EFFECT OF
TEMPEATURE ON
CONSTRUCTION
MATERIALS
THE MANNER IN WHICH A MATERIAL
RESPONDS TO FIRE CAN BE CLASSIFIED
AS
1. Chemical: Decomposition, charring.
2. Physical: Variation in density (ρ), softening, melting, spalling.
3. Mechanical: Strength as measured by yield or peak stressed ( fy for steel and fcu for concrete),
stiffness as measured by the modulus of elasticity (E), creep, thermal expansion as measured by
the coefficient of thermal expansion (α).
4. Thermal: Thermal conductivity (k), specific heat (c).
EFFECT OF TEMPERATURE ON STEEL
• Steel is used in construction as structural steel or as reinforcing steel for reinforced concrete.
• Structural steel is considered considerably more vulnerable to fire than reinforcing steels
which are encased in concrete.
• Steels are very good conductors and tend to be used in thin sections. They are, therefore, liable
to heat up very quickly in fires if not insulated.
• The rate of heating depends upon the parameters of thermal conductivity, specific heat and
density.
• The density of steel is approximately 7850 kg/m3.The thermal conductivity of steel is approx.
54 W/mK at room temperature and reduces to about half this value at 800◦C.
VARIATION OF THERMAL CONDUCTIVITY
WITH TEMPERATURE
VARIATION OF SPECIFIC HEAT WITH
TEMP
STRESS/STRAIN CURVES AT HIGH TEMP
COEFFICIENT OF THERMAL EXPANSION
• The coefficient of thermal expansion of steel, α, is around 12 × 10-6 /℃ and increases slightly with
temperature. At about 730℃, steel undergoes a phase change (energy is absorbed without
increase in temperature) and a denser structure results. The heat absorbed at phase change causes
delay in further temperature rise.
• A simple relationship provides a useful estimate of thermal expansion variation with temperature,
α = 6.1 × 10-6 +3.96-9∆T (1)
where, ∆T is the temperature change in ℃.
• The property of thermal expansion of structural steel can be both beneficial to the structure or
be the cause of great damage. For instance a steel beam anchored to a masonry wall can cause it
to collapse by expansion (during heating) or contraction (during cooling – this can make masonry
walls very hazardous in post-fire operations).
EFFECT OF TEMPERATURE ON CONCRETE
• Concrete has excellent fire resistance properties and maintains its integrity and strength in very
high temperatures. The thermal properties of concrete depend upon the aggregate type used, due
to chemical changes (crystal structure) in aggregate compounds. Three common types are;
Siliceous aggregates (gravel, granite, flint), calcareous aggregates (limestone) and lightweight
aggregates made from sintered fuel ash (Lytag) and expanded clay. Siliceous aggregate concretes
have a tendency to spall due to high thermal conductivity of such aggregate.
• Lightweight concrete (LWC) has the best thermal properties of all, i.e. less than half the thermal
conductivity (0.8 W/mK) of normal weight concrete (NWC) and consequently loses its strength at a
considerably lower rate. The thermal diffusivity of LWC is only slightly lower than NWC, so the
extra fire resistance in LWC comes not so much from reduced temperatures, but from the stability
of the light weight aggregates at high temperatures. The typical density of NWC is 2350 kg/m3 and
that of LWC is 1850 kg/m3.
THERMAL CONDUCTIVITY OF
VARIOUS CONCRETES WITH TEMP
VARIATION OF SPECIFIC HEAT WITH
TEMP
STRESS STRAIN CURVES FOR DENSE
CONCRETE
COMPRESSIVE
STRENGTH OF CONCRETE
TEMPERATURE
AT
HIGH
Temperature Compressive strength of concrete as % of initial strength with different
(°C)
types of aggregate
Siliceous Aggregate
Carbonate Aggregate
Light
Aggregate
Weight
Stressed
Unstressed
Stressed
Unstressed
Stressed
Unstressed
200
99
92
100
97
98
94
400
95
85
87
81
96
91
600
85
77
56
34
86
74
800
62
40
20
48
42
• Tests have shown that even with a 4 hour fire duration (Severe fire),
the portion of concrete affected by the fire is only for a depth of
about 150 mm from the heated surface.
• In practice, even in the case of a severe fire, temperature at which
the concrete loses its strength is rarely attained by concrete
throughout its whole mass.
• As a result, concrete often maintains its integrity and strength even
after being attacked by severe fire.
• So, in order to protect the steel from fire, BIS specify minimum
thickness of concrete cover to the steel reinforcement (IS 456 : 2000
and NBC-Part IV- Fire Protection )
Modulus of Elasticity:
• Modulus of elasticity of concrete reduces rapidly with increase in
temperature.
At 200 C
temperature
- Young’s modulus is about 70 – 80 % of that at room
At 400C
- Young’s modulus is about 40 – 50 % of that at
room temperature
• If all other parameters are equal, the deflection of a flexural member
is inversely proportional to the Young’s modulus. Hence it can be
stated that, the deflection of a concrete member will be doubled if
its temperature reaches about 400 C .
Poisson’s Ratio:
• With the available data, a generalized behaviour of Poisson’s ratio
with temperature is not possible in case of concrete. In some cases,
it was observed that Poisson’s ratio decreases with increase in
temperature.
Stress Strain Relationship:
• It can be observed that as temperature increases, the ultimate
strength of concrete decreases and the ultimate strain increases.
Creep:
• As the duration of fire is short, the short term creep
characteristics are important with regard to materials
subjected to fire.
• In case of concrete, experiments have proved that creep plays
a very limited role in the overall behaviour of concrete except
when the temperature is above 400 C .
• For temperature above 400 C, creep deformation will be
more and has influence on the structural behaviour of the
member.
Spalling of Concrete:
• The most common form of damage to concrete when subjected to
fire is spalling of concrete.
• That is, the breaking off of pieces or layer of concrete.
• It has been suggested that spalling of concrete due to temperature
rise can occur in a member due to one or more of the following
reasons.
1. Due to excessive compression or restraint of the materials
2. Due to the formation of high pressure steam in the material
3. Splitting of the aggregate used in the concrete mix.
• The coefficient of thermal expansion of concretes is of the same order as of steel, but here
again the it is considerably higher for NWC, up to 14×10-6 /◦C, while that of LWC is about
8×10-6 /◦C. Concrete thermal expansion depends considerably upon the stress in concrete, so
for large compressive stresses the thermal expansion coefficient can be considerably lower
that in unstressed concrete.
• This is caused by the fact that creep effects become very important in concrete at around
400◦C, with strains increasing considerably for small increases in temperature beyond this
point. This effect is termed as transient thermal creep and it can be so large at large compressive
stresses as to completely counter the effect of thermal expansion, even leading to contraction.
• Concrete stiffness (i.e. the slope of the stress/strain curve) also reduces significantly at high
temperatures, which also results in additional strains. These effects can cause large deflections
in concrete structural members.
• One of the most destructive effects of fire on concrete is spalling (loss of surface material),
which ranges from superficial surface damage to explosive blowout of large chunks of material.
• Spalling is characterised by lines of striation and loss of surface material resulting in chipped,
cracked, broken or cratered appearance.
• Spalled areas may also appear lighter in colour than adjacent areas through exposure of clean
subsurface material.
COLOUR CHANGES IN CONCRETE CAN BE USED AS ROUGH GUIDE TO
THE TEMPERATURE REACHED AND THE RESIDUAL STRENGTH
• • Grey: (Under 300◦C) Minimal loss of strength
• Pink, red or red/brown: (300-650◦C) 10 to 60 % of the strength is lost (for NWC).
• Grey-white: (650-900◦C) 60 to 100% loss of strength.
• Buff: (Over 1000◦C) Sintered.
Case 1 :
• Due to differential thermal expansion, excessive compression or
restraints occurs in concrete.
• If steel is present, it will restrain concrete from expanding and as
concrete is weak in tension, a concrete layer will spall off at the level
of steel.
• This kind of spalling can occur to the concrete cover provided to
structural members such as slabs, beams, columns, etc.
Case 2 :
• Concrete by nature is porous and depending on the various
parameters like water-cement ratio, aggregate gradation,
compaction, etc., there can be about 2 – 5 % voids by volume in
concrete.
• These voids are called capillary pores. The capillary pores are
distributed in random throughout the mass of concrete and in
general, they are interconnected.
Case 2 :
• Depending on the environmental conditions such as temperature,
humidity, etc. to which the member is exposed, these voids may be
fully or partially filled with water and such water is called capillary
water ( or free water in concrete).
• When a concrete surface is exposed to fire, the water present in the
capillaries near the surface will get converted into steam and
thereby develops pressure in the capillary pores.
• This pressure will cause the pore water adjacent to the heated
surface to migrate inwards, until a stage is reached when part of the
concrete has so much water in it that it can no longer move quickly
enough through the pores to relieve the steam pressure developed.
• As a result, the steam pressure will build up until the forces are
great enough to cause lateral fracture of concrete and thereby
spalling occurs.
• The quantity of pore water present in concrete at the time of fire is
the most important factor with respect to the spalling due to steam
pressure.
Case 3 :
• The thermal expansion and other thermal properties of aggregate
used in concrete is different from those of the cement paste.
• Hence, depending on the type of aggregate used, thermal instability
may occur and thereby splitting of aggregate occur.
Case 3 :
• It has been observed that Carbonate aggregates are less prone to
splitting.
Igneous rocks, lime stone, crushed bricks,
blast
furnace
dolomite are some of the carbonate aggregates used in concrete.
slag,
• Silitious aggregates are prone to aggregate splitting.
Flint, Gravel, and Sand stone are the type of
classification.
aggregates
under
this
• Light weight aggregates (Foamed slag, Crushed bricks, etc.)
have the least chance to undergo aggregate splitting.
• As the strength of cement increases, likelihood of aggregate
splitting also increases.
• Large sized aggregate may also lead to aggregate splitting when
compared with small sized aggregates
• The steel in concrete, whether it is reinforcing, prestressing or
structural steel, may get exposed due to spalling of concrete.
• Once such steel is exposed to fire, the fire resisting property
of the whole structure will be suddenly at risk.
• Hence spalling of concrete is to be prevented or controlled.
The spalling of concrete can be prevented by proper selection
of concrete mixes and its constituents –
1.
Need for a design concrete mix.
2. specifying proper cover to the steel - Depending on the
fire resistance rating required, minimum cover to the
reinforcement is specified by Bureau of Indian standards
3. providing a secondary layer of reinforcement with wire
mesh between the steel and surface of concrete, so that the
even if concrete get cracked, it will not spall off and thereby
direct exposure of steel to the fire is prevented ( IS 456 :
2000 and NBC-Part IV- Fire Protection )
EFFECT OF TEMPERATURE ON WOOD
• Wood (or timber) is a combustible material, however, it is also one of the most widely used
materials of construction. It is therefore fortunate that wood possesses certain features that
allow it to provide satisfactory performance in most building fires.
• One of these is that it is not easily ignitable, but the most important property of wood is the
formation of char after ignition.
CHAR FORMATION IN WOOD
• The concept of sacrificial timber is used in design, i.e. using a larger section of timber than
necessary for carrying the design load, with the excess sufficient to protect the member through a
given duration of fire.
Wood has many advantages as a building material such as
1. high strength to weight ratio
2. Low Thermal Expansion
3. Low Thermal Conductivity
4. Reasonable level of Fire Resistance
The behavior of wood when exposed to fire can be summarized as
follows:
• Over 100 C - Wood will become over dry, discolored, distorted
and will loose weight.
• Above 150 C - Wood can be easily ignited.
• The ignition temperature of wood depends on the type of species,
its density, intensity of heat radiation, etc. and the normal ignition
temperature ranges from 275 C to 300 C.
• Wood burns at a fairly constant rate from its ignition temperature.
• However, the rate of combustion will be less for a wood with higher
density. Further, the combustion rate depends on the heat radiation
intensity and the type of species.
• The variation of rate of burning with density of wood is as presented
below:
• Density ( kg/ m3 )
Rate of Burning ( mm / Minutes)
400 – 500
0.60 – 1.10
600 – 800
0.30 – 0.60
• When wood burns, it forms a layer of charcoal on the burnt
surface, which helps to insulate and protect the un-burnt wood
below the charred zone.
• Due to the low thermal expansion, the char layer stays in place
even with continued heating.
• Further, due to the low thermal conductivity the undamaged
timber below the char retains its strength.
• Flame retarding treatment of wood has no effect on the rate of
combustion.
• Flame retarding treatment only delays the appearance of flames
at the wood’s surface and it reduces the speed at which the
flame propagates.
• The normal practice in designing structural members with
wood is to provide extra cross section for wood when
compared with the structural requirement.
• The additional quantity required is arrived at, based on the
rate of combustion and the required duration of fire resistance,
so that, the portion of wood as per structural requirement will
be available even at the end of the required duration of fire
resistance.
Example 1
• A wooden beam requires a cross section of 200 mm x 300 mm
to resist the dead load and live load coming over it. Determine
the cross section required, if it has to resist a fire of 4 h
duration.Assume the rate of combustion as 0.3 mm / min
• Thickness required to resist 4 h duration of fire = 0.30 x 4 x
60
= 72 mm
So, required size of beam
- Depth
= 300 + 2x 72 = 444 mm
- Width
= 200 + 2x 72 = 344 mm
• So, the minimum size of beam required is 344 mm x 444 mm.
EFFECT OF TEMPERATURE ON MASONRY
 Masonry consisting of either brick-work or of concrete block-work is inherently stable in fire.
 The following is a partial list of masonry failures due to structural reasons:
• Steel/Composite beams may cause instability in a connected wall through expansion (or
contraction at cooling).
• High walls with low slenderness ratio (i.e. thin).
• Lack of lateral support
• Movement of super-structure supported on masonry.
• Differential heating due to a progressive pre-flashover fire
 Masonry can also suffer integrity failure when fire loads are excessive. Bricks can withstand
temperatures of around a 1000◦C and they melt at about 1400 ◦C. In domestic fires integrity
failures are more common.
 The reasons for masonry wall failures are often nothing to do with the fire resistance capability
of masonry materials.
EFFECT OF TEMPERATURE ON BURNT CLAY BRICKS:
• Masonry is used for the construction of walls, columns, piers, etc. in buildings. In general,
a masonry structure can be either Load bearing or Non-load bearing structure.
• Behaviour of masonry structures with respect to temperature depends on the type of
material used for making the individual masonry units as well as the type of bonding
material used for construction.
• The individual masonry units can be of different types such as
1.
2.
3.
4.
5.
6.
7.
8.
Burnt Clay Bricks ( Solid)
Burnt Clay Hollow Blocks
Stones ( different types)
Sand-Lime bricks
Lime based Blocks
Concrete Blocks ( Solid and Hollow)
Gypsum Partition Blocks
Autoclaved Cellular Concrete Blocks
• In general, masonry construction behaves well in fire
conditions possibly because of the fact that the numerous joints
present in the masonry structure prevent thermal stresses from
building up over a large area.
• A masonry wall, when exposed to fire on one side will deflect
towards the fire due to the greater expansion of the surface
layers on the side of the fire.
• The failure of such walls will be due to the excessive distortion
caused by fire(Stability failure) rather than the material failure.
• Similarly, when hollow block clay or concrete blocks are used
in floor construction, the floor will deflect towards the source of
heat and the ultimate failure of such floors will be primarily due
to this deflection.
• Clay is the commonly used material in the manufacture of bricks. In general,
compressive strength of burnt clay bricks and blocks at elevated
temperature follows a similar pattern to that of concrete.
Variation in strength:
• Up to 400 C - There is only slight deterioration in the compressive strength
• At 500 C
-
The strength starts to decrease
• At and above 600 C - The rate of decrease of strength becomes rapid.
• Behaviour of solid bricks are better than the perforated or cavity
(hollow ) bricks with an equivalent thickness (That is with the
same over all) thickness.
• The improvement expected in thermal insulation due to air
cavities in perforated or cavity bricks is insufficient to
compensate for the loss of solid materials.
• In case of masonry with burnt clay bricks exposed to fire, the
development of residual stresses may not occur.
• This is due to the fact that during the manufacturing process
itself, bricks are subjected to high temperature and later they are
cooled down at a slow rate.
• As a result, residual stresses developed, if any, would have been
released during the cooling process and development of
residual stresses may not occur when they are subjected a
temperature rise again.
BEHAVIOUR OF PLASTICS ON FIRE:
Plastic covers a wide range of materials.
• The use of plastic is increasing for various structural and nonstructural purposes in building construction such as in walls,
partitions, ceilings, floorings, roofing etc.
Plastics consists of three basic groups of materials:
1. Resins – They act as binders and they can be of synthetic, artificial or
natural
2. Fillers – Can be of organic or mineral origin and may be in solid,
liquid or gaseous state.
3. Plasticizers- Plasticizers or dispersants are additives that increase
the plasticity or viscosity of a material
• A wide range of plastics can be made out by the appropriate
combination of the above.
• Behaviour of plastics at high temperature depends on their
physical, mechanical and chemical properties.
• According to the thermal stability of resins, plastics can be
grouped into two, namely Thermoplastic and Thermosetting
plastics.
• Thermoplastic materials will lose their mechanical strength and
their physical shape very quickly when heat is applied.
• When cooled, they will regain their mechanical strength.
• Most of the plastics under this group get softened at
temperature even below 100 C and in a fire, they will burn as
a molten material.
• Thermosetting plastics, upon heating will get converted to
an insoluble and infusible state, which is irreversible.
• They retain their mechanical strength and their physical shape in
a fire until decomposition.
• Most of the plastics under this group have the maximum
temperature of destruction in the range of 250-400oC.
• In general, polymer materials possess comparatively low thermal
stability.
• Maximum temperature of softening and decomposition is about
300-400oC only.
• Fire resistance of such materials is only for a few minutes.
• Almost all plastic materials break down under fire to form
undesirable and toxic gases.
• Smoke impairs visibility and thereby affects the escape of the
occupants of a building.
• The common hazard due to plastic in buildings are due to the
use of plastic
 as the internal lining of the wall or ceiling or
 as a thermal insulating material in the form of plastic foam
• In both cases, there is a danger of rapid fire spread.
• Table 1.5 : Classification of Plastics and the Possible Hazard
Group
Thermoplastics
Thermosetting
Plastics
Uses
Plastic
Possible
Hazard
High
density
Sheet,
mouldings,
extrusions
Acrylic,
Nylon
Low
density
Flexible
foam, rigid
foam
Polyurethane
Polythene
High
Density
Sheet,
mouldings,
extrusions
Bakelite
Formica
Polyesters
Low
Density
Flexible
foam, rigid
foam
Area
formaldehyde
Polyster
PVC,
Flaming
droplet Hazard
Burns
very
rapidly
with
flaming droplet
hazard
Burns as a solid
Burns
rapidly
as a Solid
EFFECT OF TEMP ON GLASS
• The main use of glass in buildings is in glazing for windows and doors. In this role, glass has
little resistance to fire and generally cracks very quickly because of the temperature difference
across the exposed surfaces.
• Double glazing does not improve this behaviour significantly.
• Wire reinforcement does provide relatively greater integrity, however in general glazing should
not be relied upon to remain intact in a fire.
BEHAVIOUR OF GLASS ON FIRE EXPOSURE
• Depending on the purpose, various forms and types of glass
are being used in construction.
• There are several types of glass treatment and some of the
common types and their manufacturing process are-
Float Glass –
• All modern manufactured glass is float glass. Molten glass
“floats” on top of a pool of molten metal, usually tin, and the
liquid tin and the liquid glass form perfectly flat surfaces.
• The glass is drawn off one end of the molten tin, and gradually
cools and hardens into a glass sheet.
Annealed Glass –
• This type is most common.
• Annealing is part of the normal glass manufacturing process.
• Glass is cooled gradually under controlled conditions to
remove undesirable stresses and to spread the minor residual
stress evenly throughout the cooled glass.
• This minimizes the tendency for spontaneous breakage.
Heat-Treated Glass –
• Clear and tinted annealed glass can be heat treated to
increase strength and resistance to thermal stress.
• Not all coatings applied to glass are suitable for subsequent
heat treatment.
• Here, glass is heated almost to its softening temperature, and
cooled quickly to lock in compressive stresses.
• The heated glass in the centre shrinks as it cools, putting the
outer surfaces of the glass into compressive stress.
• Depending on the rate of cooling, glasses are classifies as
tempered or heat strengthened glasses.
• If the outer surfaces of glass are cooled very quickly to retain
very high compressive stress, the result is tempered glass;
otherwise, the result is heat-strengthened glass.
• Tempered glass (and to a lesser extent heat-strengthened
glass) can resist higher impact loads, wind loads, and
temperature changes than ordinary annealed glass.
• Fully tempered glass is approximately four times as strong as
annealed glass of equal thickness. Tempered glass tends to
break into small cubical pieces.
Laminated Glass –
• Laminated glass can be fabricated by bonding two or more
glass panes with a transparent, flexible interlayment material.
• When broken, laminated glass tends to remain in place with
glass particles adhered to interlayment.
Wired Glass –
• Wired glass is formed by rolling and has an embedded wire
mesh to prevent shattering and to withstand fire exposure.
• It is accepted as safety glazing in fire-rated doors and windows.
• Woven stainless steel wire is used in diamond or square mesh
pattern.
• As far as fire safety is concerned, window/door glazing systems
are important, as breaking of these glasses may influence the
fire severity in ventilation controlled fires.
• The breaking of glass when exposed to fire depends on the
properties such as the glass thickness, thermal conductivity,
thermal diffusivity, Young’s modulus, fracture stress, shading
depth, thermal expansion coefficient and the distance to the
flame.
• When a window pane of ordinary float glass is first heated, it
tends to crack when the glass reaches a temperature of about
150 - 200ºC.
• The temperature differences between the exposed glass surface and the glass
shielded by the edge mounting play the dominant role in controlling cracking.
• A temperature difference of about 80°C between the heated glass temperature
and the edge temperature is needed to initiate cracking.
• This is dependent on the thermal and mechanical properties for glass and may
vary.
• The first crack initiates from one of the edges.
• At that point, there is a crack running through the pane of glass, but there is no
effect on the ventilation available to the fire.
• For the air flows to be affected, the glass must not only crack, but a large piece
or pieces must fall out.
• Tempered glass shatters upon initial cracking, but the initial cracking
does not occur until the glass reaches rather high temperatures.
• An exposed-surface temperature of 290-380ºC has been found to be
needed, with the unexposed surface temperatures being about 100ºC
lower.
• Plain" glass was found to "break" when the exposed side reached 150175ºC, with the unexposed side being at 75-150ºC.
• Heat-strengthened and tempered glass (unspecified thickness) was
found not to break at an irradiance of 43 kW m-2.
• The latter heat flux corresponded to 350ºC on the exposed face and
300ºC on the unexposed face.
• The oldest category of the latter is wire glass. Nowadays, several types of
patented fire-resistive glasses also exist which are not wired glass.
• These are usually multi-layered structures, generally involving some
polymeric inner layers.
• Fire-resistive glasses will normally be accompanied by a laboratory report
of the endurance period.
• Such glasses can be assumed to have no ventilation flow until after their
failure time.
• It is, very difficult to predict when glass will actually break enough to fall out
in a real fire. 300°C appears to be a reasonable lower bound.
• 3 mm window glass will break around 340°C.
• For thicker, 4-6 mm glass, the mean temperature of breakage would
appear to be around 450°C.
• Double-glazed windows using 6 mm glass can be expected to break
out at about 600ºC. Tempered-glass in not likely to break out until
after room flashover has been reached.
• Factors such as window size, frame type, glass thickness, glass
defects, and vertical temperature gradient may all be expected to
have an effect on glass fall-out.
• Over-pressure due to gas explosions is an obvious glass failure
mechanism. Yet, normal fires do show pressure variations and these
could potentially affect the failure of glass panes.