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Observations about the Corrosion of Reinforcement in Marine Environments

Robert E. Melchers 1 ABSTRACT The corrosion of reinforcement bars and prestressing strands in concrete structures such as bridges and coastal and harbour facilities is a significant maintenance and renewal issue. Mostly such structures are considered to have useful lives of 2-3 decades, but, surprisingly perhaps, there are a considerable number of structures still in existence with much longer useful lives, some showing little evidence of reinforcement corrosion despite being either exposed directly to hostile marine environments or very likely to have been made with seawater as mixing water. A small number of the latter, and a recent review of the performance of many (>300) of such structures is given. That review did not support the conventional thinking that modern cements were responsible for the poorer long-term durability of modern structures. Instead it was proposed, on the basis of electrochemical arguments and observed experience, that the properties of the aggregates is a key factor. To test this hypothesis a series of exposure tests were commenced several years ago in which seawater was used as mixing water and with different aggregates, including those rich in calcium-carbonate, which the conventional wisdom suggests lowers the concrete pore solution sufficiently to initiate reinforcement corrosion. Preliminary results from these tests are described and it is shown that calcium carbonate concretes, even with seawater, do not lead to early corrosion initiation. Results for the effect of higher cement content, water cement ratio and the presence or absence of salt water in the concrete mix also are presented. KEYWORDS Corrosion, Seawater, Reinforcement, Concrete, Structures.

1 Centre for Infrastructure Performance and Reliability, The University of Newcastle. Australia,

[email protected]

Robert Melchers

2 XII DBMC, Porto, PORTUGAL, 2011

1 INTRODUCTION The long-term durability of reinforced concrete structures particularly in marine environments remains a matter of concern despite many years of practical experience with reinforced and prestressed concrete and very extensive research programs. The consensus of opinion favours very dense concretes with high cover to the reinforcement on the basis that this will reduce the diffusion of chlorides (or carbon dioxide or both) from the external environment to the reinforcing bars. Various additives also are known to further reduce the pore-spaces and hence the diffusion of water, oxygen and chlorides and/or carbon dioxide to the reinforcement. Most of the practices currently considered desirable are derived from short-term laboratory experiments and practical observations. Nevertheless, there are two broad aspects that should be of concern in the present state of understanding. One is that despite these practices, there are numerous examples of relatively modern apparently well-designed reinforced concrete structures that show signs of reinforcement corrosion giving, overall, likely shorter serviceable lives than might be expected. Particularly in coastal environments, evidence of corrosion initiation for reinforcement is observed already after a few years and for many modern structures serious cracking and spalling of the concrete occurs within 2-3 decades. Conversely, for a considerable number of reinforced concrete structures, including many that would today be considered of inferior quality, there is little or no evidence of reinforcement corrosion even after many decades of exposure in severe marine environments [Melchers & Li 2009a]. Most commonly the difference in long-term durability of reinforced concrete structures is attributed to changes in the making of cement and having had a negative impact on durability. Changes in cement-making originated in the USA during the 1930s and were aimed at producing higher strength concretes with higher earlier strengths. This was achieved with higher clinkering temperatures and finer clinker grind. The changes were adopted later in the UK and Australia - not until the 1960 - despite vocal opposition on the grounds that durability would be adversely affected. Certainly the composition of the cements changed somewhat [Nixon & Spooner 1993] but there is only anecdotal evidence that this affected durability. Using reports of the actual behaviour of reinforced concrete structures and reasonable estimates of durability, a recent study indicated only poor correlation between changes in cement making practice and reinforcement corrosion severity [Melchers and Li 2009a]. Since that study a number of other cases have become available - all examples of structures that have shown a high degree of durability despite aggressive environments and in many cases without the benefit of what is today considered to be good practice [Melchers 2010a,b]. This raises the question whether the current conventional wisdom is sufficient to explain long-term durability. The next section summarizes observations of the actual durability of older reinforced concrete structures. This is followed by a brief review of some fundamental aspects of the corrosion of steel in marine environments and the effect of the concrete surrounding the steel. It will be argued that assumptions commonly made in the reinforcement corrosion literature are not always in accord with fundamental corrosion science principles. The final section describes, briefly, on-going long-term durability tests of reinforced concrete samples made with various combinations of materials, including seawater as mixing water and seawater with additional salt added, and various types of aggregates. 2 OBSERVATIONS FROM ACTUAL STRUCTURES The present work was prompted by observations of the surprising long-term satisfactory behaviour of multiple, simple reinforced concrete elements (Fig. 1a), made without high level technology or modern quality control (Fig. 1b) and for more than 65 years subjected to North Sea exposure [Melchers & Li 2009b]. One surprising feature was that this concrete showed very high levels of chloride contamination and very high content of seashells (Fig. 1c). This raised the question whether high levels of carbonates in the concrete might be beneficial, even though at first sight this contradicts the notion of ‘carbonation’. In the event, an extensive literature survey was conducted and this found

Marine Corrosion of Reinforcemen

XII DBMC, Porto, PORTUGAL, 2011 3

many reports (300+) of cases (54 groups of data ranging from just one element to more than 1000) where reinforcement corrosion took many years to initiate and to become sufficiently active for remedial or other action to be taken.

(a) (b) (c)

Figure 1. (a) Reinforced concrete balustrade at Arbroath, Scotland (b) typical cover of balustrade railing and (c) evidence of seashells in the concrete [Melchers & Li 2009b].

Each case was assessed for two parameters, the time to initiation and the time to active corrosion, both judged from all information available. Laboratory based experimental observations, and in particular those from accelerated tests using impressed currents to stimulate the anodic corrosion reaction, were ignored. The corrosion science literature recognizes that such tests produce only relative observations, not results that can be related directly to the behaviour of actual structures, particularly when bacteria are involved [e.g. Cord-Ruwish 2000]. Because of lack of information, the influence of concrete mix and concrete cover, of average or range in temperature and local weather conditions was ignored and treated as variability in the data. Estimates for the degree of chloride contamination were considered but relative to the other influences this parameter turned out to be of only of secondary importance, particularly for the time to active corrosion [Melchers and Li 2009a]. Fig. 2 shows that despite scatter in the data, the trends for calcareous (alkaline) and normal aggregates are remarkably different.

(a)

(b)

Figure 2. Normal cumulative probability plots for (a) time to initiation and (b) time to active corrosion showing normal aggregates and 'alkaline' aggregates.

Figure 2(a) also shows that some data may have been influenced by alkali-aggregate or alkali-silicate reactivity of the aggregate. Typically these reactions cause early concrete cracking and thus loss of adequate cover. Evidently, this is a different failure mode and does not belong to the same data cohort as failures resulting primarily from reinforcement corrosion [Melchers & Pape 2010]. 3 SUBSEQUENT OBSERVATIONS In parallel to the literature survey, several older existing concrete structures were examined, in some cases as a direct result of the author being alerted to their existence by practitioners who had become

Robert Melchers


aware of the thesis underlying the present and earlier papers. A number of these structures are summarized in Table 1. Overwhelming they were found to be consistent with the findings reflected in Fig. 2. Some of structures are heritage protected and it was not possible to obtain cores but, as indicated in Table 1 and in the references, in many cases there is sufficient circumstantial evidence to indicate they were, or are likely to have been, constructed using calcareous sands, limestone and/or unreactive dolomite aggregates or, in some cases, blast furnace slag as aggregate.

Table 1. Examples of older reinforced concrete structures.

21st Street Bridge, Tacoma, Washington, USA, 1909. This early bridge has had minimal exposure to carbonation effects and chloride ions. The presence of seashells and acid soluble chloride contents around 0.2% by weight of cement indicates concrete was mixed with seawater. It was estimated that reinforcement corrosion did not commence until some 60 years after construction and most likely is the result of carbonation. Portland Cement Works, exposed to southern Pacific, Maria Island, Tasmania, Australia, 1923 Maria Island consists of limestone that was used for early cement-making and also for construction of the works. It was abandoned in 1930 and has been without maintenance until recently. The four RC cement silos and clinker house largely are free from reinforcement corrosion. Akthough there are areas of spalling, there is little sign of heavy rusting or of rust staining on the concrete.

Beach-side Railing, Mordialloc, Australia, 1928 Directly on marine environment at Port Philip Bay and not maintained until recently. There were very limited signs of reinforcment corrosion. Highly likely to have been constructed using local beach sand known to be rich in seashells. Photograph taken prior to recent maintenance.

Tongue Sands Tower, Thames Estuary, UK, 1943 Two 7.3m diameter, 18.3m high RC cellular caissons on a RC foundation pontoon, several miles off the UK coast was investigated in detai. Reinforcement corrosion was confined mainly to areas where concrete cover was much less than specified (32mm) or where inferior local mortar mixes had been used. Overall in '... remarkably good condition.' Many cores showed the presence of seashells in the concrete and high chloride levels, suggesting the use of beach sands and seawater as mixing water. High levels of ‘carbonation’ also detected.

Two-storey Pill Box Forts, Thames Estuary, UK, 1943 Constructed in groups of 4-5 and connected together with structural steelwork walkways and surmounted with steel gun platforms, given extensive exterior rust staining. These have all disappeared - presumably corroded away. Remarkably little external evidence of reinforcement corrosion. It is reasonable to assume that the reinforced concrete for these structures also used beach sand for aggregate and seawater as mixing water.



Pacific Ocean Harbor fortifications, Newcastle, Australia, 1943 Prior to its first ever maintenance in 2006 showed some loss of cover and little reinforcement corrosion. Coarse blast furnace slag (BFS) aggregate, present in concrete, was a common by-product of the iron and steel making and was widely used as an aggregate for concretes. Its a high alkali content maintains pH at around 9-10. Photograph taken just prior to recent maintenance.

1.6 km long RC Balustrade, Arbroath, Scotland, 1943 More than 90% of the 1000 original RC elements exposed to North Sea marine atmosphere and splashing show little external rust staining, spalling or cracking. The other 10% were replaced in 1968 and 1993 and nearly all of these show extensive and severe longitudinal cracking and damage. Clear evidence of seashells in 1943 concrete mix, suggesting also seawater was used as mixing water. Mulberry Harbour Caisson, Langstone Harbour, UK, 1943-4 Few ofmore than 200 Phoenix units built still survive. Built very quickly with un-skilled labour, using available materials, withoutdurability considerations. The unit at Langstone Harbour broke during beaching. It has little obvious reinforcement corrosion. Use of beach sands and flint nodules (found in chalk i.e. limestone) likely. Mulberry Harbour Caissons, Portland Harbour, UK, 1943-4 The two caissons in Portland Harbour are offshore and in completely exposed locations. They appear to be in generally good condition showing reinforcement corrosion around the mean-tide zone where cover appears low, but little reinforcement corrosion elsewhere. There is some concrete spalling. Concrete lighters, Purton, Severn Estuary, UK, 1940's Reinforced concrete lighters were used for bulk transport in fresh and tidal waterways but fell out of favour. Some are partly buried on the southern bank of the Severn Estuary to aid shore protection. They are exposed to high tidal ranges but show little reinforcement corrosion. Where there was physical damage to the concrete cover there is moderate reinforcement corrosion. The fine aggregates appear to be calcareous.

Concrete piles, Bar Beach, Newcastle, Australia, 1940’s? Concrete piles supporting a reinforced concrete slab located immediately adjacent to the Pacific Ocean. The underside of the slab shows reinforcement corrosion but the piles are essentially free from reinforcement corrosion, despite the bars being visible (at arrows) from the top and the concrete cover varying significantly. There is no rust staining. The concrete contains BFS aggregate.

Robert Melchers


Florida bridge piles (1000+) in tidal zone, Florida panhandle region, USA, 1960s? Very low incidence of signs of reinforcement corrosion, despite approx 35 mm concrete cover. Steel showed no or very light rusting. Chloride penetration varied widely and poorly correlated to cracking and corrosion. Porewater pH measurements still 11-13. Some aggregates are limestone, others 'river rock', which US Geological Survey information indicates is predominantly limestone.

Concrete Ammunition Lighters (6 + others), Sydney Harbour, Australia, 1970s Essentially RC boats of cast construction used in marine conditions. Except for one limited hairline crack, no evidence of any reinforcment corrosion. Igneous large aggregate but most likely calcareous sand from Kurnell Beach, Sydney’s only sand supply at the time. 4 EXPRIMENTAL OBSERVATIONS A long-term test program, commenced in 2003 is examining the effect of several factors on corrosion of reinforcement. Some 700 samples, 150mm x 50mm x 50 mm in size, with one centrally-placed 6 mm diam. mild steel round bar, were made in steel forms under high quality control, weight-batching of air-dried sand and aggregates and highly controlled, timed vibration. All bars were sourced from the same batch and, except as noted, the same cement, sand and aggregates were used throughout, and natural seawater was used as mixing water so as to accelerate the time to initiation of reinforcement corrosion. The specimens were cured for 4 weeks in the fog-room prior to being air-stored in the laboratory while the remaining specimens were made. The last specimens were made during June 2004. Six months later the specimens were placed in the fog room on open web plastic racks to ensure specimen were exposed on all sides. Six years later (2010) none of the specimens showed visible external evidence of reinforcement corrosion such as rust stains, cracking or spalling. After 3 years exposure (2007) one of each type of specimen was broken open and the reinforcing bars visually examined. The surface condition of the bars ranged from apparently 'as new' condition to extensive but still quite thin rust coverage. Since mass loss differentials would be problematic, the relative visual surface corrosion condition was rated using the simple scale: 0 = no rust visible, 1 = small spots of rust, 2 = rust patches over 50% of surface, and 3 = most of the surface covered by rust. Herein attention is restricted mainly to the effect of (i) saline mixing water and (ii) the use of limestone as the main aggregate. Table 2 gives some of the rating results. Figure 3 shows the relative corrosion ratings as a function of aggregate/cement ratio. Analysis of the ratings showed almost no difference as a function of water/cement ratio. It shows that limestone aggregate appears to lower the relative corrosion, that low-heat cement may be beneficial, that there is an effect apparently due to aggregate/cement ratio and, perhaps surprising in the conventional wisdom in reinforcement corrosion, that chloride content does not necessarily increase corrosion severity [Melchers 2010b].

Table 2. Selected test program and reinforcement surface rating result after 3 years exposure.

Series Mark a/c w/c Rating Series Mark a/c w/c Rating Base case B1 2 0.5 1.5 + 50%NaCl D2 4 0.8 0.5 B2 0.65 1.5 D3 0.95 1.5 B3 0.8 1.5 D5 6 0.8 1 B5 4 0.8 2 D6 0.95 1 B6 0.95 2 Limestone E2 2 0.8 0.5 B8 6 0.8 1 aggregate E3 0.95 1 B9 0.95 0.5 E5 4 0.8 2 E6 0.95 1



Figure 3. View of reinforcing bars (left) and relative corrosion rating (right) as a function of aggregate cement ratio showing effect of limestone aggregate, low heat cement and added salt

[Melchers 2010b]. 5 DISCUSSION Chlorides in seawater (or in de-icing sprays) diffusing into the concrete conventionally are considered responsible for reinforcement corrosion. The use of seawater as mixing water for concrete has been prohibited since the 1960s. This was based, in part, on (short-term) laboratory findings [e.g. Shalon & Raphael 1959] that showed earlier corrosion initiation of steel bars in calcium hydroxide solutions when chlorides were present. Similar findings were obtained for realistic concrete samples made with seawater as mixing water (and for curing) when exposed to actual marine exposure conditions, at least within the first 3 years of exposure. But after 5 years this was not the case and after 10 years both the corrosion area and the average corrosion rates were about one third of those for concretes made with fresh waters [Boqi et al. 1983]. This is consistent with the field exposure observations at Arbroath. The role of NaCl in the corrosion of steel has long been debated in the corrosion science literature [e.g. Schilling 2006]. Physical chemistry free energy considerations show that of all the possible reactions the conventional oxidation reaction of iron (Fe) with water (H2O) and oxygen (O2) is the most likely to occur. It does not involve NaCl - the chloride ions are 'spectators'. The primary rate-controlling step in the corrosion of steel in water is the rate at which the electron acceptor (usually oxygen) can reach the corroding surface. This does not depend, in a significant sense, on the salinity of the water, as shown both in laboratory and in field investigations [Mercer & Lumbard 1995, Melchers 2006]. To be sure, under low oxygen conditions FeCl2 and other corrosion products involving chlorides may be produced, such as the chloride-bearing β-FeOOH [Waseda & Suzuki 2006]. Salt is hygroscopic and this may increase the 'time of wetness' for situations where the concrete cover is no longer effective. The apparent positive influence of calcareous aggregates (shell fragments, calcium carbonate or non-reactive dolomite sand or aggregates, or blast furnace slag aggregates) has support in controlled corrosion science experiments [Davies & Burstein 1980]. Also, chemical thermodynamics show that increasing the alkali content delays corrosion initiation and reduces the rate of corrosion. The main issues can be summarized as [Melchers & Pape 2010]: 1. The alkalinity of the porewaters, that is, the buffering capacity of alkalis, determines when

corrosion initiation will occur. Conventionally this has been associated only with the alkalinity derived from the reaction of the setting cement.

2. Laboratory studies show that (calcium) carbonate has a buffering effect and that higher concentrations of calcium carbonate inhibits the initiation of corrosion.

Robert Melchers


3. Under higher concentrations of (calcium) carbonate the presence of chloride ions does not have a strong influence on the initiation of corrosion.

4. Initiation of chemical reactions, including corrosion initiation, is determined by free energy considerations. When calcium carbonate can be involved in the chemical reaction there is the possibility to form protective FeCO3 (siderite). It will tend to retain concrete pH at around 10.

5. Fundamental corrosion science, thermodynamics and field observations show that corrosion initiation cannot commence until the local pH has dropped below about 9.

6. The most likely mechanism for lowering of pH is leaching of the alkalis [Popovics, et al. 1983, Melchers & Li 2006]. Both KOH and NaOH are highly soluble. The low solubility of Ca(OH)2 is increased by several orders of magnitude in saline conditions.

7. Outward alkali leaching is simultneous with inward diffusion of chlorides and governed by the same factors - concrete permeability and external surface washing. This may be the reason that most attention has been given only to the most obvious mechanism – chloride diffusion.

8. In marine environments the presence of calcium carbonate in concrete, usually attributed to 'carbonation' is much more likely to be the direct result of deposition since seawater is well-known to be (super-)saturated with carbonates. Evidently, this will inhibit oxygen diffusion.

It is considered that these points, taken together, go a long way towards explaining the various observations for long-term exposures of reinforced concrete structures in realistic environments, as opposed to results from short-term laboratory experiments. They also cast a new light on the role of chlorides and of carbonation in the corrosion of reinforcement steel. 6 CONCLUSION Field and controlled laboratory observations show that alkali aggregates such calcium carbonate, non-reactive dolomite, or sands containing seashell fragments can add to the alkalinity of the concrete and delay the initiation and the rate of reinforcement corrosion, provided other deterioration mechanisms do not interfere. Moreover, calcium carbonate (cf. ‘carbonation’) does not lower the pH sufficiently immediately adjacent to the reinforcement to permit corrosion initiation. Additional mechanisms such as the leaching of alkalis must occur also. The chloride content immediately adjacent to the reinforcing bars has little effect on the time to active corosion since the progression of corrosion is governed by the rate at which oxidation can occur. In real seawaters this is inhibited by the build-up of carbonates. ACKNOWLEDGMENTS The support for this project provided by the Australian Research Council and by many people in industry and in various countries (acknowledged in detail elsewhere) is much appreciated. REFERENCES Boqi C, Dinghai H, Hengquan G. & Yinghao Z. 1983. Ten-Year field exposure tests on the endurance of reinforced concrete in harbor works, Cement and Concrete Res. 13: 603-610. Cord-Ruwisch, R. 2000. Microbiologically Influenced Corrosion, Environmental Microbe-Metal Interactions D.R. Lovely (Ed.), Washington: ASM Press, 159-173. Davies, D.H. & Burstein, G.T. 1980. The effects of bicarbonate on the corrosion and passivation of iron, Corrosion 36(8) 416-422.



Melchers R.E. 2006. Modelling immersion corrosion of structural steels in natural fresh and brackish waters, Corrosion Science 48(12) 4174-4201. Melchers, R.E. 2010a. Observations about the time to commencement of reinforcement corrosion for concrete structures in marine environments, Consec 2010, Mexico, CRC Press, Boca Raton, 617-624. Melchers R.E. 2010b. Observations about the performance of reinforcement in marine environments, ASEC2010, Sydney, CDROM. Melchers, R.E. & Li, C.Q. 2006. Phenomenological modelling of corrosion loss of steel reinforcement in marine environments, ACI Materials Journal, 103(1) 25-32. Melchers R.E. & Li C.Q. 2009a. Reinforcement corrosion initiation and activation times in concrete structures exposed to severe marine environments, Cement and Concrete Research, 39: 1068-1076. Melchers, R.E. & Li, C.Q. 2009b. Reinforcement corrosion in concrete exposed to the North Sea for over 60 years, Corrosion, 65(8) 554-566. Melchers R.E. & Pape T. 2010. Aspects of long-term durability of reinforced concrete structures in marine environments, Medachs2010, La Rochelle, 28-30 April 2010. Mercer A.D. & Lumbard E.A. 1995. Corrosion of mild steel in water, British Corrosion J, 30(1) 43-55. Nixon P.J. & Spooner D.C. 1993. Concrete proof for British cement, Concrete 27(5) 41-44. Popovics, S., Simeonov, B. & Barovsky, N. 1983. Durability of reinforced concrete in seawater, Corrosion of Reinforcement on Concrete Construction, Ellis Horwood, Chichester, 19-38. Shalom, R. & Raphael, M. 1959. Influence of sea water on corrosion of reinforcement, Journal of the American Concrete Institute, 30(12) 1251-1268. Schilling M. 2006. Soluble salt frenzy - junk science, Corrosion & Materials, 31(5) 10-15. Waseda Y. & Suzuki S. (Eds) 2006. Characterization of Corrosion Products on Steel Surfaces, Springer, Berlin.

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