Fundamental Research Creating Firm Foundations in Concrete Futures
Throughout his engineering life, Dr Taehwan Kim has always championed a reduced carbon footprint in construction. He has a particular interest in the creation of sustainable infrastructure materials and believes that a fundamental understanding of the chemo-physical reactions in cementitious materials is key to achieving understanding and control of the Alkali-Silica Reaction (ASR).
ASR is more ubiquitously and generally known as concrete cancer. It deleteriously affects many cement structures and even though it was first recognised in the 1930s, it remains difficult to avoid or mitigate, often resulting in expensive repair and reconstruction works and resulting in greatly reduced structural durability. It is an enemy of sustainable practice.
In Australia, ASR has affected such iconic structures as the MCG and the Dee Why coastal pool, as well as our everyday facilities; carparks, bridges, pavements and buildings. As Dr Kim notes “ASR is a worldwide problem, but its fundamental processes and mechanisms do not receive much attention here. It is a timely opportunity to conduct this type of Australian research into this problem.”
Problematically, by the time ASR is apparent in a structure, it has been active for a long time. The most common visible symptom is an intricate web of cracks that exude a hygroscopic gel.
ASR, as a particular type of concrete cancer, is caused by the alkaline solution in cement paste reacting to the silica contained in certain concrete aggregates. Aggregates are, ideally, inert materials that comprise up to 70% of concrete mass. But certain aggregates can cause ASR. The viscous gel formed as a result of this chemical reaction expands and concrete swelling occurs.
“Concrete is designed to resist compressive forces, not tensile force. Internal expansion, like that caused by ASR, generates tensile forces that concrete cannot resist and cracking occurs.” While ASR may not cause total failure in the mechanical performance of a structure, the reduced durability has profound implications for resources, cost and sustainability. “Once the cracks appear a range of chemicals can penetrate very easily. What starts with ASR then becomes corrosion or sulphate attack and the lifetime of the structure may be greatly reduced.”
The most direct way to avoid ASR is to test the potential reactivity of aggregate material. “Aggregate is one of the cheapest components of concrete and no-one wants to spend more. Construction relies on locally available material. Once you try to import or transport aggregates the cost and the carbon footprint increases. Even though we have known about ASR for over 70 years, construction enterprises are forced into a compromise by using established mitigation techniques. For example, replacing Portland cement with fly-ash or slag in the original mix.”
“While testing of aggregate reactivity is still limited, common sense tells us that testing should increase and it is increasing. But our knowledge is not extensive enough. We need to develop new, more reliable testing methods to assess aggregate reactivity.”
To complicate matters, modern concrete is more complex and various than the traditional cement-based concrete that construction has relied upon since Joseph Aspdin invented Portland cement at 1824. The process of the change is necessarily steady.
“The construction industry changes very slowly. Its not like electronics. The pace must be slow to ensure we avoid catastrophe. Concrete has served us well for 200 years and so, step by step, we try to make the process more sustainable and environmentally responsible. But some of the newer materials are not fully tested on large structures and if no proven record exists, it becomes risky to use these newer materials.
This century we have reduced the use of Portland cement by 20-30%. This type of concrete is responsible for high levels of carbon dioxide emissions. “By reducing the amount of cement in new concrete we can reduce the production of carbon dioxide. I believe that a 50% reduction in the use of Portland cement is a reasonable vision. In the next 100 years we may have completely different materials with which to build our cities.”
“New concrete materials using bio-waste and incorporating chemical admixtures may be more resistant to ASR and other problems like shrinkage.” Concrete is also now more fit for use. “there are now numerous different types of concrete, determined by purpose.” But for widespread change to occur new materials must be economical and trustworthy, stimulated by government regulation that demands sustainable construction practices and establishes codes.
Dr Kim’s long-term research goals hope to provide fundamental chemical and physical knowledge about new cementitious materials and the deterioration of concrete. As an example, he uses x-ray micro tomography and other characterisation techniques to more fully understand the sequence of ASR gel formation. “This is not so much practical research, but looking to fundamental mechanisms. Because modern concrete is complex and diverse, developing ASR mitigation techniques is also complex and diverse. By looking to the unchanging fundaments of the ASR process, then we can develop predictive tools that are applicable to all kinds of concrete, saving time, saving resources. There is so much more to discover about ASR, about why and how it happens.”
Taehwan has worked with the UNSW School of Mineral and Energy Resources Engineering and the Mark Wainright Analytical Centre to conduct experiments using the latest tomographic and chemical technology. As these experiments continue, they will create an accurate and detailed data base, generating widely applicable mathematical models. But it is the very nature of these experiments that hold the most promise. “As well as equations, the specific type of experiment I have been conducting is a very flexible method and can be applicable to many different systems. As the work progresses, I will collaborate with chemical engineers to create a predictive tool for thermodynamic modelling.”
It can seem at times that Dr Kim’s genre of research, fundamental investigation, does not have the widespread appeal of immediate and practically applicable projects that bring tangible outcomes and reveal measurable and direct impact. Certainly, he is currently conducting funded projects about the use of glass waste and rice husks in building materials, that are designed to have immediate applications. But he is an academic with a long-term view.
“It takes time to build a comprehensive and efficient tool to test construction materials, which are becoming more complex every year. A paradigm shift is required. We need to value the fundamental. If we fully understand the chemistry and the physics, then while materials may vary, their intrinsic reaction process and microstructural evolution can be predictable. We will no longer have to rely on purely empirical design, based on trial and error.”
And where could this philosophy make more sense than in construction, where a strong foundation is the most important element for building strong and lasting structures.