Reinforced concrete has always been regarded as a
material that "forgets" to age, i.e. when reference is made to its effective
functional lifetime. Although this view is, to a great extent, true,
the external environment can undoubtedly shorten the life expectancy of
reinforced concrete, when for the prolongation of its acceptable functionality,
costly repairs are created and there is, in some degree, disruption of everyday
life (of the users and the material).
When most people think of concrete as an extremely
durable material, the Pantheon in Rome comes immediately in their minds.
Actually, it is a construction that has been in operation for well over 2000
years. If, then, the Roman concrete can last for centuries, surely today's
construction materials should have a design lifetime (under the technical/engineering
meaning) of 100 years. In a way, the assumptions made about the viability of
the material may have influenced earlier specifications, which, however, fail
to adequately describe its performance over time.
As our understanding of the sustainability of
materials increases, it is generally customary to expect increased performance.
For example, in the case of major structures (e.g. milestones, monumental,
etc.), the desire for them is to be preserved indefinitely. The ultimate goal,
then, must be an approach of producing construction materials, which will allow
the construction of major projects with an extended lifespan, such as the above
mentioned Pantheon.
A huge amount of research on the resilience of
concrete was conducted from the 1970’s to the 1990’s and achieved great
technical knowledge. It is now possible to exploit this information so as to
provide a level of confidence to the owners of reinforced concrete structures.
The truth is that, as early as the 70’s, there has
been a lot of research in the area of understanding the viability of reinforced
concrete. Each relevant research approach is based on the standardisation of the
structure capacity over time. Thus, the most common probabilistic models
of this kind provide for a time nearly linear increase in probability of
failure up to 60% (approximately), where the turning point corresponds to 43
years (approximately), towards a probability of 95% (approximately)
corresponding to 100 years.
For the forecasts of these probabilistic models, of
course, there is an important role of external factors related to the macro and
micro climate as well as to the quality of the materials used, since this
generally varies. It is therefore understood that both the capacity of the
materials and their functional lifetime should be subject of a rather stochastic
evaluation.
Based on the above, it would be appropriate to clarify
a few concepts concerning the technical approach of the viability of the
structures:
· Functional
Capacity and Deterioration
The functional capacity of a structure usually means
its assessment on the basis of its functionality, as it relates to the use of
the structure. By extension, the functional capacity refers to basic operating
parameters of the structure, such as design durability, stability, safety,
morphology, etc. The functional capacity is usually studied as a quantifiable
property of structures in relation to time. Therefore, functional structure
means that, which satisfies the purpose for which it was studied and
constructed.
Accordingly, deterioration means impairment of functional
capacity in relation to time and may reasonably be regarded as the inverse of
the functional capacity. Therefore, measurement of deterioration allows for
assessing any functional capacity problems. This finding, in turn, implies that
the functional capacity threshold arises on the basis of the determination of
an acceptable deterioration ceiling. These are the so-called sustainability
limits. Of course, as in mathematics, when limit conditions are established,
the above mentioned limits may be determined in a way that is related to either
an absolute level or a level of functionality corresponding to an acceptable
level of maintenance. Thus, the maintenance time of the structure is determined
and consequently the functional requirements for it.
It is obvious that the viability of reinforced
concrete depends on the viability of its two main components, namely concrete
and reinforcement (steel). The interoperability of these two basic materials is the main prerequisite
for the viability of reinforced concrete structures. If, in other words, there
are defects in the initial fabric of reinforced concrete or if the materials
selected have quality problems, as well, if there is a (non-design) unfavorable
charge that favors the collapse, deterioration is guaranteed. Also, the
environment within which these materials have been installed and operate
decisively influences the functional capacity and respectively the deterioration
of the structure.
· Functional
Lifetime
The estimation of the functional lifetime of the
materials may be done either through their anticipated lifetime or the
acceptable maintenance period. As functional lifetime may be expressed in 3
ways, i.e. technical, functional or economical, it is obvious that relevant
criteria for evaluation of use are required. For example, the estimation of the
functional lifetime of a structure from an investment point of view is done
through techno-economical analyses, concerning the maintainability and
reliability of the operation of the structure.
Functional lifetime and maintenance are concepts
completely correlated as, in any case, some maintenance procedures are
performed during the operating time of a structure. For this reason,
maintenance work that affects the functional lifetime, deserve due attention.
It is therefore understood that this finding changes the definition of functional
lifetime, in which the maintenance condition should be added, i.e. a phrase of
the type: "...if and as long as the construction is maintained
systematically ".
It is, of course, up to the so-called Master of the
Project (MoP) or in any case the owner of the structure – in the broadest sense
of the term chosen for this text – the definition of operational and
sustainability requirements, something that ultimately defines the functional lifetime.
· Probability
of Failure
When a functional lifetime has been defined, the
stochastic viability planning should include the determination of the maximum
probability corresponding to the avoidance of a marginal situation. Such
borderline situations can be either the final marginal situation or the
marginal state of satisfaction of acceptable functionality.
There are two types of failure: viability failure and
mechanical failure (e.g. bending, buckling, hammering, creep, loosening,
thermal shock, fatigue, corrosion, cracking). However, for a material, the
failure of viability is essentially responsible for the failure due to a
mechanical cause.
In ordinary mathematical models the assessment of the
failure hazard arises by multiplying the probability of failure with the
quantified deterioration measured.
The determination of the probability of failure is
based on social, economic and environmental criteria. For the social criteria,
the essential is the importance of the structure and the consequences of
failure as they endanger human lives. For the economic criteria, the additional
– compared to the construction cost – economic consequences of the situation
created because of the failure are examined.
For environmental/ecological criteria, the assessment
is based on environmental problems caused or the circumvention of ecological
principles.
The estimation of the probability of failure is
applied both at the stage of the study of new structures and in existing structures.
In the second case, of course, the safety tolerances are smaller compared to
the first case.
· Viability
Design
Conceptually, viability design is based on safety, as the
structure must effectively address the various hazards to which it is exposed.
Surely safety is systematically examined by the application of the laws of Mechanics.
However, during the design stage, the examination of the behaviour of
construction materials has a broader view. Why? Because, precisely, what
matters are the above mentioned concepts, of viability and operational lifetime
of the structure.
Introducing the time factor in the design, it becomes
possible to study the deterioration of materials, which is, of course, a part
of the whole problem of the viability of the structures. Based on this, the time
functional approach, the desired behaviour requirements of the structure are set
out and they must be met in the long term, with a view to safety.
In this context, for reinforced concrete structures,
the main – there is further finer segregation in subcategories – categories,
which are examined are:
* Freezing/Thawing
* Influence of sulfate (S), mainly anion SO ₄ ²-
* Contact with water
* Protection of reinforcement against corrosion
· The
Environment
What is required for the optimal design it is a
thorough study of the characteristics of the environment within which a structure
will be set in operation, i.e. the construction materials will be called to be
exposed.
To this end, in the generally applicable concrete
specifications, as regards the risk of failure due to environmental conditions,
various (classified) cases regarding exposure to environmental factors are included
and they are assigned to classified categories of failure risk, as follows:
*
Zero
failure risk
*
Failure
caused due to carbonation
*
Failure
caused by chlorides not associated with seawater
*
Failure
caused by seawater chlorides
*
Freezing/Thawing
with or without external de-icing agents
Based on the above classifications, it is obvious that
any cases concerning environmental or corrosive factors, which may coexist, are
examined individually. Therefore, in the study of structures, the design
durability includes taking into account the combined effect of such risk
factors on concrete. The criterion of risk acceptance (so-called design
tolerances), which will ultimately determine the viability of the concrete is nothing
else than the cost.
In any case, experience has shown that the most
serious risk to the viability of the concrete is related to the failure of the
(integrated) reinforcement, which may well cause damage to the concrete
surrounding it. The repair of such faults is always very costly and causes the
so-called – from a techno-economic point of view – "indirect" costs.
Conclusion
The overall approach that can lead to a guarantee of
the viability of reinforced concrete structures (must) include the thorough
study of the (constantly changing) environment in which the structure will be
"exposed" and the proper production of reinforced concrete, based on
the study of materials (concrete, reinforcement), the maturation, the workability
and quality controls. Then, during operation, the role of monitoring of
cracking and proper maintenance is important, of course. Given the application
of the above approach, the viability of the concrete and the structure made of
this material is guaranteed.