Memorandum by TRL Limited (RM 16)
1. We welcome the opportunity to submit
comment on the matters under consideration by the sub-committee
the current state of repair of roads;
the steps taken by government, the
Highways Agency, and local authorities; and
possible future improvements.
2. We have written this memorandum in our
capacity as an impartial research establishment, drawing on +our
scientific knowledge rather than commenting on matters of policy.
We have put greater emphasis on our knowledge of the approach
to establishing the state of repair, rather than on the condition
itself. We felt it also important to submit comment on the appropriate
standard that should be achieved. Finally, we have commented on
sustainability issues, including the use of quieter road surfaces
and the recycling of materials, as requested. The link between
research and application in these areas is a very close one and
we have therefore attempted to set the operational context for
much of the material relating to research in the submission.
The Current position
3. The current state of repair of trunk
roads is expressed by the performance indicator for maintenance
described in the Highways Agency (HA) Business Plan and Annual
Report. Each year, the conditions of trunk roads and principal
roads are also reported separately in the National Road Maintenance
Condition Survey (NRMCS). The HA performance indicator is for
carriageways only and does not apply to other parts of the road
(eg footways or structures). The NRMCS is primarily for carriageways
but also includes a summarised representation of the condition
4. Currently, the HA performance indicator
and the NRMCS use different measures of condition for trunk roads.
We welcome the review of the NRMCS (in progress) from which it
is expected that a common reporting measure will be specified
for trunk and principal roads.
5. The inspection of some components of
the road structure that have a significant influence on the condition
of the carriageway is not included in either the HA performance
indicator or the NRMCS. For example, there is concern that the
sub-surface drainage network of some trunk and principal roads
is in need of attention but there is little hard evidence to support
6. The target level of condition for trunk
roads aims at achieving a steady state, ie that no more than 7
per cent to 8 per cent of the network requires maintenance at
any one time. The estimated proportion of trunk road network requiring
maintenance at March 2000 is 7.5 per cent. But the target condition
of principal roads in terms of the whole-life cost, or any other
measure, has not been established. A further weakness of the present
system is that, for both surface condition and pavement strength,
there is no satisfactory way of comparing trunk roads and principal
roads. The measures of condition are for asphalt roads only.
7. Current research (by TRL) is investigating
what the target condition should be so as to minimise the whole-life
cost of trunk road maintenance, evaluated in terms of maintenance
works cost and the costs of disruption to road users at roadworks
sites. All of the current reports on the state of repair of the
trunk road network are based on levels of condition; in the future
they will take into account the costs of future maintenance work.
The HA has a performance indicator for lane availability on the
road network but there is no equivalent for principal roads.
8. For trunk roads, the need for maintenance
work and appropriate remedial treatment is established following
clearly defined condition assessment procedures set out in the
Design Manual for Roads & Bridges (DMRB). This process feeds
through to the fund bidding and prioritisation requirements specified
in the HA Bidding Pack, which Managing Agents should follow to
ensure best practice in developing a programme of maintenance
work that reflects value for money. The identification of maintenance
needs and allocation of funds for principal roads follows a similar
process. But there is an added dimension in a "political"
factor; ie the final prioritisation of works is often influenced
by Members' desires and does not necessarily reflect the ranking
of the condition on the road network.
9. There is a need to ensure that sufficient
resources are allocated so as to guarantee a successful implementation
of the new NRMCS following completion of the review.
10. There is a need to improve the reporting
of the condition of the road network, to include all pavement
types (asphalt and concrete) in the network and to make it consistent
across both trunk and principal roads. The target condition should
be identified for each road type. This target should be set taking
account of minimum levels of condition (such as skidding resistance),
the whole-life costs of the maintenance works and the level of
serviceability provided to the road user and those affected by
11. All reports on the state of repair are
based on static measures of condition which are generally the
average condition. An additional indicator could show how the
condition is changing. At best, maintenance is carried out on
only a small proportion of the network in any year and as some
deterioration in condition is relatively slow, there is little
change in the average network condition each year. Research (by
TRL) for the CSS in 1997, showed that a better representation
of the change in condition is given by the "tail" of
the distribution of condition. The advantages from using this
approach, in terms of quality of reporting, have not been fully
exploited in any of the national reports.
12. Although historic results of condition
allow trends to be shown, no reports are available which predict
the likely future condition of the network. A useful extension
to the traditional reports would be the expected future condition
of the network under different assumptions, or at least under
the current assumption, of future maintenance funding.
13. The state of repair of road networks
is reported separately for each component of the road, ie carriageway,
bridges, safety fences, signage and so on. The state of repair
of both trunk roads and principal roads does not take into account
the quality of ride or journey. Highway authorities have responsibility
for all aspects and some way of reporting the combined state of
repair for all components would allow summaries of the state of
repair and performance of the whole of each network to be produced.
14. The objectives in the Ten Year Plan
have led to a significant increase in maintenance funding for
local roads. Although much of the increase will be for non-principal
roads, it is a concern that there are currently insufficient assessment
tools to report on the effectiveness of the increased maintenance
expenditure on principal roads compared to those which exist or
are being developed for trunk roads. The need for research to
develop such tools is urgent.
15. Research is also required to assess
the current state of drains on selected sections of the road network.
This might also identify the need for further work on the design
and detailing of drains. For assessing the condition of long lengths
of pipe drains, it is likely that the use of CCTV surveys will
become more widespread.
16. We consider the issues of recycling
and quieter road surfaces in paragraphs 35 to 52 below.
The Current position
17. All highway authorities undertake inspections
and structural assessments using detailed engineering standards
and procedures, most of which have been developed by the HA (and
earlier by DTp) with the close involvement of TRL. Structures
are regularly inspected to determine their physical condition
and structural assessments are carried out to determine whether
structures can safely carry the required loading.
18. Structural assessments on bridges are
being carried out as part of the national bridge assessment and
strengthening programme that is nearing completion. Assessments
have been undertaken on short span bridges designed prior to 1975
and longer span bridges ie those with a loaded length in excess
of 50m. The assessment calculations take account of the physical
condition of the structure as determined from the inspections.
This will ensure that all bridge structures currently assessed
to be sub-standard or unsafe will soon be either strengthened
or otherwise safeguarded. Structural assessments on other highway
structures are normally made where a structure shows signs of
distress. However, there are no nationally published data to indicate
how many structures are at present in an unacceptable state in
terms of their material condition.
19. Maintenance work is currently triggered
from the results of an inspection or a structural assessment.
It is divided into three categories: essential, preventative and
routine. Essential maintenance is required to maintain safety
standards eg on structures that are assessed to be inadequate
for the 40 tonne loading or because the condition of the structure
is deteriorating to an unacceptable level which may cause public
alarm or make the structural behaviour unpredictable. Preventative
maintenance is work that is not essential but which is aimed at
reducing future deterioration. Routine maintenance is work that
should be carried out at regular intervals to keep structures
in good order for example cleaning drains.
20. There is currently no recognised condition
index for highway structures. Most structures are, in respect
of their load carrying capacity and physical condition, somewhere
between the "as new" state and the "unsafe"
or "unacceptable" state but there is no means for knowing
where the structures lie within this range. There is however evidence
that some types of structure lie towards the unacceptable end
of this range. There are also some types of structure that are
difficult to assess.
21. Recent publicised figures on lighting
columns throughout the UK have raised the awareness of street
lighting engineers to the ever-increasing age and condition of
the lighting stock. There are around 6.3 million columns in the
UK, and it is estimated that around 700,000 columns are in urgent
need of replacement. Nearly 30 per cent of the current stock of
lighting columns is over 30 years of age and a further 2.2 million
columns will reach this age within the next 10 years. Because
most deterioration takes place internally and below ground the
structural condition of columns cannot easily be identified. This
is a major difficulty. Past under investment in maintenance and
replacement has generated concerns about public safety.
22. The condition of substructures such
as buried structures, and retaining walls are difficult to assess.
The reliability of structural assessments of a corrugated steel
buried structure is a problem because the method is highly empirical,
and for anchored structures it is almost impossible to assess
structural stability without information on the loads carried
by ground anchorages. For these structures, judgement is largely
based on inspection reports.
23. There are certain types of structure
on the road network which are likely to be subjected to increasing
risks in the future years. Such risk factors arise from vehicle
impacts on bridge columns or bridge parapets, corrosion on tendons
in post-tensioned concrete bridges and scour of the foundation
supporting river bridges. As bridges age so the risk of fatigue
in steel structures will also increase. Although investigations
are being carried out in respect of these risk factors, there
are at present no nationally published data to indicate the adequacy
of the structure types concerned to withstand these risks.
24. In the past, maintenance was carried
out following an inspection with the aim of returning the structure
to its original state. More comprehensive procedures are now being
introduced by the HA so that in the future maintenance is likely
to be based on whole life costs and the sustainability of resources.
25. A good deal of research has been undertaken
for the HA on the development of advice on the inspection and
assessment of buried structures. Work has also been commissioned
to determine best practice for the design, inspection, assessment
and maintenance of ground anchored structures. Some of this was
aimed at determining long-term performance. The final results
of this work should come to fruition and be implemented in the
26. A review of non-destructive techniques
for determining the condition of lighting columns is underway.
The development of a risk management strategy to enable maintenance
engineers to prioritise the use of limited funding for the testing,
maintenance, repair and replacement is also being undertaken to
minimise the risk of injury to people and property. An inventory
database has been identified as essential to compiling information
about the condition of the national lighting column stock and
it is anticipated that funds will be made available for its compilation
27. There has been a relatively high incidence
of shallow failures in embankments on the highway network. The
HA has promoted research on assessing the likely rate of failure
of slopes of different slope angles and in various types of soil.
Risk assessment models have been developed from data of this type.
28. Methods of inspecting the condition
of earthworks using remote sensing techniques are also under investigation.
The use of risk assessment models based on the results of remote
sensing surveys should become commonplace. This should help in
the efficient management of earthworks. In addition it would seem
necessary to undertake some defensive research to determine the
likelihood of deep-seated failures in earthworks. A high incidence
of such failures would have significant economic implications
for owners and maintaining agencies alike.
29. The HA is currently developing and implementing
new maintenance procedures for the Trunk Road Network. These include
the structures strategic plan which will provide future expenditure
profiles for different types of maintenance, whole life assessment
and risks and options based prioritisation of works. These procedures
could also be applied to local authority structures. A nationally
co-ordinated approach is required to implement these procedures
30. Research is required to determine the
optimum maintenance strategy for structures at the network level
to ensure that future maintenance is properly planned and peaks
in expenditure avoided. Failure to do so might result in backlogs
of work or periods of high expenditure since particular groups
of structure may need rehabilitation all at the same time. The
development of an optimum strategy would ensure that best value
was obtained from the available funds for maintenance.
31. Maintenance strategies and programmes
are required to safeguard structures against the increasing risks
from Heavy Goods Vehicles collision, flood action and steel fatigue.
Post-tensioned concrete bridges, a few of which around the world
have suffered from catastrophic failure in the past, should be
subjected to special attention.
32. Developments over the last 15 years
have led to a fragmented approach to lighting column maintenance
and to the tools that could assist in determining the structural
condition. A risk management strategy, which allocates columns
to a high, medium or low risk category is needed using data from
accurate non-destructive test methods. It will allow prioritisation
of maintenance based on real need.
33. Research is also required to determine
the life cycle costs for lighting columns. Included in this will
be the potential for reduction of the energy consumption of lamps
and carbon dioxide emissions. Together with the structural assessment
of lighting columns, a more cost-effective management strategy
for the repair and replacement of lighting columns and luminares
can then be realised.
34. There is a need to ensure that maintenance
is properly resourced not only in terms of finance but also in
terms of the availability of experienced maintenance personnel.
Developments over the last few decades have meant that centres
of expertise (local authority bridge offices, road construction
units and regional offices) have changed and the engineering expertise
has diminished. There has also been a loss of continuity due to
constant changes in responsibility for road maintenance. This
has meant that the whole process is becoming fragmented and there
is an increasing tendency for those responsible to rely more on
procedures than experience and engineering judgement. This needs
to be addressed by paying greater attention to developing and
retaining engineering expertise in the authorities and in other
35. Sustainable development can be defined
as Development that meets the needs of the present without compromising
the ability of future generations to meet their own needs (Bruntland
1987). Sustainable Development and the contribution of Sustainable
Construction practices are now at the foreground of DETR policy
development. Because this is an all-encompassing issue, many individual
themes relating to the sustainable construction and maintenance
of roads are already addressed. They include whole-life costing,
use of recycled, locally-won or alternative materials, road-user
and workforce safety, use of low-noise surfaces, environmental
management systems and so on. Some other issues are not yet adequately
addressed, for instance, the development of performance specifications
to allow innovation or alternative methods, monitoring of energy
consumption and emissions, waste-management, road-user and neighbourhood
consultation, considering the requirements of non-motorised users
and permitting access, etc.
36. All of these things have a part to play
in delivering more sustainable options for road repair, but there
remains a requirement to bring the separate themes together to
strike the right balance between sometimes conflicting objectives
and to deliver workable changes to current practice. To do this
will require a broad methodology for specification, procurement
and developing best practice guidance. Some elements are already
in place but others require development. A principal requirement
is the development of an assessment system with which to measure
progress against sustainability objectives, to benchmark and to
set targets. The production and adoption of indicators, in line
with DETR policy, is an essential part of this process and could
be used in assessment both as part of condition monitoring, scheme
appraisal and in procurement.
37. Specific issues of sustainability related
to recycling and quieter roads are considered separately as follows.
38. In this context, recycling is taken
to mean the re-use of materials already in the road, or the use
of wastes or alternative materials to imported natural resources.
Recycling road pavement materials can lead to reductions on the
use of resources including aggregates, bitumen and cement. Where
the process is performed in situ, or uses "cold-mix"
technology, reductions can also result in energy consumption and
the resulting emissions.
39. Foundation layers can be constructed
using graded, granular waste materials, or stabilised locally-won
soils etc, as an alternative to importing aggregates. Bound foundation
layers can use industrial by-products (pulverised fuel ash, granulated
blast-furnace slag) as alternatives to lime or cement.
40. Concrete can be recycled by crushing
and reuse as aggregate. This has proved more popular in some overseas
countries than in the UK. Construction and demolition waste can
be used as an aggregate in concrete and industrial by-products
can be used as alternative binders to cement. Concrete can also
be reused when an asphalt surface is required, by using techniques
such as crack-and-seat. This maintenance procedure retains the
concrete as the load-bearing layer while reducing the incidence
of cracking in asphalt overlays that would otherwise result. The
crack-and-seat method is currently finding wide application on
UK trunk roads and resulting in significant resource and cost
41. Standards and techniques exist for recycling
"conventional" asphalt pavement materials ("Repave"
and "Remix") but their use has been disappointing in
an industry that is traditionally risk-averse with respect to
new techniques and where the required capital investment is difficult
to fund. More recently, TRL, in collaboration with highway authorities
and the industry, has published procedures for the use of cold-mix
asphalt recycling, which has already been used on a number of
Local Authority schemes. Development of complementary procedures
42. Within the utilities industry there
are increasing environmental and commercial pressures to reduce
reinstatement costs. Again, there is potential for re-use of the
highway-excavated material. Where materials can be efficiently
and economically processed and stockpiled, the re-use of such
materials offers significant environmental benefits and inherent
cost savings. The re-use of excavated materials, to meet the relevant
specifications, has been made possible by the addition of stabilising
materials. In the case of water companies the aim is to ensure
that industrial by-products are used effectively to stabilise
excavated materials such that the resultant material has no adverse
effects on water-courses or the environment. Within the utilities
companies generally, there is a need to encourage a pro-active
culture of sustainable construction to support the conservation
of primary aggregates and to promote this through each company's
environmental policy. Further laboratory testing and field trials
are required to develop mix compositions with the best engineering
properties and long-term performance.
43. In addition to recycling the existing
pavement there is also the potential for the reuse and/or recycling
of other materials in asphalt. A recent example is the introduction
of 30 per cent green glass cullet into an asphalt product, in
place of natural crushed rock. However its use is restricted to
structural layers; glass cannot be used in surfacings due to poor
skidding resistance. Green glass is a waste material with very
limited markets. Tyre rubber has also been used in road surfaces
with recent success but there are doubts about its use in structural
layers. In depth studies have been undertaken and new specifications
will be available, in the near future, for the use of slag by-products,
quarry wastes from both slate and china clay sand, and bituminous
arisings (planings) in high-quality reuse applications. For surface
courses, the use of slag aggregate has been commonplace in proximity
to iron and steel works. The geographical range of use has been
limited by transport costs; this may change marginally (a further
20 mile radius has been suggested) due to the introduction of
the aggregates levy.
44. There are other materials where the
potential for reuse in the road pavement has not been fully exploited.
In particular the use of incineration residue from waste disposal
stations (despite the recent case of the Newcastle allotment site),
sewage sludge re-use, and demolition debris. The latter has developed
as a resource since the introduction of the landfill tax that
has led to improved demolition and waste disposal practices.
45. As mentioned later in paragraph 50 below,
the use of new thin surfacing products has grown enormously over
recent years. The specific issue of recycling thin surfacing systems
has not yet been addressed, as very few sites have reached the
end of their useful life. However, as with their durability, the
need to assess their recycling capability is coming to the forefront
and studies have been commissioned to research the potential to
extend the life of these products.
46. Overall, the technology required to
enable large increases in recycling is either in place, going
through approval procedures, or the subject of current research.
To secure these increases, three factors will have to be addressed.
The "newer" sources of
"waste" materials as viable pavement materials will
have to be validated.
Performance specifications will have
to be formulated that will allow contractors more freedom to innovate
in the use of materials for road construction and maintenance.
Procurement procedures will need
to be developed that encourage the implementation of recycling.
These might involve some form of scoring in the assessments of
Quieter road surfaces
47. The first "quieter" road surfacing
material was porous asphalt, developed during the 1970s and adopted,
after extensive trials, for use on trunk roads in the 1990s. Originally
developed for enhanced high-speed wet-friction on airfield runways,
it also offers the benefits of low-noise and reduced spray. However,
the HA has recently dropped porous asphalt from use due to its
high cost and some doubts about guaranteeing durability. Cost
considerations have also limited the use of porous asphalt by
Local Authorities. Continuing development of this material, for
instance in twin-layer systems, where only a thin upper-surface
is made from premium aggregate, may prove viable in the future.
48. Asphalt thin surfacing products were
first developed on the continent and introduced into the UK in
the early 1990s. Changes in the material specifications were required
to meet the UK's more stringent requirements for skidding resistance
and TRL undertook extensive trials of these surfacings, for the
HA, before they were permitted for use. The HA encouraged suppliers
to develop their own proprietary thin surfacing systems and this
led to the development of an approval scheme, operated by TRL
and the British Board of Agrément. Approval has been granted
for more than 20 products and most other highway authorities have
also adopted this scheme.
49. These thin surfacing materials are open-graded
with a degree of surface porosity, which means that, like porous
asphalt, they offer marked noise reductions over conventional
materials such as chipped hot-rolled asphalt (HRA). Generally,
porous asphalt and thin surfacing give noise reductions equivalent
to a reduction in traffic of 50-70 per cent. However, there is
recent evidence that this reduction can be significantly diminished
on wet roads. No equivalent diminution was found to be the case
for non-porous HRA.
50. Thin surfacings are now the predominant
surface course materials for UK trunk roads and Local Authority
principal roads, indeed the HA has recently issued new advice
which actively discourages the use of HRA and surface dressing
systems as alternatives. However, some issues remain open to question
in the, now widespread, use of thin surfacings. Their long-term
durability compared to HRA has yet to be proven, due to their
recent acceptance, and continued performance monitoring of older
sites is being undertaken by TRL for the HA. Another issue concerns
the sustainability of these products. Open-graded materials require
carefully sized aggregates and do not use the fine fraction of
material generated by the production process. These factors are
responsible for an increase in unused fine aggregate waste. Also,
these systems use premium, skid-resistant aggregate throughout
the layer (about 40mm thick) rather than just in the surface,
as for HRA or surface dressings. This is a sub-optimal use of
a scarce and expensive natural resource. This could be countered
to some extent by recycling of the materials after their initial
lifetime as discussed earlier.
51. A third surfacing offering a quieter
road than conventional materials is exposed aggregate, or "whisper",
concrete. This surface can provide noise reductions compared to
HRA, equivalent to a reduction in traffic of up to 40 per cent.
The reduction is much greater compared to traditional concrete
surfaces, which have recently become very unpopular amongst neighbouring
communities due to high noise levels. Concrete surfaces have potential
benefits compared to asphalt, in that, if properly constructed
and maintained they will not undergo surface deterioration rutting,
which is a recognised risk for asphalt roads. The same comments
apply with regard to the use of premium aggregates in depth as
for thin surfacings.
52. In response to public opinion regarding
concrete surfacings, the 10-Year Plan commits the HA to overlaying
all concrete stretches with quieter surfaces. The HA will prioritise
these works where the noise level from the existing concrete surface
affects the most residents. On the grounds of noise level, the
justification for overlay of whisper concrete would appear to
be weaker than for other concrete roads.
53. This memorandum has summarised, necessarily
briefly, aspects of maintenance on which research can throw light.
The matters discussed are in many cases complex and further more
extensive argumentation can be found in relevant technical reports.
23 February 2001