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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 2
J. TANESI
|
M. G. da SILVA
|
V. GOMES
type of structure, the reinforcement ratio, placing and consolida-
tion. It is usually specified in terms of slump, however, in some
cases, other requirements could be used such as slump loss over
time, segregation and initial set, as they may directly affect the
construction, concrete quality and its durability.
The requirements related to structural safety also depend on the
specific use and structure design, and among them are the com-
pressive strength, flexural strength, modulus of elasticity and creep.
When using mixtures with high level of supplementary cementi-
tious materials, the 28 day compressive strength may not be a
proper requirement, since such mixtures may develop strength
slower than plain mixtures. In this case, a 56 or 63 day compres-
sive strength may better represent the strength characteristics of
these mixtures.
3.2.2.2 Durability requirements
Specifying durability requirements is a much more difficult task
than specifying structural safety requirements. Those requirements
should be selected based on the exposure class of the structure.
Figure 2 shows the sequence for the selection of durability perfor-
mance requirements and criteria.
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Step 1:
the owner must choose the service life. Internationally,
some structures are already being designed for a minimum
service life of 100 years. However, not all structures need to
last the same period of time. Structures with higher repair and
reconstruction costs, which may cause more disturbances to
the environment and offer a greater risk to society typically,
require higher service life.
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Step 2:
the owner must specify whether or not special require-
ments are needed, such as color, texture, skid resistance,
aesthetics, reflectance, flatness and lifecycle cost analysis,
among others. In this stage, the owner must specify any sus-
tainability requirement, as well. The definition of sustainability
requirements at this stage is important because besides being
required for the performance criteria selection (step 7), it may
also influence the definition of exposure classes (step 4).
In case sustainability requirements are incorporated into Bra-
zilian specifications, it is necessary to establish performance
indicators and tools for calculating the life-cycle inventory for
conditions and materials available in Brazil.
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Step 3:
the structural designer must specify the structural
safety requirements, such as compressive strength and flex-
ural strength and modulus of elasticity, among others. In addi-
tion, depending on the reinforcement spacing, dimensions of
structure elements, the designer should define the slump or
other parameter related to workability.
In this stage, the contractor should consider construction is-
sues related to environment, such as the temperature at the
time of construction, and then specify any property required to
handle slump loss, setting time and strength development.
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Step 4:
the specifier must choose the exposure classes. Sev-
eral classes of exposure may be necessary to characterize a
structure, because multiple mechanisms of deterioration may
be involved, as, for example, in a structure that, in addition to
being subject to seawater chloride induced reinforcement cor-
rosion (class CAM), has also the risk of alkali-aggregate reac-
tion (class E).
If concrete with high levels of blast-furnace slag is used, which
could potentially provide a greater susceptibility to carbonation,
in some cases, it may be advisable to incorporate CB exposure
class, so that any mixture used is investigated with respect to
carbonation and, if necessary, measures are taken to prevent
carbonation induced reinforcement corrosion.
In addition, different parts of the structure may be subject to
different levels of severity. For example, a bridge deck could
be classified as CAM-1, while the columns could be classi-
fied as CAM-3. Different subclasses, despite having the same
performance requirements, may have very distinct perfor-
mance criteria.
At this point, it is important to establish the performance re-
quirements related to durability. The requirements should be
chosen taking into account what models will be used to predict
the service life, because these requirements should be the in-
puts in these models or should have a clear correlation with
them. Furthermore, the durability requirements should be able
to reflect the deterioration mechanisms to which the structure
is going to be subjected to.
The following are some of the durability requirements that
could potentially be specified. They were divided into require-
ments related to seawater chloride induced corrosion, carbon-
ation induced corrosion, general and special requirement
3.2.2.2.1 Discussion on possible requirements related
to chloride induced corrosion
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Apparent chloride diffusion coefficient
– this is one of the
most important durability requirements as it has a direct rela-
tionship with the ingress of chlorides and thus with reinforce-
ment corrosion. Most mathematical models for the service life
prediction, when considering chloride induced corrosion, use
the diffusion coefficient as one of its inputs.
The test used to determine this coefficient is the ASTM C 1556-
04 (ASTM [14]) (Chloride bulk diffusion test). This test takes
approximately three months and is relatively difficult to be car-
ried out. Grinding 1-2 mm layers of concrete is a very delicate
task that can lead to a high variability of test results. So, it is
necessary to develop another test that can give an indication of
the diffusion coefficient.
The rapid migration test NT Build 492: 1999 (NORDEST [15])
was developed based on the research of Tang and Nilsson [16]
and presents an equation for calculating the chloride migration
coefficient, which is commonly used as diffusion coefficient.
However, the coefficient calculated on the basis of this standard
is not the apparent diffusion coefficient used by most mathemati-
cal models. In reality, this coefficient is not even the chloride
diffusion coefficient but an approximation. This is because dif-
fusion is the movement of ions due to a concentration gradient
or chemical potential, while the migration test actually measures
the movement of ions under an external electric field.
If the diffusion coefficient is chosen as a requirement, it is rec-
ommended that models, such as Life-365, are used to estimate
the maximum acceptable diffusion coefficient for the specific
conditions of the structure under study that would guarantee
that the desired service life is achieved. Selected mixtures
should be tested and should comply with these limits.
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Total charge
– ASTM C 1202-05 (ASTM [17]) is the most
widely durability requirement used in hybrid specifications. This