Gulf Construction Online - Stainless steel key to durability

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Pier of Jeddah Port ... concrete reinforced with carbon steel.

Stainless steel key to durability

Stainless steel-concrete composite construction is a safe bet to ensure durable structures, especially when they have to interact with particularly aggressive environments says A Bennani of Germany-based Cogne’s R&D Department.

01 March 2004

The durability of a reinforced concrete structure is vital especially in a structure of strategic importance, such as a major road, bridges, viaducts, tunnels, wharfs, railway or metropolitan communication route and similar structures.

Sector researchers tend to pay increasing attention to durability, acknowledging that the steel-concrete composite is anything but stable in time especially when the structures interact with particularly aggressive environments.
In the last few years, governments have taken note of this research work, issuing norms that specifically refer to durability.
Various remedies to ensure the long-term functionality of reinforced concrete structures in aggressive environments exist and have been used for some time – mainly regarding the concrete (use of suitable concrete cover, less porous concrete, etc.) and the reinforcement (lining with epoxy resins and galvanic protection).
The use of stainless steel (SS) reinforcements instead of carbon steel rods, albeit at an initially high cost, can guarantee optimal performance for a much longer time than all the other measures adopted.

Why stainless steel?
SS belongs to a family of steels having a wide variety of characteristics with respect to physical and mechanical properties and resistance to corrosive environment. As defined in EN 1008801, it contains a minimum of 10.5 per cent chromium, a maximum of 1.2 per cent carbon, and is highly resistant to atmospheric corrosion.
SS used for reinforcement, generally has a maximum carbon content of 0.07 per cent or below, and derives its corrosion resistance from the naturally-occurring, invisible, chromium film. This film is inert, tightly adherent to the metal surface and self-healing (it re-forms instantly if the surface is damaged).
However, there are aggressive environments that give rise to localised or widespread breakdown of this passive layer resulting in corrosion of the unprotected surface. In such cases, the specific corrosive environment of the application needs to be considered by the designer in selecting the most suitable SS.
There are four ranges of SS that are distinguished by the microstructure and which possess different characteristics:
• Austenitic;
• Ferritic;
• Martensitic; and
• Duplex stainless steels, which have a microstructure with similar proportions of austenite and ferrite.
SS is processed by adding elements to iron to achieve the required compositional balance. These include chromium, nickel, manganese, molybdenum and sometimes titanium, which are added while controlling the level of carbon during the processing. These alloying elements affect the steel microstructure, as well as its mechanical properties and corrosion resistance in ways that can be exploited by engineers.
Only austenitic and duplex stainless steels are recommended for use as reinforcement to concrete because of their high corrosion resistance. Austenitic SS have chromium and nickel as the main elements alloyed with iron, whereas duplex have high chromium and lower nickel content.

Corrosion resistance of SS rebars
The presence of chromium in suitable quantities gives SS the ability to “passivate” (the ability of stainless steel to cover itself with a protective oxide film whenever in contact with a sufficiently oxidising environment) spontaneously.
Other alloying elements contribute to increase the corrosion resistance of SS, namely: molybdenum, chromium, nickel and nitrogen.
In concretes with neutral pH (that is, which are carbonated) or with alkaline pH (>12,5), SS becomes passive, and do not suffer general corrosion.
Theoretically, different forms of corrosion of SS can occur – intergranular corrosion and pitting corrosion. However, in practice only pitting corrosion is a real problem. The ingress of chlorides into concrete is the main source for pitting corrosion.
Chlorides gradually accumulate on the surface of the concrete – such as from road deicing salts or from exposure to the marine environment – and are transported through the pore structure to the level of the bars. Once the chloride concentration exceeds a critical value, then passivity is locally lost and corrosion initiates in the presence of oxygen. Corrosion tends to take the form of localised severe pitting.
The expansive nature of corroded products, which occupy up to eight times the original volume of the steel, produces a tensile stress in the concrete cover layer. These stresses produce cracking and spalling of the cover which are quite noticeable even before the entire bar is affected. Once cracking and spalling have occurred, it is necessary to undertake extensive repairs or remedial works to ensure the integrity of the structure.
Structures that are commonly affected are:
• Marine and offshore structures subject to wave action in a splash zone or tidal zone, in particular:
* Splash and tidal zones of offshore platforms;
* Port and harbour structures such as lock gates, quay walls and hard standings;
* Jetties-concrete piles in the splash and tidal zones and decks;
* Support columns to bridges spanning tidal estuaries.
• Highway structures in countries where road deicing salts are used, in particular:
* Bridge structures, particularly decks and abutments/columns subject to road spray or splashing;
* Road decks;
* Tunnel linings; and
* Single and multi-storey car parks.
The simple solution to the problem of steel reinforcement corrosion is to use SS reinforcement where potential corrosion zones are identified within the design of a given structure. It has now been proven that SS reinforcement will not corrode in aggressive conditions where chloride content of up to eight per cent by weight of cement are present at 20 deg C.

Fundamental characteristics
The behaviour of SS differs from that of carbon steels in that they do not exhibit a well-defined yield point when test pieces are submitted to tensile load (see figure below).
Austenitic and duplex stainless steels show early plastic deformation, but continue to sustain increasing load with increasing strain. For SS, this is commonly at 0.2 per cent.
According to BS 6744:2001, the tensile properties of SS rebars (reinforcing ribbed bars) depend on their range (in all grades) from 3 to 50 mm diameter.
The elastic modulus varies slightly with the chemical composition of the SS. While in general the values are similar to those for carbon steel, at about 20 deg C the value is about 200 kN/sq mm.
To obtain similar characteristics SS rebars are produced by hot rolling (diameter range 14 to 40 mm) or by the cold worked (3 to 12 mm diameter) process. The work hardening occurring during the production cycle of SS rebars doesn’t affect the fatigue strength; in fact, the principal factor affecting this property is the notch effect at the root radius of the rib.
BS6744:2001 indicates that “transverse ribs shall have a crescent shape and shall merge smoothly into the core of the product” and “the rib flank inclination … shall be greater than or equal to 40 degrees and radiused at the transition of the core of the product.”
The density of different stainless steels varies according to their chemical composition.
The thermal expansion coefficient for SS (austenitic and duplex) is slightly greater than for carbon steel without creating any problem in concrete or structures. The magnetic permeability varies also with the chemical composition, but austenitic SS is generally considered as non-magnetic material. Under these conditions, austenitic SS rebars are used in housing for electronic equipment where the field interference effects cannot be tolerated. However the relative magnetic permeability varies with production process, for example, the value for cold drawn bar is a bit higher than for warm-worked bars.
On the contrary, duplex (and ferritic) SS are magnetic as carbon steel.
One of the most important characteristics of stainless steels (particularly as concerned to reinforced bars) is its high and low temperature behaviour.
Depending on the composition and processing method, stainless steels show an increasing strength advantage with temperature, while carbon steel reinforcement shows a significant drop, especially above 500 deg C.
This suggests that concrete elements reinforced with SS behave better in fire than conventionally reinforced elements with same depth cover.