CEMENT and CONCRETE RESEARCH. Vol. 18, pp. 44-54, 1988. Pr inted in the USA. 0008-8846/88 $3.00+00 Copyr ight (c) 1988 Pergamon Journals, Ltd.

CORROSION RESISTANCE OF STEEL FIBRES IN CONCRETE UNDER

MARINE EXPOSURE

P.S. Mangat and Kr ibanandan Gurusamy Department of Engineering, Aberdeen University, Mar ischal College, Aberdeen, U.K.

(Communicated by A.J. Majumdar) (Received July 16, 1987)

ABSTRACT

This is the final paper of a series (i, 2, 3) which have reported d i f ferent aspects of a long term study on the marine durabi l i ty of steel f ibre re inforced concrete (sfrc) . Two mixes, one with and one without pfa were re inforced with three ty?es of steel fibres. The cement content of the mixes was 430 and 590 kg/m ~ respect ively. Pr ism specimens of these mixes were cured under marine exposure, both in the laboratory and at Aberdeen beach, for up to 2000 wet-dry cycles (12OO days). The state of corros ion of the steel f ibres was invest igated v isual ly and by electro- chemical analysis of f ibres exposed at f ractured surfaces of specimens after f lexural testing.

The results show that the general ly accepted act ivat ion level of 0.4% Cl by weight of cement does not apply to sfrc. Similarly, the threshold

value of O.61 for the --(Cl-----~) ratio, as proposed by Hausemann for in i t iat ion (OH-)

of Corrosion, is not val id to steel f ibre re inforced concrete. No corrosion - (C I - )

of f ibres embedded in concrete was evident at C1 and ~ levels great ly

exceeding the above values.

Int roduct ion

Concrete normal ly prov ides re inforc ing steel with excel lent corros ion protect ion due to its h igh alkal inity. In addit ion, concrete mixes can be designed to have low permeabi l i ty which min imises the d i f fus ion of corrosion inducing substances such as CI-, CO 2 and 0 2 . Low permeabi l i ty also increases the electr ical resis-

t iv i ty of concrete, which impedes the f low of e lectrochemica l corros ion currents.

In des igning for durabi l i ty, codes of pract ice st ipulate str ingent requirements regarding concrete cover in addit ion to the use of impermeable concrete. In the case of steel f ibre re inforced concrete, the f ibres are uni formly and randomly d is t r ibuted with some at the surface of the composite be ing d i rect ly exposed to the outside environment. Consequent ly the min imum cover to f ibres is ef fect ively zero.

44

Vol. 18, No. i 45 CORROSION RESISTANCE, STEEL FIBERS, SEA EXPOSURE

At the splash zone of mar ine structures the superior mechanica l propert ies of sfrc can be of greatest advantage (4). However, it is also here that the combinat ion of salt water and oxygen, wave act ion and ice results in an extremely aggressive env i ronment for f ibre corrosion. F ibres in the v ic in i ty of concrete surface are par t icu lar ly vulnerable and conclusive proof of their corros ion res istance is required before this important area of appl icat ions can be opened for sfrc.

Previous durabi l i ty research on sfrc has concentrated on exposure condit ions such as an industr ia l atmosphere and deic ing salts (5), sewage outfa l l and a coastal site (remote from splash and t idal zones) (6) and freeze-thaw cycles (7). The results of these invest igat ions have genera l ly indicated sat is factory durabi l i ty of sfrc.

A research programme was in i t iated at Aberdeen Univers i ty some years ago in order to make a fundamental study of corrosion of steel f ibres in concrete. Specimens of sfrc incorporat ing d i f ferent types of f ibres were introduced to long term marine exposure (I, 2, 3). Chemical analyses were carr ied out at regular intervals to establ ish the d i f fus ion character is t ics of acid soluble chlor ides (i, 2) and the composit ion of pore f luid with respect to Cl- and OH- concentrat ion (3). In this paper evidence regarding the state of corrosion of the var ious types of steel f ibres is presented and the results given in ear l ier papers (i, 2, 3) are related to these data in order to gain an understanding of the corros ion mechanism.

Exper imenta l

Mixes and Mater ia ls

Two types of mixes of steel f ibre re inforced concrete were used. The f irst mix (mix A) was based on OPC and in the second mix (mix B) 26 per cent of cement was replaced by pfa. The proport ions, by weight, of mix A were 1.O:1.5:O.86 with a water /cement rat io of 0.4. The cement content was 590 kg/m 3. The proport ions, by weight, of mix B were 0.26 (pfa) :0.74 (OPC) :1.51:0.84 with a water / (OPC + pfa) rat io of 0.4. The cement content of this mix was 430 kg/m 3 . Four mixes with d i f ferent steel f ibre re inforcement were manufactured for each of the above mix proport ions. Detai ls of these are given in Table i.

Ord inary Port land cement, f ine aggregate conforming to zone 2 of BS 882 and iO mm nominal size granite coarse aggregate were used. Further detai ls of mater ia ls are given in the ear l ier papers (i, 2).

Casting, Cur ing and Test ing

iOO x iOO x 500 mm pr ism specimens were made as descr ibed prev ious ly (i, 2) and were demoulded after 24 hours. They were then cured in the laboratory air for one or fourteen days as indicated in Table i. Subsequent ly the specimens were trans- ferred either to a sea water spray chamber in the laboratory or to Aberdeen beach. Further detai ls of mar ine cur ing have been given prev ious ly (i, 2).

Tests were conducted after 2000 cycles (for mix A specimens) and 12OO cycles (for mix B specimens) of wett ing and drying under mar ine exposure. These corresponded to ages of about 1250 and 640 days respectively. Three pr ism specimens per mix of Table 1 were f irst tested in f lexure and the broken halves were used to obtain samples for chemical analysis as descr ibed prev ious ly (i, 2, 3).

For specimens of mix A only, the e lectrode potent ia l of steel f ibres exposed at the f ractured faces of pr ism specimens was also measured by compar ison with a standard calomel e lectrode (half cell). The arrangement used for these measurements is shown in Fig. i. An e lectr ica l connect ion was made between a steel f ibre on the f ractured face and the calomel e lectrode v ia a high impedence voltmeter. The potent ia l d i f ference between the steel f ibre and the half cell was recorded by

46 Vo]. 18, No. ] P.S. Mangat and K. Gurusamy

placing the calomel electrode either on face 3 or face 1 of the prism, these being the side faces during casting.

The potent ia l of the fibres at O, 15, 30, 40 and 50 m/n depth relative to face 1 and 3 was then obtained by making electr ical contact with fibres at these depths. The potent ia ls were taken, as far as possible, through fibres located at the centre line of the fractured face of the specimen, which is indicated in Fig. i.

Results and Discussion

Corrosion Threshold Level of (CI-) (OH-)

Hausmann (8) suggested a threshold ratio of chloride ion activity to hydroxyl ion (Cl-)

activity, (OH-)' of O.61 in solut ion at the iron-matr ix interface, above which

corrosion is initiated. In order to check the val id i ty of this threshold value to sfrc, the data on free CI- and OH- concentrat ions in specimens of this investi- gation are considered. These results have been publ ished separately (3) and were

(Cl-) used to calculate the (OH_------~ ratios at di f ferent depths into concrete. These

(cl-) values of (OH_--------~l for mixes A and B, are plotted in Figures 2 and 3 respectively.

The vert ical scales of Figures 2a and 3a are enlarged in Figures 2b and 3b in order

TABLE 1

Detai ls of Mixes, Fibres and Curing

Fibre detai ls

1 Fibre Mix 1 d vf vf ~ type

(ram) (ram) (~)

A - - 0 O - o

AME 25 O.51 3 147 Melt extract

AMS 28.2 0.48 2.5 147 Low carbon

ACR 40 0.60 2.2 145 Corros ion resistant

B - - O O - o

BME 26.5 0.44 1.7 iOO Melt extract

BMS 28.2 0.48 1.7 iOO Low carbon

BCR 40 0.60 1.7 i12 Corrosion resistant

Curing* condit ions

Shl4, Bhl4

Shl4, Bhl4

Shl4, Bhl4

Shl4, Bhl4

Sh I , Shl4, Bhl4

Shl4

Shl4

* Shl, Shl4

Bhl4

- marine shower curing (MC) after 1 or 14 days laboratory air curing.

beach curing under tidal cycles (TC) after 14 days laboratory air curing.

Vol. 18, No. i 47 CORROSION RESISTANCE, STEEL FIBERS, SEA EXPOSURE

High impedence votfmeter-----.~

Steel fibres

Fractured face of

, /C a[ome[ electrode

sponge

crete prism

fe~Z ~ acel or 3

N i I

Fig. 1 E lectrochemica l potent ia l measurement of steel fibres.

to indicate clearly Hausmann's threshold value of O.61 in relat ion to the data of this investigation. Figure 2b shows that this threshold value is exceeded at depths of up to 29 mm and 32 mm in concrete exposed to tidal cycles and laboratory marine cycles respectively. Similar trends are evident in Figure 3b. The results in Figures 2 and 3, therefore, show that in the surface zones of specimens, the

(CI-) rat ios were several orders of magnitude higher than Hausmann's threshold (OH-) level of 0.61. Accordingly, extensive corrosion of steel fibres should have occurred in these zones. The fact that no such corrosion was evident suggests either that this threshold value is not appl icable to small diameter steel f ibres embedded in concrete or that one or more of the other requirements for corrosion to proceed is not present. Hausmann der ived the threshold level of O.61 on the basis of studies undertaken with "model" electrolytes. Page and Vennesland (9) have emphasised the unre l iab i l i ty of predict ing corrosion behaviour of steel in concrete by such methods, as it is impossible to simulate the effects which may arise either from oxygen deplet ion or from the l imited mobi l i ty of chloride ions in the cement matrix. The threshold levels are, therefore, l ikely to be much higher than the value suggested by Hausmann (iO, ii, 12).

(Cl-) It is also clear from Figures 2 and 3 that, within the surface zones, the (OH------~

rat ios are much higher in marine shower cured specimens (Sh 1 and Sh 14) as compared to beach cured specimens (Bh). This is due to the higher salt concentra-_

(C I ) t ions in the laboratory marine shower (2, 3). Figure 3 also shows that (OH-------~

rat ios are higher in specimens Sh 1 which were exposed to marine cycles after 1 day of air curing in comparison with specimens Sh 14 which were exposed to mar ine cycles after 14 days of air curing.

Corros ion Act ivat ion Level of Cl-

Var ious act ivat ion levels of acid soluble CI- above which corrosion of steel re in forcement is cons idered to init iate are used in design (13, 14, 15). The h ighest value quoted for design is 0.4 per cent Cl- by weight of cement. If this act ivat ion level is exceeded then, in the presence of oxygen and moisture, corros ion of steel in concrete is assumed to be init iated.

Acid soluble Cl- d i f fus ion curves for mar ine shower cured and beach cured

48 Vol. 18, No. I P.S. Mangat and K. Gurusamy

350

300

250

P 200 " i -

0

.~. 150

100

50

Mix A (1: 1'50:0"86:0"4) 20

18

16 .~arine spray cydes 14

i'~ 12 - r

O

~ 8

6

4

Tidal cycles ,....._/ . . , 2

"~--- - - - - i ,~ ; @61 10 20 30

Depth (ram)into concrete

i Exposure to ~ spray cycles \ ~ Conventional \ ~ reinforcement

/ \ \ Ex osure to~ ~ tidatcyc,es ~ \: q

. . . . . . . . . . . _ _--_ _~T_h~_,_sh_~d_ tenet

0 10 20 30 Oepth(mm) into concrete

(a) (b)

Fig. 2 (a) & (b) The var iat ion of (CI-)/(OH-) with depth after 2 OOO MC or TC

400

350

300

250

200 o T ~

150

100

50

Mix B (0"26:0.74:151:084:0.4) 20

18

16

Shl 1& I / k ~ 12 ,, s

~ 10

~ 8

6

.... ~ . ~ o-61 io 2n ~o

Depth into concrete (ram) (al

i spray Bh Shl

. . . . . . : . . . . . . . . .~_~_ _'q~_~e_,~_?d

10 20 30 40 Depth into concrete (ram)

(b)

Level

Fig. 3 (a) & (b) The var iat ion of (CI-)/(OH-) with depth after 1 200 MC or TC

Vol. 18, No. 1 49 CORROSION RESISTANCE, STEEL FIBERS, SEA EXPOSURE

specimens of this invest igat ion were given in ear l ier papers (1, 2). The CI- was extracted from powder samples of concrete by di lute acid (i). The results showed that in the surface zones of specimens C1 concentrat ion were almost an order of magni tude greater than the assumed act ivat ion level of 0.4 per cent. Despi te this there was no v isual evidence of corros ion of f ibres which were embedded in the surface zones of specimens. The above act ivat ion level, therefore, appears to be meaning less for sfrc.

Corros ion Res istance of Steel F ibres

Al l specimens were v isua l ly inspected before any mechanica l test ing was undertaken. It was observed that no rust ing of f ibres exposed at the concrete surface was evident in the case of melt extract (ME) fibre re inforced specimens. This was due to the fact that mel t extract f ibres are of sta in less steel which is less prone to corros ion under d irect exposure to sea water. The chemical analysis of steel f ibres are given in Table 2 which indicate a high Cr and Ni content of melt extract fibres. The corros ion res istant f ibres (CR) exposed at the surface of concrete were also unsta ined at ear ly ages of up to 1 year of exposure. Such f ibres are coated with a layer of zinc which provides anodic protect ion to steel. At later ages, however, a l imited number of rust spots appeared at the exposed surface of such f ibres probably at points where the zinc coat ing had been damaged or depleted due to the preferent ia l corros ion of zinc. Extens ive corrosion, however, was evident in the case of low carbon steel f ibres (MS) exposed at the surface of specimens which were cured under mar ine exposure. The corros ion occurred wel l wi th in 150 cycles of exposure. Corrosion, however, was local ised at the specimen surface and there was no evidence of its penetrat ion into the embedded parts of the f ibres even after 2 OOO cycles (i 250 days).

TABLE 2

Chemical Compos i t ion of Steel F ibres

Steel F ibre E lements (%)

C si Mn P S Cr Mo Ni Zn

Melt extract (ME) 0.20 2.77 0.38 0.02 O.O1 17.6 0.07 0.58

50 Vo l . 18, No. ] P.S. Mangat and K. Gurusamy

PH of pore e so[ution {OH-) Free chloride {d.~_ in pore solution Potential ~" 13"30 (my SCELIoo

-200 13.20

-300

-z.O0 40

-500 13.00

-600 30

-700 20' -800 12.60

10 -900

(i) 35 500ppm CF (ii) 355ppr. [[" / /

Genera[ " ] - - - - . . Corroslon . ~

6 , Ol , ph

corrosion

0 2 ~ 6 8 10 12 lZ~

/ (OH') ~ ~,

~ ~ ~ 8 P0fenfial -~ o (r.V SCE)

co

1'o 2'o ~o io s'o Depth (mr.) into concrete

(iii) noCF Ic- i .... }.iooo~

L, , - - _ _ ~,soo T orroslon "- -

r_. Lo J ]mmTt~'-'~" t 1000~o . , , . . , . .- 0 2 4 6 8 10 12 14ph

Pourbaix diagrams at different depths (i) surface (ii) 35mr. (iii)38-50mr.

Mix A (1 :1 '51 :0 S6 :Ok) F ib re : Low carbon s tee l IMS)

~kppr.10 3

Fig. 4 E lectrode potent ia ls after 2 OOO TC, with Pourbaix diagrams.

With this in format ion it is possib le to draw the typical Pourbaix diagrams (16) at selected depths into concrete, which represent the thermodynamic equi l ibr ium of steel in concrete. In using Pourbaix diagrams, certa in l imitat ions must be appre- c iated (17). First ly, the e lectochemcia l equi l ibr ium diagrams were obtained by Pourbaix by record ing the behaviour of pure metals in aqueous solutions. Secondly, the pH value given along the x-axis of the Pourbaix d iagrams is the pH of the solu- t ion which is in d i rect contact with the metal surface. The divergence between local pH values and bulk pH values can be considerable, par t icu lar ly in cases of local ised corrosion. Furthermore, Pourbaix h imsel f has warned that the electro- chemical equi l ibr ium diagrams must always be used in conjunct ion with other means of invest igat ion. These l imitat ions not withstanding, the use of Pourbaix d iagrams provides a valuable insight into the e lectrochemica l state of steel f ibres in concrete. Three typical Pourbaix d iagrams have, therefore, been inc luded in F igure 4. These represent the thermodynamic equi l ibr ium of steel in concrete contami- nated with 35,500 ppm, 355 ppm and zero ppm Cl- respect ively. At the surface of specimens where Cl- concentrat ion approx imates 27,000 ppm, Pourbaix d iagram (i) in Fig. 4 approx imate ly represents the state of steel fibres. If the pH at the surface is taken as approx imate ly 12.3 from Fig. 4, then it is apparent that the f ibres at the surface are in the region of imperfect pass iv i ty (IP) in the Pourbaix d iagram '(I) ' At a depth of 35 mm from the surface, where the CI- concentrat ion is approx imate ly 355 ppm, Pourbaix (ii) represents the state of steel f ibres in concrete. The poten- tial and pH values of these f ibres indicate their pos i t ion to be in the region of imperfect passivity. At depths beyond about 38 mm, where free chlor ide is not present, Pourbaix d iagram (iii) represents the thermodynamic equi l ibr ium of steel fibres. In this case the f ibres are comfortab ly with in the pass ive region and, therefore, there is no r isk of corrosion.

The values of e lectrode potent ia l (E) genera l ly accepted as represent ing the act ive and pass ive state of steel re in forcement are shown in Table 3. More recent ly Arup has def ined four states of corros ion of steel, the typical potent ia ls for in i t iat ion of p i t t ing corros ion be ing between - 200 and - 500 mV (19). In Fig. 4 the elec- t rochemical potent ia ls of steel f ibres at al l depths into concrete and especia l ly in

Vol. 18, No. i 51 CORROSION RESISTANCE, STEEL FIBERS, SEA EXPOSURE

the surface zones are more negat ive than - 250 mY. It is apparent from Table 3 that at these potentials, steel f ibres should have corroded. As already discussed above no such corros ion of the f ibres was apparent. The possib le reasons for this

are:-

(i) The e lectrode potent ia l values shown in Table 3 were obtained by using an exper imenta l arrangement where an electr ical connect ion was made to a convent ional re in forcement bar at a convenient pos i t ion enabl ing electrode potent ia ls to be measured at the surface of concrete. By moving the reference electrode about on the surface, the e lectrode potent ia l of steel embedded at a given cover could be measured at any point. The technique used to measure electrode potent ia ls in this invest igat ion, however, was quite d i f ferent as shown in Fig. i. In this case the reference electrode was f ixed at the surface and the potent ia ls of steel f ibres measured at d i f ferent depths into concrete.

(ii) The potent ia l measurements were taken on steel f ibres exposed at the frac- tured face of pr ism specimens after test ing in flexure. These electrode potent ia l values indicate the state of steel after phys ica l d isrupt ion of f ibres during pul l out and not of undisturbed steel f ibres embedded in the matrix.

(iii) The potent ia l measurements were usual ly taken after between 1 to 3 hours from the time of mechanical testing. Low carbon steel f ibres (MS) exposed at the fractured faces of specimens and left under uncontro l led condit ions in the laboratory air, showed signs of rust staining after about 24 hours. It is likely, therefore, that potent ia l measurements taken even after a few minutes of mechanica l test ing may not be the actual potent ia ls of unexposed steel fibres.

The e lectrode potent ia l measurement technique shown in Fig. 1 assumes that f ibres are isolated and not in e lectr ical contact with adjacent fibres. A check was made by measur ing the resistance between adjacent fibres, using a standard meter. This conf i rmed that the f ibres were not in e lectr ical contact.

The potent ia l measurements for specimens re inforced with corrosion res istant f ibres (CR) and melt extract f ibres (ME) are shown in Fig. 5. The potent ia l values show a similar trend of less negat ive values at greater depths from the surface as was prev ious ly noted for low carbon steel f ibres (MS). Beyond a depth of about 30 mm the potent ia l readings are fair ly constant at mean values of about - 250 mV for ME f ibres and - 600 mV for CR fibres. It is not poss ib le to compare these potent ia l values with Pourbaix d iagrams which are val id only for standard steels.

TABLE 3

Interpretat ion of Corros ion Potent ia l Measurements (18)

E (mY vs SCE) Condi t ion of Steel

> - 220

between - 220 and - 270

< - 270

Passive

Act ive or Passive

Act ive

Ava i lab i l i ty of Oxygen

In the mar ine envi ronment where chlor ide ions are l ikely to d isrupt the stable oxide layer even at high pH, the ava i lab i l i ty of oxygen at the steel surface is l ikely to be the cr i t ical contro l l ing factor for corros ion to occur. It has been shown (20) that oxygen d i f fus ion near the surface of moist concrete is very high. Hence it is reasonable to assume that at least in the surface zones of sfrc specimens suff ic ient oxygen was avai lable to support corrosion.

52 Vol. 18, No. ] P.S. Mangat and K. Gurusamy

PH of pore solution (OH')

Z'Free chloride (CF} in pore solution Potential 7o 13.30 (mV SEE)I00

60 -200 13,20

-300 50

-400 ~,0

"500 13'00

-600 30

-700- 20-

-B00 " 12-60-

10- -900-

12.00- -1000

",,,,, Mix A(1:1.50:0.86:0 4)

,"' (OH-)

(mY SEE)

~ I . ) : Potential ., (mV SCE) II

o 1o 20 30 4 s'o Depth (mm)

Corrosion resistant fibre(CR] A Melt extract fibre(ME)

~ppmx103

Fig. 5 E lectrode potent ia ls after 2 OO0 MC.

It is general ly accepted that oxygen consumed dur ing the cathodic react ion has to be in a d isso lved state (21). The effect of salt concentrat ion on oxygen solubi l i ty in pore f luid and consequent ly rate of corrosion is demonstrated in F igure 6 (21). It is evident that the CI- concentrat ion of about 78,000 ppm which occurred in the surface zones of specimens exposed to 2 OOO MC corresponds approximately to the maximum rate of corrosion. The pore f luid in the surface zones, therefore, offers an ideal environment for corrosion.

Hypothesis for F ibre Pass iv i ty

Accord ing to standard corros ion theory, steel embedded in concrete is largely in a protected state because of the a lka l in i ty of the matrix. This a lkal in i ty

~o ~

: t

i I I I I I i I i L I i

36"5 7()'S I02 132 159 185 210 232 Satinify ppm xl03

Fig. 6

Ef fect of sodium chlor ide concentrat ion on corrosion rate, after Gri f f in and Henry (21).

Vol. 18, No. i 53 CORROSION RESISTANCE, STEEL FIBERS, SEA EXPOSURE

results in the formation of a dense oxide layer which prevents current flowing from anode to cathode, thereby maintaining passivity of steel. This passivity, however, can be broken down in the presence of chloride anions, resulting in pitting corrosion (19). From the results and observations of this investigation, it is apparent that no such breakdown has taken place in the steel fibre reinforced concrete specimens. In addition, no deterioration in mechanical properties of both uncracked and precracked specimens of sfrc at small crack widths was evident after long term marine exposure (22, 23).

Page (24, 25) has suggested an alternative view of passivation. With the aid of scanning electron micrographs, he has shown that the interfacial zone separating concrete and embedded steel is composed largely of segregated lime. For reinforced concrete which is permanently submerged in a chloride-bearing aqueous environment, this lime rich layer effectively screens the metal from direct access of oxygen (24) Therefore, although the ingress of CI- from sea water destroys the passive film on steel thus creating anodic sites, the large polarization of the cathodic reaction effectively stifles the corrosion process. Hence, the limited occurrence of corrosion in permanently submerged marine structures is not on account of anodic passivation but due to the unavailabil ity of oxygen to stimulate the cathodic reaction. In conditions other than those of total immersion, however, the supply of oxygen to embedded steel is unlikely to be a limiting factor (24), more so in the case of sfrc where the cover to steel is effectively zero. It is, therefore, the availabil ity and rates of replenishment of chloride and hydroxyl ions which are liable to govern the anodic behaviour of metal. In this connection the influence of the lime-rich layer in intimate contact with steel is likely to be significant. This is because the surrounding lime provides a reservoir of OH-, thereby buffering the anodic sites at high pH. Additionally, the mobility of chloride ions and, therefore, their supply to the anodic pits is likely to be restrained by this lime rich layer (12, 26). This restraint on the diffusion of chloride ions whilst maintaining a buffered alkaline environment at the surface of steel will tend to repassivate pits once formed. The small diameter fibres, with their large surface area to volume ratio, are even more effectively screened by the lime rich layer than the large diameter bars used in conventionally reinforced concrete. This enhanced protection of the steel fibres provides an explanation for their apparent passivity in concrete even though chloride is

available in abundance, and the (CI'____~) (OH-) ratios are several orders of magnitude

higher than the threshold levels suggested by Hausmann.

Corrosion is characterised by the electro-chemical reaction which takes place between a confined "pit" (the anode) where steel is depassivated and the adjacent area of passive steel which acts as cathode (19). The corrosion rate depends on the ratio of the cathodic area to the anodic area (27). Owing to their discrete nature, the maximum cathodic area available for steel fibres is limited. Therefore, even if corrosion is initiated it is probable that the subsequent rate of corrosion will be very small.

Conclusions

The following conclusions are based on experimental results given in this paper:-

i. Low carbon steel and corrosion resistant (galvanised) steel fibres which are exposed at concrete surface are prone to corrosion under marine exposure. Melt extract (stainless steel) fibres exposed at concrete surface did not show signs of corrosion after 2 OOO cycles of marine exposure.

2. The activation level of 0.4 per cent CI- by weight of cement does not apply to steel fibres embedded in concrete since corrosion was not initiated

54 Vol. 18, No .... P.S. Mangat and K. Gurusamy

at CI- concentrations almost an order of magnitude greater than the above activation level.

(Cl-) 3. The threshold value of O.61 for the (OH_----~ ratio does not apply to steel fibres

(Cl-) embedded in concrete since corrosion was not initiated even when ]-~c~ ratios

were as high as 320. Acknowledgements

The authors gratefully acknowledge the financial support from the SERC Marine Technology Directorate for the research project on marine durability of steel fibre reinforced concrete. The authors also gratefully acknowledge the advice and facilities made available by Prof. F.P. Glasser of the Chemistry Department, Aberdeen University.

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