Driver Trett Digest Issue 21 03.2021 - Flipbook - Page 22
DIGEST | ISSUE 21
The definition of durability differs slightly between these
standards, but there is commonality between them, in
that they all require structural elements to exhibit the
following qualities if they are to be classified as durable:
They should be designed with their service environment
in mind.
They should last for their intended working life (i.e.,
design/service life).
There should be no appreciable loss of utility during the
intended working life.
There should be no need for any unforeseen maintenance
or repairs.
In essence, durability is the capability of a building, assembly,
component, structure, or product, to maintain a required
minimum performance over at least a specified time, while
in its environment of operation.
Modern design codes are increasingly based on the durability
performance of buildings and it must be ensured that adequate
performance continues throughout the service life of the
structure. A durability design approach offers considerable
benefits for both asset owners and society. By understanding
the deterioration processes affecting structures, design and
maintenance can be optimised to ensure that service life
aspirations of employers are met without unnecessary use of
resources, both during construction and while the structure
is operational. This offers improvements in sustainability,
climate change resilience and potentially the whole life cost.
DURABILITY DESIGN
DESIGNING
FOR
DURABILITY
Construction projects should be designed and specified to
provide an appropriate level of durability for their intended
service life and service environment. Failure to achieve
durability requirements frequently becomes a factor in
disputes as associated defects present themselves. Often,
technical experts are called upon to determine causation
and provide advice regarding remediation and future
maintenance.
DEFINING DURABILITY
Definitions of durability are found in the following
standards:
Jeremy Ingham, Diales Technical Expert, summarises
the use and pitfalls of durability design for construction
projects.
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Section 2.4 of BS EN 1990:2001+A1:2005 Eurocode —
Basis of structural design (section 2.4).
Section 4.1 of BS EN 1992-1-1:2004+A1:2014 Eurocode
2: Design of concrete structures - Part 1-1: General
rules and rules for buildings.
Section 3.10 of ISO 13823:2008 General principles on the
design of structures for durability.
Section 2.1.3 of fib Model Code for Service Life Design
(2006).
Section 2.3 of ACI 318-14 Building Code Requirements
for Structural Concrete.
Standards provide guidance on how to design for durability
in specific service environments and for certain minimum
lengths of working life. However, employers will sometimes
require a service life that is longer than allowed for in
standards and/or that the construction materials should
remain durable in harsher environments than allowed for by
standards. In these situations, a technical specialist (usually
a specialist materials engineer) can undertake a durability
study to determine the type of materials and protective
measures that are required. These durability studies often
involve the use of deterministic or probabilistic modelling
using predictive computer modelling tools.
It is now common on large projects for tender-stage and
design-stage durability reviews and reports to be provided
by materials engineers. These typically include:
An overview of the applicable codes and standards.
A description of the structures with breakdown into
principal structural components.
Details of the environmental and in-service exposure
conditions.
Details of the anticipated deterioration mechanisms.
A detailed strategy for achieving durability.
Specification and construction guidance for achieving
durability.
Maintenance and operational requirements for achieving
durability.
A recent high-profile example is the New Safe Confinement
built to cover and allow dismantling of the remains of the
number 4 reactor at the Chernobyl Nuclear Power Plant.
Completed in 2018 its shed-like structure comprising a steel
arch with cladding was slid into position and is the largest
moveable land-based structure ever built. Long design life
requirements (100 year minimum) combined with harsh
climatic conditions at a heavily contaminated site made
careful consideration of the durability of the materials and
structures imperative. A combination of durable materials
and special corrosion protection measures were used. This
includes treating the air around the steel arch on an on-going
basis to maintain low humidity that will prevent corrosion of
the structural steel members.
PRACTICAL APPLICATION TO CONCRETE
STRUCTURES
Concrete is the most widely used construction material in
modern buildings and civil engineering structures. When
appropriately designed and constructed, concrete structures
bring considerable sustainable, societal, economic, and
environmental benefits throughout their whole life.
Designing durability into new concrete structures is an
effective means of minimising their whole-life cost and
improving their sustainability.
For many structures this can be achieved through design
using tabulated guidance in internationally recognised
standards. However, special consideration is required for
concrete structures, if:
The service environment is particularly aggressive to
concrete (e.g., marine conditions, arid climates, etc.).
The service life is very long; greater than 100 years (up to
120 or even 150 years).
The asset is regarded as critical infrastructure where the
consequence of failure is great (e.g., bridges, tunnels,
power facilities, etc.).
In such cases, a project-specific durability study that
includes the use of predictive modelling tools, should be
undertaken. These may be deterministic or probabilistic in
nature. Durability studies that include modelling are now a
fundamental part of the tender design and detailed design
stages for major infrastructure projects.
Rusting of steel reinforcement bars that are
embedded in reinforced concrete is the most
important form of deterioration in concrete
structures worldwide (see Fig.1, page 25).
The corrosion reaction is initiated by differences in electrical
potential caused by variations in the environment along
a reinforced concrete element. Such variations include
exposure to moisture, oxygen and salts, differences in the
depth of cover concrete, stray electrical currents or where
two dissimilar metals are connected.
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