The Matrix – Concrete Science, not Science Fiction
Dr Anthea Airey, Airey Taylor Consulting
Doctor Anthea Airey gained her PhD in Chemistry at ANU, and heads the materials group within the Perth-based civil and structural engineering firm Airey Taylor Consulting. Her presentation featured forensic investigations of concrete failures, and major concrete remediation works that she has specified. The most common direct cause of concrete failure is ‘concrete cancer’ caused by corrosion of steel reinforcing, in which the greater volume of corrosion products resulting in tensile cracking and eventual spalling of the concrete.
Anthea briefly reviewed the historical use of concrete dating back to Ancient Egypt and Rome. The Pantheon (126 CE) in Rome remains the largest unreinforced concrete dome ever built. They key word is ‘unreinforced’; it is designed to be in compression. The potential for ‘concrete cancer’ is essentially inherent in the use of steel to give tensile strength to the concrete composite. However, whether or not corrosion occurs is largely dependent on the cement matrix that binds the aggregate and steel in the concrete. She reminded the audience that while the Pantheon has stood for nearly 1900 years, Australian Standard AS 3600-2009 (Concrete structures) is based on a design life of 50 years ± 20%.
Knowledge of how to make concrete was lost from around the 5th Century until the development of Portland cement around 1824. Portland cement is based on calcining limestone, and is quite different from Roman pozzolanic cement. Simplifying a complicated sequences of reactions, the main process in the setting of Portland cement is the hydration of calcium oxide. This produces nanometre-scale gel particles, surrounded by similar scale gel pores. Since the water to cement ratio normally exceeds the stoichiometric minimum of 0.38, the excess results in micrometre-scale capillary pores; these are the pores that cause most trouble as they provide a path for moisture to reach the reinforcing steel.
Anthea’s forensic investigations included some interesting examples of cracking in new concrete, where gel pores can be involved (freeze-thaw cracking), and also cases where the structural design has not provided adequately for initial shrinkage. Rapid carbonation can also cause self-healing crazing in new concrete. However, the main focus of her work has been deterioration of old concrete, in which carbonation is often a major factor.
Carbonation is a slow process by which carbon dioxide from the surrounding air reacts with calcium hydroxide, the initial product of the setting reaction, converting it to calcium carbonate. Essentially, the cement return to limestone. This is not necessarily deleterious to the cement matrix itself, but the consequence is that the pH drops from around 13, which is protective to iron, to around 9, which is not protective. In lower strength concrete, which has more capillary pores, the result is that water can penetrate to the unprotected steel, resulting in ‘concrete cancer’. Carbonation is typically fairly slow (eg, penetrating 30 mm in 20 years), but can be much more rapid where the concentration is high, notably the undersides (soffits) of underground car parks. Increasing global carbon dioxide levels area already increasing observed carbonation rates.
Anthea described a case study of remediation of a 1920s reinforced concrete building in Hay Street Perth. Investigations included core sampling and visualisation of carbonation using phenolphthalein indicator spray (pink in unaffected concrete and colourless in carbonated concrete).
Designing against concrete cancer involves reducing the water to cement ratio (minimising capillary pore size), eliminating cracking during setting, excluding contaminants (coating), good detailing for water run-off, and considering alternatives to ordinary steel reinforcing. Airey Taylor’s design for the Perth Performing Arts Centre is based on a100-year life. It involves galvanising the reinforcing, pre-cast panels for better control of manufacture, and use of high strength cement.
The large audience contributed to an extended question and answer session. Topics included use of stainless steel, coated steel and glass fibre reinforcing, and the potential applications of geopolymer cements. The common issue limiting their adoption appears to be that established engineering standards and codes are based on ordinary Portland cement and steel. The potential benefits to be gained through change have to compete against the comfort afforded by being able to design to these standards, accepting that they are based on a relatively limited designed life for a concrete structure.