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Exposure Station
Concrete in the Marine Environment-
Treat Island Marine Exposure Station
By Michelle L. Wilson, Program Coordinator, Education &
Product Development, PCA
Established in 1936, the US Army Corps of Engineers (USACE) Treat
Island Marine Exposure Station is a natural weathering facility
used to study concrete durability.
Natural Weathering Exposure Station
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| Treat Island marine exposure site. Photo
courtesy of USACE. |
Located on the Bay of Fundy near Eastport, Maine, the Treat Island
exposure station naturally imposes a combination of severe environmental
conditions. Specimens are located at mid-tide level and immersed
in seawater twice daily. The facility can be visited only at select
times because the tides vary by as much as 6.7 meters (22 feet).
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| Exposure rack immersed in seawater at
mid-tide. (IMG17952) |
During the coldest part of the winter, temperatures average around
-10°C (15°F). Depending on the specimen placement and severity
of the winter, test specimens are subjected to anywhere from 100
to 160 freeze-thaw cycles per year. In addition, the cyclic flooding
by saltwater and air drying subjects test specimens to chloride
intrusion, wetting and drying, and abrasion/erosion.
Areas of Investigation
There are approximately 22 active research programs underway (click
here for list). Areas of investigation include supplementary
cementing materials (SCMs), high-performance concrete (HPC), lightweight
concrete, fiber-reinforced concrete, alkali-aggregate reactivity,
roller-compacted concrete (RCC), prestressed concrete, polymer concrete,
and high-range water reducers.
Specimens are located on both the exposure rack (wharf) and on the
beach adjacent to the rack.
The concrete specimens are a variety of shapes and sizes, from concrete
prisms to large cubes of concrete. Data are collected yearly and
published biennially (ERDC 2004).
Property of USACE, the Treat Island exposure station is directly
maintained by the Engineering Research and Development Center (ERDC),
Geotechnical and Structures Laboratory, Engineering Systems and
Materials Division, Concrete and Materials Branch. Sponsors include
the Bureau of Reclamation, Department of Transportation, CANMET,
Degussa Admixtures, and Mobile Research and Development Corporation.
Many other sponsors have specimens and ongoing programs on Treat
Island. Although space at the marine exposure site is limited, specimens
from outside organizations can be placed there provided that certain
requirements are met (ERDC 2004).
Example Project 1—Admixtures
Since 1992, Degussa Admixtures, in cooperation with the USACE, has
performed and maintained concrete durability experiments on Treat
Island intended to complement durability research efforts for developing
new concrete admixtures.
“Our goal at Treat Island is to expose admixture-treated concrete
to a real world harsh environment to ensure that the desired durability
performance can be achieved,” says Mark Bury, Product Line
Manager for Degussa Admixtures. “For years we have been monitoring
test samples from different experiments evaluating concrete treated
with air-entraining agents, high-range water-reducing admixtures,
corrosion-inhibiting admixtures, fly ash and silica fume. We are
also monitoring the durability of newer technology such as self-consolidating
concrete and very high-early strength high-performance concrete
mixes.”
Findings
Scaling resistance
• Recommended air contents for frost resistant concrete in
ACI 201 do not assure good scaling resistance in this environment,
using the ASTM C 672 visual scaling rating (Figure 2).
• Many of the specimens placed at the island had a measured
air content greater than 6% and a spacing factor of less than 0.20
mm (0.008 in.), but exhibited more scaling than expected (Figure
3). The best performing concrete overall consisted of air-entrained
concrete (6.5% and 0.23 mm [0.009 in.] spacing factor) that contained
8% silica fume (3 year rating = 1)
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| ASTM C 672 numerical scale ratings. IIMG12330) |
Example of scaling of test specimens after
long-term exposure.
Photo courtesy of Degussa. |
Chloride ingress
• Concrete containing a calcium-nitrite based corrosion-inhibiting
admixture (CNI) had a greater chloride ingress and the concrete
with organic corrosion-inhibiting admixture (OCI) had a lower chloride
ingress relative to an untreated plain reference concrete (Figure
4). Similar trends were found in concretes containing silica fume
with both corrosion inhibitors. This supports previously published
papers on chloride ingress of corrosion-inhibiting admixtures (Miller
and Miltenberger 2001).
Graph showing effects of corrosion inhibitors on chloride ingress
obtained after three years of exposure to seawater at Treat Island:
Chloride ingress information, chloride threshold, depth of concrete
cover over steel reinforcement and other factors can be used to
model and predict the service life of steel-reinforced concrete
structures exposed to moisture and chlorides.
Example Project 2—Supplementary Cementing
Materials
Since the mid 1970s, CANMET, in collaboration with the University
of New Brunswick (UNB) and the USACE, has been investigating the
performance of various concrete formulations on Treat Island.
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| CANMET specimens on exposure rack. Photo
courtesy of CANMET. |
“Over the years, the field condition surveys and the series
of detailed laboratory investigations performed on specimens exposed
at Treat Island have provided invaluable information on the critical
parameters impacting on the performance of conventional concrete
and concrete incorporating SCM in severe marine environments,”
states Benoit Fournier, Research Scientist and Manager of the Concrete
Technology Program of CANMET-MTL. “The data generated from
that work will continue to influence specifications to improve the
performance of concrete exposed to such environments.”
Concrete mixtures, with water-to-cementitious materials ratios
(w/cm) generally ranging from 0.40 to 0.60, and SCMs ranging from
0% to 80% by mass, have been used to manufacture large block specimens
(Malhotra and Bremner 1996). Close to 300 non-reinforced and reinforced
concrete blocks have been installed at the exposure site.
Findings
• Air-entrained prisms made with CSA A5 Type 10 cement performed
satisfactorily within the range of w/cm tested (0.40 to 0.60), with
some surface scaling and loss of paste. The type of cements, Type
10 and Type 50 (equivalent to ASTM C 150 Type I and Type V), did
not affect the development of surface deterioration of the concrete
specimens.
• Generally, the scaling of air-entrained concrete specimens
incorporating silica fume, fly ash or slag increased with increasing
w/cm. Scaling was noted at very early ages and the rate of surface
deterioration was found to decrease with time. The results confirm
that, under extreme marine exposure conditions, concretes, especially
those incorporating SCM, must have
low w/cm.
• Silica fume concretes must be air entrained and the percentage
of silica fume should not exceed 10% to resist severe frost action
in a marine environment.
• The incorporation of slag at any level (25% to 65%) significantly
contributes to reducing the ingress of chloride ions into the concrete.
• The w/cm of high-volume (56% by mass of cement) fly-ash
concrete exposed to severe environments in the presence of salts
should not exceed about 0.30.
Conclusions
“Most marine exposure studies for concrete are usually not
maintained for more than a few years, and to arrive at useful information
there is a need to carry out long-term tests that run for several
decades,” says Ted Bremner, Professor Emeritus and Honorary
Research Professor from UNB. After nearly seven decades, it is expected
that experimentation on Treat Island will continue to provide valuable
information and data essential for the development of technology
to enhance concrete performance and service life.
The Treat
Island Web site should be consulted for additional information.
Point of contact is Donna C. Day, Corps of Engineers, telephone:
601.634.4101.
References
ACI Committee 201, Guide to Durable Concrete, ACI 201.2R-01,
American Concrete Institute, Farmington Hills, Michigan, 2001.
ASTM C 672, Standard Test Method for Scaling Resistance of Concrete
Surfaces Exposed to Deicing Chemicals, American Society for
Testing and Materials, 2003.
ERDC, 2004 Review of Programs, Natural Weathering Exposure Station,
Treat Island, Maine, USA, USACE Waterways Experiment Station, Vicksburg,
Mississippi, August 2004.
Malhotra, V. M. and Bremner, T. W. Performance of Concrete at
Treat Island, USA; CANMET Investigations, ACI Special Publications,
SP-163, 1996, pages 1 to 52.
Miller, B. D., and Miltenberger, M. A., “The Effects of Corrosion-Inhibiting
Admixtures on Chloride Transport in Concrete,” Ion and
Mass Transport in Cement-Based Materials, American Ceramic
Society, Westerville, Ohio, 2001, pages 367 to 376.
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