Summary Report

Determination of
Permissible Chloride Levels
in Prestressed Concrete

David Stark
Principal Research Petrographer
Construction Technology Laboratories
A Division of the Portland Cement Association
Skokie, Illinois

SIGNIFICANCE OF FINDINGS was found that the threshold water solu-

ble chloride limit above which corrosion
This study demonstrated that de- of prestressed tendons occurred during
velopment of active corrosion of pre- and immediately after curing was be-
stressing tendons depends not only on tween 0.11 and 0.17 percent by weight
the presence of chloride ion in the con- of cement. However, it was also found
crete, but also on the environment into that under prolonged uniformly moist or
which the concrete member is placed. It dry conditions, active corrosion could
not be sustained even in the presence of
water soluble chloride contents as high
NOTE: This is a Summary Report of an investi- as 0.9 to 1.0 percent by weight of ce-
gation on Research Project No. 2, "Determina- ment.
tion of Permissible Chloride Levels in Pre- This study further demonstrated that,
stressed Concrete," sponsored by the Pre- with chloride ion present, changes in
stressed Concrete Institute. The study was car- exposure conditions, such as alternate
ried out in the Construction Technology Labo-
drying and wetting, and differential or
ratories, a Division of the Portland Cement As-
localized drying, can rapidly initiate
sociation, Skokie, Illinois. The full report (84 pp.)
is available from PCI Headquarters upon re- corrosion. These changes had greater
quest at a cost of $30 per copy; $15 to PCI impact on corrosion processes than ce-
members. ment composition, curing method, or

106
stress level in the steel. Water-cement
ratio of the concrete influences corro-
Synopsis
sion as it affects moisture, oxygen, and
chloride diffusion rates. This report describes a labora-
The above findings can be extended tory investigation to determine
to field structures to explain purported chloride concentrations and expo-
corrosion-free performance of pre- sure conditions required to induce
stressed concrete members made with corrosion of prestressed tendons in
high levels of chloride ion. In these concrete beams.
cases, either drying has increased the Variables included source and
electrical resistivity of the concrete suf- concentration of chloride ion, wet-
ficiently to prevent flow of galvanic cur- ting and drying, C3A content of the
rent between potential anodic and cement, water-cement ratio of the
cathodic sites on the steel, or, under concrete, stress level in the steel,
uniform moist conditions, sufficient dis- and concrete curing method.
solved oxygen has not been present to Performance was evaluated by
sustain active corrosion. It should be electrical potential measurements,
noted that unforeseen changes in these visual examination of tendons, and
environmental conditions can introduce measurement of chloride contents.
a high risk of development of active cor- Recommendations are made for
rosion and should be considered. permissible chloride levels in pre-
To eliminate the risk of active corro- stressed concrete as they are af-
sion, it is recommended that the permis- fected by exposure conditions.
sible water soluble chloride ion content
in prestressed concrete not exceed 0.10
percent by weight of portland cement.
cement corresponds to about 1.0 percent
total chloride ion by weight of cement.
INTRODUCTION The ACI limit is based primarily on
estimates by George J. Verbeck* and ap-
Under certain exposure conditions, an
environment may develop that is con- pears to include a "safety factor" be-
ducive to galvanic corrosion of pre- cause of possible catastrophic results if
stressing steel in pretensioned concrete corrosion occurs.
members. This type of corrosion re- In view of the uncertain basis for
quires moisture, oxygen, and sufficient chloride limitations, an investigation
chloride ion at the surface of the steel to was undertaken to address two pertinent
sustain electrochemical corrosion reac- questions:
tions. The required chloride ion level 1. What is the permissible chloride
may be reached through the use of level in concrete, above which corrosion
chloride-bearing admixtures such as of prestressing steel occurs?
calcium chloride, or by migration 2. Under what exposure conditions
through concrete of chloride from exter- can calcium chloride admixture in con-
nal sources such as seawater and deicing crete induce corrosion of prestressing
salts. steel? Or conversely, under what condi-
The American Concrete Institute tions does it not induce corrosion?
(ACI) states that water soluble chloride A two-phase program was undertaken
in prestressed concrete should not ex- to obtain answers to these questions.
ceed 0.06 percent by weight of cement.'
Putting this figure into more practical *Personal communication. Mr. Verbeck was for-
terms, a 2.0 percent calcium chloride merly Director of Research, Portland Cement Associa-
(CaCl 2 • 2H20) addition by weight of tion, Skokie, Illinois.

PCI JOURNAL/July-August 1984 107

One phase included the circulation of a the top edge of each beam to help
questionnaire to members of the Pre- maintain the intended exposure condi-
stressed Concrete Institute, requesting tion. The vertical sides and ends of the
information on the occurrence of corro- beams were sealed with epoxy to
sion and the use of calcium chloride as minimize effects related to moisture
an admixture in prestressed concrete. diffusion at those surfaces.
The second phase consisted of a labo-
ratory study to determine the chloride
Test Variables
levels and environmental conditions re-
quired to develop galvanic corrosion of Six variables considered to potentially
prestressed tendons in concrete. Results affect galvanic corrosion processes in
of this work are summarized below. concrete were included in this study.
Table 1 lists the combinations of vari-
ables tested.
RESULTS OF
QUESTIONNAIRE SURVEY Materials
Very few cases of corrosion of pre- Two Type I cements, conforming to
stressed steel in concrete containing ASTM C150-80 Standard Specifications
calcium chloride as an admixture were for Portland Cement, were used to make
reported. This appears to be related the test beams. Siliceous coarse and fine
largely to lack of use of this admixture aggregate from Eau Claire, Wisconsin
by concrete producers. Corrosion prob- was used in all test beams. Total
lems that were reported apparently re- chloride content of the aggregate was
sulted from exposure to chloride so- only 0.003 to 0.005 percent by weight of
lutions in the in-service environment, the aggregate. Maximum particle size of
particularly to chloride deicer solution. the aggregate was limited to % in. (9.5
None of the producers reported plans to mm).
use calcium chloride as an admixture in All prestressing steel was stress re-
future production. lieved Grade 270K obtained from the
same lot. It consisted of '/2 in. (12.7 mm)
diameter, uncoated seven-wire strand
LABORATORY PROGRAM tendon conforming to ASTM A416-80,
Standard Specification for Uncoated
This section describes the details of Seven-Wire Stress-Relieved Steel
the experimental program, including test Strand for Prestressed Concrete.
specimens and variables, materials, Tap water containing 10 ppm chloride
concrete mix design, specimen fabrica- ion was used as mix water. Commer-
tion, and corrosive monitoring methods. cially available flake calcium chloride
(CaC1 2 • 2H 2O) was used as a set-
Test Specimens accelerating admixture.

Nineteen pretensioned concrete

beams were fabricated for this study. Concrete Mix Design
Each beam was 12 ft long, 1 ft wide, and Two concrete mix designs were
6 in. deep (3.66 x 0.305 m x 152 mm). utilized in this program. Pertinent mix
Three evenly spaced prestressing ten- design, curing, and compressive
dons were positioned 1 in. (25.4 mm) strength data are given in Table 2.
from the top surface of each beam, while
three additional tendons were located 1
in. (25.4 mm) from the bottom surface.
Specimen Fabrication
Styrofoam dikes were cemented along All test beams were cast and cured at

108
C)
C- Tnhlo I Vnrinhla rnmhinatinns in test Droaram.
0
C
Cement, Water- Stress Exposure conditions
Z
D percent cement Percent level,
C- Beam C3A ratio CaCl2 • 2 =O* Cure ksi Initial 10 to 12 months Final 11 months
C

1 11 0.35 1.0 Steam 0 Continuous damp beam dry, ½ beam damp

'/2

C
2 11 0.35 2.0 Steam 0 Continuous damp ½ beam dry,' beam damp
C
3 11 0.35 1.0 Steam 80 Continuous damp Remain as is
4 11 0.35 2.0 Steam 80 Continuous damp Remain as is
5 11 0.35 1.0 Steam 160 Continuous damp 10 days dry + 4 days damp
6 11 0.35 2.0 Steam 160 Continuous damp 10 days dry + 4 days damp
7 11 0.35 2.0 14 days moist 160 Continuous damp Remain as is
8 8 0.35 2.0 Steam 160 Continuous damp 10 days dry + 4 days damp
9 11 0.50 2.0 Steam 160 Continuous damp 10 days dry + 4 days damp
11 0.50 2.0 14 days moist 160 Continuous damp Continuous dryt
10
0.35 None Steam 0 Pond -4 percent NaCI soln. 10 days dry + 4 days 8 percent NaCI
11 11
12 11 0.35 None Steam 80 Pond -4 percent NaCl soln. 10 days dry + 4 days 8 percent NaCl
13 11 0.35 None Steam 160 Pond - 4 percent NaCl soln. 10 days dry + 4 days 8 percent NaCl
14 8 0.35 None Steam 160 Pond -4 percent NaCI soln. 10 days dry + 4 days 8 percent NaCI
15 11 0.35 2.0 Steam 160 Pond -4 percent NaCI soln. Continuous dryt
16 11 0.35 Aggr. sat. Steam 160 Continuous damp Remain as is
4 percent NaC1
17 11 0.35 2.0 Steam 160 Continuous dry-So percent RH 10 days dry + 4 days 8 percent NaCI
18 8 0.50 0.17 Steam 160 Continuous damp 10 days dry + 4 days 8 percent NaCI
19 11 0.35 None 14 days moist 160 Pond -4 percent NaCl soln. 10 days dry + 4 days 8 percent NaCI

*Expressed as percent by weight of cement. Metric (5I) convention factor: 1 ksi = 6.895 MPa.
tTwo 1V2-in. (38 mm) diameter holes were drilled to two tendons and filled with water or 4 percent NaC1 solution for short-teem study of localized shifts in potential.
Details are given in full report.

0
CD
Table 2. Summary of concrete mix design and strength data.

Compressive
strength, psi*
Cement
C
factor, Water- Calcium
Beam lb per cement chloride, Method Percent
No. cu yd ratio percent of cure air 1 day 14 days
1 705 0.35 1.0 Steam 5.2 5510 -
2 705 0.35 2.0 Steam 5.5 5240 -
3 705 0.35 1.0 Steam 5.4 3910 -
4 705 0.35 2.0 Steam 6.2 4290 -
5 705 0.35 1.0 Steam 5.0 4790 -
6 705 0.35 2.0 Steam 5.7 4010 -
7 705 0.35 2.0 Moist 5.5 - 5440
8 705 0.35 2.0 Steam 5.5 5910 -
9 493 0.50 2.0 Steam 6.3 3510 -
10 493 0.50 2.0 Moist 6.0 - 4980
11 705 0.35 0 Steam 6.2 4270 -
12 705 0.35 0 Steam 5.6 4300 -
13 705 0.35 0 Steam 5.8 4730 -
14 705 0.35 0 Steam 5.6 5550 -
15 705 0.35 2.0 Steam 6.5 4190 -
16 705 0.35 Sat. Agg.t Steam 5.0 4290 -
17 705 0.35 2.0 Steam 6.0 4070 -
18 493 0.50 0.17 Steam 5.0 3460 -
19 705 0.35 0 Moist 5.2 - 5830

*Corrected to 6 x 12 in. cylinder strengths. All data are the average of two companion cylinders.
tAggregate saturated with 4 percent NaCI solution.

Aggregate proportions per cu yd

0.35 W/C mixes - C. Aggregate 1608 lbs

F. Aggregate 1426 lbs

0.50 W/C mixes - C. Aggregate 1660 lbs

F. Aggregate 1460 lbs

Metric (SI) conversion factors: 1 lb per cu yd = 0.5933 kg/m 3 ; 1 psi = 0.006895 MPa; 1 lb = 4.448 N; 1 in. _
25.4 mm.

Construction Technology Laboratories Beams scheduled for steam curing

(CTL). Tendons were degreased with were kept under damp burlap and
xylene to insure that steel surfaces were polyethylene sheeting for 4 hours, after
clean prior to casting. which steam curing was initiated. Tem-
The fresh concrete was consolidated perature rise was 15 to 20 deg F (8.3 to
in the forms by internal vibration. The 11.1 deg C) per hour during the initial
top surface of each beam was screeded 4-hour period of steam cure. Maximum
and finished with a magnesium float. temperatures were then maintained at
Concrete cylinders [3 x 6 in. (76 x 152 155 to 165 F (68.3 to 73.9 F) for an addi-
mm)] were cast for later strength deter- tional 12 hours. Following this period,
minations and chloride measurements. the specimens were allowed to cool, in
All mixing and casting was done at 73 ± the forms, to room temperature.
3 F (23 ± 1.7 C) and 50 ± 5 percent rel- For continuously moist curing, the
ative humidity. beams were covered with wet burlap

110
and polyethylene for 14 days. Each day (76 x 152 mm) concrete cylinders that
during this period, top surfaces of the were cast and cured with the beams.
beams were wetted to minimize desic- Dry powder samples were obtained
cation due to cement hydration. from the beams at depths of 3/4 to 1 1/4 in.
After curing, and following load re- (19 to 32 mm) from the top surface to
lease and removal of forms, all beams determine chloride concentration at
were transferred to a test room main- about the level of the upper tendons.
tained at 73 ± 3 F (22.8 ± 1.7 C) and 50 Chemical analyses were made accord-
± 5 percent relative humidity. Steam ing to procedures described by Ber-
cured beams were stored under this man. 4 Nonevaporable water contents
condition for 27 days, while 14-day were measured on each powder sample
moist cured beams were stored under to correct for nonuniformities in paste-
the same condition for 14 days prior to aggregate ratios among the samples.
start of the test period.

Monitoring Methods TEST RESULTS

The following three methods were Results from the three test methods
used to monitor corrosion-related de- are summarized in this section. Electri-
velopments: cal potentials are referenced to the
1. Visual inspections were made for copper/copper sulfate half cell (CSE).
development of corrosion products on All chloride contents, including dosage
tendons extracted from the beams. of calcium chloride admixture, are ex-
2. Electrical potential mea- pressed as percent by weight of cement
surements 2 ' 3 were made following if not specified otherwise.
ASTM Designation C876-80 Standard
Test Method for Half Cell Potentials of
Reinforcing Steel in Concrete. A
Visual Examination of Tendons
copper/copper sulfate (CSE) half cell Prestressing tendons were removed
was used as the reference electrode. from eighteen of the nineteen test
The writer's experience has indicated beams and examined for the presence of
that potentials more negative than steel corrosion products. Observations,
–0.30V reflect high probabilities of cor- summarized in Table 3, show that active
rosion. This criterion was used in this corrosion had developed in all nine
study. Equally significant are differ- beams made with 2 percent calcium
ences in potential between different lo- chloride admixture.
cations on a given prestressed tendon. The nature and severity of corrosion
Differences of about 0.1OV, or more, are ranged from localized pitting in Beams 4
generally required to sustain corrosion and 15 to numerous areas with films of
cells for this condition. Potential mea- corrosion product scattered along the
surements indicate only the presence of entire length of the tendons in Beam 17.
active corrosion and not rate of corrosion Corrosion also occurred in the three
or the presence of corrosion products beams made with 1 percent calcium
from previously active corrosion cells. chloride admixture. In these beams
3. Chloride analyses were made to (Beams 1, 3, and 5) corrosion products
characterize the environment to which occurred as localized films with no evi-
the prestressed tendons were subjected. dence of pitting.
Initial and subsequent total and water Tendons from four of the five beams
soluble chloride ion contents were de- made without added chloride revealed
termined. Initial chloride contents of no evidence of corrosion, even after
the concrete were measured on 3 x 6-in. ponding with NaCl solution throughout

PCI JOURNAL/July-August 1984 111

Table 3. Summary of observations of prestressing tendons.
Beam CaCl2•
No. 2H2O* Exposure conditions Observations of upper tendons

1 1 percent Continuous damp — Several localized areas with film of

Half dry and half damp corrosion products.

2 2 percent Continuous damp-p Few localized deposits of corrosion

Half dry and half damp product scattered along full length of
tendons. Extensive, but discontinuous
film of corrosion product along tendon
in damp half of beam. Minor pitting
in this section.

3 1 percent Continuous damp Light film of corrosion product in sev-

eral 2 to 4 in. long areas 2 ft from one
end on each tendon.

4 2 percent Continuous damp Localized areas with film of corrosion

product. Pitting scattered along entire
length on two or three strands of each
tendon.

5 1 percent Continuous damp -f Film of corrosion product along 1' ft

10 days dry plus 4 days damp length on four strands near one end of
one tendon. Film on several strands at
middle of beam on each tendon.
6 2 percent Continuous damp -- Localized areas with film of corrosion
10 days dry plus 4 days damp product in 1 to 3 ft length near one end
of each tendon. Several areas, 2 to 4 in.
long, with corrosion products.

7 2 percent Continuous damp Small areas 2 in. long with film of cor-
rosion product scattered along entire
length. Film of corrosion product 2 ft
long near one end of each tendon.

8 2 percent Continuous damp-* Few spots, about' in. long, with film
10 days dry plus 4 days damp of corrosion product scattered full
length on each tendon.

9 2 percent Continuous damp -* Few spots, 2 to 4 in. long and 2 ft from

10 days dry plus 4 days damp one end of each tendon, with film of
corrosion product.

10 2 percent Continuous damp -* Few spots, 2 to 4 in. long and 2 ft from

Continuous dryt one end on each tendon, with film of
corrosion product.

See footnotes on opposite page.

the test period. One tendon in Beam 11 tained coarse aggregate presoaked in 4
displayed corrosion products. In this percent NaCI solution, displayed local-
case, a crack had formed over the tendon ized corrosion after 79 weeks under the
prior to indications of active corrosion. continuous damp exposure condition. In
Tendons from Beam 16 which con- Beam 18, which was made with 0.17

112
Table 3 (cont.). Summary of observations of prestressing tendons.

Beam CaCl2•
No. 2H20* Exposure conditions Observations of upper tendons

11 0 Pond 4 percent NaCl soln.- No corrosion observed on Tendons A

10 days dry plus 4 days pond and C. Pitting in 2 in. length in 2
8 percent NaCI soln. strands 1 to 2 ft from one end on
Tendon B.

12 0 Pond 4 percent NaCI soln.–s No corrosion observed on tendons.

10 days dry plus 4 days pond
8 percent NaCl soln.

13 0 Pond 4 percent NaCl soln.–s No corrosion observed on tendons.

10 days dry plus 4 days pond
8 percent NaCI soln.

14 0 Pond 4 percent NaC1 soln.–s No corrosion observed on tendons.

10 days dry plus 4 days pond
8 percent NaCI soln.

15 2 percent Pond 4 percent NaCl soln.–s Localized areas up to 6 in. long with
Continuous dryt film of corrosion product 2 ft from one
end. Occasional pitting scattered full
length of tendons.

16 0 Continuous damp Few localized spots on each tendon

Aggr. sat, with corrosion products.
4 percent
NaCI

17 2 percent Continuous dry Localized areas with layer of corrosion

50 percent RH – product on all tendons.
10 days dry plus 4 days pond
8 percent NaCI soln.

18 0.17 Continuous damp –s Localized areas, 2 to 4 in. long and 1 to

10 days dry plus 4 days pond 3 ft from one end, with film of corrosion
8 percent NaCl soln. product on tendons.

19 0 Pond 4 percent NaCI soln.— Beam held in reserve for possible

10 days dry plus 4 days pond further testing. Tendons not retrieved
8 percent NaCI soln. for visual inspection.

Metric (SI) conversion factors: 1 ft = 0.305 m; 1 in. = 25.4 mm.

*Expressed as percent by weight of cement.
tTwo 1 y -in. (38 mm) diameter holes were drilled to two tendons and filled with water or 4 percent NaC1
solution for short-term study of localized shifts in potential. Details are given in full report.

percent calcium chloride admixture ponding with 8 percent NaCl solution.

(which corresponds to the ACI limit of With the exception of Beam 11, none
0.06 percent water soluble chloride ion of the beams displayed cracking. Beam
for prestressed concrete), two of the 19 was held for possible further testing.
three tendons displayed localized films
of corrosion product in a 2-ft (0.61 m)
Electrical Potentials
length near one end of the beam. This
condition appeared to develop after The most significant results of electri-

PCI JOURNAL/July-August 1984 113

 BEAM NOS. 3,4, AND 12
0.50
w Nos. 3 And 4-Continuous Damp
U)
U No. 12-Continuous Pond-4% NaCI
040

0 0.30
4-

0.20
2-0
a
WIC - 0.35
0.10
Z C3A- I I %
o Stress-80 ksi
o 0 Cure-Steam

IDay 10 20 30 40 50
AGE, WEEKS
Fig. 1. Potential curves for beams made with 0, 1, or 2 percent calcium chloride
admixture.

cal potential measurements are sum- soaking the coarse aggregate in 4. per-
marized in Figs. 1 to 4. In Fig. 1, com- cent NaCI solution to simulate seawater
parisons are made for beams made with immersion. The potential curve shows
0, 1, or 2 percent calcium chloride ad- the development of an initial period of
mixture, by weight of cement. Results active corrosion that terminated at about
indicate that, where 1 or 2 percent cal- 15 weeks. This illustrates that water sol-
cium chloride was used, active corrosion uble chloride as low as 0.17 percent,
developed within 1 day of fabrication of even though introduced with the aggre-
the beam. However, it terminated gate, can initiate active corrosion.
within 3 to 10 weeks. This is indicated Fig. 2 also shows the potential curve
by shifts in potential to values less for Beam 18, which was made with 0.13
negative than –0.30V. percent total (0.11 percent water sol-
Potentials for the beam made without uble) chloride, by weight of cement. In
calcium chloride remained less negative this case, active corrosion was not initi-
than –0.30V, thus indicating that active ated until the period of ponding with
corrosion had not developed during the NaCI solution. Thus, 0.13 percent
test period. Results for this beam also chloride by weight of cement was not
indicate that sufficient chloride ion from sufficient to initiate corrosion.
the solution ponded on the top surface The effect of prolonged drying, at 73
of the beam did not reach the tendons to ± 3 F (23 ± 1.7 C) and 50 ± 5 percent
later initiate active corrosion. relative humidity, on corrosion resis-
Fig. 2 shows results for Beam 16, tance is indicated in Fig. 3 by the po-
which contained 0.20 percent total (0.17 tential curve for Beam 17, which was
percent water soluble) chloride by made using 2 percent calcium chloride
weight of cement. Virtually all of the admixture. Initial potential was –0.48V
chloride was introduced through pre- which indicates the development of ac-

114
 BEAM NOS. 16 AND 18
0.50
w No. 16 Continuous
(n
0 Continuous Damp No.1 1 8 10 Days Dry
U) 040 I^ 4 Days Pond-8% NaCI

0.30 -- ------- --
0
No. 16-0.20% CI
, Pre-Soaked Agg^
0.20

No. 16 - 11%C3A
Z 0.10 No. 18-0.13% CI 0.35 W/C

F_ No. 18- 8% C3A

0 0 0.50 W/C

IDay 20 40 60 80 o

AGE, WEEKS
Fig. 2. Potential curves for beams with 0.13 and 0.20 percent total chloride ion.

tive corrosion. During the ensuing 50- dons. This observation does not indicate
week continuous dry period, potentials whether corrosion occurred during the
progressively shifted to less negative first 20 weeks of drying, during the
values, and reached –0.18V as drying cycling period, or during both periods.
was terminated. However, potentials indicate that active
This long-term shift indicates that ac- corrosion occurred during two different
tive corrosion had stopped within about periods of time.
20 weeks. The tendons then remained Fig. 4 reveals the effects of differen-
in the passive state for the remaining tial drying on corrosion resistance in
30-week duration of the drying period. Beams 1 and 2. Beam 2 was made using
Following the period of continuous a 2 percent calcium chloride admixture.
drying, the beam was subjected to cy- After the initial 50-week period of con-
cles of 10 days drying and 4 days pond- tinuous damp exposure, one-half [6 ft
ing with 8 percent NaCI solution. The (1.83 m)] of the top surface of the beam
curve in Fig. 3 indicates that, within 1 or was exposed continuously to 50±5 per-
2 weeks of this change, potentials cent relative humidity, while the other
shifted to approximately –0.35V. This half remained in the continuous damp
indicates recurrence of active corrosion. exposure.
Potentials then remained at about Beam 1, which was similar to Beam 2
–0.33V for the duration of the 42-week except that it was made using 1 percent
cycling period. This indicates that active calcium chloride admixture, was sub-
corrosion continued throughout this jected to the same change in exposure
period. conditions. No additional chloride was
Visual examination revealed the introduced into either beam.
presence of a layer of corrosion product Two potential curves are shown for
along the entire length of the upper ten- each beam after the change in exposure

PCI JOURNAL/July-August 1984 115

 BEAM NO. 17
0.50r Continuous Dry 10 Da ys Dr
Ui
4 Days Pond-8% NaCI
V I
c, 040
U)
1030

0.20
W/C-0.35
Calcium Chloride-2%
z 0.10 Stress-160 ksi
w Cure-Steam
0
a- n

I Day 20 40 60 80
AGE, WEEKS
Fig. 3. Potential curve for beam subjected to 50 weeks of drying followed by alternate
ponding and drying.

BEAM NOS. I AND 2

0.50i-
w Damp Continuous Half Dry
o , Half Damp
040
U)
0 0.30 — — — —
> No. 2-Damp
0.20 No.2-Dry
J
a No. 1-Damp
Z
w
0.10
No. l -Dry
F-
0
O

45 50 60 70 80 90
AGE, WEEKS
Fig. 4. Potential curves for two beams subjected to continuous half dry-half damp
exposure conditions.

116
condition; one for the dry half of each 3915 lbs per cu yd (2342 and 2322 kg/m3)
beam, and one for the damp half. Unlike were used to convert percent chloride
the similarity of potentials measured by weight of concrete to percent by
along tendons under uniform exposure weight of cement for the 0.35 and 0.50
conditions, differences in potential be- water-cement ratios, respectively.
tween the two halves of each beam de- From these data, an estimate can be
veloped soon after the change in expo- made of minimum chloride concen-
sure condition. In Beam 2, maximum trations required to induce corrosion of
potential differences reached 0.1OV prestressed tendons during and im-
(-0.28V vs –0.18V) in 5 to 10 weeks. A mediately following curing. The lowest
similar pattern developed for Beam 1, water soluble chloride level measured
except that maximum potential differ- at which corrosion was initiated during
ences reached only 0.06V. this period was 0.17 percent by weight
Visual examination of tendons from of cement. In this case, chloride ion was
these two beams revealed, in addition to introduced into the fresh concrete
the few localized deposits of corrosion through prior absorption by the coarse
product present along the full length of aggregate.
tendons from the two beams, a heavier, In contrast, corrosion did not develop
continuous film of corrosion product initially when measured water soluble
along only the 6-ft (1.83 m) length of chloride levels were 0.11 percent by
tendons embedded in concrete exposed weight of cement. Thus, the water sol-
continuously to the damp condition in uble chloride level required to induce
Beam 2. This was not seen along the corrosion of prestressed tendons during
tendons from Beam 1. and immediately following curing was
Thus, when 2 percent calcium in the range of 0.11 to 0.17 percent by
chloride admixture was used, prolonged weight of cement. Respective total
periods of damp exposure followed by chloride concentrations were 0.13 per-
differential drying of the concrete in- cent and 0.20 by weight of cement.
duced corrosion of prestressed tendons. The data also indicate that chloride
Potential curves (not shown) for other levels required to induce corrosion at
beams reveal that C 3 A content of the later ages were strongly dependent
cement, stress level in the steel, water- upon type of exposure condition. For
cement ratio of the concrete, and curing example, under uniform drying condi-
method, had little effect on initial corro- tions of 50 percent relative humidity at
sion resistance where 1 or 2 percent cal- 70 to 75 F (21 to 24C), corrosion failed to
cium chloride admixture had been used. develop in concrete containing 0.87
In all cases, only initial periods of active percent water soluble chloride ion by
corrosion, similar to those shown in weight of cement. Most of this chloride
Figs. 1 and 2, developed, which were was introduced into the concrete
terminated in less than 30 weeks. through use of 2 percent calcium
chloride admixture. Continuous expo-
sure of companion concrete beams to
Chloride Contents uniformly damp conditions following
Table 4 summarizes the results of ini- curing and 1 month of drying also failed
tial and final chloride content mea- to sustain active corrosion.
surements. Values are expressed as per- In contrast, exposure to nonuniform
cent by weight of cement, and have drying conditions permitted develop-
been corrected for differences in paste- ment of corrosion. In this instance, 2
aggregate ratios by averaging percent calcium chloride admixture, by
nonevaporable water contents. Mea- weight of cement, had been utilized.
sured concrete unit weights of 3950 and Corrosion did not develop under similar

PCI JOURNAL/July-August 1984 117

8 Initial chloride content Final chloride content
Beam Source of Water Water
No. initial chloride* Initial and final exposure conditions Total soluble Total soluble
It 1 percent CaC1 2 Cont. damp, '/z beam dry, Vz beam damp 0.51 0.45 0.53 0.41
0.54 0.50
2* 2 percent CaCl 2 Cont. damp -* '/z beam dry, 1/z beam damp 0.99 0.93 0.93 0.58
0.87 0.70
3 1 percent CaCl z Cont. damp - * Remain as is 0.51 0.44 0.44 0.22
4 2 percent CaC1 2 Cont. damp -* Remain as is 0.99 0.92 0.86 0.65
5 1 percent CaCl 2 Cont. damp -* 10 days dry plus 4 days damp 0.51 0.42 0.57 0.46
6 2 percent CaC1 2 Cont. damp -+ 10 days dry plus 4 days damp 0.99 0.88 0.85 0.67
7 2 percent CaC1 2 Cont. damp, Remain as is 0.99 0.87 0.81 0.75
8 2 percent CaCl 2 Cont. damp , 10 days dry plus 4 days damp 0.99 0.83 0.81 0.54
9 2 percent CaCl 2 Cont. damp -* 10 days dry plus 4 days damp 1.00 0.86 0.67 0.41
10 2 percent CaCl 2 Cont. damp, Continuous dry$ 1.00 0.88 0.66 0.54
11 None Pond 4 percent NaCl -* 10 days dry plus 4 days 0.05 0.04 0.31 0.13
8 percent NaCl
12 None Pond 4 percent NaCI -a 10 days dry plus 4 days 0.05 0.04 0.17 0.09
8 percent NaCl
13 None Pond 4 percent NaCI -+ 10 days dry plus 4 days 0.05 0.01 0.26 0.25
8 percent NaC1
14 None Pond 4 percent NaC1-* 10 days dry plus 4 days 0.05 0.05 0.43 0.36
8 percent NaCI
15 2 percent CaCl 2 Pond 4 percent NaCI -* Continuous dry$ 0.99 0.91 1.26 1.07
16 Aggr. sat. Cont. damp - Remain as is 0.20 0.17 0.17 0.13
17 2 percent CaC1 2 Cont. dry 50 percent RH -* 10 days dry plus 4 days 0.99 0.87 1.27 1.19
8 percent NaCl
18 0.17 percent CaC1 2 Cont. damp -* 10 days dry plus 4 days 0.13 0.11 0.29 0.29
8 percent NaCI
19 None Pond 4 percent NaCI -* 10 days dry plus 4 days 0.05 0.05 0.17 0.12
8 percent NaCI
'-.''- 2 rerers to Lac,i 2 • 2ti2 u uy weight of cement. Total and water soluble chloride contents are expressed as percent by weight of cement.
tThe first figure for final chloride content is for the dry end of the beam.
#Two 1½-in. (38 mm) diameter holes were drilled to two tendons and filled with water or 4 percent NaCI solution for short term study of localized shifts in potential.
Details are given in full report.
 uble chloride level above which corro-
exposures where 1 percent calcium
chloride admixture had been used. sion occurred at later ages depended on
In beams made without calcium environmental conditions to which the
chloride admixture, ponding with 4 or 8 concrete beams were exposed.
percent NaC1 solution failed to induce 4. The laboratory tests indicated that,
corrosion, even though up to 0.36 per- under certain uniform wetting or drying
cent water soluble chloride ion was conditions, corrosion of prestressed ten-
measured at the level of the upper ten- dons was not sustained in concrete
dons. Absence of active corrosion in made with up to 2 percent calcium
these beams appeared to have been due chloride admixture by weight of cement.
to lack of dissolved oxygen, high electri- 5. Tests indicated cyclic wetting and
cal resistivity of concrete, and uni- drying, or differential drying, inducedcor-
formity of environment along individual rosion where 2 percent calcium chloride
tendons. admixture had been used.
Thus, the data indicate that a single 6. Laboratory tests also indicated that
permissible chloride level in concrete, stress levels in prestressing tendons,
above which corrosion is initiated, does C 3A content of cement, water-cement
not apply for all environments. Relative ratio of the concrete, and method of
lengths of wetting and drying periods, curing had little effect on development
diffusion rates of chloride ion and dis- of corrosion.
solved oxygen, and moisture condition
of the concrete all are significant factors ACKNOWLEDGMENT
determining the corrosion resistance of
prestressing tendons in concrete. This study was sponsored by the Pre-
stressed Concrete Institute. The Construc-
tion Technology Laboratories is grateful for
CONCLUSIONS their support and assistance.

Based on results developed in this

study, the following conclusions are
REFERENCES
drawn: 1. ACI Committee 201, "Guide to Durable
1. A survey of prestressed concrete Concrete," ACI journal, Proceedings V.
producers indicates that most occur- 74, No. 12, December 1977, p. 594.
rences of corrosion of prestressing ten- 2. Clear, K. C., and Hay, R. E., "Time-to-
dons have developed under conditions Corrosion of Reinforcing Steel in Con-
where concrete had been exposed to crete Slabs," Report No. FHWA-RD-
73-32, Federal Highway Administration,
external sources of chloride ion.
April 1973.
2. Based on laboratory tests conducted
3. Stratfull, Richard F., "Half-Cell Potentials
in this program, the threshold water sol- and the Corrosion of Steel in Concrete,"
uble chloride limit above which corro- Highway Research Record No. 433,
sion of prestressing tendons occurred Highway Research Board, 1973, pp. 12-21.
during and immediately after curing was 4. Berman, H. A., "Determination of
between 0.11 and 0.17 percent by Chloride in Hardened Portland Cement
weight of cement. Paste, Mortar and Concrete," Report No.
FHWA-RD-72-12, Federal Highway Ad-
3. Based on laboratory tests conducted
in this program, the threshold water sol- ministration, September 1972.

NOTE: Discussion of this paper is invited. Please submit

your comments to PCI Headquarters by March 1, 1985.
119
PCI JOURNALJJuly-August 1984