New Materials and Systems for Prestressed Concrete Structures"

COST Materials Action 534

WG5-P2LONG TERM DURABILITY AND SERVICE LIFE

OF POSTENSIONED STRUCTURESToulouse-November 2007

Carmen. Andrade_IETcc, Christian Tessier-LCPC,

Manuela Salta-LNEC, Ramon Novoa- Un. Vigo,

Oystein Vennesland-NTNU, Ranieri Cigna-IPROMA,

Joost Gulikers-RB, Andrea Mercalli-Autostrade

DIFFERENTIAL CARACTERISTICS OF PRESTRESSING

• Regarding the estimation of the life time of prestressing and postensionedconcrete there are several differenceswith respect to reinforced concrete.

• The main which could influence this estimation are:– The type of steel– The detailing of the reinforcement

location and therefore, the cover thickness.

– The existence of ducts and grout/ grease.

– The crack widths– The corrosion rate :The level of

tension of the reinforcements induces SSC processes.

Detailing of the reinforcement location cover thickness

CORROSION MODELLING

ttotal = t1 + t2 + t3 + t4

t1t2

CORROSION MODELLINGbrittle failure

ttotal = t1 + t2 + t3 + t4 =

= [[[[Dt]]]] � duct cover��x/10 Vcorr + [Dt]]]]�grout + Px/Vcrack

t1t2t3

t1 t2 t3

On the propagation rates, the load level has an important influence as in order the embritlement to be produced, it is necessary the existence of a load threshold

Stress Corrosion Crackingmechanism

• It introduces crucial changes– It is of not electrochemical

nature– It may progress

simultaneously to H2induced stress corrosion

– Its generation cannot be predicted yet

– The Vcrack is very high

OBJECTIVESP2-WG5

TO DEVELOP A SERVICE LIFE MODEL 1. Passivating abilities of grouts.

1. Effect of contaminants2. Chloride difusion and threshold3. Carbonation resistance4. Corrosion rates5. Techniques of charaterization6. Service life model

Passivating properties of groutsR. Novoa et al.

• Typical factors, such as applied stress, temperature or chloride content, have been explored. • Moreover, the effect of pressure waves was also taken into account.

– Some authors1-3 have shown that sonic waves, in different frequency ranges, are able to modify properties such as permeability and viscosity when applied through a porous material.

– Studies at the University of Vigo showed also that cement paste responds to applied pressure by moving water from interlamellar spaces towards void pores, which will affect the wettability degree at the rebar-mortar interface.

– Moreover, pressure waves could also affect the integrity of the passive layer because of the different Young’s Modulus involved at the interface (metal, oxide and concrete cover).

– Thus, this section gives a first insight into the relative effect of mechanical waves on the stability of the passive layer of tendons under stress.

– This aspect is of interest because in roads and bridges (but not only) the interface rebar-concrete or tendon-grouting material is under continuous exposure to pressure waves of car traffic origin.

Conclusion• This preliminary study shows that temperature changes and pressure waves have to be

regarded as important factors in the corrosion behaviour of tendons under applied stress.

• A range of energies need to be studied in order to establish whether shock waves generated by frequent automobile circulation are able to induce irreversible damage of the passive layer even in the absence of chlorides.

Influence of defects in cementitious groutingChristian Tessier with Elise Blactot, Laurent Gaillet,

Thierry Chaussadent, Fabienne Farcas

One of the main parts of the study led to better understand corrosion behaviour of tendons embedded in

unsatisfactory or defective grouting. This defective grouting can be in three different forms:

presence of voids, presence of bleeding water, presence of segregated substancesPresence of depassivating ions

Conclusions• Prestressing steel in contact with segregated cement grout can be subjected to

stress corrosion cracking phenomena.• Experiments were carried out without mechanical stress to identify the influence of

three parameters on steel SCC susceptibility:– sulphate content, – pH and– steel surface.

• It has been observed that – (a) sulphate content has little influence on SCC in these conditions (pH > 12);– (b) high pH can generate important anodic peaks in the active/passive zone; and– (c) the specific layer present on prestressing steel surface due to the cold drawn process

partially breaks up and thus generates a high susceptibility to corrosion.

Modifications to grouts to improve performance

[Ranieri Cigna, Cecilia Bartuli]

Inhibitors

• In the ducts of the post-tensioned structures the situation is further complicated by – inadequate pumping of the cementitious grout and/or– bleeding within the duct itself leading to the formation of voids.

• A consequence is that the carbon steel strands cannot be protected by the alkaline environment generated by the hydration reaction of the cement constituents.

• Inhibitors are now often used, mixed-in in new reinforced concrete structures exposed to very aggressive environments

Modifications to grouts to improve performance

[Ranieri Cigna, Cecilia Bartuli](II)Conclusions• inhibitors are added to the cement mixture in the grout, showing that the mechanism

of action of these chemicals also involves migration in the vapour phase.• A significant effect of corrosion inhibitors seems to be attested by this very

preliminary experimental evidence. • Both uniform and localized corrosion phenomena proceed at a lower rate when

migrating• Of the formulations tested the most effective appeared to be amino-alcohol 1

(liquid) and amino-alcohol 2 (powder), indicating that the physical state of the inhibitor is not an issue of any fundamental importance.

The following limitations of the work should be mentioned:• Carbon steel was tested for the present investigation instead of high strength steel normally

used for tensioned strands: the significance of the experimental results is thus to be found in the relative comparison of the aggressiveness of different environments;

• The test should be considered as accelerated, due to the relatively – Elevated temperature (30°C); the influence of this parameter is extremely significant, as

demonstrated by preliminary tests carried out at 50°C, leading to very severe degradation of all exposed materials;

– the influence of inhibitors on the rheological properties of the grout was not investigated;– longer term effectiveness of inhibitors must be investigated.

Carbonation and chloride ingress in groutsC. Andrade

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50

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0 4 8 12 16 20 24 28

Tiempo (Días)

Re

(Om

hio

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Superstresscem

Flowcable

Intacrete

•It was used the same cement with 3 different admixtures• Is remarkable the fact that the admixtures change significantly the porodity and resistivity•And therefore the carbonation and chloride penetration•The differences may be due the different reology and air entrainment introduced by each admixture• The different diffusivities and chloride thresholds will need further work to be justified.

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Critical ThresholdOystein Vennesland

Definitions

• The critical chloride content or chloride threshold value in concrete is usually defined as– The chloride content required for depassivation of the steel (definition 1).

• However, depassivation of the reinforcement may not always lead to any deterioration: e.g. in very dry concrete the corrosion rates are kept low or in water-saturated concrete the cathodicreaction is inhibited by low oxygen availability which limits the corrosion rate.

• Thus the critical chloride content can also be defined as– the chloride content associated with visible or acceptable deterioration of the reinforced

concrete structure (definition 2), e.g. cracks, spalling, a certain loss in cross section, etc.

Different chloride thresholds are obtained by using different definitions.– It has to be noted that whereas in definition 1, the depassivation depends on the chloride

concentration at the rebar, – in definition 2 the higher critical chloride contents associated with an acceptable degree of

corrosion is only the result of a longer time passing until the chloride concentration is determined. Also the term “acceptable degree” is somewhat imprecise and confusing.

However, whereas definition 1 is more precise from a scientific point of view, it is actually definition 2 that is more relevant “from an engineering point of view” and is thus preferably used in practice (e.g. durability design, modelling of service life).

Critical ThresholdOystein Vennesland

Influencing parameters- Concentration of hydroxyl ions in the pore solution (pH)- Potential of the steel- Presence of voids at the steel/concrete interface- Type of cement (chloride binding capacity)- Water to cement or binder ratio- Moisture content of the concrete- Oxygen content in the concrete- Chemical composition of the steel- Surface condition of the steel- Temperature- Type of cation accompanying the chloride ion

Permanent monitoring of corrosion in prestressedand post-tensioned concrete with embedded

sensorsElsa Vaz Pereira and Manuela Salta

• Several sensors have been proposed, in the last fifteen to twenty years, – corrosion rate, – open circuit potential, – concrete resistivity, – chloride content, – temperature or moisture content

Conclusions• The identification of the places with higher probability of corrosion is

essential for the proper detection of corrosion initiation. • Galvanic current and open circuit potential sensors seem to be good

options for detecting corrosion induced by chlorides, • As the electrical resistance is influenced by the moisture content, it

could be used as a tool to detect changes in the degree of saturation of concrete or grouts. However, it might not allow the detection of concrete contamination with chlorides.

-0.7

-0.5

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0 150 300 450 600 750

Time (days)

Ecorr vs MnO

2 (V)

Non-contaminated

Internal chlorides (3%)

External chlorides (1%)

a)

-3.0E-05

-2.5E-05

-2.0E-05

-1.5E-05

-1.0E-05

-5.0E-06

0.0E+00

5.0E-06

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Time (days)

I (A)

External chlorides (1%)

Internal chlorides (3%)

Non-contaminated b)

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1000

10000

100000

0 150 300 450 600 750Time (days)

R (O

hm)

External chlorides (1%)

Internal chlorides (3%)

Non-contaminated

c)

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Time (days)

Precipitation (m

m)

e)

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T / ºC

d)

Numerical modelling of the initiation and propagation processes

[Joost Gulikers]

0,0

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age, t [yr]

chlo

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C(x

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%m

/m]

obstruction

In 1-dimensional approach 3 subsequent stages can be distinguished:

•The initiation stage for the metal duct: chloride ingress into concrete•The propagation stage of the duct: corrosion until perforation•The initiation stage of the prestressing steel : chloride ingress through the grout

For prestressed concrete with prestressing steel embedded in ducts additional model parameters have to be included:

•Dap apparent diffusion coefficient of the grout material•Ccrit,p critical chloride content for prestressing steel.

An example of chloride ingress with temporary obstruction is shown in Figure .

Numerical modelling of the initiation and propagation processes

[Joost Gulikers] (II)Practical remarks/considerations• Generally, the reinforcing steel will start to corrode much earlier than the metal

duct as a result of the smaller thickness of the concrete cover. The required cover depth to prevent the reinforcing steel from corroding during the design service life can be calculated from:

• For prestressing steel in direct contact with the concrete the cover thickness should be such as to compensate for the lower critical chloride content.

• As an example: – for Cs = 3.0%, Ci = 0.1%, Dao = 5*10-12m2/s, n = 0.30, tdsl = 100yr, a cover

thickness of 90mm would be required to prevent the reinforcing steel from corroding.

– For prestressing steel a cover depth d = 99mm, 110mm and 128 mm would be required for Ccrit = 0.4%, 0.3% and 0.2%, respectively.

• A more pragmatic approach would be to discard the obstructing influenceof the duct

• This simplified and conservative approach can be justified when there is a lack of reliable data on the transport properties of the grouting material, the critical chloride content of prestressing steel embedded in grout, as well as the effective thickness of the grouting on the prestressing steel.

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THANKS FOR YOUR ATTENTION