Corrosion of carbon steel embedded in rammed earth

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SCUOLA DI ARCHITETTURA URBANISTICA E INGEGNERIA DELLE COSTRUZIONI CORSO DI LAUREA MAGISTRALE IN INGEGNERIA DEI SISTEMI EDILIZI

DIPARTIMENTO DI CHIMICA, MATERIALI E INGEGNERIA CHIMICA “GIULIO NATTA”

CORROSION OF CARBON STEEL EMBEDDED IN RAMMED EARTH

Relatore: Prof. Ing. Luca BERTOLINI

Correlatori: Prof.ssa Ing. Maddalena CARSANA, Prof.ssa Ing. Daniela CIANCIO

Autori:

Emanuele Raffaele Lucianò matr. 820395

Grazia Rita Orfeo matr. 824085

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Ai miei genitori e a mio fratello pilastri insostituibili della mia vita. A mia nonna, e a mio nonno che l’aspetta ancora. “Se la forma scompare la sua radice è eterna” Grazia

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Abstract (English)

Rammed earth, used as construction material is an alternative to traditional construction materials that presents several advantages including the low consume of energy, modest economic impact and the low environmental impact. This is particularly relevant to the construction industry where materials such as steel, concrete and bricks require a significant amount of energy to be mined, manufactured and utilised. The rammed earth can be stabilised with cementitious materials which make it alkaline. As in reinforced concrete structures, it is possible reinforce the rammed earth with steel bars. In literature there are not enough data concerning corrosion of steel embedded in cement stabilised in rammed earth. The purpose of this thesis is the characterization of corrosion behaviour of carbon steel bars in rammed earth mixtures (crushed limestone, recycled concrete aggregate, a natural soil mix and an artificial soil mix of crushed limestone and clay) stabilised by cement or a mix of fly ash and carbide lime. The thesis based on an experimental program which included unconfined compressive strength tests, absorption tests and pH tests, realized on different mixes. Concerning the durability of these mixtures, resistance to carbonation was evaluated and corrosion tests were executed on embedded carbon steel bars.

Results showed that related to steel protection the best solution between rammed earth batches were those where cement as stabiliser was used. In fact, it contributed to improve mechanical properties of rammed earth and slow down corrosion process, differently from fly ash and carbide lime.

Keywords: corrosion; reinforced concrete; rammed earth; electrochemical tests; corrosion rate; corrosion potential.

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Abstract (Italian)

Il “Rammed Earth” (terra battuta) usato come materiale da costruzione, risulta essere un’alternativa ai materiali tradizionali presentando diversi vantaggi, tra cui la bassa energia per essere prodotto, l’economicità e il basso impatto ambientale. Ciò è particolarmente rilevante per il settore delle costruzioni, dove materiali come l’acciaio, il calcestruzzo e i mattoni richiedono una notevole quantità di energia per essere estratti, fabbricati e utilizzati. La terra battuta può essere stabilizzata con materiali cementizi che la rendono alcalina. Come per le strutture in calcestruzzo armato è possibile rinforzare la terra battuta stabilizzata con barre di acciaio. In letteratura però si hanno pochi dati relativi alla corrosione delle armature a contatto con la terra battuta stabilizzata con cemento.

L’obiettivo di questo lavoro di tesi è la caratterizzazione del comportamento alla corrosione delle armature al carbonio in miscele confezionate (calcare frantumato, aggregati di calcestruzzo riciclati, una miscela di terreno naturale e una miscela artificiale di calcare frantumato e argilla) e stabilizzate sia con il cemento sia con un composto di ceneri volanti e carburo di calce. La tesi ha previsto un lavoro sperimentale che si è articolato in prove di caratterizzazione allo stato indurito (resistenza meccanica a compressione, assorbimento alla pressione atmosferica e misura del pH) effettuate su diverse miscele. Per quanto riguarda la durabilità di queste miscele, è stato poi valutato il grado di avanzamento della carbonatazione e si sono svolte dalle prove di corrosione su armature di acciaio al carbonio a contatto con esse.

I risultati hanno mostrato che il cemento, usato come stabilizzante, è stata la migliore soluzione in termini di protezione delle armature tra le diverse miscele. Infatti questo ha contribuito a migliorare le proprietà meccaniche della terra battuta e a rallentare il processo di corrosione, a differenza delle ceneri volanti e del carburo di calce.

Parole chiave: corrosione; calcestruzzo armato; terra battuta; misure elettrochimiche; velocità di corrosione; potenziale di corrosione.

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Acknowledgements

We would like to first thank who has allowed us to take part in this research project and allowed us to be able to do this experience in Australia.

We would like to express our sincere gratitude to our supervisors Luca Bertolini, Maddalena Carsana for their guidance and good advice during our work, and for having reviewed it.

We would like to thank also Daniela Ciancio, Chris Beckett and Alexandra Meek for the guidance and their support and all the members of the Concrete laboratory who helped us in this work during our experience in Australia.

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2.2.2 Absorption Test ...46 2.2.2.1 Samples ...46 2.2.2.2 Exposure Conditions ...48 2.2.3 Carbonation Test ...50 2.2.3.1 Samples ...50 2.2.3.2 Exposure Conditions ...50 2.2.4 Corrosion Test ...53 2.2.4.1 Samples ...53 2.2.4.2 Exposure Conditions ...56 2.2.4.3 Corrosion Potential ...57 2.2.4.4 Corrosion Rate ...58 3 Results ...62

3.1 Unconfined Compressive Strength Test ... 62

3.2 Dry density ... 63

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3.3.1 Porosity ...65

3.4 Carbonation Test ... 65

3.4.1 Carbonation Depth ...66

3.5 Preliminary Corrosion Test in Milan ... 78

3.5.1 Corrosion Potential - Fe vs SCE ...79

3.5.2 Corrosion Rate ...80

3.5.3 Potential of active titanium electrode - Ti vs SCE...82

3.6 Corrosion Tests in Perth ... 83

3.6.1 Corrosion potential per batch ...84

4 Discussion ...94

4.1 Strength and Porosity ... 94

4.2 pH ... 97

4.2.1 pH of Soil ...97

4.2.2 pH of Batches ...98

4.3 Carbonation Resistance ... 99

4.4 Steel Corrosion ...104

4.4.1 Correlation icorr vs Ecorr ... 104

4.4.2 Effect of Moisture ... 106

4.4.3 Effect of Type of Material ... 108

Conclusion ... 115

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List of Figures

Figure 1.1. Great Wall, China………....……6

Figure 1.2. Alambra, Spain……….……….…...6

Figure 1.3. Particle diameter of earth. …………..……….7

Figure 1.4. Feldman-Sereda model of C-S-H [4,6] ………....…9

Figure 1.5. Rammed Earth construction process [3] ...……….……….…12

Figure 1.6. Outline of reinforced lintels [7] ………..……13

Figure 1.7. Roof fixings [3] ……….…14

Figure 1.8. Concrete wall………...….…15

Figure 1.9. Rammed Earth wall [24] ………..………..………....15

Figure 1.10. Electrochemical mechanism of corrosion [4,6] ………....17

Figure 1.11. Schematic anodic polarization curve for a metal that shows active-passive behaviour [4,6]………19

Figure 1.12. Pourbaix Diagram of iron [4,6]………...…20

Figure 1.13. Example of corrosion by carbonation [26] ….………...…………22

Figure 1.14. Tuutti’s Model to predict initiation and propagation period in a reinforced concrete [13,14]………..…23

Figure 1.15 Schematic representation of the rate of carbonation of concrete as a function of the relative humidity of the environment, under equilibrium conditions [4,6]………..……24

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Figure 2.1. Sieve Shaker……….…29

Figure 2.2. pH meter………..…….…30

Figure 2.3. Different fractions of the aggregate used for casting rammed earth samples in Milan laboratory [10]………31

Figure 2.4. Crushed Limestone………...………...31

Figure 2.5. Recycled Concrete Aggregate ……….……...32

Figure 2.6. Dumpier Peninsula soil………..….………33

Figure 2.7. Crushed Limestone and Kaolin……….……….………34

Figure 2.8. Cement……….….……36

Figure 2.9. Fly Ash………...……….….……37

Figure 2.10. Carbide Lime……….……….………37

Figure 2.11. Batch used for rammed earth samples before ramming [10] ………….……..……...38

Figure 2.12. Cylindrical Sample………...………..…42

Figure 2.13. Cylindrical formwork for UCS test ………..………….…....44

Figure 2.14. Bosh Jackhammer used for rammed of test specimens……….………..44

Figure 2.15. UCS test setup ...………..….….47

Figure 2.16. Modified Proctor test used to evaluate the optimum water content [17]…………...48

Figure 2.17. Smooth curve that explain the optimum water content results………..49

Figure 2.18. Cubic sample ………...….………...49

Figure 2.19. Cubic formwork for absorption, carbonation and corrosion test…………..………..51

Figure 2.20. Absorption test………...53

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Figure 2.22. Reinforced specimens made in different formworks in Milan [10] ………..…....57

Figure 2.23. Reinforced sample...……….….57

Figure 2.24. Titanium electrode preparation………..….58

Figure 2.25. Steel Bar preparation………..…….….58

Figure 2.26. Steel bar and Titanium electrode………..….….59

Figure 2.27. Formwork for casting reinforced specimens……….……….59

Figure 2.28. Corrosion test cycle in Pert…..………...….….61

Figure 2.29. Corrosion test cycle in Milan ...………....………...………….….61

Figure 2.30. Schematic representation and photo of the measurement of potential of steel reinforcement……….…62

Figure 2.31. Potentiostat to measure potential and corrosion rate of steel reinforcement……….……63

Figure 3.1. Cubic specimen exposed in laboratory condition (28 days of curing)_Crushed Limestone + Cement………..71

Figure 3.2. Fracture surface of specimen exposed in laboratory condition _Crushed Limestone + Cement………....71

Figure 3.3. Carbonation depth measured on rammed earth fracture surface specimen after 0 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Crushed Limestone + Cement……….…………..72

Figure 3.4. Carbonation depth measured on rammed earth fracture surface specimen after 7 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Crushed Limestone + Cement……….……..72

Figure 3.5. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Crushed Limestone + Cement………...………72

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Figure 3.6. Carbonation depth measured on rammed earth fracture surface specimen after 49 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Crushed Limestone + Cement………...72 Figure 3.7. Cubic specimen exposed in laboratory condition (28 days of curing)_Recycle

Concrete Aggregate + Cement………..…73 Figure 3.8. Fracture surface of specimen exposed in laboratory condition (28 days of

curing)_Recycle Concrete Aggregate + Cement……….……….73 Figure 3.9. Carbonation depth measured on rammed earth fracture surface specimen after 0 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Recycle Concrete Aggregate + Cement………...………..74 Figure 3.10. Carbonation depth measured on rammed earth fracture surface specimen after 7 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Recycle Concrete Aggregate + Cement………...……..74 Figure 3.11. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Recycle Concrete Aggregate + Cement………...………..74 Figure 3.12. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Recycle Concrete Aggregate + Cement………....……….74 Figure 3.13. Cubic specimen exposed in laboratory condition (28 days of curing)_Soil Mix 1 + Cement……….…...75 Figure 3.14. Fracture surface of specimen exposed in laboratory condition (28 days of

curing)_Soil Mix 1 + Cement………..75 Figure 3.15. Carbonation depth measured on rammed earth fracture surface specimen after 0 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Cement……….75

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Figure 3.16. Carbonation depth measured on rammed earth fracture surface specimen after 7 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Cement……….75 Figure 3.17. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Cement……….76 Figure 3.18. Carbonation depth measured on rammed earth fracture surface specimen after 49 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Cement………...………..76 Figure 3.19. Cubic specimen exposed in laboratory condition (28 days of curing)_Soil Mix 2 + Cement………....77 Figure 3.20. Fracture surface of specimen exposed in laboratory condition (28 days of

curing)_Soil Mix 2 + Cement………..…77 Figure 3.21. Carbonation depth measured on rammed earth fracture surface specimen after 0 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Cement……….77 Figure 3.22. Carbonation depth measured on rammed earth fracture surface specimen after 7 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Cement……….……77 Figure 3.23. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Cement……….78 Figure 3.24. Carbonation depth measured on rammed earth fracture surface specimen after 49 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Cement……….……78 Figure 3.25. Cubic specimen exposed in laboratory condition (28 days of curing)_Soil Mix 1 + Carbide lime + Fly Ash.………..……….79

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Figure 3.26. Fracture surface of specimen exposed in laboratory condition (28 days of

curing)_Soil Mix 1 + Carbide lime + Fly Ash……….……….79 Figure 3.27. Carbonation depth measured on rammed earth fracture surface specimen after 0 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Carbide Lime + Fly Ash………....………79 Figure 3.28. Carbonation depth measured on rammed earth fracture surface specimen after 7 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Carbide Lime + Fly Ash………..………..79 Figure 3.29. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Carbide Lime + Fly Ash………...……….…80 Figure 3.30. Carbonation depth measured on rammed earth fracture surface specimen after 49 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 1 + Carbide Lime + Fly Ash………...……….80 Figure 3.31. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (56 days of curing)_Soil Mix 1 + Carbide lime + Fly Ash..………...………80 Figure 3.32. Cubic specimen exposed in laboratory condition (28 days of curing)_Soil Mix 2 + Carbide lime + Fly Ash………81 Figure 3.33. Fracture surface of specimen exposed in laboratory condition (28 days of

curing)_Soil Mix 2 + Carbide lime + Fly Ash……….…….81 Figure 3.34. Carbonation depth measured on rammed earth fracture surface specimen after 0 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Carbide lime + Fly Ash………...………..82 Figure 3.35. Carbonation depth measured on rammed earth fracture surface specimen after 7 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Carbide lime + Fly Ash………82

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Figure 3.36. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28

days of curing)_Soil Mix 2 + Carbide lime + Fly Ash………...…..82

Figure 3.37. Carbonation depth measured on rammed earth fracture surface specimen after 49 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (28 days of curing)_Soil Mix 2 + Carbide lime + Fly Ash……….………...…….82

Figure 3.38. Carbonation depth measured on rammed earth fracture surface specimen after 21 days of exposure in laboratory condition by using an alcoholic solution of phenolphthalein (56 days of curing)_Soil Mix 2 + Carbide lime + Fly Ash……….………..……...…………...82

Figure 3.39. Break of sample S47 during the Cicle V ………....…….…….95

Figure 3.40. Break of sample S35 during the Cicle IV………..………95

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List of Tables

Table 2.1. Nominal mix design of rammed earth [10]……….……….…..37 Table 2.2. Resume of batches……….…..39 Table 2.3. Summary of tests……….…….40 Table 2.4. Mix preparation for cylindrical specimens………..……….……..43 Table 2.5. UCS samples after 28 days and 56 days of curing room ………....…..45 Table 2.6. Soil preparation for cubic specimens………..…….…..50 Table 2.7. Absorption samples after 28 days of curing room ………..…...……….…..51 Table 2.8. Carbonation samples after 28 days of curing room………...………….………..54 Table 2.9. Carbonation samples after 56 days of curing room………...……….………..54 Table 2.10. Corrosion samples after 28 days and 56 days of curing room……….…..60

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List of Graphs

Graph 2.1. Particle size of aggregates used for casting in Milan some reinforced samples of rammed earth [10]……...……….……..…35 Graph 2.2. Particle size of materials used for casting the specimens of Crushed

Limestone……… ………...………37 Graph 2.3. Particle size of materials used for casting the specimens of RCA……….….38 Graph 2.4. Particle size of materials used for casting the specimens of Soil Mix #1…………...….39 Graph 2.5. Particle size of materials used for casting the specimens of Soil Mix #2……...……….40 Graph 2.6. Temperature and Relative Humidity………...…………..….58 Graph 3.1. Compressive strength (mean value and variation range) of cylindrical specimens measured after 28 and 56 days of moist curing on different type of batch…………....……….….69 Graph 3.2. Dry Density (mean value and variation range) of cylindrical specimens measured after 28 and 56 days of moist curing on different type of batch………...……….…….70 Graph 3.3. Porosity (mean value and variation range) of cubic specimens measured after 28 days of moist curing on different type of batch…… ……….71 Graph 3.4. Carbonation depth as a function of time of specimens exposed in laboratory with Crushed Limestone and Cement………..………74 Graph 3.5. Carbonation depth as a function of time of specimens exposed in laboratory with RCA and Cement……….………76 Graph 3.6. Carbonation depth as a function of time of specimens exposed in laboratory with SM1 and Cement………...….78 Graph 3.7. Carbonation depth as a function of time of specimens exposed in laboratory with SM2 and Cement………80

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Graph 3.8. Carbonation depth as a function of time of specimens exposed in laboratory with SM1, Fly ash and Carbide lime………...………...82 Graph 3.9. Carbonation depth as a function of time of specimens exposed in laboratory (depth measured within few seconds after spraying phenolphthalein) – SM2, Fly ash and Carbide lime………..84 Graph 3.10. Corrosion potentials of carbon steel in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle I)…….. ………...…..87 Graph 3.11. Corrosion potentials of carbon steel in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle II)…………..……….87 Graph 3.12. Corrosion potentials of carbon steel in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle III)……… ………...……..87 Graph 3.13. Corrosion potentials of carbon steel in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle IV)……..………87 Graph 3.14. Corrosion rate of carbon steel in rammed earth specimens during the I wetting cycle of 24 hours………...…..89 Graph 3.15. Corrosion rate of carbon steel in rammed earth specimens during the II wetting cycle of 24 hours……….…89 Graph 3.16. Corrosion rate of carbon steel in rammed earth specimens during the III wetting cycle of 24 hours………...89 Graph 3.17. Corrosion rate of carbon steel in rammed earth specimens during the IV wetting cycle of 24 hours……….89 Graph 3.18. Potentials of activated titanium electrode in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle I)...………..90 Graph 3.19. Potentials of activated titanium electrode in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle II)………....90

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Graph 3.20 Potentials of activated titanium electrode in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (Cycle III)……….……..90 Graph 3.21. Potentials of activated titanium electrode in rammed earth specimens versus an external saturated calomel electrode during the immersion of 24 hours (CycleIV)…...………….90 Graph 3.22. Corrosion potential of carbon steels in rammed earth made with Crushed

Limestone and Cement versus an external saturated calomel electrode…………..……….92 Graph 3.23. Corrosion potential of carbon steels in rammed earth made with Recycle Concrete Aggregates and Cement versus an external saturated calomel electrode……….……….93 Graph 3.24. Corrosion potential of carbon steels in rammed earth made with Soil Mix #1 and Cement versus an external saturated calomel electrode……….…….94 Graph 3.25. Corrosion potentials of carbon steels in rammed earth made with Soil Mix #2 and Cement versus an external saturated calomel electrode……….………….95 Graph 3.26. Corrosion potential of carbon steels in rammed earth made with Soil Mix#1, Fly Ash and Carbide lime versus an external saturated calomel electrode……….……….96 Graph 3.27. Corrosion potentials of carbon steels in rammed earth made with Soil Mix #2, Fly ash and Carbide lime versus an external saturated calomel electrode…………..……….97 Graph 4.1. Compressive strength after 28 days of curing and porosity for different mixtures...100 Graph 4.1.2 Estimate of Optimum Water Content for Soil Mix #1 (left) and Soil Mix #2 (right), stabilised with fly ash and carbide lime………...…..101 Graph 4.2. pH measurements of Soils (without stabilisers) made at the time before

mixing………...….103 Graph 4.3. pH measurements of Batches, made at the time of mixing (yellow) and

after 28 days of curing (blue)……….………..104 Graph 4.4. Carbonation depth measurements and pH for different mixtures, after 28 days of curing……….105

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Graph 4.5. Carbonation depth measurements after 28 days of curing and porosity for different mixtures………...105 Graph 4.6. Carbonation depth measurements as a function of moisture content of all

batches……….….107 Graph 4.7. Coefficient K calculation of all batches………..….…108 Graph 4.8. Carbonation depth, calculated by the simplified function s = K·t0.5_{, as a function of}

time and K for all batches………...…….110 Graph 4.9. Coefficient K calculation as a function of porosity for different mixtures………....…111 Graph 4.10. Corrosion rate and corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode, as a function of time exposure for specimen RE – 01….112 Graph 4.11. Corrosion rate and corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode, as a function of time exposure for specimen RE – 02….112 Graph 4.12. Corrosion rate and corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode, as a function of time exposure for specimen RE – 03….112 Graph 4.13. Corrosion rate and corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode, as a function of time exposure for specimen RE – 04….112 Graph 4.14. Relationship between corrosion potential and corrosion rate of reinforcing bars in different mixtures subjected to natural carbonation………...………113 Graph 4.15. Corrosion potentials of carbon steels in rammed earth specimens versus an external saturated calomel electrode for different mixtures - Cycle I………...……….114 Graph 4.16. Corrosion potentials of carbon steels in rammed earth specimens versus an external saturated calomel electrode for different mixtures - Cycle VII………...…….115 Graph 4.17. Corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode and Carbonation depth measured in lab condition, as a function of time exposure for Crushed Limestone and Cement………..…....117

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Graph 4.18. Corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode and Carbonation depth measured in lab condition, as a function of time exposure for Recycle Concrete Aggregates and Cement………....…….118 Graph 4.19. Corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode and Carbonation depth measured in lab condition, as a function of time exposure for Soil Mix #1 and Cement……….…118 Graph 4.20. Corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode and Carbonation depth measured in lab condition, as a function of time exposure for Soil Mix #2 and Cement………...119 Graph 4.21. Corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode and Carbonation depth measured in lab condition, as a function of time exposure for Soil Mix #1, Fly Ash and Carbide Lime………..……119 Graph 4.22. Corrosion potential of carbon steels in rammed earth versus an external saturated calomel electrode and Carbonation depth measured in lab condition, as a function of time exposure for Soil Mix #2, Fly Ash and Carbide Lime………..…120

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Introduction

Over the last decades, the world has discussed the continuing use of our limited natural resources and the production of waste and contaminating materials. This is particularly relevant to the construction industry. Construction materials such as steel, concrete and bricks require a significant amount of energy either to be mined or manufactured. They are responsible for a significant production of carbon footprint every year. The increasing of awareness related to environmental problems motivates the request for new structural materials.

In this context, rammed earth is proposed as an alternative construction technique that presents several benefits including low-embodied energy, cost-effectiveness and low environmental impact.

Rammed earth has been used for thousands of years and many historic buildings exist around the world, some of which need special care to be conserved and maintained. Nevertheless, rammed earth is also widely used as a modern construction technique. Many builders and engineers in different countries have been experimenting with new construction practices and design methods for rammed earth.

While the technique has changed little since its beginning, it is now common practice to stabilise rammed earth materials with small quantities of cement in order to improve its strength and durability. The adding of cement, however, seems to reduce the sustainability of rammed earth and increases both its cost and environmental impact. In order to obtain a more sustainable material, a mixture of lime and fly ash could be added in soil and used as stabiliser in rammed earth even if this practice is not diffused yet.

As in concrete structures, it is possible to reinforce the rammed earth with steel bars. At this regard, there are no exhaustive studies because the engineering knowledge of structural and material properties of rammed earth left significantly behind compared with the more common building materials such as concrete, masonry and steel.

In collaboration between Politecnico di Milano and Perth University of Western Australia the corrosion behaviour of steel embedded in rammed earth composed by different soils and stabilisers was studied in the Concrete Durability laboratory of the Department of Chemistry and

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Chemical Engineering “G. Natta” and in the Concrete laboratory of Perth. In particular, the main purpose is the analysis of the behaviour of reinforced rammed earth made with different soils and stabilised with fly ash and carbide lime compared with cement.

In the first part of Chapter 1, rammed earth description as construction material is exposed; history of material use, soil properties and description of stabiliser are explained. Successively, a comparison between rammed earth and concrete about their properties is illustrated. As a consequence of this analogy, the last chapter concerns corrosion of steel bars and concrete degradation.

In Chapter 2, experimental methodology used in laboratories is described. Two series of tests were carried out. Preliminary corrosion tests were carried out at Politecnico di Milano on samples previously realized and composed by a simulated soil. More detailed tests on different types of soils were realized at The University of Western Australia in Perth. In this chapter the choice of materials and the types of specimens, used for tests, will be described and the experimental procedures followed for the preparation of samples and execution test will be explained. Before starting the experimental plan, characterization tests were done on soils and their results will be analysed in the following chapter.

Chapter 3 results obtained from all tests performed are showed, both in Milan and Perth.

In the last chapter results of experimental tests will be discussed, in order to define the different batches properties related to the corrosion behaviour of the embedded carbon steel bars. The obtained analyses made it possible to show that stabilised rammed earth reinforced with steel bars could be considered as an innovative and sustainable construction material.

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1 Literature Review

Rammed earth is a building technique where moist soil is compacted in removable formworks. It has been used for thousands of years and its environmental benefits have recently been widely recognised. Although the increasing environmental awareness motivates the request for new rammed earth structures, currently rammed earth accounts only for a small proportion of the building market worldwide. It is possible to add stabilizing materials in the mix as cement. Steel bars can be embedded in rammed earth structures as reinforcement technique to improve the strength of the material, in particular to increase the tensile strength. However, steel bars in rammed earth can be damaged by corrosion processes. Studies concerning corrosion of steel and degradation phenomenon of concrete materials were carried out.

1.1 Rammed Earth

Rammed earth is a construction technique in which raw material as earth and water are mixed together and rammed to create building walls, foundations and floors. Earth has always been one of the main construction material in hot and temperate climate countries. Even nowadays, one third of the human population lives in earthen houses; in countries with low industrialization, earth built houses are more than half of the total. Earth is the most common natural construction material, and it is accessible everywhere the world [41]. It is easily obtained directly from the building site as backfill during the excavation process. In the more industrialised countries there are many abuses of resources and a large use the energy-intensive production; this is not only wasteful but it also pollutes the environment. In many countries, earth is being revived as a building material and all its qualities are being revalued.

1.1.1 History

Rammed earth has been used for thousands of years and many historic buildings existing around the world, some of which need special care to be conserved and maintained. The oldest proof of rammed earth used as construction technique is on Catahyoulk near to Konya in Turkey, around 10,000 years old. This archaeological site can show that all the buildings of this city were built

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with mud bricks of various sizes with various types of mortar, cob and rammed earth. If in Turkey it’s possible to find the most ancient application of rammed earth construction, in China and in Spain there are the most impressive ones. Most of the Great Wall of China (Figure 1.1) is either made of rammed earth or has a large component of rammed earth as its basis [25]. The Chinese empire built it 2,000 years ago and the earth in the wall is still in good condition. Also the Alhambra in Granada in southern Spain was largely built using rammed earth thousands of years ago (Figure 1.2). In recent, time it’s possible to find some examples of rammed earth construction in Europe, especially in France, where this technique is known as “pisè de terre” and it was very popular in 1800. This construction technique was not often used in Italy but in recent few years there are more construction experimentations on that [14], mostly because of its low environmental impact [1,18].

Figure 1.1. Great Wall, China Figure 1.2. Alambra, Spain

1.1.2 Soil Properties

Earth is a product of erosion from rocks in the earth’s crust. The erosion happens mainly due to the mechanical grinding of the rock by the movement of crustal plate, water and wind, or through thermal expansion and contraction of rock, or through the expansion of freezing water in the crevices of the rock. Soil is a mixture of clay, silt and sand, and sometimes contains larger aggregates like gravel and stones. Engineering science defines its particles according to diameter: particles with diameters smaller than 0.002 mm are named clay, those between 0.002 and 0.05 mm are called silt, and those between 0.05 and 2 mm are called sand as shown in Figure 1.3. Particles of larger diameter are termed gravels and stones [1,19].

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Figure 1.3. Particle diameter of earth

Rammed earth is a composite material composed by a binder and a reinforcement material. Clay and silt can be classified as a binder for all bigger particles in the soil. Sand constitutes the fillers in the soil and aggregates make the material stronger. Depending on which of the three components is dominant, it is possible to speak of a clayey, silty or sandy soil. The local conditions influence composition and varying properties of the soil. Gravelly mountainous soils, that are soils that contain large part of gravel and sand, are more suitable for rammed earth but only provided they contain sufficient clay; conversely, riverside soils are often siltier and so they are less weather resistant and weaker in compression [1]. The majority of modern rammed earth mixes contain the following ranges of percentages by mass: sand and gravel, 45-80%; silt, 10-30% and clay 5-20% [2].

Clay is a product of the erosion of feldspar and other minerals. Feldspar contains mainly aluminium oxide and silicon dioxide. One of the most common types of feldspar has the chemical formula Al2O3 · K2O · 6SiO2. If during erosion an easily soluble potassium compounds is dissolved,

a type of clay called Kaolinite is formed; Kaolinite has the chemical formula Al2O3 · 2SiO2 · 2H2O.

Clay minerals are also found mixed with other chemical compounds, particularly with hydrated iron oxide (Fe2O3· H2O) and other iron compounds, giving the clay a characteristic yellow or red

colour. Clay has a lamellar structure which allows gain and lose water easily. The properties of silt, sand and gravel are totally different from clay. They are simply aggregates without binding forces; if they are shaped from eroding stones they have sharp corners, whereas if they are formed by the movement of water they are rounded.

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Binding forces of soil particles are activated by water. Besides free water, there are three different types of water in soil: water of crystallisation (structural water), absorbed water and water of capillarity (pore water). Water of crystallisation is chemically bound and is only distinguishable if the soil is heated to temperatures between 400°C and 900°C. Absorbed water is bound to the clay minerals. Water of capillarity has entered the pores of the material by capillary action. Absorbed and capillary water are typically released when the mixture is heated to 105°C. If dry clay gets wet, his volume gets larger because water moves inside the lamellar structure. When this water evaporates, the inter-lamellar distance is reduced, and lamellas arrange themselves in a parallel pattern due to the forces of electrical attraction. Clay thus acquires a binding force if it is in a plastic state; conversely it achieves compressive and tensile strength after drying process. A high porosity of soil can be accounted by a high presence of a fine friction like clay and silt. These fine materials have a high specific surface (surface per unit mass) with a high capacity of water absorption. Consequently, during batching they demand large amount of water that, when eliminated by evaporation, leaves high porosity in rammed earth structures. Water is not the only parameter that influences the bonding forces of soil; these forces also depend on the clay content and the type of clay minerals present. The dry density of soil in rammed earth applications depends on the soil type, the moisture content during compaction and the compaction effort [1]. A range of dry density values for rammed earth, varying from 1700 kg/m3_{to 2200 kg/m}3_{is reported}

in many studies [1,3].

The compressive strength of dry building elements made of rammed earth, ranges in general from 0,5 to 2.5 MPa. This depends not only on the quantity and type of clay involved, but also on the content of silt, sand and larger aggregates, as well as on the method of preparation and compaction [2].

Measurement of porosity is a method used to determine the quality and durability of rammed earth. The degree of porosity is defined by the total volume of pores within the soil.

1.1.3 Stabilisers

It is possible to add stabilisers in rammed earth that increase strength and erosion resistance of the material. The most common stabiliser is cement, that is added directly to the earth before adding water. Lime is an alternative to cement because of its ability to hydrate. It is possible also to add pozzolanic material, like fly ash, which may react with lime or cement.

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Cement

Cements is a fine mineral powder that, when mixed with water, forms a paste that sets and hardens due to hydration reactions. Portland cement is one of the most common used cement. It is produced by grinding clinker, which is obtained by burning a suitable mixture of limestone and clay raw materials. Its main components are tricalcium and dicalcium silicates (C3S and C2S),

aluminate and ferroaluminate of calcium (C3A and C4AF, respectively) [28,30].

In the chemistry of cement, the following abbreviations are used: CaO = C; SiO2 = S; Al2O3 = A; Fe2O3

= F; H2O = H; S=SO3.

Gypsum (CS) is added to clinker before grinding, to control the rate of hydration of aluminates. Aluminates react first with water and they are mainly responsible for setting of the cement paste. The hydration of C3A and C4AF mainly gives rise to hydrated sulfo-aluminates of calcium [4,5,6].

In the presence of water, the silicate compounds of Portland cement form colloidal hydrated products of very low solubility. The major part of hardening of cement paste is governed by hydration of silicates. The hydration of C3Sand C2S gives rise to calcium silicate hydrates forming

a gel, indicated as C–S–H (Figure 1.4).

It is composed of extremely small particles with a layer structure that tend to aggregate in formations a few μm in dimension, characterized by interlayer spaces of small dimensions (<2 nm) and by a large surface area (100–700 m2_{/g). Hydration of calcium silicates also produces}

hexagonal crystals of calcium hydroxide (portlandite). They do not contribute to the strength of cement paste but they are very important with regard to protecting the reinforcement, because they cause an alkaline pH up to 13.5 in the pore liquid [4,5,6].

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Cement stabilisation has become accepted a routine practice in rammed earth constructions. There are many advantages by using cement as a stabilizer. Soil samples gain strength from the formation of a cement gel matrix that binds together the soil particles. [7]

Cement stabilisation improves the surface coating reducing erosion and the vulnerability to frost attack [3].

The amount of cement required will depend on grading and other soil characteristics [1]. It is typically used in proportions between 4% and 15% of the volume of soil; the most common use is between 6% and 10% of the volume of the soil [3]. However, the environmental impact of cement production is a significant argument against widespread use of it in rammed earth construction.

Lime

Lime is a construction material obtained by cooking at high temperature limestone or all materials that contain calcium carbonate. During the cooking carbon dioxide is released and there is a calcium oxide production called quicklime; the process is resume by Equation 1.1:

If water is added to quicklime, hydrated lime is obtained, as the following Equation 1.2:

Hydrated lime may react with carbon dioxide (CO2) and thus harden in time.From a sustainability

prospective, it could be useful to use lime produced from recycled materials. Carbide lime is a high quality hydrated lime slurry produced as a by-product of the generation of acetylene gas according to the following Equation 1.3:

CaCO3 + heat  CaO+ CO2 (1.1)

CaO3 + H2O  CaO+ Ca(OH)2 (1.2)

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Fly Ash

Fly ash is a pozzolanic material. Pozzolanic materials are a broad class of siliceous or siliceous and aluminous materials; they possess little or no cementitious particles but which could react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. Fly ash is a product of the combustion of coal powder in thermoelectric power plants. It has mainly a glassy siliceous microstructure and it may contain aluminous compounds but has a low lime (calcium oxide) content. Fly ash has no binding properties, but it acquires them in the presence of lime or cement, giving rise to hydration products similar to those of Portland cement [4,5].

The reaction between a pozzolanic material as the fly ash with lime and water is known as the pozzolanic reaction:

A mix of lime and fly ash can be uses as stabilizer to increase the strength of soil, binding together its particles. [4,6]

1.1.4 Optimum Water Content

The Optimum Water Content (OWC) is the degree or percentage of water in soil at which the soil can be compacted to its greatest density. The dry density is dependent on soil type and compaction energy during compaction, and to achieve maximum density the optimum water content is used when ramming [17]. An optimum water content is required to provide the best path to enter energy into soil and compact it. A constant value of energy applied to a particular type of soil, at optimum water content, leads to a maximum dry unit weight. Density and water content are not unique for various types of soils and vary with type of soils and the compaction energy [43].

1.1.5 Construction Technique

The soil used to create structures is compacted by a process of ramming in formworks. Steel bars are added in the weakest part as lintels or joints and in order to increase the tensile strength. It is a less common practice to add steel bars in the entire structure to improve its strength. [42,14]

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Advantages

The preparation, transport and handling of construction soil requires only 1% of the energy needed for the production, transport and handling of baked bricks or reinforced concrete [2,21]. Soil, then, produces virtually no environmental pollution. Natural rammed earth construction (without stabilisers) can be recycled an indefinite number of times over an extremely long period so it never becomes a waste material that harms the environment. The use of excavated soil means greatly reduced costs in comparison with other building materials. Even if this soil is transported from other construction sites, it is usually much cheaper than industrial building material. Provided the building process is supervised by an experienced engineer, earth constructions can usually be executed by non-professionals. Since the processes involved are labour-intensive and can require only inexpensive tools and machines, they are ideal also for do-it-yourself building.

Method of construction

The specific construction of rammed earth consists of layers of earth poured into formworks at a depth of 20 – 30 cm compacted to 30% of their initial volume (Figure 1.5). This creates a striated earthen wall. Formworks are used during the construction but these should be in place before water is added to the soil mix. The compaction was traditionally undertaken manually by a hand tamper, but over the past 50+ it years has been replaced by pneumatic, vibrating plate and sheep’s foot roller compactors. After the ramming, formworks are removed.

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Steel bars in rammed earth

Reinforcement can be distributed throughout the wall, as horizontally or vertically strengthening. Steel bars can be used in lintels of windows and doors opening and they can be also in joints between foundations and walls or walls and roof. Therefore, this strengthening can be used to resist it to the horizontal forces of wind, earthquakes and cyclones.

Lintels

Lintels provide a wall support over openings such as doorways and windows (Figure 1.6). Strength and stiffness have to be enough high to support the weight of the construction above and reduce the deflection. It may be necessary to increase the strength of the material in the zone where the lintels are applied due to the concentration of forces beneath lintels supports. The strength can be increased either by gaining the level of stabilisation or using higher strength non-earthen materials. In rammed earth construction the lintels must be propped in earth during the compaction [7,22].

Figure 1.6. Outline of reinforced lintels [7]

Wall plates and Roof Fixings

It is possible in rammed earth construction to provide a wall plate, collar beam, bond beam or roof plate, continuously around the top of walls. Wall plates enhance stability of earth walls of low tensile strength when subject to high lateral loads (wind, earthquake). In addition, wall plates provide interface between wall and roof for connection and anchorage. In the absence of a wall

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plate the roof may be tied down directly to the wall with steel bars, embedded within the wall (Figure 1.7). Bars have to resist to the atmospheric agents, so they usually are galvanized bars [3].

Figure 1.7. Roof fixings [3]

Foundations

As rammed earth is a material with low water resistance, it is important to create a base for the walls that protects them from possible absorbed water from the ground. Foundation is usually composed by a layer of concrete and rarely of masonry. Rammed earth walls are built above this layer. Steel bars connecting the foundation to the walls are used to protect the structure from the horizontal forces generated by as wind and earthquakes.

1.2 Comparison between Rammed Earth and Concrete

Rammed earth is a construction method used for thousands of years and its qualities are being revalued. However, there are very few standards that provides specifically guidelines about this construction technique. One of the best publication for rammed earth construction is The Australian Earth Building Handbook (HB 195) [5,38]. Nevertheless, the big experience acquired by literature allows to notice big analogies between cement-stabilised rammed earth and concrete. Analogies concern both materials behaviour and construction technique. Because of these few information the concrete’s standards can be used also in rammed earth.

Cement-stabilised rammed earth and concrete are similar in several physical aspects. Both of them are composed by a binder and aggregates. In concrete the binder is the cement and the aggregates are sand and gravel; in rammed earth silt, clay, sand and gravel are bound by stabilisers. The density in concrete is about 2400 kg/m3_{; in rammed earth the density is between}

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Both materials have alkaline properties; The steel embedded in concrete is, in optimal conditions, very well protected by the elevated alkalinity of the hydrated cement environment given by pore solution which, in normal conditions of a good concrete, presents pH values between 12,6 and 13,8. In this interval of pH values the reinforcing steel is passive.Rammed earth has a pH of 6-8.5 [1] but with an addition of stabiliser, the pH can achieve values of 11-12. Analogies are in construction technique too. Both materials take the form of formwork where they are rammed (in case of rammed earth, Figure 1.8) or casted (in case of concrete Figure 1.9). This characteristic makes them adaptable in different shapes and usable in several circumstances.

Figure 1.8. Concrete wall Figure 1.9. Rammed Earth wall [24]

In literature, there are no specific manuals or standards that explain the behaviour of steel bars in rammed earth.

1.3 Corrosion of Steel

Steel used as reinforced material in rammed earth may corrode in particular electro-chemical conditions weakening the entire structure. Studies about corrosion of steel bars in concrete can be used to analyse the behaviour of reinforcement in rammed earth because of the similarities between rammed earth and concrete [13].

1.3.1 General aspects of corrosion

Steel bars could be subject to corrosion processes if they are in aggressive environment. Humid corrosion is the most important type of corrosion and it happens if the steel bars are in physical contact with aqueous solutions (such as sea or fresh water) or environment with water (such as concrete, soils, humid atmosphere, etc.). The consequences of corrosion of steel involve several aspects connected with the condition of the structure, such as its esthetical appearance, service ability, safety, and structural performance [4].

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There are different types of corrosion and its consequences concern by the mode it propagates. Corrosion is often indicated by rust spots that appear on the external surface of the reinforcing bars. The rust occupies a much greater volume than the original steel bar. The volume of the corrosion in fact can be from 2 to 6 times greater than that of iron they are derived from, depending on their composition and the causes of corrosion itself.

Uniform corrosion causes the reduction of steel section. This kind of corrosion is not the most dangerous because penetration rate is slow (usually less than 100 µm/year) and similar on all steel surface. Instead in cases localized corrosion, the penetration rate is different depending on the heterogeneity of metal, environment and geometry of structure.

The effect of corrosion may concern the steel bars embedded in concrete. The cross section of the steel and its loading capacity and its fatigue strength can be significantly reduced before any sign of corrosion becomes visible at the surface of concrete. The attack can be low localized to pitting (high localized). In pitting, the rate of corrosion can reach 1 mm/year. It is typical of material contact with environment in which there are chlorides.

1.3.2 Electrochemical Aspects

The corrosion of steel in humid environment can be summarized with the following reaction. [1]

Actually, this is an electrochemical reaction and is composed of four partial processes (Figure 1.10):

-anodic process: the oxidation of iron liberates electrons in the metallic phase and gives rise to the formation of iron ions (Fe  Fe2+_{+ 2e}-_{) whose hydrolysis produces acidity:}

-cathodic process: it is the reduction of oxygen reaction that consumes these electrons (the metal phase) and produces alkalinity:

iron + oxygen + water  corrosion products (1.5)

Fe2+_{+ 2H}

2O  Fe(OH)2 + 2H+

iron + water  iron oxidation products + acidity (1.6)

O2 + 2H2O + 4e- 4OH-

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-transport of current in the metal: the electrons within the metal ranging from anodic regions (where they are produced) to the cathodic (where they are consumed) giving origin to the passage of a conventional current in the opposite direction;

-transport of current within concrete: finally, in order for the circuit to be complete, the flow of current inside the concrete from the anodic regions to the cathodic ones, transported by ions in the pore solution.

These four processes are complementary, which is to say that they occur at the same rate. In fact, the anodic current Ia (i.e., the number of electrons liberated by the anodic reaction in a unit of

time), the cathodic current Ic (i.e., the number of electrons that are consumed in the cathodic

reaction in a unit of time), the current that flows inside the steel from the cathodic region to the anodic (Im), and finally the current that circulates inside the environment from the anode to the

cathode (Ienv), should all be equal:

Figure 1.10. Electrochemical mechanism of corrosion [4,6]

The common value of all these currents (Icorr) is, in electrochemical units, the rate of the overall

process of corrosion. The corrosion rate will thus be determined by the slowest of the four partial processes.

To evaluate the progress of corrosion, the corrosion rate is expressed as mass of metal consumed in a time unit vm or as the rate with which thickness of material is reduces vp:

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Where:

vm is the rate of mass loss [g/(m²· anno)];

vp is the thinning rate [μm/anno];

Δm is the loss of mass [g] over time t [years];

A is surface affected by corrosion [m²].

Equations 1.9 and 1.10 are used only if corrosion is uniform and takes place at a constant rate over time; they become meaningless in case of localized attack.

1.3.3 Passivity

Some kind of metallic material in specific environmental conditions, show an elevate corrosion resistance because of their passive condition. A passive metal is a metal that has on its surface a protective layer of oxides (icorr~0) and a typical anodic polarization curve is shown in Figure 1.11.

In alkaline concrete and without chlorides, the reinforcing bars are protected by corrosion thanks to passive conditions; these conditions are created in contact with solutions contained in pores of material, after hydration of cement.

The potential is measured versus the saturated calomel reference electrode (SCE), whose potential is +244 mV versus the standard hydrogen electrode (SHE). Other reference electrodes used to measure the potential of steel in concrete are: Ag/AgCl, Cu/CuSO4, MnO2, and activated

titanium types. Steel has a tendency to oxidation at potentials more positive, about −1000 mV vs SCE. Therefore, below −1000 mV steel is in a condition of immunity. In the range of potentials between −800 and +600 mV vs SCE, the anodic current is very low (0.1 mA/m2_{) because the steel}

is covered by a very thin film of iron oxide that protects it completely (passive film). Thus, in this interval of potentials the dissolution rate of iron is negligible. This is known as the condition of passivity and it exists in the interval of potentials known as the passivity range. In the interval of potentials between equilibrium and about -800 mV vs SCE, the protective film does not form spontaneously. In this condition, called activity, steel can theoretically corrode.

𝑣𝑚 = |∆𝑚| 𝐴𝑡 (1.9) 𝑣𝑝= |∆𝑚| 𝜌 ∙ 𝐴𝑡 (1.10)

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Above the passivity range, that is for potentials above about +600 mV SCE, the steel is brought to conditions known as transpassivity [4].

Figure 1.11. Schematic anodic polarization curve for a steel in noncarbonated concrete without chlorides [4,6]

Pourbaix diagram describes conditions in which the formation of film of passivity on steel is possible (Figure 1.12).

Figure 1.12. Pourbaix Diagram of iron [4,6]

In alkaline solutions with pH> 11.5 and in the absence of chlorides, the iron is covered with a thin oxide film, whose thickness is of few nanometres. This diagram is useful to understand the

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behaviour of a metal in an environment and helps to identify the immunity domain, corrosion and passivation.

1.4 Degradation of Concrete

Concrete is one of the main material reinforced with steel bars. Analogies between concrete and rammed earth and the knowledge of concrete behaviour to study rammed earth properties were discussed is Paragraph 1.2. The environmental action in reinforced concrete structures can determinate a progressive damage on concrete or reinforcement. Degradation processes of concrete can be classified as: physical (caused by natural thermal variations such as freeze–thaw cycles, or artificial ones, such as those produced by fire), mechanical (abrasion, erosion, impact, explosion), chemical (attack by acids, sulphates, ammonium and magnesium ions, pure water, or alkali aggregate reactions), biological (fouling, biogenic attack), and structural (overloading, settlement, cyclic loading). The processes of deterioration of concrete and corrosion of reinforcement are closely connected.

Experience shows that corrosion of steel is the most frequent cause of degradation of reinforced cement structures.

1.4.1 Transport Processes

The deterioration of reinforced concrete structures is closely related to the properties of concrete, in particular its permeability, as this influences the transport of aggressive substances. For this reason, the environmental action influences the performance of the material as a function of exposure time. Concrete can be penetrated, through its pores, by gases and liquid substances. The term permeability indicates in general the property of concrete to allow the ingress of these substances. The permeability of concrete is not only important for water-retaining structures, but is a decisive factor in the durability of reinforced concrete. Phenomena that lead to degradation of reinforced concrete depend on the processes that allow transport of water, carbon dioxide, chloride ions, oxygen, sulfate ions and electrical current within the concrete. The movement of fluids and ions through concrete can take place according to four basic mechanisms: capillary suction, due to capillary action inside capillaries of cement paste, permeation, due to pressure gradients, diffusion, due to concentration gradients, and migration, due to electrical potential gradients. The kinetics of transport depend on the mechanism, on the properties of the concrete (e.g., its porosity and the presence of cracks), on the binding by the hydrated cement paste, of the

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substances being transported, as well as on the environmental conditions existing at the surface of the concrete (microclimate) and their variations in time [4].

The penetration of gaseous substances such as carbon dioxide or oxygen and the penetration of ionic substances such as chloride (Cl-_{) is related to the presence (or absence) of water within the}

concrete; the presence of water hinders the entry of gaseous species but promotes the penetration of ions in the liquid phase. Water therefore plays a critical role by regulating the corrosive phenomenon. It is in cement paste as capillary water in the pores or as adsorbed water, and it is related to environmental conditions where the concrete structure is exposed.

1.4.2 Corrosion of steel in concrete

Passivity conditions, developed by contact with the alkaline solution of concrete, protect the reinforcement from corrosion. Steel corrosion can however also be induced by carbonation (it takes place on the whole surface of steel in contact with carbonated concrete) or chloride ions penetration (localized with penetrating attack of limited area surrounded by not-corroded areas) [4,6].

1.4.3 Carbonation-Induced Corrosion

Carbonation is the reaction of carbon dioxide from the air with alkaline constituents of concrete [4,6]. It has important effects with regard to corrosion of embedded steel. In moist environments, carbon dioxide present in the air forms an acid aqueous solution that can react with the hydrated cement paste, which tends to neutralize the alkalinity of concrete (this process is known as carbonation).

Acidic gases like carbon dioxide react with free alkalis that may be present and cause a reduction in the pH value (values close to neutrality), as it can be seen in Figure 1.13.

The alkaline constituents of concrete are present in the pore liquid but also in the solid hydration products (Ca(OH)2, C-S-H). Calcium hydroxide is the hydrate in the cement paste that reacts most

readily with CO2. The carbonation reaction can be schematically written as:

Figure 1.16 shows the carbonated zone in pink where the steel bar is corroded and the not-carbonated where the steel bar is not corroded in the left picture, while right picture shows schematic representation of CO2 diffusion through the capillary pores.

.

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Figure 1.13. Example of carbonation [26]

Figure 1.14 shows the effect of corrosion correlated to time of a carbonated concrete element. Service life of the structure can be divided in two distinct phases. The first phase is the initiation phase, in which the reinforcement is passive but phenomena, that can lead to loss of passivity, take place. The second phase is the propagation phase, which begins when the steel is depassivated and finishes when a limiting state is reached beyond which consequences of corrosion cannot be further tolerated.

Figure 1.14. Tuutti’s Model to predict initiation and propagation period in a reinforced concrete [13,14]

The rate for uniform corrosion by carbonation can be expressed as the penetration depth in unit of time, measured in µm/year or in electrochemical units, i.e. mA/m2_{. In the case of steel, 1 mA/m}2

or 0.1 μA/cm2_{corresponds to a loss of mass equal to approximately 9 g/m}2_{year and a penetration}

rate of about 1.17 μm/year. As a general figure, the corrosion rate can be considered negligible if it is below 1-1-5 μm/year. Instead when it exceeds 2 μm/year, corrosion products accumulate on

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the interface between steel bars and concrete, causing a decrease of adherence and degradation of concrete. Corrosion rate is considered low between 2 and 5 μm/year, moderate between 5 and 10 μm/year, intermediate between 10 and 50 μm/year, high between 50 and 100 μm/year and very high for values above 100 μm/year.

During the initiation phase carbonation penetrates beginning at the surface of concrete and moving gradually towards the inner zones, where the passive film cannot be more stable. The initiation time is defined by carbonation penetration rate and concrete cover [13]. The process of carbonation front is described by Equation 1.21:

Where:

- x is the depth of carbonation [mm]; - t is time [years];

- K is carbonation coefficient and depends on both environmental factors and factors related to the concrete [mm/years1/2_];

- n is approximately equal to two.

The coefficient K, which describes the kinetics with which advances the carbonation, depends on several factors, some related to the type of concrete, to its alkalinity and its porosity (in turn influenced by the water/cement ratio and by maturation), others alloyed to the in environmental parameters such as relative humidity, the temperature and the concentration of carbon dioxide. The main factors related to concrete are:

1. Water/cement ratio and curing: the porosity of concrete has an important influence on the penetration rate of carbonation. The decrease of w/c ratio decreases the capillarity porosity of cementitious paste and thus allows to slow down the process of carbonation. It is important that the concrete has adequate curing, as inadequate wet maturation does not allow proper hydration producing a structure too porous.

2. Alkalinity: it defines the capacity of carbon dioxide to react with hydroxides in concrete. This factor depends on the type of cement used: in the case of blended cements the alkalinity will be lower because of the pozzolanic reaction.

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The coefficient of carbonation K also depends also by environmental factors such as:

3. Moisture: the development of carbonation is possible only in the presence of humidity higher than 40%. This is because carbon dioxide needs water to react with the alkaline constituents of the concrete. But a percentage of humidity higher than 80-90 % prevent the diffusion of carbon dioxide through the water saturated pores thus the carbonation is negligible. For these two reasons the most favourable range for the carbonation is between 60 % and 80 % of relative humidity, as shown in Figure 1.15.

Figure 1.15 Schematic representation of the rate of carbonation of concrete as a function of the relative humidity of the environment, under equilibrium conditions [4,6]

4. Temperature: the carbonation rate increases with increasing temperature;

5. The content of carbon dioxide: a high concentration of carbon dioxide on the surface of concrete promotes the carbonation; thus, the rate increases with increasing concentration of carbon dioxide in the atmosphere.

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Corrosion of carbon steel embedded in rammed earth