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Published : Tuesday, March 27, 2012
Reading/s : 27
Origin : transportresearchfoundation.co.uk
Number of pages: 23
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Sustainable bridges through innovative advances
2 May 2007
Presented by Professor Adrian E Long at the Joint ICE and TRF Fellows Lecture


1 Abstract 2
Introduction32
3 Sustainability issues affecting bridges 5
Development of novel in-situ test methods 84
5 Sustainable concrete bridge decks by design 13
6 A novel flexible concrete arch system for sustainable bridges 17
7 Concluding remarks 21
8 Acknowledgments 22
9 References22




















Institution of Civil Engineers Registered charity number 210252
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1 Abstract

Sustainability is now recognised as a key issue which much be addressed in the
design, construction and life long maintenance of civil engineering structures. In this
lecture, generic aspects of sustainability will be briefly discussed, but the main focus
will be on its application to bridges. Motorway bridges built in the 1960’s/70’s had
design lives of 120 years; however many were showing signs of deterioration after
only 20-40 years. This led to much debate on the issue of initial versus full life cycle
costing which is still ongoing today.

In order to address the highly complex issue of the sustainability of bridges the
author will discuss a number of specific areas which impinge on this important
subject:

1. The impact on sustainability of different forms of bridge construction and
maintenance/repair/replacement strategies. Alleviation of the indirect effects
of congestion, by repairing bridges whilst they remain in service will also be
discussed.
2. The utilisation of innovative in-situ testing equipment which will allow the
long term durability of concrete to be addressed or an indication obtained
for the remaining life of existing concrete bridges.
3. The development of innovative structural designs for bridges which
inherently have greatly extended lives at minimal, if any, additional cost. For
example:
Taking the benefit of compressive membrane action to produce bridge
deck slabs without any corrodible reinforcement or with minimal
reinforcing steel.
Increasing the use of arches which have an enviable record for longevity,
durability and strength. Here experiences with a flexible concrete arch
which requires no centring, has no corrodible reinforcement and can be
delivered to site in a flat pack form will be detailed.
Institution of Civil Engineers Registered charity number 210252
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2 Introduction

The built environment has to co-exist with the natural environment with which it is
inseparably linked. Energy, materials, water and land are all consumed in the
construction and operation of buildings and infrastructure to such an extent that
sustainable development can be said to depend on the built environment. The
world’s cities have a major impact on emissions of ‘green house gases’ and global
warming: they take up around 2% of the earth’s surface but account for nearly
80% of the carbon emissions from human activities. The urban environment
influences our living conditions, social well-being and health. Thus the performance
characteristics and quality of our infrastructure are of fundamental importance to
urban sustainability and the well-being of our environment. The significance of this
should not be underestimated especially if it is borne in mind that our infrastructure
accounts for at least 50% of our national wealth.

The burden placed by construction on our natural resources can be estimated from
the embodied energy ie the total primary energy that has to be extracted from the
2 earth to produce a specific product – usually measured per m of plan area. In
addition the operational energy used during their lifetime has to be taken into
account. The relative proportion depends on the form of construction. In general a
bridge has high embodied energy and low operational requirements whereas a
hospital with its demanding service conditions has a high proportion of operational
energy. However for bridges the relative proportions for these energies depends on
the extent of maintenance/repair during their lifetime. If minimal maintenance/
repairs are required the operational energy may be only marginal however if
extensive repairs are necessary and considerable disruption/congestion results the
energy consumed can increase dramatically. Thus the challenge for designers is to
achieve the minimum total energy used over the 120 year design life and to
persuade the client that a sustainable approach is preferable to a minimum initial
cost design.

In order to contribute to a better understanding of the highly complex issue of the
sustainability of bridges a number of specific aspects which impinge on this
important topic will be discussed:

1. The relative merits of different forms of construction from the sustainability
viewpoint and the benefits of designing bridges which can be repaired whilst
remaining in service.
2. The utilization of innovative in-situ testing equipment which will allow the
durability of concrete bridges to be enhanced or an indication obtained for
the remaining life of existing bridges of this widely used construction
material.
3. Technological innovations which could lead to much more durable and
sustainable forms of construction for concrete bridges based on:
The enhanced strength of deck slabs arising from arching action.
A novel flexible concrete arch system.
Institution of Civil Engineers Registered charity number 210252
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In the latter two instances the approach adopted in research at Queen’s University,
carried out with industry, will be placed in context.

Institution of Civil Engineers Registered charity number 210252
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3 Sustainability issues affecting bridges

The environmental impact of new bridges

The embodied energy from the use of construction materials is a source of concern
to engineers when planning, designing and constructing a bridge. However
relatively little advice or guidance is given in the literature as to the relative merits of
[1]different forms of construction. In this context a recent paper by Collings
presents the results of a comparative study, derived from an actual project. A
bridge in the UK over a river of width 120m with 66m of approach spans on each
2side was considered. The total deck area was over 4000m and this bridge allowed
consideration of the shorter spans on the approaches as well as the main river span.
Three basic forms of construction were considered for the river span; a profiled
girder; a tied arch; and a cable stayed bridge. Constant depth girders were used for
all the approach spans. Temporary works were included as was an estimate of the
likely repair, maintenance during the life time of the three basic forms of
construction; steel; concrete; and composite construction.

Useful comparative tables and graphs are included in the paper by Collings however
the results summarising the impact of the span and the form of construction on the
embodied energy are only included in this paper. The estimates of the embodied
2energy during construction (per m for bridge deck) are tabulated in Table 1. Values
2 vary from approximately 16 to 75 GJ/m of deck with the short span concrete
structural form giving the lowest values and the all steel or composite, longer span
structure the highest. The embodied energy is also presented graphically in Figure
1, where it can be clearly seen that longer spans consume greater embodied
2 2 energy/m (not unexpected as the cost/m also follows these trends). Figure 1 also
implies that a well engineered longer span bridge using local materials, recycled
steel and sustainable cement can be almost as environmentally friendly as a shorter
span structure where sustainability issues are not considered. Table 1 and Figure 1
also indicate that at the spans under consideration the more architectural solutions
(arches, cable stayed) have a higher environmental burden for all materials (as well
as a cost premium). Only basic arches/cable stayed systems were considered and
incorporating leaning or distortion of elements would have further increased the
[2]material content , construction complexity and the environmental burden. Further
comparative studies of this nature, by experienced designers, need to be
encouraged so that the most appropriate forms of construction, from the
sustainability viewpoint, are selected.

Repair, maintenance and congestion

All bridges will require some form of intervention during their lifetime and ideally
this, as well as all other aspects, should be taken into account in the design process.
Even the most basic maintenance will cause congestion but the impact of
replacement can be much greater as exemplified by the Tinsley Viaduct.

Institution of Civil Engineers Registered charity number 210252
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Energy Type Steel Concrete Composite
viaduct 17.8 15.7/16.6 16.6
girder 30.9 23.6 29.1
Minimum arch 49.8 34.3 48.8
cable-stay 40.3 21.1/22.1 37.7

viaduct 23.5 21.1/22.1 22.1
girder 39.3 30.6 37.0
Average arch 61.9 49.1 60.8
cable-stay 50.6 43.9 47.7

viaduct 30.8 28.1/28.6 29.2
girder 49.3 39.1 46.6
Maximum arch 75.6 60.9 74.4
cable-stay 62.6 54.8 59.3

2Table 1 Embodied energy during construction (GJ/m ) for Figure 1 Variations in embodied energy
various structural forms and materials with span and material type


The Tinsley Viaduct is a twin deck steel/concrete composite beam girder bridge
which carries the M1 motorway and the A631 trunk road across the Don valley near
Sheffield. The 1km long structure has 20 spans and crosses two railway lines, the
River Don and a canal. As a strategic part of the motorway network, the viaduct
carries approximately 115,000 vehicles per day. However in the late 1990’s it failed
to satisfy the requirements for the introduction of 40t lorries and decisions had to
be taken on whether to strengthen or replace the structure.

A replacement bridge was estimated to cost £200m however the associated cost of
congestion over the 2-3 years period of construction was considered to be around
£1400m. This enormous additional cost, not to mention the associated
environmental impact of congestion, was clearly unacceptable and in the end it was
decided to carry out a complex strengthening process whilst keeping the viaduct
open for traffic except for a short time each night. In the end the Viaduct was
repaired for £80m with minimal congestion resulting – a net saving of £1,500m.

From the viewpoint of the impact of congestion this extreme example demonstrates
the importance of having bridges which can be repaired whilst effectively remaining
in service. In this regard steel is more amenable to strengthening however the
availability of carbon fibre composites allows comparable action to be taken for
concrete bridges. It should also be noted that the cost/environmental impact of
congestion is an ever increasing problem as many urban bridges built in the 1960’s
are now in need of remedial action. This should be considered, even if only
approximately, in the total life cycle design of future bridges. Whilst this may
increase the design cost the long term savings could be enormous.



Institution of Civil Engineers Registered charity number 210252
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The relative importance of congestion also requires designers to think carefully
about the selection of durable materials and the most appropriate form of
construction. As a consequence in the future it will be even more important to
build bridges which require minimal maintenance and ensure that premature
replacement is avoided.

Total life cycle ‘cost’ of bridges

Whilst the initial ‘costs’ are useful it is the ‘cost’/’energy use’ over their full life that
is more significant. In this context Howard Taylor in his TRF Fellowship lecture
[3] highlights the importance of adopting Integral Bridges for relatively short spans.
Basically by designing a bridge without movement joints and which is integral from
one abutment to the other the maximum resistance to chloride penetration is
obtained. As a further step the timely and appropriate application of protective
coatings such as a silane or ‘Pavix’, which can be applied whilst it is in service, can
delay the need for bridge repair. Thus by using these methods and some of the
innovative approaches detailed later in this paper the life of specific types of bridges
can be greatly enhanced and the total life cycle ‘cost’/’energy use’ reduced.

Institution of Civil Engineers Registered charity number 210252
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4 Development of novel in-situ test methods

Background

The mechanisms of deterioration and their rate are controlled by the environment,
the paste microstructure and the fracture strength of the concrete. Environmental
factors such as seasonal temperature variations, cyclical freezing and thawing,
rainfall and relative humidity changes, and concentration of deleterious chemicals in
the atmosphere/water in contact with the concrete are the main causes of
degradation. However, the single most important parameter that leads to
[4, 5]premature deterioration is the ingress of moisture into the concrete .
Permeability of concrete to the macro-environment during its service life therefore
can be used as a measure of its durability.

[5]In the development of a holistic model for concrete deterioration, Mehta has
considered reinforced concrete with discontinuous microcracks. In this model, the
influence of environmental factors on the various deterioration mechanisms
involved causes the microcracks to propagate until they become continuous. In
essence, the permeability influences the primary method of transport of moisture
and aggressive ions into the concrete and subsequent increases in the permeation
properties are responsible for the increased rate of damage. Thereafter, crack
growth (which depends on the fracture strength) accelerates the penetration of
aggressive substances into the concrete and the spiral of deterioration continues
downwards. The interdependence of all these factors and the importance of
permeability/transport properties and strength are clearly illustrated in the holistic
[4]model in reference .

Measurement of strength

The pull-off test

The pull-off test is based on the concept that the ‘tensile strength’ of a layer of
surface concrete can be related to the compressive strength of the concrete (Figure
2). The ‘LIMPET’, developed at The Queen’s University of Belfast can then used to
measure the tensile force to ‘pull-off’ the disc and a nominal tensile strength
calculated on the basis of the disc diameter (usually 50mm). To convert this tensile
strength into a cube compressive strength an empirical correlation curve is normally
required (Figure 3). It should also be noted that the pull-off test gives a good
indication of the tensile strength or the fracture strength which is one of the most
important factors in relation to the rate of deterioration. Unlike most other partially
destructive tests, the variation in strength with depth can be determined by using
partial coring and this technique is also invaluable for assessing the bond strength
of patch repairs.

Institution of Civil Engineers Registered charity number 210252
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50 mm Dia.
Steel Disc
Epoxy Resin
Adhesive
Partial core
Typical failure
surface
Figure 2 Schematic diagram showing the Figure 3 Pull-off test results
procedures used to complete a pull-off
test

Using the pull-off test, it has been found that within-test variability is sufficiently
low to allow the natural variations in strength from one location to another to be
detected (eg bottom vs top of columns). In addition, the effects of maturity are
automatically included and useful information obtained on partial safety factors for
[6]in-situ variability .

Measurement of permeation/transport properties

The main transport processes which describe the movement of aggressive
substances through concrete can be categorised as absorption, permeability and
[7]diffusion .

Absorption and permeability testing

[8]When the ‘Clam’ test was first reported , in the early 1980s, it had been developed
to overcome some of the problems associated with the standard Initial Surface
Absorption Test (ISAT) such as achieving a watertight seal and accurately measuring
the flow rate. Initially it was only a water permeability test but it was then modified
and the ‘Universal Clam’ produced which enabled both water and air permeability
[9]to be measured. In the early 1990s Basheer completed further development work
that not only standardised all the tests but also made the whole process fully
automatic. This version of the Clam, the ‘AUTOCLAM’, is now controlled by a
microprocessor and has a complete data acquisition and transfer facility to enable
analysis of the results by computer. The ‘AUTOCLAM’ has been available
commercially for over 10 years and three types of test are now possible: water
absorption, air permeability and water permeability. All three tests are quick and
simple to perform both on-site and in the laboratory.


Institution of Civil Engineers Registered charity number 210252
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An indication of the relevance of permeability testing to durability testing can be
[9]observed from a comprehensive series of tests by Basheer . The high permeability
values obtained from the ‘AUTOCLAM’ (Figure 4) show a strong correlation with
high levels of freeze-thaw damage, and vice-versa.

25
Extremely
20 high
15
High
10
Medium
5
Low
0
Mix Numbers

Figure 4 Risk levels based on extent of deterioration caused by freezing and thawing [9]


Diffusion tests

Recently there has been an increased interest in ionic diffusion tests because the
rate at which chloride ions diffuse through concrete is closely related to the
corrosion of reinforcement. The chloride diffusion coefficient can be determined
from several types of test, but in this paper the discussion will be limited to just one
test technique which can be carried out in-situ or in the laboratory. Other tests
require cores to be extracted.

‘PERMIT ION MIGRATION TEST’: This test, recently developed at Queen’s University,
Belfast, has the advantage that the migration coefficient of concrete on site can be
determined and, cores need not be taken. Full details of the equipment are given in
[10]Andrews and the basic concept is shown in Figure 5.

Basically it consists of two concentric cylinders, placed on the concrete surface, and
then sealed them to prevent any flow between them along the surface of the
concrete. In this accelerated test a potential difference of 60v dc is applied between
the anode and the cathode which forces chloride ions to travel from the anode to
the cathode through the concrete in the near surface zone. After about six to 10
hours, depending on the quality of the concrete, a steady migration of chlorides
into the outer cell is achieved, and the chloride migration coefficient can be
[10]calculated . This correlates well with the effective diffusion coefficient (Figure 6).
Thus the Permit ion migration test, now available commercially, can be used to
determine the diffusion characteristics of concrete on site.

Institution of Civil Engineers Registered charity number 210252
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Clam WP index
(Cu.m x E-7 /¦min)
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