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Strength_ durability_ stiffness and microstructure of metakaolin and fly

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					CITY UNIVERSITY OF HONG KONG
          香港城市大學


The Proposed Hybrid Flaws Model of
Interfacial Transition Zone in HPC at
 Normal and Elevated Temperatures
高性能混凝土在正常和高溫下其界面過度
    區之混接裂紋模型提議


                 Submitted to
    Department of Building and Construction
                     建築系
    in Partial Fulfillment of the Requirements
      for the Degree of Doctor of Philosophy
                 哲學博士學位



                       by



                  Nadeem Abid




                 June 2005
                二零零五年六月
                                                                                     Abstract


                                      ABSTRACT


This thesis examines the deterioration mechanism in high performance concrete (HPC)
subjected to elevated temperatures. This has mainly been illustrated by proposing a model of
interfacial transition zone (ITZ) in HPC applicable to both normal and elevated temperatures.
HPC mixes incorporating metakaolin (MK), pulverised fuel ash (PFA) and ordinary Portland
cement (OPC) were used in the investigation. Metakaolin is a relatively new pozzolanic
material for concrete and is being increasingly investigated by many researchers all over the
world. However, it is realized that there is a lack of published literature on its elevated
temperature performance. Moreover, there is hardly any published study available on the
microstructure of HPC describing the character of ITZ and hardened cement paste (hcp)
under elevated temperatures. The studies in this thesis try to address these issues by using
Scanning Electron Microscope (SEM) and Image Analysis (IA) in addition to mechanical,
permeability and stiffness properties investigations of HPC under normal and elevated
temperatures. Variables of the test program include partial replacement of cement with MK
from 5% to 20%, PFA from 20% to 60%, temperatures from 27°C to 800°C and two types of
cooling methods.

The microstructure of ITZ in HPC is investigated by the analysis of SEM images of fractured
concrete specimens. These images show the presence of both ‘flaw’ and compact areas in ITZ
of concrete at normal and elevated temperatures. The flaws are categorized into Texture and
Orientation (TO) flaws and Local Band (LB) flaws. TO flaws are associated either to the
surface roughness of aggregate (texture flaws) or to the close-gap between two or more
aggregate particles (orientation flaws). LB flaws are associated with local insufficient
compaction of concrete and they can occur at areas in ITZ where the conditions for TO flaws
are not present. The distribution of the number of TO and LB flaws and the variations in their
porosity at various temperatures enabled the author to propose ‘The Hybrid Flaws Model of
ITZ in HPC at Normal and Elevated Temperatures’. According to the proposed model, the
character of ITZ in HPC changes gradually from a discrete or discontinuous flaw zone at
normal temperature to a continuous flaw zone at elevated temperatures. For normal to low
range elevated temperatures (27°C-200°C), there are more number of TO flaws than LB flaws.
There are compact areas between flaws which make the flaws discrete. For moderately
elevated temperatures (200°C-400°C), there is no specific increase in the number of TO flaws
but there is an increase in the number of LB flaws and LB flaws outnumber the TO flaws.
The increased number of LB flaws is due to the effect of elevated temperature. The porosity
of both types of flaws in this temperature range is higher than the porosity at temperature
range of 27°C-200°C. Some of the new LB flaws join with the original TO and LB flaws and
the discreteness of flaws reduces. Thus the ITZ at this temperature range shows semi-discrete



                                              i
                                                                                      Abstract


or semi-continuous flaws. At the high range of elevated temperatures (400°C-800°C), LB
flaws appreciably outnumber TO flaws. In this range of temperatures, the porosity of flaws
further increases and the microstructure of ITZ shows almost continuous flaws. This is a
highly disintegrated state of concrete and is manifested in other properties of concrete at
elevated temperatures.

Mechanical properties tests on compressive strength show that HPC concrete is slightly
weaker than the corresponding mortar at temperatures of 400°C or above. This is because of
the property losses in ITZ of HPC due to elevated temperatures. Chloride permeability results
shows that HPC is less permeable than corresponding mortars at normal temperatures.
However, relative permeability of HPC with respect to mortar increases with the increase of
temperature and at temperatures of 600°C or above HPC is more permeable than mortar. This
shows the changing contribution of ITZ of HPC to allow more charge to pass through ITZ at
elevated temperatures than at normal temperature.

SEM study using image analysis of the hcp of mortar specimens shows that there is an
increase in the pore area fraction with a simultaneous decrease in calcium hydroxide (CH)
and hydrated paste (HP) area fractions of hcp with the increase of temperature. The strength
loss and high permeability at elevated temperatures can be ascribed to the changes in HP and
pore area of hcp with temperature.

A relatively new method ‘stiffness damage test’ (SDT) is performed to assess the damage to
HPC due to elevated temperatures by measuring Damage Index (DI), Chord Modulus (Ec),
Unloading Stiffness (Eu), Non Linearity Index (NLI) and Plastic Strain (PS). These five
parameters are sensitive to temperature change and gave a good idea of the damage
mechanism in various HPC mixes tested. Variation in NLI with temperature in SDT shows
that the ductility of HPC increases with the increase of temperature but this occurs
simultaneously with losses in other properties of HPC.

The results from mechanical properties studies for the effect of cooling method show that the
additional loss in compressive and tensile strengths due to quick cooling is high in the range
of 300°C to 500°C. This is because the effect of thermal shock in this range of temperatures is
higher than at temperatures below 300°C or above 500°C. HPC with MK shows better
performance than other type of concrete mixes at normal temperature. At elevated
temperatures above 400°C, all HPC mixes show appreciably more loss in properties relative
to the values at normal temperature. PFA mixes exhibit better performance than other types of
mixes at elevated temperatures. HPC specimens do not suffer appreciable loss in compressive
strength up to 400°C under slow cooling. However, there is sharp loss in other properties like
tensile strength, water sorptivity, chloride permeability and stiffness of HPC at temperatures
above 200°C.


                                              ii
                                                                         Table of contents


                                   TABLE OF CONTENTS


Abstract                                                                                i
Acknowledgments                                                                         iii
List of Abbreviations                                                                   iv
Table of contents                                                                       v


Chapter 1: Introduction                                                               1-1

1.1        Background                                                                 1-1

1.2        Research objectives                                                        1-5

1.3        Research significance                                                      1-6

1.4        Scope of study                                                             1-6

1.5        Outline of the thesis                                                      1-7

Chapter 2: Literature review                                                          2-1

2.1        Concrete under elevated temperatures                                       2-1

2.2        Effect of elevated temperatures on compressive strength of concrete        2-3

2.3        Effect of elevated temperatures on tensile strength of concrete            2-7

2.4        Effect of elevated temperatures on elastic properties of concretes         2-9

2.5        Effect of cooling method on mechanical properties of concrete              2-12

2.6        Properties of PFA and MK concrete                                          2-13

2.7        Elevated temperature performance of PFA concrete                           2-15
  2.7.1 Mechanical properties performance                                             2-15
  2.7.2 Durability performance                                                        2-18

2.8        Elevated temperature performance of MK concrete                            2-19
  2.8.1 Mechanical properties performance                                             2-19
  2.8.2 Durability performance                                                        2-21

2.9        Interfacial transition zone in concrete                                    2-21
  2.9.1 Characterisation of ITZ in concrete                                           2-21
  2.9.2 USE of SEM for characterization of ITZ and hcp                                2-23



                                             v
                                                                     Table of contents



Chapter 3: Experimental details                                                   3-1

3.1      Details of concrete mixes                                                3-1
  3.1.1 Details of Materials                                                      3-1
  3.1.2 Proportioning of mixes                                                    3-5
  3.1.3 Mixing, casting and curing details                                        3-9

3.2      Mixes allocation and specimen details                                    3-10

3.3      Heating and cooling details and mechanical properties testing            3-13
  3.3.1 Heating details                                                           3-13
  3.3.2 Cooling details                                                           3-14
  3.3.3 Compressive and tensile strength tests                                    3-15

3.4      Details of SEM test                                                      3-15
  3.4.1 Specimen preparation                                                      3-15
  3.4.2 SEM specimen heating and cooling                                          3-19
  3.4.3 Scanning electron microscopy                                              3-19
  3.4.4 Image analysis methodology                                                3-20

3.5      Description of chloride permeability test                                3-21

3.6      Description of water sorptivity test                                     3-21

3.7      Details of Stiffness damage testing (SDT)                                3-22
  3.7.1 Equipment details                                                         3-23
  3.7.2 Procedure of stiffness damage test                                        3-25
  3.7.3 Calculation of SDT parameters                                             3-26

Chapter 4: Mechanical properties of metakaolin and PFA concrete under             4-1
           elevated temperatures-results and discussions

4.1      Compressive strength development of MK and PFA concrete                  4-1
  4.1.1 Concrete groups based on 28 days compressive strength                     4-1
  4.1.2 Comparison compressive strengths of various concrete mixes                4-2
  4.1.3 Comparison of compressive strength with respect to control concrete       4-5

4.2      Mass loss in metakaolin and PFA concrete under elevated temperatures 4-9
  4.2.1 Results of mass loss in concrete                                          4-9
  4.2.2 Variation of mass loss in concrete with temperature                       4-11



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                                                                         Table of contents


  4.2.3 Discussion of mass loss in concrete under elevated temperatures               4-14

4.3       Compressive strength of metakaolin and PFA concrete under elevated          4-16
          temperatures
  4.3.1 Results of compressive strength tests on concrete under slow cooling          4-16
  4.3.2 Variation of compressive strength with temperature under slow                 4-18
        cooling
  4.3.3 Results of compressive strength tests on concrete under quick cooling         4-23
  4.3.4 Variation of compressive strength with temperature under quick                4-26
        cooling
  4.3.5 Comparison of slow and quick cooling for compressive strength                 4-30

4.4       Compressive Strength of Mortar Mixes under Elevated Temperatures            4-35
  4.4.1 Results on compressive strength tests on mortar mixes                         4-35
  4.4.2 Comparison of compressive strengths of concrete and mortar                    4-37

4.5       Tensile strength of metakaolin and PFA concrete under elevated              4-40
          temperatures
  4.5.1 Results of split cube tests on concrete under slow cooling                    4-40
  4.5.2 Variation of tensile strength with temperature under slow cooling             4-42
  4.5.3 Results of split cube tests on concrete under quick cooling                   4-46
  4.5.4 Variation of tensile strength with temperature under quick cooling            4-48
  4.5.5 Comparison of slow and quick cooling for tensile strength                     4-52

4.6       Conclusions of mechanical properties studies                                4-57

Chapter 5: SEM and image analysis for microstructure of HPC under                     5-1
            elevated temperatures - results and discussions

5.1       SEM images of hcp at elevated temperatures                                  5-1
  5.1.1 hcp of OPC (PC) mortar                                                        5-2
  5.1.2   hcp of metakaolin (MK20) mortar                                             5-3
  5.1.3   hcp of PFA (FA20) mortar                                                    5-3
  5.1.4   hcp of ternary (F20M20) mortar                                              5-7

5.2       Image Analysis of hcp for all mixes at elevated temperatures                5-9
  5.2.1   Pore (P) area fraction analysis of hcp                                      5-11
  5.2.2   Hydrated paste (HP) area fraction analysis of hcp                           5-13
  5.2.3   Calcium hydroxide (CH) area fraction analysis of hcp                        5-15
  5.2.4   Unhydrated (UH)cement area fraction analysis of hcp                         5-17



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                                                                         Table of contents


  5.2.5 Relationship among hcp components at elevated temperatures                    5-19

5.3       SEM image analysis framework for ITZ                                        5-21
  5.3.1 Classification of ITZ flaws                                                   5-21
  5.3.2   Mechanism of ITZ flaws formation                                            5-23
  5.3.3 Presentation Scheme of SEM images for ITZ                                     5-25

5.4       SEM images of ITZ for OPC concrete                                          5-27
  5.4.1 Analysis of ITZ for OPC concrete at 27°C                                      5-27
  5.4.2 Analysis of ITZ for OPC concrete at 200°C                                     5-33
  5.4.3 Analysis of ITZ for OPC concrete at 400°C                                     5-40
  5.4.4 Analysis of ITZ for OPC concrete at 600°C                                     5-46
  5.4.5 Analysis of ITZ for OPC concrete at 800°C                                     5-52

5.5       SEM images of ITZ for metakaolin concrete                                   5-58
  5.5.1   Analysis of ITZ for metakaolin concrete at 27°C                             5-58
  5.5.2   Analysis of ITZ for metakaolin concrete at 200°C                            5-64
  5.5.3   Analysis of ITZ for metakaolin concrete at 400°C                            5-70
  5.5.4   Analysis of ITZ for metakaolin concrete at 600°C                            5-75
  5.5.5   Analysis of ITZ for metakaolin concrete at 800°C                            5-81

5.6       SEM images of ITZ for PFA concrete                                          5-87
  5.6.1 Analysis of ITZ for PFA concrete at 27°C                                      5-87
  5.6.2 Analysis of ITZ for PFA concrete at 200°C                                     5-93
  5.6.3 Analysis of ITZ for PFA concrete at 400°C                                     5-99
  5.6.4 Analysis of ITZ for PFA concrete at 600°C                                     5-105
  5.6.5 Analysis of ITZ for PFA concrete at 800°C                                     5-111

5.7       Discussion of ITZ image analysis for concrete mixes                         5-117
  5.7.1   Variation of the occurrence of texture and orientation flaws                5-118
  5.7.2   Variation in porosity of texture and orientation flaws                      5-119
  5.7.3   Variation of the occurrence of ‘local-band’ flaws                           5-120
  5.7.4   Variation in porosity of ‘local-band’ flaws                                 5-121
  5.7.5   Comparison of the occurrence of TO and LB flaws                             5-122
  5.7.6   The proposed hybrid flaws model of ITZ                                      5-125
  5.7.7   Comparison of the porosity of TO and LB flaws                               5-126
  5.7.8   Comparison of concrete mixes for occurrence and porosity of flaws           5-127



                                          viii
                                                                        Table of contents



5.8      Conclusions of SEM and image analysis studies                               5-128
  5.8.1 Conclusions of SEM study on hcp                                              5-128
  5.8.2 Conclusions of SEM study on ITZ                                              5-129

Chapter 6: Chloride permeability of metakaolin and PFA concrete under                6-1
           elevated temperatures–results and discussions

6.1      Results of chloride permeability tests                                      6-1

6.2      Variation of charge passed through metakaolin concrete with                 6-9
         temperature

6.3      Variation of charge passed through PFA concrete with temperature            6-12

6.4      Comparison of charge passed through metakaolin and PFA concrete             6-15

6.5      Variation of charge passed through metakaolin and PFA mortar with           6-17
         temperature

6.6      Comparison of charge passed through concrete and mortar                     6-20

6.7      Conclusions of chloride permeability tests                                  6-24

Chapter 7: Water sorptivity of metakaolin and PFA concrete under elevated
           temperatures—results and discussions                                      7-1

7.1      Results of water sorptivity tests                                           7-1

7.2      Sorptivity variation in metakaolin concrete with temperature                7-7

7.3      Sorptivity variation in PFA concrete with temperature                       7-10

7.4      Comparison of sorptivity of metakaolin and PFA Concrete                     7-13

7.5      Conclusions of water sorptivity tests                                       7-15

Chapter 8: Stiffness of metakaolin and PFA concrete under elevated                   8-1
           temperatures–results and discussions

8.1      Results of stiffness damage test                                            8-1

8.2      Variation in chord modulus ‘Ec’ with temperature                            8-6

8.3      Variation in Unloading Stiffness ‘Eu’ with temperature                      8-8

8.4      Variation in plastic strain ‘PS’ with temperature                           8-10

8.5      Variation in damage index ‘DI’ with temperature                             8-12


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                                                                   Table of contents



8.6      Variation in non-linearity index ‘NLI’ with temperature                8-14

8.7      Conclusions of stiffness damage test                                   8-15

Chapter 9: Comparative review of the studies performed                          9-1

9.1      Overview of the comparison of studies                                  9-1

9.2      Comparison of study findings                                           9-2
  9.2.1 SEM and mechanical properties tests findings                            9-2
  9.2.2 SEM and chloride permeability test findings                             9-4
  9.2.3 SEM and water sorptivity test findings                                  9-5
  9.2.4 SEM and stiffness damage test findings                                  9-6

9.3      Analytical deductions from comparative studies                         9-6
  9.3.1 Existence of ITZ of HPC at normal temperature                           9-6
  9.3.2 Character of ITZ of HPC at normal temperature                           9-8
  9.3.3 Character of ITZ of HPC up to 200°C                                     9-9
  9.3.4 Character of ITZ of HPC from 200°C to 400°C                             9-10
  9.3.5 Character of ITZ of HPC for more than 400°C                             9-11

Chapter 10: Conclusions and recommendations                                     10-1

10.1      Main conclusions                                                      10-1
  10.1.1 Character of ITZ for HPC at normal and elevated temperatures           10-1
  10.1.2 Effect of quick cooling over slow cooling                              10-3
  10.1.3 Performance of MK and PFA concrete                                     10-4

10.2      Conclusions of individual studies                                     10-4
  10.2.1 Conclusions of mechanical properties studies                           10-4
  10.2.2 Conclusions of SEM studies                                             10-7
  10.2.3 Conclusions of chloride permeability tests                             10-9
  10.2.4 Conclusions of water sorptivity tests                                  10-11
  10.2.5 Conclusions of stiffness damage tests                                  10-13

10.3      Recommendations for further studies                                   10-14

References                                                                      R-1




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