1. CORROSION OF REINFORCEMENT

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1 MAB 1033 Structural Assessment and Repair 1. CORROSION OF REINFORCEMENT Professor Dr. Mohammad bin Ismail C09-313

2 Learning Outcome At the end of the course students should be able to understand Mechanism of corrosion of reinforcement Factors that influence corrosion propagation Differences general and pitting corrosion Method of rectification

3 Effect Leakage Settlement Deflection Wear Spalling Disintegration Cracking Delamination Cause Defect Damage Deterioration Design Materials Construction Overloading Chemical spill Earthquake Fire Erosion Corrosion of metals AAR Sulphate Attack MAB 1033 Structural Assessment & Repair 3 Scaling

4 INTRODUCTION Corrosion of reinforcement is indeed one of the major cause of deterioration to concrete structures in many parts of the world The main cause is largely related to : The use of de-icing salts Chloride : a) Exposure chloride containing environments (marine environments) b) Previous use of chloride based accelerator c) Chloride contaminated materials Due to reduction in alkalinity of concrete as a result of carbonation of concrete from exposure to CO 2 in the atmosphere MAB 1033 Structural Assessment & Repair 4

5 MECHANISMS OF CORROSION OF STEEL IN CONCRETE (1) Definition of corrosion : Degradation of metals by an electrochemical reaction with the environment The electrochemical corrosion cell has 4 components : Anode : Site where corrosion occurs and electrons flow from Cathode : Site where no corrosion occurs and electron flow to Electrolyte : the aqueous environment, in contact with both the anode and cathode to provide a path for ionic conduction The electrical connection between the anode and the cathode to allow electrons to flow between them MAB 1033 Structural Assessment & Repair 5

6 MECHANISMS OF CORROSION OF STEEL IN CONCRETE (2) Electron Flow Salt Water Zinc (Anode) Copper (Cathode) MAB 1033 Structural Assessment & Repair 6

7 MECHANISMS OF CORROSION OF STEEL IN CONCRETE (3) Cracking, Spalling and Delamination Reinforcement Cracking Reinforcement Spalling MAB 1033 Reinforcement Structural Assessment & Repair Delamination 7

8 MECHANISMS OF CORROSION OF STEEL IN CONCRETE (4) Fe Fe 3 O 4 Fe(OH) 2 Fe(OH) 3 Fe(OH) 3. H 2 O Volume Change Volume (cm 3 ) MAB 1033 Structural Assessment & Repair 8

9 MECHANISMS OF CORROSION OF STEEL IN CONCRETE (5) In order for corrosion to occur the 4 basic elements (anode, cathode, electrolyte & electrical connection) are required plus the supply of O 2 & H 2 O If any of these required elements is absent, corrosion will not occur Corrosion cell : Anodic reaction : Fe (solid) Fe 2+ (ions) + 2e Cathodic reaction : O 2 + 2H 2 O + 4e 4OH - 9 Structural Assessment & Repair MAB 1033

10 Corrosion Process Concrete high alkalinity material (ph 12-13) Passive film protect Steelγ-Fe 2 O 3 When passive filmdisrupted, corrosion may take place Corrosion is defined as the deterioration of metal by reaction with species in the environment to form chemical compound Corrosion is a electrochemical process requiring an anode, a cathode an electrolyte MAB 1033 Structural Assessment & Repair 10

11 The Three-Stage Model of Corrosion Damage Initiation Period Propagation Period Accelerated Period Extent of Damag ge No evidence of Damage Corrosion initiated by chlorides or carbonation Corrosion with minor damage Widespread cracking and spalling of cover Age of Structure (Years) MAB 1033 Structural Assessment & Repair 11

12 Corrosion Inhibitors High quality concrete High ph (Alkalinity) concrete protects steel surface from corrosion Corrosion Promoters: - Oxygen. - Water - Stray electrical currents. - Uneven chemical environment around reinforcement. - Environments that lower the ph (alkalinity). - Chlorides. MAB 1033 Structural Assessment & Repair 12

13 Carbonation Carbonation is a reaction between acidic gases in the atmosphere and the products of cement hydration CO 2 + H 2 O H 2 CO 3 H 2 CO 3 + Ca(OH) 2 CaCO 3 + 2H 2 O Carbon dioxide diffuse in concrete react with calcium hydroxide and reduce ph value (ph < 10) Protective layer of the steel destroyed MAB 1033 Structural Assessment & Repair 13

14 Acidic Alkaline Corrosion Rate mm/yr ph of Concrete Relationship between ph & Corrosion rate MAB 1033 Structural Assessment & Repair 14

15 CARBONATION Induced corrosion CO 2 from the atmosphere penetrates the concrete Presence of O 2 & H 2 O 1. CO 2 react with Ca(OH) 2 to form CaCO 3 Alkalinity < Passive oxide layer lost its ability to protect the steel Steel reinforcement Reinforcement - corroded MAB 1033 Reinforcement Structural Assessment & Repair concrete 15

16 Carbonation process Delamination CO 2 H 2 O Y e a r s Corrosion takes place faster when the ph is lowered. MAB 1033 Structural Assessment & Repair 16

17 Corrosion of Reinforcement (Carbonation) MAB 1033 Structural Assessment & Repair 17

18 Chloride penetration Chloride in concrete may arise from external and internal source External ingress from sea-water, salt laden mist, deicing salt Internal added as admixture (accelerator) Chloride attack the passive layer on steel As rust layer builds, tensile forces generated by expansion of the oxide cause concrete to crack and delaminate MAB 1033 Structural Assessment & Repair 18

19 CHLORIDE Induced corrosion Chloride penetrate the concrete from de-icing salts / seawater Presence of O 2 & moisture Existing chloride admixtures / contaminated aggregates etc. Passive oxide layer lost its ability to protect the steel Steel reinforcement Reinforcement - corroded MAB 1033 Reinforcement Structural Assessment & Repair concrete 19

20 Chloride penetration Delamination/Sp all When chlorides penetrate to reinforcing steel corrosion begins. MAB 1033 Structural Assessment & Repair 20

21 Cast-in Chloride Introduced deliberately as an accelerator Natural ingredient found in some aggregates Concrete made from beach sand or mix using sea-water Chlorides occur in either water soluble or acid soluble MAB 1033 Structural Assessment & Repair 21

22 Corrosion of Reinforcement (Chloride) MAB 1033 Structural Assessment & Repair 22

23 Limit of chloride ion in concrete Service condition % of Cl to weight of cement Prestressed concrete 0.06 Conventionally reinforced concrete in a moist environment and exposed to chloride Conventionally reinforced concrete not exposed to chloride Above-ground building construction where concrete will stay dry No limit MAB 1033 Structural Assessment & Repair 23

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28 Cracks and Chloride Cracks and construction joints permit corrosive chemicals to access reinforcement ACI 224R-90 present the following table of tolerable crack width Exposure Condition Dry air, protective membrane Tolerable crack Width 0.41mm Humidity, moist air, soil 0.3 De-icing chemicals 0.18 Seawater, seawater spray 0.15 Water retaining structures 0.1 MAB 1033 Structural Assessment & Repair 28

29 Corrosion induced cracking and spalling Cracking and spalling is a function of Concrete tensile strength Quality of concrete cover Bond/condition of interface between rebar and surrounding concrete Diameter of reinforcing bar Percentage of corrosion by weight of reinforcement C/D Ratio Cover (mm) Bar size Corrosion % to cause cracking 7 89 #4 4% 3 38 #4 1% MAB 1033 Structural Assessment & Repair 29

30 Reduction in Structural Capacity The structural capacity of a concrete member is affected by bar corrosion and cracking of surrounding concrete Steel with more than 1.5% corrosion, the ult load capacity began to fall, and at 4.5% corrosion, the ult load reduced by 12%. MAB 1033 Structural Assessment & Repair 30

31 Dissimilar metal Corrosion Corrosion can take place in concrete when two different metals are cast into a concrete structure 1. Zinc 2. Aluminium 3. Steel 4. Iron 5. Nickel (galvanic) Note: shaded area denotes level of moisture penetration and active electrolyte. If chlorides are present, the process is accelerated. Cathode + Electron Flow Ion OH Flow _ Anod e MAB 1033 Structural Assessment & Repair 31

32 Post-Tension Strand Corrosion Unbonded post-tension strands are protected by protective grease and sheathing Aggressive agents can penetrate when inadequate cover damage by heavy loads Common problem -poor corrosion protection of the end anchorages due to porous or cracked anchorage plug grout MAB 1033 Structural Assessment & Repair 32

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34 Leakage Paths into Strand System Unprotected Strand without Protective Sheathing 7 Wire Strand Anchorage Plug Grout Wedge s End Anchor Casting Breakout Bars Protective Sheathing Types Individual Wires Push-Thru Heat- Sealed Grease (typical) Extruded MAB 1033 Structural Assessment & Repair 34

35 Structural Steel Member Corrosion Steel beam cast into concrete to form a composite member To provide fire protection Top flange of beam is susceptible to corrosion when a crack or construction joint intersect the flange MAB 1033 Structural Assessment & Repair 35

36 Crack or Construction Joint over Embedded Structural Steel Aggressive Environmen t MAB 1033 Structural Assessment & Repair 36

37 CONSEQUENCES OF CORROSION Reduction in the steel cross-sectional area Cracking, spalling & delamination of the concrete cover (due to expansive nature of the iron oxides) A decrease of the steel/concrete bond Possible reduction in load carrying capacity of structural member 37 Structural Assessment & Repair MAB 1033

38 ASSESSMENT OF CORROSION DAMAGED CONCRETE STRUCTURES Objectives to find the causes/as well as the extent of the corrosion problem Normally done in 2 stages : Visual inspection + limited testing Detailed testing Testing : Covermeter survey Carbonation depth measurement Chloride ion content measurement Half-cell potential measurement Resistivity measurement Degree of corrosion Other tests (rate of corrosion, analyses for cement content) 38 Structural Assessment & Repair MAB 1033

39 REPAIR OF CORROSION DAMAGED Patch repair CONCRETE 39 Structural Assessment & Repair MAB 1033

40 REPAIR OF CORROSION DAMAGED CONCRETE (Cont.) Guniting / Shotcreting Preparing slab for guniting Shotcreting on R.C. wall MAB 1033 Structural Assessment & Repair 40

41 REPAIR OF CORROSION DAMAGED Pressure grouting CONCRETE (Cont.) Hardworking Confuse.. Before In progress After MAB 1033 Structural Assessment & Repair 41

42 REPAIR OF CORROSION DAMAGED CONCRETE Another repair options Preplaced aggregates & Pressure grouting Preventive measures Surface protection Electrochemical methods Strengthening Plate bonding (steel, CFRP) Jacketing External prestressing Give up Demolish and rebuild to new & improved specification 42 Structural Assessment & Repair MAB 1033

43 CORROSION PREVENTION FOR CONCRETE STRUCTURES (1) Use of sufficient cover Use of impermeable good quality concrete Lower water binder ratio Use of mineral admixtures Use of optimum cement content Optimum compaction Early and comprehensive curing Apply surface treatments Use of durability related tests for compliance (gas & water permeability, chloride permeability, chloride diffusion) 43 Structural Assessment & Repair MAB 1033

44 CORROSION PREVENTION FOR CONCRETE STRUCTURES (2) Isolation of reinforcement from the chemical effect of corrosion by means of physical barrier or chemical inhibition Use of epoxy coated reinforcement Use of galvanised reinforcement Use of stainless steel reinforcement Use of bar primer Use of zinc rich paint 44 Structural Assessment & Repair MAB 1033

45 CORROSION PREVENTION FOR CONCRETE STRUCTURES (3) Reversing the effect of corrosion by cathodic protection (CP) It works based on the principles of eliminating the anodic sites (corrosion sites) by progressing the steel to a cathodic state Sacrificial anodes CP Impressed current (CP) 45 Structural Assessment & Repair MAB 1033

46 CORROSION PREVENTION FOR CONCRETE STRUCTURES (4) Preserving or restoring passivity (reserving the effect carbonation and chloride attack by electrochemical processes) Realkalization : Technique to introduce alkaline solution into concrete to arrest and prevent further deterioration due to carbonation. Produce hydroxyl ions & restoring ph levels Chloride extraction (Desalination) : Technique to remove ingressed or cast in chlorides in order to arrest deterioration due to carbonation 46 Structural Assessment & Repair MAB 1033

47 CONCLUSION The risk of reinforcement corrosion occuring in new construction could be reduced by understanding the cause and mechanism of corrosion and taking appropriate preventives measures in the planning and construction stages The most appropriate measure to reduce the risk of corrosion is to produce durable concrete in the first place by choosing proper materials and mix proportions as well as appropriate construction practices Successful repair to deteriorated concrete also require an understanding of the causes and mechanism of the deterioration, so that the most appropriate repair materials and techniques could be applied 47 Structural Assessment & Repair MAB 1033

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49 Department of Structures and Materials, Faculty of Civil Engineering UTM 49