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Resistance
Corrosion Resistance of Concrete
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| Corrosion of this rebar caused spalling on
a concrete step. |
The corrosion of reinforcing steel can be a problem in concrete structures.
Corrosion of steel produces rust (hydrated iron oxide), which is very
expansive (PCA 2002).
This expansion builds up internal pressure until the concrete fails
in the form of spalling. This is a leading cause of concrete deterioration
and numerous studies have been performed to prevent corrosion of reinforcing
steel.
Specifying Durable Concrete
Specifying a durable concrete begins with identifying exposure conditions.
To what deterioration mechanisms will the concrete be exposed? Three
exposure classes relating to corrosion protection of reinforcement
are defined in ACI 318: Not applicable (C0), Moderate (C1), and
Severe (C2). The most common cause of steel corrosion is chloride
ingress (see Corrosion of Embedded
Metals). Any reinforced concrete exposed to moisture and external
chlorides, be it from seawater or deicer salts, is considered to
be in severe corrosion environment. This condition requires a minimum
design strength of 35 MPa (5000 psi) and a maximum w/cm of 0.40.
In addition, supplementary cementitious material use is limited
in any environment where concrete will be exposed to both deicer
salts (C2) and very severe freezing and thawing (exposure class
F3). Current limitations include: slag – a maximum of 50%,
fly ash – a maximum of 25%, and silica fume – a maximum
of 10%. Additionally, mixtures containing three or more cementitious
or pozzolanic materials (ternary, quaternary, and so on) are limited
to no more than 50% cementitious materials by mass when slag is
included or 35% when slag is not included.
Performance specifications may be used in lieu of prescriptive mixture
proportions. Caldarone
(2005) provides a guide specification that allows for performance-based
acceptance of concrete. With regards to corrosion resistance, the
specifier indicates the method of curing and duration of aging of
test specimens and the maximum charge allowed using ASTM C1202 (AASHTO
T 277), Standard Test Method for Electrical Indication of Concrete’s
Ability to Resist Chloride Ion Penetration. The resistance
to chloride ion penetration is used, because, as mentioned earlier,
chloride ingress is the most common cause of reinforcing steel corrosion.
Other performance requirements can be specified, such as scaling
resistance, strength, and freeze/thaw durability.
Testing for Corrosion Resistance
The first line of defense against corrosion of reinforcement is
to inhibit the penetration of water, oxygen, carbon dioxide, and
salts from the concrete surface to the reinforcement. Quite a few
tests try to assess permeability, diffusion, absorption, or other
direct measures of fluid penetration resistance. The most frequently
used is ASTM C1202. This test is commonly referred to as the Rapid
Chloride Permeability Test (RCPT). The RCPT is a measurement of
the electrical charge that travels between two sides of a concrete
specimen over a six-hour period. This charge is correlated to chloride
ions travelling through the pore system. Lower values signify a
higher resistance to chloride intrusion. This test is much quicker
to run than ASTM C1556, Determining the Apparent Bulk Diffusion
Coefficient of Cementitious Mixtures by Bulk Diffusion.
ASTM C1556 is a more rigorous method to calculate concrete permeability.
Test results from C1556 typically provide lower variability in test
results. The specimens are subjected to unidirectional chloride
intrusion after 28 days of moist curing. The depth of chloride intrusion
is measured over time (beginning at 35 days soaking period) by grinding
away successive layers from the specimen and then measuring the
chloride level of each layer using ASTM C1152, Standard Test
Method for Acid-Soluble Chloride in Mortar and Concrete. This
procedure gives a direct correlation to the permeability of the
concrete and is considered a useful method for prequalification
of concrete mixtures. Unfortunately, C1556 is very time consuming
and requires about 3 months to complete.
ASTM C1543 (AASHTO T259), Standard Test Method for Determining
the Penetration of Chloride Ion into Concrete by Ponding, has
been used for decades by many highway agencies. A concrete slab
is cast and moist cured for 14 days, then air cured to 28 days.
The top surface is bermed and ponded with a salt solution for 90
days. Cores are then taken from the exposed surface and sliced into
approximately half-inch thick discs. Each disc is crushed and the
chloride content of each layer is determined. Unfortunately, this
test requires almost six months to complete and there is no clear
way provided for interpretation of the results in the method. Transport
mechanisms in this test also include undefined components of absorption,
diffusion, and wick action.
In AASHTO TP 64, Predicting Chloride Penetration of Hydraulic
Cement Concrete by the Rapid Migration Procedure, a 50-mm (2-in.)
long, 100-mm (4-in.) diameter concrete sample is saturated using
the vacuum saturation procedure of the RCPT. This test ranks multiple
concretes in the same order as ASTM C1202, but has the advantage
of not being influenced by strongly ionic admixtures, such as calcium
nitrite. As well, the specimen does not experience a temperature
rise during the test. The test also has been shown to have a somewhat
lower variability than the RCPT (Hooton 2001).
A more rapid test for permeability than the RCPT was recently developed
by Florida’s Department of Transportation (FDOT
2004). This procedure uses the Werner probe array method for
testing resistivity of concrete on 100x200 mm (4x8 in.) specimens.
ACI 222R (2001) also recommends using this method for assessing
the permeability of in-place concrete. The results of the electrical
resistivity test have been correlated to the RCPT permeability rating
system (Smith 2006).
The surface resistivity is measured in a matter of seconds thereby
allowing for a much larger sample size. The Florida standard requires
8 tests each on 3 specimens, while the RCPT can only provide a single
test per specimen because of the inherently destructive preparation
requirements.
Summary
While corrosion of reinforcing steel can be a problem in concrete,
it can be reduced using proper specifications and rapid identification
of susceptible placements. Concrete mixtures that are designed for
service exposure environments will reduce the opportunities for
corrosion. Rapid test methods assist by providing a fast and reasonable
approximation of the corrosion resistance of concrete.
References
ACI Committee 222, Protection of Metals in Concrete Against
Corrosion, ACI 222R-01, American Concrete Institute, Farmington
Hills, Michigan, 2001, 41 pages.
ACI Committee 318, Building Code Requirements for Structural
Concrete, ACI 318-08, American Concrete Institute, Farmington
Hills, Michigan, 2008, 471 pages.
Caldarone, Michael A., Peter C. Taylor, Rachel J. Detwiler, and
Shrinivas B. Bhidé; Guide
Specification for High-Performance Concrete for Bridges,
EB233, 1st edition, Portland Cement Association, Skokie, Illinois,
USA, 2005, 64 pages.
FDOT, Florida Method of Test for Concrete Resistivity as an Electrical
Indicator of its Permeability, FM 5-578, Florida Department of Transportation,
January 27, 2004.
PCA, Types and
Causes of Concrete Deterioration, IS536, Portland Cement
Association, Skokie, Illinois, 2002, 16 pages.
Hooton, R.D., “Development of Standard Test Methods for Measuring
Fluid Penetration and Ion Transport Rates”, Materials
Science of Concrete: Fluid and Ion Transport Rates in Concrete,
American Ceramic Society, 2001, pp. 1-12.
PCA, Types and
Causes of Concrete Deterioration, IS536, Portland Cement
Association, Skokie, Illinois, 2002, 16 pages.
Smith, David, The Development
of a Rapid Test for Determining the Transport Properties of Concrete,
SN2821, Portland Cement Association, Skokie, Illinois, 2006, 125
pages.
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