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:
[email protected]; 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.
[email protected]; 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
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“Material Matters” department.
Contact MP Associate Editor
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phone: +1 281-228-6281,
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or e-mail:
—K.R. Larsen