Concrete, Chlorides, Cover and Corrosion

Donald W. Pfeifer J. Robert Landgren William Perenchio Vice President Senior Engineer Senior Consultant Wiss, Janney, Elsiner Wiss, Janney, Elstner Wiss, Janney. Elstner Associates, inc. Associates, Inc. Associates, Inc. Northbrook, Illinois Northbrook, Illinois Northbrook, Illinois

considerable amount has been compared the effects of cover and A written on the subject of corrosion water-cement ratio on the intrusion of of steel in concrete; however, it is not saltwater after 48 weeks of cycling be- the purpose of this paper to present a tween saltwater ponding and drying in discussion of the history of corrosion warm air. These numerous test speci- and the development of corrosion mens are shown tinder test in Fig. 1. technology. Suffice it to say that the chloride ion has been shown to be the most common instigator of steel Materials and Concrete Mixtures corrosion in concrete and that much The aggregates used for this study has been published about the need were highly siliceous natural sand and for certain minimum amounts of gravel from Eau Claire, Wisconsin. clear cover and certain maximum wa- They were chosen specifically because ter-cement ratios. of their very low initial chloride con- The study from which the data con- tents, essentially 0.002 percent. The tained in this paper is excerpted was concretes were designed to have water- primarily a corrosion study designed to cement ratios of 0.28, 0.40 and 0.51. evaluate various means of protecting They were used at a workability of 3 to 4 embedded steel from corrosion. One in. (76 to 102 mm) of slump. A high group of nine tests utilizing 90 conven- range water-reducing admixture was tionally reinforced concrete specimens necessary to produce this workability in produced very interesting data which the 0.28 water-cement ratio mixture.

42 Synopsis Small reinforced concrete slabs Theresults show the very signifi- were tested as part of a larger study cant effect of water-cement ratio on supported by the Federal Highway salt intrusion, which can be much Administration in Project ©TFH61- greater than its effect on compressive 83-00085, "Protective Systems for strength. These results help explain New Prestressed and Substructure the typically observed greater corro- Concrete." The slabs were alternately sion resistance of precast concrete ponded with saltwater followed by which is usually made with lower drying in air. water-cement ratio concrete.

Compressive strengths at 28 days were of embedded steel, the specimens con- nominally 7500, 6000 and 5000 psi (52, tained two mats of reinforcing bars, top 41 and 34 MPa), respectively. and bottom. By ponding the top surface with saltwater, the top bars were made to act as anodes, and potentially corrode, Test Specimen Design while the bottom bars acted as cathodes, Because the primary purpose of the once corrosion started on the top mat study was to determine the effects of bars. various corrosion protection treatments Fig. 2 shows the configuration of the and procedures, including cover and specimens with top covers of 1, 2 and 3 water-cement ratio, on time-to-corrosion in. (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^BARS OR 1/2' STRANDS it NO. 4 REBARS

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

FORMED SURFACE

DIKE

:PDXY COAT NLL SIDES

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

44 of the specimens were varied to produce top bar on at least a monthly basis. the different covers so that the distance Also each month, "instant off" po- and hence the total electrical resistance tentials and AC electrical resistance remained constant, for a given concrete were measured between the top and mixture at the same conditions of tem- bottom steel mats, perature and humidity, between the two Tests for acid-soluble chloride ion mats of bars. Also, the cover over the were made when a surge in corrosion bottom bars was maintained constant at current indicated the start of macrocell 1 in. (25 mm) so that, as far as possible, corrosion of the top bars. Chloride con- constant conditions would he main- tents were also determined near the tained at the cathode. conclusion of the 48-week test period. The two top bars and the four bottom Powder samples were obtained for these bars were joined into top and bottom chloride tests by drilling three holes "mats" electrically by means of external each into the two sides of the slabs in buss bars. The two mats were then the top surface plane of the top bars so as joined by means of a resistor, to provide not to disturb the top slab surface. The for measuring potential differences drilled hole diameter was' in. (6 mm). across the resistor and calculating corro- The hole depth was 2 in. (51 rnm), with sion current flows. the powder sample test portion taken as shown in Fig_ 3. The holes were filled with a sanded epoxy resin before testing Test Procedure was resumed. To simulate realistic field conditions, all of the slabs were moist cured for 3 TEST RESULTS days, followed by 25 days of air drying at 60 to 80°F (16 to 27°C) prior to cyclic The start of macrocell corrosion in testing. During the air-drying period, all slabs with 1 in. (25 mm) of cover varied sides and the protruding ends of bars from 2 to 16 weeks from start of testing; were given two coats of epoxy to however, the anticipated effect of longer minimize lateral moisture movement time-to-corrosion with lower water- during the cyclic tests. Dikes to retain cement ratio was generally not ob- ponded saltwater on the top surface served. None of the slabs with 2 or 3 in. were also added at this time, and elec- (51 or 76 mm) of cover displayed any trical connections were made. measurable corrosion activity during the The saltwater used was a 15 percent 48 weeks of testing. sodium chloride solution, a solution having about five times the chloride ion content of ocean water. The weekly test Time-to-Corrosion cycle consisted of: In the interest of brevity, all of the 1. 100 hours of ponding under salt- corrosion measurements are not in- water at 60 to 80°F (16 to 27°C). cluded in this paper. However, Fig. 4 2. Removal of saltwater. illustrates the onset of corrosion as de- 3. Rinse with fresh water, followed termined by calculation of actual corro- by vacuum removal of water. sion current from voltage drops across 4. 68 hours of drying at 100°F (38°C). the resistor connecting top and bottom This weekly cycle was repeated 48 groups of reinforcing bars. Actual corro- times. The corrosion current was deter- sion current was also measured inde- mined once each week for each speci- pendently. men. Copper-copper sulfate half-cell This curve clearly indicates the onset tests were made at three locations along of corrosion at about 8 weeks. Instant-off the concrete above the length of each potentials 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 SAMPLES FOR TEST

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

46 300

250

I- z w Ir 200 D U z 0_ U' 150 O I IN. COVER Cr 4r 0.50 WIG 0 U UNPROTECTED GRAY BAR °w 100

U7 w 50

06 10 20 30 40 50 WEEKS OF TESTING

Fig. 4. Measured corrosion current activity of unprotected control specimen.

generally in agreement with corrosion Table 1. Average chloride ion content for current data as to when corrosion started various water-cement ratio concretes. and how it varied with time. Chloride ion content al Chloride Content at Initiation of tin, depth at initiation Water-cement of corrosion, percent Corrosion ratio, by weight by weight of concrete The average chloride ion content at 0.51 0.023 the onset of corrosion for the slabs with 0.40 0.038 tin. (25 mm) of cover at all three water- 0.28 0.030 cement ratios was 0.032 percent by weight of concrete. The average values are shown in Table 1 and the individual values ranged from 0.018 to 0.049 per- 0.21, 0.26 and 0.17 percent chloride ion cent hut, again there was no relationship by weight of portland cement. These between chloride ion percent and average values correlate well with pre- water-cement ratio. vious threshold values reported by These average corrosion threshold Lewis' and Clear.' In related work values are equivalent to 0.91, 1.50 and done for private industry, 48 specimens 1.19 lbs of acid-soluble chloride ion per made with 1 in. (25 mm) of clear cover cubic yard of concrete (0.706 kglm3). with a water-cement ratio of 0.50 had an Using the cement contents of the con- average of 0.024 percent chloride ion by crete mixtures, these values produce weight of concrete at intiation of corro-

PCI JOURNAL/July-August 1986 47 0 0 Q 0.06 Z N 7 m 005 W W O ~ F W a= Uz 0.04 UZ)Oa d U W moo 0.03 0I

p W M m LU 0.02

zo 0.01 z 0 U U 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 contents by analyzing powder from 1/4 in. (6 mm) diameter drilled holes.

sion, equivalent to 0.22 percent by 2. The powder from between the weight of cement. depths of I and 2½ in, (25 and 63 mm) Due to concern about the small sam- was retained. ple size brought about by the '/4 in. (6 3. The powder samples from the three mm) drill bit used to make the sample holes on each side ofeach cylinder were depth specific, a small supplemental combined into one composite sample. test series was done to evaluate the ef- 4. Each of the twelve samples from fect of drill and sample size. Concrete these six cylinders (two per cylinder) specimens with a 0.51 water-cement was analyzed for chloride ion. ratio were made which contained The results of the tests are shown in known quantities of added chloride ion, Fig. 5. The average values for the six ranging from about 0.01 to 0.05 percent holes drilled in each cylinder show a by weight of concrete. A total of six cyl- good relationship between the sampled inders [6 x 12 in. (152 x 305 mm)1 were and the as-mixed concrete chloride cast. Two cylinders were cast from the contents, indicating a reasonably accu- concrete with the lowest chloride con- rate sampling and testing procedure. tent and one each was cast for the others. The sampling technique was evaluated Chloride Profiles After 44 Weeks as follows: 1. Three '/4 in. (6 mm) diameter holes of Cyclic Testing were drilled into opposite sides of each Chloride ion contents at the end of 44 cylinder. weeks of cyclic testing for the 90 speci-

48 Table 2. Average chloride contents after 44 weeks. Water- Mean value, Standard deviation, Coefficient cement Cover, No. of percent by percent by of variation, ratio in. slabs weight of concrete weight of concrete percent 0.51 1 16 0.451 0.061 13.4 2 16 0.0193 0.0135 69.9 3 14 0.0106 0.0074 69.8

0.40 1 4 0.0973 0.050 51.6 2 14 0.0064 0.0036 56.3 3 4 0.0043 0.0005 11.6

0.28 1 4 0.025 0.0096 38.4 2 14 0.0113 0.0077 68.1 3 4 0.0065 0.0019 29.2

mens in this group are listed in Table 2. 830 daily applications of 3 percent Note that the chloride test method has a sodium chloride. These FHWA data are limit of detectability of about 0.004 per- superimposed on the data of Fig. 6, in cent by weight of concrete. Fig. 7. The trends in the data from these The average chloride ion contents are two test programs are surprisingly simi- plotted versus the depth of cover for lar, considering the differences in the each concrete water-cement ratio in Fig. lengths and conditions of exposure. 6. The most dramatic differences be- tween the three concretes occur at a clear cover of tin. (25 mm). At the 2 and Discussion of Test Results 3 in. (51 and 76 mm) depth levels, the The data developed during this study differences in chloride content are in- do much to point out the value of significant or nil. water-cement ratio and concrete cover The range in chloride ion content cor- in providing protection against chlo- rosion threshold values as determined ride-induced electrolytic corrosion of in this study is indicated by horizontal steel. They support the current re- lines near the origin in Fig. 6. None of quirements of the American Association the measured chloride ion contents at 2 of State Highway and Transportation or 3 in. (51 or 76 mm) cover penetrate up Officials (AASHTO) and the American into this range and indeed none of the Concrete Institute (AC!) for maximum specimens with 2 and 3 in. (51 or 76 mm) water-cement ratios of 0.44 to 0.40, re- cover showed active corrosion during spectively, for reinforced concretes the 48 weeks. The curve for 0.51 water- which will he exposed to external chlo- cement ratio, however, indicates that rides in service. the onset of corrosion with 2 in. cover If the final 0.451 percent chloride ion may have been near for specimens made content by weight of concrete of the 0.51 with that concrete. water-cement ratio concrete specimens The values obtained in this study and at the 1 in. (25 mnin) depth level is as- shown in Fig. 6 can be compared to data sumed to be a control value of 100 per- developed and reported by the Federal cent, the final 44 week chloride ion Highway Administrations for large con- contents at the 1 in. (25 mm) depth for crote slabs exposed outdoors in the the 0.40 and 0.28 water-cement ratio Washington. D.C. area and subjected to concretes were 22 and 6 percent, re-

PCI JOURNAL/July-August 1986 `l9 0 F w W/C=0.51 aU w U Z Q 0.4 W U Z ^ O U = 0.3 U LU Ld Cf3 ^

UJ o 0.2 o_ N C_] Y a LQ w 0.40 w 0.1 C^ Q a ^ 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 chloride rent, instant-off potential and half-cell content difference between the 0.40 and potentials. 0.51 water-cement ratio specimens at Since the 1 in. (25 mm) cover, 0.40 and the 1 in. (25 mm) depth was almost 500 0.28 water-cement ratio concrete slabs percent, the use of the high-range wa- showed surprisingly early corrosion ac- ter-reducer (superplasticizer) produced tivity, these two test conditions were re- an 1800 percent difference in chloride peated and these four specimens were content, between the 0.28 and 0.51 wa- duplicated and retested. After 4 weeks ter-cement ratio concretes. of testing, one of the duplicate 0.28 The effect of cover is also clearly water-cement ratio slabs also exhibited shown in the data of the present study, corrosion activity. The two slabs with which was too short-term to show the the 0.40 water-cement ratio showed cor- added benefit of the increase in cover rosion activity after 12 and 16 weeks of from 2 to 3 in. (51 to 76 mm). The data testing during these supplemental from the FHWA study' show this bene- studies. fit, particularly in the concretes with These data show that with 1 in. (25 water-cement ratios of 0.5 and 0.6. mm) of concrete cover, the water- The six slabs with 1 in. (25 mm) cover cement ratio differences did not provide and water-cement ratios of 0.51, 0.40 significant differences in time-to- and 0.28 all evidenced corrosion activity corrosion of the test slabs. The average after as little as 2 to 8 weeks of cyclic time-to-corrosion period is shown in testing. At the start of corrosion activity, Table 3 for these 1 in. (25 mm) cover these slabs exhibited significant and conditions. simultaneous increases in corrosion cur- These time-to-corrosion data are cer-

50 0.5 W/C=0.54 -AVERAGE CI" AFTER 830 DAILY SALT APPLICATIONS

----AVERAGE CI AFTER 44 w 0.4 z F WEEKS w w N a Z V oz 00 L 0 0.3 UL 0 W -ii m^ J w 0.2

m 0.40 U^ a `` ^` 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-corrosion study and present study.

tainly not what was anticipated. One specimens after the 48 week testing could reasonably expect that a lower period may explain both of these seem- water-cement ratio paste, with its atten- ingly anomalous results. dant lower permeability, would provide The specimens were broken by impos- greater protection against chloride pen- ing line loads directly above and below etration. Also, the fact that these speci- the top steel, thereby splitting the mens exhibited corrosion after only a specimens along the centerlines of the very few weeks while the 2 in. (51 mm) steel bars. In several instances, the only cover specimens were still inactive after visible corrosion products on the bars 48 weeks is bothersome. An observation were directly below the imprint of a made during examination of the broken large coarse aggregate particle which nearly spanned the 1 in. (25 mm) clear distance between the surface which had been ponded and the top surface of the Table 3. Average time-to-corrosion period bars. for various water-cement ratio concretes. This leads to the conclusion that at Average time- least some aggregate particles were Number of Water-cement to-corrosion much more permeable than the sur- specimens ratio period, weeks rounding paste and acted like channels carrying the chloride solution to within 2 0.51 6.5 a fraction of an inch of the steel. The 4 0.40 9.5 randomness of aggregate placement 3 0.28 3.0 could explain the lack of correlation

PCI JOURNAL/July-August 1986 51 between water-cement ratios and the and ponding with 15 percent sodium unanticipated early age time-to- chloride solution. Increasing the con- corrosion. The very great difference in crete cover to 2 or 3 in, (51 or 76 mm) corrosion protection performance be- totally prevented corrosion of bars in tween 1 and 2 in. (25 and 51 mm) of similar concretes with water-cement cover is much easier to understand if in- ratios of 0.28, 0.40 and 0,51 after 48 stead of I in. versus 2 in. (25 mm versus weeks of cyclic testing. 51 mm) of cover, the situation is actually Sampling of the concrete at the on- equivalent to A in. versus 1 1/4 in. (6 mm set of corrosion showed that the aver- versus 32 mm) of effective concrete cover. age corrosion threshold value for chlo- ride ion content was about 0.20 per- There is no doubt that the water-ce- cent by weight of cement, regardless of ment ratio has a strong influence on water-cement ratio. However, similar chlorideingress into concrete. While chloride content tests after 44 weeks of the time-to-corrosion and chloride ion cyclic testing showed great differences corrosion threshold levels for the 0.51, in chloride uptake, particularly between 0.40 and 0.28 water-cement ratio con- the 0.51 water-cement ratio concrete cretes with I in. (25 mm) cover were rel- and the low 0.40 and 0.28 water-cement atively constant, the final chloride ion ratio concretes, at I in. (25 mm) of contents at the I in. (25 min) level at 44 cover. weeks were drastically different. The Differences in chloride uptake at 0,51 water-cement ratio concrete even- depths of concrete cover of 2 or 3 in. (51 tually absorbed at the 1 in. (25 mm) level or 76 mm), however, were insignificant about 18 times as much chloride ion as or nil. The results of tests for chloride the 0.025 threshold chloride levels uptake in this study compared closely found at the same depth at time-to- with results obtained in a FHWA study corrosion. done outdoors by making daily salt so- The 0.40 water-cement ratio concrete lution applications of concretes of vari- eventually absorbed about three times ous water-cement ratios. as much chloride ion as the 0.035 While the difference in 28-day com- threshold chloride levels found at that pressive strength between the 0.40 and same depth at time-to-corrosion. The 0.51 water-cement ratio concretes was 0.28 water-cement ratio concrete did not only 20 percent, the difference in ab- absorb any measurable additional chlo- sorbed chloride ion at the 1 in. (25 mm) ride ion at the 1 in. (25 mm) depth level depth after 44 weeks of cyclic testing between the time-to-corrosion tests at 2 was almost 500 percent. This large dif- to 4 weeks and the final chloride testing ference helps explain the typically ob- at 44 weeks. served greater corrosion resistance of precast concrete which is usually made CONCLUDING REMARKS with relatively low water-cement ratios such as 0.40. The use of a superplasti- Reinforced concrete laboratory cizer to achieve a water-cement ratio of specimens having water-cement ratios 0.28 produced an 1800 percent differ- of 0.28, 0.40 and 0.51 with 1 in. (25 mm) ence between the final chloride content of clear cover over the reinforcing bars at the 1 in. (25 mm) depth for the con- can be made to support steel corrosion cretes made with 0.51 and 0.28 water- in 2 to 16 weeks of alternately drying cement ratios.

52 REFERENCES

1, Lewis, D. A., "Some Aspects of the Corro- istratiun, February 1974, 49 pp. sion of Reinforcing Steel in Concrete in 3. Clear, K. C., "Time-to-Corrosion of Rein- Marine Atmospheres," Corrosion, V. 15, forcing Steel in Concrete Slabs—V. 1: Ef- No. 7, July 1959, pp. 60-66. fect of Mix Design and Construction Pa- 2. Clear, K. C., "Evaluation of Portland Ce- rameters," FHWA Interim Report ment Concrete for Permanent Bridge FHWA-RD-73-52, Federal Highway Ad- Deck Repairs," Federal Highway Admin- ministration, 1976, 55 pp.

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

PCI JOURNAL1July-August 1986 53