fires as a cause of concrete deterioration

1

SEEM 531 - CONCRETE DURABILITY

FIRES AS A CAUSE OF CONCRETE DETERIORATION

MOHAMED NABIL – 201326007213 April, 2015

BRITISHUNIVERSITY IN

DUBAI

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

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


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.


Concrete Deterioratio

n

Physical Causes

Chemical Causes

Mechanical Causes

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.



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.


Fire Development


Fire Development

Fully Developed Fire


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.



• 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.


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.


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.


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.



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.



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.


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.



Fire resistance

Hour

Nominal cover - mm

Beams Floors Ribs

Simply

supportedContinuous

Simply

supportedContinuous

Simply

supportedContinuous

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

4.0 80 70 65 55 75 65



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



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.



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.



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.


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.



• 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.



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.



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.



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.



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.



• 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.


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.


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.


Thank you

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