TECHBRIEF An FHWA Special Study: 

Post-Tensioning Tendon Grout 

Chloride Thresholds

FHWA Publication No.: FHWA-HRT-14-040

FHWA Contacts: Paul Virmani, HRDI-60, (202) 493-3052, paul.

virmani@dot.gov and Hamid Ghasemi, HRDI-60, (202) 493-

3042, hamid.ghasemi@dot.gov.

This document is a technical summary of the Federal Highway 

Administration report, An FHWA Special Study: Post-Tensioning 

Tendon Grout Chloride Thresholds (FHWA-HRT-14-039).

Introduction

Since the 1970s, the number of pre-tensioned and posttensioned (PT) concrete bridge structures utilizing high-strength 

seven-wire strands has increased steadily. For bonded PT 

tendons, the grout functions as the last layer of corrosion protection for the highly stressed seven-wire strands by providing 

a high pH environment to form a protective oxide film on the 

strand surface and also acting as a physical barrier to water, 

oxygen, and chloride ions. Corrosion of strands initiates when 

the protective oxide film is compromised due to chloride attack 

or carbonation of the surrounding grout upon exposure to 

water and air. Once corrosion starts, it propagates at a certain 

rate controlled by many factors such as oxygen availability, 

moisture content, electrical resistance, grout pH, and chloride 

concentration. In this TechBrief, a prestressed seven-wire strand 

will be simply referred to as a “PT strand.”

Corrosion problems have been observed in some PT bridges 

in Florida, Virginia, and Minnesota mainly due to grout voids, 

poor quality grout, water infiltration, and duct holes/cracking. 

These deficiencies can lead to severe corrosion or failure of the 

PT tendons. A recent discovery of a grout product with elevated 

levels of chloride used in a PT concrete straddle cap located in 

Corpus Christi, TX, resulted in a full investigation of a potential 

corrosion problem. Chloride concentrations were reported to 

be as high as 5.27 percent by weight of cement.(1,2) These 

numbers exceed the current limits set by all domestic and 

international regulatory committees. For example, the chloride 

limits for prestressed concrete imposed by American Concrete 

Institute committees are either 0.06 percent water-soluble 

chloride by weight of cement or 0.08 percent acid-soluble 

chloride by weight of cement. These chloride limits are arbitrary 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

 www.fhwa.dot.gov/research
2

concentrations determined by the code authorities and specifications after taking into account 

a factor of safety. Conversely, chloride threshold

is the actual amount of chloride needed to initiate 

corrosion. There is limited information reported 

in the literature pertaining to the chloride threshold of the PT tendons. As chloride information is 

needed to assess corrosion risk of the PT bridges 

containing the chloride contaminated grout, the 

Federal Highway Administration sponsored a 

6-month accelerated corrosion testing program 

to determine chloride threshold(s) of PT strands 

exposed to chloride-contaminated grout. This 

TechBrief explains how two chloride threshold values of 0.4 and 0.8 percent by weight of 

cement were determined for corrosion initiation 

and corrosion propagation, respectively, using 

supporting test results.

Approach

Test Specimens

A 6-month accelerated corrosion testing program employed three types of laboratory specimens as shown in figure 1 through figure 3. 

The first type was single king (i.e., center) wire 

specimens that were extracted from the regular PT strands. More than 50 specimens were 

tested in aqueous solutions having a low pH of 

9 to simulate carbonated grout and a high pH of 

13.6 to simulate highly alkaline normal grout, 

each with 10 chloride concentrations. Some of 

them were stressed at 60 percent guaranteed 

ultimate tensile strength (GUTS), and the others 

were placed in a no stress condition. The second 

type was 12-inch-long single-strand specimens 

embedded in grout. A total of 40 specimens 

were fabricated, and some were stressed at 

60 percent GUTS and/or contained an artificial 

gout void. The effects of stress, grout void, and 

chloride concentration were to be evaluated in 

an ideal condition.

The third type was 10-ft-long multi-strand specimens encased in steel loading frames. A total 

of eight specimens were prepared to evaluate 

the effects of stress, void, and chloride concentration in the macro-cell corrosion condition, 

which has been responsible for the observed 

severe corrosion problems in the field. To create 

simulated macro-cell corrosion, each specimen 

contained four strands stressed at 60 percent 

GUTS and was passed through an artificial void 

near the top anchor plate. This served as the 

macro-anode. Additionally, a bundle of five 7-ftlong unstressed strands were completely buried 

in grout, which served as the macro-cathode. 

The grout mixes were admixed with eight acidsoluble chloride concentrations ranging from 

0 to 2.0 percent by weight of cement. 

Test Conditions

The single-wire specimens were tested in an 

ambient laboratory condition. The single-strand 

Figure 1. Single-wire specimen.

Figure 2. Single-strand specimens.
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and multi-strand specimens were placed in 

two environmental chambers and went through 

a full exposure cycle of 104 °F and 90 percent 

relative humidity (RH) (hot and humid (H & H)) 

to 77 °F and 60 percent RH (ambient) to 14 °F 

and 40 percent RH (freezing and dry (F & D)) to 

77 °F and 60 percent RH (ambient). Each subcycle lasted 2 weeks, and three full cycles were 

repeated during the 6-month testing period. In 

the middle of the exposure testing, a water 

recharging event was simulated by adding 

0.17 fl oz of distilled water to some of the 

voided single-strand specimens and 5.1 fl oz to 

all of the multi-strand specimens.

Performance Evaluation

Corrosion performance data were collected 

weekly to monitor progressive changes in the 

specimens. Corrosion potential data indicated 

thermodynamic state of corrosion to indicate 

whether corrosion initiation was possible. 

Corrosion kinetics indicating how fast corrosion 

progressed was determined by corrosion rate 

data (single-wire and single-strand specimens) 

or macro-cell corrosion current density data 

(multi-strand specimens). In addition, apparent 

grout resistivity (single-strand specimens) and 

apparent grout resistance (multi-strand specimens) were measured to determine conductivity of the grout which can be related to corrosiveness of the environment. Upon completion 

of the 6-month accelerated corrosion testing, 

an autopsy of the specimens embedded in the 

grout was carried out followed by a thorough 

corrosion damage assessment of the extracted 

strands. Actual damage conditions observed on 

individual specimens exposed to eight chloride 

concentrations were compared to the corresponding non-destructively collected data of the 

specimens. From this, chloride threshold values 

were determined.

Major Findings

The corrosion rate of the single wires tested in 

pH 13.6 solutions was less than 0.05 mil/year at 

chloride concentrations up to 0.6 percent and 

increased at higher chloride concentrations. For 

the specimens tested in pH 9.0 solutions containing 0.04 percent and higher chloride concentrations, typical active corrosion behavior was 

observed—corrosion potentials became more 

negative, and corrosion rates increased. These 

test results suggest that PT strands can tolerate 

chloride contamination without significant corrosion up to 0.6 percent by weight of cement 

in carbonation-free (high pH) grout, whereas as 

low as 0.04 percent chloride by weight of cement 

can initiate active corrosion in the carbonated 

(low pH) grout. This is below the American 

Association of State Highway and Transportation 

Officials specified chloride limit of 0.08 percent 

by weight of cement.(3) Once corrosion starts in 

the low pH environment, the level of corrosion 

damage depends on chloride concentration.

For single-strand specimens, chloride concentration of 0.4 percent by weight of cement was the 

lowest concentration to make the strands more 

prone to corrosion in most conditions. Mean 

corrosion potential gradually became negative 

starting from 0.4 percent chloride. Mean corrosion rate also became progressively higher 

starting from 0.4 percent chloride. Dependence 

of corrosion rate on temperature was observed 

(i.e., higher corrosion rate in H & H cycles and 

negligible corrosion rate in F & D cycles regardless of chloride concentration). The limited data 

suggest that the stressed strands exposed to 

water in the grout void may be more susceptible 

to corrosion than the unstressed ones if chloride 

concentration is higher than 0.08 percent and 

the risk of corrosion is elevated with increased 

Figure 3. Multi-strand specimens.
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chloride concentration. All control specimens 

experienced steadily decreasing corrosion rates 

as their corrosion potential data also indicated 

the passive behavior. 

The first rust spot (considered a sign of corrosion initiation) observed on the single-strand 

specimens began to appear when chloride 

concentration was 0.4 percent. Measurable pits 

deeper than 2 mil (considered a sign of corrosion 

propagation and also the lowest measurement 

limit of a digital pit gauge) were observed when 

chloride concentration was 0.8 percent. Figure 4

through figure 7 show physical conditions of 

the corroding wires removed from 0.4 and 

0.8 percent chloride concentration specimens, 

respectively. The red arrow in figure 4 indicates 

a rust stain matching with the corrosion spot 

marked in the red circle. As chloride concentration increased, severity of the corrosion worsened, particularly for the stressed specimens 

with void. Mean pit depths ranged between 3.6 

and 6.2 mil. A combination of a void, recharged 

water, and elevated chloride concentration can 

be detrimental, evidenced by the markedly 

higher number of pits found on the stressed 

2.0 percent chloride specimen with void. 

Corrosion potential and corrosion rate data of 

single-strand specimens containing 0.4 and 

0.8 percent chloride concentrations are shown 

in figure 8 through figure 11. 

Figure 4. Pitting corrosion on a 0.4 percent chloride 

concentration single-strand specimen.

Figure 5. Close-up view of cleaned rust spot on a 

0.4 percent chloride concentration single-strand 

specimen.

Figure 6. Pitting corrosion on a 0.8 percent chloride 

concentration single-strand specimen. 

Figure 7. Second view of pitting corrosion on a 

0.8 percent chloride concentration single-strand 

specimen.

Figure 8. Electrochemical data of 0.4 percent 

chloride concentration single-strand specimens: 

corrosion potential versus time. 

Figure 9. Electrochemical data of 0.4 percent 

chloride concentration single-strand specimens: 

corrosion rate versus time.

PSE = Pseudo-reference electrode.
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For multi-strand specimens, chloride concentration was a more influential parameter for 

corrosion potential than exposure condition. 

As the macro-cell corrosion setup intended, 

stressed strands exhibited more negative corrosion potentials than unstressed ones, making 

the stressed strands anodic to the unstressed 

ones at any chloride concentration. It is unclear 

if presence of stress or localized corrosion 

at the void/grout interface or both made the 

entire stressed strands active. When chloride 

concentration increased to 0.4 percent, mean 

corrosion potentials of the stressed strands 

decreased to the 90th percentile of corrosion 

probability. Mean macro-cell corrosion current

density also began to increase at 0.4 percent 

chloride. Figure 12 and figure 13 show the 

mean corrosion potential data of the stressed 

strands and mean macro-cell corrosion current 

density data, respectively.

Even though it was not included, the unstressed 

strands exhibited active corrosion potential at 

2.0 percent chloride concentration. The H & H 

and F & D cycles produced the highest and 

the lowest mean macro-cell corrosion current 

densities, respectively, independent of chloride 

concentration. Water recharging did not influence the mean corrosion potentials and mean 

macro-cell current densities significantly. 

An autopsy of the multi-strand specimens 

revealed that, in general, stressed segments 

started to show superficial rust at 0.4 percent 

chloride concentration and heavy rust requiring 

Figure 11. Electrochemical data of 0.8 percent 

chloride concentration single-strand specimens: 

corrosion rate versus time.

Figure 12. Electrochemical data of multi-strand 

specimens: mean corrosion potentials.

Figure 13. Electrochemical data of multi-strand 

specimens: mean macro-cell corrosion current 

densities.

Figure 10. Electrochemical data of 0.8 percent 

chloride concentration single-strand specimens: 

corrosion potential versus time.
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acid cleaning at 0.8 percent chloride concentration. This observation could be made with 

both of the void/grout interface segments and 

in-grout segments. Mean pit depths measured 

on the stressed in-grout strands increased 

from 4.3 to 9.4 mil when chloride concentration increased from 0.4 to 2.0 percent. For 

unstressed strands, superficial rust began to 

show up at 0.6 percent chloride concentration, and none of the pits exceeded 2 mil 

regardless of chloride concentration. Figure 14 

and figure 15 show the numbers of pits 

counted in the in-grout segment of stressed 

and unstressed strands and mean pit depths 

of the stressed strands as a function of chloride concentration, respectively. 

For the interface segments, mean pit depth 

was approximately 6 mil when chloride 

concentration was less than 1.0 percent. At 

2.0 percent chloride concentration, mean pit 

depth increased to 11 mil. The largest measured 

pit depths were 60 and 36 mil for the interface 

segments and in-grout segments, respectively. 

Figure 16 and figure 17 show pitting corrosion 

damage found on 0.4 and 0.8 percent chloride 

concentration multi-strand specimens, respectively. At 0.4 percent chloride concentration, 

the mean pit depth was 4.1 mil with a maximum pit depth of 9.0 mil. At 0.8 percent chloride 

concentration, mean pit depth was 5.9 mil with 

a maximum pit depth of 11 mil.

Crevice corrosion, another form of localized 

corrosion, was also observed on many of the 

multi-strand specimens containing 0.4 percent 

and higher chloride concentrations. Similar to 

the observed pitting corrosion, crevice-induced 

corrosion got worse as chloride concentration 

increased. Figure 18 and figure 19 show typical

crevice corrosion damage found on 0.4 and 

0.8 percent chloride specimens, respectively. 

At 0.4 percent chloride concentration, mean pit 

depth was 4.5 mil with a maximum pit depth of 

8 mil. At 0.8 percent chloride concentration, 

mean pit depth was 4.5 mil with a maximum pit 

depth of 6 mil.

A general trend observed from the multi-strand 

specimens with normal grout at the interface is that more pits were found on the segments closer to the interface when chloride 

concentration was 0.8 and 1.0 percent. When 

chloride concentration was between 0.08 and 

0.6 percent, no measurable pits were found at 

all. This finding suggests that stressed strands 

near the interface were subjected to the most 

intensive macro-cell corrosion if chloride concentration was 0.8 percent chloride. At this level 

of chloride, a small surface area of the interface 

became the most anodic in the presence of 

abundant dissolved oxygen and moisture in the 

void. The number of pits tended to decrease as 

a segment was further away from the interface. 

It is thought that this phenomenon was related 

to weakened macro-cell corrosion current in the 

deeper grout. Mean pit depth data did not follow the same trend in that it was fairly uniform 

around 6 mil along the most part of the stressed 

strand length including the interface section.

Figure 14. Number of pits measured on multistrand specimens.

Figure 15. Mean pit depths measured on multistrand specimens.
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Conclusions

The corrosion process of PT strands is divided 

into two stages: corrosion initiation and corrosion

 propagation. Based on the test results obtained 

through a 6-month accelerated corrosion testing 

program and a subsequent autopsy of the specimens, two chloride threshold values, one for 

each stage, were determined for the PT strands 

fully encased in normal grout. 

Figure 16. Pitting corrosion observed on a stressed wire of 0.4 percent chloride concentration multi-strand 

specimen.

Figure 17. Pitting corrosion observed on a stressed wire of 0.8 percent chloride concentration multi-strand 

specimen.

Figure 18. Crevice corrosion between two stressed wires on 0.4 percent chloride concentration multi-strand 

specimen.

Figure 19. Crevice corrosion between two stressed wires on 0.8 percent chloride concentration multi-strand 

specimen.
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Researchers—This study was performed by the Center for Advanced Infrastructure and Transportation of Rutgers 

University, 100 Brett Road, Piscataway, NJ 08854.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being 

made to the Divisions and Resource Center.

Availability—The TechBrief may be obtained from the FHWA Product Distribution Center by e-mail to report.

center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.gov/research.

Key Words—Chloride threshold, Chloride, Corrosion initiation, Corrosion propagation, Seven-wire strand, Posttensioning, Grout, Autopsy, Accelerated corrosion testing, Sulfate ions, Void.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in the 

interest of information exchange. The U.S. Government assumes no liability for the use of the information contained 

in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ 

names appear in this TechBrief only because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides high-quality information to serve 

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reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

MARCH 2014 FHWA-HRT-14-040

HRDI-60/03-14(600)E

The first threshold is 0.4 percent chloride concentration by weight of cement, which is the 

lowest amount to initiate corrosion of strands. 

At this threshold, rust spots formed, and small 

number of pits started to form beneath some 

of the rust spots. It is a conservative number, 

and there is still sufficient time to mitigate future 

corrosion problems if necessary. Therefore, it 

can serve as the lower limit in assessing longerterm corrosion risk of PT bridges. 

The second critical chloride threshold is found in 

association with corrosion propagation. At this 

threshold, corrosion started to intensify in term 

of number of pits and pit depth. Test data and 

autopsy results obtained in this study indicated 

that 0.8 percent chloride by weight of cement 

is the critical threshold. This threshold should 

be considered as the upper limit in corrosion 

risk assessment. Once chloride concentration 

exceeds the critical threshold, significant corrosion damage can occur rapidly, and structural 

integrity of the PT bridges may be compromised 

in the near future. The actual deterioration rate 

can be influenced by many factors such as moisture content, temperature, oxygen availability, 

and grout resistivity. 

It should be emphasized that the threshold 

values are applicable to normal grout condition

only. They may not be adequate in other conditions such as carbonated grout, segregated 

grout, duct cracks, grout voids filled with water 

with or without chloride ions, and free sulfate ions in contact with the strands. In these 

circumstances, corrosion should start below 

the proposed threshold values. 

References

1. Merrill, B.D. (2010). Memorandum—Grout 

Testing and Analysis of Carbon Plant 

Road Over IH 37, Texas Department of 

Transportation, Austin, TX, 

2. Holt, R. (2014). Results from the Sika Grout 

Study, Presentation at the FHWA Long-Term 

Bridge Performance Program Workshop, 

93d Transportation Research Board Annual 

Conference, Washington DC.

3. American Association of State Highway and 

Transportation Officials. (2010). LRFD Bridge 

Construction Specifications, 3d Edition, 

AASHTO, Washington, DC.