PROTECTION OF NEW CONTAINER TERMINAL AT ZEEBRUGGE AGAINSTCHLORIDE INGRESS

ir L. Schueremans

Catholic University of Leuven

Department of Civil Engineering

W. de Croylaan 2

3000 Leuven

prof. dr. ir. D. Van Gemert

Catholic University of Leuven

Department of Civil Engineering

W. de Croylaan 2

3000 Leuven

KEYW ORDS: Chloride Ingress, Wa ter-repellent, Hydrophobic Agent

ABSTRACT

Treatment of the concrete surface by impregnation with a water-repellent reduces the absorption and transport of water and the salts

dissolved in it, without hindering the water damp diffusion through the pore system. With the appropriate water-repellent this leads

to an improved resistance against the damage mechanisms : water erosion, pitting corrosion and alkali-aggregate reaction. The

effectiveness criteria of an hyd rophob ic agent are : deep and active penetration; applicability in successive treatments wet on wet

or wet on dry as part of a multi-layer system; easy application on the job site within large temperature spans; impact on environ ment.

Today the hydrophobic treatment is no longer considered as a remedial treatment, but it is considered to be an integral part of the

construction process of a concrete structure. As an example of this new philosophy the application of highly concentrated, solvent-

free hydropho bic agents at the construction of the new container terminal at Zeebrugge in 1992-1993 are discussed. In 1996 cores

were taken from the concrete wall at different locations in and above the tidal zone . The effectiveness of the hydrophobic treatment

as a barrier against chloride ingress was checked.

INTRODUCTION

Chloride ingress is one of the most imp ortant actor s in the corrosio n process o f concrete re inforceme nt rods. Esp ecially for off-

shore and marine constructions the exposition to chlorides from sea water and marine air p lay an impo rtant role. Also, the co ncrete

of swimming -pools or sa nitarian installations is heavily expo sed to water rich at chlorides. The ingress of chloride ions into the

pores of the concrete is caused by diffusion through the pores if they are filled with water or by capillary suction if the pores are

empty. When the chloride ions reach the reinforcement bars, the passivating oxide layer may be depassivated, which initiates the

local corro sion of the rod s.

The protection of concrete structures against chloride ingress should no lo nger be see n as a last reme dy in order to protect an d repair

the depassivated conc rete and corrode d steel, but as a full phase in the construction project as a preventive protection of the new

and non-exposed concrete. Such a preventive protection can be realized by means of a protective coating on the total exposed

surface, which might b e a quite exp ensive unde rtaking. A more elegant and cheaper solution is the applicatio n of a hydro phobic

treatment.

A brief overview is given of the durability problem of concrete structures in marine environment, on-shore as well as off-shore.

The production and the working mechanisms of hydrophobic agents are discussed, and the proced ures to evalua te their effectiveness

are described. The application of a protective h ydropho bic treatmen t by means o f highly concen trated, solven t-free hydrop hobic

agents during construction of the new co ntainer termina l at Zeebrugg e is discussed .

In 1996 cores were taken from the concrete quay-wall at different locations in and above the tidal zone. This allowed to check the

effectiveness of the hydrophobic treatment as a barrier against chloride ingre ss. The pe rformance of the hydrop hobic age nt is

evaluated a nd comp arisons are m ade betw een treated and non-trea ted zones.

DURABILITY OF CONCRETE

The most frequently occurring deterioration causes of concrete structures can be classified in five categories [Polen, 1993]. It often

happens that different causes appear at the same time, are located at different places or amplify each other :

1) damage by ex ternal impact on the structure in accidents,

2) dama ge caused by moisture a nd water, du e to bad fun ctioning of wate r drains or b y inapprop riate conce pt,

3) carbonation damage,

4) corrosion of steel reinforcements induced by chlorides (pitting corrosion),

5) damage caused by alkali-aggregate reaction (AAR).

Reinforced concrete durability is determined by its resistance against various chemical and physical pro cesses. In nor mal concr ete

the reinforcement stays in an alkaline environment with a pH-value of 12.5 to 13 and under these c onditions a d ense and w ell

adhering p rotective pa tina is formed on the reinfor cement surfa ce. This layer with a thickness of ap proxima tely 10 nm p revents

the reinforce ment from corrosio n [Gonz ález et al, 19 96, I & I I].

Under influence of carbon dioxide (CO2) from the air, the free lime in the co ncrete cement pa ste is bound, b y which the alka linity

is lost. This is the well-known carbonation process (annexe 1). The pH of the concrete drops to values form 8,5 to 9.

When the carbon ation dep th equals the co ncrete cov er depth, the r einforcem ent can start to c orrode o r rust.

For corrosion to happen, the presence of moisture and the ingress of oxygen from the air is needed (annexe 2). Concrete exposed

Figure 1 : Different kind s of salts in concr ete

to the outside climate contains enough moisture to keep the corrosion process running and the concrete cover allows the penetration

of a sufficient amount of oxygen. Of course, the less permeable and the thicker the concrete cover, the longer will be the time

needed for corrosion to appear, and hence, the longer will be the service life of the reinforced concrete structure concerned.

The chlorides in the concrete either come from the components of the fresh mix (water, sand, aggregates, additives), or from

external contamination agents (marine environment, de-icing salts).

The chlorides, present in the mix, react in different ways [Kissel & Pourbaix, 1996] (figure 1) :

- a part (.5%) forms insoluble salts or is locked in a pore of the silicates that are insoluble in water,

- a part (.85 till 90 % ) forms solub le salts (Salt of Frie del : C3A.CaCl.10H2O),

- a part (.5%) can be found in the concrete as free chlorides, in solution, or easily soluble by adding water.

The chlorides coming from external con tamination ag ents after the hardening of the concrete, react very little with the solid phase

of the concrete, and can be found in the concrete as free chlorides. The water soluble salts (Salt of Friedel) act as a stock of free

chlorides. In the presen ce of these salts, the water in the po res will enrich itself with ch lorides until a final concentration is reached

that equals the product of solub ility. The chlorides that stand for the corrosion risk of the reinforce d concrete structure are th e

chlorides in the pore water (the free chlorides) and a part of the soluble chlorides (Salt of Friedel).

At the same time it is a fact that the risk for rus t, with a given amo unt of chlorides in the pore water, will be much higher for a

carbona ted concr ete structure.

When aggressive products, like chloride ions, are present in the concrete, the corrosion also takes place in the above-mentioned

high alkaline en vironmen t. Aggressive e lements caus e fast and loca lized corro sion, called p itting-corrosio n (annexe 3 ).

The most important sources of chlorides for marine constructions are de-icing salts and the contact with marine air and splash-water.

The adding of calcium chloride or other chloride containing mixtures as a hardening accelerator mean a supplementary chloride

source for th e concrete .

For constructions in a marine environment or for constructions attacked by de-icing salts, the chlorides will penetrate through the

concrete cover in a relatively short time, and when they reach the steel reinforcement surface, the corrosion p rocess can start.

Depending on the amount of chlorides, the corrosion process can take place in an alkaline environment. A subsequent phase is the

spalling-off of the concrete cover due to the expansive corrosion reaction, which leads to the full exposition of parts of the

reinforcement rods to the aggressive environment. In most cases the concrete will give enough warnings before endangering the

structure. In prestressed concrete this is generally not the case : the prestressing cables or strands will break without previous visual

damage and warnin g.

In the neighbo urhood of cracks an d fissures the co rrosion pro cess is faster because of the easier access of CO2 and because of the

reinforcem ents residing in a low alkaline en vironmen t.

The reaction of alkalies with reactive aggregates, also called AAR , is in contrary with the above-mentioned processes, not a surface

reaction (annexe 4). The p rocess happens in the concrete mass. The alkali Na and K, which are present in the cement in the form

of their oxides, re act with the reac tive comp onents of the aggregates if the alkali content exceeds a certain threshold. The reaction

produces an alkali-silica gel. By uptake of water this gel swells and causes expansive forces which can destroy the concrete from

inside. The degree of swelling depends on many factors, but especially on the moisture content of the concrete. Tests have shown

that it is possible to obtain a stable situation of the concrete, even at alkali contents above the threshold, if the moisture content can

be kept low enough [Van Gemert, 1989].

ACTION PRINCIPLE OF SILANES

Various generations of protective products have been developed to counteract the aggressive actions of the environme nt against

concrete. Good results have been obtained with barrier-penetrants : after penetration in the concrete they form a barrier ag ainst

water and the salts diss olved in it. Different families of these hydrophobic agents or waterproofs are already being used for many

years in construction indus try : silicones, siloxan es, silanes, ... . The silan es used for w aterproo fing are mostly alk yl-trialkoxy-

Figure 2 : Chemical steps in the production of silicon-organic compounds

silanes and thus monom er products. The siloxa nes are oligomer or po lymer alkyl-alkoxy-siloxanes.

The starting product for all silicon-organic compounds is alkyl-trichloro-silane, figure 2. In figure 2, the alkyl-group is represented

by the symbol R. By transformation of this silane with alcohol (R’-OH) only, the corresponding alkyl-trialkoxy-silane is produced

together with separatio n of hydrog en-chloride . The reac tion with alcohol and water gives oligomer or polymer siloxanes, depending

on the amount of water used. The last two products differ in their degree of polymerization, see figure 2.

A classification of waterproofing agents is possible on the basis of their active ingredient content. The active ingredient content

of most siloxane based c ompound s is generally lower than 10 mass perc ent, especially because of two reaso ns :

- lower cost p rice : the lower th e active ingred ient, the lower the cost of the hydr ophob ic agent,

- smaller risk of ad hesion pro blems betw een the hydro phobic su rface and the protective su rface coating .

The hydrophobic treatment reduces the absorption and transport of liquid water and salts dissolved in it. Whereas the penetration

of water in liquid form should be entirely prevented, the waterproofing should hardly reduce the diffusion of water vapor. The water

repellents must penetrate as deep as possible into the concrete substrate to obtain a guaranteed long-term durability. At the same

time a maximum penetration depth is an essential prerequisite for an effective protection against chloride ingress and chloride

induced corrosion of the reinforcements. The penetration capacity of the waterproofing agent and its concentration gradient depend

on system specific parameters such as molecular size of the active organo-silicon compound and on the type of solvent used for

dilution. Besides, the penetration and the concentration of the active ingredient also depend on the porosity a nd the perm eability

of the concr ete substrate, o n the amou nt of waterpro ofing material a pplied, on the water con tent of the substra te ... .

Furthermore, a good water-repellent is characterized by a high alkali resistance and improves the resistance o f the substrate against

freeze-thaw action. Finally, there should be no und esirable side effects such as color cha nges of the surface or film formation.

Waterp roofs generally do not provide effective protection against carbonation caused by atmospheric CO2. A waterproofing can

only affect the carbonation behavior through changes in the water content of the substrate [Fliedner, 1995].

System Active ingredient Content active ingredient [w%] Water absorption [%]

1 Siloxane 7,6 100 reference

2 Siloxane 6,0 95

3 Siloxane 7,6 80

4 Silane (IBTE O-based com pounds) 40,0 40

5 silane/siloxane micro emulsion 19,0 100

6 siloxane 20,0 95

7 silane/siloxane 20,0 50

8 silane/siloxane (IBTE O-based com pounds) 100,0 30

9 silane/siloxane micro emulsion 20,0 50

10 siloxane 5,9 80

Table 1 : Relative wate r absorptio n after 28 d ays in concre te, treated with d ifferent waterpr oofing agen ts

Of the silanes, isobutyl-triethoxy-silane (IBTEO) based water-repellents have been found to be particularly suitable for

waterproofing, especially if app lied on low p orosity cons truction mater ials, table 1. Isobutyl-triethoxy-silane penetrates much

deeper in the concrete substrate than other silicon-organic compounds, in particular compared to siloxanes. As a consequ ence, a

Figure 3 :C hloride pe netration in co ncrete

cubes, at immersion in salt water [Fliedner,

1993]

Figure 4 : Test set-up for forced Cl--diffusion in con crete

much longer durability is achieved.

The deep penetration of such a product also provides an effective protection against chloride ingress and pitting corrosion, figure

3.

The good performances are confirmed in forced chloride penetration tests

[Van Gemert, 1995], [Van Gemert & Horckmans, 1992]. The principle of the

test is shown in figure 4.

A compar ison of test results is giv en in table 2. Th e given chlo ride penetra tion time CP T is the time in hours needed for the chloride

ions to reach the steel rod in the middle of the concrete test specimen. The times given in table 2 show that the surface penetrating

waterproofing treatment at non completely saturated concrete is more effective than a polymer cement concrete with 10 % polymer

to cement ra tio.

Figure 5 : Reaction mechanism of isobutyl-triethoxy-silane

Material W/C Conservation CPT [h]

Reference

+EVA

+EVA + SCA

IBTEO

IBTEO

IBTEO

0,5

0,5

0,5

0,5

0,5

0,5

saturated

saturated

saturated

saturated

immersed

dried at 105 °C

235

178

227

105-106

>140 (5V)

>800

Legend : Voltage : 3V Conservation in a 3% NaCl-solution before tes t Cement : P40/CEM I

4.5

EVA : Ethylene-vinyl-acetate SCA : silane coupling agent

Table 2 : Chloride penetration time for different concretes

The action of hyd rophob ic agents stems from their du al character w ith both hydro phobic as well as hydro philic properties, figure

5. The efficiency, stability and durability of the molecule are determined by its nature and magnitude . The hydr ophilic com ponents

react with water. At this reaction, ethanol is separated. The produced silanol can be described as an alkyl-silica-com pound. This

highly reactive c ompou nd reacts with n eighboring molecules to make po lymers.

The silanol is also able to react with the inorganic surfaces in the concrete pores. The thus chemically bonded alkyl-groups form

a protective coating for the concrete. Capillary suction is not only counteracted, but even reversed. The capillary forces prevent

the penetration of liquid water, even if the water is pressurized. This effect can be compared with the working of a water-proof

raincoat. Unlike siloxa nes, and than ks to its dual character, the isobutyl-triethox y-silane is also app licable on w et concrete

substrates without remarkable influence on the penetration depth. The adhesion between the treated surface and an eventually

additional coating is not affected as well. In the series of methyl-, ethyl-, propyl- and isobutyl-triethoxy-silane, the IBTEO

waterproofing agent has the smallest hydrolysis reaction speed, which enables its maximum penetration. Moreover, this better

penetration capacity is further improved by the lower surface tension of the solvent-free silane, compared to solvent containing,

diluted silane mixes. The lower hydrolysis speed keeps the IBTEO hydrophilic during a longer time, an d allows it to pe netrate into

wet concrete pores before the hydrophobic interfaces are formed.

CASE STUDY : PROTECTION OF NEW CONTAINER TERMINAL AT ZEEBRUGGE

The construction of the container terminal at Zeebrugge was ordered by the Min istry of the Flemish Community, Sea-Harbour

Division and finished in 1993. Figure 6 gives a general view of the quay-wall. This quay-wall is constructed on top of cylindrical

sunk down reinforced cells (caissons).

The original concept of preventive corrosion protection provided in the use of epoxy coated steel reinforcements. For practical

reasons of ease of application it was decided to apply an alternative solution, consisting of a hydroph obic treatm ent with a highly

concentrated solvent-free co mpound based on isobutyl-triethoxy-silan e in order to prevent it for damage caused by (1) chloride

penetration , (2) pitting cor rosion and (3)AAR .

Figure 6 : Plan view and cross-section of the quay-wall and locations of the cores

By means of the preliminary re search pro gram [V an Ton gelen et al, 199 4], the effectiven ess of the hydro phobic agent was evaluated

by the following criteria, which had to be fulfilled to label the product as a quality product and a usable product for practice :

1) deep and active penetration to obtain a strongly improved long-term protection;

2) being applicable in combination with additional protective coatings in a multi-layer system, where the additional layers are

applied to slow dow n the progr ess of the carb onation fron t. The hydrophobic agent applied on the concrete surface may not reduce

the adhesion of the coating film;

3) easy application on the site: no special conditions should be required concerning : pH, moisture content and temperature of the

concrete substrate (-15°C till 45 °C);

4) being environmental friendly. A solvent-free product should be preferred.

The design engin eers of the Se a-Harbo ur Division b ased their de cision on the r esults of the research program mentioned above,

on additional tests executed at the Reyntjens lab oratory of K.U.Leu ven and on spec ific tests, executed in the laboratories of Hüls

A.G. in T roisdorf.

Samples of the specific concrete were prepared and used in the laboratory to determine the penetration depth of the water-repellent

and the total consumption per m2. The results fo r the specific q uay-wall concr ete are given in table 3. A trea tment consists o f a

15 seco nds immer sion of a sam ple in the water -repellent.

Sample Second treatment

after

Penetration

depth [mm]

Immersion time [s] Consum ption [g/m 2]

1st treatment 2nd treatment Total

1

2

3

4

5

2 days

4 days

7 days

7 days

7days

6

7

8

8

9

2 x 15

2 x 15

2 x 15

2 x 15

2 x 60

138

169

89

102

156

187

285

182

204

244

325

454

271

306

400

Table 3 : Penetration depth and consumption per m2 as function of time between first and second treatment

Figure 7 : A irless spraying o f water-repellen t with

plunger pump at low pressure

Figure 8 : T aking of cylind rical cores o n top of the q uay-

wall

Figure 9 : pH-values at different depths of the cores

At the construction site, the following application scheme has been adopted :

- a first application of the water-repellent was done immediately after demolding. This prevented the surface drying of the concrete,

as well as the eventual penetration of salt water;

- a second application was executed 7 days after demolding, in order to obtain a deep penetration in the concrete.

The waterproofing has been applied by means o f airless spraying at low pressures, using a plunger pump, figure 7. The consumption

was measured to be 0,35 liters per m2, which coincided very well with the preliminary tests in the laboratory on concrete cubes,

prepared on the construction site. The treatm ent was don e at the concr ete structure ab ove water le vel as well as in the splash-zone.

The total project included a surface of 15.000 m2 to treat with hydro phobic a gent.

In 1996, the in site reached performance of the IBTEO water-repellent was evaluated [Van Gemert & Schueremans, 1996] on

cylindrical cores taken at different locations in the quay-wall (figure 8) : treated (location 2) and non-treated (loca tion 1) locations,

in the tidal zone , above wa ter level and o n top of the q uay-wall, figure 6.

To get a global view of the effectiveness of the hydrophobic agent, following characteristics were measured : penetration depth,

porosity, pH at following de pths : 0-9 mm, 11-20 mm, 40- 60 mm, carbonation depth, compressive strength, cement content and

chloride content. T he pH-va lues, figure 9, are somewha t higher at the non-treated location. However, all pH-values are in the

region of p assivation.

Fully corresponding resu lts are obtained by the measure d carbonation de pths, table 4. For the non-treated loc ations, the carbonation

depth is limited to 1 mm, for the treated locations a carbonation depth up to 5 mm is measured, which declares the lower pH-values

in the first zone which are mean values for a depth from 0 till 9 mm. The used water-repellent has no protective influence on the

carbonation process of the concrete, as mentioned before. The measured penetration depth was limited to 3.5 mm, which is lower

than the results o btained in the laborator y tests (table 3).

Figure 10 : Water-soluble chloride content at different depths in the cores

Porosity

n [vol%]

Dry density of the

concrete [k g/m3]

Compressive

strength [N/mm2]

Cement content

[kg/m3]

Location of cores Carbonation

depth [mm]

Non-treated

(location 1,

figure 6)

15,6 2205 44,5 284 in tidal zone 1

above tidal zone 0

Treated

(location 2,

figure 6)

16,2 2215 43,1 260 in tidal zone 5

above tidal zone 4

on top of q uay-wall 5

Table 4 : Characteristics and carbonation depth of the cores. CEM III-42.5

The chloride content was measured at the following depths : 0-9 mm, 11-20 mm, 40-60 mm. The content pro files of water solub le

chlorides are shown in figure 10. The chloride contents were determined by means of wet chemical analysis, according to the

Belgian Standard NBN B15-257. The chloride content, obtained by wet chemical analysis, equals the amount of free chlorides and

a great deal of the chlorides, bound under the form of the Salt of Friedel which dissolves in the water during extraction. In fac t,

these water so luble chlorid es mean the real dange r for corros ion of the reinfo rcement.

The relation water-soluble to acid-soluble chloride content amounts 94 %. Thus, the chloride content, present at the binding and

transformed into insoluble chlorides is very restricted. The chloride loading is for the greater part caused by the external

contamination, i.e. the marine env ironment. It can be ob served that the chloride co ntent in the non-tre ated zone s is significantly

higher than in the treated zo nes. The absence of an effective barrier, combined with the relatively high porosity, allowed the

chlorides to penetrate very deep in the concrete. O n the contrar y, the chloride c ontents in the trea ted zones a re still at accepta ble

level, and co mbined w ith the low carb onation de pth, they prese nt no dange r for reinforce ment corro sion.

Direct exposition in the tidal zone and exposition in the splash-zone deliver the biggest chloride loading. The difference between

them is less significant. The chloride attack on top of the quay-wall, due to marine air and further removed from direct exposition,

prove to be a remarkable lower chloride loading.

CONCLUSIONS

Hydrop hobic treatment of concrete surfaces is an excellent and elegant way to protect r einforced a nd prestres sed conc rete

constructions against aggressive environments. T he use of an ap propriate hydropho bic agent assu res the prote ction against a

multitude of damaging elements, such as water penetration, chloride penetration , alkali-aggrega te reaction. T ests have shown the

effectiveness of highly concentrated, solvent-free compounds based on isobutyl-triethoxy-silane. Chloride contents after 3 years

of real life exposure in the tidal zone at seaside are considerably lower in treated than in non treated area s. These p ositive results

must stimulate design ers to stop co nsidering a hyd rophob ic treatment as a last remedy for treatening damage. On the contrary, they

should consider the hydrophobic treatment as a full phase in the construction project. In this way it will be possible to improve

durability of co ncrete con structions in an e legant and e conom ic way.

REFERENCES

Fliedner Ch. (1993) Hochkonzentrierte und Lösingsmittelfreie Silane als Hydrophobierungsmittel in Mehrkomponenten-

Oberfläc hen-Schutz systemen für B eton, Hüls AG, 1995.

Fliedner, Ch. (1995)Co ncrete protection with organo silicon compound s, Proceedings VIIIth ICPIC Congress Oostende,1995, 603-

610.

Gonzá lez J.A., Feliú S., Rodríguez P., Ramírez E., Alonso C. en Andrad e C. (199 6), Some questions o n the corro sion of steel in

concrete - Part I : when, how an d how much steel co rrodes, Materials and Structures, Vol. 29, 1996, 40-46.

González J.A., Feliú S., Rodríguez P., López W., Ramírez E., Alonso C. en Andrade C. (1996), Some questions on the corrosion

of steel in concrete - Part II : Corrosion mechanism and monitoring, service life prediction and prote ction metho ds, Materials and

Structures, Vol 29, 1996, 97-104.

Kissel J. en Pour baix A. (19 96), Les effe ts de la teneur en chlorure et de l’alcalinité des bétons sur la corrosion de l’acier, Centre

Belge d’étude de la corrosion, Vol. 165, RT. 315, 1996.

Polen, J. (1 993) V an inspectie to t herstelling : de toe stand in Vlaa nderen, FEREB Conference , Brussels, 1993.

Van Gemert, D . (1989) Alkali-aggreg ate reaction in Belgium : C oncrete H ighway and Office Building, 8th International Conference

on Alkali-Aggregate Reaction, Kyoto, July 1989.

Van Gemert, D (1992) Betonbescherming tegen chloridenindringing, De Bouwkroniek, November 1992, 8-10.

Van Gemert, D and Horckmans, J. (1992) Com parison of two methods for measurement of chloride ion diffusion in PCC,

Proceeding s of the 7th ICPIC Congress , Moscow, 1992, 162-177.

Van Gemert, D . and Schu eremans, L. (1996) Determination o f Chloride In gress Pro files in Quay-wall a t Container T erminal in

Zeebrug ge, Internal Report PV28521, Reyntjens Laboratory, KULeuven, 1996.

Van Tonge len J., Van G emert D. en Fremout J. (1994) Preventieve bescherming van de nieuwe containerterminal te Zeebrugge

tegen de schadelijke invloed van chloride-ionen, De Bouwkron iek, April 1994, pp. 30-35.

ANNEXES

Annexe 1 : Carbonation

CO2 + H2O 6 H2CO3

H2CO3 + Ca(OH)2 6CaCO3 + 2H2O

At the moment the carbonation process is completed there will be a CaCO3/Ca(HCO3)+ balance at which the pH-value drops to 8,3.

The water-soluble Ca(HCO3)2 may initiate the erosion of the concrete surface.

Annexe 2 : Corrosion

A corro sion elemen t consists of a cath ode and an anode at which followin g reactions tak e place :

Anode : Fe 6 Fe2+ + 2e- (iron in solution)

Cathode : 4e- + O2 + 2H2O 6 4(OH)-

Fe2+ + 2(OH)- 6 Fe(OH)2

2Fe3+ + 6(OH)- 6 2Fe(OH)3

2Fe(OH)3 6 Fe2O3 (rust) + 3H2O

Annexe 3 : Pitting corrosio n due to chloride ingress

Anode : Fe 6 Fe2+ + 2e- (iron in solution)

Cathode : 4e- + O2 + 2H2O 6 4(OH)-

2Fe3+ + 6(OH)- 6 Fe2O3 + 3H2O

In an intermediate stadium hydrochloric acid (HCl) is formed by which the pH-value drops to 5 :

Fe2+ + 2Cl- + 2H2O 6 Fe(OH)2 + 2HCl

HCl 6 Cl- + H+

Annexe 4 :Alkali-aggregate reaction

SiO2 + 2NaOH + nH2O 6 Na2SiO3.nH2O

Na2SiO3.nH2O + Ca(OH)2 + H2O 6 CaSiO 3.mH2O + 2NaOH

The pro duct CaS iO3.mH2O is a gel with expansive properties