Presentation outline Introduction to the subject What is the causes of fire? Physical and chemical response to fire Spalling of concrete Factors influencing the explosive spalling How to improve the concrete structures in the fire resistance? Case study - Concrete structure subjected to a fire in U.A.E Rehabilitation methods and the repairing plan Recommendation and conclusions

1. SEEM 531 - CONCRETE DURABILITY
FIRES AS A CAUSE OF CONCRETE DETERIORATION
MOHAMED NABIL – 2013260072
13 April, 2015
BRITISH
UNIVERSITY IN
DUBAI
1

2. SEEM531 – CONCRETE DURABILITY
FIRES AS A CAUSE OF CONCRETE DETERIORATION
By Mohamed Nabil, student in MSc in structural engineering
British University in Dubai
Module Tutor: Professor Abid Abu Tair, professor of Structural Engineering BUID
FIRES AS A CAUSE OF CONCRETE DETERIORATION 2

3. Presentation outline
• Introduction to the subject
• What is the causes of fire?
• Physical and chemical response to fire
• Spalling of concrete
• Factors influencing the explosive spalling
• How to improve the concrete structures in the fire resistance?
• Case study - Concrete structure subjected to a fire in U.A.E
• Rehabilitation methods and the repairing plan
• Recommendation and conclusions
FIRES AS A CAUSE OF CONCRETE DETERIORATION 3

4. Introduction to the subject
• The main causes of the concrete structures
deterioration can be classified into three
categories which are mechanical, chemical and
physical causes.
• The fires are considered as one of the physical
causes of the concrete deterioration.
FIRES AS A CAUSE OF CONCRETE DETERIORATION 4
Concrete
Deterioration
Physical
Causes
Chemical
Causes
Mechanical
Causes

5. Introduction to the subject
• The concrete as a building material has a very good
behavior when it exposed to fire, especially when it
is compared to any others building materials like
wood and steel.
• But this is not mean that the concrete has infinite fire
resistance, in some levels of fires when the concrete
exposed to high temperature up to 900oc, significant
changes in the mechanical properties of the structural
elements like stiffness and strength will be occur.
FIRES AS A CAUSE OF CONCRETE DETERIORATION 5

6. FIRES AS A CAUSE OF CONCRETE DETERIORATION 6
What is the causes of fires?
Oxygen
Combust-
ible
Materials
Fire
Fire
Source
Fire Triangle
• For a fire to start there are three elements should be
present, oxygen, combustible materials and a fire source.
These three elements represent what is called the fire
triangle.

7. FIRES AS A CAUSE OF CONCRETE DETERIORATION 7
Fire Development

8. FIRES AS A CAUSE OF CONCRETE DETERIORATION 8
Fire Development

9. FIRES AS A CAUSE OF CONCRETE DETERIORATION 9
Fire Development
Fully Developed Fire

10. FIRES AS A CAUSE OF CONCRETE DETERIORATION 10
Physical and chemical response to fire

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Physical and chemical response to fire
• 100 to 140°C - Evaporation of the free water inside the concrete mix.
• 300°C - The cement paste will start to shrink due to water evaporation and the
aggregate will expand. This will cause which is called the spalling of concrete.
• 400 to 600°C - the calcium hydroxide in the cement paste breaks to calcium oxide
and water. The resultant water from the chemical reaction, start to evaporate. This
will cause a significant reduction in the concrete strength.
• Starting from 550°C - the aggregate in the concrete will start to decompose
causing significant loss in the concrete strength.

12. FIRES AS A CAUSE OF CONCRETE DETERIORATION 12
Physical and chemical response to fire
• This reaction may vary based on the aggregate type and the different thermal
expansion between the cement matrix and the aggregate.
• The cooling operation after the fire, cause also physical and chemical reactions in
the concrete such as cracks, moisture absorption and rehydration of the calcium
oxide.
• All these chemical and physical reactions in both heating and cooling are
depending on the cement type, admixture, the aggregate type and the
interaction between the concrete mix materials. For example the using of
thermally stable aggregate of low thermal expansion like basalt and granite and
the using of supplementary cementitious materials like blast furnace slag, this can
improve significantly the concrete fire resistance.

13. FIRES AS A CAUSE OF CONCRETE DETERIORATION 13
Spalling of concrete
The spalling of concrete is the breaking and splitting of
the concrete elements surface layers due to high thermal
exposure. The spalling can be classified to three types.
A. Aggregate spalling is caused by the failure and the
splitting of the small aggregate pieces which are close
to the surface of the structural elements.
B. Corner & surface spalling which is the falling of
large corner pieces of the concrete due to tensile
cracks. and this type of spalling usually happen in the
decay stage.

14. FIRES AS A CAUSE OF CONCRETE DETERIORATION 14
Spalling of concrete
C. Explosive spalling, the ejection of concrete pieces from
the heated surface at high temperature. Explosive
spalling is the most dangerous type of all the types of
concrete spalling and it may cause the collapse of the
building.
Generally spalling cause a reduction in the structural
elements cross section and causing higher stress in the
remaining area of concrete. For example if the spalling
happened to the concrete column, the column may collapse
due to the increasing in the compressive stresses on the
remaining concrete section or may collapse due to buckling.

15. FIRES AS A CAUSE OF CONCRETE DETERIORATION 15
Factors influencing the explosive spalling
1. Heating Rate, the probability of explosive
spalling to occur increasing with the
increasing of the heating rate.
2. The exposure of the element to the fire, the
more faces of structural elements are exposed
to fire, the probability of the spalling to occur
increased. For example, slabs have better
resistance than the beams this because there
is only one face of the slabs are exposed to
fire unlike beams 3 faces exposed to fire.

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Factors influencing the explosive spalling
3. Moisture content, generally the explosive spalling occur to the concrete with moisture contents
less than 3% by weight. However, the spalling in the high strength concrete can occur in lower
moisture contents 2.3 to 3% by weight. This is because of the low porosity and permeability,
making it more difficult for the moisture to escape. This is generate higher pore pressure and
internal tensile stresses, increasing the risk of spalling.
4. Age of concrete structure, most of the research papers indicate that the probability of the
concrete spalling decrease with the increasing of the structure age. This is because when the
concrete structure age increase, the moisture content is decreasing. As a result of that significant
decrease will happened in the generated pore pressure and the internal tension stresses.

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Factors influencing the explosive spalling
5. Aggregates type, the probability of spalling decrease when low thermal expansion aggregates
are used.
6. Aggregate size, most of research papers and the results from the experiments indicates that the
greater size of the aggregate, more likely explosive concrete spalling is to occur.
7. Cover to reinforcement, the bigger concrete cover has a higher probability for spalling. It is
founded that if the concrete cover is more than 50mm, spalling must be feared. On the other
hand, the concrete cover with thickness less than 15mm has high probability for the spalling of
the concrete cover this is because of the unsupported concrete is small.

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How to improve the concrete structures in the fire resistance?
1. Applying all the civil defense requirements in the architecture design stage, like the
minimum corridor width and the minimum number of escaping stairs in the building.
2. Applying the standard codes requirements in the structural design stage. Like the
minimum thickness and minimum concrete cover to the steel reinforcement of the
structural elements to achieve the required time of fire resistance. The following table
shows the minimum concrete cover required to achieve the required time for the fire
resistance as per BS8110.

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How to improve the concrete structures in the fire resistance?
Fire resistance
Hour
Nominal cover - mm
Beams Floors Ribs
Simply
supported
Continuous
Simply
supported
Continuous
Simply
supported
Continuous
0.5 20 20 20 20 20 20
1.0 20 20 25 20 35 20
1.5 35 20 30 25 45 35
2.0 60 35 40 35 55 45
3.0 70 60 55 45 65 55

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How to improve the concrete structures in the fire resistance?
3. Define the fire protection system which will be used in the structure with respect to
building use and cost.
Active fire protection system Passive fire protection system
Smoke detectors Fire rated walls
Sprinklers Fire rated floors
Duct detectors Fire rated doors
Fire alarms
BALANCE DESIGN

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How to improve the concrete structures in the fire resistance?
4. The improvement of the concrete mix materials.
Cement
• High alumina and a lesser pozzolanic and blast furnace slag cements have a better performance
in the fire resistance compared to the normal Portland cement.
• When the concrete is subjected to a fire, the cement paste start to shrink due to water
evaporation and dehydration. On the other hand, the particles of the aggregates expand because
of high thermal exposure. This contrary physical reactions sets up internal stresses explain why
it is recommended to use a low aggregate - cement ratio.

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How to improve the concrete structures in the fire resistance?
Aggregates
• The siliceous aggregates can cause
spalling and its performance with
fire is badly compared with the
other types. This table shows the
minimum thickness to achieve the
required fire resistance period for
different types of aggregate.

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How to improve the concrete structures in the fire resistance?
Water
• The water content in the hardened concrete exposed to a fire is playing very important role,
this is because there is more heat required to evaporate the saturated water which will enhance
the concrete fire resistance.
Admixtures
• The using of SCM can improve the concrete density which is considered a very important
factor in the fire resistance improvement.
• The using of polypropylene fibers in the concrete mix can improve the concrete fire resistance
especially the resistance to the spalling phenomena.

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Case study - Concrete structure subjected to a fire in U.A.E
• The chosen building is the Torch Tower,
which is one of the tallest residential towers in
Dubai.
• the tower consist 86 floors and its height is
almost 336 m.
• On 21 February, 2015 a fire started from the
floor number 50, and because of high wind
speed in that day the fire was spread rapidly in
the other floors.

25. FIRES AS A CAUSE OF CONCRETE DETERIORATION 25
Case study - Concrete structure subjected to a fire in U.A.E
• The smoke detectors and fire alarms started
to work immediately. And the civil defense
started to containment the fire by using all
the modern tools and techniques.

26. FIRES AS A CAUSE OF CONCRETE DETERIORATION 26
Case study - Concrete structure subjected to a fire in U.A.E
Rehabilitation and repairing plan of the building
A. Concrete structure assessment
 Visual inspection
• In this stage it is very easy to check if there is spalling in the concrete elements. As well
as, the formation of cracks due to the generate tensile stresses.
• It is possible to make an approximation assessment of the maximum temperature
reached during the fire.
Pink or red for temperatures between 300 °C and 600 °C.
Grey‐white for temperatures between 600 °C and 900 °C.
Dull or light yellow for temperatures over 900 °C.

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Case study - Concrete structure subjected to a fire in U.A.E
 Testing of concrete structure
• Examination of concrete compressive strength,
by using nondestructive tests like Schmidt
hammer test.
• Carrying out acoustic test to detect the
formation of internal cracks.
• Boring and extracting core samples to carry out
compression tests and carry out both
petrographic and microscopic examinations.

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Case study - Concrete structure subjected to a fire in U.A.E
B. Repairing operation
• The first scenario is the concrete structure
defects are only in the exposed surface layers
and the steel reinforcement still without
significant defects. As well as, the tests shows
that there are no formation of internal cracks in
the concrete structure elements and the
concrete strength still sufficient. the suitable
method of repairing in this case is to clean the
defected surface, apply chemical materials for
the bonding between the old and new concrete,
then place the concrete by using shotcrete
technique.

29. FIRES AS A CAUSE OF CONCRETE DETERIORATION 29
Case study - Concrete structure subjected to a fire in U.A.E
B. Repairing operation
• The second scenario if the concrete damage
and defects is very deep and there is impact on
the steel reinforcement and the concrete
strength. The suitable method for the repairing
in this case in addition to the previous method
is to sticking metal plates or carbon fiber strips
to the surface of the damaged concrete for
strengthening the concrete element.

30. FIRES AS A CAUSE OF CONCRETE DETERIORATION 30
Case study - Concrete structure subjected to a fire in U.A.E
• The third scenario if the concrete
deterioration is huge and the cost of
repairing is too much compared to the
demolition and construction of new
structure. In that case the recommended
solution is to demolish the structure and
construct new one.

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Recommendation and conclusions
• The fires is one of the major causes of the
concrete structures deterioration (Physical
causes). Therefore, it is important to apply
all the requirements and specification in the
design and construction stage to improve
the concrete fire resistance.
• Dubai announced that they will apply the
project of green concrete, this is by using
environmentally friendly cementitious
materials to improve the concrete
structures sustainability and to reduce the
emission of the carbon dioxide.

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Recommendation and conclusions
 The advantages of using green concrete;
• Increasing the concrete structure life time by
almost 20 years.
• Decrease the emission of the carbon dioxide by
80%.
• Eliminate the cracks in the plastic stage of
concrete.
• Produce low permeability concrete and reduce the
probability of the steel reinforcement corrosion.
• Improve the concrete fire resistance properties.

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