Concrete – The Sustainable choice for Structures in Fire

H. P. G. Backes

State Director WA, Cement Concrete & Aggregates Australia, 45 Ventnor Avenue, West Perth,

Western Australia 6005, Australia (E-mail: harry@ccaa.com.au)

ABSTRACT

Structural engineers have the option to design buildings using concrete, steel or timber as the main

construction material. This paper provides a brief overview of the performance of these materials

under fire conditions, with an emphasis on the sustainable benefits of using reinforced concrete

structural elements for buildings subjected to fire. Key subject areas addressed are:

• Sustainability; Concrete buildings have the ability to withstand fire, negating or minimising the

resources required for rebuilding thereby enhancing their sustainable performance.

• Fire Performance of Buildings in Bushfire Areas; A brief overview is provided of beneficial

design and construction details that have superior performance in bushfire areas, highlighting

the sustainable benefits of using concrete and non-combustible material.

• Buildings Exposed to Internal Fires; In Australia there have been no failures of major concrete

framed buildings subjected to fire, which represents an excellent sustainable outcome. AS 3600-

2009 provides relatively simple fire design methods for normal concrete and now also includes

high strength concretes which require some special considerations as described in this paper.

KEYWORDS Bushfire; Concrete; Fire; Structures; Sustainable

INTRODUCTION

Structural engineers have the option to design buildings using concrete, steel or timber as the main

construction material. These materials behave differently under fire conditions and therefore will

provide varying performances when evaluated from a sustainable perspective under fire conditions.

To provide a brief understanding of the performance of these materials under fire conditions, the

following summarised extracts have been obtained from technical literature.

Concrete The Concrete Centre’s (2012) perspective on concrete is that it performs well in fire, both as an

engineered structure, and as a material in its own right. Because of concrete’s inherent material

properties, it can be used to minimise fire risk for the lowest initial cost while requiring the least in

terms of ongoing maintenance. In most cases, concrete does not require any additional fire-

protection because of its built-in resistance to fire. It is a non-combustible material (i.e. it does not

burn), and has a slow rate of heat transfer. Concrete ensures that structural integrity remains, fire

compartmentation is not compromised and shielding from heat can be relied upon.

Steel Carleton University (2009) in its short course on the performance of steel structure in fire stated that

unprotected steel members perform poorly in fires. Steel has higher thermal conductivity values

than most other materials and when unprotected steel structures are exposed to fire, the steel

temperatures increase, strength and stiffness reduces, which leads to deformation and potential

failure. The thermal expansion of steel members can also cause damage to other parts of the

building. Steel members therefore need protection to achieve fire resistance, which can include a

number of alternative passive fire protection systems which reduce the temperature increase in steel

structures exposed to fire. These include concrete encasement, board systems, spray-on systems,

intumescent paint and concrete filling.

Timber Wood Solutions (2012) indicates that fire ratings can be achieved with timber in a number of ways.

The three most common are protecting timber by covering it with a good insulator such as fire-rated

plasterboard which means that the timber takes longer to get to ignition temperature and can remain

functional for a longer period while the fire is burning. Secondly, use oversized timber which will

allow for loss of material charring throughout the burn period, and there will still be enough timber

remaining in the cross-section to give it the required strength. Thirdly, treat timber with fire-

retardant chemicals, which delays the initiation of combustion, and can prevent the spread of flame.

SUSTAINABILITY OF BUILDINGS EXPOSED TO FIRE

Buildings exposed to fire behave differently depending on the materials used to construct them.

Once they have been exposed to a fire, their performance from a sustainability perspective can be

evaluated according to the Waste Hierarchy.

Waste Hierarchy

A common definition of the Waste Hierarchy is shown in Figure 1 below, as extracted from

Wikipedia (2012).

Figure 1. Waste Hierarchy - Wikipedia (2012).

Based on the Waste Hierarchy, the most favoured outcome for a building exposed to fire would be

the “prevention” of waste i.e. the effects of the fire on the building are not of structural significance

permitting its continued use.

Moving down the scale, possibly some repair to the building may be required after the fire, which

then permits its “reuse”, this remains a positive sustainable outcome.

The least sustainable choice would be “disposal”, whereby the building exposed to fire is damaged

beyond repair requiring its demolition, resulting in the facility not being able to be used for an

extended period of time until it has been fully reconstructed. This is a poor sustainable outcome

causing the loss of accommodation, building contents and in extreme cases the loss of life.

Sustainable Material Choice

In terms of the above discussion, concrete can provide a sustainable solution for the construction of

structures, typically not requiring additional fire protection. It does not burn, adding fuel to the fire,

emit toxic fumes, smoke or drip molten particles under fire conditions. Cement Concrete &

Aggregates Australia’s (2010) briefing on Sustainable Concrete Buildings also indicates that

concrete structures generally do not require fire protection, if appropriately designed, because of

their inherent fire resistance. This removes the time, cost, additional materials and labour required

to provide separate fire protection measures.

Concrete structures subject to fire can often be repaired and are ready for reuse within a relatively

short period after the fire. These factors indicate that concrete is a superior sustainable structural

material of choice warranting discussion of important sustainable aspects when using concrete for

our buildings that require fire risk analysis.

BUILDINGS IN BUSHFIRE-PRONE AREAS

A useful document “Building in Bushfire-prone Areas” has been produced by Cement Concrete &

Aggregates Australia (2009), which outlines important considerations for building elements in

Bushfire risk areas. The sustainable performance of buildings can be improved if the major

structural elements are constructed as follows:

Floors It is advisable to keep buildings low, where possible following the contours of the ground. Where

cut and fill is necessary, the cut should be maximised and the fill depth minimised. A floor, laid as a

concrete footing slab or slab-on-ground is efficient, avoiding the possibility of floor ignitions and

fire access underneath a building.

Suspended floors should also be reinforced concrete (insitu or precast), especially where sections of

the raised floor project beyond support walls. Where sub-floor voids occur, the perimeter of the

building should have a robust, non-combustible sub-floor wall. The junction of the building and

ground is where debris can accumulate, making non-combustible material such as a concrete slab

edge and/or masonry or concrete walling essential.

Walls

Primarily, the external walls of any building should be of a non‑combustible material. Brick is

traditionally favoured, and when laid with flush mortar joints, performs well under fire exposure.

However, to resist thermal wind conditions experienced during bushfires, it is necessary to

structurally tie the roof structure to the slab or footing. This is easier to achieve when hollow

concrete masonry is used. Hollow concrete blocks with partial reinforcing and grout fill acts as a

structural tie between roof and footing. Reinforced masonry, in the form of single-leaf block

walling is an established, successful form of construction in cyclonic wind areas, with habitable

spaces requiring insulation.

Reinforced concrete panel walls offer an alternative to masonry, with this method of construction

comprehensibly covered in the “Concrete Panel Homes Handbook” produced by Cement Concrete

& Aggregates Australia (2001). The fire resistance of solid concrete panel walling systems are

good, withstanding the effects of fire and remaining structurally sound for a relatively long period

of time. Concrete wall panels may be cast either on-site as tilt-up panels, or offsite in a precast

factory. Both types are readily available and often used for commercial buildings, but are practical

and economically applicable for houses, garages or sheds in bushfire-prone areas.

Insitu reinforced concrete cast within vertical formwork (or rammed earth walls) is another option

for structurally suitable, non combustible walling.

Doors and window openings in all wall types require appropriate bushfire-protection by using

screens, shutters, seals and metal or aluminium frames.

Roofs The roof structure needs to be designed for high positive and negative wind loads caused by bush

fires and must be structurally connected, through the walling, to the footing. Typically, pitched,

gabled or skillion roofs (preferably with a maximum pitch of 25°) are framed with timber and/or

steel. As the roof structure must not be exposed externally, its fire protection relies on the roof

covering, sarking, facia and eaves lining materials.

The available non-combustible coverings for such roofs are concrete or terracotta tile, fibre cement

shingle or sheet, and galvanised/painted steel sheet. Steel sheet may be dented by impact, while the

high-wind fixings will reduce, but not eliminate, the possibility of it being buckled by heat. Since

the battens are above the sarking they need to be non-combustible steel battens with self-tapping

screws (eg tek-screw) for tile clips.

The optimum roof for bushfire exposed buildings is a reinforced concrete slab. This can be formed

with permanent steel or conventional formwork and placed insitu, or can be precast, with or without

an insitu concrete topping. A roof slab requires a waterproofing membrane, which in turn needs fire

protection. This can be provided by rigid insulation boards covered with precast paving slabs. This

gives an excellent thermal performance at all times – while protecting the membrane and slab from

high winds, embers and the heat and flame of bushfire exposure.

To achieve sustainable construction in bushfire prone areas, the use of concrete components for the

major structural elements, including floors, walls and roof, will ensure a superior sustainable

outcome. This inturn will negate the need for substantial rebuilding after a bushfire, as indicated

graphically in Figure 2 below.

Figure 2. House constructed using non-combustible materials after a bushfire.

Cement Concrete & Aggregates Australia (2005)

BUILDINGS EXPOSED TO INTERNAL FIRES

In Australia there have been no failures of major concrete framed buildings subjected to fire, which

represents an excellent sustainable outcome.

The common view of concrete structures amongst both engineers and regulators is that these

structures are inherently fire resistant and that high levels of fire resistance can be achieved by

adopting certain member dimensions and cover to reinforcement. The reason for this is that

concrete has both low thermal conductivity and high heat capacity, and that concrete elements are

therefore naturally resistant to temperature rise due to fire exposure.

Simple Design and Sustainable Construction Process Many major buildings, irrespective of the materials of construction, will be the subject of a fire

engineering design. However, for concrete building structures, it is not often that the fire

engineering process considers the building structure. This is because the prescriptive requirements

i.e. the deemed-to-satisfy (DTS) provisions of the Building Code of Australia (2009) are usually

easily met and durability requirements often govern the required axis distance to the steel

reinforcement.

The design of typical concrete structures for fire resistance is therefore a straightforward process,

merely requiring selection of the correct member dimensions and covers for each member within

the fire compartment. Resulting in an efficient design process with a superior sustainable outcome

because no additional materials are required to fire protect a concrete structure. This reduces

material, time, energy and labour requirements during the construction process.

The positive perception of concrete structures and fire resistance has also been reinforced by design

standards for concrete structures, which in the fire design section contain relatively simple tabular

solutions for various members – beams, floors, walls and columns.

High Strength Concrete (HSC) To improve the sustainable performance of concrete buildings there has been an increasing interest

in high-performance concretes to achieve greater strengths and performances. This has been driven

by the desire to construct buildings taller (to better utilise the land footprint) with smaller columns

and thinner core walls (to increase the rentable area and functionality) and with minimum floor-to-

floor height (to provide more floors within the building envelope). This has largely been achieved

through the use of high-strength concrete (HSC) columns and post-tensioned floors.

It is important to understand the behaviour of HSC structures in fire, to ensure that the enhanced

building sustainability is achieved in a safe manner. To assist with the sustainable fire design of

concrete buildings, research funded by Cement Concrete & Aggregates Australia (CCAA) has been

completed and published in CCAA’s Fire Safety of Concrete Buildings (2010), which:

• Investigates the behaviour of various elements of construction in fire;

• Explains and develops simplified fire engineering procedures for concrete structures;

• Investigates the effect of real fires on concrete structures;

• Investigates fire impact on HSC

• Can be used as a guide to enhance understanding in combination with other publications such as

AS 3600 (2009)

The findings contained in this publication, based on a review of the accounts of major fires in

concrete buildings, concluded that:

• Serious fires in buildings are rare and there have been few cases of significant building failure in

concrete buildings.

• The behaviour and resistance of concrete under elevated temperature conditions is complex.

Sufficient testing has, however, been undertaken to provide the basis for describing the

reduction in strength with temperature and simplified stress-strain relationships that can be used

for design or advanced analysis.

• The possibility of spalling has been long recognised in relation to both normal-strength and

high-strength concrete, with the likelihood of spalling increasing as the permeability of the

concrete reduces. This is usually the case for high strength concrete although there is some

beneficial increase in the tensile strength of the concrete.

• The addition of polypropylene fibres to the high-strength concrete mixes has a dramatic effect in

reducing the level of spalling, as shown in Figure 3 below. This is consistent with overseas test

data. A dosage rate of approximately 1.2 kg/m3 of 6-mm monofilament polypropylene fibres is

recommended.

• It is considered that the adoption of such measures for slabs which may be more prone to

spalling (eg stressed slabs with river gravel aggregates) will also provide an appropriate

measure to minimise such behaviour.

No fibre–Significant Spalling With Fibre-Corner Spalling Only

Figure 3. High Strength Concrete Columns exposed to fire, with and without Fibre.

CCAA, Fire Safety of Concrete Buildings (2010)

Case Study

Craig B F (2005) provided a case study presentation at Engineers Australia, Western Australia that

detailed a fire which occurred in a building that consisted of a single storey brick office with a

gang-nail timber roof truss system, a steel portal framed and sheeted factory, with a steel extension

containing concrete panel boundary fire walls.

In 2002 the complete facility was gutted by fire when furniture grade dried timber that was stacked

from floor to roof caught fire. It took some 11 hours to bring the fire under control, with the

following outcome:

• Brickwork walls to office: The walls failed with the mortar between the bricks disintegrated to a

powder. Both bed joints and perpends had been badly compromised structurally, or had

collapsed.

• Timber Framed Clawplate roof; The office roof structure consisted of treated pine fabricated

trusses held together with steel claw plates which penetrated the timber by 10mm. The heat

from the fire was conducted through the steel plates causing the claws to heat and char the

timber to the depth of the spikes on the claws, which resulted in a completely failed roof truss.

• Steel Framed Portal structure: The steel frame had completely failed as shown in Figure 4

below.

• Tilt-up Concrete Wall Panels: The inside face of the panels had a maize of tension cracks and

dished, like big saucers, away from the fire by approximately 400mm. All of the exposed steel

beam connection bolts had failed but none of the cast-in base ferrules had failed, ensuring that

the panels remained upright, cantilevering from the floor as shown in Figure 4 below. The

concrete had afforded a considerable amount of heat shielding from the fire proving to be an

effective fire wall. The benefit of using a cantilever base connection under fire conditions is also

highlighted, as detailed in Figure 5.

Figure 4. Failed Steel Frame exposed to fire with the Tilt-up Concrete Walls remaining upright.

Craig B F (2005)

Figure 5. Section: Tilt-up Panel Cantilever Connection to Floor Slab.

Craig B F (2005)

CONCLUSION

Concrete provides a superior sustainable solution when used as the structural material for buildings

subjected to fire. When assessed against the sustainable “Waste Hierarchy”, concrete structures

provide a superior sustainable response because they can typically be repaired and are ready for

reuse within a relatively short period after being exposure to a fire.

Concrete structures typically do not require additional fire protection, they do not burn, adding fuel

to the fire, emit toxic fumes, smoke or drip molten particles under fire conditions. The benefits of

using concrete as the main structural material under both bushfire and internal fire conditions have

been explained by discussing both design and construction details and providing an actual case

study.

Concrete is therefore a superior sustainable structural material warranting consideration by

structural engineers when choosing their construction material for buildings which require fire or

bushfire assessment.

REFERENCES Australian Buiding Codes Board, 2009. Building Code of Australia (BCA)

Craig B F (2005). Concrete Panel Performance in Fire. Presentation at Engineers Australia,

Western Australia.

Carleton University, Ottawa, Ontario. Short Course - Performance of Steel Structures Exposed to

Fire (2009). http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/oral962.pdf (accessed 16 January 2012)

Cement Concrete & Aggregates Australia (2009). Briefing 10, Building in Bushfire-prone Areas.

Sydney, Australia see web: http://www.ccaa.com.au/publications/pdf/Briefing10_WEB.pdf

Cement Concrete & Aggregates Australia (2010). Fire Safety of Concrete Buildings. Sydney,

Australia see web: http://www.ccaa.com.au/publications/pdf/FireSafety.pdf

Cement Concrete & Aggregates Australia (2001). Concrete Panel Homes Handbook. Sydney,

Australia see web: http://www.ccaa.com.au/publications/pdf/CPH5-9.pdf

Cement Concrete & Aggregates Australia (2005). Tilt-up Presentation. Perth, Australia

Standards Australia (2009). AS3600 Concrete Structures.

The Concrete Centre. Fire Resistance of Concrete.

http://www.concretecentre.com/technical_information/performance_and_benefits/fire_resistance.as

px (accessed 16 January 2012)

Wikipedia. Waste hierarchy. http://en.wikipedia.org/wiki/Waste_hierarchy (accessed 24 January

2012)

Wood Solutions. Design for Fire. http://www.woodsolutions.com.au/Resources/fire-design

(accessed 16 January 2012)