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

mailto:[email protected]

mailto:[email protected]

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


(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


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.


[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


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.

REFERENCES

Kamaitis Z (2002). Damage of concrete bridges due to reinforcement corrosion, Transport, 17:4,

137-142.

W.J. Chitty, P. Dillmann, V. L’Hostis and C. Lombard, 2005. Long-term corrosion resistance of

metallic reinforcements in concrete - A study of corrosion mechanisms based on archaeological

artefacts. Corrosion Science 47, 1555– 1581.

M. Criado, S. Martínez-Ramirez and J.M. Bastidas, 2015. A Raman spectroscopy study of steel

corrosion products in activated fly ash mortar containing chlorides. Construction and Building

Materials 96, 383–390.

I. Zafar and T. Sugiyama, 2014. Laboratory Investigation to Study the Corrosion Initiation of Rebars in

Fly Ash Concrete. Magazine of Concrete Research 66(20), 1051-1064.

Peter C. T, Mohamad A.N and David A. W, 1999. Threshold chloride content for corrosion of steel in

concrete. A literature Review. Portland Cement Association, www.portcement.org.

Marcotte T.D, 2001. Characterization of chloride-induced corrosion products that form in

steel-reinforced cementitious materials. A thesis of doctor of philosophy in mechanical engineering,

Waterloo, Ontario, Canada.


study on corrosion of reinforcing steel in concrete slab

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