CS_-_Assessment.pdf

17th March 2008 Section 3.4.4 amended

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Draft Technical Report ??

Assessment, design and repair of fire-damaged concrete structures

Final draft

March 2008


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Contents Members of the Working Party Acknowledgements List of Figures List of Tables Foreword 1 Introduction

1.1 Scope 1.2 Process 1.3 Health and safety

2 Assessment of damage 2.1 Objectives and methodology of assessment 2.2 Materials

2.2.1 Effects of high temperature on concrete strength and elastic modulus 2.2.2 Mineralogical changes in concrete 2.2.3 Cracking of concrete in fires 2.2.4 Spalling of concrete in fires 2.2.5 Residual thermal movement cracks 2.2.6 High-alumina cement 2.2.7 Reinforcing and prestressing steel 2.2.8 Degradation of other materials

2.3 Testing of fire damaged reinforced concrete 2.3.1 On-site inspection 2.3.2 Non-destructive testing (NDT) 2.3.3 Petrographic examination 2.3.4 Thermoluminescence tests 2.3.5 Core test 2.3.6 Tests on samples of reinforcement 2.3.7 Other laboratory tests

2.4 Assessment of fire damaged structures 2.4.1 Introduction 2.4.2 Testing 2.4.3 Assessment of fire severity 2.4.4 Heat transfer

2.5 Presentation of data 3 Design

3.1 Design philosophy 3.1.1 Objectives 3.1.2 Building regulations

3.1.3 Codes of practice 3.1.4 Design assumptions 3.2 Structural analysis and member design

3.2.1 Structural analysis 3.2.2 Element design

3.3 Repair criteria


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3.3.1 Reduced material strengths 3.3.2 Residual strength factor 3.3.3 Bond strength

3.3.4 Bar size and spacing 3.3.5 Shear reinforcement 3.4 Member design 3.4.1 General

3.4.2 Beams and slabs bending 3.4.3 Beams shear

3.4.4 Columns 3.4.5 Walls 3.5 Design output 3.5.1 Demolition and construction sequence drawings 3.5.2 Key plans 3.5.3 Design details 3.5.4 Specifications 3.5.5 Design calculations 3.5.6 Method statements 3.6 Load tests 4 Repair methods

4.1 General 4.2 Health and safety 4.3 Quality control 4.4 Surface cleaning 4.5 Breaking out 4.6 Reinforcement

4.6.1 Bar size and spacing 4.6.2 Connecting reinforcement

4.7 Mortar 4.8 Flowable micro-concrete and concrete 4.9 Sprayed concrete

4.9.1 General 4.9.2 Health and Safety 4.9.3 Substrate preparation 4.9.4 Layer thickness 4.9.5 Surface finishing 4.9.6 Curing 4.9.7 Repair details

4.10 Resins 4.11 Strengthening with fibre composites

References Further reading Appendix A Case studies A1 Fires in buildings A2 Fires under bridges Appendix B Worked examples


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B1 Introduction B2 Example 1 Continuous slab B3 Example 2 Simply supported tee beam B4 Example 3 Axially loaded column

Appendix C Historical information C1 Design codes C2 Specification and strength of historic concrete

C3 Reinforcement C3.1 Early reinforcement systems C3.2 Standards and strengths C3.3 Detailing symbols


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Members of the Working Party Full Members Florian Block Buro Happold John Clarke The Concrete Society (Secretary) Brian Cole Buro Happold (Chairman) Susan Deeny Edinburgh University Alexander Heise Arup Fire Jeremy Ingham Halcrow Group Limited Nigel Pierce CRL Surveys Corresponding Members Simon Bladon CRL Surveys Pal Chana British Cement Association Stuart Matthews Building Research Establishment Ganga Prakhya Sir Robert McAlpine Acknowledgements The Concrete Society is grateful to the following for the provision of photographs:

Buro Happold: [To be checked.] Concrete Repairs Limited: Figures 16, 2124 Jeremy Ingham: Figures 1, 2, 4, 5, 11, 15, 1719.

List of Figures Figure 1 The interior (left) and exterior (right) of a concrete framed structure shortly

after a major fire during construction. Figure 2 View of the same structure as Figure 1.1 after repair of fire damage. Figure 3 Typical effect of heat upon the compressive strength of dense aggregate

concrete after cooling. Figure 4 Appearance of flint aggregate concrete cores which have been heated for

hour (upper row) and 2 hours (lower row), at the temperatures indicated. Figure 5 View of floor below the fire showing thermal expansion cracks on the slab

soffit. Figure 6 Surface crazing. Figure 7 Explosive spalling. Figure 8 Sloughing off. Figure 9 Spalling of a slab soffit owing to fire-damage of embedded plastic

reinforcement bar spacers. Figure 10 Yield strength of reinforcing steels at room temperature after heating to an

elevated temperature. Figure 11 Buckled bars. Figure 12 Tensile tests on untreated 0.76% carbon steel wire at high temperatures. Figure 13 Temperature effects upon relaxation of untreated cold-drawn prestressing wire. Figure 14 Ultimate strength of prestressing steels at room temperature after heating to an

elevated temperature. Figure 15 Melting of aluminium formwork supports indicating that the fire reached

temperatures in excess of 650C.


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Figure 16 A fire-damaged reinforced concrete slab soffit showing pink/red discolouration of flint aggregate particles.

Figure 17 Technicians diamond drilling core samples through the full thickness of a fire-damaged concrete floor slab.

Figure 18 View of spalled and discoloured fire-damaged concrete slab soffit, showing the location of a core sample (centre) that was taken to aid determination of the depth of fire damage.

Figure 19 A photomicrograph of fire-damaged concrete seen through the optical microscope.

Figure 20 Typical section of key diagram classification. Figure 21 Breaking out small area of concrete using hand-held equipment. Figure 22 Breaking out using hydro-demolition. Figure 23 Hand-applied mortar repair. Figure 24 Sprayed concrete application. Figure 25 Sprayed concrete repairs to beams. Figure 26 Sprayed concrete repairs to columns. Figure 27 Sprayed concrete repairs to floor slabs. Figure B1 Original slab profile. Figure B2 Damaged slab. Figure B3 Temperature profiles. Figure B4 Repaired section. Figure B5 Original beam profile. Figure B6 Damaged beam profile. Figure B7 Temperature profiles. Figure B8 Temperature profile at corner. Figure B9 Support of added main bars. Figure B10 Anchorage of shear links. Figure B11 Repaired section. Figure B12 Original column profile. Figure B13 Damaged column profile. Figure B14 Temperature profiles. Figure B15 Temperature profile at corner. Figure B16 Repaired section. List of Tables Table 1 Stages in the assessment and repair process. Table 2 Mineralogical and strength changes to concrete caused by heating. Table 3 Assessment of temperature reached by selected materials and components in

fires Table 4 Notional rate of charring for the calculation of residual section. Table 5 A guide to the selection of test methods for fire-damaged reinforced concrete. Table 6 Cycle of effects upon reinforced concrete structures. Table 7 An example of a visual damage classification scheme for reinforced concrete

elements. Table 8 Initial repair classification. Table 9 Typical section of schedule for damage classification shown in Figure 20. Table 10 Features of methods of breaking out concrete. Table 11 Advantages and disadvantages of sprayed concrete


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Table C1 The development of design codes. Table C2 Reinforcement standards and associated strengths. Table C3 Detailing symbols.


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Foreword Concrete has good inherent fire-resistant and concrete structures are generally capable of being repaired after a fire, even a severe one. The initial guidance on assessment and repair was published by The Concrete Society in 1978 as Technical Report 15, Assessment of fire-damaged concrete structures and repair by gunite(1). In the late 1980s The Society was concerned that the guidance should remain useful and a Working Party was set up to update TR 15 and to include methods of repair other than gunite (sprayed concrete), which by then had its own Code of Practice. This led to the publication in 1990 of Technical Report 33, Assessment and repair of fire-damaged concrete structures(2). In 2007 The Society again reviewed the guidance given in TR 33 and concluded that much of it was still sound and that the Technical Report was widely used. However, there was a need to bring the material in the document into line with current Standards and repair techniques. The emphasis in TR 33 was still on the use of sprayed concrete, with little mention of other repair methods. In addition, assessment techniques, such as petrography, and analytical methods had advanced significantly. A small working party was formed, which prepared the present Technical Report. The emphasis of this report is on methods for assessing a concrete structure following a fire and hence for determining the extent of the required repairs. The design approaches used to assess the strength of repaired elements, illustrated by design examples, are in accordance with the relevant Eurocodes(3, 4, 5). The chapter on repairs is somewhat more limited than in previous versions of the Technical Report as the working party considered that techniques are common to all concrete repairs, irrespective of the cause of the damage, and not simply to the repair of fire-damaged concrete structures. Finally the report includes summaries of a number of case studies of the assessment and repair of structures damaged by fire.


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1 Introduction Concrete is inherently fire-resistant and concrete structures are generally capable of being repaired after a fire, even a severe one. In the 1980s, Tovey and Crook(6, 7) summarised the information gathered from over 100 fire-damaged structures. They concluded that, almost without exception, the structures performed well during and after the fire. Most of the structures were repaired and returned to service; when structures were demolished and replaced, it was generally for reasons other than the damage sustained during the fire. Some more recent case studies are given in Appendix A of this Technical Report, which outline the damage caused by the fire and the subsequent investigation. The examples include residential buildings, commercial buildings and bridges. In all but one case the structure was successfully repaired. 1.1 SCOPE The emphasis of this report is on methods for assessing a concrete structure following a fire and hence for determining the extent of the required repairs. The design approaches used to assess the strength of repaired elements, illustrated by the design examples in Appendix B, are in accordance with the relevant Eurocodes(3, 4, 5). In addition to structural damage, there may be smoke damage to partitions, electrical and mechanical systems etc. Although the associated costs of cleaning or replacing such systems can be significant, they are not considered in this report. The focus of this report is on fires in reinforced concrete buildings, including multi-storey structures, warehouses and factories, but the principles are equally applicable to civil engineering structures, such as bridges. However, tunnels are specifically excluded as an assessment of their performance will require specialised geotechnical input, which is beyond the scope of this report. There is a major difference between designing a structure to withstand a fire, allowing for safe evacuation and fire fighting, and assessing the extent of damage caused by a fire so that repair methodologies can be proposed. While designing structures is predicting performance during a future event, assessing structures is determining its residual strength after such an event. Hence, the focus in the latter case and in this report is on methodologies to measure on site the residual strength and deformations and to obtain evidence of the temperatures reached during the fire. Calculation methodologies are presented that may assist during the evaluation process, but the working party felt that any assessment needs to be based mainly on an on-site evaluation of the fire damaged structure, which is supplemented as necessary by laboratory testing, examination or numerical assessment. In all cases, it is important that the assessment work is carried out by a competent person, who is aware of the limits of applicability for any methodology and whether special considerations for certain construction methods are required. The competent person needs to be aware that material properties and calculation methodologies presented in Eurocode 2 may not be applicable to the specific situation, since effects such as cooling of the structure or restraint and residual stresses need consideration after a fire event. This means that although the structure may have served its purpose according to Building Regulations and allowed for safe evacuation and fire fighting, considerable effort may be required to strengthen the structure for future occupation after a fire.


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A brief chapter on repair techniques is included, which makes reference to more detailed guidance. The working party considered that techniques are common to all repairs, irrespective of the cause of the damage, and not simply to the repair of fire-damaged concrete structures. Finally appendices to the report includes summaries of a number of case studies of the assessment and repair of structures damaged by fire, worked examples and historical information on design and material properties given in British Standards and other documents. 1.2 PROCESS After a fire the focus is on immediate measures for securing public safety. In the UK, the fire brigade will usually secure the building; if they have any doubts about the stability of the structure they will call in the local Building Control Officer to make an assessment. After a serious fire the Building Control Officer may require parts of the structure to be demolished or stabilised before anyone else can enter. If part of a damaged building is to remain occupied while repairs are carried out elsewhere, it will be necessary to establish that the remaining escape routes, fire separation, fire protection systems etc are adequate throughout. The responsible person, as defined in the Regulatory Reform Order(8), is required to assess whether the building is deemed safe. The Fire and Rescue Authority can request that compliance with the requirements of the fire safety order is demonstrated. The fire authority has the powers to take enforcement action where requirements of the order are breached or where a serious risk to life exists. Often the authority will also be notified by the police, who may investigate arson. Finally, the insurers may commission an investigation of the damage. The insurer will often have a major interest in finding the most cost effective solution for repairing the structure. When a fire has occurred, the requirements are generally for an immediate and thorough appraisal to be carried out, with clear objectives. Such an appraisal should begin as soon as the building can be entered safely and generally before the removal of debris. The competent person needs to establish whether the building is safe or not and propose propping of the structure, if required. Propping might not be required if the structure is too damaged for repair and demolition is proposed. The spalled and discoloured or blackened concrete surfaces and exposed reinforcement generally apparent after a severe fire often present a picture which suggests widespread and significant damage; Figure 1 shows the aftermath of a fire in a concrete multi-storey building under construction (see Ingham(9)). However, in practice the damage may be much less severe. It is necessary to be strictly objective and to consider the effect of high temperature upon the properties of the materials concerned. This is considered in Chapter 2.


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Figure 1: The interior (left) and exterior (right) of a concrete framed structure shortly after a major fire during construction. Remedial measures are likely to be based on a comparison of the cost of removal and the need for replacement. Experience shows that, following detailed appraisal, reinforced concrete structures can nearly always be repaired by means of a selection of suitable techniques. In the case of severe damage, replacement of some elements may be necessary. However, the actual fire resistance of a concrete structure is frequently well above minimum requirements due to the structural continuity present in most buildings. These reserves of strength may enable the structure to survive severe fires and still be reinstated. Reinstatement by repair will usually be preferable to demolition and rebuilding as it may require less capital expenditure, and may also produce a direct saving as a result of earlier reoccupation. As an example, Figure 2 shows the completed 10-storey concrete framed residential building mentioned earlier that was extensively damaged by a fire during construction (see Figure 1) and subsequently repaired, allowing earlier occupation than if it had been demolished and reconstructed(9). Figure 2: View of the same structure as Figure 1 after repair of fire damage.


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Table 1 is intended as a simple guide to the process of assessment and repair; reference is made to the main parts of the report dealing with the various activities. The main contractor and, if appropriate, a repair specialist should be appointed as early as possible so that they can participate in the preparation of the programme and the strategy for the site work. Table 1: Stages in the assessment and repair process. Stage Activities Reference 1 Carry out preliminary inspection of the structure. Take

immediate steps to secure public safety and the safety of the structure; it may be necessary to prop members that are in a critical condition.

Section 1.3

2 Carry out on-site assessment of the structure to determine the extent of damage (by visual inspection, breakouts and/or non-destructive testing). Decide which elements require cosmetic repairs only (e.g. cleaning and repainting) and which, if any, will require further assessment under Stage 3. (At this stage the decision may be taken to demolish and rebuild parts or all of the structure.)

Sections 2.3 and 2.5 and Table 7

3 Break out spalled concrete to determine depth of fire damage. Carry out laboratory testing of concrete and reinforcement samples to determine their residual strengths and confirm depth of fire-damage, supplemented by thermal modelling where appropriate. Possibly carry out dimensional surveys to determine the extent of deflections of beams and slabs and lateral movements of columns.

Sections 2.3 and 2.4

4 Determine structural capacity of members that are to be repaired, using reduced residual material properties, and hence determine additional concrete and reinforcement required to reinstate original capacity. The opportunity may be taken to upgrade parts or all the structure to modern standards. (As at Stage 2, the decision may be taken to demolish and rebuild parts of the structure if the quantity of additional material required makes repair uneconomic or impractical.)

Chapter 3 and Appendix B

5 Select appropriate repair methods and carry out work to restore the structure to its original capacity.

Chapter 4

1.3 HEALTH AND SAFETY All repair work will be subject to the requirements of the Construction (Design and Management) Regulations(10), generally known as the CDM Regulations. The Regulations require a planning co-coordinator to be appointed if the construction work is longer than 30 days duration or requires more than 500 person days. All operatives should wear the appropriate personal protective equipment (i.e. safety helmet, gloves, boots etc) when carrying out repairs. Further aspects of health and safety are covered in Sections 4.2 and 4.9.2; the latter is specifically concerned with the use of sprayed concrete.


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It is essential to consider the safety of the structure at all stages, from the initial assessment phase through to the final repair. Where necessary, beams and slabs should be propped, with temporary bracing. Phased breaking out may be required in some circumstances. Special consideration is needed if the structure is to be left for some time before repair work is carried out, during which time further deterioration may occur. The obvious risk of collapse of the structure should be considered, as well as the risk to third parties, for example from falling debris. During the course of the remedial works risks, such as falling concrete during breaking out, should be assessed and appropriate actions specified to mitigate any identified events that could arise. Safe access to the area being repaired should be provided for personnel. Repair materials and equipment should be stored in a suitable location, taking account of any additional loads they may apply to the weakened structure. Temporary falsework may be urgently required to secure the structure, not just for individual members, but for the stability of the structure as a whole. Remaining applied loads and all dead loads should be calculated for all doubtful members. Special care is required to avoid the transfer of excessive loads and stresses to other members. This applies particularly where falsework is being used to relieve a column at an intermediate floor level. Relieving falsework may have to be carried through to foundation level to avoid creating excessive stresses in adjoining parts of the structure. Installation of the falsework should precede the detailed appraisal described in the following chapter. Attention is drawn to BS 5975, Code of practice for falsework(11).


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2 Assessment of damage The aim of an assessment of a fire damaged structure is to propose appropriate repair methods or to decide whether demolition of elements or the whole structure is more appropriate. 2.1 OBJECTIVES AND METHODOLOGY OF ASSESSMENT The total feasibility of repair is dependent on parameters such as the extent of local and global damage to the building, but also to direct losses, such as damage to the faade or mechanical and electrical (M&E) installations, and indirect losses to business, for example by relocation of people, interruption of trade and the costs of cleaning smoke and combustion products, which may include cleaning to provide acceptable air quality in future. The focus of this report is on the assessment of the damage of the concrete structure only. The respective stakeholders need to decide from their point of view if the suggested repair methodology is appropriate. A systematic approach will result in a damage classification for the various parts of the affected structure, which may be used in the selection of appropriate repair techniques. At best members may need no structural repair as they have sufficient residual strength, and at worst demolition will be required. The assessment can follow two methodologies, which can be used separately or combined depending on the nature of the fire and of the structure, as follows:

1. Test the fire damaged concrete to directly assess the concrete quality. 2. Estimate the fire severity so as to deduce temperature profiles and hence to calculate

the residual strength of the concrete and the reinforcement.

Following the first methodology, there are several levels and methods to test fire damaged concrete:

Visual inspection and hammer soundings Non-destructive testing (e.g. rebound hammer, ultrasonic pulse velocity (UPV)) Coring, sampling and subsequent laboratory testing (e.g. petrographic examination,

strength testing of concrete and reinforcement samples). The second methodology involves three steps, which should be confirmed by tests:

1. Evaluation of fire severity This can be performed based on debris or applying numerical evaluation methods, such as computational fluid dynamics.

2. Determination of temperature-profiles This maybe performed applying numerical methods or simpler calculation techniques as provided for instance in Part 1-2 of Eurocode 2(5).

3. Assessment of residual strength of the concrete See Section 2.2.2. With both methodologies, the result will be a damage classification, which ideally should be provided on drawings showing the actual condition of the fire damaged structure. It is advisable to assess the strength of the unaffected concrete to confirm the design assumptions. The assessment needs to provide sufficient information to finally prepare detailed drawings with instructions on how to repair the structure. 2.2 MATERIALS


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The following sections outline the material properties of concrete, reinforcing steel and pre-stressing wires to facilitate an understanding of the residual strength after a fire. 2.2.1 Effects of high temperature on concrete strength and elastic modulus After cooling to ambient temperatures it has been observed that the strength of concrete may be further reduced from its strength at high temperature. Effectively during the cooling period a further loss of strength takes place because of continuing disintegration of the microstructure, see for example Schneider(12). This is one reason that a more conservative strength reduction factor to assess the residual strength of the concrete than that given in Part 1-2 of Eurocode 2 is proposed, see Figure 3.

[Note: Change Bazant et al to Bazant and Kaplan.] Figure 3: Typical effect of heat upon the compressive strength of dense aggregate concrete after cooling. The stress:strain curves in the Eurocode are based on steady state as well as transient state tests. For this reason the stress:strain relationships given are solely valid for heating rates between 2 and 50K/min. Creep effects are not explicitly considered. Therefore, strictly speaking the Eurocode curves are not valid for the cooling phase, see Franssen(13). It should be noted that there is a great variation in the residual strength of concrete after cooling depending on factors, including the following:

The maximum temperature attained Duration of heating exposure Mix proportions Aggregates Conditions of loading during heating and stress level.

0.00

0.20

0.40

0.60

0.80

1.00

0 200 400 600 800 1000

temperature oC

resi

dual

str

engt

h fa

ctor

% 1 hr exposure (Bazant et.al)

2 hr exposure(Bazant et.al)

residual strength


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Concretes containing certain synthetic lightweight aggregates, such as sintered pulverised-fuel ash are though to offer good fire resistance, provided that the concrete is dry. However, poor performance has been observed in conditions were the concrete is saturated at the time of the fire. The cement type and cement blend also influence behaviour of concrete in fire. Modern concretes often include a pozzolanic mineral addition in the binder such as fly ash (pulverised-fuel ash or pfa) ground granulated blastfurnace slag (ggbs). Their use is generally thought to give a slight improvement in heat resistance owing to the fact that they reduce the amount of calcium hydroxide (portlandite) within the hydrated binder. However, in the case of microsilica, its use significantly increases the risk of spalling due to the fact that it leads to very low permeability to the hardened concrete. Figure 2.1 also shows data from Bazant and Kaplan(14) giving the residual strength of concrete samples exposed to the same temperatures, but for different exposure times. It can be seen that a longer exposure to higher temperatures results in lower residual strength. In simple terms, for temperatures up to 300C, the residual compressive strength of structural-quality concrete is not significantly reduced, while for temperatures greater than 500C the residual strength may be reduced to only a small fraction of its original value. Consequently, the design methodology in the Eurocode discounts the strength of concrete exposed to temperatures higher than 500C. On the basis of the uncertainties regarding the assessment of the residual strength of concrete discussed above, this report recommends a more conservative approach, discounting the residual strength for concrete exposed to temperatures above 300C. 2.2.2 Mineralogical changes in concrete caused by heating Heating concrete causes a progressive series of mineralogical changes that can be investigated by petrographic examination to determine the maximum temperature attained and deduce the depth to which the concrete has been damaged. A compilation of the changes undergone by Portland cement concrete as it is heated is presented in Table 2, which is based on Ingham(9).

Table 2: Mineralogical and strength changes to concrete caused by heating.

Heating temperature

Changes caused by heating Mineralogical changes Strength changes

7080C Dissociation of ettringite, Ca6Al2(SO4)3(OH)1226H2O causing its depletion in the cement matrix.

105C Loss of physically bound water in aggregate and cement matrix commences causing an increase in the capillary porosity and minor

microcracking.

Minor loss of strength possible

(


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450500C Dehydroxylation of portlandite, Ca(OH)2 causing its depletion in the cement matrix.

Red discolouration of aggregate may deepen in colour up to 600C. Flint aggregate calcines at 250-450C and will eventually (often at

higher temperatures) change colour to white/grey. Normally isotropic cement matrix exhibits patchy yellow/beige colour in cross-polarised light, often completely birefringent by

500C.

573C Transition of -to -quartz , accompanied by an instantaneous increase in volume of quartz of about 5% in a radial cracking

pattern around the quartz grains in the aggregate.

600800C Decarbonation of carbonates; depending on the content of carbonates in the concrete, e.g. if the aggregate used is calcareous, this may cause a considerable contraction of the concrete due to

release of carbon dioxide, CO2; the volume contraction will cause severe microcracking of the cement matrix.

Concrete not structurally useful

after heating in temperatures in excess of 550

600C 8001200C Complete disintegration of calcareous constituents of the aggregate

and cement matrix due to both dissociation and extreme thermal stress, causing a whitish grey colouration of the concrete and severe

microcracking. Limestone aggregate particles become white.

1200C Concrete starts to melt.

13001400C Concrete melted. The colour of concrete can change as a result of heating, which is apparent upon visual inspection. In many cases a pink/red discolouration occurs above 300C, which is important since it coincides approximately with the onset of significant loss of strength due to heating. Any pink/red discoloured concrete should be regarded as being suspect and potentially weakened. In addition to the maximum temperature reached, the actual heat-induced concrete colour changes depend on the mineralogy of aggregate present in the concrete. Colour changes are most pronounced for siliceous aggregates and less so for limestone, granite and sintered pulverised-fuel ash lightweight aggregate (which shows very little colour change). Striking colour changes are produced by flint (chert); Figure 4, taken from Ingham(9), illustrates the colour changes of flint aggregate concrete. Figure 4: Appearance of flint aggregate concrete cores which have been heated for hour (upper row) and 2 hours (lower row), at the temperatures indicated.


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The pink/red colour change is a function of (oxidizable) iron content and it should be noted that as iron content varies, not all aggregates undergo colour changes on heating. Concrete which has not turned pink/red is not necessarily undamaged by fire. Also, due consideration should always given to the possibility that the pink/red colour may be a natural feature of the aggregate rather than heat-induced. In concrete containing aggregate that does not undergo colour change or is naturally pink/red, other mineralogical and physical indicators should be used for determining the presence of fire-damage. It should also be noted that the cement paste can also be discoloured by carbonation and this should not be confused with heat-induced discolouration. This is discussed further in Section 2.3.1. 2.2.3 Cracking of concrete in fires At high temperatures, the unrestrained thermal expansion of steel reinforcement is greater than that of most concretes. This can lead to bursting stresses and cracking around the steel in heavily reinforced members. Experience suggests that such cracks concentrate at positions where, incipient cracks due to drying shrinkage, flexural loading, etc. were present (see Figure 5). In addition, the thermal incompatibility of aggregates and cement paste causes stresses which frequently lead to cracks, particularly in the form of surface crazing, see Figure 6. Figure 5: View of floor below the fire showing thermal expansion cracks on the slab soffit.

[Photograph required.] Figure 6: Surface crazing. 2.2.4 Spalling of concrete in fires Spalling involves the breaking off of layers of concrete from the exposed surface at high and rapidly rising temperatures. Spalling is complex and there are many parameters influencing


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the process, see for example fib Bulletin 38(15). It is believed that the main process involved in spalling is vapour pressure, which is released from physically and chemically bound water at the beginning of the heating process through concrete pores. Due to the small capacity of the pores pressure builds up, which eventually may lead to spalling. Whether the tensile stresses from the vapour pressure within the pore spaces exceed the tensile strength of the cement matrix resulting in spalling is dependent on the amount of moisture, the rate of heating, permeability, porosity and pore distribution, as well as inherent tensile stresses of the structure. Three main types of spalling can be recognised. Explosive spalling (see Figure 7) occurs early in the fire (typically within the first 30 minutes) and proceeds with a series of disruptions, each locally removing layers of shallow depth. Aggregate spalling, also occurring in the early stages, involves the expansion and decomposition of the aggregate at the concrete surface causing pieces of the aggregate to be ejected from the surface.

Figure 7: Explosive spalling. Sloughing off or corner spalling (see Figure 8), occurs in the later stages of the fire when temperatures are lower. It occurs chiefly in beams and columns, as tensile cracks develop at planes of weakness such as the interface between the reinforcement and the concrete. As this type of spalling occurs in the advanced stages, the concrete is already significantly weakened and thus there are no implications for structural performance. Due to the lateness of the onset of this type of spalling, the interior concrete and the reinforcement are unlikely to have been subjected to high temperatures, even though the latter is often exposed. Figure 8: Sloughing off.


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Spalling of concrete surfaces can be caused by the deterioration of materials embedded in concrete other than reinforcement bars. Ingham and Tarada(16) indicate that plastic reinforcement bar spacers are one of the more commonly encountered examples of this (see Figure 9). The deterioration temperatures of materials other than concrete and reinforcement are given in Section 2.2.8 Figure 9: Spalling of a slab soffit owing to fire-damage of embedded plastic reinforcement bar spacers. Further loss of concrete may also take place after the fire has been extinguished and as the concrete cools. In such cases this concrete has remained in place long enough for the rise in temperature of internal concrete and reinforcement to be restricted. 2.2.5 Residual thermal movement cracks During a fire concrete structures undergo deformations due to a number of reasons. Generally heating causes expansion and this can push columns, particularly edge columns, out of plumb. Differential temperatures through concrete elements, particularly slabs, can lead to thermal bowing. As discussed in previous sections the increase in material temperatures leads to a reduction in stiffness which again leads to increased deflection under gravity loads. Experience shows that these deflections are not fully recovered once the fire is extinguished. For example, as described by Chana and Price(17), the concrete building at Cardington underwent a fire test involving only four structural bays; the perimeter columns were all pushed out and in the worst case the residual horizontal deflection at slab level was 67mm. Such residual deflections should be recorded. These may have an important impact on the future serviceability of the structure. For example if a deflected slab compromises floor to ceiling heights it may require replacement even if it is structurally adequate. Similarly a column with a large out of plumb due to fire effects may attract substantially greater bending


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moments than assumed in normal design and this will need to be considered in any structural assessment. 2.2.6 High-alumina cement concrete Special care should be taken when fire has damaged a structure containing high-alumina cement (HAC), also known as calcium aluminate cement, as the concrete may already have been weakened by the process of HAC conversion. In addition to the specific fire damage investigations it is recommended that the condition of the HAC concrete is checked in accordance with BRE guidance(18). It should be remembered that fire fighting water could potentially initiate or worsen mechanisms of HAC concrete deterioration such as sulfate attack or alkaline hydrolysis. 2.2.7 Reinforcing and prestressing steel The effect of elevated temperatures and subsequent cooling on the residual strength of steel has been researched in detail, for example by Stevens(19) and Holmes et al(20). Significant loss of strength may occur while the steel is at high temperature and this is usually responsible for any excessive residual deflections. However, recovery of yield strength after cooling is generally complete for temperatures up to 450C for cold worked steel and 600C for hot rolled steel. Above these temperatures, there will be a loss in yield strength after cooling. The actual loss in strength depends on the heating conditions and type of steel but the conservative values given in Figure 10, for temperatures up to 700C, will be sufficient for most purposes. Values above 700C are not given due to the additional variations in properties that can occur due to phase changes in the steel. Therefore where the temperature of the steel has exceeded 700C or where determination of the strength is critical to assessment, the matter should be discussed with the reinforcement manufacturer if known or, alternatively, tests carried out on samples taken from the member. Loss in ductility may occur after exposure to particularly high temperatures. Figure 10: Yield strength of reinforcing steels at room temperature after heating to an elevated temperature. The initial survey should identify what type of steel was used in the original structure, i.e. cold worked or hot rolled. For older structures, Appendix C gives some information on the types and strengths of bars that were available at various times. For other materials, such as


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stainless steel reinforcement, it will be necessary to seek specialist advice. If the production process cannot be identified or for the very early (pre 1920s) proprietary reinforcement systems mentioned in Appendix C, it will be necessary to carry out tests on samples removed from the structure. An on-site indentation (hardness) test might be considered as an indirect means of measuring strength. However the hardness of the surface of the reinforcement may be different from that at the centre of the bar due to the possible effects of quenching that result from fire fighting. Any such tests should therefore be used with caution. Buckling of reinforcing bars often occurs as a result of compressive stress induced at high temperatures by restraint against thermal expansion. Figure 11 shows bars that have buckled out from the soffit of a slab and are no longer bonded to the concrete in this region. Figure 11: Buckled bars. The effect of high temperature is more critical on prestressing steel than on reinforcing steel. The strength of prestressing steel during heating is likely to be reduced to less than 50% of the normal strength when the steel temperature reaches about 400C. In terms of re-use, a more important factor is the effect of heat upon the tension of the steel. Loss of elastic modulus in the concrete, increased relaxation due to creep and non-recoverable extension of tensions all contribute to this loss of tension. The loss in strength for untreated wire when hot is shown in Figure 12.


23

Figure 12: Tensile tests on untreated 0.76% carbon steel wire at high temperatures. Also shown in the Figure is the reduction in the limit of proportionality while the steel is hot; if the stress at which plastic elongation occurs is reduced at a particular temperature, the prestressing force will be similarly and immediately reduced unless it is already below the new limit of proportionality. Loss of prestress due to relaxation then proceeds from this new value of initial stress, as indicated in the diagrams of Figure 13. These two figures will be useful when assessing the residual prestress and when determining performance of a member under serviceability conditions. Figure 13: Temperature effects upon relaxation of untreated cold-drawn prestressing wire. The residual strength of prestressing steels on cooling from an elevated temperature is shown in Figure 14 which is derived from Holmes et al(20). This will be useful when assessing the residual strength of a member under ultimate conditions. Figure 14: Ultimate strength of prestressing steels at room temperature after heating to an elevated temperature. The maximum temperature reached in the steel, together with the temperature distribution and duration, are therefore clearly far more critical than in the case of reinforced concrete. In addition, it is necessary to consider factors such as the effect of temperature when hot and after cooling upon the elastic modulus and creep of the concrete, together with the effects of expansion and any restraints against expansion which may be present.


24

For these reasons, precise guidance on the assessment and repair of prestressed concrete is beyond the scope of this Technical Report, but it should be noted that some fire-damaged prestressed concrete structures have been satisfactorily assessed and repaired, see Appendix A. 2.2.8 Degradation of other materials The condition of other debris may be useful in determining the history and characteristics of fire (see for example Figure 15, which shows aluminium formwork supports that have melted during a fire). Table 3 gives a guide to the approximate temperatures which cause various materials and components to degrade in building fires. Figure 15: Melting of aluminium formwork supports indicating that the fire reached temperatures in excess of 650C. Table 3: Assessment of temperature reached by selected materials and components in fires Substance Typical examples Conditions Approximate

temperature (C) Paint Deteriorates 100

Destroyed 150 Polystyrene Thin-wall food containers, foam, light

shades, handles, curtain hooks, radio casings

Collapse 120 Softens 120140 Melts and flows 150180

Polyethylene Bags, films, bottles, buckets, pipes Shrivels 120 Softens and melts 120140

Polymethyl methacrylate

Handles, covers, skylights, glazing Softens 130200 Bubbles 250

PVC Cables, pipes, ducts, linings, profiles, handles, knobs, house ware, toys, bottles (Values depend on length of exposure to temperature.)

Degrades 100 Fumes 150 Browns 200 Charring 400500

Cellulose Wood, paper, cotton Darkens 200300


25

Wood Ignites 240 Solder lead Plumber joints, plumbing, sanitary

installations, toys Melts 250 Melts, sharp edges rounded

300350

Drop formation 350400 Zinc Sanitary installations, gutters, downpipes Drop formations 400

Melts 420 Aluminium and alloys

Fixtures, casings, brackets, small mechanical parts

Softens 400 Melts 600 Drop formation 650

Glass Glazing, bottles Softens, sharp edges rounded

500600

Flowing easily, viscous

800

Silver Jewellery, spoons, cutlery Melts 900 Drop formation 950

Brass Locks, taps, door handles, clasps Melt (particularly edges)

9001000

Drop formation 9501050 Bronze Windows, fittings, doorbells,

ornamentation Edges rounded 900 Drop formation 9001000

Copper Wiring, cables, ornaments Melts 10001100 Cast iron Radiators, pipes Melts 11001200

Drop formation 11501250 An indication of the duration and severity of the fire may be obtained by examining charred timber that has remained in place throughout the fire. As a rough guide the char increases at a rate of 40mm per hour in the standard fire test. Variations from this can be seen in Table 4, which is based on Section 4.1 of BS 5268(21). From this Table, it can be seen that, if a piece of oak is found with a char depth of 45mm, it has been exposed to a fire equivalent of 1 hours under furnace conditions. The information is less relevant if the timber had not remained in place and has fallen to the ground with the other debris. Table 4: Notional rate of charring for the calculation of residual section.

Timber Depth of charring in 30 minutes 60 minutes

All structural species listed in Appendix A of BS 5268-2: 1989 except those listed below

20mm

40mm

Western red cedar 25mm 50mm Hardwoods having a nominal density not less that 650kg/m3 at 18% moisture content

15mm

30mm

2.3 TESTING OF FIRE DAMAGED REINFORCED CONCRETE There are a number of on-site and laboratory-based techniques available to aid in the diagnosis of reinforced concrete condition. Techniques conducted on site include visual inspection, non-destructive testing and the removal of concrete and reinforcement samples,


26

which may subsequently be examined and/or tested in the laboratory. A guide to selected test methods appropriate for investigating fire-damaged concrete is provided in Table 5. Table 5: A guide to the selection of test methods for fire-damaged reinforced concrete. Test location

Test type

Test method Information gained Colour changes

Lateral extent of damage

Depth of damage

Compressive strength of undamaged concrete

Tensile strength of reinforcement bars (damaged and undamaged)

On-

site

Non

-de

stru

ctiv

e Visual inspection 9 9 9 Hammer soundings 9 9 Rebound Hammer 9 Ultrasonic Pulse Velocity

9

Parti

ally

de

stru

ctiv

e

Breakout/ drilling 9 9

Labo

rato

ry Petrographic

examination 9 9

Thermoluminescence 9 Core Test 9 Reinforcement test 9

2.3.1 On-site inspection It may be sufficient to take soundings on the damaged concrete to determine the degree of deterioration. The ring of sound concrete and the dull thud of weak material are readily distinguished, and this test may be successfully done with a hammer and chisel. Removing concrete with a hammer and chisel can therefore be used to determine the depth of the pink/red layer. Figure 16 shows red/pink discolouration on the soffit of a fire-damaged slab. Figure 16: A fire-damaged reinforced concrete slab soffit showing pink/red discolouration of flint aggregate particles. Where it is difficult to assess the depth of the pink/red layer, a small-diameter core or a drilled hole can be made to determine it accurately, see Figures 17 and 18. Core samples may be required anyway for laboratory examination and/or testing.


27

Figure 17: Technicians diamond drilling core samples through the full thickness of a fire-damaged concrete floor slab. Figure 18: View of spalled and discoloured fire-damaged concrete slab soffit, showing the location of a core sample (centre) that was taken to aid determination of the depth of fire damage. As mentioned in Section 2.2.2, discolouration can occur as a result of carbonation and therefore care needs to be taken when investigating older concrete. Carbonation depth may be found by spraying a freshly broken surface with phenolphthalein indicator. If the depth of visual discolouration is beyond the layer shown by the phenolphthalein then it is clearly due to the effects of the fire. If it coincides with the layer shown by the phenolphthalein then it may be due to carbonation and not the fire. The boundary for the pink/red zone may be taken as being on the 300C temperature profile and hence the strength loss and equivalent duration of the fire may be determined. An alternative on-site technique is the drilling resistance test, which uses a hammer drill to determine the depth of weakened concrete, see Felicetti(22).


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It is considered that hammer and chisel testing should still form the basis of investigations. Other methods are available for the assessment of strength and these are outlined in the following sections together with the comments on their particular use. 2.3.2 Non-destructive testing (NDT) Visual inspection, hammer tapping and breakouts are the principal on-site methods of fire damage assessment. However, in certain situations there may be benefit in supplementing the normal on-site regime with some non-destructive testing (NDT). Guidance on the use of NDT methods is given in BS 6089: 1981(23) and BS 1881: Part 201(24). (Note that these Standards are in the process of being replaced by European Standards.) The NDT methods most commonly used for on-site condition assessment of fire-damaged concrete structures are the rebound test (Schmidt hammer) and the ultrasonic pulse velocity (UPV) test. The rebound test gives a measure of the surface hardness of the concrete surface. Although there is no direct relationship between this measurement of surface hardness and strength, an empirical relationship exists. Due to the need for a flat surface to test and as a large number of tests is desirable to reduce the effects of variability, the rebound hammer is not generally suitable for use on spalled surfaces, which is often the case with fire damaged concrete. The results of this test on fire-damaged concrete, even on flat surfaces, are somewhat variable and this is perhaps due to skin hardening effects that appear to occur. The apparatus is, however, commonly available and the method of test is given in Part 2 of BS EN 12504(25). The UPV test for the estimation of concrete strength is well established and, although there is no fundamental relationship between pulse velocity and strength, an accepted equation exists and the method is covered by Part 4 of BS EN 12504(26). Although an estimation of strength can be obtained by correlation, the method has perhaps a greater potential for comparing known sound concrete with affected concrete. This test also requires a flat surface and is, therefore, generally only appropriate for unspalled surfaces. The method has been found to be particularly suitable for use on the ribs of waffle and trough floors and for assessing the extent of damage of a localized fire. The UPV test can also be used to give an indication of depth of seriously weakened concrete. Most of the major testing companies have UPV apparatus. 2.3.3 Petrographic examination Petrographic examination is the definitive technique for determining the depth of fire damage in concrete. It is performed in the laboratory by experienced concrete petrographers, using optical microscopes in accordance with ASTM C856(26). Concrete core or lump samples are subjected to visual and low-power microscopical examination. Followed this, samples are selected for thin-section preparation and more detailed examination with a high-power microscope. Petrographic examination provides a great deal of information and is offered by all of the major testing companies. A typical commercial examination would be expected to determine the following:

Type, mineralogy and approximate grading of the coarse and fine aggregates Cement type Presence of mineral additions and fillers Cement content Water:cement ratio Air void content


29

Depth of carbonation Presence of defects or deterioration Identification of deleterious reactions such as alkali-silica reaction.

Petrographic examination is invaluable in determining the heating history of concrete as it can determine whether features observed visually are actually caused by heat rather than some other factor. In addition to colour changes of the aggregate, the heating temperature can be cross-checked with changes in the cement matrix and evidence of physical distress such as cracking and microcracking. Careful identification of microscopically observed features allows thermal contours (isograds) to be plotted through the depth of individual concrete members. In the most favourable situations contours can be plotted for 105C (increased porosity of cement matrix), 300C (red discoloration of aggregate), 500C (cement matrix becomes wholly isotropic), 600C (- to -quartz transition), 800C (calcination of limestone) and 1200C (first signs of melting). Figure 19 shows an example of some of the microscopical features that may be observed in fire-damaged concrete. Some aggregate particles have been reddened indicating that the concrete has reached at least 300C at that point. Particles of flint have been calcined (brown mottled) and so have been heated to 250450C. The cement matrix is bisected by numerous fine cracks (white) within the cement matrix (dark), some of which radiate from quartz grains (white) in the fine aggregate fraction. This deep cracking and cracking associated with quartz suggest that the concrete has reached 550575C. Overall we can deduce that the concrete has been heated to approximately 600C in the area represented by the sample. Figure 19: A photomicrograph of fire-damaged concrete seen through the optical microscope. In recent years, research has been conducted into the application of image analysis techniques to assessment of colour changes caused to concrete by heating, see for example Lin et al(27) and Short et al(28). These methods involve using computer software to analyse the colours of digital images captured from finely ground slices of concrete. Reliance on these methods to determine the depth of fire-damage has a number of drawbacks and should always be cross-checked with microscopical examinations, see Ingham(9). The limitations of petrographic examination are that it is relatively expensive and usually takes at least two weeks to complete. Also, although petrography will determine concrete condition in qualitative terms, it does not provide numerical values of concrete strength. When applied to fire-damaged concrete, petrography will usually:


30

Provide details of the concrete ingredients and mix proportions Identify potentially deleterious ingredients (e.g. high-alumina cement) Provide an assessment of general concrete workmanship and general concrete

condition, including identification of any underlying problems Describe the effects of fire-damage and determine the depth of damage.

2.3.4 Thermoluminescence tests The basis of this technique for investigation of fire damaged concrete is the measurement of the residual thermoluminescence in small samples of sand drilled from the concrete, see Placido(29) and Chew(30). A major loss of thermoluminescence occurs at around the same temperature that concrete begins to lose significant strength. This test has the advantage that only small holes are required for sampling of drilled dust on site and temperature profiles may be determined. However, it requires specialist laboratory equipment and experienced operators; the usefulness of this technique is somewhat reduced by its limited availability and relatively high cost, see Smart(31). 2.3.5 Core test The most direct method of estimating the compressive strength of in situ concrete is by testing cores cut from the structure. The test procedure is given in Part 3 of BS EN 12390(32). Large cores should not be taken from positions where they would cause a significant loss in structural strength. 2.3.6 Tests on samples of reinforcement The effect of elevated temperatures to the reinforcement has been considered in Section 2.2.7. To confirm the limit of deterioration, samples should be taken in the first instance from members damaged beyond repair (if any). If the tests are not satisfactorily further samples should be taken from representative elements. Obviously, the damage to the reinforcement should be evaluated when taking the samples, which need to meet requirements of test facilities with respect to their length. They should be tested for yield, elongation and tensile strength. The results should be compared with the relevant British Standard (see Appendix C) for the grade of steel concerned and if a reduced strength compared to code requirements is observed a re-assessment with modified properties should be performed. 2.3.7 Other laboratory tests A number of other microscopical and chemical analysis methods have been used to investigate fire-damaged concrete. These include scanning electron microscopy (SEM) and mineralogical analysis by X-ray diffraction (XRD), see Handoo et al(33) ]. Thermal analytical methods used include differential thermal analysis (DTA), thermal gravimetric analysis (TGA) and derivative thermogravimetric analysis, see Alarcon-Ruiz et al(34) and Handoo et al. To date, these methods have been used mainly for academic research and are not routinely used to investigate fire-damaged structures commercially. 2.4 ASSESSMENT OF FIRE DAMAGED STRUCTURES 2.4.1 Introduction


31

First, it is useful to try to build up a picture of the nature of fire and thus the likely nature and extent of any damage. An assessment of the materials burnt and the disposition of the fire can give information about likely temperatures developed and the duration at any location. Fire debris can also give useful guidance as to the temperatures experienced by evaluating which types of materials, e.g. plastics, glass, aluminium, or timber that have deformed, melted or burnt, see Section 2.2.8 and Tables 3 and 4. These observations are rarely enough to evaluate the extent of damage directly but are a useful guide in planning more specific examination and testing. Consideration of the fire characteristics may also prompt other specific issues, such as whether toxic or deleterious combustion products have been given off. The burning of extensive quantities of polyvinyl chloride (PVC), for example, may give off enough hydrogen chloride to initiate corrosion of steel elements or reinforcement. The three principal concerns in evaluating the effect of the fire on concrete structure are:

Depth of damage (spalling) or loss in strength of the concrete Loss in strength of steel reinforcement or embedded structural steel elements Damage or distress to the structure from movement, settlement or imposed loads.

Reinforced concrete members exposed to a fire of a severity insufficient to cause collapse are likely to have undergone a cycle which is summarised in Table 6. Table 6: Cycle of effects upon reinforced concrete structures. Stage Probable effects On heating 1. Rise in surface temperature Surface crazing

2. Heat transfer to interior concrete Loss of concrete strength, cracking and spalling

3. Heat transfer to reinforcement (accelerated if spalling occurs)

Reduction of yield strength Possible buckling and/or deflection increase

On cooling 4. Reinforcement cools Recovery of yield strength appropriate to maximum temperature attained (Figure 10) Any buckled bars remain buckled.

5. Concrete cools Cracks close up Reduction in strength until normal temperature is reached Deflection recovery incomplete for severe fire Further deformations and cracking may result as concrete absorbs moisture from the atmosphere.

For prestressed concrete members the sequence of events is broadly similar to that for reinforced concrete, but the position is made more complex by the lower temperature at which prestressing steel looses strength and by the reduced modulus of elasticity, which results in a reduction in tension (see Section 2.2.7). Both of these effects may be offset by the greater amount of cover generally provided to the tendons in prestressed concrete. Outwardly, damage to the concrete will be seen most obviously as spalling, see Section 2.2.4. This will vary depending upon the location in the structure and the severity of the fire. Typically, soffits and thin ribbed slabs show more damage than the tops of slabs and the


32

lower portions of columns. The absence of spalling, however, does not necessarily signify that no damage or loss in strength has occurred. As mentioned earlier, the assessment of fire damaged structures can follow two methodologies, which can be combined:

1. Testing of fire damaged concrete to directly assess the concrete quality 2. Estimation of fire severity to deduce temperature-profiles and hence calculate the

residual strength of the concrete or the reinforcement. 2.4.2 Testing It is sensible to define a procedure to test the structure. The aim is to establish zones in ceiling areas or damage classes of individual structural members. The zones are defined from the seat of the fire to parts where no heat impingement has been observed. If possible, samples that have been taken from concrete that has not been affected by elevated temperatures should be tested for comparative purposes. The amount of testing, as well as the methodology, will be governed by the availability of equipment, the budget, the time available and other restraints. Health and safety must be considered at all times, particularly when extracting material samples from critical locations. Care should be taken not to cause more damage than the fire. 2.4.3 Assessment of the fire severity Deduction of the temperatures reached in the concrete should be based as much as possible on observations. The history of a fire may help to determine the pattern of the fire intensity. Video footage and observations from the fire brigade are important sources of information. Often the duration, intensity and extent can be determined from eye-witness accounts. The aim is to determine the condition of the structure as accurately as possible. An examination of the debris (see Section 2.2.8) may not give as accurate a picture of the temperature of the fire and its effect upon the structure as would a detailed petrographic investigation or a visual inspection of the discoloration in the concrete. The condition of the concrete is a direct indication of the effect the fire had upon the materials, whereas the condition of the debris is subject to local conditions and flame temperature. In addition, predictive fire engineering tools, such as empirical equations or computer modelling used in design, can be used to assess the fire severity in the building, based on the fire load in the building, ventilation conditions, compartment size and shape and properties of wall linings. An estimate of the fire time:temperature curve can be based on the heat-release, the characteristic temperatures at flashover, the expected gas temperatures during a fully developed phase of the fire and the area of window openings providing ventilation to the fire. From eyewitness accounts it may, for example, be possible to determine when the fire started, how many windows were open and when flashover occurred. This allows estimates of the pre-flashover heat release rate. Further estimates of the heat release rate for the post-flashover phase may be obtained from temperatures within the compartment, deduced from debris. An assessment with a finite element Computer Fluid Dynamics program might then allow hot spots to be determined and may provide confidence in data obtained from testing and debris. The results can then be used to determine the amount and locations for testing and may confirm evidence provided by tests.


33

2.4.4 Heat transfer Once a credible time:temperature distribution within the compartment is determined, an assessment of the temperatures within the concrete is possible without relying solely on testing. As a result of heat transfer analysis it may be possible to reduce the amount of testing and the determination of required depths of samples. Temperatures greater than 900C are frequent in fires in buildings. But, in a concrete member, only the temperature of the outside layers is drastically increased and the temperature of the internal concrete may be comparatively low. Temperature distributions in dense concrete elements in standard fire tests are given in Annex A of Part 1-2 of BS EN 1992(5), as follows:

Figure A.2: slabs (and walls exposed to fire on one side) Figures A.3A.10: beams of various sizes Figures A.11A.15: 300 300mm columns Figures A.16 A.20: 300mm diameter circular columns.

It is important to note that the development of temperature in a real fire differs from the standard temperature time curve. Hence it is difficult to deduce the effects of a real fire from standard tests. A more straight forward way is to estimate realistic time:temperature distributions within the compartment and conduct heat transfer analysis. Such an assessment is more realistic than comparing results of standard tests with the situation after a real fire. Heat transfer calculations of concrete structures can be performed according to methods outlined in Part 1-2 of BS EN 1992. The numerical calculations are based on finite elements and can be performed with standard software. The required thermal properties of normal and lightweight concrete are provided in Part 1-2 of BS EN 1992. 2.5 Presentation of data Before determining the necessary repairs to the structure, the degree of damage to the various elements must be determined. This section presents one approach, though other approaches may be more appropriate depending on the type of structure being considered and the nature of the fire. The degree of damage may be determined simply on the basis of a visual assessment of the structure. An example of a visual classification scheme is given in Table 7, which leads to Classes of Damage ranging from 0 to 4. However, visual inspection will generally be supplemented by the following evidence:

The condition of the concrete o Temperature reached o Depth of penetration of fire damage o Proportion of section needing renewal o Evidence from temperature of fire, duration of fire, sounding, colour, spalling,

physical tests The condition of the steel

o Temperature reached o Reduction to yield point, ultimate strength, Youngs modulus, ductility o Loss of tension (prestressing tendons)


34

o Evidence from loss of concrete cover, colour of surrounding concrete, samples of steel

The quality of the original construction o Concrete: low strength, poor compaction, weak or contaminated aggregate,

type of cement o Reinforcement: poor detailing, inadequate cover, closeness of bars o General: inaccuracies of line or level, non-axiality of column storey heights,

poorly executed construction joints.


35

Table 7: An example of a visual damage classification scheme for reinforced concrete elements. Class of damage

Element Surface appearance of concrete Structural condition Condition of plaster/finish

Colour Crazing Spalling Exposure and condition of main reinforcement*

Cracks Deflection/ distortion

0 Any Unaffected or beyond extent of fire 1 Column Some peeling

Normal

Slight Minor None exposed None None Wall Floor Beam Very minor exposure

2 Column Substantial loss

Pink/red**

Moderate Localised to corners Up to 25% exposed, none buckled None None Wall Localised to patches

Up to 10% exposed, all adhering Floor

Beam Localised to corners, minor to soffit Up to 25% exposed, none buckled 3 Column Total loss

Pink/red** Whitish grey***

Extensive Considerable to corners Up to 50% exposed, not more than one bar buckled

Minor None

Wall Considerable to surface Up to 20% exposed, generally adhering Small Not significant Floor Considerable to soffit Beam Considerable to corners, sides,

soffit Up to 50% exposed, not more than one bar buckled

4 Column Destroyed

Whitish grey*** Surface lost Almost all surface spalled Over 50% exposed, more than one bar buckled Major Any distortion Wall Over 20% exposed, much separated from

concrete Severe and significant

Severe and significant Floor

Beam Over 50% exposed, more than one bar buckled

Notes: * In the case of beams and columns the main reinforcement should be presumed to be in the corners unless other information exists

** Pink/red discolouration is due to oxidation of ferric salts in aggregates and is not always present and seldom in calcareous aggregate, see Section 2.2.2 *** White-grey discolouration due to calcination of calcareous components of cement matrix and (where present) calcareous or flint aggregate.


36

As a general guide, it is suggested that the repair that is required for the various Classes of Damage should be as indicated in Table 8. Table 8: Initial repair classification. Class of damage

Repair classification

Repair requirements

0 Decoration Redecoration if required 1 Superficial Superficial repair of slight damage not needing fabric

reinforcement 2 General repair Non-structural or minor structural repair restoring cover to

reinforcement where this has been partly lost. 3 Principal repair Strengthening repair reinforced in accordance with the load-

carrying requirement of the member. Concrete and reinforcement strength may be significantly reduced requiring check by design procedure.

4 Major repair Major strengthening repair with original concrete and reinforcement written down to zero strength, or demolition and recasting.

An example of how this information might be summarised for part of a building is shown in Figure 20, and the accompanying Table 9.

[Dimensions for columns sizes (380 380) and spacing (4.25m in NS direction and 5m in EW direction) to be added to Figure so that it ties up with the design examples.]

Note: Circled numbers are damage class for each member.

Figure 20: Typical section of key diagram classification.


37

Table 9: Typical section of schedule for damage classification shown in Figure 20.

Location: Ground floor columns and first floor beams and slabs Element Class of damage Member reference number Columns 1 1, 2, 11, 21, 31

2 5, 12, 15, 22, 25, 32, 34, 35 3 3, 4, 13, 23, 24, 33 4 14

Beams 1 11, 111, 211, 311, 112, 212, 312 2 21, 121, 331, 152 3 31, 41, 141, 221, 241, 341, 321, 132, 142, 232, 242, 252, 352 4 131, 231

Slabs 1 101, 201, 301 2 102, 202, 203, 302, 303, 304 3 104, 204 4 -

The above approach is somewhat simplistic as it only considers the requirements for individual elements. Some adjustments would be required in the light of the overall performance of the structure, considering aspects such as:

The continuity of members Stability including the need for robustness Excessive residual distortion; the structure will probably have incorporated

inaccuracies in the original construction, hence check by reference to items (e.g. lift guides) of known alignment prior to the fire.

The method of carrying out the repairs will depend on a number of factors including the following:

Accessibility for the proper application of sprayed concrete The performance of other repair materials in the event of a subsequent fire The practicability of adding the number and size of reinforcing bars or links needed to

restore the strength of the member. If the repair is Class 3 or Class 4 it may be necessary to consider whether it will be quicker and/or cheaper to carry out the repair or to demolish and reconstruct. It is also important to consider the general condition of the structure with respect to durability. This may influence the choice of repair method, and other aspects such as the provision of protective coatings.


38

3 Design This section of the report deals with aspects of the structural design of the reinforced concrete elements within the fire damaged building where the damage assessment procedure outlined in the previous sections has shown that repair is a viable option. The recommendations apply to conventionally reinforced structures. Similar principles apply to prestressed concrete; however, specialist advice may be necessary on how to deal with loss of prestress. Worked examples illustrating the design approach outlined in this chapter, applied to a slab, tee-beam and column, are given in Appendix B. 3.1 DESIGN PHILOSOPHY 3.1.1 Objectives

Repairs to a fire damaged concrete structure should provide the strength, fire resistance, durability and appearance appropriate to the proposed use and projected design life of the building. The intended use for the structure and the objectives for the repair should be agreed with the building owner before commencing the design of the repair work.

3.1.2 Building regulations The designer should consult with the local authority regarding the need for approval under the Building Regulations for the proposed reinstatement and repair works. It is possible that upgrading of both structural and non-structural aspects of the building may be required as part of the repair works. 3.1.3 Codes of practice

In general the design of the repaired sections of the building should comply with current codes of practice. However, the damaged structure may have been designed to codes of practice which are out of date; for example, there have been significant changes to the requirements for shear reinforcement and provision for robustness in more recent structural codes. Where this is the case it may be necessary to formulate a strategy for the structural design of the repaired section of the building which is compatible with the original design. For example, limitations may be imposed on the restoration of listed buildings. Alternatively, if the assessment of the fire damaged elements of a building indicates significant deficiencies in the original design, it may be necessary to enhance the complete structure. However, before taking such a step it will be necessary to determine if there is any evidence to suggest that the structure was in any way inadequate or that there had been problems with similar structures.

Wherever possible the original structural design drawings for the building should be obtained as these will be of considerable help in assessing the original properties of the structure; the steel and concrete strengths in older buildings were lower than those commonly used at present. Appendix C lists historical information, including the dates at which various structural codes were introduced, the steel strengths that were current at the time and the symbols used for reinforcement detailing.


39

3.1.4 Design assumptions

Any assumptions used in the design of the repaired sections of the building should take due account of the materials and methods used in carrying out the repair 3.2 STRUCTURAL ANALYSIS AND MEMBER DESIGN 3.2.1 Structural analysis

Structural analysis of the repaired structural frame should be based on methods of analysis and load arrangements as set out in current codes of practice. In addition to normal assumptions, the analysis should take due account of any dimensional changes, lack of verticality and residual forces which could have resulted from the elevated temperatures during the fire. As discussed in Section 2.2.5, a column with a large out of plumb will be required to carry substantially greater bending moments than would be assumed in normal design. 3.2.2 Element design

The design of the reinstated and repaired concrete elements should be based on design methods as set out in current codes of practice.

Repaired concrete elements will comprise a combination of the remaining section of the existing member and the repair materials. Modified material parameters may have been established in the fire damage assessment process and must be taken into account in the redesign of the repaired members. The strength properties of the repair material should be used where appropriate. 3.3 REPAIR CRITERIA 3.3.1 Reduced material strengths

In the design of the repaired section of the structure it is necessary to take account of the reduced strength of the remaining concrete and reinforcement which may have suffered from the elevated temperatures during the fire. Ideally this reduction in strength should be based on the results of tests on samples taken from the sections of the fire damaged zone of the structure which are to remain after the repairs are complete. The concrete strength should be based on compression tests on concrete cores, see Section 2.3.5. Similarly the design strength of the steel reinforcement should be based on tensile tests on reinforcement samples taken from the fire zone, see Section 2.3.6.

Due account may taken of the fact that the test results give an indication of the actual strength of the concrete and steel by making appropriate modifications to the material strength factors. Further information may be found in the Institution of Structural Engineers document Appraisal of existing structures(35) and the Highways Agency Assessment of concrete highway bridges and structures(36). 3.3.2 Residual strength factor


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As an alternative to the above, where there is some knowledge of the fire temperature, residual strength factors obtained from Figures 3 and 10 may be used. The factors are to be applied to the design stresses in the residual concrete and reinforcement. An average factor for the whole member may be obtained by considering separate layers in a cross-section. Stresses in compression, tension, shear, torsion and bond are to be reduced in this way. If no data exists regarding the original strength of the concrete it should be determined from cores taken from existing undamaged concrete. The strength of the reinforcement should be determined from samples taken from sections of the undamaged concrete structure. 3.3.3 Bond strength Provided the full cover to the existing reinforcement, along with any shear reinforcement, is reinstated during the repair, full bond between the existing reinforcement and surrounding repair material may be assumed and hence lap and anchorage lengths will not be compromised. Where existing bars have buckled (see Section 2.2.7 and Figure 11) they will no longer be adequately bonded to the concrete. In this situation, it will be necessary to remove sufficient concrete from behind the bars so that the repair material fully surrounds the reinforcement, to ensure full composite action. 3.3.4 Bar size and spacing Bar spacing should be sufficient to ensure full compaction of the repair material, see Section 4.6.1. Bar spacing should also consider the direction from which the sprayed concrete or other material is to be applied. Some reduction in the spacing may be considered where there is access from several faces. Adequate compaction is vital to successful repair, and any deviations from the recommended minimum should be discussed with the repair contractor. 3.3.5 Shear reinforcement Additional shear reinforcement should be anchored in such a way to enable it to function properly with the undamaged portions of the member concerned, see Section 4.6.2 and Figure B.10. 3.4 MEMBER DESIGN 3.4.1 General

Member design should take into account the stress history of the remaining concrete and steel. Beams and slabs may be propped during the repair process and the undamaged concrete may therefore be assumed to be unstressed at the time of the repair. However the remaining concrete in columns and walls may be highly stressed at the time of the repair by loads from the structure above. In these cases careful consideration needs to be given to the distribution of loads between the remaining concrete and steel and the new repair materials. 3.4.2 Beams and slabs bending Where appropriate, beams and slabs may be repaired and strengthened to resist the applied bending moments by adding repair concrete and tension reinforcement. The added reinforcement must be sufficient to resist the redesign tensile forces, minus the permissible


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tensile force which can be resisted by the original tension reinforcement, taking into account the residual strength. The approach is illustrated by Examples 1 and 2 in Appendix B. 3.4.3 Beams shear Where appropriate, beams may be strengthened to resist the redesign shear forces by adding shear reinforcement. The added reinforcement must be sufficient to resist the redesign shear force minus the permissible shear forces, which can be resisted by the original shear reinforcement taking into account the residual strength for both the steel and the concrete. The approach is illustrated by Example 2 in Appendix B. 3.4.4 Columns Columns may be strengthened to resist the redesign loads by adding reinforcement and repair concrete. The resultant section comprising the repair materials and the original concrete and reinforcement, taking account of the residual strengths, must be sufficient to resist the redesign axial load and bending moments. Example 3 in Appendix B shows the approach for an axially loaded column. Special consideration should be given to columns with a permanent lack of verticality and the resulting additional P- effect should be included in the design based on measured imperfections. The design should also account of any potential permanent reduction of the stiffness of the concrete and reinforcement due to the fire. This becomes particularly important for a slender column, whose failure mode would be dominated by stability rather then strength.

Designers should note the possible difficulties in adding longitudinal reinforcement due to overhead obstructions from upper floor slabs, beams, etc, particularly where bending is a dominant influence in the column design and it is necessary to provide adequate anchorage beyond the point at which the additional steel is no longer required. 3.4.5 Walls The general principles of column strengthening may be applied to walls but there may be difficulty in threading new bars through the remaining floors. It is recommended that vertical wall reinforcement should be therefore be nominal, terminating above and below the floor slabs, and that the original, undamaged concrete acting together with the repair should accordingly be capable of achieving the desired strength as an unreinforced wall wherever possible. This recommendation will not apply to shear walls which may be heavily reinforced to resist lateral forces and for which special treatment to ensure maintenance of continuity of the steel reinforcement may be required in designing the repair. 3.5 DESIGN OUTPUT 3.6.1 Demolition and construction sequence drawings The designer should prepare drawings which clearly show the extent of demolition of the fire damaged structure and should include details of any associated temporary propping which may be required in the temporary condition to enable the repair works to be carried out safely.


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In some instances it will be necessary to provide temporary propping beyond the extent of the damaged structure (e.g. where demolition could cause increased design forces in adjacent spans). Where critical to the design assumptions the designer should prepare construction sequence drawings showing the timing of the installation and removal of any temporary propping and the sequence of the repair operation. 3.5.2 Key plans Key plans should be prepared at each floor level showing the location of the repair work and the positioning of the detailed sections. 3.5.3 Design details Design details are required at each of the repair locations. These will be determined on the basis of testing (as described in Chapter 2) along with practical considerations such as the method of breaking out the damaged concrete and the subsequent method of repair (see Chapter 4). The details should include the following information:

The extent of breaking out of fire damaged concrete and removal of fire damaged reinforcement steel.

Requirements for preparation of concrete surfaces that are to receive repair concrete including any special requirements to prevent feathered edges.

Details of new steel reinforcement including lap length and splicing with original bars, mechanical anchorage, cover etc.

Any fabric reinforcement that may be required to hold the repair concrete in place in the temporary condition, including means of supporting the fabric and the required concrete cover.

The thickness and the properties of the repair concrete. Some typical repair details, using sprayed concrete, are shown in Section 4.9.7. Similar details will be used for hand-applied repair materials or concrete cast in formwork. 3.5.4 Specifications In addition to the design drawings and details the designer should prepare detailed material and workmanship specifications for the repair work. This should include full information on the repair materials and include means for ensuring quality control. Information on suitable repair methods is given in Chapter 4. 3.5.5 Design calculations The designer should prepare design calculations for the repair works. The calculations should clearly set out all assumptions used in preparing the repair in

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