(Microsoft PowerPoint - 2 - Lecture N\374rnberger, PPP.ppt)

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 1

Ulf NUlf Nüürnbergerrnberger

PrestressingPrestressing steelsteel-- CorrosionCorrosion damagesdamagesand and applicationapplication of of stainlessstainless steelsteel tendonstendons

ContributionContribution of of thethe groupgroup „„Metallic Metallic TendonsTendons““U. Nürnberger, Y. Wu, University of Stuttgart, GermanyM. C. Alonso, F. J. Recio, Instituto Eduardo Torroja, Madrid, Spain

COST 534 FINAL WORKSHOP, 26COST 534 FINAL WORKSHOP, 26--27 November 2007, TOULOUSE, FRANCE27 November 2007, TOULOUSE, FRANCE

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 2

Collapse of a prestressed concretebeam of a laboratory roof after35 years in use

fracture of the prestressing cablesin the moment tension zone

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 3

Reasons of damages of prestressing steel

• insufficient design (poor construction)

• incorrect execution of planned design (poor workmanship)

• unsuitable mineral building materials

• unsuitable post-tensioning system components including the prestressing steel

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 4

Hydrogen induced cracksand fractures of cold deformed wirewrapped around a concrete tube

Preconditions for HPreconditions for H--SCC:SCC:- sensitive material or state,- sufficient high tension load,- at least a slight corrosion attack.

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 5

Mechanism of hydrogen assisted stress corrosion cracking

electrolyte

metal

AA Electrochemical corrosion processes on the steel surface withcathodic hydrogen evolution:

anodic reaction : Fe →→→→ Fe++ + 2e- (iron dissolution) cathodic reaction: 2H+ + 2e- →→→→ 2H (discharging hydrogen)

BB absorption and diffusion of atomic hydrogenCC hydrogen assisted crack formationDD crack propagation

A B C A B C DD

plasticzone

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Mechanism of pitting inducedstress corrosion cracking of high strength steel

crack

in in corrosioncorrosion pitpit: : hydrolysis

FeCl2 + 2H2O →→→→ Fe(OH)2 + 2HClcorrosionpit

corrosion product (rust)

crack

pre-crackscrack

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Pitting induced stress corrosion cracking (H-SCC)

pitting

crack

mill scale

corrosionpit

precrack

1cm ==== 30 µm1cm ==== 30 µm

surface (scanning microskope) metallographical slip

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- Advantages of stainless steels in comparison to other corrosionprotection methods in hevealy chloride contaminated concrte.

- Successful application of high strength stainless steel strands forropes and cables for bridges, roofs etc..

- Successful introduction of stainless of steel reinforcement in carbona-ted and chloride containing reinforced concrete because of very high corrosion resistivity.

Reasons for application of prestressing stainless steel

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 9

The question is:Can we transfer the positive experiencesfrom• high strength stainless steel cables• and stainless steel reinforcementto prestressed concrete?

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Application of stainless steel spiral ropesas hangers on a foot-bridge in Stuttgart

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 11

0 20 40 60 80 100 cold-deformation in %

tens

ilest

reng

thR

min

N/m

m2

min. Rm

• Austenitic chromium-nickel steelshave a pronounced tendencytowards work hardening.

• Therefore the strength of wires canbe increased by cold-deformation.

• To reach a notified strength of 1450 N/mm2 the wires must bedeformed till a degree of 50 to 70%.

• Not too high alloyed stainless steelswith a structure instability tend toformation of martensite.

• Martensite in the austenitic struc-ture increases the strength.

• However, it may lead to a worsecorrosion behaviour.

Production of high strengthstainless steel strands

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 12

1.4301 (X5CrNi 18-10)no application for ropes in Germany

1.4401 (X5CrNiMo 17-12-2)application for ropes in not contaminated urban atmosphere

1.4436 (X3CrNiMo 17-13-3)application for ropes in chloride contaminated atmosphere(de-icing salt spray)

1.4439 (X2CrNiMoN 17-13-5)application for ropes in offshore structures

Investigated materials for prestressed concreteand their present application for ropes outside concrete

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 13

Test results of the magnetic permeability �r of stainless prestressing steel(permeability = ferromagnetic behaviour, corresponds to the austenite stability

and with the martensite portion in the austenitic structure after cold deformation)

1.4401CrNiMo 17-12-2�r = 1.2

1.4439CrNiMo 17-13-5�r = 1.0

austenite-stability increaseswith the alloy elements as follows:

AS ≅≅≅≅ 14Cr + 10Ni + 19Mo

1.4301: µr = 17.7 much martensite1.4401: µr = 1.2 very low martensite1.4439: µr = 1.0 no martensite

1.4301CrNi 18-10�r = 17.7

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• high strength and sufficiently high proof stress• sufficiently high ductility• sufficiently relaxation behaviour under performance load• good behaviour in (wedge) anchorage systems• sufficiently high dynamic loadability• high corrosion resistance against

- pitting corrosion- anodic SCC- hydrogen assisted SCCunder atmospheric corrosion conditions (transport, handling on site etc.)and in carbonated and/or chloride containing concrete

Necessary performance characteristics of prestressing steelfor application in concrete

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• The pitting corrosion potentials ofcold drawn high strength stainless steel wires 1.4301 - 1.4401 - 1.4439were determined by potentiostaticcurrent-potential measurements with mortar electrodes in

- alkaline concrete - carbonated concrete

with 5% chloride relative to cement weight.

• The results have been compared with those of cold drawn low strength reinforcing steels (deformation degree 36 %)of the same composition and similar surface condition.

Potentiostatic determination of the pitting corrosion potentialon high strength stainless steel wires with mortar electrodes

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 16

1.4439 1.4401 1.4301 1.4439 1.4401 1

CrNiMo CrNiMo CrNi CrNiMo CrNiMo CrNi17-13-5 17-12-2 18-10 17-13-5 17-12-2 18-10

pitt

ing

corr

. pot

entia

l Ep

in m

V-c

al

200

0

400

600 alkaline carbonated

Pitting corrosion potential of cold-drawn stainless steels in alkaline and carbonated concrete with 5 mass-% Cl�

• EP decreases in the order1.4439 - 1.4401 - 1.4301.

• In carbonated concretethe EP is significantlylower than in alkalineconcrete.

• The difference betweenEP - reinforcing steel andEP - prestressing steelincreases in the order1.4439 - 1.4401 - 1.4301(reason: martensite).

• It is expected that EP ofprestressing steel 1.4301in carbonated concrete(tests not finished)will be unacceptable low.

reinforcing steelprestressing steel

After these tests the prestressing steel 1.4401 can be recommended for highlychloride-contaminated concrete: The higher deformation grade does not very badly affect the pitting corrosion behaviour.

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 17

pitt

ing

corr

. pot

entia

l Ep

in m

V-c

al

1.4301 (X5CrNi 18-10)1.4436 (X3CrNiMo 17-13-3)1.4401 (X5CrNiMo 17-12-2)

pH 13,2

pH 8,5

0 0,2 0,4 0,6 0,8 1,0 chloride concentration in Mol

Pitting corrosion potential forhigh strength stainless steel in solutions representing alkalineand carbonated concrete pollu-ted with chlorides

University of Stuttgart, Germany, and Instituto Eduardo Torroja, Madrid, Spain 18Anodic stress corrosion tests in saturated chloride solutions

The threshold temperatures of stressed single wires in saturated chloride solutions were determined by isotherm exposure tests on bending specimens.

The threshold temperature is that temperature, below that no SCC can occur.

solution 3: acetic acid/sodium-acetate, pH=4.5 buffer solution + saturated NaCl-solution

30°C – 40°C – 50°C – 60°C – 80°Ctemperature

> 15 000 htest time

solution 2: carbonate/bicarbonate, pH=8.5buffer solution + saturated NaCl-solution

solution 1: calciumhydroxide solution, pH=12.1+ saturated NaCl-solution

solution condition

1.4439 (X2CrNiMoN 17-13-5)

1.4436 (X3CrNiMo 17-13-3)

1.4401 (X5CrNiMo 17-12-2)

1.4301 (X5CrNi 18-10)

material

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bending specimen

broken specimen: • pH = 8.5• temperature = 80°C• steel =1.4401• lifetime = 765h

new

broken

Specimens for anodic SCC-test

100 µm

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5000

1000

10 000

15 000

40 50 60 80

life

time

in h

The resistance to anodic Cl�- SCC rises with • increasing pH-value • decreasing temperature of media• increasing steel quality (1.4401 1.4439)

After these teststhe prestressing steel 1.4401 can berecommended for highly chloride-contaminated concrete.

Results of SCC - tests of cold-drawn high-strength stainless steel wiresin chloride-saturated solutions of different temperature and pH-value

temperature in °C

pH 4.5

1.4401pH 12.1

1.4439

pH 4.5

pH 12.1

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SCC-tests on centricallystressed wiresin aqueous solutionsunder hydrogen charge

temperedtest vessel

stressed wire

testing frame

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Hydrogen - induced SCC behaviour of prestressing stainless steel wires (mean value of life times in h from 3 tests)

no fracture within 4000 h860 h102 h

FIP - test with cathodic polarisation

to -1000mV

no fracture within 4000 h250 h91 h

FIP - test with pre-corrosion

by MgCl2 spots

no fracture within 4000 h2830 h145 h

FIP - test:R = 80% Rm, 50°C,

20% NH4SCN-solution

1.4439 CrNiMo17-13-5

1.4401 CrNiMo17-12-2

1.4301 CrNi18-10

test conditions

materials

• the lifetime decreases in the order 1.4439 - 1.4401 - 1.4301,• It seems that martensite reduces considerably the hydrogen resistance, • the lifetime of 1.4401 exceed those of conventional prestressing steels.

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Conclusion

• All tests carried through have shown that high strengthstainless steel wires of quality 1.4401 are resistantenough to withstand pitting and stress corrosioncracking in strongly chloride contaminated concrete.

• In case of 1.4301 deformation martensite increases in a not acceptable degree the suceptibility to all kinds of corrosion.

• Our results correspond with the results from theinvestigations of Instituto Eduardo Torroja in Madrid