Effect_Crack_Embedded_Reinforcement_Hartt.pdf

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EFFECT OF CONCRETE CRACK WIDTH ON CORROSION OF EMBEDDEDREINFORCEMENT

William H. Hartt, PhD, PE

Hartt and Associates, Inc.

20914 Morada Court

Boca Raton, Florida 33433

March 18, 2009

BACKGROUND

It is generally recognized that concrete cracks above a certain width facilitate chloride ingress and

initiation of active corrosion of embedded reinforcement sooner than would otherwise occur in soundconcrete. This has led professional societies and certification authorities to establish design criteria such

that width of cracks on the exterior concrete surface does not exceed a specified value. For a sea water

splash zone exposure, multiple authors have recommended that this maximum crack width be 0.3 mm.1,2,3

At the same time, the correlation between crack width and extent of corrosion is not particularly strong.4,5

If it can be demonstrated that reinforcement corrosion in hostile exposures is not strongly sensitive tocrack width, then a case can be made for relaxing current maximum crack width criteria to 0.45 mm

provided corrosion resistant reinforcement such as MMFX2 is specified. The purpose of this commentary

is to review the existing literature pertaining to the effect of concrete crack width on reinforcement

corrosion in environments such as exist in the Middle East and project appropriateness of such a

relaxation.

Both marine and inland structures invariable encounter different exposure zones. In the former case

(marine), these are submerged, tidal, splash, and atmospheric. For the latter (inland), zones are

categorized as 1) buried below the water table, 2) buried in the water table transition zone (depth range

where the water table level cycles), 3) buried in aerated soil, and 4) atmospheric (above ground level).

For situations where the soil and ground water contain significant concentrations of chloride and sulfates,

as is likely to be the case in many Middle East locations, similarities exist between zones of the two typesof exposure (marine and inland); however, focus in the present commentary is placed upon structures that

are located inland. Figure 1 provides a schematic illustration of an inland located concrete element that

extends through all four of the above zones. The propensity for reinforcing steel corrosion at concrete

cracks in each of these four zones is described below.

EFFECT OF CORROSION ZONE

Zone 1

Here, ground water with chlorides and sulfates rapidly penetrate to the full depth of cracks and

potentially exceed the threshold concentration required to depassivate the reinforcing steel and initiate

active corrosion. Rodriguez and Hooton6

reported, based upon experiments involving concrete specimenswith artificially created cracks submerged in chloride solutions that chloride uptake was independent of

crack width in the range investigated (0.08 to 0.68 mm) and was uniform with crack depth. Irrespective

of chloride and sulfate concentration, however, corrosion rate is nil in this zone because dissolved oxygen

concentration, a necessary requisite for corrosion, is zero in ground waters. Hence, concrete cracks are

not a concern here.


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Figure 1: Schematic illustration of a reinforced concrete element that transitions traversing

different inland corrosion zones.Zone 2

In this zone, the concrete cycles between being wet and saturated during periods when the water

table is high and relatively dry when the water table is low. Consequently, chlorides readily accumulate

within concrete cracks as for Zone 1 during the former period (concrete is wet and saturated) and

dissolved oxygen is readily available during times when the water table is low. Even when the water

table is high, corrosion may occur because of a macro-cell between the reinforcement at the crack tip

(anode) and more elevated reinforcement in dryer, oxygenated concrete (cathode). The extent of

corrosion here is likely to be controlled by, first, the range of water table variability and, second, concrete

quality and cover to the extent that these restrict macro-cell activity. Also, this corrosion may be limited

somewhat by cathodic polarization via the more negative potential of reinforcement in Zone 1.7

Zone 3

Enhanced corrosion in this zone requires that chlorides and sulfates from the soil diffuse into the

concrete along the crack faces. This is likely to most pronounced during periods when the soil is moist

from rain. Consequently, the risk of corrosion is less here than for Zone 2.

Zone 4

In this zone, chlorides and sulfates are present on the exterior concrete surface and to a lesser extent

within cracks as a consequence of contact by airborne particulates. The detrimental ions can then diffuse

into cracks and potentially initiate corrosion.

CORROSION MECHANISM AT CONCRETE CRACKS

The generally accepted mechanism whereby reinforcement corrosion occurs at concrete cracks

involves a macro-cell where steel exposed at the crack tip and immediately thereto becomes an anode and

actively corrodes once the critical chloride concentration is exceeded here with adjacent passive rebar

serving as a cathode for oxygen reduction. Figure 2 illustrates this process schematically. In order to

maintain charge conservation, the anodic corrosion reaction can occur only as rapidly as oxygen is

Ground

Level

Zone 2

Zone 1 (Ground Water)

Zone 4 (Atmospheric)

Zone 3 (Aerated Soil)


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reduced at cathodic sites. Consequently, good concrete quality (high resistivity) and greater concrete

cover have been identified as important in limiting the extent of this macro-cell activity.8,9

Figure 2: Schematic illustration of macro-cell corrosion activity at a concrete crack.

SUMMARY OF RELEVANT LITERATURE

Schiessl and Raupach

These authors10 performed corrosion studies on mechanically cracked concrete specimens with crack

widths in the range 0.1 0.5 mm. They observed that, while corrosion initiated earlier the wider the

crack, in all cases macro-cell current activity generated by reinforcement corrosion at the crack decreased

with time and in the long-term (two years) was independent of crack width. They concluded that concrete

cover and quality (mix design) were much more important in controlling corrosion at cracks than was

crack width.

Debate: Crack Width, Cover, and Corrosion11

Concern regarding appropriateness of the ACI Building Code applicable in 1985, as this pertained to

the interrelationship between concrete design, cracks, and corrosion, prompted a formal discussion byrecognized authorities (D. Darwin, D.G. Manning, E. Hognestad, A.W. Beeby, P.F. Rice, and A.Q.

Ghowrwal) which was subsequently published as indicated above. In this, Hognestad pointed out that

results from Corps of Engineer experiments involving loaded reinforced concrete beams exposed at Treat

Island for 25 years failed to indicate any correlation between crack width and extent of deterioration.

This led him to conclude, the Treat Island tests warn us not to get over-excited about crack widths and

steel stresses. This conclusion agrees with that of Schiessl and Raupach discussed above.

Sags, Kranc, Presuel-Moreno, Rey, Torres-Acosta, and Yao12

These authors conducted a comprehensive investigation of corrosion on eight different coastal

(marine exposed) reinforced concrete bridges in Florida. As a part of this study concrete cracks were

identified and pairs of cores were acquired where one was taken centered on a crack and a second fromsound concrete to the side of the crack at the same elevation. Some cores were from the splash and the

remainder from the atmospheric zone. Crack widths were measured prior to coring, and powdered

concrete samples were taken from each core pair at a common depth into the core from what had been the

exposed surface and analyzed for acid soluble chlorides using the standardized Florida Department of

Transportation method.13 The samples from the cracked cores were from along the crack tip. It was

assumed that chloride concentration in the vicinity of the crack tip represents a measure of corrosion

propensity for the reinforcement here. Figure 3 plots the results as chloride concentration ([Cl-])

Concrete Crack

Concrete

Rebar 2


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determined from sound concrete divided by [Cl -] at the same depth at the crack ([Cl -] (Sound))/([Cl-]

(Crack)) in units of pounds per cubic yard (pcy) versus crack width. The data are broadly scattered with

no indication that chloride concentration at the crack depended upon crack width.

0.1

1.0

10.0

100.0

0 0.1 0.2 0.3 0.4 0.5

[Cl-](Solid)/[Cl-](Crack)

Crack Width, mm

HF Bridge CC Causeway

Dames Point Bridge Bay Side Bridge

Safety Harbor Bridge New Smyrna Bridge

New Pass Bridge BC Bay Bridge

Figure 3: Data from Florida bridges showing a lack of correlation between chloride

concentration within concrete cracks and crack width.

Berke, Dellaire, Hicks, and Hoopes14

In this study, the authors performed experiments on four cracked CEN (European Committee forStandardization) reinforced concrete specimens but with a crack being introduced by loading in three

point bending. The CEN specimen uses smooth bars (no deformations), and so some slippage of the bars

during loading is expected such that the crack extends to a greater depth than would be the case for

deformed bars. While still loaded, a shim was positioned in the crack to minimize closure. The

specimens were ponded with a 3 wt% NaCl solution on a two week wet two week dry cycle. They were

dissected after 16 months and both the corroded area on the top bar and chloride concentration in the

concrete at the top bar near the crack were measured. Figure 4 shows a plot of these data and reveals a

crack width for three of the specimens as 0.25 mm (0.01 in.) and for the fourth 0.51 mm. (0.02 in.).

Obviously, data scatter for correlating corroded area with crack width is large, indicating the variability

that can be encountered here. Thus, for two of the smaller crack specimens, the corroded area was

relatively small, whereas for the third, the corroded area was about the same as for the larger crack width

specimen. Particularly noteworthy, however, is that chloride concentration at the crack tip region, which

is the major factor in promoting depassivation of the reinforcement, was essentially the same for the two

crack widths.


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0

1

2

3

4

5

6

7

8

9

10

0

2,000

4,000

6,000

8,000

10,000

12,000

0 0.1 0.2 0.3 0.4 0.5 0.6

ChlorideConc.

AtCra

ckTip,

kg/m3

CorrodedArea,mm

2

Crack Width, mm

Corroded Area

[Cl-]

Figure 4: Correlation of the extent of corrosion and chloride concentration with crack

width as reported by Berke et al.

PERFORMANCE OF MMFX2 COMPARED TO BLACK BAR IN CRACKED CONCRETE

Presuel-Moreno and Roarke15

Results from a research study by these authors involved G109 type specimens fabricated with a

simulated concrete crack of a specific width and reinforced with either BB and galvanized bar (GV). No

MMFX2 reinforcement was employed in this study. The simulated crack was introduced by placing a 1.6

mm (0.063 in.) thick stainless steel sheet in the form to depth of the upper bar prior to concrete placement.

The sheet was removed after initial concrete set. As such, crack width (1.6 mm) was constant along theentire span of the reinforcement cover. The specimens were ponded on a two week wet two week dry

cycle with 15 wt% NaCl, which is considered a very harsh exposure.

Figure 5 shows a plot of macro-cell current (proportional to corrosion rate) versus time for three

concrete slab specimens reinforced with BB and three with GV. A sustained current of 10 A was taken

as indicative of corrosion initiation. The data show that for both the BB and GV reinforcements there was

a tendency for repassivation subsequent to activation, consistent with the reporting by others that

corrosion at cracks often arrests after a period of time, apparently because of corrosion products sealing

off the crack mouth.10,16 This makes definition of time-to-corrosion, Ti, difficult; however, Table 1 lists

the projected Ti values for these specimens along with the ratio of T i for GV compared to that for BB

[Ti(GV)/Ti(BB)] based on the three specimen average Ti for the two reinforcement types, where the latter

parameter assumes that the GV reinforced specimens activated at the indicated Ti.

From experiments on reinforced concrete specimens exposed to chlorides and data collected from

cracked locations on concrete bridge decks subjected to deicing salts in Kansas, Darwin et al. 17 projected

that corrosion of BB reinforcement with 76 mm cover initiates on average after 2.3 years, with galvanized

rebar (GV) after 4.8 years, and with MMFX2 14.8 years. The corresponding Ti ratios relative to BB are

Ti(GV)/Ti(BB) = 2.1 and Ti(MMFX2)/Ti(BB) = 6.4. No crack widths were reported in this study;


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however, the Ti(GV)/Ti(BB) value reported above by Presuel-Moreno and Roarke (4.9) is generally

consistent with the one determined by Darwin et al.15 (2.1), considering differences in the two

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000 3500

Current,microA

Exposure Time, days

CT41-GV-CRK1

CT41-GV-CRK2

CT41-GV-CRK3

CT41-BB-CRK1

CT41-BB-CRK2

CT41-BB-CRK3

Figure 5: Macro-cell current (corrosion rate) versus time for cracked concrete specimens

reinforced with BB and GV.

Table 1: Listing of Ti and average Ti(GV)/Ti(BB) for the data in Figure 6.

Specimen Ti Ti(GV)/Ti(BB)

CT41-BB-CRK1 756CT41-BB-CRK2 657

CT41-BB-CRK3 290

CT41-GV-CRK1 2,765

CT41-GV-CRK2 >2,765

CT41-GV-CRK3 >2,035

4.9

experimental approaches. On this basis, the projection by Darwin et al. of Ti(MMFX2)/Ti(BB) being 6.4

(see above) seems appropriate, even considering the relatively wide crack width involved (1.6 mm).

Hartt, Powers, Lysogorski, Liroux, and Virmani18

The above analyses aside, it can be reasoned that the most conservative approach to evaluating the

effect of crack width on durability of reinforced concrete is to consider that T i is zero (corrosion initiates

upon initial exposure) and service life is determined by the time for corrosion to propagate to the point

where repair or rehabilitation is required. Results of a study by the above authors allow for a comparison

of corrosion rates at simulated cracks for concrete specimens reinforced with MMFX2 compared to black

bar (BB). In this, specimens were exposed outdoors in south Florida approximately 300 m inland from

the Atlantic Ocean while ponded with a 15wt% NaCl solution on a one week wet one week dry cycle.


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Figure 6 provides a schematic illustration of the general specimen geometry, and Figure 7 shows a

photograph of two specimens under test. For three BB and three MMFX2 specimens, a simulated

concrete crack that intersected the top three bars was part of the fabrication. The cracks were produced

the same as described above for the experiments of Presuel-Moreno and Roarke and were also 1.6 mm

(0.063 in.) wide. Thus, as above, crack width was constant along the reinforcement cover. Macro-cell

current between the three top and three bottom bars, which is directly proportional to corrosion rate of the

top bars, was measured weekly.

Figure 6: Schematic illustration of the reinforced concrete specimens.

Figure 7: Photograph of two slab specimens with ponding baths under test.

Figure 8 shows a plot of macro-cell current density versus time for the three BB and three MMFX2

concrete cracked specimens. The saw-toothed nature of the data resulted because acquisitions were

alternately made when specimens were wet and then dry (high values correspond to the slabs being wet).

Particularly noteworthy is that current density for the BB specimens tended to stabilize (become constant)

after about 200 days, whereas for MMFX2 current density was lower than for BB and decreased withtime beyond about 200 days and stabilized at a relatively low value after about 400 days. This difference

is attributed to formation of a more dense corrosion product film within the crack on the MMFX2 which

provided greater corrosion protection than for BB. Beyond 400 days, the average current density for the

three BB specimens was 0.945 A/cm2, where for the three MMFX2 specimens the average current

density was 0.0165 A/cm2. Consequently, the BB specimens were corroding 5.7 times more rapidly

than the MMFX2 ones.

Crack Front

Simulated

Crack

NaCl

Pond

All dimensions

in cm.

1530

30


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0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

0 100 200 300 400 500 600

CurrentDensity,

A/cm

2

Exposure Time, days

3-CCON-BB-1

3-CCON-BB-2

3-CCON-BB-3

3-CCON-MMFX-1

3-CCON-MMFX-2

3-CCON-MMFX-3

Figure 8: Macro-cell current density data for three specimens each of BB and MMFX2.

Subsequent to these exposures, one specimen with each reinforcement was dissected and the

reinforcement examined for corrosion. Figure 9 shows the reinforcement and adjacent concrete for

specimen 3-CCON-BB-1 and Figure 10 does the same for 3-CCON-MMFX-1. The extent of corrosion

Figure 9: Photograph of reinforcement from the three BB reinforced specimens.

product staining is generally consistent with conclusions reached from the macro-cell current data (Figure

8); that is, corrosion was much less developed for the MMFX2 reinforcement compared to BB.

Conclusions

While intuition suggests that corrosion of reinforcement at the base of concrete cracks should

increase with crack width, there is evidence in the literature that crack width does not significantly

influence long-term durability of reinforced concrete. From the available information, as reviewed above,

a conservative estimate is that MMFX2 reinforcement initiates corrosion at the base of concrete cracks at


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Figure 10: Photograph of reinforcement from the three MMFX2 reinforced specimens.

exposure times about six times greater than for BB. This is likely to be the case for cracks as wide as 1.6

mm (0.063 in.). Also, corrosion rate in cracked concrete, once initiated, is about six times less for

MMFX2 compared to BB. This also is projected to apply for crack widths as large as 1.6 mm (0.063 in.).

Figure 11 provides a schematic illustration of this difference assuming that BB initiates corrosion at time

3 (arbitrary scale) and MMFX2 at time 18 (six times longer). It is concluded from this that it is

appropriate to relax existing maximum crack width criteria to 0.45 mm provided corrosion resistant

reinforcement such as MMFX2 is specified.

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100 120

CumulativeCorrosionDa

nage,

arbitraryunits

Time, arbitrary units

BB

MMFX2

Figure 11: Schematic illustration of cumulative corrosion damage to BB and MMFX 2

reinforcement at concrete cracks assuming a six-fold greater time-to-corrosion and a

six-fold reduction in corrosion rate, once initiated, for MMFX2.


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References

1 R.D. Browne and A.F. Baker, The Performance of Structural Concrete in a Marine Environment, in

Development of Concrete Technology, Ed. F.D. Lyden, Applied Science Pub., London, 1979, p. 111.2 R.D. Browne, Low Maintenance Concrete Specification versus Practice?, in Proc. 2

ndIntl. Conf. on

Maintenance of Marine Structures, Thomas Telford, UK, 1986.3 A.L. Marshall,Marine Concrete, Van Nostrand Reinhold, NY, 1990.4 A. Bentur, S. Diamond, and N.S. Berke, Steel Corrosion in Concrete, E&FN Spon, London, 1997.

5 H. Martin and P. Schiessel, The Influence of Cracks on the Corrosion of Steel in Concrete, Intl.

Symp. on Durability of Concrete, Rilem, 1969, p. D205.6 O.G. Rodriguez and R.D. Hooten, Influence of Cracks on Chloride Ingres into Concrete, ACI

Materials Journal, March-April, 2003, p. 120.7 R.E. Tanner and W.H. Hartt, Specimen Type and Test Protocol for Corrosion Performance

Characterization of Reinforced Concrete Marine Pilings, paper no. 08320, CORROSION/08, NACE

International, Houston, 2008.8 E.A. Baker, K.L. Money, and L.R. Sanborn, Marine Corrosion Behavior of Bare and Metallic-Coated

Steel Reinforcing Rods in Concrete, in Chloride Corrosion of Steel in Concrete, STP 629, American

Society for Testing and Materials, Phila., 1977, p. 30.

9 F.M. Lea and C.M. Watkins, The Durability of Reinforced Concrete in Sea Water, Research PaperNo. 30, National Building Studies, Dept. of Scientific and Industrial Research, Her Majestys

Stationery Office, London, 1960.10 P. Schiessl and M. Raupach, Laboratory Studies and Calculations on the Influence of Crack Width on

Chloride-Induced Corrosion of Steel in Concrete, ACI Materials Journal, Vol. 94(1), January-

February, 1997, p. 56.11 Debate: Crack Width, Cover, and Corrosion, Concrete International, May, 1985, p. 20.12 A.A. Sags, S.C. Kranc, F. Presuel-Moreno, D. Rey, A. Torres-Acosta, and L. Yao, Corrosion

Forecasting for 75-Year Durability Design of Reinforced Concrete, Final Report No. BA502

submitted to Florida Department of Transportation by University of South Florida, December 31, 201.13 Florida Method of Test for Determining Low-Levels of Chloride in Concrete and Raw Materials,

FM 5-516, Florida Department of Transportation, September 1994.14 N.S. Berke, M.P. Dellaire, M.C. Hicks, and R.J. Hoopes, Corrosion of Steel in Cracked Concrete,

Corrosion, Vol. 49, 1993, p. 934.15 F.J. Presuel-Moreno and D. Rourke, Towards Achieving the 100 Year Bridge Using Galvanized

Rebar, paper presented at the International Corrosion Congress, Las Vegas, May, 2008.16 P. Schiessel, Admissible Crack Width in Reinforced Concrete Structures, contribution no. II,Inter-

Association Colloquium on the Behavior of Interservice Concrete Structures, Preliminary Report V.II,

Liege, 1995, p. 3.17 D. Darwin, J. Browning, M. OReilly, and L. Xing, Critical Chloride Threshold for Galvanized

Reinforcing Bars, research report 07-2 submitted to the International Lead Zinc Research

Organization, Inc. by University of Kansas, December, 2007.18 Corrosion Resistant Alloys for Reinforced Concrete, Report No. FHWA-HRT-07-039, Federal

Highway Administration, Washington, DC, July, 2007.

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