The development of a resistivity measurement has enabled Professor Zongjin Li, at the Hong Kong University of Science and Technology's Department of Civil Engineering, to monitor and interpret the hydration process of cement-based materials.

This testing process is non-contacting and non-destructive and produces highly accurate results and is regarded as important to the building and construction industry where strong and durable materials are essential for reliable and sustainable development.

Professor Li says that although concrete has been in widespread use for 100 years, the hydration process of cement based products is still not fully understood.

"We liken our studies of concrete to genetics… no two people in the world have a common genetic structure but fundamental pattern. The same principle applies to cement-based materials where the micro-structure varies incredibly. However, some basic configurations should exist which can lead to problems associated with durability and cement cracking. This in turn affects the life of a building as well as the cost of construction, maintenance and on going repairs.

"Our projects have helped us understand the influence of the water/cement ratio on the development rate and strength development rate in Portland cement paste, a commonly used, time-dependent component of mortar and concrete."

Professor Li and his team have gone on to identify the hydration stage itself, finding that hydration occurs at the minimum point of resistivity. The resistivity measurement is closely related to the setting time of concrete and the microstructure development in concrete or cement based material.

The next stage of the study is to find the relationship between the microstructure and the porosity inside concrete/cement based materials.

"This knowledge is important in terms of enabling us to predict how long concrete needs to set, when the formwork on a building can be removed and the actual long term strength of concrete."

Controlling the quality of concrete brings numerous environmental advantages. In the past, developers have to tear down some buildings floor when disqualified concrete underneath is discovered after a few weeks of constructions. By being able to record clearly the quality of a concrete pour, the construction industry will save on raw materials, energy consumption and overall environmental degradation.

The degree of hydration in concrete is now regarded as a direct indicator of strength. Resistivity, in turn, is an indirect measure of strength. By applying a non-contact electrical resistivity method to measure the resistivity development of fresh concrete and the pore solution within concrete mixes, contact problems between electrodes and surrounding materials have been eliminated.

The accuracy of the testing procedures and measurements has already been recognised by many universities and research institutions and the actual resistivity testing method has been developed as a commercial product.

Professor Li's project has produced five journal articles, three conference papers and trained two PhD students. It will provide the foundation for on-going research projects on the nature and characteristics of cement-based construction products and the development of novel and sustainable cement-based materials.

Professor Li Zongjin
Department of Civil Engineering
The Hong Kong University of Science & Technology
zongjin@ust.hk