Ongoing improvements to engineered cementitious composites (ECC) have the potential to make them the ideal repair solution for concrete structures that are vulnerable to spalling or cracks.
According to Dr. Farhad Nabavi, senior technical director at Xypex Australia, dimensional compatibility with substrates is the key to the development of effective concrete repair materials.
"The most important factor in the selection process for repair materials is the 'dimensional compatibility' of repair materials with the concrete substrate," said Nabavi. "Considerable savings in the cost for the repair and rehabilitation of deteriorated concrete structures can be realised if the performance criteria of the selection repair materials are compatible with the existing concrete substrate.
"Compatibility is the balance of physical, mechanical, chemical, and electrochemical properties between the repair material and the existing substrate phase. This balance ensures that the system resists all anticipated stresses induced by volume changes, chemical, and electrochemical effects without distress and deterioration over a desired period of time."
Nabavi notes that cementitious materials have long been considered one of the best options for concrete repairs on the basis of their high level of substrate compatibility.
"The criteria for the selection of a high performance repair material include but are not limited to durability, compatibility with the concrete substrate, which in turn includes the coefficient of thermal expansion, compressive strength, stiffness, deformation behaviour and modulus of elasticity; as well as strong bonding with the concrete substrate," he said. "Considering these criteria, cementitious-based materials are a proper solution for the repair and rehabilitation of concrete structures."
Engineered cementitious composites mark a further improvement upon pre-existing cementitious repair solutions, using the addition of polymeric fibres to achieve a marked improvement in their physical properties and performance.
"ECC were first developed in the mid-1980s, and consist of a mixture Portland cement, water, sand, and short, discontinuous polymeric fibres, whose volume fraction in the mix is limited to approximately two per cent by volume of concrete (fractional volume)," said Nabavi. "Over the past 30 years, they have undergone continuous refinement and improvement. The important thing when it comes to composite materials is not just the ingredients, also but the interface or interaction between them which affects performance.
"Various types of synthetic fibres such as PolyVinyl Alcohol, Polyethylene, and Polypropylene can be used in cementitious-based materials, considering the interfacial properties between fibres and the cement matrix to achieve the unique behaviour of ECCs."
One of the chief advantages of ECCs is their high level of ductility, making the material far less susceptible to the various forms of cracking or degradation that afflict stronger yet more brittle forms of concrete.
"Concrete, especially high strength concrete, is a brittle material which makes it vulnerable to cracking," said Nabavi. "This has been the motivation behind the development of high ductile cementitious materials (ECC) for structural applications.
"ECC with a tensile strain capacity of between three and seven per cent provides a high ductile composite that is not susceptible to cracking due to its higher tensile strength, and serves as non-shrinkable cementitious-based mortar."
Other advantages include improved bonding capability and heightened water resistance.
"The polymers provide a strong bond between mortar and concrete substrate - about 2.5 MPa. The polymer and cement hydration products commingle and create two interpenetrating matrices that work together as an interlocking skeleton, which provides a strong bond," said Nabavi.
"The polymers can also make concrete extremely watertight, while crystalline technology can increase the durability of the repair mortar by blocking the pores and healing any potential micro-cracks as well."
Nabavi notes that ECCs have already been applied with great success to a range of highly challenging environments across Australia.
"We’ve employed ECC materials across a broad variety of environments in Australia, from infrastructure works like bridges to water treatment plants, where they’ve all proved highly effective," he said.