The efficiency of four commercial concrete coatings (a polymer modified cementitious mortar and three elastomeric coatings) against chloride-induced corrosion is discussed by means of steel corrosion longterm monitoring and by chlorides penetration profiles in concrete. The cement-based coating shows the best effect on delay chlorides penetration in concrete by acting as a physical barrier in addition to concrete cover. Despite its lower polymer content, the higher thickness guarantees a longer time-to-corrosion with respect to organic coatings. Once corrosion has started, corrosion rate is lower in the presence of coatings, due to their ability to reduce water ingress in concrete.
Trang 1Long-term chloride-induced corrosion monitoring of reinforced concrete
coated with commercial polymer-modified mortar and polymeric
coatings
A Brenna, F Bolzoni, S Beretta, M Ormellese⇑
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Via Mancinelli 7, 20131 Milano, Italy
h i g h l i g h t s
a r t i c l e i n f o
Article history:
Received 4 December 2012
Received in revised form 23 July 2013
Accepted 25 July 2013
Available online 24 August 2013
Keywords:
Coating
Concrete
Corrosion
Chloride
Diffusion
a b s t r a c t
The efficiency of four commercial concrete coatings (a polymer modified cementitious mortar and three elastomeric coatings) against chloride-induced corrosion is discussed by means of steel corrosion long-term monitoring and by chlorides penetration profiles in concrete The cement-based coating shows the best effect on delay chlorides penetration in concrete by acting as a physical barrier in addition to concrete cover Despite its lower polymer content, the higher thickness guarantees a longer time-to-cor-rosion with respect to organic coatings Once cortime-to-cor-rosion has started, cortime-to-cor-rosion rate is lower in the presence
of coatings, due to their ability to reduce water ingress in concrete
Ó 2013 Elsevier Ltd All rights reserved
1 Introduction
On ordinary steel reinforcements embedded in alkaline
con-crete, a thin protective film (the so-called passive film) is
thermo-dynamically stable and is formed spontaneously, as Pourbaix
reported in his Atlas[1] Corrosion of steel reinforcements
repre-sents the most widespread form of deterioration of concrete
struc-tures and its accurate knowledge is compulsory in order to predict
the service life of reinforced concrete structures, which can be
di-vided in two phases[2]
The first is corrosion initiation, during which steel is in passive
condition and processes which can lead to steel depassivation
(concrete carbonation or chloride penetration in the concrete
cov-er) are taking place In carbonation, carbon dioxide, diffusing into concrete, it neutralizes its alkalinity by the reaction with calcium hydroxide, so that the pH of the concrete pore solution decreases
to a value lower than 9 In this condition, the passive film is not thermodynamically stable Chloride ions penetrate into concrete
in water solution causing a local breakdown of the passive film if their concentration at the metal surface reaches a critical threshold (in atmosphere between 0.4% and 1% by cement weight)[2] The duration of this phase depends on concrete cover thickness and penetration rate of aggressive species
The second phase is corrosion propagation, which starts after corrosion initiation and ends when a limit state is achieved, over which corrosion cannot be further accepted Once the passive layer
is destroyed, corrosion occurs only in the presence of water and oxygen on the metal surface
Prevention of corrosion is firstly assured by casting an high quality concrete (i.e., proper concrete mixture proportion, W/C ra-tio and cover) If proper concrete cover cannot be assured, when a very long service life is required or in very severe environmental 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd All rights reserved.
⇑Corresponding author Tel.: +39 02 2399 3118; fax: +39 02 2399 3180.
E-mail addresses: andrea.brenna@chem.polimi.it (A Brenna),
fabio.bolzoni@po-limi.it (F Bolzoni), silvia.beretta@chem.polimi.it (S Beretta),
marco.ormellese@po-limi.it (M Ormellese).
Contents lists available atScienceDirect Construction and Building Materials
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c o n b u i l d m a t
Trang 2exposure, the durability of the structure could be increased by
adopting a specific preventative measure that modifies the
charac-teristics of concrete, reinforcement, external environment or the
structure itself These techniques act by preventing aggressive
spe-cies penetration in concrete or by controlling the corrosion process
through inhibition of the anodic process or of the galvanic flow in
the electrolyte Nowadays, different preventative measures are
claimed to prevent, or at least to reduce, steel corrosion in concrete
[2] In this wide context, concrete surface treatments and corrosion
inhibitors[3]offer a possible way to improve concrete structures
durability By the way, the efficiency of organic substances as
cor-rosion inhibitors and concrete coatings on chloride-induced
corro-sion was investigated during a research project started in 1997 and
initially granted by an Italian company leader in the production of
adhesives and chemical products for building While the
effective-ness of organic substances in preventing chloride-induced
corro-sion in synthetic alkaline pore solution and in concrete was
recently discussed elsewhere[4,5], in this paper results about the
effect of polymer-modified mortar and elastomeric coatings on
time-to-corrosion and corrosion rate of reinforced concrete are
reported
Commonly, it is possible to distinguish four principal classes of
surface treatments for concrete[2]: organic coatings that form a
continuous film, hydrophobic treatments that line the surface of
the pores, treatments that fill the capillary pores and cementitious
layers Their effect is twofold: to reduce the permeability of
aggres-sive agents in concrete and to decrease the water content of
con-crete with the resulting decrease of concrete electrical
conductivity and corrosion rate[6–13] In this work, the effect of
three organic coatings and a polymer modified cementitious
mor-tar on chloride induced corrosion of reinforced concrete was
inves-tigated Organic coatings are commonly used to block the
penetration of carbon dioxide or chloride ions by forming a
contin-uous polymeric film on concrete surface of thickness from 0.1 to
1 mm They are based on various types of polymers (e.g acrylate,
polyurethane, and epoxy), pigments and additives and their
effec-tiveness is related to the absence of pores or defects Cementitious
coatings form a layer of low permeability and thickness of a few
millimetres, typically lower than 10 mm The mortar is generally
fine grained and modified with polymers to decrease its
permeabil-ity and to increase its bond to concrete European standardizations
[14,15]report several methods to test coating characteristics, as for
instance water absorption, water vapour, chlorides and carbon
dioxide permeability, adhesion, crack-bridging and self-repairing
properties, mainly to short-term tests There is a lack of data about
long-term behaviour of organic and cement-based coatings This
work reports almost seven years corrosion monitoring of
rein-forced concrete specimens coated with cementitious and organic
coatings Coating efficiency was investigated by means of corrosion
potential and corrosion rate monitoring and by chlorides
penetra-tion profiles in concrete
2 Materials and methods
2.1 Concrete coatings
Four commercial concrete coatings were tested:
Coating A: two-components mortar based on cementitious binders,
fine-grained selected aggregates, special additives and synthetic acrylic polymers
dispersed in water with polymer-to-cement ratio 0.33.
Coating B: hydro-dispersed fibrous coating, based on elastomeric acrylic
emul-sions cement-free.
Coating C: cement-free and elastomeric acrylic-based fibrous coating mixed
with graded sand.
Coating D: single-component acrylic resin-based paint in water dispersion
which forms a flexible film on the concrete surface due to the action of natural
Polymeric coatings were applied on concrete after a previous surface treatment with a specific primer which aims to improve coating-to-concrete bonding While the thickness of organic coatings (Coating B, C and D) is in the range between 0.2 and 1 mm, the mortar (Coating A) has a thickness between 1 and 3 mm The tested coatings were commercial products available on the market on 2005; technical data are in conformity with the requirements of international standard tests.
2.2 Concrete mix design
Concrete coatings (Section 2.1 ) were tested on laboratory concrete specimens prepared with a Portland-limestone cement, type CEM II A/L 42.5R that, according
to EN 197-1 [16] , indicates a cement containing from 6% to 20% by mass of lime-stone and with a minimum compressive strength of 20 and 42.5 MPa after 2 and
28 days, respectively Concrete specimens were prepared with two water-to-ce-ment ratios (W/C), 0.55 and 0.65, in order to study coating efficiency in concretes with different porosity Limestone aggregates were used with maximum diameter
16 mm An acrylic plasticizer was mixed to fresh concrete in order to guarantee S5 workability, according to EN 206-1 [17] (slump P220 mm) Concrete mix design
is reported in Table 1 Two prismatic reinforced concrete specimens (Sample A and B,
340 250 50 mm, Fig 1 ) were prepared for each experimental condition Five car-bon steel reinforcements (diameter 10 mm and length 290 mm) with chemical com-position and mechanical properties according to EN 10080 [18] were placed in each concrete specimen; the ends (40 mm) of each steel bar were coated with heat shrink-age sleeve, so that only a length of 210 mm was exposed to concrete Concrete cover was 20 mm A Ti-MMO reference electrode and two stainless steel wires (diameter
2 mm) were placed next to each rebar for corrosion rate measurements Specimens were cured for 28 days at 20 °C and 95% relative humidity, before the application of the coating on the top surface The organic coatings (A, B and C) were applied only
on concrete specimens prepared with the highest water-to-cement ratio (0.65) Plain concrete cubic samples (side 150 mm) were also prepared in order to allow concrete cores extraction for chloride concentration profiles determination Concrete mix de-sign and exposure condition are the same of reinforced specimens.
2.3 Exposure condition
Both plain cubic samples and reinforced specimens were exposed to accelerated chlorides penetration wet–dry cycles, i.e ponding cycles A ponding cycle consist of one week wetting with a 5% sodium chloride solution (almost 30,000 mg/L chloride ions), and two weeks drying The test solution was placed in contact with the upper surface of the specimen by putting the solution in a plastic box fixed on the top of the specimen.
According to EN 206-1 [17] , cyclic ponding to chloride solution can be classified
as XS3 and XD3 exposure classes, which refer to corrosion induced by chlorides from seawater and other than from seawater, respectively.
It should be pointed out that W/C ratio and concrete cover thickness (Section 2.2 ) were not adopted in agreement with standards requirements recommenda-tions [17,19] , in order to reduce chlorides penetration time into concrete, compat-ibly with the laboratory research time schedule.
2.4 Corrosion monitoring
Almost seven years long tests were performed Steel reinforcements corrosion was monitored by open circuit potential (in the following called corrosion potential,
E CORR ) measurement with respect to a saturated calomel reference electrode (SCE, +0.244 V SHE) placed on the upper surface of the concrete specimen by means of
a wet sponge, and by linear polarization resistance (LPR) measurement, which is considered an accurate and rapid way to determine the instantaneous corrosion rate of steel reinforcements [20] In LPR technique a small potential scan defined with respect to corrosion potential (DE = E E CORR ) is applied to the metal and the polarization current (which varies approximately linearly with potential within
a few millivolts from E CORR ) is recorded Corrosion current density (i CORR , mA/m 2
) is related to the specific LPR (Xm 2
) by Stern–Geary equation [21] :
where B is the Stern–Geary coefficient (related to anodic and cathodic Tafel slopes) which assumes approximately a value of 26 mV or 52 mV for steel in active or pas-sive condition, respectively [2,21] LPR measurement was carried out by means of an
Table 1 Concrete mix design.
W/C = 0.55 W/C = 0.65 CEM II A/L 42.5 R kg/m 3
Trang 3EG&G 273 Potentiostat/Galvanostat applying a potential scan in the range ±10 mV
with respect to E CORR with a scan rate of 0.16 mV/s [21] The polarization current
was supplied by means of two stainless steel counter-electrodes placed on both sides
of the reinforcement ( Fig 1 ) During LPR test, potential was measured with respect to
the internal Ti-MMO reference electrode, in order to minimize the ohmic drop
con-tribution LPR was calculated by a linear regression of the potential–current curve
with the PowerCORRÓ software.
2.5 Chlorides penetration
Concrete cores (30 mm in diameter) were extracted from cubic concrete
speci-mens, cut into 10 mm slices and then crushed and dissolved in nitric acid Chlorides
content was measured by potentiometric titration with AgNO 3 0.01 N [22]
Chlo-rides penetration in concrete is due to the presence of different mechanisms, mainly
diffusion and capillary sorption Nevertheless, for comparison purposes,
experimen-tal profiles were interpolated using the analytical solution of the second Fick’s law
of diffusion, valid in non-stationary condition Supposing that chloride
concentra-tion at the concrete surface (C s ) is constant with time and considering an effective
chlorides diffusion coefficient (D eff ) that does not vary with time and space,
i.e.con-crete is homogeneous, the analytical solution is:
Cðx; tÞ
p
ð2Þ where C (x, t) is the chloride concentration at the depth x after time t.
3 Results
The effect of the coatings on chloride-induced corrosion has
been investigated by means of corrosion potential and corrosion
rate (calculated by LPR measurements) monitoring and by the
determination of chlorides penetration profiles in concrete
Coat-ing A (Section2.1) was tested on concrete specimens with both
water-to-cement ratios (0.55 and 0.65), while elastomeric coatings
were applied on concrete specimens prepared only with W/C ratio
0.65 Overall, the corrosion behaviour of ten steel reinforcements
for each condition was monitored
3.1 Corrosion monitoring
At the end of the wetting week of the ponding cycle, corrosion potential was measured with respect to a SCE reference electrode
by a high impedance voltmeter Corrosion potential measurement allows to assess the corrosion state of the reinforcement: generally, corroded and passive steels in concrete show a difference in corro-sion potential up to 500 mV, due to the different electrochemical behaviour of active and passive areas In practical applications on extended structures, potential measurement allows to locate cor-roding reinforcements, corresponding to the most negative values (half-cell potential mapping)[23] As reported by the standard ASTM C876-09[24], if potential is more positive than 200 mV CSE (Cu/CuSO4 saturated reference electrode, +318 mV SHE), the probability that no reinforcing steel corrosion is occurring at the time of the measurement is greater than 90% Otherwise, if poten-tial is in the range from 200 to 350 mV CSE corrosion activity is uncertain and if potential is more negative than 350 mV CSE there is a probability greater than 90% that steel is in corrosion condition This criterion (derived empirically and so not univer-sally applicable), provides an indication of the corrosion behaviour
of steel in atmospherically exposed concrete but does not indicate steel corrosion rate Steel corrosion rate (CR, mm/y) was calculated
by means of LPR measurements[25]:
where EW is carbon steel equivalent weight (27.92), iCORRis the cor-rosion current density (lA/cm2) calculated assuming 26 mV as Stern–Geary coefficient (B in Eq (1)), q is the metal density (7.86 g/cm3) and K is a constant (3.27 103mm g/lA cm y) For mild steel, a corrosion current of 1 mA/m2 (0.1lA/cm2) corre-sponds to a corrosion rate of 1.17lm/y Generally, corrosion rate can be considered low if lower than 5lm/y and negligible if lower Fig 1 Reinforced concrete specimens geometry.
Trang 4than 1lm/y[26], considering the usual minimal acceptable service
life of new designed structures (50 years)
It should be specified that corrosion rate by means of Eq.(3)is a
mean value calculated on the entire surface area of the rebar, and
its use as an ‘‘exact’’ value can be misleading in the case of
local-ized corrosion, as pitting corrosion, depending on macrocell
forma-tion and on the cathodic-to-anodic surface ratio, in particular if the
concrete resistivity is low[2] Nevertheless, the initiation of
corro-sion can be detected by the decrease in the values of LPR of at least
one order of magnitude with respect to passive condition
3.1.1 Concrete with W/C ratio 0.65
3.1.1.1 Polymer modified cementitious mortar (Coating A).Figs 2
and 3show corrosion potential (mV SCE) and corrosion rate (lm/
y) monitoring of reinforced concrete specimens (Sample A and B,
respectively) with water-to-cement ratio 0.65, each one containing
five steel reinforcements Data are compared with steel corrosion
potentials and corrosion rate in the reference condition, without
concrete coating Almost seven years monitoring was performed
Initially, concrete is chlorides-free and carbon steel reinforcements
are in passive condition Corrosion potential is approximately in
the range between 0 and 150 mV SCE, which corresponds to a
corrosion probability lower than 10%[24]and mean corrosion rate
is between 0.1 and 0.3lm/y Generally, corrosion potential is
determined by the intersection of anodic and cathodic potential–
current density curves While the anodic process (described by
the anodic passive curve) could be considered the same for all
the reinforcements (in the absence of chloride ions), the variation
of oxygen concentration corresponding to steel surface can lead
to corrosion potential variation Indeed, corrosion potential is determined by the availability of oxygen to the metal surface Gen-erally, in concrete exposed to atmosphere, steel reinforcements show corrosion potential between +100 mV and 200 mV SCE [2] Nevertheless, no remarkable differences of corrosion potential data in passive condition between coated and uncoated specimens are observed A corrosion potential drop is measured after five ponding cycles (about 100 days) for the reinforced uncoated spec-imens Steel potential decreases to values between 500 mV and
600 mV SCE, with a net drop of about 500 mV The potential de-crease can be related to passive film breakdown with the formation
of localized anodic area (pits) on the metal surface surrounded by not corroded regions Accordingly, corresponding to the corrosion potential drop, corrosion rate increases from the initial values up
to 30lm/y Corrosion initiation is so confirmed by the simulta-neous decrease of corrosion potential and the increase of corrosion rate The presence of Coating A allows to increase time-to-corro-sion with respect to the uncoated condition Nevertheless, an odd behaviour can be observed comparing the two specimens (Sample
A and Sample B) prepared with the same water-to-cement ratio For Sample A (Fig 2) corrosion potential never goes down
276 mV SCE (350 mV CSE, dotted line), which corresponds to
a corrosion probability greater than 90%[24] A corrosion potential drop is observed for two reinforcements, which potential decreases
to about 250 mV SCE, corresponding to an uncertain corrosion condition Conversely, corrosion rate never exceeds 1lm/y, i.e corrosion is negligible On the other hand, Sample B shows the
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 300 600 900 1200 1500 1800 2100 2400 0 300 600 900 1200 1500 1800 2100 2400
Time (days)
W/C = 0.65 Uncoated concrete
Fig 2 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating A (Sample A) during ponding cycles tests Monitoring on uncoated concrete specimen is also shown.
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 300 600 900 1200 1500 1800 2100 2400 0 300 600 900 1200 1500 1800 2100 2400
Time (days)
W/C = 0.65 Uncoated concrete
Fig 3 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating A (Sample B) during ponding cycles
Trang 5worst corrosion behaviour: after 49 ponding cycles (about
1000 days testing), corrosion potential decreases to values lower
than 276 mV SCE for all the reinforcements, indicating corrosion
initiation Then, after about one year during which corrosion
po-tential remains below this value (reaching a minimum value of
465 mV SCE), it increases and shifts to more noble values, up to
130 mV SCE The increase of corrosion potential could be
associ-ated to the variation of the anodic and/or cathodic curves of steel in
concrete, as well as to the ohmic drop contribution The growth of
corrosion products on steel surface and the variation of oxygen
availability can cause a change of corrosion potential, even if,
un-der active corrosion in concrete exposed to atmosphere, a relevant
increase of corrosion potential should principally be related to the
increase of overpotential of anodic reaction Nevertheless, passivity
conditions are not restored as well as it can be obtained with other
protection methods, mainly cathodic protection and
electrochem-ical chloride removal, or with conventional repair methods based
on replacing of all contaminated concrete, less effective due to
the pitting auto-catalytic mechanism Corrosion rate
measure-ments are in agreement with corrosion potential data (Fig 3)
3.1.1.2 Polymeric coatings (Coating B, Coating C, Coating D) Figs 4
and 5show corrosion potential and corrosion rate data of
rein-forced concrete specimens (Sample A and B, respectively) coated
with Coating B (Section2.1) compared with steel corrosion
poten-tials and corrosion rate in the reference condition, without
con-crete coating Initially, concon-crete is chlorides-free and carbon steel reinforcements are in passive condition
In the presence of Coating B, time-to-corrosion increases signif-icantly: corrosion starts after 750 days, and after 800 days (about two years of exposure) only one reinforcement is still in passive condition Corresponding to the corrosion potential drop, corrosion rate increases of more than one order of magnitude from the initial values up to 10lm/y After corrosion initiation, similarly to con-crete coated with the cement-based coating, corrosion potential in-creases gradually, even though it never reaches the initial values of passive condition Corrosion tests were interrupted after four years
of exposition: at the end of the test corrosion potential is in the range from 130 to 200 mV SCE and corrosion rate never exceeds
10lm/y, remaining always lower than corrosion rate measured in the reference condition
Figs 6 and 7show corrosion potential and corrosion rate mon-itoring of reinforced concrete specimens (Sample A and B, respec-tively) coated with Coating C (Section2.1) The earliest corrosion potential drop occurs after 450 days (15 months) A strong disper-sion in time-to-corrodisper-sion is observed After 30 months two steel reinforcements are still in passive condition while the remaining eight rebars are in corrosion condition showing potentials lower than 400 mV SCE Corrosion tests were interrupted after four years of exposition: at the end of the test corrosion potential varies from 200 to 500 mV SCE Corresponding to the corrosion poten-tial drop, corrosion rate increases of more than one order of mag-nitude from the initial values (lower than 1lm/y) up to 30lm/y
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 150 300 450 600 750 900 1050 1200 1350 1500 0 150 300 450 600 750 900 1050 1200 1350 1500
Time (days)
Uncoated concrete
W/C = 0.65
Fig 4 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating B (Sample A) during ponding cycles tests Monitoring on uncoated concrete specimen is also shown.
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 150 300 450 600 750 900 1050 1200 1350 1500 0 150 300 450 600 750 900 1050 1200 1350 1500
Time (days)
Uncoated concrete
W/C = 0.65
Fig 5 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating B (Sample B) during ponding cycles
Trang 6for Sample A, which shows the worst corrosion behaviour (Fig 6).
Conversely, for Sample B (Fig 7), corrosion rate is lower than the
one measured in the reference condition and only for one steel
reinforcement goes over 10lm/y
Figs 8 and 9show corrosion potential and corrosion rate data
referred to concrete specimens (Sample A and B, respectively)
coated with Coating D In the presence of the coating, passive
con-dition is maintained up to about 600 days of exposition
(20 months) showing corrosion potential always more positive
than 100 mV SCE Then, corrosion rate increases from the initial
passive values (lower than 1lm/y) up to values in the range from
1 to 30lm/y After 830 days (40 ponding cycles) nine of ten steel reinforcements are in corrosion condition Whereupon, corrosion potential increases slowly up to values in the range from 200
to 280 mV SCE After four years exposition only one steel rein-forcement (Sample A,Fig 8) is in passive condition with a potential higher than 100 mV SCE
3.1.2 Concrete with W/C ratio 0.55 Figs 10 and 11show corrosion potential (mV SCE) and corro-sion rate monitoring of reinforced concrete specimens (Sample A and B, respectively) with water-to-cement ratio 0.55 coated with
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 150 300 450 600 750 900 1050 1200 1350 1500 0 150 300 450 600 750 900 1050 1200 1350 1500
Time (days)
Uncoated concrete
W/C = 0.65
Fig 6 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating C (Sample A) during ponding cycles tests Monitoring on uncoated concrete specimen is also shown.
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 150 300 450 600 750 900 1050 1200 1350 1500 0 150 300 450 600 750 900 1050 1200 1350 1500
Time (days)
Uncoated concrete
W/C = 0.65
Fig 7 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating C (Sample B) during ponding cycles tests Monitoring on uncoated concrete specimen is also shown.
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0
100
Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 150 300 450 600 750 900 1050 1200 1350 1500 0 150 300 450 600 750 900 1050 1200 1350 1500
Time (days)
Uncoated concrete
W/C = 0.65
Fig 8 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating D (Sample A) during ponding
Trang 7Coating A Initially, concrete is chlorides-free and carbon steel
rein-forcements are in passive condition, i.e corrosion rate is negligible
In the uncoated specimens, the earliest corrosion potential drop
occurs after 7 ponding cycles: steel potential decreases to values
between 500 mV and 600 mV, with a net drop of about
500 mV and corrosion rate increases of about two orders of
magni-tude Otherwise, for both the coated specimens no remarkable
dif-ference in corrosion potential data can be observed with time Only
for three steel reinforcements (Figs.10a and11a), a slight corrosion
potential decrease is observed to about 200 mV SCE
Neverthe-less, corrosion rate never exceeds 1lm/y for both the specimens
and the decrease of corrosion potential does not find a significant confirmation by corrosion rate measurements
3.2 Chlorides penetration Chlorides penetration profiles were determined on concrete cores:
At the 52nd ponding cycle on reinforced concrete specimen coated with the polymer modified mortar (Coating A)
At the 40th ponding cycle on cubic samples
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Time (days) Uncoated concrete
W/C = 0.65
0.1 1.0 10.0 100.0
0 150 300 450 600 750 900 1050 1200 1350 1500 0 150 300 450 600 750 900 1050 1200 1350 1500
Time (days)
Uncoated concrete
W/C = 0.65
Fig 9 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.65 in the presence of Coating D (Sample B) during ponding cycles tests Monitoring on uncoated concrete specimen is also shown.
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0
100
Time (days) Uncoated concrete
W/C = 0.55
0.1 1.0 10.0 100.0
0 300 600 900 1200 1500 1800 2100 2400 0 300 600 900 1200 1500 1800 2100 2400
Time (days)
W/C = 0.55 Uncoated concrete
Fig 10 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.55 in the presence of Coating A (Sample A) during ponding cycles tests Monitoring on uncoated concrete specimen is also shown.
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0
100
Time (days) Uncoated concrete
W/C = 0.55
0.1 1.0 10.0 100.0
0 300 600 900 1200 1500 1800 2100 2400 0 300 600 900 1200 1500 1800 2100 2400
Time (days)
W/C = 0.55 Uncoated concrete
Fig 11 Corrosion potential (a) and corrosion rate (b) monitoring on concrete specimens cast with W/C ratio 0.55 in the presence of Coating A (Sample B) during ponding
Trang 8Fig 12shows chlorides penetration profiles in reinforced
con-crete in the presence of Coating A varying concon-crete
water-to-ce-ment ratio (0.55 and 0.65) after 52 ponding cycles (about 3 years
exposure) Data are compared with concentration profiles in the
uncoated specimens obtained after 40 ponding cycles
Coating A reduces chlorides containing solution penetration
into concrete: the best effect is observed in combination with the
lowest W/C ratio (0.55) At the rebar level (20 mm) chlorides
con-centration is about 0.35% by cement weight, below the critical level
usually considered for concrete in atmosphere (in the range from
0.4% to 1% by cement weight) Conversely, the uncoated specimen
with W/C 0.55 shows chlorides concentration almost one order of
magnitude greater, close to 2.5% by cement weight
Considering the highest water-to-cement ratio (0.65), a strong
difference between the two coated specimens is observed: chloride
concentrations in Sample B are greater than in Sample A, which
shows a penetration profile slightly higher than those obtained
with W/C equal to 0.55 Chlorides concentration in Sample B at
the concrete cover thickness is about 2.3% by cement weight,
lar-gely over the critical level In the uncoated specimens, chloride
concentration is very high (close to 4% at the rebar level)
As expected, water-to-cement ratio has a great effect on
pene-tration rate of aggressive agents from the external environment,
i.e chlorides penetration rate increases by increasing
water-to-ce-ment ratio due to the higher cewater-to-ce-ment paste porosity Chloride
pro-files are in good agreement with corrosion potential and corrosion
rate data discussed previously (Figs 2 and 3): rebars under active
corrosion are embedded in the concrete specimen B, in which the
highest chloride penetration was measured
In order to understand this odd behaviour, coating thickness
measurements (Table 2) were carried out on the concrete cores
ex-tracted for chlorides profile measurement by means of a Leica
ste-reomicroscope As expected, the lowest coating thickness
(1.26 mm) was measured for Sample B with W/C equal to 0.65,
probably due to the low accuracy in the manual application of
the coating carried out by the furnisher Coating thickness shows
a great effect in establishing concrete coating efficiency, which
does not depends only on its permeability and waterproofing
prop-erties but also on the barrier effect provided by the coating In
or-der to compare all the tested coatings, chlorides penetration
profiles (Fig 13) were determined after 40 ponding cycles (2 years
of exposure) by extracting a concrete core from cubic samples with
W/C 0.65 In all the specimens, chlorides content is lower with
re-spect to the one measured in the uncoated concrete The lowest chloride concentrations are measured in concrete with the cemen-titious mortar, while no strong differences are observed comparing the three polymeric coatings (Coating B, C and D) At the rebar level (20 mm) chlorides content is close to 0.25% by cement weight for concrete with the cementitious mortar, while it is in the range from 0.4% to 0.7% in concrete coated with polymeric coatings
4 Discussion Concerning service life of reinforced concrete structures sub-jected to corrosion, concrete coatings performance can be de-scribed in terms of their ability to influence time-to-corrosion initiation and corrosion propagation time (by reducing corrosion rate) These effects will be discussed separately
4.1 Effect on time-to-corrosion Time-to-corrosion was measured as the ponding cycle corre-sponding to which the simultaneous drop of corrosion potential and the increase of corrosion rate was observed.Table 3reports time-to-corrosion of reinforced concrete specimens in the tested conditions All concrete coatings seem to be able to increase corro-sion initiation time in concrete subjected to accelerated chlorides penetration As discussed previously, time-to-corrosion depends
on concrete W/C ratio: mean values vary from 5 to 9 considering the uncoated concretes prepared with W/C 0.65 and 0.55, respec-tively W/C ratio seems to be a significant parameter also if the polymer modified mortar (Coating A) is applied on concrete sur-face Nowadays, corrosion is not occurring in the presence of W/
C equal to 0.55 Conversely, in concrete prepared with W/C 0.65 steel corrosion started after 44 ponding cycles, with a mean value (calculated considering only rebars in corrosion condition) of 46, about nine times greater than that measured for the uncoated
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
Depth (mm)
W/C = 0.55 - Sample A W/C = 0.55 - Sample B W/C = 0.65 - Sample A W/C = 0.65 - Sample B W/C = 0.55 - Uncoated W/C = 0.65 - Uncoated
Coating A
Cycle # 52
Fig 12 Chlorides penetration profiles on concrete specimens coated with Coating
A (W/C = 0.65 and W/C = 0.55) Profiles of uncoated samples were obtained after 40
Table 2 Coating A thickness measurements.
W/C Sample Coating thickness (mm)
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
Depth (mm)
Coating A Coating B Coating C Coating D Uncoated
Cycle # 40
Fig 13 Chlorides penetration profiles on concrete specimens coated with all the tested coatings (W/C = 0.65) after 40 cycles of ponding Profile of uncoated sample
Trang 9specimens The cementitious mortar seems to provide the best
effi-ciency to delay time-to-corrosion In the presence of polymeric
coatings, mean time-to-corrosion varies in the range from 26
(Coating C) to 30 (Coating B), about six times the initiation time
measured in the reference condition
Generally, the increase of corrosion initiation can be related to
two different effects:
Reduction of chlorides transport rate in concrete (Section4.1.1)
Increase of critical chlorides content for the initiation of
corro-sion (Section4.1.2)
4.1.1 Effect on chlorides penetration
Chloride penetration rate depends on concrete quality
(poros-ity, related primarily to W/C ratio, compaction and curing
condi-tion), on exposure condition and on the presence of additional
protective measures, as concrete coatings Indeed, the application
of a concrete coating provides an additional physical barrier to
the chloride-containing solution penetration in the concrete cover
In order to compare the efficiency of the tested coatings to
re-duce chlorides penetration, chloride concentration profiles
(Fig 13) were analysed by means of the analytical solution of the
Fick’s second law (Eq.(2)) Nevertheless, some considerations are
necessary The analytical solution of Fick’s second law is valid in
non-stationary condition, supposing that chlorides concentration
at the concrete surface (Cs) is constant in time (C = Cs for x = 0
and any t) and that chlorides diffusion coefficient (D) does not vary
with time and space, i.e concrete is homogeneous While Fick’s law
describes only diffusion transport, chlorides penetration in
con-crete occurs in general by means of different mechanisms, as
cap-illary sorption due to the capcap-illary action of concrete pores, and
depends also on chlorides binding by concrete In particular, pure
diffusion transport is verified only in concrete completely and
per-manently saturated with water Accordingly, Fick’s second law is
not universally applicable to describe chloride transport in
con-crete and an effective diffusion coefficient (Deff) is usually
consid-ered in order to take into account the existence of different
transport mechanisms Moreover, the boundary condition
ex-pressed by the invariance of chlorides concentration at the
con-crete surface (Cs) in time could be too restrictive in the tested
conditions Cs is the chloride concentration at the interface
be-tween coating and concrete and increases with time due to the
presence of the coating that acts as a physical barrier to chlorides
penetration Although the boundary condition on surface
concen-tration is not verified, the fitting operation of chlorides peneconcen-tration
profile was carried out only for comparison purpose The calculated
diffusion coefficient assumes exclusively the significance of
regres-sion parameter and its use as an intrinsic property of concrete
could be misleading
Chloride concentration profiles (Figs 12 and 13) were analysed
by fitting the analytical solution of Fick’s second law to the exper-imental data by using chlorides surface concentration (Cs) and effective diffusion coefficient (Deff) as fitting parameters using the least squares method (Tables 4 and 5) Diffusion coefficient de-creases by decreasing the W/C ratio (Table 4) that, as expected, represents a crucial parameter influencing penetration rate and time-to-corrosion Once chloride-containing solution penetrates through coating thickness, it proceeds in the concrete cover until steel reinforcements are reached By using an electrical analogy, concrete coating and concrete cover represent two resistors which act in series as barrier to chlorides penetration and which resis-tance depends on chlorides permeability and thickness In the un-coated specimens, Deff increases from 10.9 108 to 29.8 108cm2/s varying the W/C ratio from 0.55 to 0.65 Surface chlorides concentration (Cs) provides information about the barrier effect of the coating on reducing chlorides penetration from the external environment Without coating, Csassumes values higher than 4% by cement weight, similar to that in the splash zone of a marine structure where evaporation of water leads to an increase
in the chloride content at the concrete surface[2] In the presence
of Coating A, Csis reduced of about one order of magnitude up to 0.6% by cement weight (Table 4) Data obtained at the 40th pond-ing cycle (Table 5) show that all the tested coatings reduce diffu-sion coefficient of about one order of magnitude with respect to the uncoated concrete Deff varies in the range from 2.2 to 6.4 108cm2/s for polymeric coatings and is 2.3 108cm2/s for the polymeric modified cementitious mortar (Coating A) The lowest chlorides content at the concrete-coating interface (Cs) is measured in the presence of Coating A which provides the best barrier effect against chlorides penetration
Results allow to state that all the tested concrete coatings show
a ‘‘physical-barrier’’ effect, by reducing the chlorides ingress in concrete from the external environment The best efficiency is shown by Coating A, in agreement with corrosion potential and corrosion rate monitoring
Table 3
Time-to-corrosion data.
Coating W/
C
# Passive rebars
# Corroded rebars
Time-to-corrosion (ponding cycle)
Min Max Mean Median
Table 4 Surface chloride content (C s ) data obtained by interpolation of experimental concentration profiles of Fig 12 by non-linear regression with Fick’s second law Coating W/C Ponding
cycle
Chlorides profile measured in reinforced specimens
D eff (10 8 cm 2 / s)
C s (% by cement weight)
a
Mean of Sample A and B considered.
b
Only Sample A considered.
Table 5 Chloride effective diffusion coefficient (D eff ) and surface chloride content (C s ) data obtained by interpolation of experimental concentration profiles of Fig 13 by non-linear regression with second Fick’s law.
Coating W/
C Ponding cycle
Chlorides profile measured in cubic specimens
D eff (10 8 cm 2
/ s)
C s (% by cement weight)
Trang 10In order to estimate coating effectiveness on reduce chlorides
ingress in concrete, an empirical parameter was introduced to
cal-culate the resistance of unit coating thickness to chlorides
penetra-tion This parameter (called ‘‘specific coating penetration
resistence, rcoat’’) does not depend on coating thickness and is
pro-portional to the concentration gradient in the coating, assumed
lin-ear because of the lower thickness of coatings with respect to the
concrete cover Specific coating penetration resistance (Table 6)
is defined as:
where Cs,extis the surface chloride concentration at the interface
be-tween the coating and the external environment, Csis the chloride
concentration at the concrete-coating interface (Table 5) and tcoatis
the coating thickness Cs,extwas considered equal to that measured
for the uncoated specimen in the same exposure condition (4.4% by
cement weigh) Due to the highest thickness, Coating A shows the
lowest specific coating resistance with respect to organic coatings
The highest value is shown by the organic paint (Coating D): the
thin layer of the paint (0.22 mm) and the high polymer content
pro-vide an effective barrier with a great chlorides concentration drop
for unit thickness Polymeric coatings exhibit the highest specific
coating resistance to chlorides penetration Nevertheless, the
cementitious mortar, despite its higher porosity, provides the best
effect against chlorides penetration due to the higher thickness
(2.33 mm,Table 6) This confirms that for concrete coatings, a
min-imum thickness value must be assured on the entire concrete
sur-face in order to guarantee the protective effect of the coating and
then the design lifetime of the structure
4.1.2 Effect on critical chlorides threshold
Critical chlorides threshold was calculated taking into account
the measured time-to-corrosion (Table 3) by using the parameters
obtained by chloride profiles interpolation (Deff, Cs, Table 5) In
respective to the type of the applied coating, the obtained value
is close to 0.5% with respect the cement weight Moreover, values
are in the typical interval generally considered for concrete
struc-tures exposed to atmosphere (0.4–1% by cement weight): coatings
have no effect on critical chloride threshold
4.2 Effect on corrosion rate
The effect of concrete coatings on corrosion propagation time
can be related to their ability to reduce water penetration in
con-crete, increasing concrete electrical resistivity (ohmic control of
the corrosion process) In any case, concerning the designed service
life of a reinforced concrete structure subjected to
chloride-in-duced corrosion, the conservative approach is to assume the time
at which repair actions should be planned on the structure equal
to the initiation time Propagation time may be shorter than
initi-ation time and is traditionally not taken into account because of
the uncertainty with regard to the consequences of localized
corro-sion Nevertheless, depending on the limit state and on the actual
corrosion rate, propagation time could be sufficiently long to be
considered, at least for economic reasons.Fig 14shows the cumu-lative frequency curve of corrosion rate of steel reinforcements in the coated concrete specimens Generally, corrosion rate can be considered negligible if below 1lm/y, low between 2 and 5lm/
y, moderate between 5 and 10lm/y and very high for values above
100lm/y[26] Corrosion rates of steel reinforcements in the uncoated concrete are more dispersed and the 70% of data are greater than 10lm/y, corresponding to a quite high corrosion rate In the presence of the cementitious mortar (Coating A), this value decreases to about the 15% The best behaviour is shown by Coating B and D: the 90% of corrosion rate values are lower than 7 and 9lm/y, respectively Conversely, corrosion rate data distribution in the presence of Coating C presents a great dispersion and the 40% of data are
high-er than 10lm/y
Concrete coatings seem to be able to reduce corrosion rate of steel reinforcements since they the increase concrete electrical resistivity as a consequence of the reduction of the water content into concrete Although corrosion rate is not negligible, steel rein-forcements corrosion is in ohmic control and corrosion rate is lim-ited to lower values with respect to the uncoated condition The adoption of concrete coatings as a preventative corrosion method can extend corrosion propagation time by a reduction of corrosion rate
Nevertheless, it should be pointed out that the calculated corro-sion rate is the mean value on the entire surface of the reinforce-ment and does not take into account the localized mechanism of chloride-induced corrosion Pitting penetration rate can be higher than corrosion rate calculated from LPR measurements depending
on the ratio between anodic and cathodic area[27,28]
5 Conclusions The efficiency of four commercial concrete coatings (a polymer modified cementitious mortar and three elastomeric coatings) against chloride-induced corrosion was studied by means of steel corrosion long-term monitoring and by chlorides penetration pro-files in concrete The polymer containing mortar shows the best ef-fect on delay chlorides penetration in concrete by acting as a physical barrier in addition to concrete cover Despite its lower polymer content with respect polymeric coatings, the higher thick-ness guarantees a longer time-to-corrosion In the presence of aggressive environments or for long service life, the use of coating
Table 6
Specific coating resistance to chlorides penetration (r coat ).
Coating t coat C s,ext C s DC = C s,ext C s r coat = DC/
t coat
mm % by cem.
weight
% by cem.
weight
% by cem.
weight
(% Cl )/
mm
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Corrosion rate (µm/y)
Uncoated Coating A Coating B Coating C Coating D
Fig 14 Cumulative frequency of corrosion rate for rebars embedded in uncoated concrete and concrete with coatings.