According to the China Building Energy Consumption Research Report (2020), the total carbon emissions of the whole process of construction in China in 2018 was 4.93 billion tons, accounting for 51.3% of the country’s carbon emissions. Among them, the building materials production phase accounted for 28.3%, the construction operation phase accounts for 21.9%. At the same time, the carbon emissions of China’s steel industry account for about 15% of the country’s total carbon emissions, which is the largest industry in 31 manufacturing categories. China’s total steel production for construction will reach 580 million tons in 2020. Reducing overhaul and maintenance will contribute to reducing national steel consumption, It plays an important role in the realization of “double carbon targets – 2030, China’s steel industry will achieve the carbon peak target, 2050, the global building to achieve zero carbon emissions.”
In general, carbon steel, which is used in large quantities in construction, has its Achilles’ heel: poor corrosion resistance. Carbon steel bars are easy to corrode, which results in high maintenance cost, degradation of structure performance and shorten service life of structure seriously. The corrosion of rebar has caused great losses in the world. Although there are many ways to solve the problem of steel corrosion, such as increasing the thickness of concrete protective layer, cathodic protection, using marine concrete and so on, these methods can only delay the corrosion of steel, and can not solve the corrosion problem completely. The difference of stainless steel bar is that it contains more nickel and chromium, which solves the corrosion problem of steel bar fundamentally and provides better mechanical properties. Internationally, it has become the ultimate option for many super projects to reduce the building’s life cycle cost.
- Stainless steel reinforcement is a sharp tool to combat corrosion
At present, the stainless steel reinforcement has been very mature in many super projects at home and abroad, in China’s Hong Kong and Macao more common. For example, the Hong Kong-Zhuhai-Macao Bridge, Stonecutters Bridge and Tseung Kwan O Bridge. Stainless steel rebar has such an irreplaceable position, first of all because of its own strong corrosion resistance, can ensure to meet the service life of more than 120 years, reduce maintenance and maintenance costs, reduce the life cycle cost. Secondly, the iterative version of stainless steel rebar has very excellent mechanical properties, while subjected to more than 5 million fatigue tests, the yield strength can be higher than 500 MPa. And stainless steel rebar also has the advantages of fire prevention, recyclability and so on, very in line with the “carbon neutral” development concept.
Internationally, stainless steel rebar was first incorporated into the British Standard Association BS6744 standard in 1986, and the American Society for Materials and Testing ASTM A955 standard in 1996. Adopted by Germany DEUTSCHES INSTITUT FUR BAUTECHNIK(DIBt） The two stainless steel rebar standards adopted by the UK and UK CARES have received European Technical Certification (ETAs). In 1999, the Institute of Concrete published Technical Report No. 51, “Guide to the Use of Stainless Steel Rebar,” which indicated that the electrochemical coupling problem had been resolved. In 2002, the British Highways Agency has written this into BA 84 / 02 “Application of Stainless Steel Rebar in Highway Components,” allowing the thinning of concrete protection layer, and the electrochemical coupling problem has been eliminated. The 2004 edition of Eurocode 2, Design of Concrete Structures, also specifies the use of stainless steel reinforcement for thinning concrete cladding.
China issued in May 2004, the “concrete structure durability design and construction guide,” pointed out that Stainless steel rebar can be used in special projects with more than one hundred years of service life, which is an important component and strategic measure to improve the durability of major engineering structures in China. The reasonable application of stainless steel has been proposed in Europe and the United States and other countries: stainless steel reinforced concrete can be partially or completely used for important structures in high corrosion environment. At present, the national standard GB / T33959-2017 “stainless steel reinforcement for concrete” is widely used, and the standard of the American Society for Testing of Materials ASTMA955/A955M-2020《Standard Specification for Deformed and Plain Stainless-Steel Bars For Concrete Reinforcement》 British Standard BS6744-2016《Stainless steel bars – Reinforcement of concrete – Requirements and test methods》。 The latest edition of “Highway Bridge and Culvert Construction Technical Specification” JTG / T 3650-2020 adds the technical content of stainless steel reinforcement in the chapter of sea bridge.
- Duplex stainless steel bars stand out
According to the state of the organization of stainless steel into martensitic steel, ferritic steel, austenitic steel, duplex stainless steel and precipitation hardening stainless steel. Among them, the most suitable steel for concrete structures is duplex stainless steel. Duplex stainless steel is a kind of steel in which ferritic and austenitic phases coexist. It has excellent mechanical properties and resistance to chloride stress corrosion. Nearly ten years, duplex stainless steel smelting technology mature, cost continuously reduced, performance gradually improved at the same time, changed the direction of stainless steel application development.
The common duplex stainless steel is 2205, 2304, etc., and 2304 is the most widely used concrete structural stainless steel. First of all, 2304 contains 23% chromium metal, 4% nickel metal, will form a layer of self-repair chromium oxide film on the surface of the steel bar, ensuring strong corrosion resistance. At the same time, the yield strength of 2304 is up to 600 MPa after the process adjustment, and the elongation, elastic modulus and other aspects are also excellent. The most critical is that the price of 2304 is lower than 2205 and 316, etc., reducing the dependence on nickel and molybdenum resources at the same time, can show better life cycle costs. Therefore, duplex stainless steel has become the current trend of use.
- Durability is the key to cost reduction
The earliest international application of stainless steel reinforced concrete project, is the construction of 1937-1941 Mexico Progreso pier, the bridge in tropical marine environment, with serious corrosion. 220t 30mm AISI 304 stainless steel rebar was used in the pier of the offshore structure. To date, the site using stainless steel bars has not undergone a single repair, and no obvious signs of deterioration have been found. Later, countries around the world began to use stainless steel in numerous projects.
The Federal Highway Administration (FHWA) compared the cost of three bridge projects in Illinois using plain, epoxy-coated and stainless steel rebar. The application of stainless steel rebar can increase the cost by 6% ~ 16%, but for the whole life economic analysis, the maintenance cost is saved. At the same time, the test shows that the application of stainless steel rebar can delay the time of crack formation by 6.5 ~ 13 times than that of ordinary steel, thus effectively improving the service life and reducing the cost of the structure in the whole service life. Completed in 2004, located in Oregon, USA, replacing the old Haynes Inlet Slough Bridge, In a marine environment, the 400T duplex stainless steel bar 2205 is used as the key structural component of the bridge, with a design life of 120 years, 2.5 times the life of the old bridge. The total cost was $12.5 million, of which stainless steel accounted for 13% of the total price and did not require maintenance throughout the service life. Compared with ordinary reinforced concrete bridges, the service life can be extended by 50 years. In terms of economic costs, it is equivalent to building a new bridge. With the continuous improvement of the comprehensive cost, the proportion of the cost raised by the use of stainless steel reinforcement in the harsh environment is often less than 5%.
In a domestic case, the Hong Kong-Zhuhai-Macau Bridge first used stainless steel bars in 2012. The Hong Kong-Zhuhai-Macao Bridge is a super-large cross-sea bridge connecting Zhuhai, Hong Kong and Macao. The total length of 55 kilometers, the total investment of the project is over 120 billion yuan, the channel span is 4100m, the design service life of 120 years, can resist 8 earthquakes, 16 typhoons. Since December 2012, the Hong Kong-Zhuhai-Macau Bridge has used over 10,000 tons of stainless steel reinforcement, making it the largest project in the world. The bridge is made of 2304 duplex stainless steel, which is mainly used in many parts of the bridge, such as the pile cap, the tower, the pier, the bridge abutment, the cushion stone and so on.
At present, stainless steel reinforcement has been widely used in the industry, its performance and application have been recognized by many owners and designers. In terms of price concerns, duplex stainless steel 2304 grades than the traditional perception of 316 stainless steel grades to be much lower, the cost is closer to 304 stainless steel, and its performance was better integrated high score. Although the price is 4-5 times that of the ordinary carbon steel bar, it is only used in some areas, and the service life can be up to 120 years. The use of stainless steel rebar can save high maintenance costs and even reconstruction costs, reduce the cost of additional anti-corrosion measures, and reduce the impact of repair on traffic. If you want to control the cost, you can use stainless steel bars on the key structural components. The total cost of the project is only increased by about 2%, but it can fundamentally reduce the life cycle cost.