NACE International, Vol. 52, No. 6 June 2013 MATERIALS PERFORMANCE 17

Continued on page 18

Corrosion-resistant rebar extends service life of concrete bridge structures

The new Huguenot Bridge over the James River in Richmond, Virginia is being

constructed with Class I CRR microcomposite reinforcing steel bars. Photo

courtesy of VDOT.

A common cause of reinforced con-

crete bridge deterioration is corrosion of

the reinforcing steel bars due to exposure

to chloride ions from deicing salts and

marine environments that permeate the

concrete. In the absence of chloride ions,

concrete’s higher pH promotes the pas-

sivation of the reinforcing steel, where a

passive oxide flm is formed on the steel’s

surface that protects the metal from cor-

roding. When chloride ions, in combina-

tion with moisture and oxygen, penetrate

the concrete and interact with the rein-

forcing steel, its protective passive flm is

compromised and the steel is susceptible

to corrosion.

To combat rebar corrosion and

lengthen the service life of bridges in the

United States, several state departments

of transportation have evaluated the use

of corrosion-resistant reinforcing (CRR)

steel bars for concrete bridge decks and

18 MATERIALS PERFORMANCE June 2013 NACE International, Vol. 52, No. 6

M A T E R I A L M A T T E R S

Continued from page 17

other bridge structures. The Structure

and Bridge Division of the Virginia De-

partment of Transportation (VDOT)

(Richmond, Virginia) made a decision to

discontinue the use of epoxy-coated re-

The Class I CRR microcomposite reinforcing steel bars are being used to construct the deck and railing of the Route 675

bridge (Beulah Road) in Fairfax County, Virginia. Photos courtesy of VDOT.

inforcing (ECR) steel bars and galva-

nized reinforcing steel bars based on

research completed by the Virginia

Transportation Research Council and

Virginia Tech.1 Field observations indi-

cated coating failures in <20 years and

section loss of reinforcing steel bars of up

to 100%. Since September 2010, VDOT

has designed all bridge projects with

CRR steel bars, including new construc-

tion, widening, superstructure replace-

ment, repairs, and rehabilitation, to

achieve a 75-year or longer service life

for its bridges. In 2012, VDOT reported

that it used ~8.8 million lb (4 million kg)

of CRR steel bars in the past two years,

and 20% of the CRR steel bars used was

stainless steel (SS).

Although the quality of concrete has

improved over the years and high-perfor-

mance concretes today resist salt intru-

sion, the concrete is going to crack on a

bridge deck at some stage and saltwater

will intrude, says Julius Volgyi, assistant

state structure and bridge engineer for

VDOT’s Structure and Bridge Division.

If the reinforcing steel bars are black

carbon steel (CS) or ECR, he says, corro-

sion will begin, and the resulting ferrous

oxide (FeO), because it has a larger vol-

ume than the steel reinforcing bars, will

put additional pressure on the concrete

deck and cause further cracking that al-

lows even more chloride intrusion. This

repetitive, cyclical process will continue

until no reinforcing steel is left, Volgyi

comments, noting that the move to CRR

steel bars is expected to extend the life of

the concrete bridge deck. “To us, an

NACE International, Vol. 52, No. 6 June 2013 MATERIALS PERFORMANCE 19

Continued on page 20

investment in CRR steel bars is going to

pay off in the long run. We won’t get

spalling associated with the rusting of

rebar,” he says.

According to Michael Sprinkel, associ-

ate director of the Virginia Center for

Transportation Innovation and Research

(Charlottesville, Virginia), CRR steel bars

are available in many formulations that

affect performance, service life, and cost.

VDOT uses three types of deformed

CRR steel bars, which are categorized

into three classes based on corrosion

performance data.

Class I, improved corrosion resistance

steel bars, has the lowest cost and lowest

anticipated service life (60-plus years) and

includes low-carbon chromium steel re-

inforcing bars that meet ASTM A1035/

A1035M2 and UNS S32101 solid duplex

SS reinforcing bars that meet ASTM

A955/A955M.3 Class II, moderate cor-

rosion resistance steel bars, includes clad

SS reinforcing bars that meet AASHTO

M 329M/M 329-114 and UNS S24100

SS reinforcing bars that meet ASTM

A955/A955M. Class III, high corrosion

resistance steel bars, has the highest cost

and longest anticipated service life (100-

plus years) and includes UNS S24000,

S30400, S31653, S31603, S31803, and

S32304 solid SS reinforcing bars that

meet ASTM A955/A955M.

The class of CRR steel bars specifed

for bridge structures constructed for

VDOT depends on the structure’s func-

tional classifcation. Sprinkel notes that

bridges on interstate highways and pri-

mary roads typically have the highest

volume of traffc, including truck loads,

and consequently are exposed to more

deicing applications and stress than typi-

cal bridges on lower-volume roads. Usu-

ally, the rural bridges on the roads with

lowest traffc volume are subject to the

least amount of deicing applications and

stress. For high-traffc roads, including

interstate highways, freeways, and prin-

cipal arterial roads in both rural and ur-

ban areas, highly corrosion-resistant SS

reinforcing bars (Class III) are used.

VDOT specifes that Class II reinforcing

steel bars be used for minor arterial roads

and Class I reinforcing steel bars be used

for local rural and urban roads with lower

traffc use. Spending more for CRR steel

bars is easily justifed for all functional

classifcations of roadways, says Sprinkel,

because the cost to redo one bridge deck

can exceed the initial extra cost to use

CRR SS bars. However, he adds, given

that lane closure and user costs will be the

least on low-volume roads, the use of

Class I CRR steel bars can be justifed

based on lower initial costs. Also, because

of less traffc stress and fewer applications

of deicing chemicals on lower-volume

roads, one bridge deck overlay in 100

years may not be required for bridge

decks constructed with Class I CRR steel

bars. “The risk of spending more money

on low-volume roads by not using stain-

20 MATERIALS PERFORMANCE June 2013 NACE International, Vol. 52, No. 6

M A T E R I A L M A T T E R S

Continued from page 19

less is low, and the risk of spending more

money on high-volume roads by not us-

ing stainless is high,” Sprinkel comments.

Bridge components constructed with

CRR steel bars include concrete deck

slabs, parapets, rails, raised medians,

terminal walls, prestressed concrete slabs,

and reinforced concrete slab spans, says

Volgyi. He notes that for all functional

classifcations, Class I CRR steel bars are

used exclusively for abutment neat work,

which applies to all abutment types, and

for all prestressed concrete beam continu-

ity reinforcement bars and other rein-

forcement that extends into the concrete

deck slab. The strands inside the pre-

stressed concrete beam, however, are

black CS.

Over the past few years, several studies

were conducted on the corrosion resis-

tance of a high-strength, low-carbon

chromium microcomposite steel reinforc-

ing bar commercialized by MMFX

Technologies Corp. (Irvine, California),

which conforms to the requirements of

ASTM A1035/A1035M and is accepted

as a Class I CRR steel bar by VDOT.

This alloy’s composition and manufactur-

ing process was designed to provide im-

proved corrosion resistance and me-

chanical properties as compared to

conventional CS (e.g., ASTM A6155 and

ASTM A7066) when fabricated as a rein-

forcing steel bar product. The microcom-

posite has a minimum yield strength of

100 ksi (690 MPa) and minimum tensile

strength of 150 ksi (1,030 MPa).

The nanotechnology used in the al-

loy’s manufacturing process, which is

based on 25 years of research at the

University of California, Berkeley and

patented by MMFX, manipulates the

structure of the steel at the nanoscale to

create a microstructure without carbides

and secondary particles. Typical CS

comprises a matrix of carbides and fer-

rites at the grain boundaries that are

chemically dissimilar. When exposed to

moisture, the carbides and ferrites in the

steel form microgalvanic cells that initiate

galvanic corrosion, explains Salem Faza,

vice president of engineering and specif-

cation for MMFX.

The low-carbon chromium steel alloy

contains a maximum carbon content of

0.15%, a chromium content of ~9.5%,

and low amounts of other carbide-form-

ing elements such as tungsten, molybde-

num, vanadium, titanium, niobium,

tantalum, and zirconium. During the

cooling step in the alloy’s fabrication

process, a fne lath martensite microstruc-

ture is formed where the presence of

carbides is almost eliminated, says Faza.

This is possible, he notes, because the

alloy’s low-carbon content reduces the

excess carbon available in the matrix,

which can combine with the chromium

present in the alloy to form chromium

carbides and reduce the corrosion resis-

tance of the alloy by depleting the chro-

mium from the matrix. Additionally, the

low-carbon content combined with the

NACE International, Vol. 52, No. 6 June 2013 MATERIALS PERFORMANCE 21

low content of other carbide-forming ele-

ments prevents the formation of second-

ary particles in the microstructure that

can initiate microgalvanic cells and gal-

vanic corrosion.

When exposed to chloride ions in

concrete, the corrosion-resistant proper-

ties of the low-carbon, chromium micro-

composite steel reinforcing bar are two-

fold, Faza says. The chromium in the

steel facilitates the formation of a surface

oxide flm that is more resistant to chlo-

ride ions and has a higher chloride thresh-

old level—the chloride concentration

level that initiates the breakdown of the

passive film on the steel reinforcing

bars—than an uncoated CS reinforcing

bar. If the chloride threshold level is

reached, the microcomposite steel rein-

forcing bar can start corroding; however,

the corrosion rate will be lower because

of the carbide-free microstructure.

A report prepared for MMFX in 2006

by AMEC Earth & Environmental

(Burnaby, British Columbia, Canada)7

examines the results of 10 different stud-

ies by universities and state transportation

departments on the performance of the

microcomposite steel reinforcing bar

compared to other reinforcing steel bars

in accelerated corrosion tests. According

to the report, the test results reviewed

indicate that the low-carbon chromium

microcomposite steel provides better cor-

rosion resistance in chloride environ-

ments than conventional uncoated CS,

although the relative amount of improve-

ment varies based on the accelerated test

method used in each study. The test re-

sults also show that most types of SS

provide better corrosion resistance than

the low-carbon chromium microcompos-

ite steel.

Data from a corrosion study con-

ducted by the Virginia Transportation

Research Council, as noted in the AMEC

report, show that a bent microcomposite

steel bar has a chloride threshold of 2,700

to 2,730 ppm, which compares to a chlo-

ride threshold of 460 to 580 ppm for CS

bars; 1,550 to 1,560 ppm for a UNS

S32101 duplex SS bar; >4,630 ppm for

a zinc-sprayed, epoxy-coated CS bar; and

>5,630 ppm for UNS S30400, S31653,

and S32205 solid SS bars and a SS-clad

CS bar. Results of an accelerated chloride

threshold test done at Texas A & M

University indicated that the mean criti-

cal chloride threshold values were 4.6 kg/

m3 for the microcomposite steel vs. 0.20

kg/m3 for black CS (ASTM A706); 5.0

kg/m3 for Type 304 SS (UNS S30400);

and 10.8 kg/m3 for Type 316LN SS

(UNS S37653). Results of a study by the

University of Kansas concluded that the

corrosion rate for a conventional CS bar

(ASTM A615) was 3.6 to 4.4 times higher

than the corrosion rate for the microcom-

posite steel bar.

Contact Julius Volgyi, VDOT—e-mail:

Julius.Volgyi@VDOT.Virginia.gov; Michael

Continued on page 22

22 MATERIALS PERFORMANCE June 2013 NACE International, Vol. 52, No. 6

Sprinkel, Virginia Center for Transportation

Innovation and Research—e-mail: Michael.

Sprinkel@VDOT.Virginia.gov; and Salem

Faza, MMFX—e-mail : salem.faza@

mmfx.com.

References

1 “Corrosion Resistant Reinforcing Steels

(CRR),” Virginia Dept. of Transporta-

tion, Structure and Bridge Division, IIM-

S&B-81.5, August 2012.

2 ASTM A1035/A1035M-11, “Standard

Specifcation for Deformed and Plain,

Low-carbon, Chromium, Steel Bars for

Concrete Reinforcement” (West Con-

shohocken, PA: ASTM).

3 ASTM A955/A955M-12e1, “Standard

Specification for Deformed and Plain

Stainless-Steel Bars for Concrete Rein-

forcement” (West Conshohocken, PA:

ASTM).

4 AASHTO M 329M/M 329-11, “Stan-

dard Specification for Stainless Clad

Deformed and Plain Round Steel Bars for

Concrete Reinforcement” (Washington,

DC: AASHTO, 2011).

5 ASTM A615/A615M-12, “Standard

Specification for Deformed and Plain

Carbon-Steel Bars for Concrete Rein-

forcement” (West Conshohocken, PA:

ASTM).

6 ASTM A706/A706M-09b, “Standard

Specification for Low-Alloy Steel De-

formed and Plain Bars for Concrete

Reinforcement” (West Conshohocken,

PA: ASTM).

7 “Comparative Performance of MMFX

Microcomposite Reinforcing Steel and

Other Types of Steel with Respect to Cor-

rosion Resistance and Service Life Predic-

tion in Reinforced Concrete Structures,”

AMEC Earth & Environmental, AMEC

VA06451, February 2006.

M A T E R I A L M A T T E R S

Continued from page 21

MP welcomes news

submissions and leads for the

“Material Matters” department.

Contact MP Associate Editor

Kathy Riggs Larsen at

phone: +1 281-228-6281,

fax: +1 281-228-6381,

or e-mail:

kathy.larsen@nace.org.

—K.R. Larsen