C7.

Project title: Stainless Steel Reinforcing Bars as a Green Material

With governments all over the world now pouring money into infrastructural development under the current financial crisis, there will be an increase in demand for the use of steel reinforcing bars for construction projects.

Stainless steel is an environmentally friendly material.  It is extremely durable and is almost 100% recyclable. 

Deterioration of carbon steel reinforcing bars in concrete structures is a worldwide problem. The corrosion product (metal oxide) has a lower density than the metal and this increase in volume causes cracking and subsequent spalling of the surrounding concrete. This often results in premature failure of buildings and structures, and has even culminated in accidents causing injury or death to people.  

In order to minimize the need for costly repair or replacement of defective structures, there is now a general shift of emphasis on life cycle costs for new construction projects, as opposed to considering only the initial capital costs. This approach helps to focus attention on total costs, including the frequency and cost of future maintenance and replacement work.  Many analyses have shown that the use of stainless steel reinforcing bars may reduce overall cost in the long run.  Durability also leads to less frequent demolition of structures with the associated benefit of minimizing environmental pollution.

Type 304 is one of the most common materials used in stainless steel reinforcing bars.  In the vast majority of applications involving the use of this steel, it is used in a metallurgical state called "solution treated" condition and has a relatively low tensile strength and high ductility compared with the carbon steels used in concrete reinforcing bars.   

Manufacturers of stainless steel reinforcing bars have tried to overcome this lack of strength by producing high strength grades of stainless steel reinforcing bars.  Unlike the carbon steels, type 304 stainless steel cannot be hardened by phase transformation such as the formation of martensite by rapid cooling from a high heat treatment temperature.  Consequently, increase in strength is achieved by a series of hot and cold working operations to vastly increase the dislocation density in the material.  For example, in one commercial type 304 stainless steel reinforcing bar, the yield strength is increased from 250MPa to 650MPa with a reduction of elongation from 50% to 30%. 

Relatively little information is available on the properties of these high strength stainless steels, e.g. their long term corrosion behaviour, notch tensile and fatigue properties, and their behaviour in the presence of fire.

This project aims to enhance our present knowledge of these high strength stainless steels and generate new information for the design of structures involving the use of these steels. 

Supervisor/Associate Supervisor: Prof Joseph K L Lai (apjoelai@cityu.edu.hk)/ Dr C H Shek (apchshek@cityu.edu.hk)

Suitable for: M.Phil./Ph.D.