Ferritic steels, concrete

Other properties. The coefficient of thermal expansion of concrete is about 10 °C . That of ferritic steels is not very different (about 1.2 X 10 as in usual carbon-steel reinforcement) that of austenitic steels is higher (about 1.8 X 10 ) austenitic-ferritic steels are in an intermediate position. The higher thermal expansion of austenitic and duplex stainless steels is not believed to cause any problems in concrete, and no cases of damage due to differential expansion have been reported [6]. Furthermore, the thermal conductivity of austenitic stainless steel is much lower than that of carbon steel and thus the increase in temperature throughout the steel is delayed. 

Stainless steels can be divided into four categories, based on their microstructure ferritic, austenitic, martensitic and austenitic-ferritic (duplex). Only specific grades of austenic and duplex stainless steel are currently used in concrete, although also a ferritic type with 12% chromium has been proposed [5-9]. In some countries also clad bars, i. e. bars with a carbon-steel core and an external layer of stainless steel are used. 

McDonald et al. studied the performance of solid stainless steel rebars (types 304 and 316) and found that they performed well while ferritic stainless steels (types 405 and 430) developed pitting (15). Studies by McDonald et al. reported investigations on a 10-year exposure of 304 stainless steel in Michigan and Type 304 stainless steel clad rebar in a bridge deck in New Jersey and found no corrosion (15). In a study by Virmani and Clemena, the type 316 stainless steel-clad rebar extended the estimated time to the cracking of the concrete beyond 50 years, but not as much as solid types 304 and 316 stainless steels (100 years) (16). 

Field studies (exposure tests) in marine or simulated marine environments demonstrated the much better corrosion resistance of stainless steels in concrete. After 4.5 years in natural marine conditions no cracking and no pitting corrosion occurred on an Fe-11% Cr alloy (Hewitt and Tull-min, 1994). Under accelerated chloride ingress the same alloy showed some pitting corrosion after one year, whereas specimens with plain carbon steel had already cracked. A 9.5 years exposure program on steels embedded in concrete containing up to 3.2% chloride additions with respect to the cement content showed that ferritic stainless steel with 13 % Cr showed corrosion at chloride levels over 1.9% (Treadaway etal., 1989). 

Summing up these results it can be concluded that stainless steels improve the durability of reinforced concrete constructions considerably. Depending on the expected severity of the environment different steel grades should be selected for carbonated, chloride-free concrete ferritic stainless steels with low chromium content (e.g. DIN... 

Almost no engineering material is safe from or immune to microbial corrosion. In Chapter 8 the vulnerability and susceptibility of copper and cupronickels, duplex stainless steels and concrete will be discussed in a brief and informative manner. I had my reasons for picking these materials copper and its alloys have the reputation of being poisonous to micro-organisms, duplex stainless steels are known for their high resistance to corrosion thanks to their duplex microstructures of ferrite and austenite, and concrete is widely used in both the marine and water industries because of its good performance and cost effectiveness. 

The potential for corrosion of steel is increased in a chloride environment that is subjected to CO2. In the presence of CaCl2, Friedel s salt of formula 3CaO Al2O3 CaCl2 10 H2O and its ferrite analogue are formed. If these chlorides disassociate during the service life of concrete, the release of chloride and reduced pH may pose even a more increased risk for steel corrosion. In an examination of concrete exposed to CO2, DTA was applied to identify and estimate the products of reaction. Friedel s salt was identified by an endothermal peak in the range of 300-350°C. The... 

---