Construction and Building Materials 83 (2015) 19–25
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Time dependence and service life prediction of chloride resistance
of concrete coatings
Guo Li ⇑, Boyuan Yang, Changsheng Guo, Jianmin Du, Xiaosuo Wu
Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, China University of Mining & Technology, Xuzhou 221116, China
h i g h l i g h t s
Concrete chloride resistance can be remarkably improved using coatings.
Organic film coatings usually deteriorate faster than infiltrating coatings.
Service lives of concrete coatings are closely related with solar irradiance.
a r t i c l e
i n f o
Article history:
Received 13 November 2014
Received in revised form 24 February 2015
Accepted 2 March 2015
Available online 12 March 2015
Keywords:
Concrete coating
Chloride resistance
Time dependence
Coulomb electric flux
a b s t r a c t
This study aims to determine the effects of coating category and degradation on chloride resistance and
service life of concrete coatings. Four typical coatings were applied on concrete specimens, and aged
under outdoor natural climate conditions and indoor artificial accelerated experiments using ultraviolet
light radiation and wetting/drying cycle. Coulomb electric fluxes of the specimens were periodically tested to determine their chloride resistance before and during aging. The chloride resistance of concrete is
remarkably improved with the use of coatings, and organic film coatings provide superior improvement
to infiltrating coatings. The chloride resistance of coatings is time-dependent, and organic film coatings
exhibit faster aging than infiltrating coatings. The experimental time needed for coating degradation can
be shortened through artificial accelerated aging experiments. The service lives of concrete coatings
against chloride resistance are closely related and can be predicted through sunlight irradiance of the
service environment.
Ó 2015 Elsevier Ltd. All rights reserved.
Steel bar corrosion, a major problem that deteriorates the durability of concrete structures, is primarily caused by penetration of
chloride ions [1,2]. Adding protective surface coatings to concrete
structures is an important method to improve their durability; this
method is convenient, simple, and feasible, as well as suitable for
new and deteriorated concrete structures. However, different
coatings correspond to different protective mechanisms, protective
efficacies, and applicable conditions [3–7]. Organic film-forming
coatings can form a protective barrier with 0.1–1-mm depth on
the concrete surfaces to isolate aggressive substances from the
outside [3,4]. Cementitious coatings, particularly polymer-modified
ones, can form a physical barrier of up to 10-mm depth on the concrete surface to reduce the permeability of the concrete, thereby
decreasing the penetration of moisture and corrosive medium.
Cementitious coatings also exhibit breathing and good weathering
resistance [5–7]. Organic silane-based water repellents can change
⇑ Corresponding author. Tel.: +86 516 15150030916; fax: +86 516 83884843.
E-mail address: guoli@cumt.edu.cn (G. Li).
http://dx.doi.org/10.1016/j.conbuildmat.2015.03.003
0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
the hydrophilicity of concrete, thereby significantly improving
water absorption and permeability, which are beneficial to frost
resistance, carbonation, and chloride resistance of concrete [8–10].
Applying surface coatings can generally improve concrete
durability to a certain degree, but the protective efficacy cannot
be maintained and gradually degrades over service time; thus,
the protective effect of coatings on concrete is time-dependent
[11–13]. Six concrete coatings were implemented and tested for
5 years in the Persian Gulf tidal zone, in which epoxy polyurethane
and aliphatic acrylic are the most efficient coatings and the performances of surface coatings are time-dependent [11]. Accelerated
wet–dry cycle experiments using a salt solution were conducted
on concrete specimens with four commercial coatings (a polymer-modified cementitious mortar and three elastomeric coatings)
for 7 years; the cement-based coatings exhibited an optimal effect
on delayed chloride penetration in concrete by acting as a physical
barrier in addition to the concrete cover [12]. Carbonation experiments using concrete with four types of surface coatings were
tested after natural exposure and indoor accelerated aging; the
20
G. Li et al. / Construction and Building Materials 83 (2015) 19–25
carbonation resistance of concrete with epoxy resin or polyurethane paint decreases with the increasing aging time and
minimally changes for concrete with cement-based permeable
crystallization or organic silicon coating [13].
Extensive research has been conducted on the protective
performance of coatings for metals and their weathering ability
[14–17]; however, coatings for metal significantly differ from those
for concrete. Studies should be performed on the application and
aging laws of coatings for concrete structures compared with traditional metal coatings [18,19]. Correct assessment and evaluation
of the time-varying performance of concrete coatings is important
to calculate their service life. In this study, four commonly used
coatings for concrete were selected. The chloride resistance of
the coated specimens was periodically tested under outdoor natural and indoor accelerated aging using ultraviolet radiation and
wetting/drying cycle. Thus, the relationship between anti-chloride
capabilities of coatings with aging time can be obtained.
1. Experimental
[20]. After 6, 9, 12, and 15 months of exposure, the specimens were
obtained and periodically subjected to the Coulomb electric flux test. The
average value of three pieces of each generic coated specimen was obtained
as the representative value.
(2) Accelerated ultraviolet radiation aging experiment: Four generic coated
concrete specimens were placed in an ultraviolet radiation climate chamber
with a constant temperature of 60 °C and relative humidity of 10%. Two
400-W ultraviolet lamps were installed in the climate chamber, and the distance of the specimen surface from the ultraviolet lamp was 20 cm. After
120, 240, 360, and 480 h of exposure, the specimens were obtained and
periodically subjected to the Coulomb electric flux test. The average value
of three pieces of each generic coated specimen was obtained as the representative value.
(3) Accelerated wetting/drying cycle aging experiment: a wet/dry cycle device
was designed to accomplish the aging experiment; the device comprised six
275-W infrared lamps to simulate the drying process and a self-suction
pump that sprays tap water to simulate the wetting process. In the wet/
dry cycle regime, one cycle constituted water spraying for 3 min and infrared light drying for 57 min. The distance of the specimen surface from the
infrared light was 30 cm, and the temperature of the specimen surface
during drying was 40 ± 10 °C. After 240, 480, 720, and 960 h of exposure,
the specimens were obtained and periodically subjected to the Coulomb
electric flux test. The average value of three pieces of each generic coated
specimen was obtained as the representative value.
1.1. Raw materials
1.4. Measurement of Coulomb electric flux
Raw materials, such as PO 42.5 ordinary Portland cement which is similar to
type I cement meeting ASTM C 150 requirement, natural river sand with fineness
modulus of Mx = 2.6, crushed limestone with a size of 5–20 mm, and tap water were
utilized to prepare concrete specimens. The designed compressive strength of the
concrete was 25 MPa with water/cement ratio of 0.6. The specific concrete mixture
proportion was 1:2.47:4.03:0.6 for cement:fine aggregates:coarse aggregates:water. The specimens were fabricated in cylinders with 100-mm diameter and 250mm height.
Four commonly used commercial coatings were selected and implemented on
the concrete surface. These coatings included two types of film-forming organic
coatings, namely, PLH52-3 epoxy glass-flake paint (EP coating) and PLS52-2 polyurethane paint (PO coating), as well as two types of infiltrating coatings, namely, BH502 cement-based permeable crystallization waterproof coating (CE coating) and
silane-based water repellent coating (SI coating).
1.2. Fabrication of coated concrete specimens
The specimens were cut into 50-mm thick and 100-mm diameter concrete
slices after 28 d standard curing (with curing temperature of 20 ± 2 °C and relative
humidity more than 95%) to satisfy the size requirement for Coulomb electric flux
test. The concrete slices were initially oven dried at 60 °C for 48 h. Each end surface
of the concrete slice was polished using sandpaper initially to remove grease and
cleaned with a moist cloth to remove dust. Thereafter, the slices were artificially
painted using a pig hairbrush according to manufacturer requirements described
in Table 1. Each level of brushing interval was 24 h. The slices for infiltrating coating
were maintained in a climate chamber with a relative humidity of 70% for 24 h
before brushing.
1.3. Plan of aging experiments
Three coating aging methods, namely, outdoor natural exposure, indoor accelerated ultraviolet radiation, and wetting/drying cycle, were used to determine the
aging resistance of the coatings.
(1) Natural exposure aging experiment: Four generic coated concrete specimens were placed on the roof of a three-story building in the China
University of Mining & Technology campus; this setup was used to expose
the specimens to natural environmental factors, such as sunlight, rain,
wind, temperature, and relative humidity, with an annual average temperature of 13.9 °C and a relative humidity of 72% in the Xuzhou region
Table 1
Description of coatings used in this study.
Coating
category
Coating system
Dosage (kg/
m2)
EP coating
Primer
Top coat
Primer
Top coat
–
–
0.13
0.40
0.13
0.40
1.2
0.16
PO coating
CE coating
SI coating
Seal primer twice
Epoxy glass flake paint twice
Seal primer twice
Polyurethane paint twice
Twice
Twice
The Coulomb electric flux of each coated specimen was tested and calculated in
accordance with the PRC national standard to determine the chloride resistance of
the coated concrete specimens before and during aging [21]. The procedure is
described as follows: the specimens were initially saturated under vacuum conditions with water and then fixed on the sink. After the installation, 3.0% NaCl solution and 0.3 mol/L NaOH solution were separately injected into the sealed water
tanks at both ends. The current value It was recorded at 30-min intervals after a
60-V direct current electric potential was implemented. Finally, Coulomb electric
flux can be calculated according to formula (1)):
Q ¼ 900ðI0 þ 2I30 þ 2I60 þ þ 2It þ þ 2I300 þ I330 þ I360 Þ;
ð1Þ
where Q indicates the total Coulomb electric flux at 6 h/C, I0 represents the initial
current value/A, and It denotes the current value corresponding to time/A.
The risks of anti-chloride penetration of concrete with different Coulomb
electric fluxes are shown in Table 2 based on the evaluation standard of ASTM
C1202 [22].
2. Experimental results
2.1. Chloride resistance of coated concrete specimens before aging
The Coulomb electric flux of each generic coated specimen was
tested before the start of the aging experiments. The results are
presented in Fig. 1.
Application of coatings can improve the durability performance
of concrete to a certain extent. Fig. 1 shows that the Coulomb electric fluxes of the specimens with coatings are significantly lower
than those of the uncoated specimens (blank). According to the
ASTM C1202 judgment, the penetration probabilities of chloride
ions in the blank specimen and in the specimens with coating
are in the middle and low grades, respectively. The specimen with
EP coating obtains the lowest probability, followed by the specimens with PO, SI, and CE coatings with approximately 1/12, 1/10,
1/5, and 1/4 of the blank specimen, respectively. In the literature
[23], the chloride resistance of concrete is highly related to its
Coulomb electric flux; thus, high Coulomb electric flux results in
a high chloride ion diffusion coefficient, indicating poor chloride
resistance. By contrast, a low Coulomb electric flux corresponds
Table 2
Probability evaluation standards of ASTM C1202.
Coulomb electric flux
of 6 h/C
<100
100–
1000
1000–
2000
2,000–
4000
>4000
Probability of chloride
penetration
Negligible
Very
low
Low
Moderate
High
21
3000
2500
2000
1500
1000
500
0
Blank
CE
coating
SI
coating
PO
coating
EP
coating
Fig. 1. Coulomb electric fluxes of coated specimens before aging.
to low chloride ion diffusion coefficient and better chloride ion
resistance. Therefore, the organic film-forming and infiltrating
coatings can significantly enhance the chloride resistance of
concrete and their ability to resist chloride penetration follows
the order of EP coating > PO coating > SI coating > CE coating.
The Coulomb electric fluxes of the specimens with EP and PO
coatings, as well as with SI and CE coatings, are similar. The values
of the infiltrating coatings are approximately two to three times
higher than those of the organic film coatings. Thus, concrete with
organic coatings exhibits higher chloride resistance than infiltrating coatings because of the superior physical barrier performance
of the former. This finding is consistent with the results presented
in the literature [3,4].
Film-forming coatings, such as EP and PO, can form a membrane
on the concrete surface that can effectively prevent the aggressive
substances from attacking the concrete; thus, EP and PO coatings
exhibit good chloride resistance. SI coating can penetrate into the
concrete through cracks and micropores, causing the concrete to
be hydrophobic, which significantly suppresses capillary water
absorption and improves the durability of the concrete. However,
this coating cannot seal the concrete surface; thus, the resistance
to chloride ion diffusion is inferior to that of the film-forming coatings. The main binder of the CE coating can penetrate into concrete
at a particular depth, react with non-dissolved calcium silicate, and
eventually produce insoluble crystal calcium sulfoaluminate. The
ultra-micro colloid silicate fills the pores as a microfiller and generates a compact microstructure. However, the CE coating cannot
completely seal and fill the micropores in the concrete surface.
2.2. Coulomb electric flux of coated specimens under natural aging
Fig. 2 shows the development of the Coulomb electric flux of the
coated specimens under natural exposure aging.
Environmental temperature and relative humidity, as well as
sunlight, rainfall, wind, and frost in natural environment, can play
Coulomb electric flux/C
2.3. Coulomb electric fluxes of coated specimens under accelerated
aging
According to the data obtained from accelerated aging experiments, the degradation of the Coulomb electric fluxes of the coated
concrete specimens at varying exposure times are shown in Figs. 3
and 4.
Ultraviolet light is an invisible light within the high-energy
region and accounts for only 7% of the solar radiation spectrum;
however, this light exhibits the highest energy and the most serious damage to polymers in organic coatings [15]. As shown in
Fig. 3, the Coulomb electric fluxes of the two organic coating specimens significantly increased after 20 d of ultraviolet radiation; EP
and PO coatings increased by 2.7 and 1.5 times, respectively.
Conversely, the two infiltrating coatings minimally increased, in
which the CE and SI coatings increased by 6.7% and 11.3%, respectively. The Coulomb electric fluxes of the EP and PO coatings
approached or surpassed those of the CE and SI coatings, implying
that the chloride resistance of the EP and PO coating specimens
was exceeded by that of the CE and SI coating specimens.
Temperature and moisture are two main factors that cause
coating deterioration. According to Fig. 4, the Coulomb electric
fluxes of the two organic coating specimens evidently increased
during the wet–dry cycles, whereas those of the two infiltrating
coatings minimally increased. After 40 d of accelerated aging
experiments under the wetting/drying cycle, the Coulomb electric
fluxes of EP, PO, CE, and SI coatings increased 2.3 times, 1.5 times,
8.3%, and 11.9%, respectively. The Coulomb electric fluxes of the
two organic coatings grew faster than those of the two infiltrating
coatings and approached or exceeded those of the CE and SI coatings after 40 d of wet–dry cycles.
Although the aging methods differ, the development of
Coulomb electric fluxes with the aging time of a similar generic
900
900
750
CE coating
600
SI coating
450
EP coating
300
PO coating
150
0
important roles in the aging of concrete coatings. Fig. 2 shows that
all Coulomb electric fluxes of each generic coated specimen undergo certain changes during 15 months of natural exposure, and two
organic film coatings evidently increase. The Coulomb electric flux
of EP coating specimen increases 2.2 times from 222.4 C to 709.6 C,
whereas the PO coating specimen increases 1.1 times from 274.1 C
to 565.3 C. However, the changes in the two infiltrating coating
specimens are not evident; the CE coating increases by 7.1% from
771.2 C to 826.2 C, and the SI coating increases by 5.8% from
549.9 C to 582 C. The increasing Coulomb electric fluxes of the concrete specimens exhibit decreasing chloride resistance. The results
from natural exposure indicate that different coatings exhibit different weathering resistances. Generally, infiltrating coatings are
more resistant to weathering than organic coatings.
Coulomb electric flux/C
Coulomb electric flux/C
G. Li et al. / Construction and Building Materials 83 (2015) 19–25
750
CE coating
600
SI coating
450
EP coating
300
PO coating
150
0
0
3
6
9
12
15
Time/month
Fig. 2. Development of Coulomb electric fluxes of coated specimens under natural
exposure.
0
5
10
15
Time/day
20
Fig. 3. Development of Coulomb electric fluxes of coated specimens under
ultraviolet radiation.
22
G. Li et al. / Construction and Building Materials 83 (2015) 19–25
of the concrete. This reaction produces insoluble silicate compounds that can fill the pores in the concrete, thereby resulting
in a compact structure. The hydration products are cement-like
and cannot be easily influenced by environmental climate.
Therefore, the CE coating demonstrates better durability than the
organic film-forming coatings. The main component of the SI
coating is octyltriethoxysilane, a hydrophobic material. After
hydrolysis of silane, the reactive silanol groups can anchor to the
cementitious materials or aggregates, causing the surfaces to be
hydrophobic [7]. The Si–O key, which is the main chain in organic
silane-based coatings, is close to the ionic bonds because of the
electronegativity differences between silicon and oxygen; thus,
this key requires high energy during electrolysis and exhibits high
stability. After the key reacts with the concrete material to generate a stable covalent bond, this coating demonstrates good oxidation, heat, and radiation resistances. Among the four generic
coatings in this paper, the SI coating exhibits superior durability
against environmental climate action.
The quantitative degradation of the four generic coatings with
aging time was further analyzed. A statistical regression analysis
and optimal fitting of the Coulomb electric flux–aging time were
performed for each coated specimen under natural climate, ultraviolet radiation, and wetting/drying cycle exposure conditions.
The regression equations of Coulomb electric flux (Q)-aging time
(t) and the correlation parameter R2 are shown in Table 3.
Table 3 presents the well-fit regression equations of the coated
specimens under the three aging conditions, and the lowest correlation coefficient R2 is 0.91. Thus, high correlation exists between
the Coulomb electric flux of the coated specimens and aging time.
The chloride resistances of the specimens with similar coating
evidently vary with different aging times, indicating that the protection of coatings to the specimens from chloride attack is timedependent. The increased Coulomb electric flux of the CE and SI
coatings present a linear relationship in which their chloride resistance degrades linearly with the increased service time.
Conversely, the increased Coulomb electric flux of the EP and PO
coatings demonstrates a non-linear relationship in which their
chloride resistance degrades faster with the increased service time.
According to the regression equations presented in Table 3, the
Coulomb electric flux increase rates of each coated specimen are
shown in Table 4.
The aging rate of each coating is significantly hastened under
artificial accelerated conditions compared with that under natural
exposure. Even if the further increase of the degradation rate of
Coulomb electric flux/C
900
750
CE coating
600
SI coating
450
EP coating
300
PO coating
150
0
0
10
20
30
Time/day
40
Fig. 4. Development of Coulomb electric flux of coated specimens under wetting/
drying cycle.
coating between accelerated ultraviolet radiation and wetting/
drying cycle are similar as shown in Figs. 3 and 4; the results are
also consistent with the findings under natural exposure.
Generally, as the aging process proceeds, the Coulomb electric flux
of each coated specimen gradually increases and corresponds to
the decreasing chloride resistance of the coatings.
3. Discussion
3.1. Time dependence of chloride resistance of concrete coatings
Experimental results have indicated that the chloride resistance
of each specimen with organic film coatings cannot be maintained
constantly. Ultraviolet light, temperature, and moisture are three
main factors that affect the aging of conventional paintings [15].
The poor durability of film-forming coatings may be due to their
volatile organic compounds. The main components of the EP coating are aromatic structures, which can easily absorb ultraviolet
light, as well as results in color change and degradation of the coatings. Thus, the coating surface gradually becomes thin and eventually loses protection to the concrete structures. During drying, the
EP coating produces a specific shrinkage accompanied by internal
stresses and therefore usually causes microcracks that can deteriorate the EP coating [18].
The PO coating can also absorb ultraviolet light and cause some
keys, including N–C key and C–O key in the carbamic acid ester
group fracture, thereby significantly decreasing the molecular
weight of the polymers and leading to coating degradation.
However, because of ammonia ester bonds in the molecular structure of polyurethane coatings, the formation of hydrogen bonds
between polymer molecules is a necessary condition. The presence
of numerous hydrogen bonds results in significant cohesion, which
improves the aging resistance of materials. Therefore, the PO coating exhibits better anti-aging ability than the EP coating.
The main constituent of the CE coating is inorganic calcium silicate. During hydration, SiO2
4 ions in the coating react with soluble
Ca2+, Mg2+, and Al3+ ions, which remain on the surface and interior
Table 4
Coulomb electric flux increase rates of each coated specimen/C/d.
Category
Natural exposure
Ultraviolet
radiation
Wetting/drying
cycle
EP coating
PO coating
SI Coating
CE coating
0.43 + 0.0028t
0.1 + 0.0026t
0.07
0.13
18.96 + 1.2t
8.29 + 1.34t
3.28
2.48
8.24 + 0.26t
3.95 + 0.36t
1.74
1.75
Table 3
Regression analysis of Coulomb electric flux–aging time of coated specimens.
Category
Natural exposure
Regression equation
EP coating
PO coating
SI Coating
CE coating
2
Q = 0.0014t + 0.43t + 225
Q = 0.0013t2 + 0.10t + 269
Q = 0.07t + 548.4
Q = 0.13t + 770.9
Note: t represents the aging time of coating/d.
Ultraviolet radiation
2
R
0.99
0.97
0.91
0.98
Regression equation
2
Q = 0.60t + 18.96t + 212
Q = 0.67t2 + 8.29t + 262
Q = 3.28t + 555.0
Q = 2.48t + 774.5
Wetting/drying cycle
R
2
0.99
0.99
0.93
0.97
Regression equation
2
Q = 0.13t + 8.24t + 214
Q = 0.18t2 + 3.95t + 260
Q = 1.74t + 549.3
Q = 1.75t + 775.5
R2
0.99
0.98
0.97
0.92
G. Li et al. / Construction and Building Materials 83 (2015) 19–25
23
natural exposure to forecast the service life of coatings. Artificial
accelerated aging experiments can significantly shorten the aging
time and enhance the efficiency of aging experiments.
3.2. Service-life prediction of concrete coatings
Fig. 5. Development of Coulomb electric fluxes of coated specimens under natural
exposure with the solar irradiance.
organic film coating with aging time is not considered, the aging
acceleration under ultraviolet radiation reaches 83 and 40 times
for the EP and PO coatings, respectively, whereas those under the
wetting/drying cycle attain 44 and 19 times, respectively. The
ultraviolet radiation and wetting/drying cycle can also enhance
the aging of infiltrating coatings in which the SI coating increases
18 and 12 times and the CE coating 46 and 24 times, respectively.
The aging rates of the coatings are generally faster under ultraviolet radiation than under the wetting/drying cycle; thus, the
damage effects on the coatings under ultraviolet radiation are
higher than those under the wetting/drying cycle.
Although aging speeds differ under the three methods, the
degradation of each coating presents a high degree of consistency.
This finding suggests that the aging degradation of the coatings is
similar under ultraviolet radiation, wetting/drying cycle, and natural exposure. Thus, accelerated aging experiments under ultraviolet radiation and wetting/drying cycle can be used instead of
Several climatic factors, such as sunlight, rainfall, temperature,
and relative humidity, exist in the natural environment, and different regions have different climatic conditions, which affect the service life of concrete coatings. Previous experimental results
showed that ultraviolet radiation and wetting/drying cycle play
important roles in coating degradation. Solar irradiance is an
important indicator that reflects a specific amount of sunlight
energy per unit area in a region; solar irradiance indicates not only
the ultraviolet radiation but also the heat effects of sunlight. These
factors significantly lead to coating degradation. This study
attempts to establish a relationship between solar radiation and
coating degradation. The total solar irradiance under natural exposure for 6, 9, 12, and 15 months can be calculated based on the
monthly average sunlight irradiation from the meteorological data
in the Xuzhou area. Thus, the development of Coulomb electric flux
with solar irradiance for each coated specimen under natural exposure and the statistical regression correlation analysis were
obtained, as shown in Fig. 5.
Fig. 5 presents a significant linear correlation of Coulomb electric flux development with solar irradiance in the EP and PO coating specimens, and the correlation coefficient R2 is higher than
0.99. Thus, sunlight radiation critically affects the degradation of
organic film coatings. The correlation coefficients R2 of the CE
and SI coatings are 0.87 and 0.81, respectively, indicating that they
are also significantly affected by sunlight radiation.
The Coulomb electric flux values of the specimens under natural
exposure are substituted into the regression equations presented
in Table 3 to establish the aging relationship between natural
Fig. 6. Relationship of accelerated aging time and solar irradiance.
24
G. Li et al. / Construction and Building Materials 83 (2015) 19–25
Table 5
Service-life prediction of coatings under natural environment (Xuzhou).
Ultraviolet radiating
Wetting/drying cycle
Coating
Accelerated aging time/d
Equivalent solar irradiance/MJ/m2
Predicted service
life/y
EP coating
PO coating
CE coating
SI coating
EP coating
PO coating
CE coating
SI coating
50.82
54.87
790.5
664.6
111.5
106.4
1119.7
1256.1
18,102.1
18,722.8
211,881.1
371,138.9
17,968.8
18,638.3
214,164.9
381,612.1
3.71
3.84
43.4
76.1
3.68
3.82
43.9
78.2
climate and two artificial climates with ultraviolet radiation and
wetting/drying cycle. Thus, accelerated aging times can be
obtained. The relationships between solar irradiance and accelerated time for each coated specimen are shown in Fig. 6.
Fig. 6 demonstrates the close correlation between accelerated
aging time under ultraviolet radiation and wetting/drying cycle
with the amount of solar irradiance. The correlation parameter R2
of the two types of organic coatings is higher than 0.9, whereas
that of the SI and CE coatings is higher than 0.8. For the same coating, the accelerated aging time required to achieve different degradation degrees (Coulomb electric flux) is linearly related to the
amount of solar irradiance absorbed in the natural environment.
A high degradation degree of coatings requires longer accelerated
aging time and corresponds with more solar irradiance.
With the gradual degradation of the chloride resistance for the
coatings, the Coulomb electric fluxes of the coated specimens
gradually approach that of the blank specimen. The value
(2,735 C) of the Coulomb electric flux of the reference blank specimen is designated as the criterion for coating failure or recoating;
thus, the accelerated aging time required for this criterion can be
calculated through the regression equations presented in Table 3.
The equivalent amount of solar irradiance can be calculated from
the regression model in Fig. 6. The service life of each coating
can be predicted using 4877 MJ/m2 as the criterion of the average
annual amount of solar irradiance in the Xuzhou area. The results
are shown in Table 5.
Table 5 shows that the service lives calculated from the two
types of artificially accelerated aging experiments are similar.
This finding implies that either of these two methods can be used
to predict the service life of coatings. The average service lives of SI
and CE coatings are 77 and 43 years, respectively, which approach
or exceed the normal designed service life of concrete structures.
However, the average service lives of EP and PO coatings are only
3.7 and 3.8 years, respectively, which are lower than the traditional
designed service life. Although the chloride resistance of infiltrating coatings initially exhibits inferior behavior to that of organic
film-forming coatings, their expected service life is longer than
that of organic film coatings because of their superior anti-aging
properties; thus, infiltrating coatings provide longer protection
for concrete structures [12].
Solar irradiance plays a decisive role in the expected service life
of coatings, and the amount of solar irradiance largely differs in different regions; a similar type of coating exhibits different predicted
service lives in different regions. According to the amount of annual average solar irradiance, the service lives of concrete coatings
can be calculated through the Coulomb electric flux regression
models established in Fig. 6. For example, the amount of annual
average solar irradiance in Beijing, Chongqing, and Lhasa cities
are 6281.9, 3076.1, and 8703.8 MJ/m2, respectively [24], so the
expected service lives of EP coatings used in this study are 2.87,
5.87, and 2.07 years in these cities, respectively. For the same coating, the predicted service life in Chongqing is 2.8 times higher than
that in Lhasa. The expected service lives of coatings are relatively
short in the strong sunlight region and relatively longer in the
moderate sunshine regions.
4. Conclusions
The experimental research in this study leads to the following
conclusions:
(1) The chloride resistance of concrete can be remarkably
improved using coatings. Organic film-forming coatings,
such as EP and PO, exhibit better chloride resistance than
infiltrating coatings, such as CE and SI, before aging.
(2) Organic film-forming and infiltrating coatings demonstrate
time-dependent chloride resistance. Organic film-forming
coatings deteriorate rapidly with increased exposure time,
whereas infiltrating coatings deteriorate slowly. With the
aging process, the chloride resistance of infiltrating coatings
gradually exceeds that of the organic coatings, thereby providing longer protection for concrete structures.
(3) The aging performance of the coatings is comparable under
natural climate exposure and accelerated ultraviolet radiation and wetting/drying cycle conditions. Conducting an
artificial accelerated aging experiment can shorten the
experiment time needed to investigate coating degradation,
thereby enhancing the efficiency of the experiment. The
aging speed of coating under ultraviolet radiation is
approximately two times faster than that under wetting/
drying cycle.
(4) Coating degradation is closely related to the amount of solar
irradiance at service environments. Higher accumulated
solar irradiance corresponds to a higher degree of coating
degradation and longer accelerated aging experiment time.
According to the method established in this study, the
expected service life of coating to chloride resistance can
be determined through accelerated aging experiment and
solar irradiance of the service environment where coating
is applied.
Acknowledgments
This study was funded by the China National Natural Science
Foundation (No. 51178455) and the Jiangsu Province Key
Laboratory (No. JSKL2012YB01).
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