. This method (known as the PCI method) recognises the behaviour under fire conditions, of the principal structural elements of beams and slabs. As the temperature in the steel rises the value of the moment of resistance of the flexural members decreases and the new moment of resistance can be computed from the properties of the concrete and steel at the elevated temperatures. By the nature of fire, the attack on beams and slabs is to affect the tensile reinforcement at an early stage, leaving the compression zones relatively unaffected except in the case of thin constructions and fires of great intensity or long duration. With the use of charts giving
the temperature at specific distances within slabs and beams for a given fire test time, the tensile reinforcement temperature and the temperature of the concrete in the compressive block are estimated and their reduced strengths estimated from temperature/strength relationship charts. At the end of the fire resistance period the reduced moment of resistance of the heated section has to be greater than or equal to the applied moment due to the service load. The PCI rational design method has been extended and amplified in the 'Guidelines'. Calculation sheets 1,2 and 3 are copied from Example 1 given in the 'Guidelines' and illustrate the type of calculations that are required under option 3 to designers in the UK. The temperature charts and strength relationship charts in the 'Guidelines' are based on the results of the PCA tests. Consequently calculations of reduced moment of resistance in heated sections equate by either the PCI method or UK 'Guidelines' method. However, the calculation methods in the UK 'Guidelines' extend the concept of the fire design of fiexural members. Chapter 8 on design deals with three modes of failure for flexural members related to the non-dimensional property of the ratio of shear span to effective depth. This property is given the value Kv and calculation methods are supplied: (i)
where the influence of shear is negligible and failure occurs in flexure only (Kv exceeds 6); (ii) where the combined effect of shear and flexure governs failure (Kv lies between 2 and 6); (iii) where the shear is predominant and flexure can be ignored (Kv is less than 2).
The guidelines lay great stress on the need for correct detailing and the provision for adequate reinforcement to ensure the safe re-distribution of loading during fire attack in the manner the design requires. The 'Guidelines' distinguish between dense, or normal, concretes and lightweight aggregate concretes, as they are based on the PCA charts. This is quite reasonable as most UK structural lightweight aggregate concretes lie in the range 1 6 0 0 - 2 0 0 0 k g / m 3, being based on the principal supply, a pelletised sintered PFA, marketed under the name of Lytag. The main impact of the two reports, ie 'FIP/CEB methods of assessment' (for international use) and the UK 'Guidelines' has been to focus the attention of the designer on a more fundamental approach to the fire resistance of concrete structures which can now be designed from first principles, or rational methods, without recourse to interpretation of a large variety of test results from tables, the derivation of which are not always well documented.
FIRE RESISTANCE C O N S T R U C T I O N - L I G H T W E I G H T CONCRETE A N D STEEL STRUCTURES The international technical press has recently given details of two examples of the uses of lightweight aggregate concrete in tall structures incorporating structural steelwork. Brussels--Cit6 Administrative Tower The last phase of this government office building complex is at presenl under construction and is scheduled for completion in
85
An international review of the fire resistance of lightweight concrete
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An international review of the fire resistance of lightweight concrete
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An international review of the fire I resistance of lightweight concrete
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An international review of the fire resistance of lightweight concrete
1982 (see Figures 2, 3 and 4). This building incorporates the latest Belgian recommendations in fire design which on a world wide analysis are exceptionally high and arise from government action following the notorious fire in the L'lnnovation store in Brussels in May 1967. (In this fire more than 350 people died and structural failure of the steel frame began within half an hour of the start of the fire.) The main tower structure consists of two 14m square service and staircase shafts in reinforced concrete surrounded by a structural steel frame with lightweight reinforced concrete floors, poured on Holorib permanent steel forms. An additional V-shaped lift tower in reinforced concrete is connected by bridges at each floor level. The outer steel frame consists of 24 perimeter columns, formed from 480 x 4 3 0 m m beams covered by 6 0 m m concrete to form columns of 600 x 550mm, and eight internal columns formed from pairs of l O 0 0 m m beam sections plated together and also covered by 6 0 m m of concrete, forming square columns of 1200 x 1200 mm. Cover to slab steel is 15 mm. The technical space above the acoustical decorative ceiling of each of the 27 office floors is divided into four compartments for fire control purposes. The ceiling of each floor is equipped with 170 smoke and heat detectors, each covering approximately 2 0 m 2.
The fire protection of the floor beams to give a resistance of 2 hours in accordance with test-method NBN 713.020 is actually in application. This fire shield consists of a spray-applied blend of plaster, expanded vermiculite, asbestos micro-fibres and proprietary binders, known as 'Vulcanit'. Because of the presence of a para-corrosion paint on the steel beams and the melting risk of this paint, the fire protection is sprayed over a rectangular shaped metal lath enveloping the beams and attached to the Holorib steel floor. The thickness of the fire shield varies from 20 to 45 mm according to the massiveness of the beams. London--National
Westminster
Figure 2 Cit~ Administrative Building, Brussels. Detail of reinforcement and fire-protection of the steel beams
/, !
BETON LEGER
/
B a n k Headquarters
The 40 storey office section of the 200 m high Nat-West rower utilises a structural steel frame supporting the floors and elevations around the massive dense concrete central core (see Figure 5). The fire resistance of the office floors is 1 hour to comply with Section 20 of the London Building Acts. To meet a 1 hour fire resistance and to keep the structure loads to a minimum, the floors, of 120 mm thickness, were constructed of Lytag concrete supported on unprotected metal decking and acting compositely with the steel floor beams. The fire resistance of this construction was determined in accordance with BS476, Part B (equivalent to ISO/R.B34) by the Institute TNO Bouwmaterialen en Bouwconstructies in
STRUCTUREL
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89
An internationa/ review of the f/~'eresistance of /ightweight concrete
Figure 3 Cit6 Administrative Building, Brussels. Last stage under construction (by courtesy of New Civil Engineer, London)
Delft, Holland and achieved a 78 minute satisfactory test under its design load of 3.85kg/m z (801b/ft2)--the required service load for offices in London. The test specimen registered a maximum temperature of 100°C at a point on its non-exposed surface (average temperature was 980°C). This illustrates the very high insulation properties of Lytag concrete and closely follows the predictions on temperature rise in lightweight concretes in international recommendations. FIRE A I - r A C K S O N L I G H T W E I G H T CONCRETE STRUCTURES In the preparation of this paper enquiries were made, through the RILEM delegates in 25 countries, of experience of actual fires that have occurred in lightweight concrete structures. Of the few replies received it was apparent that no national authority distinguishes between dense and lightweight aggregates when reporting on fires in concrete structures--not even in such a fire conscious country as the UK. Where reports have been obtained the existence of a lightweight
90
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Figure 4 Cit6 Administrative Building, Brussels. Detail view during construction (by courtesy of New Civil Engineer, London)
concrete has only emerged from an engineering appraisal of the residual safety in the structure. In all such cases the fire resistance of the structure has been exceedingly good and the building restored to usefulness with a minimum of repair or even no repair necessary. Fire in "Castlemaine' tower block, Battersea, London Under the heading 'Structure Undamaged in Fatal Tower Block Fire'--the New Civil Engineer (Journal of the UK Institution of Civil Engineers) reported, in March 1979, that the 21-storey tower block had suffered a severe fire at 16th Floor level (see Figures 6 and 7). The fire killed one tenant and injured four others, three of them firemen. The structure of the tower block is described as a concrete 'egg-crate' by its structural designer W. Atkins of W. A. Atkins Associates and was built in Lytag concrete for high fire resistance and minimum foundation loading. The concrete walls and floors exposed to the fire exhibited no spalling and a screwdriver could be pushed no more than 6 mm into the area of the soffit of the 17th floor exposed to the greatest
An international review of the fire resistance of lightweight concrete
heat. Detailed structural checking of the residual strength of the concrete and reinforcing steel is currently in hand.
Fire at Tivoli Court, 239 Bourke Street, Melbourne, Australia In December 1978 a fire occurred on the 1 lth Floor of this 15-storey office building. It was built in 1 9 7 1 - 7 2 before the sprinkler requirement in multistorey buildings was introduced. However, the building was equipped with heat detectors and operation of one of these gave an automatic alarm to the Fire Brigade. In a report on the incident the structure was described as lightweight concrete with floors of flat slab design with unreinforced drop panels (over the columns)
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100mm deep and 2.59m square. The main slab is 200 mm thick with 19 mm cover. All spalling in the main slab was shallow, no reinforcement was exposed, and did not exceed 5mm thickness. A larger degree of spalling can be seen in the drop panels especially on their corners (see Figure 8). The remedial work consisted of replacing the spalled concrete with vermiculite spray to restore the required insulation to the floor reinforcement.
Fire in housing tower block, Melbourne, Australia A recent report reveals that a fire occurred in one of the living rooms of a tenant in the upper levels of a 30-storey
Figure 5 National Westminster Bank Headquarters, London. 40-storey offices sectionwith lightweight concrete floors (by courtesyof Lytag, U nited Kingdom)
91
An international review of the fire resistance of lightweight concrete
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tower block of flats of lightweight concrete structure (see Figure 9). Despite complete.burn out of the room's contents, no structural damage was reported, but further details have been requested from the designer of the structure. Three fires in lightweight concrete structures in Japan In a report--'Spalling of Concrete in Actual
Fire' Messrs. Shirayama, Tomosawa and Kawase of the Building Research Institute, Ministry of Construction, Tsukuba Science City, Japan, gave descriptions, with special emphasis on spalling, on three fires in lightweight concrete structures. 1. Spa/ling in fire o f a reinforced concrete and steel structure building nearing completion The building was
a six-storey steel structure with a basement of reinforced concrete, having 6 0 0 0 0 m z total floor area. The floor slab of each storey was of reinforced lightweight aggregate concrete except that normal concrete was used in the basement. Fire occurred at the basement, just before the opening of the building when the concrete was approximately 6 months old. Spalling of the concrete was observed at the underside of each floor slab affected and to the reinforced concrete beams of the basement. It was so severe in the slabs that the lower reinforcing bars were entirely exposed. It was considered that the cause of this extensive spalling could be a high content of free water in the concrete, especially in the basement, and the long duration of fire owing to the large amount of combustible materials present.
Figure6 CastlemaineTower Block, Battersea, London. Fire at 16th Floor level--General view (by courtesy of New Civil Engineer, London) Figure 7 Castlemaine Tower Block, Battersea, London. Fire at 16th Floor level-Detail view (by courtesy of New Civil Engineer, London)
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An international review of the fire resistance of lightweight concrete
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Figure 8 Tivoli Court, Melbourne, Australia. Fire at 1 1th Floor--Damage to column drop (by courtesy of L Reddaway--Messrs. Irwin Johnston & Partners Pry Ltd.)
2. Spalling after gas explosion in a precast concrete structure apartment house The building was of a composite structure of H section steel frame and precast concrete panels. The large panels were made of steam cured lightweight aggregate concrete. Fire occurred after a gas explosion in an apartment on the 6th floor and spread to 5 adjoining apartments. Though the damage from the gas explosion was so heaw as to destroy the walls and slabs, spalling was observed only at the lower corner of the beams and around the ventilation openings. The incidence of only slight spalling was considered to be due to the low content of free water in the concrete from steam curing the panels. 3, Spa/ling in fire of a reinforced lightweight aggregate concrete apartment house The apartment house was
constructed of lightweight aggregate concrete, cast in-situ, of 11 storeys. Fire occurred in March 1976, 6 years after completion, in an apartment on the 8th floor which was fully burnt out. Spalling could scarcely be observed except at the under side of the balcony slab of the upper storey. The spalled par t was about 0.8 m x 0 . 5 m x 2 O m m deep. This spalled section had been attacked directly by flames emanating from the broken window, and the amount of free water in this balcony slab could well have been higher than in interior elements. ACKNOWLEDGEMENT The author gratefully acknowledges the help and guidance of colleagues in the UK and from overseas who supplied information for the compilation of this paper.
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An international review of the fire resistance of lightweight concrete
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Figure 9 Housing Tower Block, Melbourne, Australia. Fire reported to tenants flat at high level (by courtesy Cembureau--Lightweight Concrete and Messrs. W. P. Brown and D. C.Taylor - - W . P. Brown & Partners Pty Ltd.)
94
An international review of the fire resistance of lightweight concrete J. C. M. Forrest*
June 1980
INTRODUCTION Since the First International Congress on Lightweight Concrete held in London just 1 2 years ago in May 1 968, there have been extensive advances in the use of lightweight concrete for building structures. Among the many examples of published works are some concerning the tallest buildings in Europe, the United States and Australia where lightweight structural concrete has been the only economical structural material to meet the design's objective. Lightweight concrete is generally chosen for its lower dead load than normal, or dense, concretes. Nevertheless, in building structures, its intrinsic thermal properties of high insulation result in buildings with a higher fire resistance than those constructed from normal concretes. It is no coincidence that the increasing use of all concretes, normal or lightweight, for good fire resistance properties has proceeded apace with the introduction of more stringent fire regulations for buildings, as the deficiencies in older constructions are revealed under fire attack.
FIP/CEB Reports The last few years have seen good progress in the publication of national and international reports covering the fire resistance of concretes, both for normal and lightweight aggregates. In 1 9 7 5 the 'FIP/CEB Recommendations for the Design of Reinforced and Prestressed Concrete Structural Members for Fire Resistance' was published and followed, in May 1 978 by the 'FIP/CEB Report on Methods of Assessment of the Fire Resistance of Concrete Structural Members'. These two reports were prepared under the direction of the FIP Commission for Fire Resistance and represent current international recommendations.
*J. c. M. Forrest is a Chartered Civil and Structural Engineer in private practice in the United Kingdom with Consulting Engineers, Kenchington Little and Partners. He has for the last fifteen years taken a special interest in the use of lightweight concretes and the fire resistance of concrete structures. His practice has contributed to the development of technology in these two special aspects of concrete and particularly in the work of the Concrete Society. tn the past John Forrest has been Chairman of the Lightweight Concrete Committee of the Concrete Society and is currently the Chairman of the Society's Fire Resistance Committee. He was a member of the Group of Engineers who provided the two reports in 1975 and 1978 on the Fire Resistance of Concrete Structures, both issued jointly by the Concrete Society and the Institution of Structural Engineers. He is also a member of the CEB/FIB Commission on Fire Resistance. © The Concrete Society, 1980 0142-0968/80/02220081/$02.00
U K ' G u i d e l i n e s ' In the United Kingdom a group of fire specialists brought together by the Institution of Structural Engineers and the Concrete Society produced a r e p o r t - - ' T h e Fire Resistance of Concrete Structures' in 1975 and this was followed, in May 1978, by a further publication of the Institution of Structural Engineers, jointly with the Concrete Society, of a more comprehensive report 'Design and Detailing of Concrete Structures for Fire Resistance'. This latter report, referred to as 'The Guidelines', is our first national definitive guide on the design of fire resistance of concrete structures and closely follows the philosophy and recommendations incorporated in the FIP/CEB international reports.
CEB/FIP Manual In 1977 'CEB/FIP Manual of Lightweight Aggregate Concrete Design and Technology' was published. It adopted the RILEM recommendation LC2 for the functional classification for lightweight concretes as set out in Table 1. The great majority of structures designed in concrete using lightweight aggregates will have concrete classified as I per this manual, and consequently this paper is confined to the use of such classification of LAC in structures. It is necessary to realise that there are few countries with more than one lightweight aggregate used for structural purposes under classification I. In the United Kingdom sintered PFA is used, in the USA, expanded
81
An international review of the fire resistance of /ightweight concrete
Table 1 Functionalclassificationof lightweight concretes-RILEM recommendationLC2
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3. Empirical relationships based on fire tests which enable (i)
Class
Type of lightweight concrete Oven dry density (kg/m 3) Compressive
I
II
III
Structural
Structural and insulating Not stipulated > 3.5
Insulating
Not stipulated > 0.5
< 0.75
< 0.30
< 2000 > 15
strength (N/mm z) Coefficient of thermal conductivity (W/mK)
--
shale and slate is in common use and in Australia a natural material called scoria is used. With such variations in aggregate types and properties, an international review such as this can only generalise and not attempt to be conclusive.
THE DESIGN OF FIRE RESISTANCE OF L I G H T W E I G H T CONCRETES From an international viewpoint the design for fire resistance of all concretes is now contained in two reports. 'FIP/CEB Guide to Good Practice--Recommendations for the design of reinforced and prestressed concrete structural members for fire resistance' (1975). 'FIP/CEB report on methods of assessment of the fire resistance of concrete structural members' (1978). No separate international guidance is available on the fire resistance of lightweight concretes as these are treated in parallel with normal or dense concretes in both reports. During the 12 years since the first International Congress the technology of fire resistance based on fire resistance test data and tabulated data for minimum section sizes and concrete cover has not changed. What has occurred is a large extension of international test programmes enabling much greater certainty to be put to the figures in the tabulated data. Quite recently empirical relationships have been established for the calculation of fire resistance, both on compartments of buildings and elements of structure, and these techniques are now emerging in national guidelines such as those recently published in the United Kingdom.
Options open to the designer The options currently open for the determination of the fire resistance of any concrete constructions are: 1. Fire resistance test data from the laboratory. 2. Tabulated data on minimum section sizes and concrete covers to reinforcement compiled from programmes of fire testing similar components.
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The direct extrapolation or interpolation of the test data. (fi) The estimation of reinforcement temperature with different arrangements of concrete cover and aggregates. (/i/) The estimation of the effect of alteration in test load, boundary and support conditions or in material properties. 4. Computational techniques based on fire tests and other research studies which enable a greater understanding of the behaviour of a structural component or a complete structure exposed to fire and which can help in the estimation of one or more of the following factors: (i) Fire severity from the building and its contents. (fi) Fire severity as related to the ISO/R.834 fire resistance tests. (iii) Heat transfer into the affected sections. (/v) Alteration in the material properties. (v) Structural analysis of the component or structure. (vi) Residual strength after heating.
Use of International Reports The FIP/CEB report on methods of assessment covers the present international recommendations for design. Reference back to the FIP/CEB recommendations is made, but the designer is given greater scope by the new information on such factors as continuity within the structure, thermal restraint and the Appendices dealing with methods of calculation. For lightweight aggregate concretes both FIP/CEB reports are very conservative in their approach. Because of the large variation in the properties of lightweight aggregates used internationally, neither report can be specific without relating the recommendations to a particular aggregate. Consequently, the advantages of higher fire resistance arising from the better insulation afforded by concretes made with lightweight aggregates are only marginal. For example Tables 2 and 3 are extracts of Tables 4.5 and 4.6 respectively from the 'FiP/CEB Report on Methods of Assessment for the fire resistance of normal and lightweight concrete beams'. The footnote to Table 4.6 is qualified by a statement in the text as follows: The values given in the tables apply to normal concrete with a moisture content of 2 to 3% by weight of concrete and made with siliceous aggregates. If carbonaceous aggregates are used the minimum dimension either of the cross-section or the value of the axis distance may be reduced by between 5 and 10%. With lightweight concrete the reduction may be 20% with density of 1.2din ~. but 10% when the density is 1.85tim 3, with linear interpolation for intermediate values. Generally in walls and columns the reduction is only recommended with respect to the minimum dimension.
Design from fire testing For practical design purposes of the fire resistance of lightweight concretes such tables as in the 'FIP/CEB Method of Assessment' can only be a guide and consequently the designer will need to turn to the aggregate manufacturer in order to obtain the data required for his design.
An international review of the fire resistance of lightweight concrete
Table 2.1
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Reinforced and prestressed concrete beams.
Dense aggregate c o n c r e t e Beam width (b) in mm and corresponding minimum axis distance (a) in mm
Fire resistance
b a
80 25
120 15
160 10
200 10
80
b a
120 40
160 35
200 30
300 25
100
b a
150 55
200 45
280 40
400 35
100
b a
200 65
240 55
300 50
500 45
120
b a
240 80
300 70
400 65
600 60
140
b b
280 90
350 80
500 75
700 70
160
F30
F60
F90 F120
Minimum web thickness (ram)
F180 F240
ast = a + 10 m m
/ a
-T b
ast = a
Table 2.2 Reinforced and prestressed concrete beams Lightweight aggregate concrete*
Beam width (b) in m m and corresponding minimum axis distance (a) in mm
Fire resistance
80 20
120 15
160 10
200 10
80
100 40
160 30
200 25
300 20
80
a
120 55
200 40
280 35
400 30
80
F120
b a
160 65
240 50
300 40
500 35
100
F180
b a
190 80
300 65
400 55
600 50
115
F240
b
225 90
350 75
500 65
700 55
130
F30
F60
F90
b
Minimum web thicknes (nun)
a
b a
b
a
ast = a + l O m m
~ - ~ 0.2 bw
bmio
=
(4 3bw)
', ast = a
* Note: For lightweight concrete with a density of about 1.2 t/m 3 , the requirements may be reduced to the values given in the table. For higher densities (which in all probability will be required in practice), dimensions should be increased.
83
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An international review of the fire resistance of lightweight concrete
published in December 1972 and Figure 1 illustrates the results from these tests.
A series of such tests have been carried out in the United States by the Fire Research Section of the Research and Development Division of the Portland Cement Association. In their Research and Development Bulletin--'Fire Resistance of Lightweight Insulating Concretes', Messrs. Gustaferro, Abrams and Litvin described a programme of tests covering lightweight concretes with densities in the region of 5 0 0 - 1 6 0 0 kg/m 3 (30-100pcf). This bulletin gives in chart form the relationships between slab thickness and fire endurance based on 250°F temperature rise of the unexposed surface--the insulation criteria--for lightweight concretes of various densities, Where manufacturers of lightweight aggregates of similar properties have formed commercial consortia it is economical for fire resistance tests on concrete constructions to be carried out on their behalf by a national testing agency. In the USA Structural Fire Testing for lightweight aggregate concrete floor slabs has been undertaken by the Underwriters Laboratories for the Expanded Shale Clay and Slate Institute. These tests were based on the insulation property of the concrete under international test recommendations. The programme of tests is described in information Sheet No. 16 of the Expanded Shale Clay and Slate Institute
Influence of restraint and continuity afforded by the structure The United States was also among the first
to introduce into its construction standards a distinction in the fire resistance of restrained as against unrestrained structural elements. As a result of the tests at the Portland Cement Association Laboratories a report-'Fire Tests of Concrete Members: An Improved Method for Estimating Restraint Forces' (ASTM STP 464) was published in 1970. This report showed that the magnitude of thermal thrust for a given expansion varied directly with the 'heated perimeter' and the concrete modulus of elasticity. Appendix B of the 'FIP/CEB Methods of Assessment' gives a set of nomograms relating to thermal thrust, expansion, and heated perimeter for normal and lightweight concrete members that are reinforced or pre-stressed. One other structural property under fire conditions, now internationally recognised in building codes and regulations, is the influence of continuity. Both the FIP/CEB Recommendations and the 'Methods of Assessment' give clear guidance on the enhanced fire resistance available from favourable continuity, but
250
10
!
9
t .s
8
i
175
7 6
150
f
L
4
125 .
100
f
THICKNESS IN INCHES
j
3
2"5
200
J
~
J
THICKNESS IN M M 75
f
I
S0
15
2 FIRE RESISTANCE
3 IN
4
HOURS
Figure 1 Fire resistance of expanded shale clay and slate structural concrete floor slabs (by Underwriters Laboratories for ESCSI)
84
~
6
7
8
9
25 10
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An international review of the fire resistance of lightweight concrete
stipulate necessary conditions for the additional reinforcement required in beams. For continuous solid slabs, restrictions on a higher fire resistance for two way spanning are imposed and continuous hollow slab with combustible material in voids can only be classified by fire tests.
Design in the United Kingdom In the United Kingdom the fire resistance of concrete structures is controlled by the Building Regulations, except in the Inner London Boroughs where the 'London Building Acts' apply. In the Regulations current form schedule 8 specifies minimum dimensions for such elements as columns, walls, beams and slabs. This suffices for a large majority of constructions but as the schedule is mandatory problems arise where the constructions are not covered by the Schedule. The data contained in publications such as codes of practice and technical publications by aggregate manufacturers are a help. Also facilities do exist within the regulations for designers to submit alternative proposals for fire resistance design, based either on actual fire tests on the proposed construction or on a method of assessment of fire resistance based on first principles and/or rational design procedure. In the latter case the recently published 'Guidelines' have been of great assistance to the design profession. The contents of the 'Guidelines' enlarge substantially the past accepted practice of fire resistance classification in the United Kingdom. Prior to the report the designer either complied with Schedule 8 of the Building Regulations; made reference to relevant previous fire testing, or special fire tests were made on the proposed construction. This last course was time consuming as well as expensive and consequently fire testing happened very infrequently and only when a lengthy design time permitted a suitable test specimen to be designed, tested and results promulgated for use in the proposed construction. With the joint report a new era of fire resistance design arrived and the designer now has three practical options: 1. 2. 3.
Complywith Schedule 8 ofthe Building regulations. Make interpretations from preceding fire testing or carry out a new fire test and interpret accordingly. Calculate the required fire resistance from the recommendations of the 'Guidelines'.
In respect of option 3 the recommendations in the 'Guidelines' closely follow the work of A. Gustaferro and others in the Portland Cement Association of the USA in the development of a method of rational design for fire resistance. This method (known as the PCI method) recognises the behaviour under fire conditions, of the principal structural elements of beams and slabs. As the temperature in the steel rises the value of the moment of resistance of the flexural members decreases and the new moment of resistance can be computed from the properties of the concrete and steel at the elevated temperatures. By the nature of fire, the attack on beams and slabs is to affect the tensile reinforcement at an early stage, leaving the compression zones relatively unaffected except in the case of thin constructions and fires of great intensity or long duration. With the use of charts giving
the temperature at specific distances within slabs and beams for a given fire test time, the tensile reinforcement temperature and the temperature of the concrete in the compressive block are estimated and their reduced strengths estimated from temperature/strength relationship charts. At the end of the fire resistance period the reduced moment of resistance of the heated section has to be greater than or equal to the applied moment due to the service load. The PCI rational design method has been extended and amplified in the 'Guidelines'. Calculation sheets 1,2 and 3 are copied from Example 1 given in the 'Guidelines' and illustrate the type of calculations that are required under option 3 to designers in the UK. The temperature charts and strength relationship charts in the 'Guidelines' are based on the results of the PCA tests. Consequently calculations of reduced moment of resistance in heated sections equate by either the PCI method or UK 'Guidelines' method. However, the calculation methods in the UK 'Guidelines' extend the concept of the fire design of fiexural members. Chapter 8 on design deals with three modes of failure for flexural members related to the non-dimensional property of the ratio of shear span to effective depth. This property is given the value Kv and calculation methods are supplied: (i)
where the influence of shear is negligible and failure occurs in flexure only (Kv exceeds 6); (ii) where the combined effect of shear and flexure governs failure (Kv lies between 2 and 6); (iii) where the shear is predominant and flexure can be ignored (Kv is less than 2).
The guidelines lay great stress on the need for correct detailing and the provision for adequate reinforcement to ensure the safe re-distribution of loading during fire attack in the manner the design requires. The 'Guidelines' distinguish between dense, or normal, concretes and lightweight aggregate concretes, as they are based on the PCA charts. This is quite reasonable as most UK structural lightweight aggregate concretes lie in the range 1 6 0 0 - 2 0 0 0 k g / m 3, being based on the principal supply, a pelletised sintered PFA, marketed under the name of Lytag. The main impact of the two reports, ie 'FIP/CEB methods of assessment' (for international use) and the UK 'Guidelines' has been to focus the attention of the designer on a more fundamental approach to the fire resistance of concrete structures which can now be designed from first principles, or rational methods, without recourse to interpretation of a large variety of test results from tables, the derivation of which are not always well documented.
FIRE RESISTANCE C O N S T R U C T I O N - L I G H T W E I G H T CONCRETE A N D STEEL STRUCTURES The international technical press has recently given details of two examples of the uses of lightweight aggregate concrete in tall structures incorporating structural steelwork. Brussels--Cit6 Administrative Tower The last phase of this government office building complex is at presenl under construction and is scheduled for completion in
85
An international review of the fire resistance of lightweight concrete
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An international review of the fire resistance of lightweight concrete
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An international review of the fire I resistance of lightweight concrete
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An international review of the fire resistance of lightweight concrete
1982 (see Figures 2, 3 and 4). This building incorporates the latest Belgian recommendations in fire design which on a world wide analysis are exceptionally high and arise from government action following the notorious fire in the L'lnnovation store in Brussels in May 1967. (In this fire more than 350 people died and structural failure of the steel frame began within half an hour of the start of the fire.) The main tower structure consists of two 14m square service and staircase shafts in reinforced concrete surrounded by a structural steel frame with lightweight reinforced concrete floors, poured on Holorib permanent steel forms. An additional V-shaped lift tower in reinforced concrete is connected by bridges at each floor level. The outer steel frame consists of 24 perimeter columns, formed from 480 x 4 3 0 m m beams covered by 6 0 m m concrete to form columns of 600 x 550mm, and eight internal columns formed from pairs of l O 0 0 m m beam sections plated together and also covered by 6 0 m m of concrete, forming square columns of 1200 x 1200 mm. Cover to slab steel is 15 mm. The technical space above the acoustical decorative ceiling of each of the 27 office floors is divided into four compartments for fire control purposes. The ceiling of each floor is equipped with 170 smoke and heat detectors, each covering approximately 2 0 m 2.
The fire protection of the floor beams to give a resistance of 2 hours in accordance with test-method NBN 713.020 is actually in application. This fire shield consists of a spray-applied blend of plaster, expanded vermiculite, asbestos micro-fibres and proprietary binders, known as 'Vulcanit'. Because of the presence of a para-corrosion paint on the steel beams and the melting risk of this paint, the fire protection is sprayed over a rectangular shaped metal lath enveloping the beams and attached to the Holorib steel floor. The thickness of the fire shield varies from 20 to 45 mm according to the massiveness of the beams. London--National
Westminster
Figure 2 Cit~ Administrative Building, Brussels. Detail of reinforcement and fire-protection of the steel beams
/, !
BETON LEGER
/
B a n k Headquarters
The 40 storey office section of the 200 m high Nat-West rower utilises a structural steel frame supporting the floors and elevations around the massive dense concrete central core (see Figure 5). The fire resistance of the office floors is 1 hour to comply with Section 20 of the London Building Acts. To meet a 1 hour fire resistance and to keep the structure loads to a minimum, the floors, of 120 mm thickness, were constructed of Lytag concrete supported on unprotected metal decking and acting compositely with the steel floor beams. The fire resistance of this construction was determined in accordance with BS476, Part B (equivalent to ISO/R.B34) by the Institute TNO Bouwmaterialen en Bouwconstructies in
STRUCTUREL
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t
15
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'
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soud~ ~ 8(BES0} 1Sx2SCm
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Gouions
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HEB 450
89
An internationa/ review of the f/~'eresistance of /ightweight concrete
Figure 3 Cit6 Administrative Building, Brussels. Last stage under construction (by courtesy of New Civil Engineer, London)
Delft, Holland and achieved a 78 minute satisfactory test under its design load of 3.85kg/m z (801b/ft2)--the required service load for offices in London. The test specimen registered a maximum temperature of 100°C at a point on its non-exposed surface (average temperature was 980°C). This illustrates the very high insulation properties of Lytag concrete and closely follows the predictions on temperature rise in lightweight concretes in international recommendations. FIRE A I - r A C K S O N L I G H T W E I G H T CONCRETE STRUCTURES In the preparation of this paper enquiries were made, through the RILEM delegates in 25 countries, of experience of actual fires that have occurred in lightweight concrete structures. Of the few replies received it was apparent that no national authority distinguishes between dense and lightweight aggregates when reporting on fires in concrete structures--not even in such a fire conscious country as the UK. Where reports have been obtained the existence of a lightweight
90
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Figure 4 Cit6 Administrative Building, Brussels. Detail view during construction (by courtesy of New Civil Engineer, London)
concrete has only emerged from an engineering appraisal of the residual safety in the structure. In all such cases the fire resistance of the structure has been exceedingly good and the building restored to usefulness with a minimum of repair or even no repair necessary. Fire in "Castlemaine' tower block, Battersea, London Under the heading 'Structure Undamaged in Fatal Tower Block Fire'--the New Civil Engineer (Journal of the UK Institution of Civil Engineers) reported, in March 1979, that the 21-storey tower block had suffered a severe fire at 16th Floor level (see Figures 6 and 7). The fire killed one tenant and injured four others, three of them firemen. The structure of the tower block is described as a concrete 'egg-crate' by its structural designer W. Atkins of W. A. Atkins Associates and was built in Lytag concrete for high fire resistance and minimum foundation loading. The concrete walls and floors exposed to the fire exhibited no spalling and a screwdriver could be pushed no more than 6 mm into the area of the soffit of the 17th floor exposed to the greatest
An international review of the fire resistance of lightweight concrete
heat. Detailed structural checking of the residual strength of the concrete and reinforcing steel is currently in hand.
Fire at Tivoli Court, 239 Bourke Street, Melbourne, Australia In December 1978 a fire occurred on the 1 lth Floor of this 15-storey office building. It was built in 1 9 7 1 - 7 2 before the sprinkler requirement in multistorey buildings was introduced. However, the building was equipped with heat detectors and operation of one of these gave an automatic alarm to the Fire Brigade. In a report on the incident the structure was described as lightweight concrete with floors of flat slab design with unreinforced drop panels (over the columns)
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100mm deep and 2.59m square. The main slab is 200 mm thick with 19 mm cover. All spalling in the main slab was shallow, no reinforcement was exposed, and did not exceed 5mm thickness. A larger degree of spalling can be seen in the drop panels especially on their corners (see Figure 8). The remedial work consisted of replacing the spalled concrete with vermiculite spray to restore the required insulation to the floor reinforcement.
Fire in housing tower block, Melbourne, Australia A recent report reveals that a fire occurred in one of the living rooms of a tenant in the upper levels of a 30-storey
Figure 5 National Westminster Bank Headquarters, London. 40-storey offices sectionwith lightweight concrete floors (by courtesyof Lytag, U nited Kingdom)
91
An international review of the fire resistance of lightweight concrete
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tower block of flats of lightweight concrete structure (see Figure 9). Despite complete.burn out of the room's contents, no structural damage was reported, but further details have been requested from the designer of the structure. Three fires in lightweight concrete structures in Japan In a report--'Spalling of Concrete in Actual
Fire' Messrs. Shirayama, Tomosawa and Kawase of the Building Research Institute, Ministry of Construction, Tsukuba Science City, Japan, gave descriptions, with special emphasis on spalling, on three fires in lightweight concrete structures. 1. Spa/ling in fire o f a reinforced concrete and steel structure building nearing completion The building was
a six-storey steel structure with a basement of reinforced concrete, having 6 0 0 0 0 m z total floor area. The floor slab of each storey was of reinforced lightweight aggregate concrete except that normal concrete was used in the basement. Fire occurred at the basement, just before the opening of the building when the concrete was approximately 6 months old. Spalling of the concrete was observed at the underside of each floor slab affected and to the reinforced concrete beams of the basement. It was so severe in the slabs that the lower reinforcing bars were entirely exposed. It was considered that the cause of this extensive spalling could be a high content of free water in the concrete, especially in the basement, and the long duration of fire owing to the large amount of combustible materials present.
Figure6 CastlemaineTower Block, Battersea, London. Fire at 16th Floor level--General view (by courtesy of New Civil Engineer, London) Figure 7 Castlemaine Tower Block, Battersea, London. Fire at 16th Floor level-Detail view (by courtesy of New Civil Engineer, London)
92
An international review of the fire resistance of lightweight concrete
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Figure 8 Tivoli Court, Melbourne, Australia. Fire at 1 1th Floor--Damage to column drop (by courtesy of L Reddaway--Messrs. Irwin Johnston & Partners Pry Ltd.)
2. Spalling after gas explosion in a precast concrete structure apartment house The building was of a composite structure of H section steel frame and precast concrete panels. The large panels were made of steam cured lightweight aggregate concrete. Fire occurred after a gas explosion in an apartment on the 6th floor and spread to 5 adjoining apartments. Though the damage from the gas explosion was so heaw as to destroy the walls and slabs, spalling was observed only at the lower corner of the beams and around the ventilation openings. The incidence of only slight spalling was considered to be due to the low content of free water in the concrete from steam curing the panels. 3, Spa/ling in fire of a reinforced lightweight aggregate concrete apartment house The apartment house was
constructed of lightweight aggregate concrete, cast in-situ, of 11 storeys. Fire occurred in March 1976, 6 years after completion, in an apartment on the 8th floor which was fully burnt out. Spalling could scarcely be observed except at the under side of the balcony slab of the upper storey. The spalled par t was about 0.8 m x 0 . 5 m x 2 O m m deep. This spalled section had been attacked directly by flames emanating from the broken window, and the amount of free water in this balcony slab could well have been higher than in interior elements. ACKNOWLEDGEMENT The author gratefully acknowledges the help and guidance of colleagues in the UK and from overseas who supplied information for the compilation of this paper.
93
An international review of the fire resistance of lightweight concrete
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Figure 9 Housing Tower Block, Melbourne, Australia. Fire reported to tenants flat at high level (by courtesy Cembureau--Lightweight Concrete and Messrs. W. P. Brown and D. C.Taylor - - W . P. Brown & Partners Pty Ltd.)
94