STUDY ON CORROSION OF REINFORCING STEEL IN CONCRETE SLAB ...
Transcript of STUDY ON CORROSION OF REINFORCING STEEL IN CONCRETE SLAB ...
STUDY ON CORROSION OF REINFORCING STEEL IN CONCRETE
SLAB CONTAINING FLY ASH
Sristi Das Gupta1*, Takafumi Sugiyama1
1*Graduate School of Engineering, Hokkaido University, Japan<[email protected]> 1Faculty of Engineering, Hokkaido University, Japan<[email protected]>
ABSTRACT Corrosion of steel reinforcement initiated by chloride contamination has become a common type of
deterioration for RC slab used for road bridges in snowy cold regions. Chlorides are present in the RC
slabs from the exposure to de-icing road salts. The chloride ion diffuses into RC slab to contact the
reinforcements and initiate the corrosion process. Recently fly ash concrete has been practically
employed to RC slab in snowy cold region so that the durability of the RC slab can be enhanced. In the
present study fly ash concretes in RC slab at two replacement levels of 15% and 30% for cement were
examined with regard to the corrosion of the steel bars. The fly ash concretes showed lower chloride
concentration and Cl-/OH- ratio in the vicinity of rebar than that of normal concrete. This study also
focused on the characterization of corrosion products in the reinforcement in normal and fly ash
concrete. Raman spectroscopy was used to characterize the corrosion products developed on the surface
of reinforcing steel embedded in RC slab. Two regions of different colours (Yellow, Grey) on the
surface were identified as different corrosion products among oxides and oxyhydroxides compounds
like hematite (-Fe2O3), magnetite (Fe3O4), wüstite (FeO), maghemite (-Fe2O3), goethite (-FeOOH)
and lepidocrocite (-FeOOH).
Keywords: Normal concrete, Fly ash concrete, Raman spectroscopy, Corrosion products
INTRODUCTION
Corrosion of reinforcement has been established as the predominant factor causing widespread
premature deterioration of concrete structures worldwide. Especially in the snowy cold regions RC slab
in the road bridges where de-icing salts are used during the winter months causes the corrosion of the
steel bars with the ingress of chlorides into the concrete followed by the destroy of the protective film of
reinforcement in the concrete. Recently, fly ash has been used for the partial replacement for cement in
RC slab for durability purposes. Concrete mixes that contain fly ash can increase the resistance to the
penetration of chloride ions and strengths at later ages as compared with non-fly ash concrete mixes.
This is a result of the fly ash reacting with the CH resulting from the pozzolan reaction. Although
pozzolanic reaction improves the denseness and discontinuity in pore network, it also reduces the alkali
content in the concrete. Reduction in hydroxyl content at the vicinity of rebars leads to initiate corrosion
at a lower chloride level. The ratio of Cl-/OH-is sometime used for the probability of the reinforcing
steel bars to cause passivation or corrosion (Taylor P, et.at. 1999). The study on characterization of
corrosion products in reinforced concrete is an important issue with a view to assessing the corrosion
state within the concrete. Raman spectroscopy is a preferred method to study the corrosion products on
the steel surface (Criado M, et.al. 2015). According to Chitty et al. (2005) corrosion system was made
up of a multilayer structure constituted of a dense corrosion product layer (DPL). Dense Product Layer
is mainly made of iron oxi-hydroxides (goethite, lepidocrocite and akaganeite) and iron oxides
(maghemite and magnetite) (Chitty J, et.at. 2005). Lepidocrocite plays an important role in the
corrosion mechanism (Criado M, et.al. 2015). It is mainly developed in the initial stage of corrosion,
when iron oxihydroxides in the presence of aggressive impurities transform into lepidocrocite product.
After that, due to aggressiveness of the medium (the amount of chloride present in the steel surface) the
corrosion products will be transformed to more stable phase; such as lepidocrocite product to transform
stable goethite and akaganeite (Criado M, et.al. 2015). Therefore, the concentration of chloride ions
influenced the nature of iron phase. To yield the maximum benefit from using fly ash in RC slab it is
1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7
now burning issue to understand the corrosion process for fly ash concrete. In this regard, this study was
done to examine the corrosion of reinforcing steel in RC slab containing fly ash with chloride
application. Fly ash is produced locally and specified in accordance with Type II in JIS A 6201.
EXPERIMENTAL METHODOLOGY
Specimen Preparation
In this research, fly ash concrete in RC slab was prepared. For comparison ordinary Portland cement
(OPC) concrete (NA, W/C=63.5%) was also prepared. Two different replacement levels of fly ash of
15% and 30% for OPC were studied namely, F15and F30. The reinforcing steel used for RC slab was
deformed steel bar. All reinforcements had a diameter of 19 mm. A total of 30 specimens with 6 groups
were prepared. All specimen series were cured in water for 91 days. The details of the mix proportions
for N, F15 and F30 are found in Table 1. In this research two different dimensions of RC slabs were
studied, namely Type A and Type B. The size and the cross sections of the specimen are shown in
Figure 1. Every specimen contains two longitudinal reinforcements one is 30 mm cover distance from
top surface named as up rebar and another is 30 mm cover distance from bottom surface named as down
rebar. An Acrylic canister (having opening in top and bottom) of 50 x 100 mm in cross section had been
set on the middle of the specimen with transparent type glue. After allowing the glue sufficiently drying
and hardening, NaCl solution of 10% in concentration was poured in that canister.
Table 1. Mix proportions of concrete
Type Gmax
(mm)
Slump
(cm)
Air
(%)
W/
(C+FA)
(%)
s/a
(%)
Unit weight
(kg/m³)
Strength
(N/mm²)
W C FA S G 28
Days
91
days
N 20 9.5 4.7 63.5 50.0 142 224 0 1,002 995 30.2 33.3
F15 20 9.5 4.8 58.5 48.0 132 192 34 969 1042 28.1 33.9
F30 20 9.5 4.8 49.5 46.0 126 178 76 919 1071 29.6 37.5
Measurements For the measurement of chloride concentration, chloride analysis was conducted after corrosion
initiation of rebar was confirmed by the half-cell potential monitoring and polaraization resistance
method accoroding to ASTM C-876 criteria. Approximately, 10 grams of concrete powder was taken
out from three points in the near of up rebar and down rebar in each side of the normal and fly ash
concrete specimen of the entire chloride application zone (50mm x 100mm) for chloride analysis
(Figure 2a). Thereafter, chloride ion concentration in each point of the specimen has been measured
using an automatic chloride ion titration device (GT-100).
(Unit: mm)
(a)
Type A Type B
( Unit: mm)
(b)
[Fig.1] Schematic presentation of concrete specimen used for corrosion test
1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7
(a)
Corroded Area (Light region: Yellow
color; Dark region: grey color)
(b)
[Fig.2] (a) Schematic presentation of concrete powder taken out for chloride analysis
(b) Reinforcement used for Raman test
Experimental chloride profile has been obtained by plotting the chloride ion concentration (kg/m3)
against the depth of penetration from the concrete surface. A nonlinear regression analysis with
computer software has been used to fit the experimental profile with Eq. 1. Indeed Eq.1 is the solution
of the Fick’s 2nd law which represents the diffusion of chloride ion in concrete.
(1)
Where, is chloride ion concentration at depth x in time t (kg/m3), is chloride ion
concentration at concrete surface (kg/m3), is depth from the penetration surface (m), Dap is apparent
diffusion coefficient (cm2/year), t is period of exposure to chloride ion (year) and erf is error Function,
provided that
(2)
After chloride concentration measurement, the concrete powder that was taken out from the vicinity of
rebar was used further to quantify the Ca(OH)2 (Portlandite) content in hardened concrete using the
Thermo- Gravimetry/ Differential Thermal Analyzer (TG/DTA). The analysis was carried out with the
temperature range of 20-1000oC with an increment rate of 10oC/min. Nitrogen(N2) gas was used for the
test with a flow rate of 200ml/min. A TG/DTA analysis was carried out in the computer data logger
system. The decomposition of Ca(OH)2 occurs in between the temperature range (mostly in between
400-450oC). From the TG/DTA graph, Ca(OH)2 was estimated from the weight loss measured from the
TG curve between the initial and final temperature of the corresponding TG peak by considering the
decomposition reaction stated in Eq.3.
Ca(OH)2 (s) CaO (s) + H2O (g) (3)
A dependency of Cl/OH ratio on mass basis can be calculated by knowing the CH content in the
concrete. The gravimetric proportion of Ca+2 and OH- in portlandite is as follows in Eq.4;
Ca(OH)2 Ca+2 + 2OH- (4)
By calculating the chloride concentration in the vicinity of rebar, Cl/OH ratio in mass basis can be
estimated.
To study the corrosion products formed on steel embedded in normal and fly ash concrete with 15% and
30% cement replacement in the presence of chloride ions Raman spectrometer with wavelength 532 nm
was used. Since several oxyhydroxides can be formed as corrosion product with laser power >1mW, in
this study the laser power was kept within 1mW in order to discern the corrosion products without laser
irradiation transformations. Raman spectra was obtained directly from the steel surface analysed just
after corrosion initiation of RC slab. The Raman spectrometer is equipped with a microscope and a
10mm
50
10%
NaCl
1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7
CCD camera. The measurements were done directly in the sample with the sample length 9cm (Figure
2b).
RESULTS AND DISCUSSIONS
Chloride Analysis
For the F15 and F30 RC slab, total of 6 specimens (F15A2, F15B4, F30A1, F30A2, F30B2 and F30B4)
showed corrosion initiation, based on ASTM C-876 in the presence of chlorides while in NA and NB
RC slab, all specimen showed corrosion initiation. Therefore, only six fly ash specimens among twenty were analysed for chloride profiling. Replacing cement in concrete with fly ash constricts the pore size
and increased the tortuosity which leads to reduction of diffusion coefficient and initiate corrosion to
lower chloride threshold level in fly ash RC slab compared to normal RC slab. The diffusion
coefficients of fly ash and normal concretes are shown in Figure 3.
[Fig.3] Diffusion coefficients for normal and fly ash RC slab
The average chloride diffusion rate was found 0.57cm2/yr in F30 concrete which is low compared to
NA, NB category. As the chloride diffusion coefficient of fly ash concrete found small the amount of
chloride concentration was low inside the concrete even the experimental chloride analysis did not
show evidence of presence of chloride more than 40 mm deep in fly ash concrete. After cutting of fly
ash concrete it was observed that only up rebar of fly ash concrete specimen was corroded while it was
found that all NA and NB concrete both up rebar and down rebar was corroded. However, the
concentration of chloride in down rebar of normal concrete was found less comapared to up rebar due to
high concrete cover depth in down rebar. On the other hand, after chloride concentration measurement amount of Ca(OH)2 content in concrete was also measured and it was observed that CH content was
found lowest in the F15 and F30 concrete than the NA and NB concrete categories It is well known that
CH content in concrete depends on the amount of cement used. The cement content of fly ash concrete
was lower compared to normal concrete and moreover the pozzolanic reaction of fly ash with hydrated
cement paste consumes some of CH content and finally reduce the CH content in fly ash concrete.
1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7
[Fig.4] Summary of Cl/OH ratio for normal and fly ash RC slab
Based on CH content in concrete amount of OH- content was calculated using Eq.4. From Figure 4, it is
seen that Cl/OH ratio in mass basis is less in F30 concrete specimen compare to the other concrete
series. The highest Cl/OH ratio is found in NB category concrete. However, in the fly ash concrete the
amount of CH content is less which also reduces the amount of OH- content due to pozzolanic reaction
in fly ash concrete which may improve the microstructure in the interface with reinforcing steel bars in
the fly ash concrete.
Corrosion products detection
Since the characterization of corrosion products by Raman microscopy was performed in a small area
(laser spot 4m2), a quantitative contribution of each phase is not possible even probing different
regions. However, it was possible to associate the colour of the oxide to a particular phase. Different
iron compounds were observed on the reinforcement extracted from normal and fly ash reinforced
concrete. Spectra for the light and dark regions obtained with a laser energy of 532 nm and the variation
of corrosion products in both dark and light region with the different chloride concentration has been
summarized in Table 2. From Table 2, it can be concluded that presence of iron oxide can vary with the
colour of corrosion.
Table 2. Corrosion products in different region of normal and fly ash concrete
Type of concrete Corrosion phase Corrosion products
N
Dark region G, L, Mg, W, H, MH, Fh, Mh
Light region L, O, W, H, MH
F15 Dark region G, L, O, H
Light region ----
F30
Dark region Mh, Fh, H
Light region L, O, H
Where, G-Goethite(-FeOOH), L- Lepeidocrocite (-FeOOH), H-Hematite (-Fe2O3), Mh-Meghemite
(-Fe2O3), Mg-Magnetite (Fe3O4) Fh-Ferrihydrite, W-Wousite (FeO), O-iron Oxyhydroxide Regarding the influence of fly ash in concrete it was noticed that the main corrosion products generated
on the surface of steel embedded in fly ash concrete in the presence of chlorides were poorly crystallised
phases of iron oxyhydroxides, goethite (-FeOOH), lepidocrocite (-FeOOH) and hematite (-Fe2O3)
while in normal concrete the main corrosion products in rebar surface was found strong
goethite(-FeOOH) compound with iron oxyhydroxides, lepidocrocite (-FeOOH), hematite
(-Fe2O3) and wusite (FeO). However, small peaks of magnetite compound were also found in normal
concrete. Moreover, the phase of dark region was found stable than the light region. In the initial stage
1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7
of corrosion, the corrosion products are mainly several iron oxyhydroxides with low intensity, however,
these corrosion products are transformed to several strong products such as akaganite compound
depending on the aggressive chemical substance (chlorides, sulphates) (Marcotte, 2001). However,
neither Fe3O4 nor akaganeite were detected in all the fly ash specimen, indicated that the amount of
chlorine was not enough in the DPL to stabilize this phase (Criado M, et.al. 2015). Because, to stabilize
the dense corrosion product layer(DPL) it is required to present the goethite, lepidocrocite, akaganeite
product with Iron oxides compound (magnetite, hematite) (Chitty J et.at. 2005).
CONCLUSIONS
1. A significant drop of chloride diffusion coefficients in fly ash concrete was confirmed compared to
those of normal concretes.
2. Cl/OH ratio in mass basis was found less in fly ash concrete compared to normal concrete.
3. Different corrosion products were identified for both normal and fly ash concretes. The corrosion
products in dark region were different from those in light region.
ACKNOWLEDGEMENT
The research support provided by recent graduate student Mr. E. Momono and undergraduate student
Mr. S. Miyanaga from the laboratory of environmental material engineering of Hokkaido University is
gratefully acknowledged.
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1st International Conference on Research and Innovation in Civil Engineering (ICRICE 2018), 12 –13 January, 2018, Southern University Bangladesh (SUB), Chittagong, Bangladesh ISBN: 978-984-34-3576-7