Anaerobic Corrosion of Reinforcement

Ross O’Donovan2,a, Brian D O’Rourke1,b , Kieran D Ruane2,c and John J Murphy1,d

¹ Department of Civil, Structural & Environmental Engineering, Cork Institute of Technology,

Bishopstown, Cork, Ireland.

² RPS Consulting Engineers Ltd, Innishmore, Ballincollig, Cork Ireland.

aross.odonovan@rpsgroup.com, bbrian.orourke@cit.ie, ckieran.ruane@rpsgroup.com, djohn.justinmurphy@cit.ie

Keywords: reinforced concrete deterioration, anaerobic corrosion, black rust Abstract Anaerobic corrosion of steel reinforcement is rarely reported and limited literature is available on the subject. Corrosion of steel is an electrochemical process requiring a supply of oxygen in the presence of moisture. Steel corrosion product usually occupies a much larger volume than the un-corroded steel resulting in cracked or spalled concrete. If the supply of oxygen is restricted, black rust may be formed by the process of anaerobic corrosion. Black rust is not expansive, which makes it particularly difficult to detect in reinforced concrete. This paper presents a case study of anaerobic corrosion in the Mizen Bridge, with an in-depth review of anaerobic corrosion of reinforcement in concrete, outlining black rust formation, characteristics and detection methods. Introduction

This research paper presents a case study of the occurrence of anaerobic corrosion of reinforcement concrete. The Mizen Footbridge was one of the oldest reinforced concrete bridges in Ireland. Anaerobic corrosion of the reinforcement contained within the bridge members was first reported in 2002 and as a result the bridge was eventually demolished and replaced in 2009-2011 [1]. The demolition of the bridge at the end of its service life provided a unique opportunity to extract samples for analysis and investigation. Evaluated Structure

Mizen Head and Cloghan Island are located on the south west coast of Ireland (Fig. 1). The bridge was constructed in 1908-1909 to provide access to a fog signal station on Cloghan Island. The bridge incorporated the bridge designer’s patented Ridley-Cammel corrugated dovetail sheeting [2]. Restricted access to the bridge site necessitated a staged construction for each bridge member (Fig. 2) using different construction techniques such as precast concrete, in-situ concrete and composite in-situ concrete [2]. The estimated 28 day concrete cube strength for the concrete mix used at the time was 18.7 N/mm² [3].

Fig. 1 – Mizen Head and Cloghan Island [2].

Fig. 2 – Bridge Members [2].

Service Life During its service life of almost 100 years, the bridge underwent various maintenance, inspections and repairs as follows: 1. Prior to 1939, evidence of reinforced concrete deterioration was present and repairs were carried

out. The repairs consisted of cutting away defective areas of concrete and reinstating with new concrete [2].

2. In 1972 extensive remediation work was carried out on the bridge, which consisted of cutting away defective concrete and treating of exposed surfaces with a fungicide to remove organic growth. All of the structural members were coated with a bituminous material coated with mica and a fine hard granite aggregate. This may have been an attempt to repair visible defects and to seal exposed concrete surfaces from further chloride ingress [2].

3. Throughout the 1980’s and 1990’s, the bridge again showed signs of deterioration due to reinforcement corrosion. Previous remediation coatings were removed and concrete around corroded reinforcement was cut out. Concrete surfaces were cleaned, and the steel and concrete surfaces primed with polymer cement slurry. Finally, the concrete profile was restored with a modified cement mortar and finished with masonry paint. The strategy may have been an attempt to carry out localised repairs and to re-seal exposed concrete surfaces from chloride ingress [2].

Bridge Inspection In 2002 following significant visible rust staining on the bridge, the bridge owners, The Commissioners of Irish Lights employed MCOS Consulting Engineers (now RPS Consulting Engineers) to carry out an inspection and assessment of the bridge. High chloride levels were noted from concrete dust samples taken, and it was concluded that the structure was suffering from chloride attack [4]. Subsequent breaking of the concrete, revealed remains of the reinforcement comprising a gritty black rust. The surrounding concrete appeared in good condition (Fig. 3) [5]. Cathodic Feasibility Study A cathodic feasibility study was commissioned in 2005 which included electrical continuity testing, that involved identifying reinforcement locations using a covermeter and subsequent exposure by drilling into the concrete. Upon drilling to expose reinforcement, a moist black paste (black rust) was observed in one of the trestles (Fig. 4). The test results revealed that of the 129 individual reinforcement bars located on the bridge, 111 gave a reading of greater than 1� (ohm), indicating that the reinforcement had lost section and was no longer effective [6]. The bridge was eventually demolished and replaced in 2009 – 2011.

Fig. 3 – Gritty black rust observed in breakout at hanger. [5]

Fig. 4 – Black rust on drill bit during electrical continuity tests. [6]

Observations During Demolition Immediately prior to the demolition of the bridge, rust staining was visible through the bituminous material and several layers of finish coat. The bituminous material showed localised signs of failure due to material loss (Fig. 5) and in some instances trapped moisture was found between the concrete and bitumen membrane. Both black and dark green corrosion products were observed in encased reinforcement (Fig. 6, Fig. 7 & Fig. 8). Once exposed, the green rust later changed to red rust. The black rust was observed, both as a spongy product and as a gritty substance (Fig. 6 & Fig. 7).

Fig. 5 – Extensive rust staining through the external coating system.

Fig. 6 – Gritty black rust observed at underside of edge beam member.

Fig. 7 – Black rust as a spongy product.

Fig. 8 – Dark green rust on reinforcement.

Concrete Testing During the demolition of the bridge, reinforced concrete samples were extracted and a suite of concrete tests was carried out. Tests included concrete compressive strength to IS EN 12504-1:2000, chloride content to BS 1881-124:1988:10.2 and concrete pH to BS 1377-3:1990. Previous studies had identified that little or no carbonation of the cover layer had occurred [4]. The concrete pH was found to be in the range of 12.4 to 12.6 and it was found that the concrete compressive strengths were generally lower in flexural and tension members than in compressive members. Chloride profiles for a number of bridge elements were measured from dust samples taken at 25, 50 and 75mm depths. High chloride levels were noted (2.56% average chloride content by weight of cement). Typically, higher concentrations of chloride are be expected at the external surface with chloride concentrations reducing with increasing depth. In this instance the measured chloride concentrations were uniform throughout the concrete section (Fig. 9) which may be accounted for by the use of sea water or sea sand in the concrete mix [4]. Electrical resistivity tomography testing was carried out on some of the bridge elements. The results indicated an unexpected decrease in resistivity moving inwards from the surface (Fig. 10) [7]. This may be explained by leaching of free chlorides contained in the concrete mix, presence of concrete repairs or the presence of soluble black rust occupying voids in the concrete.

00.5

11.5

22.5

3

25 50 75Depth from external surface (mm)

% C

hlor

ide

by w

eigh

t of

cem

ent

Handrail Deck slab Edge beam

Hanger Post Arch rib

Fig 9 – Measured chloride content [2].

Fig. 10 – Electrical resistivity tomography mapping for bridge hanger element [7].

Anaerobic Corrosion Limited literature is available on anaerobic corrosion of steel in concrete and the formation of black rust. Studies of anaerobic corrosion of steel have involved diverse areas such as nuclear waste repositories, boiler feed water systems, underground pipe lines and archaeological samples [8]. Anaerobic corrosion in reinforced concrete is thought to occur when the oxygen supply is limited at active anodes typically resulting in non-expansive corrosion products. Corrosion under oxygen deficient conditions is considered to be more serious than normal aerobic corrosion (red rust), as it may be active for some time before there is any visible evidence at the surface of the concrete [9], [10]. Anaerobic Conditions Three simultaneously occurring conditions are believed to be necessary for anaerobic corrosion to take place. 1. Oxygen availability at the cathode [9]; 2. A path to allow free flow of electrons from anodic to cathodic sites [11]; 3. The steel at the anode must remain in solution in a depleted oxygen environment [12].

In order to obtain a flow of electrons from anodic to cathodic sites, the electrical resistivity of the concrete must be reduced to 12 k�cm or less [13]. This may occur through the presence of highly conductive ions such as chlorides or nitrates [11]. Corrosion rates of steel under anaerobic conditions are reported to vary from 0.1 to 7µm per year [8]. In the corrosion cell, it is essential that a flow of oxygen is present at the cathode. However, the anode does not require oxygen and it is at the anode where the reaction (corrosion activity) takes place. If the anode and cathode are separated and the anode is starved of oxygen the iron will stay in solution. This means there will be no expansive forces to disrupt the concrete [11], [12], [14]. The resulting corrosion product may be observed as white, green or black in colour (commonly referred to as green or black rust) [9]. The green product is believed to be a chloride complex while the black product is a combination of ferric and ferrous oxide (Fe2O3, FeO respectively) and is chemically similar to the mineral magnetite (Fe3O4) [11]. Corrosion of steel is an electro-chemical process and under anaerobic conditions the reaction of interest is the anodic dissolution of Fe [8], [12]:

Fe � Fe²+ + 2e� (1),

and the cathodic reduction of H2O

H2O + e� � ½ H2 + OH� (2).

The overall corrosion reaction can be written as

Fe + 2H2O � Fe(OH)2 + H2 (3).

The Schikkor reaction converts Fe(OH)2 to Fe3O4

3Fe(OH)2 � Fe3O4 + 2H2O + H2 (4).

Alternatively, the formation of Fe3O4 can be written as

3Fe + 4H2O � Fe3O2 + 4H2 (5). Fig. 11 and Fig. 12 show other anaerobic corrosion mechanisms from the literature for the formation of black and green rusts [7], [14].

Fig. 11 – Formation of black rust in culverts [15].

Fig. 12 – Generalised reaction scheme for the formation of green rusts (GR1, GR2) [8].

With reference to Fig. 12, it is worth noting that the oxide, lepidocrocite (�-FeOOH) may be reduced to generate magnetite when the presence of oxygen content is lowered, according to the following equation [16]:

8FeOOH(S) + Fe2+(ac) + 2e� � 3Fe3O4(S) + 4H2O(I) (6).

Reaction Products Black rust is described as being a spongy product, and soluble in water. The black rust is not a crystalline material and it will occupy voids in concrete spaces without exerting expansive pressure on the concrete. Rust staining may occur on the concrete surface. It is a relatively unstable product and readily converts to ferric oxide (red rust, Fe2O3) when exposed to oxygen. Therefore the initial period following the breakout of concrete may be crucial if the presence of anaerobic corrosion is to be correctly identified [11], [12], [15].

Anaerobic conditions in reinforced concrete, may occur when the anode is starved of oxygen by factors such as the presence of concrete protective membranes, concrete waterproofing systems, build-up of the rust layer, coating on steel reinforcement or within submerged concrete [12], [13], [17]. There are confirmed cases of anaerobic corrosion in bridge structures, prestressing cables in bridge beams (Fig. 13) and tidal culverts (Fig. 14) [11], [15], [18]. Following hydro-demolition on a marine jetty in south-west Ireland in 2012, black rust was observed on the freshly exposed reinforcement (Fig. 15) [19]. The presence of anaerobic corrosion may be indicated by the following detection methods: • Observation of red rust staining present on the concrete surface [5]; • Measurement of half cell potentials, electrical continuity testing, or electrical resistivity

tomography [9]; • Covermeter surveys of known reinforcement locations [15]; • Chloride testing in the concrete [2], [15], [16]; • Concrete drilling/breakouts [2], [10], [15].

Fig. 13 – Green rust on bridge beam prestressing strand [18]

Fig. 14 – Black rust on culvert reinforcement [15].

Fig. 15 – Black rust in recently exposed reinforcement, in concrete jetty [19].

Phenomenological Model It has been proposed that anaerobic corrosion of steel occurs at an advanced stage of the corrosion process due to the build up of corrosion product, preventing supply of oxygen to the anode. The phases of marine corrosion can be represented by a phenomenological model. A description of the different phases of marine corrosion is presented in Table 1 with reference to the phenomenological model shown in Fig. 16 [17], [20].

Fig. 16 – Proposed Phenomenological model for reinforcement corrosion in concrete [17]

Phase D1 D2/C0 C1 C2 C3 Description Diffusion of

chloride into the concrete. Early structural thermal and fatigue cracks present. Migration of chloride into the concrete.

Further diffusion ongoing. Chlorides have reached the steel. A chloride/hydroxide ratio threshold is reached resulting in initial corrosion of steel and formation of corrosion products.

Increasing corrosion activity with radial stresses, building longitudinal cracks.

Increasing longitudinal cracking and concrete spalling which enhances oxygen inflow.

Occurrence of anaerobic corrosion as the supply oxygen to the corroding bars is prevent by the build up of the rust layer.

Diagram

Table 1 – Chloride induced corrosion and cracking [20]. Conclusions In the case of the Mizen bridge the structure was effectively sealed from the external environment and further chloride ingress for a period of 37 years, after repair work in 1972, but the condition of the bridge continued to deteriorate. High chloride levels were recorded in the concrete in 2009-2011 and may have been introduced in the concrete mix or through diffusion from the marine environment. Under these conditions it is suggested that prior to 1972, chloride attack was at an advanced stage of propagation. This may have facilitated anaerobic corrosion of the reinforcement in later years from either the application of the concrete protective system, or due to the advanced corrosion of the reinforcement, or a combination of both. Limited literature is available on the subject of anaerobic corrosion in reinforced concrete. In recent years cases of anaerobic corrosion in reinforced concrete structures have been reported. The obvious danger of anaerobic corrosion in reinforced concrete is that there may be little or no sign of corrosion meaning that it can go undetected eventually leading to structural failure. Inspection standards and guidelines should be made aware of the conditions where anaerobic corrosion may exist. Further research into the formation of black rust is warranted so that methods for its detection, identification and treatment may be developed. References [1] K.D. Ruane, R. O’Donovan, N. O’Keeffe, E. Lehane, M. Coleman, K. Power, B. Minihane, E.

Collery, The Design and Construction of the New Mizen Head Footbridge, Paper accepted for publication in the Proceedings of the Institution of Civil Engineers, publication date September 2013.

[2] R.O’Donovan. B.D. O’Rourke. K.D. Ruane. J.J. Murphy, Reinforced Concrete Deterioration of a 100 year old Structure in a Marine Environment, Proceedings of Bridge and Concrete Research in Ireland Conference, September 2012.

[3] F.W. Taylor, S.E. Thompson, A Treatise on Concrete, The Plimpton Press, 1905. [4] K.D. Ruane, A. Healy, Assessment testing Mizen Head Footbridge Ireland, Proceedings of the

Institution of Civil Engineers, Bridge Engineering 157, September 2004 Issue BE3, 2004. [5] K.D. Ruane, Mizen Head Structural Evaluation, RPS MCOS Consulting Engineers, 2005. [6] K.P. Woodland, J. Preston, Mizen Head Footbridge, Cathodic Protection Feasibility and

Continuity Testing Report, Freyssinet Ltd, 2005.

[7] R. du Plooy, A. Azadeh, C. McNally, M. Richardson et al, Destructive and Non-destructive Condition Assessment of a 100 year Old Concrete Bridge, ICDS12 – International Conference Durable Structures, June 2012

[8] F. King, Technical Report 08-12 corrosion of carbon steel under anaerobic conditions in a repository for SF and HLW in Opalinus Clay, Nagra, October 2008.

[9] C. Arya, L.A. Wood, The Relevance of Cracking in Concrete to Corrosion of Reinforcement, Concrete Society, Technical Report No. 44, 1995.

[10] A. Karimi, N.R. Buenfeld, Service life prediction of concrete structure based on automated monitoring, Imperial College London, 2007.

[11] R.J. Cope, Concrete Bridge Engineering: Performance and Advances, Elsevier Applied Science, 1987.

[12] J.P. Broomfield, Corrosion of Steel in Concrete – Understanding, investigation and repair, E & FN Spon, 2003.

[13] CTI Consultants Pty Ltd, Corrosion of Steel Reinforcement in Concrete, CTI Technical Note C1, January 2004.

[14] Concrete Society Working Party, Diagnosis of deterioration in concrete structures Identification of defects, evaluation and development of remedial action, Concrete Society, Technical Report No. 54, 2000.

[15] H. Fatemi, M. Moore, R. Khatri, Importance of Detection of Black Rust Formation, Proceedings of Austroroads 8th Bridge Conference, June 2011.

[16] R. Vera, M. Villarroel, A.M. Carvajal, E. Vera, C. Ortiz, Corrosion products of reinforcement in concrete in marine and industrial environments, Material Chemistry and Physics 114, 2009.

[17] Melchers R.E., Li C.Q., Phenomenological modelling of reinforcement corrosion in a marine environment, ACI Materials Journal, January-February 2006.

[18] T.M Pape, R.E. Melchers, The Effects of Corrosion on a 45-year-old pre-stressed concrete bridge, Structure and Infrastructure Engineering, Vol 7, Nos 1-2, January-February 2011,

[19] Private communication between the authors with Mr Niall Mc Phillips, Lemac Ltd, January 2013

[20] R. Arndt, F. Jalinoos, NDE for corrosion detection in reinforced concrete structures – a benchmark approach, Conference proceedings Non-Destructive Testing in Civil Engineering, 2009.