Corrosion Mechanisms, Monitoring, and Service Life Enhancement of Reinforced Concrete Structures

Transcription

1 Corrosion Mechanisms, Monitoring, and Service Life Enhancement of Reinforced Concrete Structures Radhakrishna G. Pillai Concrete Materials Research Group (Building Technology & Construction Management Division) Department of Civil Engineering Indian Institute of Technology Madras (IITM) Chennai, India

2 Outline Significance of corrosion Corrosion mechanisms in concrete structures Corrosion monitoring in the field Ways to enhance and estimate service life

3 The Golden Gate Bridge, San Francisco, USA is facing a lot of corrosion issues

4 After just 13 years of service, tendons in the Sunshine Skyway bridge, Florida failed

5 Wire mesh to protect the spalled concrete from falling onto the vehicles underneath the Mercier Bridge, Montreal, Canada

6 The cost of corrosion is significant In 2012, the direct cost of corrosion in India was Rs. 2 lakhs crores/year! A corrosion protection strategy to minimise the repair and maintenance costs is a MUST Corrosion cost split-up in the USA (FHWA 1998) FHWA 1998, George Hays 2004

7 Outline Significance of corrosion Corrosion mechanisms in concrete structures Corrosion monitoring in the field Ways to enhance and estimate service life

8 Why does iron corrode? 2 + 4H 2 O + 4Fe 4 ( ) 2 2O Fe OH ( OH ) 2 + O 2 + 2H 2 O 4Fe( ) 3 4Fe OH Iron Ore (Fe 2 O 3 ) Blast Furnace Molten material Rolling/ other manufacturing processes Dust to dust... back to its lowest energy state Corrosion = Extractive metallurgy in reverse ( OH ) 2Fe O H O 4 Fe ( OH ) 2 + O 2 + 2H 2 O 4Fe( ) 3 4Fe OH 2 + 4H 2 O + 4Fe 4 ( ) 2 2O Fe OH Exposure conditions

9 Concrete An Introduction Concrete microstructure and pore solution C-S-H Ca(OH) 2 ; NaOH; KOH Many other complex chemical compounds ph = -log 10 [H + ] Acidic Neutral Alkaline ph of concrete pore solution ph scale

10 Steel An Introduction Ferrous alloys with various elements such as C, Mn, P, S, Si, Ni, Mo, Cr, V etc.

11 Why steel embedded in uncontaminated concrete does not corrode? Steel does not corrode due to high ph of concrete pore solution A protective layer ( Passive film ) is formed A thin, invisible, and stable layer of initial corrosion products (i.e., iron oxides and hydroxides). However, corrosion can occur when exposed to aggressive conditions Hoar (1967)

12 What are the essential parts of a corrosion cell? ELECTROLYTE METAL Note: Current flows in the opposite direction as the electrons move.

13 What are typical electrochemical half-cell reactions associated with the corrosion of steel in concrete? Anodic (oxidation) reaction 2 Fe Fe + 2e + Cathodic (reduction) reaction 1 2 2( ) O + H O + e OH At the splash zone, corrosion can occur at a higher rate

14 Corrosion of Steel in Water with Oxygen Water 1 2 O H 2O + 2e OH Iron Cathode 2OH - 2OH - Fe(OH) 2 Fe 2+ Fe 2+ 2e - 2e Fe Fe + 2e Anode Anode & cathode coexist on the same piece of metal!

15 Two major types of corrosion in concrete structures Carbonation-induced corrosion General or uniform section loss Chloride-induced corrosion Localized, pitting or non-uniform section loss

16 Carbonation-induced corrosion

17 Chloride-induced corrosion Cl - Flaws in passive layer and high chloride content Pits growing at various locations

18 Chloride-induced corrosion Concrete 1 2 O H 2O + 2e OH FeCl 2 + 2H 2 O 2HCl + Fe (OH) 2 Fe Cl - FeCl 2 2H Cl - Iron 2e e + Fe Fe The process is regenerating and instead of spreading along the bar, corrosion continues at local anodes and deep pits are formed.

19 Pitting corrosion on strands and deformed bars due to chloride attack 7-wire strand Deformed bar

20 Typical locations for chloride induced corrosion and solutions?

21 Typical locations for chloride induced corrosion and solutions?

22 A huge concern about the epoxy-coated steel It is more dangerous to use scratched/damaged epoxy-coated steel than conventional uncoated steel Courtesy: M. Thomas

23 Volume of rust is ~6 times more than that of steel

24 Outline Significance of corrosion Corrosion mechanisms in concrete structures Corrosion monitoring in the field Ways to enhance and estimate service life

25 How to detect corrosion in concrete structures? ASTM C876 Standard Test Method for Half-Cell Potentials of Uncoated Reinforcement in Concrete If E < V, 90% chance of corrosion If E > V, 90% chance no corrosion If V < E < V, then? When using Cu/CuSO 4 reference electrode Mehta and Monteiro

26 Some commercially available equipment to detect corrosion using potential mapping Canin+

27 Some commercially available equipment to measure corrosion rate GECOR6 Connection to reinforcement Photo courtesy: James Instruments

28 Linear polarization resistance and guard ring techniques to measure corrosion rate Mehta and Monteiro

29 Outline Significance of corrosion Corrosion mechanisms in concrete structures Corrosion monitoring in the field Ways to enhance and estimate service life

30 The cost of corrosion is significant In 2012, the direct cost of corrosion in India was Rs. 2 lakhs crores/year! A corrosion protection strategy to minimise the repair and maintenance costs is a MUST Corrosion cost split-up in the USA (FHWA 1998) FHWA 1998, George Hays 2004

31 Service life = the time during which the structure is able to safely meet the user requirements Damage level The First corrosion Last initiation phase repair is typically repair the longest phase in the Maximum Allowable Damage Level service life of a structure Determining the duration of this phase is very important in determining a structure s service life Initiation Phase Propagation Phase Repair Phase Time Overall Service Life

32 Core-crete Strength Cover-crete Durability Cover-crete with low permeability is essential to achieve durability

33 Critical durability parameters 1. Rate of penetration (diffusion) of chlorides through covercrete (not corecrete) Cl surface a) w/c b) SCMs c) SPs 2. Cover depth (covercrete) 3. Chloride threshold of steel 4. Chlorides at the surface of concrete Cover depth Cl threshold

34 Adding excess water will reduce the strength and durability of concrete w/c = low w/c = high Thomas and Bentz 2001, Life 365

35 Adding SCMs will improve the strength and durability of concrete C S + C3S + H CH 2 + Water Cement Calcium hydroxide + S SCMs C -S- more H C -S- H Concrete becomes stronger and less permeable when silica fume, fly ash and/or slag are added

36 Adding silica fume will improve the strength and durability of concrete Pozzolanic reactions produce more C-S-H Thomas and Bentz 2001, Life 365

37 Adding fly ash and/or slag will improve the strength and durability of concrete y g Diffusion Coefficient, Dt (m 2 /s) 1E-10 1E-11 1E days 1E Thomas and Bentz 2001, Life 365 Age (years) PC 30% SG 40% FA m ( ) D t ref = Dref m = (%FA/50 + %SG/70) Pozzolanic reactions produce more C-S-H t t

38 Critical chloride threshold levels of various steels Critical Chloride Threshold Value, (lb/yd 3 ) A706 A615 Microcomposite SS304 SS Critical Chloride Threshold Value, (kg/m 3 ) Pillai and Trejo, 2005

39 Tools are available to estimate the service life of concretes with various SCMs

40 Effects of w/c and SCMs on service life Case No. w/c Fly ash (%) Silica fume (%) Time to corrosion initiation (years) Silica fume 0.6 concrete 0 was 0used in 40 5 Bandra-Worli Sealink

41 Life-365 software

42 Life-365 software

43 Life-365 software probabilistic version

44 Life-365 software

45 Life-365 software

46 Life-365 software

47 Life-365 software

48 Life-365 software Results in tabular format

49 Sensitivity of corrosion initiation time on input parameters Slope 54 o Slope 30 o Slope -35 o Pillai and Annapareddy 2013

50 Nomograph for estimating corrosion initiation time (based on Life-365 software) Pillai and Annapareddy 2013

51 Summary Significance of corrosion Carbonation-induced corrosion Chloride-induced corrosion Corrosion monitoring in the field Ways to enhance and estimate service life

52 Courtesy: Prof. Devanshu Pandit Where there is a will, there is a way! Thank You (pillai@iitm.ac.in)

53 A great text book with an emphasis on the durability of concrete With a lot of photos, videos, and lecture aids