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Concrinox TM
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Using stainless steel in concrete reiforcement structures
Available products |
DUCTILITY
Due to their ductile nature, austenitic and austenitic-ferritic stainless steels show not only high percentage elongations to rupture but also ratios ft/fy rather high being either annealed or work-hardened. The strain-stress curve is rather different from that of carbon steel as shown on picture 5.
Picture 5 - Comparison between carbon steel and stainless steel in the strain-stress curve
Bearing in mind that the area subtended by the two curves is proportional to the mechanical energy absorbed in the tensile test, we can easily understand the relevant difference in the capacity of dissipating energy linked to events which are subject to important deformations.
In stainless steel rebars, the relevant difference of both materials in terms of percentage elongation to rupture is to be noticed ( almost 12% for carbon steel, almost 20% for stainless steel) as reported in the table.
| Carbon steel | 1.4301 | 1.4436 | DM. 9.1.96 | |
| Structure | Austenitic | Austenitic | FeB44k | |
| Rp0,2 [N/mm2] (yield characteristic strenght fyk) | >=430 | 670 Ø <= 6  12 mm540 Ø >12 mm | >=430 | |
| Rm [N/mm2] (Rupture strenght ftk) | >=540 | 810 Ø <= 6 12 mm 780 Ø >12 mm | >=540 | |
| Elongation A5% | >=12 | 20 Ø <= 6 12 mm 35 Ø >12 mm | >=12 | |
| Ratio (ft/fy)k | >=1.13 | 1.20 Ø <= 6 12 mm 1.40 Ø >12 mm | V. Tab 5a e 5b | |
| Elastic modulus [kN/mm2] | 190 | 200 | ||
| Tab. 4 - Mechanical characteristics of steels applied for stainless steel rebars. | ||||
By ductility, we mean the capacity the material to undergo high deformation with reduced resistance either under monotonic or cyclic loads.Considering stainless steel from a ductilitity point of view the classification relates to final stress (on drawing 5 indicated as “maximum tensile strength”)
u and the ratio ft/fy turns out to be interesting. Model Code 90 gives the three categories B, A , S subject to increasing ductility and advises use of Grade S steel in seismic areas provided that ratio between rupture and yield strength shall not overcome 1.3 value (Table 5a.)Eurocode 2, on the contrary, considers two different stainless steel rebars, called High ductility (HD) and normal ductility (ND) (Table 5 b).
On the other hand greater restrictions are provided in Eurocode 8 for stainless steel rebars in buildings in a seismic area (still in Table 5b).
| CEB Model Code 90 | |||
| Category B | Category A | Category S | |
| u | >=2.5% | >=5% | >=6% |
| ft/fy | >=1.05 | >=1.08 | >=1.15 |
| Tab. 5a - Minimum values of resistance ratios and of final stress according to Model Code 90 | |||
| Eurocode 2 | Eurocode 8 | |||
| ND | HD | DC-M | DC-H | |
| u | >=2.5% | >=5% | >=6% | >=9% |
| ft/fy | >=1.05 | >=1.08 | >=1.15 | >=1.20 |
| Tab. 5b - Minimum values of final resistance and stress according to EC2 and EC8. | ||||
However we should point out that a high material ductility does not correspond, of course, to a high structural ductility since in rebars other phenomena appear, linked to cross section behaviour or to the structural element and to other specific problems capable of affecting the ductility itself. On that point studies are being carried out aimed at improving concrete quality.