New Materials and Systems for Prestressed Concrete Structures" COST Materials Action 534

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1 New Materials and Systems for Prestressed Concrete Structures" COST Materials Action 534 WG5-P2 LONG TERM DURABILITY AND SERVICE LIFE OF POSTENSIONED STRUCTURES Toulouse-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

2 DIFFERENTIAL CARACTERISTICS OF PRESTRESSING Regarding the estimation of the life time of prestressing and postensioned concrete there are several differences with 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.

3 Detailing of the reinforcement location cover thickness CORROSION MODELLING t 2 t 1 t total = t 1 + t 2 + t 3 + t 4

4 CORROSION MODELLING brittle failure t total = t 1 + t 2 + t 3 + t 4 = = [Dt] duct coverx/10 V corr + [Dt] grout + P x /V crack t 1 t 2 t 3 t 1 t 2 t 3 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

5 Stress Corrosion Cracking mechanism It introduces crucial changes It is of not electrochemical nature It may progress simultaneously to H 2 induced stress corrosion Its generation cannot be predicted yet The V crack is very high

6 OBJECTIVES P2-WG5 TO DEVELOP A SERVICE LIFE MODEL 1. Passivating abilities of grouts. 1. Effect of contaminants 2. Chloride difusion and threshold 3. Carbonation resistance 4. Corrosion rates 5. Techniques of charaterization 6. Service life model

7 Passivating properties of grouts R. 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 rebarconcrete 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.

8 Influence of defects in cementitious grouting Christian 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 substances Presence 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.

9 Inhibitors Modifications to grouts to improve performance [Ranieri Cigna, Cecilia Bartuli] 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

10 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.

11 Carbonation and chloride ingress in grouts C. Andrade Superstresscem Flowcable./0).').. *+, (-+) %&'( )') 300 Intacrete Re (Omhios) !!"#!!$ # Tiempo (Días) 0,18./0).').. *+, (-+) %&'( )') 0,16 Flow cable Pore volume (ml/g) 0,14 0,12 0,1 0,08 0,06 Superstressem Intacrete 4!!"#!!$ $ # 0,04 0, Pore diameter (um) 0,1 0,01 0, #3456 ##!7# 12 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.

12 Definitions Critical Threshold Oystein Vennesland 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 cathodic reaction 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).

13 Critical Threshold Oystein 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

14 E E E E E E E E Non-contaminated External chlorides (1%) Internal chlorides (3%) External chlorides (1%) Non-contaminated Internal chlorides (3%) b) d) e) c) Permanent monitoring of corrosion in prestressed and post-tensioned concrete with embedded sensors Elsa 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. I (A) R(Ohm) E corr vsmno 2 (V) T/ ºC Precipitation(mm) Time (days) Time (days) Time (days) Time (days) Time (days) Non-contaminated a) External chlorides (1%) Internal chlorides (3%)

15 Numerical modelling of the initiation and propagation processes [Joost Gulikers] 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. chlo rid e co ntent, C (x=d ) [% m /m ] 1,2 1,0 0,8 0,6 0,4 0,2 0,0 obstruction age, t [yr]

16 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 influence of 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.

17 THANKS FOR YOUR ATTENTION