Concrete, Chlorides,Cover and Corrosion

Donald W. PfeiferVice PresidentWiss, Janney, Elsiner

Associates, inc.Northbrook, Illinois

J. Robert LandgrenSenior EngineerWiss, Janney, Elstner

Associates, Inc.Northbrook, Illinois

William PerenchioSenior ConsultantWiss, Janney. Elstner

Associates, Inc.Northbrook, Illinois

A considerable amount has beenwritten on the subject of corrosion

of steel in concrete; however, it is notthe purpose of this paper to present adiscussion of the history of corrosionand the development of corrosiontechnology. Suffice it to say that thechloride ion has been shown to bethe most common instigator of steelcorrosion in concrete and that muchhas been published about the needfor certain minimum amounts ofclear cover and certain maximum wa-ter-cement ratios.

The study from which the data con-tained in this paper is excerpted wasprimarily a corrosion study designed toevaluate various means of protectingembedded steel from corrosion. Onegroup of nine tests utilizing 90 conven-tionally reinforced concrete specimensproduced very interesting data which

compared the effects of cover andwater-cement ratio on the intrusion ofsaltwater after 48 weeks of cycling be-tween saltwater ponding and drying inwarm air. These numerous test speci-mens are shown tinder test in Fig. 1.

Materials and Concrete MixturesThe aggregates used for this study

were highly siliceous natural sand andgravel from Eau Claire, Wisconsin.They were chosen specifically becauseof their very low initial chloride con-tents, essentially 0.002 percent. Theconcretes were designed to have water-cement ratios of 0.28, 0.40 and 0.51.They were used at a workability of 3 to 4in. (76 to 102 mm) of slump. A highrange water-reducing admixture wasnecessary to produce this workability inthe 0.28 water-cement ratio mixture.

42

Synopsis

Small reinforced concrete slabswere tested as part of a larger studysupported by the Federal HighwayAdministration in Project ©TFH61-83-00085, "Protective Systems forNew Prestressed and SubstructureConcrete." The slabs were alternatelyponded with saltwater followed bydrying in air.

Theresults show the very signifi-cant effect of water-cement ratio onsalt intrusion, which can be muchgreater than its effect on compressivestrength. These results help explainthe typically observed greater corro-sion resistance of precast concretewhich is usually made with lowerwater-cement ratio concrete.

Compressive strengths at 28 days werenominally 7500, 6000 and 5000 psi (52,41 and 34 MPa), respectively.

Test Specimen DesignBecause the primary purpose of the

study was to determine the effects ofvarious corrosion protection treatmentsand procedures, including cover andwater-cement ratio, on time-to-corrosion

of embedded steel, the specimens con-tained two mats of reinforcing bars, topand bottom. By ponding the top surfacewith saltwater, the top bars were madeto act as anodes, and potentially corrode,while the bottom bars acted as cathodes,once corrosion started on the top matbars.

Fig. 2 shows the configuration of thespecimens with top covers of 1, 2 and 3in. (25, 51 and 76 mm). The total depths

Fig. 1. Test specimens during cyclic wetting and drying corrosion testing.

PCI JOURNAUJuly-August 1986 43

12"

CLEAR COVER =1"

NO.4 RF^BARSOR 1/2' STRANDS

itNO. 4 REBARS

2 I/4 2 I/2" 2 I/2" 2 ^112-2

FORMED SURFACE

DIKE

:PDXY COATNLL SIDES

Fig, 2. Cross section of time-to-corrosion specimens.

44

of the specimens were varied to producethe different covers so that the distanceand hence the total electrical resistanceremained constant, for a given concretemixture at the same conditions of tem-perature and humidity, between the twomats of bars. Also, the cover over thebottom bars was maintained constant at1 in. (25 mm) so that, as far as possible,constant conditions would he main-tained at the cathode.

The two top bars and the four bottombars were joined into top and bottom"mats" electrically by means of externalbuss bars. The two mats were thenjoined by means of a resistor, to providefor measuring potential differencesacross the resistor and calculating corro-sion current flows.

Test ProcedureTo simulate realistic field conditions,

all of the slabs were moist cured for 3days, followed by 25 days of air drying at60 to 80°F (16 to 27°C) prior to cyclictesting. During the air-drying period, allsides and the protruding ends of barswere given two coats of epoxy tominimize lateral moisture movementduring the cyclic tests. Dikes to retainponded saltwater on the top surfacewere also added at this time, and elec-trical connections were made.

The saltwater used was a 15 percentsodium chloride solution, a solutionhaving about five times the chloride ioncontent of ocean water. The weekly testcycle consisted of:

1. 100 hours of ponding under salt-water at 60 to 80°F (16 to 27°C).

2. Removal of saltwater.3. Rinse with fresh water, followed

by vacuum removal of water.4. 68 hours of drying at 100°F (38°C).This weekly cycle was repeated 48

times. The corrosion current was deter-mined once each week for each speci-men. Copper-copper sulfate half-celltests were made at three locations alongthe concrete above the length of each

top bar on at least a monthly basis.Also each month, "instant off" po-tentials and AC electrical resistancewere measured between the top andbottom steel mats,

Tests for acid-soluble chloride ionwere made when a surge in corrosioncurrent indicated the start of macrocellcorrosion of the top bars. Chloride con-tents were also determined near theconclusion of the 48-week test period.Powder samples were obtained for thesechloride tests by drilling three holeseach into the two sides of the slabs inthe top surface plane of the top bars so asnot to disturb the top slab surface. Thedrilled hole diameter was' in. (6 mm).The hole depth was 2 in. (51 rnm), withthe powder sample test portion taken asshown in Fig_ 3. The holes were filledwith a sanded epoxy resin before testingwas resumed.

TEST RESULTS

The start of macrocell corrosion inslabs with 1 in. (25 mm) of cover variedfrom 2 to 16 weeks from start of testing;however, the anticipated effect of longertime-to-corrosion with lower water-cement ratio was generally not ob-served. None of the slabs with 2 or 3 in.(51 or 76 mm) of cover displayed anymeasurable corrosion activity during the48 weeks of testing.

Time-to-CorrosionIn the interest of brevity, all of the

corrosion measurements are not in-cluded in this paper. However, Fig. 4illustrates the onset of corrosion as de-termined by calculation of actual corro-sion current from voltage drops acrossthe resistor connecting top and bottomgroups of reinforcing bars. Actual corro-sion current was also measured inde-pendently.

This curve clearly indicates the onsetof corrosion at about 8 weeks. Instant-offpotentials and half-cell potentials were

PC! JOURNAU7uly-August 1986 45

2 I/4 2 I/2' 2 1/2 2 I/2" 2 I/4"

HOLE DIA.= 1/4"

2" \

POWDER SAMPLESFOR TEST

Fig, 3. Location for drilled powder samples to measure chloride ion content at initiationof macrocell corrosion and at end of test period.

46

300

250

I-zwIr 200DUz0_U' 150OCr4r0U

°w 100

U7

w50

06

I IN. COVER0.50 WIGUNPROTECTED GRAY BAR

10 20 30 40 50

WEEKS OF TESTING

Fig. 4. Measured corrosion current activity of unprotectedcontrol specimen.

generally in agreement with corrosioncurrent data as to when corrosion startedand how it varied with time.

Chloride Content at Initiation ofCorrosion

The average chloride ion content atthe onset of corrosion for the slabs withtin. (25 mm) of cover at all three water-cement ratios was 0.032 percent byweight of concrete. The average valuesare shown in Table 1 and the individualvalues ranged from 0.018 to 0.049 per-cent hut, again there was no relationshipbetween chloride ion percent andwater-cement ratio.

These average corrosion thresholdvalues are equivalent to 0.91, 1.50 and1.19 lbs of acid-soluble chloride ion percubic yard of concrete (0.706 kglm3).

Using the cement contents of the con-crete mixtures, these values produce

Table 1. Average chloride ion content forvarious water-cement ratio concretes.

Chloride ion content altin, depth at initiation

Water-cement of corrosion, percentratio, by weight by weight of concrete

0.51 0.0230.40 0.0380.28 0.030

0.21, 0.26 and 0.17 percent chloride ionby weight of portland cement. Theseaverage values correlate well with pre-vious threshold values reported byLewis' and Clear.' In related workdone for private industry, 48 specimensmade with 1 in. (25 mm) of clear coverwith a water-cement ratio of 0.50 had anaverage of 0.024 percent chloride ion byweight of concrete at intiation of corro-

PCI JOURNAL/July-August 1986 47

00Q

0.06ZN7

m 005W

W O ~F W

aU 0.04a= z

UZ)Od UW

moo 0.030I

p WM m LU 0.02

zo0.01

z0UU 0 7 0.01 0.02 0.03 0.04 0.05 0.06

AS-MIXED CI - CONTENT,% BY WEIGHT OF CONCRETE

Fig. 5. Relationship between "as-mixed" and "as-tested" chloride ion contentsby analyzing powder from 1/4 in. (6 mm) diameter drilled holes.

sion, equivalent to 0.22 percent byweight of cement.

Due to concern about the small sam-ple size brought about by the '/4 in. (6mm) drill bit used to make the sampledepth specific, a small supplementaltest series was done to evaluate the ef-fect of drill and sample size. Concretespecimens with a 0.51 water-cementratio were made which containedknown quantities of added chloride ion,ranging from about 0.01 to 0.05 percentby weight of concrete. A total of six cyl-inders [6 x 12 in. (152 x 305 mm)1 werecast. Two cylinders were cast from theconcrete with the lowest chloride con-tent and one each was cast for the others.The sampling technique was evaluatedas follows:

1. Three '/4 in. (6 mm) diameter holeswere drilled into opposite sides of eachcylinder.

2. The powder from between thedepths of I and 2½ in, (25 and 63 mm)was retained.

3. The powder samples from the threeholes on each side ofeach cylinder werecombined into one composite sample.

4. Each of the twelve samples fromthese six cylinders (two per cylinder)was analyzed for chloride ion.

The results of the tests are shown inFig. 5. The average values for the sixholes drilled in each cylinder show agood relationship between the sampledand the as-mixed concrete chloridecontents, indicating a reasonably accu-rate sampling and testing procedure.

Chloride Profiles After 44 Weeksof Cyclic Testing

Chloride ion contents at the end of 44weeks of cyclic testing for the 90 speci-

48

Table 2. Average chloride contents after 44 weeks.

Water-cement

ratioCover,

in.No. ofslabs

Mean value,percent by

weight of concrete

Standard deviation,percent by

weight of concrete

Coefficientof variation,

percent

0.51 1 16 0.451 0.061 13.42 16 0.0193 0.0135 69.93 14 0.0106 0.0074 69.8

0.40 1 4 0.0973 0.050 51.62 14 0.0064 0.0036 56.33 4 0.0043 0.0005 11.6

0.28 1 4 0.025 0.0096 38.42 14 0.0113 0.0077 68.13 4 0.0065 0.0019 29.2

mens in this group are listed in Table 2.Note that the chloride test method has alimit of detectability of about 0.004 per-cent by weight of concrete.

The average chloride ion contents areplotted versus the depth of cover foreach concrete water-cement ratio in Fig.6. The most dramatic differences be-tween the three concretes occur at aclear cover of tin. (25 mm). At the 2 and3 in. (51 and 76 mm) depth levels, thedifferences in chloride content are in-significant or nil.

The range in chloride ion content cor-rosion threshold values as determinedin this study is indicated by horizontallines near the origin in Fig. 6. None ofthe measured chloride ion contents at 2or 3 in. (51 or 76 mm) cover penetrate upinto this range and indeed none of thespecimens with 2 and 3 in. (51 or 76 mm)cover showed active corrosion duringthe 48 weeks. The curve for 0.51 water-cement ratio, however, indicates thatthe onset of corrosion with 2 in. covermay have been near for specimens madewith that concrete.

The values obtained in this study andshown in Fig. 6 can be compared to datadeveloped and reported by the FederalHighway Administrations for large con-crote slabs exposed outdoors in theWashington. D.C. area and subjected to

830 daily applications of 3 percentsodium chloride. These FHWA data aresuperimposed on the data of Fig. 6, inFig. 7. The trends in the data from thesetwo test programs are surprisingly simi-lar, considering the differences in thelengths and conditions of exposure.

Discussion of Test ResultsThe data developed during this study

do much to point out the value ofwater-cement ratio and concrete coverin providing protection against chlo-ride-induced electrolytic corrosion ofsteel. They support the current re-quirements of the American Associationof State Highway and TransportationOfficials (AASHTO) and the AmericanConcrete Institute (AC!) for maximumwater-cement ratios of 0.44 to 0.40, re-spectively, for reinforced concreteswhich will he exposed to external chlo-rides in service.

If the final 0.451 percent chloride ioncontent by weight of concrete of the 0.51water-cement ratio concrete specimensat the 1 in. (25 mnin) depth level is as-sumed to be a control value of 100 per-cent, the final 44 week chloride ioncontents at the 1 in. (25 mm) depth forthe 0.40 and 0.28 water-cement ratioconcretes were 22 and 6 percent, re-

PCI JOURNAL/July-August 1986 `l9

0 F w W/C=0.51U wa

UZ Q 0.4W UZ ^OU

Ld

= 0.3U LU

Cf3 ^

J 0.2U o

o_ NC_] Ya LQww 0.1C^Qa ^

0.40

Cl - Threshold 0 28

CLEAR COVER DEPTH, IN.

Fig. 6. Chloride contents after 44 weeks of testing versus clear cover depth.

spectively, While the final chloridecontent difference between the 0.40 and0.51 water-cement ratio specimens atthe 1 in. (25 mm) depth was almost 500percent, the use of the high-range wa-ter-reducer (superplasticizer) producedan 1800 percent difference in chloridecontent, between the 0.28 and 0.51 wa-ter-cement ratio concretes.

The effect of cover is also clearlyshown in the data of the present study,which was too short-term to show theadded benefit of the increase in coverfrom 2 to 3 in. (51 to 76 mm). The datafrom the FHWA study' show this bene-fit, particularly in the concretes withwater-cement ratios of 0.5 and 0.6.

The six slabs with 1 in. (25 mm) coverand water-cement ratios of 0.51, 0.40and 0.28 all evidenced corrosion activityafter as little as 2 to 8 weeks of cyclictesting. At the start of corrosion activity,these slabs exhibited significant andsimultaneous increases in corrosion cur-

rent, instant-off potential and half-cellpotentials.

Since the 1 in. (25 mm) cover, 0.40 and0.28 water-cement ratio concrete slabsshowed surprisingly early corrosion ac-tivity, these two test conditions were re-peated and these four specimens wereduplicated and retested. After 4 weeksof testing, one of the duplicate 0.28water-cement ratio slabs also exhibitedcorrosion activity. The two slabs withthe 0.40 water-cement ratio showed cor-rosion activity after 12 and 16 weeks oftesting during these supplementalstudies.

These data show that with 1 in. (25mm) of concrete cover, the water-cement ratio differences did not providesignificant differences in time-to-corrosion of the test slabs. The averagetime-to-corrosion period is shown inTable 3 for these 1 in. (25 mm) coverconditions.

These time-to-corrosion data are cer-

50

w 0.4z Fw wN aZ Voz00L 0 0.3UL

0W-iim^J w 0.2

mU^a

0.5W/C=0.54 -AVERAGE CI"' AFTER 830

DAILY SALT APPLICATIONS

----AVERAGE CI AFTER 44WEEKS

0.40

`` ^` 0.50 0.60

CI' Threshold 0.40 ^ N

V

0.28--

0

CLEAR COVER DEPTH, IN

Fig. 7. Comparison of chloride ion content profiles from 1976 FHWA time-to-corrosionstudy and present study.

tainly not what was anticipated. Onecould reasonably expect that a lowerwater-cement ratio paste, with its atten-dant lower permeability, would providegreater protection against chloride pen-etration. Also, the fact that these speci-mens exhibited corrosion after only avery few weeks while the 2 in. (51 mm)cover specimens were still inactive after48 weeks is bothersome. An observationmade during examination of the broken

Table 3. Average time-to-corrosion periodfor various water-cement ratio concretes.

Average time-Number of Water-cement to-corrosionspecimens ratio period, weeks

2 0.51 6.54 0.40 9.53 0.28 3.0

specimens after the 48 week testingperiod may explain both of these seem-ingly anomalous results.

The specimens were broken by impos-ing line loads directly above and belowthe top steel, thereby splitting thespecimens along the centerlines of thesteel bars. In several instances, the onlyvisible corrosion products on the barswere directly below the imprint of alarge coarse aggregate particle whichnearly spanned the 1 in. (25 mm) cleardistance between the surface which hadbeen ponded and the top surface of thebars.

This leads to the conclusion that atleast some aggregate particles weremuch more permeable than the sur-rounding paste and acted like channelscarrying the chloride solution to withina fraction of an inch of the steel. Therandomness of aggregate placementcould explain the lack of correlation

PCI JOURNAL/July-August 1986 51

between water-cement ratios and theunanticipated early age time-to-corrosion. The very great difference incorrosion protection performance be-tween 1 and 2 in. (25 and 51 mm) ofcover is much easier to understand if in-stead of I in. versus 2 in. (25 mm versus51 mm) of cover, the situation is actuallyequivalent to A in. versus 1 1/4 in. (6 mmversus 32 mm) of effective concretecover.

There is no doubt that the water-ce-ment ratio has a strong influence onchlorideingress into concrete. Whilethe time-to-corrosion and chloride ioncorrosion threshold levels for the 0.51,0.40 and 0.28 water-cement ratio con-cretes with I in. (25 mm) cover were rel-atively constant, the final chloride ioncontents at the I in. (25 min) level at 44weeks were drastically different. The0,51 water-cement ratio concrete even-tually absorbed at the 1 in. (25 mm) levelabout 18 times as much chloride ion asthe 0.025 threshold chloride levelsfound at the same depth at time-to-corrosion.

The 0.40 water-cement ratio concreteeventually absorbed about three timesas much chloride ion as the 0.035threshold chloride levels found at thatsame depth at time-to-corrosion. The0.28 water-cement ratio concrete did notabsorb any measurable additional chlo-ride ion at the 1 in. (25 mm) depth levelbetween the time-to-corrosion tests at 2to 4 weeks and the final chloride testingat 44 weeks.

CONCLUDING REMARKS

Reinforced concrete laboratoryspecimens having water-cement ratiosof 0.28, 0.40 and 0.51 with 1 in. (25 mm)of clear cover over the reinforcing barscan be made to support steel corrosionin 2 to 16 weeks of alternately drying

and ponding with 15 percent sodiumchloride solution. Increasing the con-crete cover to 2 or 3 in, (51 or 76 mm)totally prevented corrosion of bars insimilar concretes with water-cementratios of 0.28, 0.40 and 0,51 after 48weeks of cyclic testing.

Sampling of the concrete at the on-set of corrosion showed that the aver-age corrosion threshold value for chlo-ride ion content was about 0.20 per-cent by weight of cement, regardless ofwater-cement ratio. However, similarchloride content tests after 44 weeks ofcyclic testing showed great differencesin chloride uptake, particularly betweenthe 0.51 water-cement ratio concreteand the low 0.40 and 0.28 water-cementratio concretes, at I in. (25 mm) ofcover.

Differences in chloride uptake atdepths of concrete cover of 2 or 3 in. (51or 76 mm), however, were insignificantor nil. The results of tests for chlorideuptake in this study compared closelywith results obtained in a FHWA studydone outdoors by making daily salt so-lution applications of concretes of vari-ous water-cement ratios.

While the difference in 28-day com-pressive strength between the 0.40 and0.51 water-cement ratio concretes wasonly 20 percent, the difference in ab-sorbed chloride ion at the 1 in. (25 mm)depth after 44 weeks of cyclic testingwas almost 500 percent. This large dif-ference helps explain the typically ob-served greater corrosion resistance ofprecast concrete which is usually madewith relatively low water-cement ratiossuch as 0.40. The use of a superplasti-cizer to achieve a water-cement ratio of0.28 produced an 1800 percent differ-ence between the final chloride contentat the 1 in. (25 mm) depth for the con-cretes made with 0.51 and 0.28 water-cement ratios.

52

REFERENCES

1, Lewis, D. A., "Some Aspects of the Corro-sion of Reinforcing Steel in Concrete inMarine Atmospheres," Corrosion, V. 15,No. 7, July 1959, pp. 60-66.

2. Clear, K. C., "Evaluation of Portland Ce-ment Concrete for Permanent BridgeDeck Repairs," Federal Highway Admin-

istratiun, February 1974, 49 pp.3. Clear, K. C., "Time-to-Corrosion of Rein-

forcing Steel in Concrete Slabs—V. 1: Ef-fect of Mix Design and Construction Pa-rameters," FHWA Interim ReportFHWA-RD-73-52, Federal Highway Ad-ministration, 1976, 55 pp.

NOTE: Discussion of this paper is invited. Please submit yourcomments to PCI Headquarters by March 1, 1987.

PCI JOURNAL1July-August 1986 53