Improving Fire Resistance of Reinforced Concrete Columns

ŖŨŹŗƒƆƚŪƗŒŗŶƆœŞƃŒ

Islamic University of Gaza

œƒƄŶƃŒŘœŪŒŧťƃŒŖťœƆŵ

Higher Education Deanship

ŗŪťƈƌƃŒŗƒƄƂ

Faculty of Engineering

ŗƒƈťƆƃŒŗŪťƈƌƃŒƅŪſ

Civil Engineering Department

Design and Rehabilitation of Structures

ŘŉŬƈƆƃŒ¾ƒƋŋřƍƅƒƆŮřŝƆœƈŧŕ

Improving Fire Resistance of Reinforced Concrete Columns

ϖϳήΤϠϟΔΤϠδϤϟ΍ΔϴϧΎγήΨϟ΍ΓΪϤϋϷ΍ΔϣϭΎϘϣϦϴδΤΗ

By Khaled Mohammed Nassar

Supervised By Prof. Samir Shihada

A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Civil Engineering –Rehabilitation and Design of Structures

ϡ˰ϫ

úČ»Æ÷¥þwÆ÷¥7¥úÌ©

\ \\ \` ` ] ] \ ā \ `] ]\ \ ā ] ā \ ` \ &¥¡ ÚØÄ×æå{Þë¤ [ ^ ^ ×åÚÔàÛæàÛyÞë¤×âØ×ÂË¦ë ^ ^ ^ ( Δϳϵ΍ΔϟΩΎΠϤϟ΍ Ϣϴψόϟ΍Ϳ΍ϕΪλ

\ \ \ Ă \ ]\ \ \ ] ` ] ` \ ] ] ] \ \ ` ] \\ \ ] ā \ \\ \ ]\ ` ] \ Ú×Ãè×Ýå¡¦«åÝæàÛ~Ü×åâ×æ«¥åÚÔØÜÃâØ×ç¦ì¬ËæØÜÃÖÏå ^^ ^ ^ ^ ( \ ]\ ` \ ` ]`] \ ` ] ]ă\]\ \ \ ā \ `\ ` &ÝæØÜÄÚàÓÜÚÔàìË¡ä°×åìÈ× ^ ^ ^ Δϳϵ΍ΔΑϮΘϟ΍ Ϣϴψόϟ΍Ϳ΍ϕΪλ

Dedication

I would like to dedicate this work to my family specially my mother and my father who loved and raised me, to my loving wife and daughters and to my brothers and sisters, for their sacrifice and endless support

Abstract


Abstract: Fire has become one of the greatest threats to buildings. Concrete is a primary construction material and its properties of concrete to high temperatures have gained a great deal of attention. Concrete structures when subjected to fire presented in general good behavior. The low thermal conductivity of the concrete associated to its great capacity of thermal insulation of the steel bars is the responsible for this good behavior. However, there is a fundamental problem caused by high temperatures that is the separation of concrete masses from the body of the concrete element " spalling phenomenon ". Spalling of concrete leads to a decrease in the cross section area of the concrete column and thereby decrease the resistances to axial loads, as well as the reinforcement steel bars become exposed directly to high temperatures. With the increase of incidents caused by major fires in buildings; research and developmental efforts are being carried out in this area and other related disciplines. This research is to investigate the behavior of the reinforced concrete columns at high temperatures. Several samples of reinforced concrete columns with Polypropylene (PP) fibers were used. Three mixes of concrete are prepared using different contents of Polypropylene ;( 0.0 kg/m³, 0.5 kg/m³ and 0.75 kg/m³). Reinforced concrete columns dimensions are (100 mm x100 mm x300 mm). The samples are heated for 2, 4 and 6 hours at 400 C°, 600 C° and 800°C and tested for compressive strength. Also, the behavior of reinforcement steel bars at high temperatures is investigated. Reinforcement steel bars are embedded into the concrete samples with 2 cm and 3 cm concrete covers, after heating at 800°C for 6 hours. The reinforcement steel bars are then extracted and tested for yield stress and maximum elongation ratio. The analysis of results obtained from the experimental program showed that, the best amount of PP to be used is 0.75 kg/m³, where the residual compressive strength is 20 % higher than of that when no PP fibers are used at 400 C for 6 hours. Moreover, a 3 cm of concrete cover is in useful improving fire resistance for concrete structures and providing a good protection for the reinforcement steel bars, where it is 5 % higher than the column samples with 2 cm concrete cover at 6 hours and 600 C°.

I

Abstract


ΔλϼΨϟ΍

ϦѧѧϣϲѧѧγΎγ΃ήμѧѧϨϋήѧѧΒΘόΗΔϧΎѧѧγήΨϟ΍ϥ΃ΚѧѧϴΣϭˬϲϧΎѧѧΒϤϟ΍ΩΪѧѧϬΗϲѧѧΘϟ΍έΎѧѧτΧϷ΍Ϣѧѧψϋ΃ϦѧѧϣΓΪѧѧΣ΍ϭϖѧѧ΋΍ήΤϟ΍ήѧѧΒΘόΗ ϡΎϤΘϫϻ΍Ϧϣ΍ήϴΒϛ΍έΪϗΐδΘϜϳΔόϔΗήϣΓέ΍ήΣΕΎΟέΪϟΎϬοήόΗΪόΑΎϬμ΋ΎμΧϭΎϬϛϮϠγϥΈϓ˯ΎϨΒϟ΍ήλΎϨϋ ϞϴѧλϮΘϟ΍ΔϠϴΌѧοΎѧϬϧ΃ΚѧϴΣΔѧόϔΗήϣΓέ΍ήѧΣΕΎΟέΪѧϟΎϬѧοήόΗ˯ΎѧϨΛ΃΍ΪѧϴΟΎϛϮϠѧγϱΪѧΒΗΔϧΎѧγήΨϟ΍ϥΈѧϓϡΎϋϞϜθΑ ΕΎѧѧΟέΩΎϬΒΒδѧѧΗΔϴѧѧγΎγ΃ΔϠϜθѧѧϣϙΎѧѧϨϫϦѧѧϜϟϭ΢ϴϠδѧѧΘϟ΍ΪѧϳΪΣϥΎΒπѧѧϗϦѧѧϋΓέ΍ήѧѧΤϠϟΪѧѧϴΟϝίΎѧѧϋήѧѧΒΘόΗΎѧѧϤϛΓέ΍ήѧΤϠϟ ϲѧϓϥΎμѧϘϧΙϭΪѧΣϰѧϟ·ϱΩΆѧϳΎѧϣϲϧΎѧγήΨϟ΍ήμϨόϟ΍ϢδΟϦϋΔϴϧΎγήΧϞΘϛϝΎμϔϧ΍ϲϓϞΜϤΘΗΔόϔΗήϤϟ΍Γέ΍ήΤϟ΍ ΪѧѧϳΪΣϥΎΒπѧѧϗ΢ΒμѧѧΗϚϟάѧѧϛˬϪѧϴϠϋΔϴѧѧγ΍ήϟ΍ϝΎѧѧϤΣϷ΍ϲѧѧϓΔѧѧϴϟΎϋΓΩΎѧѧϳίϲϟΎѧѧΘϟΎΑϭϲϧΎѧѧγήΨϟ΍ΩϮѧѧϤόϠϟϊѧѧτϘϤϟ΍ΔΣΎδѧѧϣ ΍άϫϭˬΎϬΘηΎθϫϦϣΪϳΰϳϭΪθϟ΍ϯϮϗϞϤΤΗϰϠϋΎϬΗέΪϗϞϠϘϳΎϤϣΔόϔΗήϤϟ΍Γέ΍ήΤϠϟήηΎΒϣϞϜθΑΔοήόϣ΢ϴϠδΘϟ΍ ΔϴϧΎγήΨϟ΍Ε΂θϨϤϟ΍ϲϓΕ΍έΎϴϬϧϻ΍ΙϭΪΣϲϓήηΎΒϣϞϜθΑΐΒδΘϳΪϗϪϠϛ ϭϝΎѧΠϤϟ΍΍άѧϫϲѧϓϱήѧΠΗΔѧΜϴΜΣΔѧϳϮϤϨΗϭΔѧϴΜΤΑ΍ΩϮѧϬΟϥΈѧϓˬϲϧΎѧΒϤϟ΍ϰѧϠϋΎϫήτΧϭϖ΋΍ήΤϟ΍ΙΩ΍ϮΣΩΎϳΩί΍ϊϣ ΔϴϟΎόϟ΍Γέ΍ήΤϟ΍ΕΎΟέΪϟΔΤϠδϤϟ΍ΔϧΎγήΨϟ΍ΔϣϭΎϘϣϦϴδΤΘϟΔϟϭΎΤϣϲϓΔϠμϟ΍Ε΍ΫΕϻΎΠϤϟ΍ ΕΎѧϨϴϋϲѧϓϦϴϠΑϭήѧΑϲϟϮѧΒϟ΍ϑΎѧϴϟ΃ϡ΍ΪΨΘѧγ΍ϢѧΗΪѧϗϭˬΔΤϠδѧϤϟ΍ΔϴϧΎγήΨϟ΍ΓΪϤϋϷ΍ϙϮϠγΔγ΍έΩΚΤΒϟ΍΍άϫϝϭΎϨΘϳ ϲϟϮѧѧΒϟ΍ϑΎѧѧϴϟ΃ϦѧѧϣΔѧѧϔϠΘΨϣΕΎѧѧϴϤϛϰѧѧϠϋϱϮѧѧΘΤΗΔϴϧΎѧѧγήΧΕΎѧѧτϠΧΙϼѧѧΛΩ΍Ϊѧѧϋ·ϢѧѧΗϭΔΤϠδѧѧϤϟ΍ΔϴϧΎѧѧγήΨϟ΍ΓΪѧϤϋϷ΍ ΩΎόΑϷ΍Ε΍ΫΔϴϧΎγήΧΓΪϤϋ΃ΐλϭ;( 0.75 kg/m³ and, 0.5 kg/m³ 0.0 kg/m³)ϲϫϭϦϴϠΑϭήΑ 400C°, 600C°, ΔѧѧϔϠΘΨϣΓέ΍ήѧѧΣΕΎΟέΪѧѧϟνήόΘΘѧѧγϲѧѧΘϟ΍ϭmm x100 mm x 300 mm) .ήδϜϟ΍έΎΒΘΧ΍ςϐπϠϟΎϬϠϤΤΗΓϮϗκΤϓϢΛϦϣϭˬΔϋΎγˬˬ ΓΪϤϟϭ800C° ϢѧγϭϢѧγϖѧϤϋΪѧϨϋΪѧϳΪΤϟ΍ϥΎΒπѧϗϦϓΩϢΗΚϴΣˬ΢ϴϠδΘϟ΍ΪϳΪΣϰϠϋΔόϔΗήϤϟ΍Γέ΍ήΤϟ΍ήϴΛ΄ΗΔγ΍έΩϢΗϚϟάϛ ΪѧϳΪΣϥΎΒπѧϗΝ΍ήΨΘѧγ΍ϢΗϚϟΫΪόΑˬΕΎϋΎγΓΪϤϟ800 C°Γέ΍ήΣΔΟέΪϟΎϬπϳήόΗϢΛΔϴϧΎγήΨϟ΍ΓΪϤϋϷ΍ϲϓ ΔϟΎτΘγϻ΍ΔΒδϧΔϓήόϣϭΪθϟ΍ϞϤΤΗέΎΒΘΧ΍ϖϴΒτΗϭΔϴϧΎγήΨϟ΍ΓΪϤϋϷ΍Ϧϣ΢ϴϠδΘϟ΍ ΪѧѧϘϓ΢ϴϠδѧѧΘϟ΍ΪѧѧϳΪΣϥΎΒπѧѧϗΕΎѧѧϨϴϋϰѧѧϠϋϭΔϴϧΎѧѧγήΨϟ΍ΓΪѧѧϤϋϷ΍ϰѧѧϠϋΔѧѧϣίϼϟ΍Ε΍έΎѧѧΒΘΧϻ΍ϖѧѧϴΒτΗϦѧѧϣ˯ΎѧѧϬΘϧϻ΍ΪѧѧόΑ ςϐπѧϟ΍ϯϮϗϞϤΤΘϟήΒϛ΃ΔϣϭΎϘϣϱΪΒΗϦϴϠΑϭήΑϲϟϮΑ0.75 kg/m³ϰϠϋϱϮΘΤΗϲΘϟ΍ΕΎϨϴόϟ΍ϥ΃Ξ΋ΎΘϨϟ΍ΕήϬχ΃ ΔѧϴϨϣίΓΪѧϣϭ C°Γέ΍ήΣΔΟέΩΪϨϋϚϟΫϭϦϴϠΑϭήΑϲϟϮΒϟ΍ϰϠϋϱϮΘΤΗϻϲΘϟ΍ΕΎϨϴόϟ΍ϦϣΔΒδϨΑ ϯϮѧѧϘϟΔѧѧϣϭΎϘϣϱΪѧѧΒΗϢѧѧγΔϴϧΎѧѧγήΧΔѧѧϴτϐΗΎѧѧϬϟϲѧѧΘϟ΍ΔϴϧΎѧѧγήΨϟ΍ΓΪѧѧϤϋϷ΍ΕΎѧѧϨϴϋϥΈѧѧϓϚѧѧϟΫϰѧѧϠϋΓϭϼѧѧϋˬΕΎϋΎѧѧγ ΔѧϴϨϣίΓΪѧϣϭ C°Γέ΍ήѧΣΔΟέΩϚϟΫϭϢγΔϴϧΎγήΧΔϴτϐΗΎϬϟϲΘϟ΍ΕΎϨϴόϟ΍ϦϣΔΒδϨΑήΜϛ΃ςϐπϟ΍ ΕΎϋΎγ

II

ACKNOWLEDGMENT I would like to extend my gratitude and my sincere thanks to my honorable, esteemed supervisor, Assoc. Prof. Samir M. Shihada, for his exemplary guidance and encouragement. Also, I would like extend my sincere appreciation to all who helped me in currying

out this thesis. I would like to thank all my lecturers in the Islamic University of Gaza from whom I learned much and developed my skills. My deepest appreciation and thanks to every one who helped me in the completeness of this study, especially to the staff of Material & Soil Laboratory in the Islamic University of Gaza, and the staff of Sharaf factory.

III

TABLE OF CONTENTS ABSTRACT…………………………………………………………………………………

I

ACKNOWLEDGMENT ………….……………………………………………………….

III

TABLE OF CONTENTS ………………………………………………………………….

IV

LIST OF FIGURES ……………………………………………………………………….

VII

LIST OF TABLES ………………………………………………………………………...

IX

LIST OF APPREVIATIONS …………………………………………………………….

X

CHAPTER 1. INTRODUCTION 1.1 Introduction ….................................................................……………………….

1

1.2 Statement of Problem……………...……………………………………………

2

1.3 Research Objectives …………………………………………………………….

3

1.4 Research Methodology ………………………………………………………….

4

1.5 Thesis Organization…………………..………………………………………….

5

CHAPTER 2. LITERATURE REVIEW 2.1 Introduction …………………………………………………...............................

6

2.2 Concrete………………………………………………………………………….

6

2.2.1 Benefits of concrete under fire ……….…………................................

8

2.3 Physical and chemical response to fire ………………………………………….

8

2.4 Spalling ………………………………………………………………………….

11

2.4.1 Mechanisms of Spalling ……………………………………………….….

12

2.5 Spalling Prevention Measures ……………………………………………….…..

14

2.5.1 Polypropylene fibers ………………………………………………………

14

2.5.2 Thermal barriers ……………………………………………………………

15

2.6 Cracking …………………………………………………………………………..

15

IV

2.7 Effect of Fire on Concrete ………………………………………………………..

17

2.8 Performance of reinforcement in fire …………………………………………….

22

2.9 Effect of Fire on Steel Reinforcement …………………………………………...

22

2.10- Effect of Fire on FRP columns …………………………………………………

24

CHAPTER 3. EXPERIMENTAL PROGRAM 3.1 Introduction………………………………………………………………………

25

3.2 Materials and Their Quality Tests ………………………………………………..

25

3.2.1 Aggregate Quality Tests ………………………...………………………….

26

3.2.1.1 Unit Weight of Aggregate …………………………………………..

26

3.2.1.2 Specific Gravity of Aggregate ……………………………………...

27

3.2.1.3 Moisture content of Aggregate ……………………………………..

29

3.2.1.4 Resistance to Degradation by Abrasion & Impaction test …………..

29

3.2.1.5 Sieve Analysis of Aggregate ………………………………………..

31

3.2.1.6 Cement ………………………………………………………………

32

3.2.1.7 Water ………………………………………………………………..

33

3.2.1.8 Polypropylene Fibers (PP) …………………………………………..

33

3.3 Mix Proportions …………………………………………….…………………….

34

3.4 Sample Categories ………………………………………………………………..

35

3.5 Mixing, casting and curing procedures …………………………………………..

38

3.5.1 Mixing procedures …………………………………………………………

38

3.5.2 Casting procedures …………………………………………………………

38

3.5.3 Curing procedures ………………………………………………………….

39

3.6 Heating Process …………………………………………………………………..

39

3.7 Compressive and Tensile Strength Tests …………………………………………

41

3.8 Reinforcing Steel Tests …………………………………………………………..

42

V

CHAPTER 4. Results & Discussion 4.1 Introduction……………………………………………………………………...

43

4.2 Effect of polypropylene content………………………………………………….

43

4.2.1 Unheated Columns ………………………………………………………

43

4.2.2 Heated Columns …………………………………………………………

44

4.3 Effect of concrete cover ………………………………………………………….

48

4.3 Effect of high temperature on steel reinforcement ……………………………….

50

4.3.1 Yield Stress ……………………………………………………………….

50

4.3.2-Elongation ………………………………………………………………..

51

CHAPTER 5. CONCLUSION & RECOMMENDATIONS 5.1 Introduction………………………………………………………………………

53

5.2 Conclusions………………………………………………………………………

53

5.3 Recommendations……………………………………………………………….

54

CHAPTER 6. REFERENCES …………………………………………………………….

55

ABBENDIX A : RESEARCH PHOTOS ………………………………………………….

A

VI

LIST OF FIGURES

Fig.1.1 Reinforced Concrete Column subjected to Fire "Al Sultan Tower – Jabalia……...…..

2

Fig.2.1 Surface cracking after subjected to high temperatures…………………………………

7

Fig.2.2 Structural failure………………………………………………………………………..

7

Fig. 2.3 Concrete in fire – physiochemical process…………………………………..…………

9

Fig. 2.4 Spalling in concrete column subjected to fire, Alsultan Tower, Jabalia, North Gaza Strip……………………………………………………………………………………

12

Fig. 2.5 The spalling mechanism of concrete cover…………………………………………….

13

Fig. 2.6 Polypropylene fibers provide protection against spalling ……………………………..

14

Fig.2.7 Thermal cracks in a concrete column subjected to high temperature …………………

16

F Fig.3.1 Mold of Unit Weight test…………………………………………………………… 27 Fig.3.2 Specific Gravity test equipments………………………………………………………

28

Fig. 3.3 Los Angeles Abrasion Machine………………………………………………………..

30

Fig. 3.4 Sieve analysis of aggregate…………………………………………………………….

32

Fig.3.5 Polypropylene Fibers…………………………………………………..………………

33

Fig.3.6 Dimension and Reinforcement Details of Samples……………………………………

35

Fig.3.7 Mechanical mixer ……………………………………………………………………..

38

Fig 3.8 Form of the moulds used for preparing specimens…………………………………….

38

Fig 3.9 Curing process for hardened concrete …………………………………………………

39

Fig 3.10 Electrical Furnace for Burning process………………………………………………...

39

Fig 3.11 Heating process Flow char ……………………………….............................................

40

Fig 3.12 Compressive strength Machine ………………...………………………………….......

41

Fig 3.13 Tensile strength test for reinforcement steel ……...…………………………………...

42

Fig 4.1 Relationship between axial load capacity and burning periods at 400 C°…....………..

46

Fig 4.2 Relationship between axial load capacity and burning periods at 600 C°….………......

46

VII

Fig 4. 3 Relationship between axial load capacity and burning periods at 800 C°…………….

47

Fig 4. 4 Relationship between axial load capacity and heating duration at 600 C° for different concrete covers………………………………………….………………

49

Relationship between yield stress of steel reinforcement and concrete coverFig 4. for 800 C° temperature at 6 hrs………………………………………………………..

50

Fig 4.6 Relationship between elongation of steel reinforcement and concrete cover for 800 C° at 6 hrs …………………………………………………………………….

51

Fig A.1 Mechanical Mixer ……………………………………………………………………..

A-1

Fig A.2 Adding of Polypropylene to the mix …………………………………………………..

A-1

Fig A.3 Column samples in water ……………………………………………………………...

A-1

Fig A.4 Column samples in the open air………………………………………………………..

A-1

Fig A.5 Electrical Furnace……………………………………………………………………...

A-2

Fig A.6 Column samples in the electrical Furnace …………………………………………….

A-2

Fig A.7 Cracks and Spalling after heating ……………………………………………………...

A-2

Fig A.8 Compressive Strength test and column samples after test……………………………...

A-3

Fig A.9 Yield stress test for the steel reinforcement ……………………………………………

A-4

VIII

LIST OF TABLES

Table 2.1

Mineralogical Composition of Portland Cement………………………

10

Table 3.1

Capacity of Measures………………….…………………………….....

26

Table 3.2

Unit weight test results…………………………………………………

27

Table 3.3 Specific gravity of aggregate…………………………………………...

28

Table 3.4

Moisture content values………………………………………………..

29

Table 3.5

Number of steel spheres for each grade of the test sample.....................

30

Sieve analysis of aggregate…………………………………………….

31

Table 3.7

Ordinary Portland cement properties "Test Results"…………………..

32

Table 3.8

Properties of Polypropylene fibers……………………………………..

33

Table 3.9

mix design of the concrete column samples ……………………………….….

34

Table 3.10

Concrete without PP and cover 2.0 cm………………………….……..

36

Table 3.11 Concrete with 0.5 Kg/m³ PP and cover 2.0 cm………………………...

36

Table 3.12

Concrete with 0.75 Kg/m³ PP and cover 2.0 cm….…………………...

37

Table 3.13

Concrete with concrete cover 3.0 cm…………………………………..

37

Table 3.14 Concrete without PP……………………………………………………

37

Table 3.6

Table 4.1 The axial load capacity test results for the columns samples with polypropylene fibers ……………………………..........................

44

Table 4.1 Percentage of reduction in axial load capacity at different % of PP, and different temperatures…................................................................

45

Table 4.3 the axial load capacity test results at 600 Cº for 2 cm and 3 cm concrete cover…………………………………………………….

48

Table 4.4 Percentages of reduction in axial load capacity at 600 Cº for 2 cm and 3 cm Concrete cover……………………………..

48

Table 4.5 the effect of high temperature on yield stress of steel reinforcement……………………………………………………..

50

Table 4.6 The effect of heating on the elongation of steel reinforcement…..…….

51

IX

LIST OF APPREVIATIONS RC

Reinforced Concrete

PP

Polypropylene

°C

Degree Celsius

f

' c

Compressive Strength of concrete cylinders at 28 days

UTM

Universal Testing Machine

ASTM

American Society for Testing and Materials

ACI

American Concrete Institute



Unit Weight

Abs.

Absorption

FRP

Fiber-Reinforced Polymers

C-S-H

Hydrated Calcium Silicate

SSD

Saturated Surface Dry

S.G

Specific Gravity

BSG

Bulk Specific Gravity

Wt

Weight

OPC

Ordinary Portland Cemen

w/c

Water cement ratio

C60

Concrete compressive strength of 60 MPa

X

INTRODUCTION

XI

CH.1: Introduction


Introduction

1.1- Introduction Fire impacts reinforced concrete (RC) members by raising the temperature of the concrete mass. This rise in temperature dramatically reduces the mechanical properties of concrete and steel. Moreover, fire temperatures induce new strains, thermal, and transient creep .They might also result in explosive spalling of surface pieces of concrete members. Fire is a global disaster in the sense that it disrupts the normal human activities and leaves behind its scars for some time to come [1]. Concrete is a good fire-resistant material due to its inherent non-combustibility and poor thermal conductivity. Concrete is specified in buildings and civil engineering projects for several reasons, sometimes cost, and sometimes speed of construction or architectural appearance, but one of concrete’s major inherent benefits is its performance in fire, which may be overlooked in the race to consider all the factors affecting design decisions. Concrete usually performs well in building fires. However, when it’s subjected to prolonged fire exposure or unusually high temperatures, concrete can suffer significant distress. Because concrete’s pre-fire compressive strength often exceeds design requirements, a modest strength reduction can be tolerated. But large temperatures can reduce the compressive strength of concrete so much that the material retains no useful structural strength [2]. A study of RC columns is important because these are primary load bearing members, and a column could be crucial for the stability of the entire structure.

CH.1: Introduction


1.2- Statement of Problem During the recent war in the Gaza Strip, the veil uncovered on the extent of disasters on constructions. It was noted the large scale of destruction resulting from fires which were either from direct missile hits, or through flammable materials and burning and flammable (e.g. gasoline) in the residential, and industrial compounds. The Sultan Tower, located in Jabalia – was damaged by burning of flammable materials. Concrete structures when subjected to fire showed good behavior in general. The low thermal conductivity of the concrete associated with its great capacity of thermal insulation of the steel bars is responsible for this good behavior. However a phenomenon such as the concrete spalling may compromise the fire behavior of the elements.

Fig.1.1: Reinforced Concrete Column subjected to Fire "Al Sultan Tower – Jabalia"

CH.1: Introduction


Spalling of concrete leads to a decrease in the cross section area of the concrete column and thereby decrease the resistances to axial loads, as well as the reinforcement steel bars become exposed directly to high temperatures. To avoid spalling phenomenon, several studies have been performed worldwide for the development of concrete compositions of enhanced fire behavior. Concretes with polypropylene fibers showed good behavior at elevated temperatures and controlling spalling of concrete [3]. In this research polypropylene fibers (PP) will be used in the concrete mix in order to improve the resistance of RC columns to compression loads, and also prevent spalling of concrete in columns at elevated temperatures. The polypropylene fibers (PP) will be used in the reinforced concrete columns at different contents (0.5 kg/m³ and 0.75 kg/m³). Also, different concrete covers are to be used in order to study the effect of concrete cover on elevated temperatures resistance of reinforced concrete columns.

1.3- Research objectives:

The main objective of this research is to increase the fire resistance of reinforced concrete columns by preventing the spalling phenomenon of concrete. Access to fire-resistant concrete, especially main structural elements such as concrete columns through the following: 1. Study the effect of concrete cover on enhancing fire resistance of reinforced concrete columns. 2. Determine the best amount of polypropylene fibers (pp) to be used in the concrete mix for improving fire resistance of RC columns to compression loads. 3. Study the effect of elevated temperature and duration on the mechanical properties and elongation of the reinforcement steel bars and the effect of concrete covers of 2 cm and 3 cm.

CH.1: Introduction


1.4-Research Methodology: 1. Study the available researches related to the subject of the study. 2. Execute a program of tests in laboratories inside and outside the Islamic University of Gaza. Testing program will include the following: 

Define the properties of constitutive materials of concrete (cement, fine aggregate, coarse aggregate, steel reinforcement ….etc).



Examine the impact of polypropylene fibers (pp) on the reinforced concrete casting with different contents (0.0 kg/m³, 0.5 kg/m³, and 0.75 kg/m³) of polypropylene fibers.



Heating columns specimens in an electric furnace for different periods of time (2, 4, and 6 hours), at temperature degrees (400 C°, 600 C° and 800 C°).

3. Tests will be studying the capacity of the reinforced concrete columns under axial load at materials and soil laboratory in the Islamic University of Gaza. 4. Examination of reinforcement steel bars after exposure to elevated temperature through the extraction of steel bars from column specimens, then subjecting those columns to yield stress test and maximum elongation ratio. 5. Test results and data analysis. 6. Conclusions and the recommendations of the research work, based on the experimental program results and data analysis.

CH.1: Introduction


1.5- Thesis Organization The thesis contains 6 chapters as follows: Thesis Organization Chapter 1 (Introduction): This chapter gives some background on fire effect on concrete structures, especially reinforced concrete columns. Also it gives a description of the research importance, scope, objectives, and methodology, in addition to the report organization. Chapter 2 (Literature Review): This chapter reviews a number of studies and scientific research on the impact of fire on reinforced concrete columns, and ways to improve the resistance of concrete columns against fire load. Chapter 3 (Experimental program): determine the basic properties of materials consisting of concrete, such as" aggregate, sand, cement…" , preparation of concrete columns samples , heating process , and test of samples after heating. Chapter 4 (Results & Discussion): This chapter discusses the results of the tests that were performed on reinforced concrete specimens and reinforcement steel bars samples. Chapter 5 (Conclusions and Recommendations): This chapter includes the concluded remarks, main conclusions and recommendations drawn from the research work. Chapter 6 (References).

LITERATURE REVIEW

CH.2: Literature Review


Literature Review

2.1-Introduction: Fire remains one of the serious potential risks to most buildings and structures. The extensive use of concrete as a structural material has led to the need to fully understand the effect of fire on concrete. Generally concrete is thought to have good fire resistance. The behavior of reinforced concrete columns under high temperature is mainly affected by the strength of the concrete, the changes of material property and explosive spalling. The hardened concrete is dense, homogeneous and has at least the same engineering properties and durability as traditional vibrated concrete. However, high temperatures affect the strength of the concrete by explosive spalling and so affect the integrity of the concrete structure. In recent years, many researchers studied the fire behavior of concrete columns. Their studies included experimental and analytical evaluations for reinforced concrete columns such as Paulo [3], Shihada [15], Ali [16] and Sideris [22]. This chapter discusses a number of researches and previous studies which were conducted to study the effect of fire on reinforced concrete columns. 2.2- Concrete Concrete is a composite material that consists mainly of mineral aggregates bound by a matrix of hydrated cement paste. The matrix is highly porous and contains a relatively large amount of free water unless artificially dried. When exposing it to high temperatures, concrete undergoes changes in its chemical composition, physical structure and water content. These changes occur primarily in the hardened cement paste in unsealed conditions. Such changes are reflected by changes in the physical and the mechanical properties of concrete that are associated with temperature increase. Deterioration of concrete at high temperatures may appear in two forms: (1) Local damage (Cracks) in the material itself and Fig (2.1). (2) Global damage resulting in the failure of the elements Fig (2.2) [4].



Fig. 2.1: Surface cracking after subjected to high temperatures [4].

Fig.2.2: Structural failure [4].

One of the advantages of concrete over other building materials is its inherent fire resistive properties; however, concrete structures must still be designed for fire effects. Structural components still must be able to withstand dead and live loads without collapse even though the rise in temperature causes a decrease in the strength and modulus of elasticity for concrete and steel reinforcement. In addition, fully developed fires cause expansion of structural components and the resulting stresses and strains must be resisted [5].



2.2.1- Benefits of concrete under fire [6]. The benefits of concrete under fire include, but not limited to the following 

It does not burn or add to fire load.



It has high resistance to fire; preventing it from spreading thus reduces resulting environmental pollution.



It does not produce any smoke, toxic gases or drip molten particles.



It reduces the risk of structural collapse.



It provides safe means of escape for occupants and access for firefighters as it is an effective fire shield.



It is not affected by the water used to put out a fire.



It is easy to repair after a fire and thus helps residents and businesses recover sooner.



It resists extreme fire conditions, making it ideal for storage facilities with a high fire load.

2.3- Physical and chemical response to fire Fires are caused by accidents, energy sources or natural means, but the majority of fires in buildings are caused by human errors. Once a fire starts and the contents and/or materials in a building are burning, then the fire spreads via radiation, convection or conduction with flames reaching temperatures of between 600 Cº and 1200 Cº. Fig (2.3) illustrates the behavior of reinforced concrete during heating and the degree of effect at each degree of heating after one hour of exposure on three sides, where the increasing of temperature degree increases the deterioration of concrete member Harm is caused by a combination of the effects of smoke and gases, which are emitted from burning materials, and the effects of flames and high air temperatures [6].



Fig.2.3: concrete in fire – physiochemical process for one hour duration [6].

Chemical changes in the structure of concrete can be studied with thermogravimetrical analyses. The following chemical transformations can be observed by the increase of temperature. At around 100 Cº the weight loss indicates water evaporation

from

the

micro

pores.

Dehydration

of

ettringite

(3CaOAl2O3·3CaSO4·31H2O) occurs between 50 Cº and 110 Cº.The mineralogical composition of Portland cement shown in Table (2.1).



At 200 Cº further dehydration takes place which causes small weight loss in case of various moisture contents. The weight loss was different until the local pore water and the chemically bound water were gone. Further weight loss was not perceptible at around 250-300 Cº. During heating the endothermic dehydration of Ca (OH)2 occurs between 450 Cº and 550 Cº (Ca (OH)2 → CaO + H2O. Dehydration of calciumsilicate-hydrates was found at the temperature of 700 Cº [4]. Table 2.1: Mineralogical Composition of Portland cement. Mineralogical composition

contribution ratio ( % )

C3S (3 CaO . SiO2)

38.95

C2S (2 CaO . SiO2)

30.55

C3A (3 CaO . Al2O3)

9.91

C4AF (4 CaO . Al2O3 . Fe2O3)

11.98

Some changes in color may also occur during the exposure for temperatures. Between 300 Cº and 600 Cº color is to be light pink, for temperatures between 600 Cº and 900 Cº color is to be light grey, and for temperatures over 900 Cº color is to be dark Beige "Creamy". The alterations produced by high temperatures are more evident when the temperature surpasses 500Cº. Most changes experienced by concrete at this temperature level are considered irreversible. C-S-H gel, which is the strength giving compound of cement paste, decomposes further above 600 Cº. At 800 Cº, concrete is usually crumbled and above 1150 Cº feldspar melts and the other minerals of the cement paste turn into a glass phase. As a result, severe micro structural changes are induced and concrete loses its strength and durability. Concrete is a composite material produced from aggregate, cement, and water. Therefore, the type and properties of aggregate also play an important role on the properties of concrete exposed to elevated temperatures.



The strength degradations of concretes with different aggregates are not same under high temperatures. This is attributed to the mineral structure of the aggregates. Quartz in siliceous aggregates polymorphically changes at 570 Cº with a volume expansion and consequent damage. In limestone aggregate concrete, CaCO3 turns into CaO at 800–900 Cº, and expands with temperature. Shrinkage may also start due to the decomposition of CaCO3 into CO2 and CaO with volume changes causing destructions. Consequently, elevated temperatures and fire may cause aesthetic and functional deteriorations to the buildings. Aesthetic damage is generally easy to repair while functional impairments are more profound and may require partial or total repair or replacement, depending on their severity [7]. 2.4- Spalling One of the most complex and hence poorly understood behavioral characteristics in the reaction of concrete to high temperatures or fire is the phenomenon of “spalling”. This process is often assumed to occur only at high temperatures, yet it has also been observed in the early stages of a fire, and at temperatures as low as 200 Cº. If severe, spalling can have a deleterious effect on the strength of reinforced concrete structures; due to enhanced heating of the steel reinforcement see Fig (2.4). Spalling may significantly reduce or even eliminate the layer of concrete cover to the reinforcement bars, thereby exposing the reinforcement to high temperatures, leading to a reduction of strength of the steel and hence a deterioration of the mechanical properties of the structure as a whole. Another significant impact of spalling upon the physical strength of structures occurs via reduction of the cross-section of concrete available to support the imposed loading, increasing the stress on the remaining areas of concrete. This can be important, as spalling may manifest itself at relatively low temperatures, before any other negative effects of heating on the strength of concrete have taken place [8].



Fig. 2.4. Spalling in concrete column subjected to fire (Alsultan Tower, Jabalia, North Gaza Strip).

2.4.1- Mechanisms of Spalling Spalling of concrete is generally categorized as: pore pressure induced spalling, thermal stress induced spalling or a combination of the two. Spalling of concrete surfaces may have two reasons: (1) Increased internal vapor pressure (mainly for normal strength concretes) and. (2) Overloading of concrete compressed zones (mainly for high strength concretes). The spalling mechanism of concrete cover is visualized in Fig (2.5).



Fig.2.5.The spalling mechanism of concrete covers [4].

As concrete is heated, the free water vaporizes at100 Cº and expands; thereby resulting in increasedpore pressures. Migration of some of this vapor to theinterior of the concrete member, where it cools andcondenses, will result in an increasingly ‘wet’ zonesometimes referred to as moisture clog. At somedistance from the hot surface the vapor frontreaches a critical point at which a maximum porepressure is achieved (further movement will result ina reduction in pressure). The distance of this pointfrom the heated surface will depend on other concrete’s permeability. Pore pressure spalling occurs ifthe maximum pore pressure is greater than the localtensile strength of the concrete. However, no porepressures have yet been measured which would exceed the tensile strength of concrete which suggests that pore pressure in isolation does not lead to theoccurrence of spalling. Strong thermal gradients develop in concrete as it isheated, due to its low thermal conductivity and highspecific heat. These thermal gradients induce compressive stresses close to the surface due to restrained thermal expansion and tensile stresses in thecooler interior regions [9]



It is most likely that spalling occurs due to the combination of tensile stresses induced by thermal expansion and increased pore pressure. Much debatestill surrounds the identification of the key mechanism (pore pressure or thermal stress). However, it is noted that the keymechanism may change depending upon the sectionsize, material and moisture content [5]. 2.5- Spalling Prevention Measures: There are some principal methods by which the incidence of spalling can be reduced. 2.5.1- Polypropylene fibers One well-known method is the addition of polypropylene (pp) fibers to the concrete mix. This approach works on the basis that, as the concrete is heated by fire, the Polypropylene fibers melt at about 160 Cº - 170 Cº thus creating channels for vapor to escape and thereby release pore pressures. The influence of compressive load during heating is important Fig (2.6).

Fig.2.6 Polypropylene fibers provide protection against spalling [10].

Tests have indicated that, for unloaded concrete, 1kg of fibers per m³ of concrete may be sufficient to eliminate spalling. For a load of 3 N/mm² the fiber content needs to be increased to 1.5-2 kg/m³ and for a load of 6 N/mm² a further increase to 3 k/m³ may be required to combat explosive spalling. Although concrete segments are lightly stressed under normal conditions, it should be pointed out that a circumferential compressive hoop stress will develop in the concrete during heating which is a function of the thermal expansion of the aggregate [10].



2.5.2- Thermal barriers Another anti-spalling measure is to place thermal barrier over the concrete surface a method sometimes used in tunnel construction. Thermal barriers reduce the rate of heating (and peak temperatures) within the concrete and thus reduce the risk of explosive spalling as well as loss of mechanical strength. They are therefore the most effective method (pp fibers do not reduce temperatures). However, there are two potential drawbacks: (a) the cost of the insulation is likely to be more than that of the fibers and (b) with some of the manufacturers there has been a problem with delaminating during normal service conditions. The design criteria normally are to apply a sufficient thickness of coating so as to reduce the maximum temperature at the surface of the concrete to below about 300 Cº and the maximum temperature at the steel rebar to about 250 Cº within 2- hours of the fire. It should be noted that experience indicates that while 25 mm of coating may be adequate for concrete strength up to about C60 a coating thickness of 35mm may be required for high strength concrete to avoid explosive spalling. Also, there are some methods as: 1- Spray coating of finished concrete with a substance that slows down the rate of heat transfer from fire. It is the rate of temperature change in the concrete that has been proven to be at least as important a cause of spalling as the ongoing exposure to high temperature itself. 2- The relatively new concept to counter the spalling threat is to provide vents in the concrete to alleviate pore pressure [9]. 2.6- Cracking The processes leading to cracking are generally believed to be similar to those which generate spalling. Thermal expansion and dehydration of the concrete due to heating may lead to the formation of fissures in the concrete rather than, or in addition to, explosive spalling. These fissures may provide pathways for direct heating of the reinforcement bars, possibly bringing about more thermal stress and further cracking. Under certain circum stances the cracks may provide path ways for fire to spread between adjoining compartments Fig (2.7).



The penetration depth of the crack is related to the temperature of the fire, and that generally the cracks extended quite deep into the concrete member. Major damage was confined to the surface near to the fire origin, but the nature of cracking and discoloration of the concrete pointed to the concrete around the reinforcement reaching 700 °C. Cracks which extended more than 30 mm into the depth of the structure were attributed to a short heating/cooling cycle due to the fire being extinguished [11]. The importance of the stress conditions in the concrete should be noted. Compressive loads which may arise from thermal expansion can be very beneficial in compact the material and suppressing the formation of cracks; this results in much smaller degradation of compressive strength and elastic modulus than in specimens bearing reduced loading [12].

Fig.2.7. Thermal cracks in a concrete column subjected to high temperature [13].



2.7- Effect of Fire on Concrete Concrete is arguably the most important building material, playing a part in all building structures. Its virtue is its versatility, i.e. its ability to be molded to take up the shapes required for the various structural forms. It is also very durable and fire resistant when specification and construction procedures are correct. Concrete can be used for all standard buildings both single storey and multistory and for containment and retaining structures and bridges. Under normal conditions, most concrete structures are subjected to a range of temperatures no more severe than that imposed by ambient environmental conditions. However, there are important cases where these structures may be exposed to much higher temperatures (e.g., building fires, chemical and metallurgical industrial applications in which the concrete is in close proximity to furnaces, some nuclear power-related postulated accident conditions, and buildings that subjected to bombing and arson). Concrete’s thermal properties are more complex than for most materials because not only is the concrete a composite material whose constituents have different properties, but its properties also depending on moisture and porosity. Exposure of concrete to elevated temperature affects its mechanical and physical properties. Elements could distort and displace, and, under certain conditions, the concrete surfaces could spall due to the buildup of steam pressure. Because thermally induced dimensional changes, loss of structural integrity, and release of moisture and gases resulting from the migration of free water could adversely affect plant operations and safety, a complete understanding of the behavior of concrete under long-term elevatedtemperature exposure as well as both during and after a thermal excursion resulting from a postulated design-basis accident condition is essential for reliable design evaluations and assessments. Because the properties of concrete change with respect to time and the environment to which it is exposed, an assessment of the effects of concrete aging is also important in performing safety evaluations. [14]



Paulo et al. [3] presented the results of a research program on the behavior of fiber reinforced concrete columns under fire. Polypropylene fibers were included in the concrete in order to enhance the fire behavior of the columns and avoid concrete spalling. Polypropylene fibers under elevated temperatures will create a network of micro-channels for the escape of the water vapor and the use of polypropylene in concrete improves the behavior of the columns in fire. Thus, the use of polypropylene fibers can control the spalling. Shihada. [15] Investigated the effect of Polypropylene fibers on fire resistance of concrete. In order to achieve this, three concrete mixtures are prepared using different percentages of Polypropylene; 0 %, 0.5 % and 1%, by volume. Out of these mixes, cubes (100 × 100 × 100 mm) in dimension were cast and cured for 28 days. The cubes were then burned at 200 C°, 400 C° and 600 C°, for 2, 4 and 6 hours for each of the three temperatures, and tested for compressive strength. Based on the results of the experimental program, it is concluded when Polypropylene fibers are used in certain amounts they improve fire resistance of concrete. Furthermore, it is observed that concrete mixes prepared using 0.5 %, by volume reserve more than 84 % of the initial compressive strength when burned at 600 C° for 6 hours. On the other hand, samples prepared using 0 % Polypropylene reserve about 50 % of their initial strength under the same temperature and duration.

Ali et al. [16] presented the results of a major research executed on high and normal strength concrete elements. The parametric study investigated the effect of restraint degree, loading level and heating rates on the performance of concrete columns subjected to elevated temperatures with a special attention directed to explosive spalling. The study included a useful comparison between the performance of high and normal strength concrete columns in fire. Using polypropylene fibers in the concrete (3 kg/mᶟ) reduced the degree of spalling from 22% to less than 1% .Also the study illustrated a method of preventing explosive spalling using polypropylene fibers in the concrete.



Hodhod et al. [17] investigated the effect of different coating types and thicknesses on the residual load capacities of reinforced concrete loaded columns when subjected to symmetrical temperature rise up to 650 C° for 30 minutes. Seventeen RC column specimens with concrete cover of 1.0 cm and a specimen dimensions (100 mm x150 mm x700 mm) were cast and reinforced with 4Ö6 mm longitudinal reinforcement bars and 7 Ö 3.5 mm stirrups equally distributed along the length. Five different types of coating were used in this study; namely traditional-cement plaster, perlite-cement, vermiculite-cement, LECA-cement and perlite–gypsum. Three different thicknesses of 1.5, 2.5 and 3.5 cm were used. Every column specimen was equipped with eight thermocouples to measure the temperature distributions in side the specimen at mid height. An electric furnace has been used in this work. Every specimen was exposed to the simultaneous effect of the axial service load and temperature of 650 Cº for 30-minutes period. The readings of applied loads and thermocouples were recorded at 5-minuts interval. Testing the columns after cooling to obtain their residual axial load capacity showed that, perlite proves to be the most effective plaster in increasing the loaded column’s resistance to elevated temperature. Specimens coated with vermiculite cement came in the second place. Specimens coated with LECA-cement came in the third place. Finally, specimens coated with traditional-cement came in the last place.

Hertz and Sørensen. [18] discussed a new material test method for determining whether or not an actual concrete may suffer from explosive spalling at a specified moisture level. The method takes into account the effect of stresses from hindered thermal expansion at the fire-exposed surface. Cylinders are used for testing the compressive strength. Consequently, the method is quick, cheap and easy to use in comparison to the alternative of testing full-scale or semi full-scale structures with correct humidity, load and boundary conditions. A number of concretes have been studied using this method, and it is concluded that sufficient quantities of polypropylene fibers of suitable characteristics may prevent spalling of a concrete even when thermal expansion is restrained.



Jau and Huang. [19] investigated the behavior of corner columns under axial loading, biaxial bending and a symmetric fire loading. They concluded that, under a longitudinal stress ratio of 0.10 f`c the residual strength ratios of the columns after fire loading show: (a) The 2 and 4 hours fire loadings resulted in residual strength ratios of 67% and 57%, respectively. Compared with unheated columns a 10% reduction on residual strength results as the duration changes from 2 to 4 hours; (b) Increasing the thickness of concrete cover caused lower residual strength ratios. It was also found that the temperature distribution across the cross-section was not affected by concrete cover thickness. The residual strengths can be used for future evaluation, repair and strengthening.

Chen et al. [20] investigated the compressive and splitting tensile strengths of concretes cured for different periods and exposed to high temperatures. The effects of the duration of curing, maximum temperature and the type of cooling on the strengths of concrete were also investigated. Experimental results indicated that after exposure to high temperatures up to 800 C°, early-age concrete that was cured for a certain period can regain 80% of the compressive strength of the control sample of concrete. The 3-day-cured early-age concrete was observed to recover the most strength. The type of cooling also affected the level of recovery of compressive and splitting tensile strength. For early-age affected concrete, the relative recovered strengths of specimens cooled by sprayed water were higher than those of specimens cooled in air when exposed to temperatures below 800 C°, while the changes for 28-day concrete were the converse. When the maximum temperature exceeded 800 C°, the relative strength values of all specimens cooled by water spray were lower than those of specimens cooled in air.



Chen et al. [2] they studied an experimental research on the effect of fire exposure time on the post-fire behavior of reinforced concrete columns. Nine reinforced concrete columns (45 mm x 30 mm x 300 mm) with two longitudinal reinforcement ratios (1.4% and 2.3%) were exposed to fire for 2 and 4 hours with a constant preload. One month after cooling, the specimens were tested under axial load combined with uniaxial or biaxial bending. The test results showed that the residual load-bearing capacity decreased with increasing fire exposure time. This deterioration in strength which is followed by an increase in fire exposure time can be slowed down by the strength recovery of hot rolled reinforcing bars after cooling. In addition, the reduction in residual stiffness was higher than that in ultimate load; consequently, much attention should be given to the deformation and stress redistribution of the reinforced concrete buildings subject to earthquakes after afire.

Komonen and Penttala. [21] investigated the effect of high temperature on the residual properties of plain and polypropylene fiber reinforced Portland cement paste. Plain Portland cement paste having water/cement ratio of 0.32 was exposed to the temperatures of 20, 50, 75, 100, 120, 150, 200, 300, 400, 440, 520, 600, 700, 800, and 1000 C°. Paste with polypropylene fibers was exposed to the temperature of 20, 120, 150, 200, 300, 440, 520, and 700 C°. Residual compressive and flexural strengths were measured. The gradual heating coarsened the pore structure. At 600 C°, the residual compressive capacity (fc600 C°/fc20 C°) was still over 50% of the original. Strength loss due to the increase of temperature was not linear. Polypropylene fibers produced a finer residual capillary pore structure, decreased compressive strengths, and improved residual flexural strengths at low temperatures. According to the tests, it seems that exposure temperatures from 50 C° to 120 C° can be as dangerous as exposure temperatures 400–500 C° to the residual strength of cement paste produced by a low water cement ratio. Sideris et al. [22 ] studied the mechanical characteristics of Fiber Reinforced Concrete subjected to high temperatures . Fiber reinforced concrete is produced by addition of Polypropylene fibers in the mixtures at dosages of 5 kg/m³. At the age of 120 days, specimens are heated to maximum temperatures of 100, 300, 500 and 700 C°.

Improving Fire Resistance of CH.2: Literature Review Reinforced Concrete Columns Specimens are then allowed to cool in the furnace and tested for compressive strength. Residual strength is reduced almost linearly up to 700 C°. The study recommended the use polypropylene fibers as part of a total spalling protection design method in combination with other materials such as external thermal barriers. Otherwise the overall thickness of the concrete members should be increased to provide a sacrificial layer who will be removed after fire in order to efficiently repair the structure 2.8-Performance of reinforcement in fire The performance of steel during a fire is understood to a higher degree than the performance of concrete, and the strength of steel at a given temperature can be predicted with reasonable confidence. It is generally held that steel reinforcement bars need to be protected from exposure to temperatures in excess of 250-300 Cº. This is due to the fact that steels with low carbon contents are known to exhibit “blue brittleness” between 200 and 300 Cº. Concrete and steel exhibit similar thermal expansion at temperatures up to 400 Cº; however, higher temperatures will result in significant expansion of the steel com pared to the concrete and, if temperatures of the order of 700 Cº are attained, the load-bearing capacity of the steel reinforcement will be reduced to about 20% of its de sign value. Bond failure may be important at high temperatures, as discussed in section Physical and chemical response to fire. Reinforcement can also have a significant effect on the transport of water within a heated concrete member, creating impermeable regions where moisture may become trapped. This forces the water to flow around the bars, increasing the pore pressure in some areas of the concrete and therefore potentially enhancing the risk of spalling. On the other hand, these areas of trapped water also alter the heat flow near the reinforcement, tending to reduce the temperatures of the internal concrete [8]. 2.9- Effect of Fire on Steel Reinforcement Ünlüoðlu et al. [23] investigated the mechanical properties of steel reinforcement bars after the exposure to high temperatures. Plain steel, reinforcing steel bars embedded into mortar and plain mortar specimens were prepared and exposed to 20 C°, 100 C°, 200 C°, 300 C°, 500 C°, 800 C° and 950 C° temperatures for 3 hours individually. A concrete cover of 25 mm provides protection against high temperatures up to 400 C°. The high temperature exposed plain steel and the steel with 25-mm cover has the

Improving Fire Resistance of CH.2: Literature Review Reinforced Concrete Columns same characteristics when the reinforcing steel is exposed to a temperature 250 C° above the exposure temperature of plain steel.

Mamillapalli.[6] investigated the impact of the elevated temperatures on reinforcement steel bars by heating the bars to 100° C°, 300 C°, 600 C°, 900 C°. The heated samples were rapidly cooled by quenching in water and normally by air cooling. The changes in the mechanical properties are studied using universal testing machine (UTM). The impact of elevated temperature above 900 C° on the reinforcement bars was observed. There was significant reduction in ductility when rapidly cooled by quenching. In the same case when cooled in normal atmospheric conditions the impact of temperature on ductility was not high.

Topҫu and IºIkdag. [24] investigated the mechanical properties of reinforcement steel bars after exposure of elevated temperatures. The mortar was prepared with CEM I 42.5N cement and fired clay. The S 420a, B16 mm ribbed steel bars were used to prepare (56 x 56 x 290) mm, (76 x 76 x 310) mm, (96 x 96 x 330) mm and (116 x 116 x 350) mm specimens with concrete covers of (20, 30, 40 and 50) mm against elevated temperatures up to 800 Cº. The reinforcement steel bars were embedded in mortars, and then specimens were exposed to 20, 100, 200, 300, 500 and 800 Cº temperatures for 3 hours, individually. After the cooling process, the specimens were cured for 28 days. The mechanical tests were conducted on cooled specimens, and the ultimate tensile strength, yield strength and elongation of mortar specimens at various temperatures were also determined at the end of the experiments. It is observed that a cover of 20, 30, 40 and 50 mm thickness provides a protection to rebar in exposure of high temperatures. The cover reduces the losses in yield and tensile strengths of rebar and ensures 15% higher strength compared to rebar without cover. For temperatures up to 300 C°, rebar with cover had the same yield and tensile strengths with that of the rebar without cover in exposure to elevated temperatures. However, when the temperature increases up to 800 C°, the rebar without cover loses an average of 80% of its strength capacities compared with a 20% loss for the rebars with cover. It was observed that 20, 30, 40 and 50 mm cover thickness was not sufficient to protect the mechanical properties of rebars in exposure of temperatures above 500 C°.



2.10- Effect of Fire on Fiber Reinforced Polymers (FRP) columns Kodur et al. [25] presented the results of a full-scale fire resistance experiments on three insulated FRP-strengthened reinforced concrete reinforced concrete columns. A comparison was made between the fire performances of FRP-strengthened RC columns and conventional unstrengthened reinforced concrete columns. Data obtained during the experiments is used to show that the fire behavior of FRPwrapped concrete columns incorporating appropriate fire protection systems was as good as that of unstrengthened RC columns. Thus, satisfactory fire resistance ratings for FRP-wrapped concrete columns could be obtained by properly incorporating appropriate fire protection measures into the overall FRP-strengthened structural systems. Fire endurance criteria and preliminary design recommendations for fire safety of FRP-strengthened RC columns were also briefly discussed. The performance of protected FRP-strengthened square RC columns at high temperatures can be similar to, or better than, that of conventional RC columns. Chowdhurya et al. [26] demonstrated that fiber-reinforced polymers (FRPs) could be used efficiently and safely in strengthening and rehabilitation of reinforced concrete structures. In there study they were presented the recent results of an experimental study of the fire performance of FRP-wrapped reinforced concrete circular columns. The results of fire tests on two columns were presented, one of which was tested without supplemental fire protection, and one of which was protected by a supplemental fire protection system applied to the exterior of the FRP-strengthening system. The primary objective of these tests was to compare fire behavior of the two FRP-wrapped columns and to investigate the effectiveness of the supplemental insulation systems. The thermal and structural behaviors of the two columns were discussed. The results show that, although FRP systems are sensitive to high temperatures, satisfactory fire endurance ratings could be achieved for reinforced concrete columns that were strengthened with FRP systems by providing adequate supplemental fire protection. In particular, the insulated FRP-strengthened column was able to resist elevated temperatures during the fire tests for at least 90 minutes longer than the equivalent uninsulated FRP-strengthened column.

EXPIRIMINTAL PROGRAM

CH.3: Experimental Program


Experimental Program 3.1- Introduction The experimental program consists of fire endurance tests. These tests will examine the effect of high temperatures on reinforced concrete columns and testing their compressive strength. The program consist of 3 types of small scale reinforced concrete columns (100 mm x 100 mm x 300 mm) ,one type of specimens is free from polypropylene and the other two types contain 0.5 kg/m³ and 0.75 kg/m³ of polypropylene fibers , samples of this group have the same concrete cover (2.0 cm). The samples will be tested at (ambient temperature, 400 C°, 600 C° and 800 C°) at (0, 2, 4 and 6 hours) exposure. The other groups have a concrete cover of 3.0 cm and will be tested at 600 C° for 6 hours to determine the behavior of RC column with deferent concrete covers at high temperatures. In addition, the behavior of steel reinforcement under high temperature 800 C°, with different concrete covers (0 cm, 2 cm, and 3 cm) is tested.

3.2-Materials and Their Quality Tests: It is very important to know the properties and characteristics of constituent materials of concrete, as we know, concrete is a composite material made up of several different materials such as aggregate, sand, water, cement and admixture. These materials have properties and different characteristics such as "Unit weight, Specific gravity, size gradation

and

water

content

...

etc".

.

We must therefore work out necessary tests on these components, and that to know the unique characteristics and their impact on the strength of concrete. The necessary tests are conducted in the laboratory of materials and soil in the Islamic University and in accordance with ASTM "American Society for Testing and Materials".



3.2.1-Aggregate Quality Tests. 3.2.1.1- Unit Weight of Aggregate: Unit weight (

 ) can be defined as the weight of a given volume of graded aggregate.

It is thus a measurement and is also known as bulk density, but this alternative term is similar to bulk specific gravity, which is quite a different quantity, and perhaps is not a good choice. The unit weight effectively measures the volume that the graded aggregate will occupy in concrete and includes both the solid aggregate particles and the voids between them. The unit weight is simply measured by filling a container of known volume and weighting it based on ASTM C 566 [27]. However, the degree of compaction will change the amount of void space and hence the value of the unit weight. Table.3.1: Capacity of Measures

Max.Aggregate

Capacity of

Size (mm)

Measure (Liter)

12.5

2.8

25

9.3

37.5

14

75

28

The sample shall be in oven dry condition and the capacity of measures are shown in Table (3.1), the molds of unit weight test for coarse and fine aggregate are shown in Fig (3.1).



Fig.3.1: Mold of Unit Weight test [IUG-Lab]

The unite weights of coarse and dine aggregate are shown in Table (3.2).

Table 3.2: Unit weight test results

Coarse aggregate

Fine aggregate “sand”

Dry unit weight

1444.00 kg/ m3

SSD unit weight

1460.4 kg / m3

Dry unit weight

1440.00 kg/ m3

SSD unit weight

1445.40 kg / m3

3.2.1.2- Specific Gravity of Aggregate: The density of the aggregates is required in mix proportioning to establish weight volume relationships. The density is expressed as the specific gravity. Specific gravity is defined as the ratio of the weight of a unit volume of aggregate to the weight of an equal volume of water. Specific gravity expresses the density of the solid fraction of the aggregate in concrete mixes as well as to determine the volume of pores in the mix. Specific Gravity (S.G) = (density of solid) / (density of water)

Improving Fire Resistance of CH.3: Experimental Program Reinforced Concrete Columns Since densities are determined by displacement in water, specific gravities are naturally and easily calculated and can be used with any system of units. The specific gravity tested for coarse and fine aggregate are shown in Fig (3.2). The specific gravity of aggregate is to determine the volume of aggregates in a concrete mix as well as to determine the volume of pores in the mix based on ASTM C127 and ASTM C128 [28,29].

Fig.3.2: Specific Gravity test equipments [IUG-Lab].

The specific gravity of coarse and fine aggregate is shown in Table (3.3).

Table.3.3: Specific gravity of aggregate

Coarse aggregate

Fine aggregate “sand”

Bulk (Gs) SSD

2.63

Bulk (Gs) dry

2.55

Apparent Bulk (Gs)

2.632

Bulk (Gs) SSD

2.16

Bulk (Gs) dry

2.15

Apparent Bulk (Gs)

2.17

CH.3: Experimental Program 3.2.1.3- Moisture content of Aggregate:


Since aggregates are porous (to some extent), they can absorb moisture. However, this is a concern for Portland cement concrete because aggregate is generally not dry and therefore the aggregate moisture content will affect the water content and thus the water-cement ratio also of the produced Portland cement concrete and the water content also affects aggregate proportioning (because it contributes to aggregate weight), based on ASTM C127 and ASTM C128 [28,29]. The moisture content values of coarse and fine aggregate are shown in Table (3.4). Table.3.4: Moisture content values

Coarse aggregate

2.8 %

Fine aggregate “Sand”

0.994 %

3.2.1.4-Resistance to Degradation by Abrasion & Impaction test. The Los Angeles test is a measure of aggregate resulting from a combination of actions including abrasion, impact, and grinding in a rotating steel drum containing a specified number of steel spheres. The Los Angeles test has a wide use as an indicator of aggregate quality, shown in Fig (3.3), based on ASTM C131 [30]. The charge shall consist of steel spheres averaging approximately 46.8 mm in diameter and each weighing between 390 and 445 g, details are shown in Table (3.5). Abrasion Loss shall be not more than 50%.



Fig.3.3: Los Angeles test (Abrasion) Machine. [30]

The charge, depending on the grading of the test sample, shall be as follows: Table.3.5: Number of steel spheres for each grade of the test sample

Grading

Number of Spheres

Weight of Charge, g

A

12

5000 +/- 25

B

11

4584 +/- 25

C

8

3330 +/- 20

D

6

2500 +/- 15

A

12

5000 +/- 25

% Abrasion Loss = ( Woriginal –Wfinal ) / Woriginal * 100 Original weight = 5000 gm Final weight = 3977.8 gm Abrasion = (5000 - 3977.8 / 5000) * 100 = 20.44 % < 50 %

OK.



3.2.1.5-Sieve Analysis of Aggregate: The size of aggregate particles differs from aggregate to another, and for the same aggregate the size is different. So in this test we will determine the particle size distribution of fine and coarse aggregate by sieving. This method is used to determine the compliance of the aggregate gradation with specific requirements, ASTM C136 [31]. Table (3.6) and Fig (3.4) illustrate the results of sieve analysis test for coarse and fine aggregate. Table 3.6 : sieve analysis of aggregate SIEVE SIZE

Wt.

% Retained

% Passing

NO.

size ( mm )

Retained,(gm)

1.5"

37.5

0

0.00

100.00

1"

25

350

4.71

95.29

3/4"

19

2092.5

28.18

71.82

1/2"

12.5

3122.5

42.05

57.95

3/8"

9.5

3727.5

50.20

49.80

#4

4.75

4797.5

64.61

35.39

# 10

2

192.3

27.32

27.55

# 16

1.18

293.5

41.69

22.11

# 30

0.6

390.1

55.41

16.90

# 40

0.425

456.9

64.90

13.31

# 50

0.3

492.9

70.01

11.37

# 100

0.125

562.4

79.89

7.63

# 200

0.075

638.5

90.70

3.53

-

ASTM C136 maximum and minimum limits for aggregate size distribution . Sieve Passing

1.5" 100%

1" 75 - 100

3/4" 60 - 90

#4 30 - 65

# 200 5.0 - 10



Sieve Analysis Curve 100 90

Test Sample ASTM Min Limit

80

ASTM Max Limit

% Passing

70 60 50 40 30 20 10 0 0.01

0.1

1

10

100

Dia. (mm)

Fig 3.4 : Sieve Analysis of Aggregate

3.2.1.6-Cement:

The cement type which was used for this research is Portland Cement “EN 197-1 CEM I 42,5 R & EN 197-1 CEM I 42,5 N " produced in Turkey , and the properties of cement met the requirement of ASTM C 150 specifications. Table (3.7) summarizes the properties of this cement [32]. Table.3.7: Ordinary Portland cement properties "Test Results"

No. 1-

Description

Sample Results

Normal Consistency Setting Time

2-

1-Initial Setting (min) 2-Finial Setting (min)

Specification ASTM C-150

27 % 100

>45

165