Carbonation of Reinforced Concrete

Carbonation is the single most common cause of reinforcement corrosion in above ground structures and although many construction professionals and contractors are familiar with the progression of carbonation through concrete and the consequential effects on reinforcement, some may not be familiar with research carried out in recent years.  This research suggests that loss of reinforcement passivity, as the carbonation front progresses, occurs at a higher ph than previously thought.

New Concrete

The ph of new concrete is typically 12-13, which surrounds embedded reinforcement with a passivating layer of highly alkaline cement, protecting reinforcement against corrosion.  The rate of carbonation in new concrete will be affected by the water/ cement ratio and the cement content; the connectivity of the capillary pore structure and size of the pores in concrete with a W/C ratio ≤ 0.4 and a cement content ≥ 400kg/m² will be reduced when compared to concrete with a W/C ratio > 0.4 and a cement content ˂ 400kg/m², making progress of the carbonation front slower in cement rich concrete with low W/C ratio.

The Chemical Process of Carbonation – accepted wisdom

Atmospheric carbon dioxide reacts with calcium hydroxide (Ca(OH)2 + CO2 =CaCO3 + H2O), a cement hydration product in the cement paste. The reaction produces calcium carbonate.  This reduces the alkalinity of the concrete to a level where the cement paste no longer provides a passive environment for embedded steel, this is said to occur when the ph of concrete falls to approximately 8.6.  Steel reinforcement is then though to be susceptible to corrosion.  The reaction of carbon dioxide and calcium hydroxide only occurs in solution and so in very dry concrete carbonation will be slow. In saturated concrete the moisture presents a barrier to the penetration of carbon dioxide and again carbonation will be slow. The most favourable condition for the carbonation reaction is when there is sufficient moisture for the reaction but not enough to act as a barrier.  See also the later paragraph by the Concrete Society headed Loss of Passivity (Concrete Society – Carbonation of concrete)

The presence of oxygen initiates surface corrosion of the reinforcing steel through oxidation, where iron oxides form on the steel’ s surface. These oxides, although porous and flaky, have a larger volume than the original steel—up to six times greater depending on the composition of the corrosion products, Broomfield notes. The iron oxides expand against the concrete, and the resulting stress causes the concrete cover to crack and eventually spall. He comments that a steel loss of only 0.05 to 0.10 mm (0.002 to 0.004 in) will cause concrete to spall. (Broomfield)

Some doubt has been cast on the assumption that a ph of ˂ 9 is required before CO₂ induced corrosion is initiated and some believe that a phenolphthalein test, which provides evidence of carbonation of concrete at a ph of ˂ 9, may be only a guide  to corrosion risk.  It is now thought that corrosion is initiated some 5-10mm ahead of the carbonation front as indicated by phenolphthalein testing (or 20mm in concrete containing chlorides), where the ph of the concrete may be up to 11 or higher.  Investigations suggest that reinforced concrete structures are not at risk of carbonation induced corrosion if the un-carbonated depth, as revealed by a phenolphthalein test, is further than 5–10 mm from the reinforcement. Thermogravimetric analysis (TGA) of drillings at 5mm depth intervals and phenolphthalein tested samples were taken from the inside and outside of a 36 year old in-situ concrete framed building.  The phenolphthalein results and the TGA data for complete carbonation indicated that outdoor exposure was more severe but TGA data for the deepest penetration of carbon dioxide suggested that that the thickness of concrete with a reduced ph (9-11) was greater inside, suggesting that sufficient CO₂ may be present in concrete to initiate corrosion at a higher ph threshold than may be detected by a phenolphthalein test. (L.J.Parrott, D.C.Killoh)

Loss of passivity – recent wisdom

Loss of passivity occurs at about pH 11. Carbonation of the concrete, caused by carbon dioxide in the atmosphere, has the effect of reducing the pH.  Hence there may be corrosion in the zone ahead of the front defined by the phenolphthalein indicator. In general the change in pH occurs in this zone, which is only a few millimetres ahead and the phenolphthalein method provides a good indication of the location of the de-passivation front. (Concrete Society – Carbonation depth)

Effect of Carbonation on the Threshold for Chloride Concentrations Required to Initiate Corrosion

In concrete with a ph of 12-13, about 7,000 to 8,000 ppm of chlorides are required to start corrosion of embedded steel.  This threshold is reduced is to below 100ppm in concrete with a ph of 10-11.  Conversely the progress of carbonation was significantly slowed down due to the existence of chlorides in concrete samples; the depth of carbonation boundary was decreased and the profile of consumed OH- became modest.

Conclusions

Results of studies suggesting CO₂ induced corrosion may occur 5-10mm ahead of the carbonation front as indicated by the phenolphthalein test are clearly a cause for concern, especially with rising global CO₂ concentrations being likely to increase the threat of corrosion in reinforced concrete and construction professionals should consider the results of such studies and the implications for acceptable reinforcement cover. It may be that an increased depth of reinforcement cover will be required (if previously specified cover was based on research where the phenolphthalein test has been used to determine behaviour) if the desired service life of a structure is to be achieved.

Ronacrete Products for Concrete Repair and Protection

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RonaBond HB25 high build polymer modified repair mortar

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Ronafix Pre-packed Render/ Screed waterproof polymer modified thin screed and render

RonaBond Anti Carbonation Coating WB water based acrylic coating

RonaBond Crack Bridging Anti Carbonation Primer S solvent based primer

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Further reading

Carbonation Depths in Structural-Quality Concrete – an assessment of evidence from investigations of

structures and from other sources, Currie R.J., Building Research Establishment Report, Building Research

Station, Garston, Watford WD 2JR, Great Britain

A Review of Carbonation in Reinforced Concrete, Parrott L.J., Cement and Concrete Association, Wexham

Springs, Slough SL3 5PL, Great Britain

Carbonization, Corrosion and Standardization, Parrott, L.J., Protection of Concrete, Proceedings of the

International Conference held at the University of Dundee, Scotland, UK, September 1990, edited by

Ravindra K. Dhir & Jeffrey W. Green, Chapman and Hall, 2-6 Boundary Road, London SE1 8HN

Determining and Extending the Remaining Service Life of Reinforced Concrete Structures John P. Broomfield, Broomfield Corrosion Consultants at e-mail: JohnPBroomfield@aol.com.

Effective Cost Analysis for Repairing of Corrosion Damaged Reinforced Concrete Structures,” Department of Trade and Industry, http://projects.bre.co.uk/rebarcorrosioncost (September 3, 2015).

1. Tilly, “Past Performance of Concrete Repairs,” Concrete Solutions: Proceedings of the 2nd International Conference (St. Malo, France: BRE Press, 2006).

Carbonation in a 36 year old, in-situ concrete

LJ Parrott, DC Killoh – Cement and Concrete Research, 1989