Influence of Corrosion on Bond Degradation

8/13/2019 Influence of Corrosion on Bond Degradation

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INFLUENCE of CORROSION on BOND DEGRADATION in REINFORCED CONCRETE

Ghandehari M.1, Zulli M.2, Shah S.P.3

1Assistant Professor, Polytechnic University, Brooklyn, NY.

2PhD Student, University of Roma La Sapienza, Rome, Italy.3Professor, Northwestern University, Evanston, Ill.

ABSTRACT

Accelerated constant current corrosion tests are performed on reinforced concrete

cylinders. The influence of dimensional parameters (rebar diameter and concrete cover)

on corrosion penetration and the splitting of concrete is investigated. A fracture

mechanic splitting model indicates correlation between the measured extent of

corrosion and the splitting strength of concrete cylinders. The effect of cracking on

specimen impedance, and corrosion rate are explored.

INTRODUCTION

Corrosion of steel is accompanied by its volumetric expansion. The corrosion

induced pressure caused by this expansion at the rebar-concrete interface causes

splitting of the concrete cover. When assessing structural integrity, reduction in bond

strength by splitting can be a bigger factor than the loss of rebar tensile load capacity by

cross section reduction. Corrosion induced splitting of concrete is the focus of this

study.

Due to carbonation and in the presence of chlorides, corrosion is initiated by the

breakdown of a passive layer. The composition, pore structure and size of the bulk

concrete cover play a big role in corrosion initiation period as well as rate of corrosion.


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The rate of corrosion is often modeled using Faradays law. The pH driven corrosion

reaction rate is current density dependent. The resulting mass loss to oxidation for a

constant current time interval according to Faradays law is:

W =tA/ZF (1)

Where : W = weight loss (grams)

= corrosion current (amp)

t = time (sec)

A = atomic weight of iron (56g)

Z = valency of the metal (2)

F = Faraday constant (96500 Amp-sec)

By inserting the specific values of A, Z, and F into eq. (1), the rebar mass conversion as

a function of time and corrosion current is:

W =25t (1a)

Taking into account the steel density (approx 0.0078 g/mm3), the rebar length and

diameter, the extent of corrosion penetration is:

x =3197it (see figure 5.) (2)

Where: i = /dL= current density (A/mm2) [Table 1]

t = time (days)

The effects of the concrete pore solution (electrolyte conductivity) and the conductivity

of other phases, such as corrosion product and interfacial transition zone (ITZ), is

reflected in changes in the corrosion current density.

Corrosion products are multi-layered and depending on their composition they will

have different transport and mechanical properties. Contrary to the passive oxide layer,


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which is mainly crystalline, corrosion products are amorphous and their mechanical

response, which is moisture dependent, has been difficult to assess. 1An expansion

parameter of 2 to 3 is typically found in the literature.

The splitting of concrete and the associated loss of bond strength is not only a

function of the degree of corrosion, but it is also a function of the material parameter

(e.g. toughness) and of the size and geometry of the member cross section. The effect

of concrete cover and corrosion level on splitting and bond strength of corroded rebar

has also been examined in other research.2,3,4,5 It has been shown that in the presence

of transverse reinforcement the effect of corrosion on bond strength may be negligible.

However in the absence of transverse reinforcement or other confining mechanisms

that effect can be substantial.

In the study presented here, constant current accelerated corrosion tests are

performed on rebars embedded in concrete cylinders. The influence of rebar size and

cylinder size, on the extent of corrosion and on the mechanical fracture of the specimen

is explored. The effect of the above parameters on the rate of corrosion is observed and

compared to that described by Faradays law. Subsequently, the influence of corrosion

and cracking on the specimen resistance is investigated.

EXPERIMENTS

Specimens

Figure 1. shows the specimen geometry. The concrete batch was mixed in

proportions of 0.5, 1, 2, 2 parts by weight of water, cement, sand and gravel

(dmax=8mm), achieving a 28 day compressive strength of 40 Mpa. The rebars were

placed in plastic molds, the concrete was cast and cured for 28 days at a temperature of


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23C and 95% relative humidity. A total of 4 different specimen geometry were cast. Two

cylinder diameters (100mm and 150mm), and two rebar diameters (9.5mm and 20mm)

were used as shown in Figure 1. and Table 1. The specimens are labeled

CsRs,CsRl,ClRs, and ClRl, where C and R stand for cylinder and rebar, and the

subscripts s and l stand for small and large respectively. Epoxy resin was applied to the

exposed rebar for corrosion protection during curing. After curing, silicon adhesive was

applied to the top and bottom of the concrete cylinders, providing an axi-symetric

corrosion condition, and effectively capturing the influence of the cylinder diameter.

Corrosion cell and test procedure

Four specimens of each geometry type were cast corresponding to four target levels

of corrosion. The current required to achieve the target corrosion levels (2.5%, 5%, 7.5&

, and 10% by weight) was determined by Faraday's law assuming 100% current

efficiency (Eq.1). The corresponding calculated current is shown in Table 1. The

corrosion cell consists of a power supply, a copper plate (cathode), and the specimen

(anode) submerged in a 5% by weight NaCl solution. In order to compare the measured

weight loss with that predicted by the Faraday law, a constant current driving circuit was

devised. The fluctuations in the electric current caused by changes in the specimen

resistance were monitored by measuring the voltage across a fixed resistor placed in

series with the specimen. An tunable resister was used to adjust and maintain the

constant current. This procedure was repeated at 6 hour intervals for 4 weeks.


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Specimen Evaluation

In addition to continuous observation of the specimens for surface cracking, the

specimens were cut in 50mm slices at the end of each corrosion level (week 1,2,3 and

4). The rebar in each slice was tested in pullout to obtain the effect of corrosion on bond

strength. Weight loss measurements were performed using the ASTM procedure C.3.1,

where the rebar was wire brush cleaned with a solution of HCl, Sb2O3, and SnCl2. The

procedure was repeated several times removing as much rust as possible.

RESULTS

Impedance Measurement

Figure 3 shows the influence of rebar corrosion and concrete cracking and how it is

reflected by changes in the specimen's resistance (Rs). The initial increase in the

resistance is attributed to initial oxidation of the rebar surface. Rs reaches a maximum

level and maintains that level until the specimen begins cracking. All specimens reach

the point of maximum resistance at approximately 5 days. The elapsed time up to first

cracking is also noted in the Figure. It indicates the time that a single visible crack

emerged on the specimen surface. The following pattern is observed in the figure:

-The small specimen with large rebar (CsRl) cracked first at 6 days. At that time the

specimen resistance dropped by about 50% in a period of about two days.

- The large specimen with the same rebar size (ClRl) cracked next, at 13 days

followed by approximately the same ratio of resistance drop, reaching a steady state in

about a week.


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- The small specimen with small rebar (CsRs) cracked at 19 days, followed by a

resistance drop twice in magnitude, but with the same ratio (50%) of the pre-crack

resistance. The resistance of this specimen also reached steady state in about a week.

- The large specimen with small rebar (ClRs) never cracked and maintained

constant resistance after the end of week 1.

The slower resistance drop after cracking for the specimen with larger cover (ClRl

vs. CsRl) suggests that increasing the concrete cover is beneficial with regards to the

rate of ionic penetration. This effect is meaningful when the time scale of the

accelerated corrosion is transformed to the actual life cycle time scale. Therefore

increasing the concrete cover plays a role in strength and in durability.

On the other hand, comparison of the large specimen with different rebar sizes

(ClRs vs ClRl) indicates that decreasing rebar size increases the corrosion/splitting

resistance. This is not only due to the influence of size of the rebar as a flaw in

bond/splitting fracture7, but it is also due to the effect of smaller expansive forces

generated by the smaller rebar as predicted by the Faradays law. The effect of

corrosion on bond strength is isolated and will be discussed later in Figure 6.

Results of the microscopic measurement of total crack opening due to corrosion

(w) in each slice near the rebar is shown in Figure 8. Based on these measurements it

was verified that the specimen ClRl had not cracked by the end of week 1. The next

crack opening measurement of this specimen at the end of week 2 revealed internal

cracking in addition to the externally observed crack. The gradual drop in the specimen

resistance between these two measurements suggests that small internal radial cracks

had formed near the rebar between week 1 and 2 influencing the specimen resistance.

The slices from specimen CsRs revealed no cracks at the end of week 1 and end of


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week 2. Finally, observation of the specimen ClRs confirmed that cracks had not formed

throughout the 4-week test.

Weight Loss Measurement

The procedure for the evaluation of the extent of corrosion by weight loss

measurement was outlined earlier. Given the measured weight loss (Wm), the

corrosion penetration (xm) can be deduced:

xm (Wm/W)d/4 (3)

Where: d= rebar diameter

W= original rebar weight

A comparison of the measured corrosion penetration (xm), and the predicted corrosion

penetration (x) is shown in Figure 5. The predicted penetration is based on Faradays

law, assuming corrosion was initiated immediately at the start of the test (Eq. 2). The

rate of corrosion indicated by the slope of the dashed lines in Figure 9 is simply a

function of the corrosion current density (Table 1). (The current density shown in the

table is selected to achieve the same percent weight loss for all specimens.

modeling

A strong motivation for this study was to determine the time-to-cracking for different

specimen geometry. Based on time-to-cracking (Figure 3), it was shown that corrosion

resistance increases with increasing cover dimension and decreasing rebar diameter. In

order to explain these trends, the specimens are modeled as a hollow cylinder subject

to internal pressure. This type of analysis was carried out in previous research 7, in


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order to evaluate the bond/splitting resistance of ribbed rebars tested in pullout. An

iterative non-linear fracture mechanics analysis was carried out prevously, for the

evaluation of internal pressure caused by the rebar pullout. It is similarly carried out

here for the evaluation of corrosion pressure. The procedure outlined below is

performed using the Finite Element program FRANC2D 8as follows.

1. Start with some small initial guess for crack length (two symmetric cracks).

2. Guess an initial crack tip opening displacement (CTOD).

3. Assume linear crack profile.

4. Apply crack flank traction based on an exponential material constitutive model.

The constitutive model developed by Gopalaratnam9 describes the relation

between the crack opening displacement (COD) and the crack flank traction (T).

5. Apply the splitting force until the crack tip stress intensity factor KI= KIc. (KIcfrom

Jenq and Shah 10).

6. Check the corresponding numerical crack profile (COD).

If the output crack profile (COD) and the input T satisfy the material model within a

stipulated accuracy, then consistency is achieved.

Grow the crack to a new length and repeat steps 1 through 6.

Else guess a new CTOD and repeat steps 3 to 6 using the new crack profile as

given by the current iteration.

That this iterative algorithm leads to consistent results was established by adopting

the same scheme for wedge pullout specimens whereexperimental data for the splitting

force were available for comparison.

Table 2 shows a correlation between the trends observed in the time-to-splitting

caused by corrosion, and the predicted pressure required for splitting a hollow core


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concrete cylinder. Based on this comparison it would be possible to isolate the sources

of damage whether by corrosion or by mechanical action.

Bond Strength

The effect of the corrosion of rebar on bond strength is shown in Figure 6. The bond

strength of the corroded rebar is shown normalized with respect to the bond strength of

the non-corroded rebar, and therefore reduction in the indicated strength reflects only

the influence of corrosion. Bond strength is evaluated based on the pullout of rebars

from 50mm long concrete sample slices.

The influence of increasing the cover dimension is evident in the Figure. Keeping the

rebar size constant, moderate exposure to corrosive currents (here less than 1 week)

does not influence the pullout strength of a large specimen nearly as much as a small

specimen. This effect is not only due to the anticipated larger splitting capacity of the

larger cylinder (Table 2), but may also be due to the effectiveness of the large concrete

cover in the inhibition of corrosion progress. Furthermore, it is shown that for the small

cylinder with different rebar sizes the effect of corrosion of the large bar (CsRl) on bond

strength is more pronounced than corrosion of the small rebar (CsRs).

CONCLUSIONS

Constant current accelerated corrosion tests on reinforced concrete cylinders were

performed:

- A comparison of the measured rate of corrosion penetration with that predicted by

Faradays law indicates an overall agreement. The assessment of a proper initiation

time would substantially improve those predictions.


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- The rate of decrease in the specimen impedance after cracking depends on the

cover dimension.

- Corrosion resistance increases with increasing cover dimension and decreasing

rebar diameter.

ACKNOWLEDGEMENTS

Generous support from the National Science Foundation Center for Advanced

Cement Based Materials, and the Department of Civil Engineering at Polytechnic

University is gratefully acknowledged. The support of the scholarship from Ferdinando

Filauro Foundation and University of LAquila, Italy is also greatly appreciated.

REFERENCES

1. Wang, K., Monteriro P.J., Corrosion Products of Reinforcing Steel and their Effects

on the Concrete Deterioration, Third CANMET/ACI International Conference on

Performance of Concrete in Marine Environment, pp. 83-97.

2. Cabrera J. G., Deterioration of Concrete due to Reinforcement Steel Corrosion,

Cement and Concrete Composites, 18 (1996) 47-59.

3. Andrade, C., Alonso F., Molina F.J., Cover Cracking as a Function of Bar Corrosion

Part I-Experimental Test, Materials and Structures 1993, V.26, pp. 453-464

4. Al-Sulaimani, G. J., Kaleemullah, M., Basunbul, A., Rasheeduzzafar, Influence of

Corrosion and Cracking on Bond Behavior and Strength of Reinforced Concrete

Members, ACI Structural Journal, March-April 1990, pp. 220-230.

5. Amleh L., Mirza S., Corrosion Influence on Bond Between Steel and Concrete, ACI

Structural Journal, May-June,1999


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6. Ghandehari, M. Krishnaswamy S., Shah S.P., A Technique for Evaluating

Kinematics Between Rebar and Concrete, Journal of Engineering Mechanics, Feb

1999 V.125, no.2, pp. 234-241

7. Ghandehari M., Krishnaswamy S., Shah S.P., Effect of Specimen Dimensions on

Bond Failure in Reinforced Concrete Journal of Applied Mechanics, accepted for

publication.

8. FRANC2D, 1995, A Two Dimensional Crack Propagation Simulator, vol.2.7,

Cornell University.

9. Gopalaratnam, V.S., and Shah, S.P., 1985, Softening Response of Plain Concrete

in Direct Tension, ACI Journal, vol.82, no.3, pp310-323.

10. Jenq, Y.S., and Shah, S.P., 1985,. A Two-parameter Fracture Model for Concrete,

J. Eng. Mech. ASCE, vol.111, no4, pp1227-1241.

LIST OF FIGURES

Figure 1 - Specimen geometry

Figure 2 - Corrosion cell

Figure 3 - Relation of corrosion level with specimen resistivity and cracking

Figure 4 - Dilation and cracking at the rebar

Figure 5 - Corrosion penetration obtained from measured weight loss

Figure 6 - Influence of corrosion on bond strength

LIST OF TABLES

Table 1 - Specimen dimensions and corrosion current levels

Table 2 - Splitting strength vs. corrosion level


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5% NaCl Solution

Copper Plate

Specimen

Power Supply

e

Resistor

(Cathode)

(Anode)

+-

by weight of Water

Current

Constant current

circuit

CsRs CsRl

ClRlClRs

D

d

L

Figure 1 Specimen geometry

Figure 2 Corrosion cell


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TABLE 1 SPECIMEN DIMENSIONS AND CORROSION CURRENT LEVELS

dimensions current design corrosion level

specimen Cylinderdiameter

(D)

Rebardiameter

(d)

CylinderLength

(L)

I i 1week

2week

3week

4week

mm mm mm mA A/m2 % % % %

CsRl 102 19 203 75 6.16

ClRl 152 19 304 115 6.16

CsRs 102 9.5 203 20 3.29

ClRs 152 9.5 304 30 3.29

2..5 5 7.5 10

Corrosion level (weight loss) according to Faradays law

0

50

100

150

200

250

300

0 1 2 3 4 5Time (week)

Resistance

(Ohm)

CsRl

ClRl

CsRs

ClRs

Crack CsRs

Crack CsRl

Crack ClRl

Figure 3 Relation of corrosion level with specimen impedance and crack


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Figure 5 Corrosion penetration obtained from measured weight loss

Corrosion product

Original rebar

Penetration (x)

w

Figure 4 Dilation and cracking at the rebar

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5

Time (week)

Penetration (x)

(micron)

CsRl

CsRs

ClRl

ClRs

Faraday, large rebar

Farafay, small rebar

CsRlcrack

ClRlcrack

CsRs

crack


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Table 2 - Splitting strength vs. corrosion levelSPECIMEN Splitting strength Penetration at cracking (x) Time to splitting

Mpa micron Week

CsRl 14 70 0.8

ClRl 17 90 1.9CsRs 35 160 2.8

ClRs 48 150 @ 4 weeks) not split

Splitting strength from cohesive crack model [Ghandehari (1999)]

Figure 6 - Influence of corrosion on bond strength

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4 5

Time (week)

NormalizedBond

Strength

CsRl

ClRl

CsRs

ClRs

cracked

cracked

cracked

no

crack

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Influence of Corrosion on Bond Degradation