Driver Trett Digest Issue 21 03.2021 - Flipbook - Page 25
DIGEST | ISSUE 21
Chloride contamination is the most common cause of
reinforcement corrosion. Chlorides can be cast into concrete
with marine aggregates, saline mix water or chemical
admixtures. More commonly, concrete is exposed to chlorides
from external sources such as seawater (marine structures),
de-icing salts (highway structures) or industrial processes
(e.g., certain building structures such as abattoirs).
Consequently, the use of predictive tools for determining
chloride ingress for reinforced concrete structures and
estimating the time taken to reach critical levels have become
an important part of the tender and/or detailed design
stages of new build structures, as well as of the condition
assessment of existing structures and of asset management
planning for deteriorating concrete structures.
By using durability modelling tools, the service life of a
structure is estimated by assessing the resistance capacity
of the examined concrete (i.e. durability that is associated
with a specific concrete type and specified concrete cover
to reinforcement) against certain environmental and
special exposure conditions or ‘actions’ (e.g. temperature,
humidity, chloride-ion exposure, carbonation, freeze/thaw
mechanisms, etc.) on the basis of a desired limit state, such
as the initiation of attack or the partial or full deterioration of
the structure or element under consideration. Specifically,
a time-dependent load or action distribution (S) is compared
to the corresponding time-dependent resistance distribution
(R) of the structure.
As shown in Fig.2, the intersection of the S(t) and R(t) curves
gives the deterministic solution for the mean service life, t, of
the examined structure or element.
The deterministic modelling approach is based on the
analysis of a defined set of input parameters (e.g. concrete
cover depth, chloride penetration resistance of concrete
mix, exposure conditions), which by being fed into a model,
gives a unique, consistent output that is certain (occurrence
risk: 100%), neglecting any risk that is associated with the
variation of these parameters. Conversely, the probabilistic
approach is a statistical way to analyse deterministic models
and comprises the estimation of the probability that the
predefined limit state will not be exceeded during the service
life of a structure.
vulnerable to deterioration. As an example, for concrete this
involves eliminating or reducing details which are likely to
make concrete placement and full compaction difficult to
achieve, particularly overly congested reinforcement.
ACCOMPANYING
FIGURES
This should not be neglected, and key to this is liaison and cooperation between the structural engineer and the materials
engineer, to ensure nothing is missed. The engineers should
also consider the ‘buildability’ of their design to assist the
constructer in reducing workmanship defects.
Whilst the primary durability strategy
is the principal means of achieving
durability (through quality of materials,
detailing and additional protective
measures), it is prudent in the case
of critical infrastructure to have a
secondary durability provision as an
‘insurance policy’.
Fig.1., Chloride-induced steel reinforcement corrosion in a highway structure
This can be used to achieve the required service life, or to
provide the option of an extension of the service life beyond
that originally envisaged. In the case of reinforced concrete
structures this can be by means of making provision for
future cathodic protection of the structure to be incorporated
as a contingency measure in the event of unforeseen
circumstances. This involves ensuring in the design that the
reinforcement within each element is in electrical continuity,
and installing connection points where a cathodic protection
system can be retrofitted to stop corrosion, if the structure
is found to be deteriorating while in service by routine asset
condition inspections.
PITFALLS AND SOLUTIONS
The accuracy of durability modelling exercises is limited
by the quality of data that is input to the model; this
is based upon assumptions, laboratory or fieldwork
data. The most significant parameters include surface
chloride concentration, chloride migration coefficient,
critical threshold for corrosion and age factor. Incorrect
assumptions will result in inaccurate durability predictions.
This can be mitigated to a degree by undertaking ‘sensitivity
analysis’, whereby a range of input parameters are modelled
to determine the durability significance of varying the values.
An essential part of any durability strategy is having a
structural design that avoids non-durable features that are
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1.
Sfikas IP and Ingham J P. 2016. Service life design of
concrete structures using probabilistic modelling tools:
statistical analysis of input parameters. In: Grantham
MG, Papayianni I and Sideris K (Eds), Concrete Solutions:
Proceedings of the 6th International Conference on Concrete
Repair. 20-23 June 2016, Thessaloniki, Greece-CRC Press,
Taylor & Francis Group. London. pp. 437-446. ISBN: 978-1138-03008-4, 2016.
Fig.2., Estimation of service life of reinforced concrete1
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