Consideration of environmental factors is of key importance when selection methods for the protection of concrete structures and the prevention of degradation or spalling.
“Concrete cancer” or spalling has become a highly problematic issue for buildings in many parts of Australia – in particular those coastal regions where so much of country’s urban population is concentrated.
Last year Strata Community Australia warned of an “epidemic” of concrete cancer throughout the Gold Coast that has cost millions of dollars in property damage, and called for the state governments to heighten regulation of building standards in order to prevent the problem from worsening.
According to Dr. Farhad Nabavi, senior technical director of Xypex, concrete can succumb to disparate forms of corrosion depending on the environmental conditions to which building structures are exposed.
“Concrete structures suffer from different forms of attack and degradation,” said Nabavi to Sourceable. “There are essentially two kinds of degradation of concrete - one is degradation of the concrete matrix itself and the other is the corrosion of the steel reinforcement.”
The specific form of degradation that affects concrete structures depends upon their geographic location. The corrosion process that affects concrete in maritime environments differs from that which can degrade the same materials situated in inland areas where the soil is acidic.
“If the structure is situated in a coastal area then the most dominant factor for deterioration is chloride diffusion, which can result in the corrosion of the steel reinforcement,” said Nabavi.
While corrosion of steel reinforcement in coastal areas is perhaps the most notorious form of concrete spalling in Australia, Nabavi points out that buildings located inland are also susceptible to other forms of degradation.
“In Australia there are some inland areas where the soil is acidic and contains sulphate,” he said. “If a building structure is located inland and comes into contact with certain forms of acidic soil, then you can have sulphate attacks on building materials which will result in the degradation of the concrete matrix due to the creation of gypsum.
“While sulphate attacks degrade the concrete matrix, they do not necessarily affect the steel reinforcement. “
Because different environmental conditions lead to different forms of concrete degradation, it’s vital to take this issue into consideration when selecting concrete protection solutions.
“The key to the selection of effective coating systems is identification of the specific environmental conditions to which buildings structures are exposed,” said Nabavi.
“Protective coatings or repair coatings should be resist that specific form of environmental exposure. For example if we want to introduce a coating system in coastal areas, that particular system should address chloride diffusion.
“If you use a coating for concrete near acidic sulphate soils, it should be capable of resisting sulphate attacks.”
Another critical factor to take into consideration when selecting concrete protection solutions is the behaviour of coating systems relative to the properties of the underlying concrete substrate.
“One of the most important requirements for coatings is that they possess similar characteristics and properties to the concrete substrate,” said Nabavi.
“For example epoxy coatings are polymer materials that are highly popular option when it comes to concrete protection.
“These coatings can differ from the concrete substrate when it comes to strength or thermal coefficients for expansion, so if the temperature goes up to 40 degrees, the elongation or expansion of the concrete can differ from that of the epoxy.
“This difference in the thermal coefficient or expansion can generate force and pressure along the surface of the concrete structure. “
Crystalline coatings are another popular option for concrete protection, and can avoid the problem of disparity in physical properties relative to the substrate because of their cementitious nature.
“Crystalline materials can possess similar characteristics or similar behaviour to the concrete substrate because they are cementitious in nature,” Nabavi said.
“When it comes to the mechanical properties of the coating material and the substrate, this is one of the most important differences between crystalline coating systems and epoxy coating systems.”