Corrosion of Reinforced Concrete in a Tropical Marine Enviroment and in Accelerated Tests

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Commtction and Building Mat&k, Vol. 11, No. 2, pp. 75-al,1997

@I1997 Elsevier Science Ltd

PI I:SO950-0618(97MOOO9-3

Printed in Great Britain. All rights reserved

0950-0618/97 $17.00 + 0.00

Corrosion of reinforced concrete in a tropical

marine environment and in accelerated tests

P. Castro , L. Weva and M. Balancim

Department of Applied, Physics, Centre for investigation and Advanced Studies

(CiNVESTAv3, M ida Unit, A.P. 73 Cordemex, C.P. 97310, M ida, Yucathn,

Mt+xico

Received 7 October 1996; revised 10 March 1997; accepted 17 March 1997

Concrete cylinders with an embedded reinforcing bar and different water/cement ratios were cured for

different periods and then exposed in a salt spray chamber (according to 180-9227). Cylinders fromthe same batches were also exposed to a marine atmosphere for 24 months at a location 50 m from

the shoreline. In both exposures the corrosion rate, the corrosion potential and the chloride content

close to the reinforcing bars were monitored as a function of the exposure time, in order to obtain

information about the corrosion kinetics. These data allowed us to find relationships between exposure

time in marine natural weathering (field tests) and in a salt spray chamber (accelerated tests).

Therefore, rapid surveys (in periods of 30-45 days) of the type of reinforced concrete evaluated here

can be made using 7 days of curing. The results showed that the salt spray chamber tests modified

the corrosion kinetics of reinforced concretes with curing times below 7 days. 0 1997 Elsevier

Science Ltd.

Keywords: corrosion; reinforced concrete; accelerated tests

Introduction

It is important to know the behaviour against corrosion

of different reinforced concrete types. This allows the

designer to take into account several aspects about

durability before recommending a particular reinforced

concrete type. However, this is an almost impossible

task due to the tests duration and the variables that

could interfere in natural weathering evaluations. For

example, a specific type of concrete could be evaluated

after 3 or 4 years but there are still a sufficiently high

number of water/cement (w/c) ratios and curing times

(tc) that could make impossible a complete natural

evaluation. A possibility of reducing the test time and

obtaining rapid information about the material deteri-

oration is the use of accelerated tests. These kind of

tests can be done through different methods: applica-

tion of electrical potential measurements’, cyclic wet-

ting and drying in 3.5% salt solution2*3, introducing a

chloride content in the concrete4,5, ponding at regular

intervals with a sodium chloride solutior , salt spray

chamber tests&‘, etc. The use of accelerated tests is an

alternative to do rapid surveys provided that the corro-

*Corresponding author

sion kinetics do not change. However, under some

circumstances and exposure conditions it can be modi-

fied. For this reason, it is difficult to find reliable

relationships between exposure times in natural and

accelerated weathering. Having this in mind, it was

decided to evaluate a commonly used type of concrete

cylinder with an embedded reinforcing bar that was

exposed to atmospheric conditions in a tropical marine

environment and to accelerated tests in a salt spray

chamber. This was done in order to evaluate whether

the natural environmental conditions could be repro-

duced in an accelerated manner without significantly

changing their corrosion kinetics. In this way, realistic

information from the same type of reinforced concrete

but with different w/c ratios than those tested could be

obtained rapidly, given specific conditions.

Experimental

Mater ial s and specimen preparation

Ordinary Portland Cement (Type I, ASTM) concrete

and crushed aggregates were used in all concrete mixes.The reinforcing steel received a previous treatment

75



76 Corrosion of reinforced concrete in a tropical marine enviornment and in accelerated tests: P. Castro et al.

before casting consisting of weighing it as received

from the factory, applying an epoxy coating and a tape

to limit the area of study and to isolate its surface in

the interface concrete and steel with air. This was done

to avoid crevices that could lead to undesirable pene-

tration of chloride’.

Figure I shows a sketch of the specimen used in this

research. The concrete cylinders were 7.5 cm in diam-

eter and 15 cm in height with a 9.5 mm diameter steel

bar embedded in the centre and traversing the cylinder

from face to face. Between concrete and the steel bar

surface, and exactly at the middle of the bar, an acti-

vated titanium rod (ATR) was positioned to serve as

the reference electrode (RB) during the corrosion

measurements. The ATR was continuously calibrated

against a saturated calomel electrode (SCE) as was

previously characterised’. The water/cement (w/c> ra-

15

tios tested were 0.76, 0.70, 0.53, 0.50 and 0.46. In this

region it is common to use w/c ratios as high as those

of 0.76 and 0.70 in structures like houses where there is

no supervision. Other w/c ratios such as 0.53 and 0.50

are commonly used in some public buildings such as

schools, and a w/c ratio of 0.46 is rarely specified in

our environment. A curing time (tc> of 7-28 days is

specified in government buildings and schools, although

it has to be recognised that, in the best case, the

concrete is cured for 7 days only. These are the reasons

why 1, 3 and 7 days of tc were evaluated in this work.

A pair of cylinders was manufactured and tested for

each w/c ratio and curing time. Before exposure, the

concrete cylinders were painted with an epoxy coating

in order to limit the areas that had to be exposed to the

environment, and to protect those in which the action

of the aggressive agents was undesirable as shown in

Working Electrode(Corrugated bar 3)

Copper wire to make

electric contact with the

RE

-Counter Electrode

Conductive Mesh

Activated Titanium rod

acting as Reference

Electrode (RE)

Epoxy Beads

Epoxy film to restrict

the desired concrete

area permeable to CI

and CO2

Epoxy iilm to protect the

steel from crevices,

diierential aereation,

bleeding or segregation

Figure 1 Sketch of the specimen used in this investigation.



Corrosion of reinforced concrete in a tropical marine environment and in accelerated tests: P. Castro et al. 77

Table 1 Comparison of the slope from the regression curves

Time of curing (days) Type of exposure No. of tests

Higher

Value of b

Lower Avg.

Avg. ratio b(m.e.)/b(s.s.)

7

Marine environment (m.e.1

Salt spray chamber (s.s.)

5 9E-7 0.7E - 7 3.14E - 7

6.375 30E-7 lOE-7 20E-7

Figure 1. More details about the characterisation of the termined by means of an ion selective and a reference

materials and concrete, as well as the specimen design, electrode (Orion@, models 9417-00 and 9002-00, re-

have been published elsewhere”. spectively).

Type of exposure (nat ural and accel erat ed t ests)

The specimens with embedded steel bars have been

exposed to natural weathering since August 1993 in the

marine atmosphere of the Port of Progreso, located on

the coast of the Yucatan Peninsula (21” 18’ N, 89” 39’

W), M&co, which is characterised as a tropical humidclimate”. The frames in which the specimens were

exposed (in a vertical position) are 50 m from the

shoreline.

Results and discussion

Concretes w it h 7 days of curi ng

A parallel set of cylinders from the same concrete

were subjected to accelerated tests in a salt spray

chamber (Atlas e, model SF-500). These tests were done

according to IS0 922712 at continuous exposure, 5%

NaCl and 35°C.

Corrosi on measurement s and sal t spray chamber moni -

toring

A piece of aluminium, as a counter electrode (CE), anda conductive mesh surrounding the specimen surface

were used to make the electrical contact with the

concrete as shown in Figure 1. A corrosion monitoring

system (Gamry @, CMS 100) was used to measure the

corrosion potential (E,,,,) and the corrosion intensity

(I,,,,) through potentiodynamic polarisation curveslo.

From the E,,,,, 10 mV in cathodic direction were

applied at a scan rate of 0.06 mV s-l to obtain the

polarization resistance. A resistivity meter (Nilsson

400@) was used to measure the ohmic drop that was

compensated when evaluating the corrosion current

data. These parameters were taken every 3 months in

the specimens exposed to the natural weathering. Theduration (days) of the salt spray chamber tests was

different for each w/c ratio. It depended on the time

for the steel depassivation.

Figur es 2 and 3 show the data of I,,, as a function of

the duration of exposure in the natural environment

and the salt spray chamber, respectively, after 7 days of

curing. The initial period in which chlorides reach the

reinforcing bar and corrosion starts is large in the

natural environment but relatively short in the acceler-

ated tests. Only significant values for the corrosion

process (higher than 0.001 PA cmm2) were taken into

account for correlation purposes. For both cases, the

corrosion kinetics can be represented by the same

bilogarithmic equation of the type log(ZO,,> = a + b

log(t), where a and b are constants, and t is the

duration of exposure for marine tests (months) and for

salt spray chamber tests (days). It appears that the saltspray chamber reproduces the corrosion behaviour

observed in natural weathering for w/c ratios cured for

7 days. The constant b value (curve slope) obtained in

the natural environment, is in the interval between

OX - 7 and 9E - 7, as shown in Table 1. For the

accelerated tests, this constant is lower and fluctuates

between 10E - 7 and 30E - 7. In this way, the value of

constant b shows that the acceleration rate of the

corrosion processes studied is more than six times

higher in a salt spray chamber than in natural weather-

ing. In both exposure regimes, the higher Zcorr values

corresponded always to higher w/c ratios. The data

from tests in the natural environment have more scat-ter (lower values of regression coefficients R in Table

Table 2 Comparison of regression coefficients RI

Facti on and determinat ion of chlor ides

After the cylinders had finished their designated cycles

in the salt spray chamber, they were cut along the

longitudinal axis to extract the reinforcing bar and to

examine the parts in which the corrosion was produced.

In such parts, the concrete close to the reinforcing bar

was sampled and pulverised until it passed through a

number 50 sieve13. The concrete dust was then ana-lyzed for chloride content through an acid extraction

technique that matches well those of the ASTM C 114

and UNE 217-91. The chloride concentration was de-

Type of exposure w/c ratio Duration of curing

7 days 3 days 1 day

Marine environment 0.76 0.87 0.96 0.920.70 0.77 0.97 0.830.53 0.78 0.85 0.950.50 0.77 0.91 0.990.46 0.93 0.81 0.79

Salt spray chamber 0.76 0.98 0.82 0.920.70 0.97 0.95 0.980.53 0.95 0.90 0.910.50 0.93 0.87 0.920.46 0.95 0.86 0.93




LOEMU

l.OE+OO

0

3 l.OE-01

P

i l.OE-02

sw/c = 0.76

j]

l.OE-ib3/ I:

:_ - -w&c:.;

___A- - - w/c=o.so

I ’ r.--.-- wk=0.46

l.OE-04

1 10 100

Time (months)

Figure 2 I,,,, vs. duration of exposure in marine environment (7

days of curing)

l.OE+Ol

l.OE +OO

1 OEM

3’

::0 l.OFxl2s

l.OJ c-03

l.OE-04

r-w/c = 0.76

.- - --w/c=o.70

- -w/c = 0.53

‘, A- - - w/c=o.50

o-.--..-.,4~=,,.46

I

1 10 100

Time (days)

Figure 3 ICorr vs. duration of exposure in salt spray chamber (7 days

of curing).

2) than those of the accelerated tests due to the

influence of uncontrolled variables such as temperature

and RH, which are fixed in the salt spray chamber.

The quantity of chlorides close to the reinforcing

bars is shown as a function of the w/c ratio in Figure 4

and ,rr in Figure 5. Figure 4 shows that chloridecontent increases with the duration of exposure and

there is a similar trend in both environments, however,

more data points are required in the accelerated tests

to make any conclusions. In Figure 5 it is observed that

the ZC”,,value, for 7-day cured specimens exposed to

marine environments, tends to a plateau after 2 years

of exposure at values close to 10 PA cm-*, irrespective

of the chloride content and the w/c ratio. It is also

observed that a higher quantity of chlorides close to

the reinforcing bar is present in the salt spray chamber

tests than in the natural environment, due to the

greater and constant RH and salinity values.

Figure 6 presents the correspondence between theduration of exposure in the natural environment and in

the salt spray chamber for similar values of I,,,,,. The

correlation is evidenced through a linear equation Y =

1

0.9

0.8

-2L 0.7

s

0.6

0.5

0.4

0 4 8 12 16 20

Chloride content close to the rebar

(kgC17m3 of concrete)

Figure 4 Chloride content close to the rebar vs. w/c ratio (7 days ofcuring).

l.OEtO2

l.OE+Ol

*- l.OE+OO

5 l.OE-01

::

E l.OE-02

l.OE-03

.

+0

0

.

. .0

12 mooths4.76

n 12 m0nths O.70

12 mods-O.53

x l2moaths~.SO

A 12 monthd.46

0 24 monthrLl.76

+ 24 monthr-O.70

0 24 montk.0.53

n 24 m0athr4.50

.24 moathr4.46%d. 0 saltpray chanbcr

l.OE-04

0 5 10 15


(kgC17m3of concrete)

Figore 5 Chloride content close to the rebar vs. IcOr, (7 days ofcuring).

a + bX; where a and b are constants, Y is the exposure

time in the salt spray chamber (days) and X is the

exposure time in the natural environment (months).

The correlation coefficient R fluctuated between 0.92and 0.98, and the dispersion was mainly due to scatter

of the data taken in the natural weathering. There is a

good correlation between the two exposure durations

but depending on the w/c ratio. Therefore, a rapid

corrosion evaluation of the type of reinforced concrete

tested here, with 7 days of curing and other w/c ratios,

can be done without changing in practical terms its

kinetics.

Concretes w it h 1 and 3 days of curi ng

Differences between the behaviour of the specimens in

the natural environment (Figur e 7) and in the saltspray chamber (Figure 8) were observed when evalu-

ating the results in the same circumstances, but with a

3-day curing time (t c). The data for natural weathering




. -w/c - 0.76

.- - -w/c=o.m

x- . . - .w/c-0.53 R-0.95,

A-. -W/C-0.S.l /’

4 8 12 16 20 24

Tic (months in marine environmmt)

Figure 6 Correspondence between exposure times for similar values

of ‘co,,.

show a similar behaviour to that observed in the speci-

mens with 7 days of curing. In this case the variation of

Zcorr with the w/c ratio (higher I,,,, with higher w/c

ratios) as well as its deceleration tendency (during the

test duration) are clearer than in the case of the

specimens cured for 7 days. Figur es 2 and 7 indicate

that the I,,,, value was higher at 3 days of curing for

nearly all w/c ratios. Then, the corrosion kinetics for

specimens exposed to the natural weathering are

present with the expected dependence among the

parameters studied as the w/c ratio, tc and I,,,. The

data of salt spray chamber tests presented in Figur e 8show that, under this controlled environment, the t c

has, also, an important role in the corrosion kinetics.

Under accelerated conditions, the 3-day cured speci-

mens show a great initial resistance to corrosion, but

once started it continues without stopping even at

similar values than those observed during the data

plateau in natural weathering. It is apparently clear

that the initiation period at which the chlorides reach

the steel becomes significant in concretes with 3 days of

curing and that it affects the processes occurring not

only in the steel but in the concrete. Therefore, the

corrosion process is delayed due to some modification

to the concrete structure. One possibility is that the

accelerated conditions caused the cement components

that did not hydrate during the 3 days of curing to

finish their hydration during the accelerated test. This

would mean that chlorides that were reaching the bar

had, in the first instance, to form stable bindings as

chloroaluminates, in such a way that once all the

possible bindings were produced, then chlorides started

to accumulate in the quantity needed to produce corro-

sion of the steel. Table 3 shows that at 1 and 3 days of

curing the quantity of chlorides detected close to the

reinforcing bar is smaller than that detected for 7 days

of curing. This could be the reason why the specimensresisted for so long before starting to corrode and why

the corrosion onset was so dramatic and did not show a

plateau tendency during the time of the test. Although

l.OE+Ol

l.OE+OO

l.OE-03

’ 2:’ Ax11: ;,___._.,.g

t..ii . - w/c - 0.76.-- . w/c - 0.70

0 I x - - - - w/c-o53

I’A- - -w/c-034l0 -.-.-. w,c-0.46

l.OFA4

1 10 100

Time (months)

Figu~ 7 I,,,,curing).

vs. exposure time in marine environment (3 days of

this is a valid hypothesis, we do not have references of

such quantities of chlorides binding in an experiment

of this type. In natural conditions, the same phenom-

ena did not occur during the evaluated period because

the chloride penetration was not so fast. In addition to

this circumstance, in the natural environment other

humidity and temperature conditions exist (i.e. dry and

wet cycles) that are affecting the chloride penetration

and the corrosion process.

On the other hand, it is important to emphasise that

the detected quantity of chlorides in Table 3 corre-

spond to different exposure times in the salt spraychamber. For this reason, the detected chloride quan-

tity was different for each particular case when Z_

showed the onset of reinforcing steel corrosion. For

example, in the case of the w/c ratio of 0.76, significant

onset values of EC,,, and I,,,, were detected at 41, 33

and 14 days for 1, 3 and 7 days of curing, respectively.

Although one might think that a possible carbonation

depth before exposure could influence the detected

quantity of chlorides close to the reinforcing bar, our

measurements of carbonation depth from the speci-

mens exposed to the natural environment are not

showing significant differences for the three curing

times”. The results of 3-day cured specimens are

Table 3 Choride concentration close to the rebar in the corrodedareas at 2 w/c ratios and 3 tc

w/c ratio Time of curing

(days)

Cl- concentration closeto the rebar in the

corroded areas(kg Cl- mm3

of concrete)

0.46 7 14.263 7.321 7.51

0.76 7 19.753 8.041 7.08




l.OE+Ol l.OE+Ol

l.OE+OO

6 OE-01

P’

K

8 l.OE-02I

l.OEa3

..- w/c = 0.76

.- - . wk = 0.70f T

x. . . . w/c-o.53

A- . -w/c=o.50

0 _..-.._w/e=0.46

l.OE.041 10 100

Time (days)

Figure 8 I,.,,,, vs. exposure time in salt spray chamber (3 days ofcuring). (Curves of w/c = 0.50 and 0.46 are superimposed.)

l.OE+Ol

.AI 1

.

l.OE+OO/ . x. y-x

, :- Q

“8 l.OE -01

, , :‘.,, ._-.;.-

_’’ 0

?II ;’,-8.--

- -A

F lOE-02r I x

8z . * Ic’- W/F - 0.76I

.- - -dc=O.70I

x. . . . w/c-0.53

l.OE-03,

A-- - wlc = 0.50

0 -----.- w,c.=o.‘l6

l.OE-04

1 10 100

Time (months)

Figure 9 I,,, vs. exposure time in marine environment (1 day of

curing).

showing a clear difference in the corrosion kinetics of

the two exposure regimes. Therefore, no good correla-

tion can be obtained.

As shown in Figur es 9 and 10 the l-day cured

samples behaved similarly to their counterparts for 3

days and the trends were similar for different w/c

ratios. These results showed again a lack of correlationbetween the two exposure regimes. Due to the fact that

the corrosion process is delayed (corrosion kinetics

change) by the salt spray chamber tests at curing times

below 7 days, it is not possible to obtain reasonable

correlation between marine environment and salt spray

chamber results.

Chlor ide content and cur ing time

In natural weathering and after a year of exposure, the

chloride content close to the bars according to the w/c

ratio and tc occur as illustrated in Figure 11. It shows a

tendency to increase together with the w/c ratio butwithout a clear influence of the curing time. No influ-

ence of the curing time for periods close to 3 years has

been reported in the literature14. Therefore, an influ-

l.OE+OO

-;” l.OE-01

::0 l.OE-02

Y

l.OE-03

l.OE-04

.- w/r = 0.76

.- - - r/c=0.70

x. . . . . n/c = 0.53

b- - - w/c-o.50

o--.---- w,e=O.

1 10 100

Tie (days)

Figure 10 I vs. exposure time in salt spray chamber (1 day ofcuring).

1

0.9

0.8

.S

e 0.7Y$

0.6

0.5

0.4

--tc=1

- *-tic3

-tc=7

1

0 1 2 3 4 5 6


kgCf/m3 of concrete)

Figure 11 Chloride distribution for five w/c ratios and three curingtimes at 1 year of exposure to the marine environment.

ence of the curing time can be detected only at the

initial exposure stages. In this way, and due to the high

salinity in the salt spray chamber, the entrance chloride

process is accelerated and such an influence is difficultto detect in terms of short duration tests. However,

significant differences were detected in I,,,, and E,,,,

values for both exposure regimes as discussed above.

Conclusions

The results can be summarised in the following conclu-

sions.

(1) The accelerated tests served to establish a correct

order of qualities in an Ordinary Portland Cementconcrete with 7 days of curing. The tests in the

salt spray chamber according to ISO- do not

change significantly the kinetics regarding those in



(2)

3)

4)

5)


natural marine weathering, but they strongly ac-

celerate the reinforcing bar activation. A clear

correlation seems to exist between both types of

tests that also classifies the materials in the same

order (Figures I and 2 for example).

Once the passive/active transition is produced at

a critical chloride concentration, the corrosionbehaviour does not depend on the chloride con-

centration.

The corrosion kinetics of the specimens cured for

7 days, exposed in field and in accelerated tests,

indicates that the curve of Z_ vs. exposure time

0) can be modelled by a mathematical equation,

which fits very well with a bilogarithmic function:

log&,,,) = a + b log(t). The correlation between

the exposure times in field and in accelerated tests

can be then modelled by a linear function: Y=

a +bX.

There is no resemblance between trends of I,,,,vs. time observed for marine environment and

accelerated tests for samples cured for less than 7

days. There exist two different mechanisms and,

under these circumstances, an interpretation of

accelerated data to predict the behaviour in nor-

mal exposure is not correct.

In the field tests, which were carried out in a

marine environment, the corrosion kinetics show

that the higher values of I,,,, correspond to

smaller curing times and greater w/c ratios. This

tendency is better defined for the w/c ratio than

for the curing time.

Acknowledgements

The authors wish to acknowledge CINVESTAV-U,

M&da and the Consejo National de Ciencia y Tec-

nologia (CONACyT), M&dco, Contracts 0527-A9109

and FO5119110 for support in conducting various phases

of this investigation. The opinions and findings of this

investigation are those of the authors and not necessar-

ily those of the supporting organisations. The authors

are indebted to Dr. Roberto Centeno and Mr. Fran-cisco Duarte from the Autonomous University of Yu-

catan for their assistance in the mixture design

specimens casting.

and

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Documents

Corrosion of Reinforced Concrete in a Tropical Marine Enviroment and in Accelerated Tests