Date, time & venue
2019-10-24;6:00 pm – 6:30 pm (Refreshment), 6:30 pm – 7:30 pm (Talk);Chan Yat Mei Sophie Room – 9/F

Speakers: Dr. Y. Qian, Department of Civil Engineering, The University of Hong Kong.

Synopsis:

Concrete is the most widely used infrastructural material, where 3 billion tons were consumed globally in 2010. Modern concrete is a complex material consisting not only cement, aggregates and water, but also chemical admixtures and supplementary cementitious materials (SCMs) for concrete sustainability.

Compared with conventional vibrating concrete, self-consolidating concrete (SCC) is more flowable and can consolidate under its own weight. Therefore, it introduces many advantages in construction applications. These include decreasing labor work for casting, better quality control and enhancing hardened properties. However, challenges still remain, such as issues relating to formwork pressure and multi-layer casting. Each of these issues is closely related to the property of thixotropy. For SCC, as well as other concrete systems, it is about balancing sufficient flowability during casting and rate of structural buildup after placement. For instance, relating to the issue of SCC formwork, it is ideal for the material to be highly flowable to achieve rapid casting, but then exhibiting high rate of structural buildup to reduce formwork pressure. This can reduce the cost of formwork and reduce the risk of formwork failure. It is apparent that accurately quantifying the two aspects of thixotropy, i.e. structuration and destructuration, is key to tackling these challenges in field application.

I have been extensively working on defining and quantifying thixotropy since PhD program. Firstly, I applied creep recovery test to accurately measure the static yield stress and probe tack test to quantify cohesion. Nanoclay addition increases both static yield stress and cohesion and thus decreases SCC formwork pressure, as well as improve static segregation and stability. Secondly, after accurately measuring both aspects of thixotropy, I tied thixotropy to the discrepancy between static and dynamic yield stress. The higher the thixotropy, the higher the discrepancy. I also define an index to effectively quantify thixotropy. Thirdly, results of thixotropic index indicate that water reducing agents decrease thixotropy while nanoclay increases it. Finally, with a good balance of water reducing agents and nanoclay, I have developed mixtures with high flowabiltiy yet high stability after placement. It is an ideal mixture for 3D printing cementitous materials. System and material control for successful 3D concrete printing are also studied.

About the Speaker:

Dr. Qian is currently Assistant Professor at the University of Hong Kong. He worked as Research Fellow at Singapore Centre for 3D Printing. Before that, he also worked as postdoctoral Research Fellow at Ghent University in Belgium. He has obtained his Ph.D. at Columbia University in the City of New York, his M.S. at Hokkaido University in Japan, and his B.S. at Huazhong University of Science and Technology in China.

He specializes in cement and concrete rheology, especially developing characterization methods to probe and control the viscoelastic properties of fresh cementitious systems and design of cementitious systems to improve processing during construction. He is also interested in nanoparticles, and sustainable construction materials.

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