Influence of Fly Ash on the Corrosion of Steel Reinforcement in Concrete A Review. by Theodore (Ted) Bremner University of New Brunswick, Canada

Transcription

1 Influence of Fly Ash on the Corrosion of Steel Reinforcement in Concrete A Review by Theodore (Ted) Bremner University of New Brunswick, Canada

2 Corrosion occurs when two different metals, or metals in different environments, are electrically connected in a moist or damp concrete. Rust forms with a volume larger than the metal consumed and in reinforced concrete the concrete cover spalls off, exposing the metal directly to the aggressive environment.

3 This will occur when: 1. Steel reinforcement is in contact with an aluminium conduit. 2. Concrete pore water composition varies between adjacent or along reinforcing bars. 3. Where there is a variation in alloy composition between or along reinforcing bars. 4. Where there is a variation in residual or applied stress along or between reinforcing bars. 5. Where there are imposed stray electrical currents.

4 Corrosion in Reinforced Concrete O 2 H 2 O secondary reaction Fe 2 O 3 H 2 O (rust) O 2 2Fe(OH) 2 anodic reaction 4(OH - ) cathodic reaction H 2 O 2Fe ++ 4e - - electron transfer anodic dissolution of iron cathodic region O 2 H 2 O

5 Can corrosion be avoided in reinforced concrete? Yes if: (a) Concrete is always dry, then there is no H 2 O to form rust. Also aggressive agents cannot easily diffuse into dry concrete. (b) Concrete is always wet, then there is no oxygen to form rust. (c) Cathodic protection is used to convert all the reinforcement into a cathode using a battery. This is not easy to implement because anodic mesh is expensive, and this technology is not easy to install and maintain.

6 (d) A polymeric coating is applied to the concrete member to keep out aggressive agents. These are expensive and not easy to apply and maintain. (e) A polymeric coating is applied to the reinforcing bars to protect them from moisture and aggressive agents. This is expensive and there is some debate as to its long- term effectiveness. (f) Stainless steel or cladded stainless steel is used in lieu of conventional black bars. This is much more expensive than black bars.

7 Can we avoid corrosion? No, not entirely: Concrete is not usually under water or continuously dry. Aggressive agents such as carbon dioxide, de-icing agents and/or sea water can diffuse into the best of moist concrete, and corrosion will eventually result.

8 If corrosion cannot always be avoided and economical solutions are required, the effects of corrosion can be minimized by making a better concrete. As will be shown later, Fly Ash added to a low w/c concrete produces a much enhanced corrosion resisting structure with no significant increase in cost.

9 The intrinsic nature of concrete is to be very protective of embedded steel. As soon as steel is placed in the high ph concrete (>12), a thin dense passive layer forms that is virtually continuous and the subsequent rate of attack is so low as to be insignificant.

10 Unfortunately when the carbonation front reaches the steel or when chlorides diffuse into the steel and reach a threshold level, this coherent protective layer is replaced by a porous incoherent expansive coating. The formerly protective oxide layer becomes an expansive porous oxide layer which causes cracking and eventually spalling of the concrete cover layer.

11 The Economical Solution: We must make concrete more protective of the steel reinforcement so that it will protect the passivating oxide layer. Making better concrete, using only Portland cement, will not make a substantial improvement. Fortunately Fly Ash added to a properly designed and cured concrete mixture will.

12 The key to protecting the protective passive layer is to make a much more impermeable concrete than we have made in the past. This can be done using a Fly Ash concrete with very low permeability, which will delay the arrival of carbonation and chlorides at the level of the steel reinforcement.

13 Fly Ash makes concrete almost impermeable Fly Ash is a finely divided silica rich powder that, in itself, gives no benefit when added to a concrete mixture, unless it can react with the calcium hydroxide formed in the first few days of hydration. Together they form a calcium silica hydrate (CSH) compound that over time effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride ions. By reducing ion diffusion, the electrical resistance of the concrete also increases as Datta, Garg and Rehsi have shown (1).

14 How does Fly Ash reacting with Ca(OH) 2 reduce permeability? The CSH compound formed in the presence of moisture in the void space within the hydration products effectively reduces the number and continuity of the capillary pores. This reaction, because it entails diffusion, takes place in a moist environment at a decreasing rate that can take a decade or more to reach completion.

15 The Role of Water in Concrete Water in concrete exhibits a Dr. Jekyll and Mr. Hyde behaviour. Water is essential for the proper hydration of a Portland cement pozzolan combination such as one containing Fly Ash. Also fully saturated cement paste is almost impermeable to both the oxygen needed for corrosion, and to carbon dioxide that destroys the passivating layer.

16 On the other hand, too much water leaves the concrete highly permeable. Also the excess water in the pores contributes to the corrosion process. In addition, it provides a medium whereby chloride ions can diffuse easily toward the reinforcement and destroy the passive layer.

17 An antidote for excess water in concrete is usually in the form of a superplasticizer.

18 Factors other than permeability influencing corrosion (i) A portion of the chloride ions diffusing through the concrete can be sequestered in the concrete by combining them with the tricalcium aluminate to form a calcium chloro-aluminate (Friedel s salt). Dass and Rajj (2) have shown that this can be a significant effect in reducing the amount of available chlorides thereby reducing corrosion.

19 (ii) In low-strength concrete and in hot dry climates, carbon dioxide can diffuse into the concrete, react with calcium, sodium and potassium to form carbonates and in the process lower the ph to a level where the passivating oxide layer is no longer stable. (iii) The chloride levels needed to initiate corrosion decrease as the level of cement replacement increases (3).

20 Fortunately increased impermeability of concrete made with Fly Ash more than compensates for the above three negative effects when Fly Ash is used to replace cement as the following experiments will show.

21 Test #1 - Uncracked Concrete Malhotra et al. (4) carried out tests on a series of slabs 150-mm thick made with 20 mm maximum size aggregates and with reinforcing bars 20, 40, 60 and 80 mm from the top surface. After moist curing for seven days and air drying for 21 days, the slabs were continuously ponded for eight years with a 4% calcium chloride solution.

22 Test # 1 Uncracked Concrete (cont d) The effect of fly ash in reducing chloride ions ingress measured at 8 years at 18 mm (and 30 mm) below the surface. Mixture w/(c + fa) fa/(c + fa) % Cl, % by mass of concrete Cl, % by mass of cementitious materials (-) 0.06 (-) (.02) 1.07 (0.12) (-) 0.14 (-) (0.12) 1.19 (0.84) (0.01) 2.43 (0.08) (0.24) 2.60 (2.02) The very low Cl levels in Mixtures 1 and 3 clearly indicate the very long service life that can be expected with high volume fly ash concrete.

23 Test # 1 Uncracked Concrete (cont d) The steel reinforcing bars in concrete incorporating moderate (25%) and high (~55%) volumes of fly ash and ponded with 4% calcium chloride solution for 8 years showed superior performance in regard to the corrosion activity compared with those in the corresponding control Portland cement concrete.

24 Test # 1 Uncracked Concrete (cont d) Of the various mixtures tested, only the W/C = 0.57 concretes showed evidence of corrosion of the embedded reinforcement when the slabs were broken open at eight years. The reinforcement embedded in the concrete with 25% replacement of cement with fly ash that had a cover of 20 mm showed signs of corrosion whereas the control specimens with only Portland cement showed signs of corrosion with both 20 and 40 mm of cover.

25 Test #2 Uncracked Concrete Gu et al. (5) in 1999 and Quian (6) in 2003 reported on a series of concrete slabs 153 mm thick made with 19 mm maximum size aggregates that had been moist cured for 7 days followed by exposure to laboratory air for approximately 50 days. The top surface was ponded with 3.4% sodium chloride solution in the laboratory.

26 Test # 2 Uncracked Concrete (cont d) After 5.3 years the following results were obtained for bars with 25 mm cover. Min. No. Type of SCM SCM % W/C Corrosion Rate μa cm 2 N1 N2 N7 N8 N3 N5 N6 Fly Ash Type F Fly Ash Type C Silica Fume Slag Where <0.1 is passive condition and >1.0 is high corrosion. <<0.1 <<0.1 <<0.1 <0.1 < to >1.0 >1.0 These tests clearly show the advantage of using supplementary cementing material (SCM) such as fly ash in low w/c concrete.

27 Test # 3 Uncracked Concrete Thomas et al. (3) in 1986 installed a series of steel-reinforced concrete prisms at Shoeburyness in the Thames estuary, England and after ten years the prisms were evaluated. The reinforcement steel mass loss in % is given below for reinforcement with 20 mm cover. Mass Loss Fly Ash % W/C + Fly Ash

28 Test # 3 Uncracked Concrete Conclusions: Fly Ash concretes showed substantial increased resistance to the penetration of chlorides which resulted in reduced corrosion of steel bars embedded in the concrete. Chloride ion threshold levels were 0.70, 0.65, 0.50 and 0.2 (% by mass of cement and Fly Ash) for Fly Ash contents of 0, 15, 30 and 50% respectively.

29 Uncracked Concrete Conclusion Aggressive agents diffuse into the concrete and when they reach the embedded reinforcement at a threshold level, corrosion takes place. Eventually this results in cracking in the plane of the bar leading to spalling of the concrete cover. Fly Ash, when used properly in a concrete mixture, will substantially delay this distress

30 Like Death and Taxes Concrete will crack Beeby (7) quotes results from exposure tests done mainly in Germany to arrive at the conclusion that the crack width has no significant influence on the amounts of corrosion that will occur during the life of the structures. Beeby goes on to state: a crack perpendicular to the line of the bar does not constitute a corrosion risk in practice, while the existence of longitudinal crack may.

31 Cracking of reinforced concrete members Step 1. Flexural cracks form in properly designed members. Step 2. Cracks act as portals for aggressive agents to reach the reinforcement. Step 3. Micro corrosion takes place on the exposed steel at the crack and more damagingly along the steel/concrete interface adjacent to the crack faces. Step 4. Corrosion products that form on this interface cause cracking to form in the plane of the reinforcing bars and spalling ensues.

32 Francois and Aaliguie (8) mapped the progressive carbonation and chloride ingress in a simply supported beam subjected to a load at mid-span. The aggressive agents can be seen to move along the crack and along the reinforcement where the crack intersected a bar. The following figures show the appearance of the carbonated area and the area contaminated with chlorides that is at right angles to the crack with carbonation spreading in the concrete adjacent to the steel from the point of convergence of the crack and along the longitudinal reinforcement under stress.

33 Carbonated concrete zone in front of a crack (Ref. 8)

34 Shape of the chloride contaminated concrete zone in front of crack (Ref. 8)

35 Francois and Aaliguie (8) also showed that there is about a 30% increase in chloride content on the tensile surface as compared to the compressive surface of a beam and this increases to 100% at a depth of 35 mm from the surface of a flexural member. Carbonation depth is influenced in a similar manner, as can be seen in the following figure.

36 Extent of carbonation at support (end of beam section) and at mid-span (middle section (Ref. 8)

37 Test # 1 Cracked Concrete Montes (9) tested a series of 27 slabs that were cast with and without fly ash and with two 15ø x 260- mm long reinforcing bars intersecting either a construction joint or preformed cracks that were 0.25 and 0.50 mm wide. The reinforcing bars had a cover of 20 mm, and a 12.5 mm maximum size coarse aggregate was used. The cement was Type I normal portland with 8% silica fume.

38 Test # 1 Cracked Concrete (cont d) After 7 days moist curing and 21 days drying in laboratory air the concrete was subjected to two cycles per day at 26ºC in the wet cycle and 55ºC in the dry cycles. Also a series of 54 slabs were cast and placed slightly below high tide level at Treat Island, Maine. The results are as follows:

39 Corrosion current density at 12 months in μa/cm 2 - Laboratory Tests (averaged over the bar) Fly Ash Replacement % w/c + Fly Ash With 0.0 mm Preformed Crack Construction Joint With 0.25 mm Preformed crack * With 0.50 mm Preformed crack * * * Improperly Consolidated Concrete

40 Corrosion current density at 2 years at Treat Island μa/cm 2 Fly Ash Replacement % w/c + Fly Ash With 0.0 mm Preformed Crack Construction Joint With 0.25 mm Preformed Crack With 0.50 mm Preformed Crack

41 Number of bars with longitudinal cracks caused by corrosion at transverse cracks in 0.45 W/C + Fly Ash specimens Laboratory testing Fly Ash % Months Accelerated Testing Note: Specimens with w/c + Fly Ash of 0.29 and 0.37 had no longitudinal cracks

42 At 12 months, all of the specimens subjected to accelerated testing were broken open to measure areas of corrosion and amount and depth of corrosion pitting. Fly Ash appears to have no influence on these parameters. Pitting depths ranged from 0.2 to 1.7 mm and areas of corrosion ranged from 0.4 to 91.1 cm 2.

43 Specimens with deep pitting had generally smaller areas of corrosion. Testing of specimens at Treat Island Marine Exposure Site will continue, and in time should provide more specific information.

44 Corrosion spreading along the reinforcing bar at either side of the crack appears to be caused by crevice corrosion. Corrosion occurs at crevices where oxygen and chlorine ion concentration is lowest. Perhaps this type of concentration cell arises because the mobility of iron (ionic radius of 0.74 Å) is greater than that of the oxygen (1.40 Å) and chlorine (1.81 Å). Fly Ash is very effective in closing the coarser pores but may not be as effective in closing the finer pores that provide a conduit for iron in ionic form.

45 Conclusion Fly Ash concrete, if properly proportioned, placed and cured, makes a substantial improvement in enhancing the protection of embedded reinforcing steel from corrosive agents. It is also effective in reducing the longitudinal cracks that form as a result of crevice corrosion that develops at transvers flexural cracks.

46 Crevice Corrosion Oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal. Oxygen content of liquid at the mouth of the crevice which is exposed to the air is greater, so a local cell develops in which the anode, or area being attacked, is the surface in contact with the oxygendepleted liquid (from Corrosion Basics NACE).

47 Crevice Corrosion of Rebar Has Some Similarities with Filiform Corrosion The head of the advancing filament becomes anodic, with a low ph and a lack of oxygen, as compared with the cathodic area immediately behind the head where oxygen is available through the semipermeable film. Corrosion proceeds as the cathode follows behind the anodic head (from Corrosion Basics NACE).

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