Study on Cover Depth for Prestressed Concrete Bridges in ... Journal/2006/March_and_April_2006/Study...prestressed concrete is of higher quality because the cover depth and concrete

42 PCIJOURNAL

Study on Cover Depth for Prestressed Concrete Bridges in Airborne-Chloride Environments

Yoshiki TanakaSenior Research EngineerBridge Structure Research TeamPublic Works Research InstituteTsukuba, Japan

In Japan, prestressed/reinforced concrete bridges

often suffer deterioration from steel corrosion in the

chloride-laden air of marine environments with high

concentrations of airborne chloride particles. Only a

few decades after erection, several prestressed concrete

bridges have already been replaced due to significant

corrosion in the main girders. To develop a reasonable

design policy for building more durable concrete

bridges that require less maintenance, the performance

of conventional concrete cover as corrosion protection

in coastal environments needs to be properly evaluated.

In this paper, the authors address the effectiveness of

conventional concrete cover with a water-cement ratio

of 0.36 and 0.43 in airborne-chloride environments

and propose a realistic evaluation method based on

Japanese surveys of airborne chloride distribution and

deterioration in existing concrete highway bridges.

The proposed evaluation method uses the chloride

diffusion coefficient for uncracked concrete exposed

to the choride atmosphere and the boundary chloride

level based on its relationship to airborne chloride

levels. The results of this research provide the basis

for a cogent policy for corrosion protection in coastal

areas for concrete bridges with 100-year design lives.

Hirotaka Kawano, Ph.D.Director for Materials and Geological

Engineering Research GroupPublic Works Research Institute

Tsukuba, Japan

Hiroshi Watanabe, M.S.Team LeaderStructure Management Technology Research TeamPublic Works Research InstituteTsukuba, Japan

Tomoyoshi NakajoPrincipal Bridge Engineer

Japan Prestressed Concrete Contractors Association

Tokyo, Japan

durability

March–April2006 43

Chlorides that can lead to premature deterioration of reinforced and prestressed concrete bridges are abun-dantly supplied by marine environs or by roadway

deicing chemicals, creating a corrosive environment for steel reinforcing. Chloride-induced deterioration often results not only in increased bridge maintenance but also in a shortened structural life for reinforced, post-tensioned, and precast, prestressed concrete structures. In the worst cases, significant corrosion has led to replacement of concrete bridges.1 Thus, corrosion protection is a high priority for transportation au-thorities concerned about improving concrete bridge sustain-ability in corrosive environments.

Since the late 1960s in the United States, premature and serious deterioration of bridge decks has often occurred as a result of the increased use of deicing salts.2 In the mid-1980s, premature, chloride-induced deterioration of concrete bridge piers containing epoxy-coated steel reinforcing bars was documented for bridges in the Florida Keys.3,4 In research to find solutions for early infrastructure deterioration, numerous studies were carried out involving various methods of corro-sion protection, such as increased concrete cover depth, use of low-permeability concrete, designs with epoxy-coated re-inforcing bars, and cathodic protection. Recently, new metal-lic reinforcing bars similar in material properties to stainless steels have also been studied as alternative designs.5–7 Several of these investigations reported that conventional concrete with increased cover specifications and lowered water-ce-ment ratios (w/c) seemed effective against chloride ingress.8,9 Currently in Florida, use of low-permeability concrete is rec-ommended as the most effective corrosion protection for new

bridge construction in coastal areas.10

In contrast to the U.S. experience, significant corrosion has been observed in concrete bridge superstructures near the coast of Japan since the late 1970s.11,12 The Guide Specifica-tions on Corrosion Protection for Highway Bridges recom-mended an increased cover for conventional concrete struc-tures.13 Although these guidelines led to improved durability in general for concrete bridges in coastal areas, the adequacy of the 1984 specifications was not verified with respect to ensuring a 100-year design life. In efforts to create a valid design policy for more durable and lower-maintenance con-crete bridges, the in-place effectiveness of the conventional concrete cover design needs to be evaluated. In Japan, the United States, and other countries, the common concern re-garding premature chloride-induced deterioration of concrete bridge structures will require ongoing research.

For predicting chloride ingress, some evaluation tools have been established.14,15 To date, these tools have some difficul-ties in determining the valid chloride boundary condition at the concrete surface. For structures in marine locations, it has long been recognized that invisible airborne chloride particles are mainly responsible for concrete bridge deterioration.13 In 1993, a nationwide survey established a relationship be-tween a concentration level of invisible chloride particles and the chloride boundary condition.16 A quantitative evaluation method for adequate concrete cover depth in relation to air-borne chloride levels, however, has not yet been developed.

In this paper, the authors address the effectiveness of con-ventional concrete cover as corrosion protection against air-borne chloride through a proposed realistic evaluation meth-

Fig. 1. Schematic presentation of chloride attack in airborne-chloride environments and factors that influence corrosion in reinforced concrete.

– Inside of concrete is generally not corrosive because of high- alkaline environment

– Much chloride becomes corrosive even in concrete

– Supply of oxygen and water have an effect, as well as chloride

Corrosion in concrete– Regional difference in same structure

Chloride on surface

– Decided at design– Error in formwork– Slippage during concreting

Cover depth

Depends on:– Quality of concrete – Type of concrete – Water-cement ratio – Construction conditions – Deterioration such as cracking,

carbonation

Chloride permeability

– Regional difference caused by wind, wave, and topography

– Reduction depending on distance from coastline

Airborne chloride

44 PCIJOURNAL44 PCIJOURNAL

od based on research and field survey data. The evaluation method uses the chloride diffusion coefficient obtained from exposure tests of uncracked concrete in the airborne-chloride environment and a boundary chloride level based on its rela-tionship with airborne chloride. The values of the diffusion coefficients are compared with distributions of diffusion co-efficient data obtained from field surveys of existing bridge girders. Predicted cover depths are comprehensively verified by comparison with deterioration in existing concrete bridg-es. From the results of this research, a corrosion protection specification for a 100-year design life in concrete bridges in coastal areas is presented.

CorroSioN oF CoNCrETE BriDgES iN JAPAN

By 2001, approximately 140,000 highway bridges with a bridge length of more than 49 ft (15 m), including about 80,000 concrete bridges, have been constructed in Japan. It is estimated that about 15% of these structures were locat-ed within 1640 ft (500 m) of the coastline, where airborne chloride has caused corrosion problems in concrete bridges. Several prestressed concrete bridges, including both post-tensioned and precast/prestressed, were replaced due to sig-nificant corrosion only a few decades after completion.17,18 In some cases, the cumulative maintenance costs at replacement of the deteriorated bridges exceeded the initial construction cost.1

A schematic of chloride-induced deterioration in un-cracked concrete is illustrated in Fig. 1. Micropores and gel water, typically present in hardened concrete, serve as routes for chloride ion transport. The concrete matrix pore structure depends on the type of cement used, mixture proportions, and concrete quality. In the airborne-chloride environment of coastlines, a large amount of chloride adheres to con-crete surfaces and chloride ions permeate the concrete, reaching the steel reinforcing bars. Chloride ions can break the passive oxide film on black reinforcing steel and initi-ate corrosion—even under corrosion-deterrent conditions of

Fig. 2. Corrosion-induced deterioration of post-tensioned concrete bridge girders before the first repairs, only 16 years after completion.

Table 1. Minimum Specified Cover Depth for Highway Bridges.19

Types of Members

Deck, Curb, Railing, Slab Bridges with Span of Less Than 10 m

Beams

Precast Concrete Members

Beams Except Precast Con-crete, Slab

Bridges with Span of More

Than 10 m

Minimum cover depth

30 mm 25 mm 35 mm

Note: 1 mm = 0.0394 in.; 1 m = 3.28 ft.


high alkalinity in the concrete. The expansion of corroded reinforcing bars causes severe cracking and spalling of the surrounding concrete. Once large cracks typical of chloride-induced corrosion are present, even major repairs may prove ineffective. It is, unfortunately, difficult to detect the exis-tence of early products of corrosion in uncracked concrete with typical bridge inspection methods.

Design Cover Depth Before 1984

An adequate concrete cover depth is required to protect steel reinforcing bars from corrosion, maintain sufficient concrete-to-steel bond strength, and facilitate concrete placement. Particularly in coastal areas, the cover depth is specified for corrosion protection. The minimum cover depths prescribed in the 1978 Design Specifications for Highway Bridges are shown in Table 1.19 Most existing Japanese highway bridge superstructures in marine conditions were designed accord-ing to 1978 specifications (or earlier guidelines with similar cover-depth minimums) until publication of new guidelines in 1984.

Nationwide Survey

Corrosion-induced deterioration of concrete bridges has been evident on Japan’s coast since the late 1970s. Instances of severe deterioration in post-tensioned and precast, pre-stressed concrete bridges are shown in Fig. 2 and 3. In 1981, significant corrosion of post-tensioning tendons in a bridge

above seawater was found during the initial repair. A dissec-tion conducted after replacement of the 34-year-old bridge revealed that only 21% of the original cross-sectional tendon area remained at the most severely deteriorated section of the girder.17

In 1982, the Public Works Research Institute in Tsukuba, Japan, conducted a nationwide survey to identify damage in concrete highway bridges located within 1640 ft (500 m) of the coast. Figure 4 shows a survey map of deficient bridges. Regional differences in the levels of deterioration of concrete bridges were identified. High levels of bridge deterioration were often located in Regions A and B (Fig. 5). Distribution of deteriorated bridges is shown in Fig. 6a and 6b, with the distance from the coastline and service life. It can be seen that most of the significantly deteriorated bridges are located within 330 ft (100 m) of the coast.

It is important to note that the data in Fig. 6a and 6b in-dicate that corrosion-induced deterioration of precast, pre-stressed concrete bridges was seldom found, even with pre-cast, prestressed concrete’s relatively shallower cover depth compared with that of post-tensioned and reinforced concrete bridges. These results indicate that precast, prestressed con-crete is generally more durable than cast-in-place concrete and that this durability is a result of precast concrete’s inher-ent manufacturing quality controls. Manufactured precast, prestressed concrete is of higher quality because the cover depth and concrete quality are ensured through accurate plant

Fig. 3. Corrosion-induced deterioration of precast, prestressed concrete bridge girders before repairs, 13 years after construction.


location of reinforcing steel and lower forming and placing tolerances than that of cast-in-place concrete.

Two decades later, in 2000, a follow-up survey focused on repaired bridges in Japanese coastal areas.1 The chloride profile data from concrete cores taken from existing concrete girders during repair work were gathered. The data provided distributions of the chloride ion diffusion coefficients for each type of structure, as shown in Fig. 7. The study assumed that all cores were made of ordinary concrete because the ad-dition of pozzolans is uncommon for bridge superstructures in Japan. From Fig. 7, it can be seen that precast, prestressed concrete typically presented a much lower diffusion coeffi-cient. This data agreed with earlier research results that found a lower w/c demonstrated a lower diffusion coefficient.20–22 Typically, precast, prestressed concrete is specified at a lower w/c (for obtaining high-early strength) than that for cast-in-place concrete.

Design Cover Depth After 1984

Based on the results of the bridge deterioration survey, the Guide Specifications were published in 1984.13 The major

Fig. 4. Survey map of deficient bridges located within 1640 ft (500 m) of Japan’s coastline in 1982.

Fig. 5. Map identifies regions referenced in the 1984 Guide Specifications on Corrosion Protection for Highway Bridges.13

The distance from Tokyo to Osaka is about 250 miles (400 km), and to Okinawa, about 990 miles (1600 km).The total length of coastline in Japan is about 22,000 miles (35,000 km) according to the national coastal statistics


Fig. 6a. Distribution of deficient concrete bridges in Regions A and B except precast, prestressed concrete bridges. Note: 1 m = 3.28 ft.

Fig. 6b. Distribution of deficient precast, prestressed concrete bridges in Regions A and B. Note: 1 m = 3.28 ft.


recommendations in the Guide Specifications were to in-crease the cover depth, depending on the distance from the coastline in each region, as shown in Tables 2, 3, and 4 and Fig. 5; restrict the maximum w/c allowable; control flexural cracking; and use the optional measures of epoxy-coated re-inforcing bars or coating on concrete surfaces.

A follow-up survey showed that before 1995, more than 400 concrete highway bridges were completed according to the 1984 Guide Specifications.1 About two decades have passed since completion of bridges using the 1984 guidelines, and significant bridge deterioration has not been observed, with the exception of a few cases of very poor workmanship. As a result of this survey, the Guide Specifications has been acknowledged for its contributions to improved durability of concrete bridges—especially in light of the fact that the

majority of significant deterioration of existing bridges built before the 1984 guidelines occurred within 15 years of con-struction. To ensure the durability of bridges designed for a 100-year life, however, the required cover depths need to be reassessed according to an evaluation method incorporating extended design life.

CovEr DEPTH EvAluATioN METHoD

The process of chloride ingress into concrete is usually ex-pressed by Fick’s equation. For evaluating the required cover depth, a basic formula was prepared based on Crank’s solu-tion of the equation as shown in Eq. (1):

C x t C erfx

D tC Co

c

init corr,( ) = −

+ <1

2 (1)

wherex = depth from concrete surfacet = timeCo = boundary chloride level at concrete surfaceerf = error functionDc = diffusion coefficientCcorr = chloride threshold level to initiate corrosionCinit = initial chloride level in concrete

Various models for predicting chloride ingress have been presented in literature to date. The complex models, how-ever, often have difficulty in establishing validity because time-dependent data required for analyzing the influences of hydration and environmental conditions on chloride ingress (such as temperature, humidity, carbonation, and chloride ions’ absorption into hydrated compounds) have yet to be ob-tained. Although Eq. (1) may not always precisely express the actual process, it is the simplest equation available to express the process of chloride ingress with a minimum number of essential parameters x, t, and Co corresponding to environ-mental inputs and Dc dependent on cement type and mixture proportions. Since this simple model has been applied in the analysis of numerous tests and field investigations, a relative abundance of data for the surface chloride level and the dif-

Table 2. Regions Requiring Preventative Measures Against Chloride Attack.

Legends Regions Requiring Preventative Measures

A Okinawa Prefecture

B Areas within 300 m of coast and above sea in regions shown in Fig. 5

C Areas within 200 m of coast and above sea ex-cept the Regions A and B

Note: 1 m = 3.28 ft.

Table 3. Measure Levels.

Regions Distance from Coast Level

ALess than 100 m and above sea I

Except the above region II

B

Less than 100 m and above sea I

100–200 m II

200–300 m III

C

Above sea I

Less than 100 m II

100–200 m III

Level of corrosion protection required based on geographic location, with I being the most protection required and III being the least.Note: 1 m = 3.28 ft.

Fig. 7. Diffusion coefficients of chloride ions for concrete cores taken from existing bridges. Ordinary design concrete strength is shown in parentheses. Note: 1 MPa = 145 psi; 1 m2/s = 10.7 ft2/s.


fusion coefficient based on this model have been amassed. Therefore, it was decided to adopt Eq. (1) as the most practi-cal and valid evaluation method for durability design avail-able at this time.

In this paper, the required cover depth is defined as the minimum cover depth for preventing chloride concentration at reinforcing steel surface in concrete from exceeding the chloride threshold level during the specified design lifetime.

Airborne Chloride

For three years, beginning in 1984, a nationwide survey of the airborne chloride distribution in coastal areas was con-ducted.16 The mass of chloride ions captured on a stainless steel plate with a 3.9 in. × 3.9 in. (0.1 m × 0.1 m) area was measured at 266 observation points within 10 km (6.2 miles) of the coast every month. In this paper, the airborne chloride level at each observation point is represented by the annual average. In Fig. 8, the result in Region B is shown as an ex-ample of the measured airborne chloride in relation to the distance from the coastline. It can be seen that the logarithm of airborne chloride, Cair, has a linear relation to the loga-rithm of the distance. From the nationwide data, it was found that the slope of the relation was typically constant regardless of region, whereas the point interceptions varied by region. Consequently, Eq. (2) was proposed for predicting the air-borne chloride level.

C C dair = −1

0 6. (2)

whereCair = airborne chloride level (mdd of Cl-) Note: The unit “mdd” is an abbreviation of “mg/100 cm2/

day.”C1 = regional coefficientd = distance from the coastline (km)

The regional coefficient, C1, is to express the airborne chlo-ride level in each region. The coefficient for regions where se-vere corrosion-induced deterioration was frequently observed was larger than that of the other regions. It was concluded that sea winds and wave conditions were mainly responsible for the regional differences recorded. The values for C1 for Regions A, B, and C were determined to be 0.6, 0.6, and 0.2, respectively.

Boundary Chloride level

Blocks made of conventional concrete with w/c of 0.39 and 0.58 were simultaneously exposed to airborne chloride at 76 observation points.11,16 The surface chloride level (the boundary chloride level, Co) of each block was determined in relation to the airborne chloride level, Cair. In Fig. 9, this correlation can be represented by Eq. (3):

C Co air=1 5 0 4. . (3)

whereCair = airborne chloride level (mdd of Cl-)Co = boundary chloride level on concrete surfaceNote: The equation was determined with a unit of Co de-

fined as “kg of Cl-/m3.”

In other investigations, it was found that the surface chlo-ride level of concrete in the splash zone ranged from about 17 lb/yd3 to 34 lb/yd3 (10 kg/m3 to 20 kg/m3).1,10

Table 4. Minimum Cover Depth Described in the Guide Specifications (mm).13

Level

Superstructure Substructure

Bottom of Deck, Curb, Railing

Beam

Beam ColumnPrecast, PrestressedConcrete

Other Type

I 50 50 70 70 70

II 40 35 50 50 50

III 30 25 35 35 40

Note: 1 mm = 0.0394 in.

Fig. 8. Survey results for Region B show the relationship between measured airborne chloride and the distance from the coastline.Note: 1 km = 0.622 miles.


Diffusion Coefficient

The diffusion coefficient, Dc, of chloride ions in concrete depends on many factors, including concrete pore structure, mixture proportions, cement types, and curing conditions. In particular, the w/c is the most significant factor influencing the diffusion coefficient in uncracked ordinary concrete.

In Fig. 10, the relationship between the diffusion coef-ficient and w/c is plotted. The resulting data were obtained from an exposure test of high-early-strength portland cement

concrete under the airborne-chloride environment in coastal areas.20 It can be seen that with the w/c ranging from 0.25 to 0.55, a lower value of w/c results in a lesser diffusion coef-ficient, especially notable for concrete with w/c values less than 0.4. This data agrees with survey results indicating that precast, prestressed concrete bridges with w/c of 0.3 to 0.4 rarely deteriorated. In addition, results from the exposure test were similar to the average values of the diffusion coefficient distributions shown in Fig. 7.

Research on the exposure tests revealed that the diffusion coefficients obtained were about one-tenth less than typical ponding test data despite use of the same concrete batch and w/c.20 It should be noted that the relationship shown in Fig. 10 is not applicable for structures submerged in chloride solu-tions or seawater. In this study, the effects of pozzolans were not considered, though other research has shown that ground granulated blast furnace slag, silica fume, and fly ash are ef-fective in creating a tight concrete pore structure if proper curing procedures are used.

Chloride Threshold level

Worldwide, an accepted common value of the chloride threshold level for black steels has not been developed. In the United States, chloride threshold value is commonly recog-nized as 1.2 lb/yd3 (0.7 kg/m3) of acid-soluble chloride ions.6,23 In Japan, the acceptable value is from 2.0 lb/yd3 to 4.2 lb/yd3 (1.2 kg/m3 to 2.5 kg/m3). These chloride threshold value differ-ences may be the result of testing conditions, a lack of precise definitions for corrosion level, and a non-standard definition of the threshold level.

Fig. 9. Relationship between Co and Cair. Note: 1 kg/m3 = 1.68 lb/yd3.

Fig. 10. Relationship between Dc and w/c. Note: 1 m2/s = 10.7 ft2/s.

AirbornechlorideCair(mddofCI-)

C0=1.5Cair0.4

C0

(kg

ofC

I- /m3 )

Dc=5×10-11e-1.6(c/w)

Diff

usio

nco

effic

ient

Dc(

m2 /

s)

w/c


Chloride threshold values in Japan were initially obtained from two experimental studies.24,25 Some investigations of existing structures showed that corrosion of black steels was seldom observed in concrete with a chloride content of less than 2.0 lb/yd3 (1.2 kg/m3) and that concrete surrounding sig-nificantly corroded steel was often contaminated by chloride ion concentrations of more than 4.2 lb/yd3 (2.5 kg/m3).12,17 In this paper, the chloride threshold value is assumed to be 2.0 lb/yd3 (1.2 kg/m3).

Since corrosion of black steel also depends on the supply of oxygen and the moisture conditions—even within the con-crete—the depth and the permeability of concrete cover may influence the chloride threshold level. The impact of these factors, however, has not yet been identified.

initial Chloride level

Currently, the initial chloride content in fresh concrete should be controlled to less than 0.50 lb/yd3 (0.3 kg/m3) in Japan. A survey showed that most Japanese ready-mixed concrete plants control the initial chloride level to less than 0.17 lb/yd3 (0.1 kg/m3).26 Another survey focusing on precast, prestressed concrete manufacturing facilities indicated that these operations can control the chloride level to less than 0.17 lb/yd3 (0.1 kg/m3).11

CoMPrEHENSivE vEriFiCATioN

In order to verify the proposed evaluation method, required cover depths were calculated using Eq. 1, 2, and 3 with test parameters as follows:

C1 = 0.6 for Regions A and B and 0.2 for Region CCcorr = 2.0 lb/yd3 (1.2 kg/m3)Cinit = 0.5 lb/yd3 (0.3 kg/m3)Dc was defined by the function with w/c as shown in

Fig. 10.w/c = 0.36 and 0.43, corresponding to precast, prestressed

concrete and post-tensioned concrete, respectively.Design lifetime = 50 years

Comparison with Bridge Deterioration Data

Calculated cover depths for 50-year design life are shown in Fig. 11a and 11b. The following results were obtained:

1. Until establishing the Guide Specifications in 1984, the minimum cover depth for post-tensioned concrete bridges was 1.4 in. (35 mm), even in highly corro-sive coastal areas. From data in Fig. 11a, research-ers concluded that 1.4 in. (35 mm) of concrete cover was insufficient for concretes with w/c of 0.43 within several hundred meters of the coast in Regions A and B. Similarly, Fig. 11b shows that 1.0 in. (25 mm) of concrete cover prescribed for precast, prestressed concrete bridges was insufficient in the same areas. These results help to explain why significant corro-sion was frequently observed on concrete bridges completed in coastal areas in Japan before 1984.

2. Calculated cover depths for 50-year design life were similar to the minimum cover depth recommended in the Guide Specifications as drawn by a stepped line in each figure. The results agree with survey results

indicating that corrosion signs were rarely observed on bridges built after 1984. Consequently, the mini-mum cover depths recommended in the 1984 Guide Specifications were reassessed as adequate for 50-year bridge design life according to the evaluation method.

3. The required cover depths for concrete with w/c of 0.43 located at more than 1000 ft (300 m) in-land from the coastline were less than the minimum cover depth of 1.4 in. (35 mm). This cover depth is adequate because it is supported by results show-ing that significant corrosion was negligible on post-tensioned concrete bridges in these areas.

Fig. 11a. Concrete cover depth required for post-tensioned concrete with w/c of 0.43. Note: 1 mm = 0.0394 in.; 1 km = 0.622 miles.

Fig. 11b. Cover depth required for precast, prestressed concrete with w/c of 0.36. Note: 1 mm = 0.0394 in.; 1 km = 0.622 miles.


As a result of this study, the proposed evaluation method and test parameters were deemed acceptable to appropriately estimate the required cover depth for prestressed concrete su-perstructures in coastal areas. The results for 50-year design life structures increased understanding of chloride-induced deterioration in concrete bridges in a marine environment.

CorroSioN ProTECTioN For 100-YEAr DESigN

According to the proposed evaluation method, the cover depths required for 100-year design life of uncracked con-crete were predicted as illustrated in Fig. 11a and 11b. The results indicated that the minimum cover prescribed in the 1984 Guide Specifications was insufficient for 100-year bridge designs. Researchers concluded that the extension of bridge design life to 100 years requires increased concrete cover depth in coastal areas. Results indicate that the area re-quiring increased concrete cover depth for bridges should be expanded several hundred meters inland. In addition, within several dozen meters of the coast, cover should be increased to more than 2.8 in. (70 mm) or alternative corrosion-protec-tion measures should be taken. Particularly for superstruc-tures in these areas, it may be advisable to use less-permeable concretes or use epoxy-coated, stainless clad, or stainless steel reinforcement; increased cover can significantly add to superstructure dead loads and a greater risk of shrinkage-in-duced cracking.

In the United States, stainless clad or stainless steel re-inforcing bars have been used in several bridge decks. A five-year-long U.S. Federal Highway Administration study revealed that the chloride threshold level for stainless steels (Type 304 and 316) in concrete was from 18 lb/yd3 to 30 lb/yd3 (11 kg/m3 to 18kg/m3).5–7 In aggressive marine environments, it may be valuable to consider use of stainless steel reinforcing as a practical way to improve durability of bridge superstructures, even though it is more expensive than black steel.

Based on study results, a proposed specification for corro-sion protection for 100-year design in coastal areas is sum-marized in Fig. 12. It is noteworthy that the follow-up sur-vey of existing concrete highway bridges revealed that a few bridges designed according to the 1984 Guide Specifications were inadvertently built with failure to meet cover tolerance.1 Therefore, quality control assessment of as-built concrete cover depth should be improved by effective post-construc-tion inspection involving nondestructive testing.

CoNCluSioNS AND rECoMMENDATioNS

In this paper, the required cover depth for prestressed con-crete bridges in coastal areas was assessed according to the proposed evaluation method and verified by comparison with bridge deterioration surveys and follow-up research. The results of this investigation indicated that the conventional cover depths of 1 in. to 1.4 in. (25 mm to 35 mm) were not sufficient as a corrosion protection for concrete bridges lo-cated in coastal areas. The minimum cover depths prescribed in the 1984 Guide Specifications, however, are sufficient for bridges with 50-year design life.

The cover depth required for 100-year design life indi-cated that the 1984 Guide Specifications were insufficient. Results showed that the cover depth prescribed in the Guide Specifications should be increased about 0.8 in. (20 mm). Further, regions where bridges need increased concrete cover should be expanded about several hundred meters in-land. It was also suggested that an increased cover alone may no longer be appropriate for superstructures within several dozen meters of the coast.

Precast, prestressed concretes have a greater potential du-rability against chloride attack because of inherently lower w/c values for satisfying high-early strengths, as well as good manufacturing quality control. This study showed that concretes with lower w/c values (less than 0.4) typically provided durability advantage in precast, prestressed con-crete based on as-built data obtained from several surveys of existing bridges.

The quantitative evaluation of the airborne chloride plays a crucial role in establishing a valid and economically feasible durability design for bridge superstructures in coastal areas.

Note: Although cracking due to shrinkage or flexure may affect the durability of concrete structures, only corrosion-in-duced cracking is addressed in this paper because the research focused primarily on the durability of prestressed concrete superstructures—where prestressing forces are expected to mitigate shrinkage and flexural cracking.5,8,27

ACKNoWlEDgMENTS

The authors thank the many staff members in the Ministry of Land, Infrastructure, and Transport (formerly the Ministry of Construction and the Hokkaido Development Bureau); the Okinawa General Bureau; and all prefecture offices that as-sisted with the surveys presented in this paper. The authors express their gratitude to the PCI Journal reviewers for their constructive comments.

Fig. 12. Schematic summary of proposed corrosion protection policy for concrete highway bridges with 100-year design life in coastal areas.


rEFErENCES1. Public Works Research Institute, March 2001, Investigation

on Life Cycle Cost of Concrete Bridges—Deterioration and Maintenance Cost of Concrete Bridge, Technical Memorandum of PWRI No. 3811, Tsukuba, Japan. (in Japanese)

2. Virmani, Y. P. and Clemena, G. G., September 1998, “Corrosion Protection—Concrete Bridges,” Report No. FHWA-RD-98-088, Federal Highway Administration, Washington, D.C.

3. Kessler, R. J. and Powers, R. G., September 1987, “Corrosion evaluation of substructure Long Key Bridge,” Interim Report, Florida Department of Transportation.

4. Kessler, R. J. and Powers, R. G., August 1988, “Corrosion of Epoxy Coated Rebar Keys Segmental Bridges Monroe County,” Report No. 88-8A, Florida Department of Transportation.

5. Pfeifer, D. W., 2000, “High Performance Concrete and Reinforcing Steel with a 100-Year Service Life,” PCI Journal, V. 45, No. 3, May–June 2000, pp. 46–54.

6. McDonald, D. B.; Pfeifer, D. W.; and Sherman, M. R., December 1998, “Corrosion Evaluation of Epoxy-Coated, Metallic Reinforcing Bars in Concrete,” Report No. FHWA-RD-98-153, Federal Highway Administration, Washington, D.C., 137 pp.

7. McDonald, D. B.; Sherman, M. R.; Pfeifer, D. W.; and Virmani, Y. P., 1995, “Stainless Steel Reinforcing as Corrosion Protection,” Concrete International, V. 17, No. 5, May 1995, pp. 65–70.

8. Poston, R. W.; Carrasquillo, R. L.; and Breen, J. E., 1987, “Durability of Post-tensioned Bridge Decks,” ACI Materials Journal, July–August 1987, pp. 315–326.

9. Smith, J. L. and Virmani, Y. P., August 1996, “Performance of Epoxy Coated Rebars in Bridge Decks,” Report No. FHWA-RD-96-092, Federal Highway Administration, Washington, D.C.

10. Sagues, A. A., et al., May 1994, “Corrosion of Epoxy Coated Rebar in Florida Bridges,” Final Report, Florida Department of Transportation.

11. Public Works Research Institute, December 2000, “Study on Prestressed Concrete Bridges Minimizing Maintenance—Required Cover Depth for Concrete Highway Bridges,” Joint Research Report No. 258, Tsukuba, Japan. (in Japanese)

12. Public Works Research Institute, March 2001, “Study on Prestressed Concrete Bridges Minimizing Maintenance—Corrosion Protection for Prestressed Concrete Bridges,” Joint Research Report No. 270, Tsukuba, Japan. (in Japanese)

13. Japan Road Association, February 1984, The Guide Specifications on Corrosion Protection for Highway Bridges. (in Japanese)

14. Thomas, M. D. A. and Bentz, E. C., October 2000, “Life-365 Computer Program for Predicting the Service Life and Life-Cycle Costs of Reinforced Concrete Exposed to Chlorides.”

15. Japan Society of Civil Engineers, January 2000, Standard Specifications for Concrete Structures. (in Japanese)

16. Public Works Research Institute, March 1993, “Nationwide Survey on Airborne Chlorides,” Technical Memorandum of PWRI No. 3175, Tsukuba, Japan. (in Japanese)

17. Tanaka, Y.; Kawano, H.; Watanabe, H.; and Kimura, T., June

2001, “Chloride-Induced Deterioration and Its Influence on Load Carrying Capacity of Post-Tensioned Concrete Bridges,” Proceedings of Third International Conference on Concrete under Severe Conditions, Vancouver, B.C., pp. 495–502.

18. Tanaka, Y.; Kawano, H.; Watanabe, H.; and Suzuki, M., November 1999, “Bending Behavior of Prestressed Concrete Girders Damaged by Salt Attack,” Proceedings of 15th U.S.-Japan Bridge Engineering Workshop, Technical Memorandum of PWRI No. 3694, Public Works Research Institute, Tsukuba, Japan, pp. 291–300.

19. Japan Road Association, 1978, Design Specifications for Highway Bridges. (in Japanese)

20. Tanaka, Y.; Fujita, M.; Cheong, H.; Watanabe, H.; and Kawano, H., October 2002, “Chloride Permeability of High-Strength Concrete,” Proceedings of fib 2002 Osaka Congress, pp. 145–154.

21. Sherman, M. R.; McDonald, D. B.; and Pfeifer, D. W., 1996, “Durability Aspects of Precast, Prestressed Concrete—Part 2: Chloride Permeability Study,” PCI Journal, V. 41, No. 4, July–August 1996, pp. 75–95.

22. Sherman, M. R.; McDonald, D. B.; and Pfeifer, D. W., 1996, “Durability Aspects of Precast, Prestressed Concrete—Part 1: Historical Review,” PCI Journal, V. 41, No. 4, July–August 1996, pp. 62–74.

23. Hope, B. B. and Ip, A. K. C., 1987, “Chloride Corrosion Threshold in Concrete,” ACI Materials Journal, July–August 1987, pp. 307–314.

24. Miyagawa, T., February 1985, Early Chloride Corrosion of Reinforcing Steel in Concrete, Doctoral thesis, University of Kyoto.

25. Otsuki, N.; Yokoi, T.; and Shimozawa, O., August 1985, “Influence of Chlorides on Passivation Film on Surface of Steel Bars in Mortar,” Proceedings of JSCE, No. 360, Japan Society for Civil Engineers, pp. 111–118. (in Japanese)

26. Public Works Research Institute, November 2001, Second Investigation on Ready-Mixed Concrete Quality in 1999, Technical Memorandum of PWRI No. 3838, Tsukuba, Japan. (in Japanese)

27. Rodriguez, O. G. and Hooton, R. D., 2003, “Influence of Cracks on Chloride Ingress into Concrete,” ACI Materials Journal, March–April 2003, pp. 120–126.

APPENDix: NoTATioNx = depth from concrete surfacet = timeCo = boundary chloride level at concrete surfaceDc = diffusion coefficientCcorr = chloride threshold level to initiate corrosionCinit = initial chloride level in concreteCair = airborne chloride levelC1 = regional coefficientd = distance from the coastline

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