Prof. László Gáspár – Dr. Zsolt Bencze, KTI (Institute for Transport Sciences Non-Profit Ltd), Budapest, Hungary
The ambition for building long-life pavements (LLPs) of roads is obvious in every country due to the ever-increasing traffic demand and funding constraints. LLP is an efficient way to reach the aimed goals of lower whole life cost and lower user costs (traffic operation cost, time delay cost and accident cost). There are a lot of differences world-wide in the definition of highway pavements of long duration (long-life pavements), however, related to its American equivalent, the “perpetual pavement” – the most widely used versions say: “a perpetual asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation, and needing only periodic surface renewal in response to distresses confined to the top of the surface” [Newcomb, 2002], and “a Long-life Pavement is a type of pavement where no significant deterioration will develop in the foundations or the road base layers provided that correct surface maintenance is carried out” [Ferne et al., 2006]
Road infrastructure investment has increased less in many countries than road traffic. If these trends continue, the outcome will be increasing intensity of road traffic on road networks in the future. These trends support the view that there will be increasing numbers and proportions of roads that are highly trafficked and therefore candidates for more durable pavements at higher construction costs.
A survey of member countries shows that pavements in use on high-traffic roads are typically re-surfaced every ten years (depending on local conditions). Within the ten-year period, there may be some other road maintenance closures for pavement repairs like patching and sealing. Indeed, the initial construction costs of a pavement are often surpassed by the costs of its life-cycle maintenance and operation. From a roads-budget viewpoint, maintenance work incurred in future years may seem preferable to increased capital expenditure now.
However, apart from the direct costs of maintenance funded from road administration budgets, road maintenance also imposes significant costs on users. On highly trafficked roads, in particular, road maintenance is likely to cause traffic congestion and disruption to normal traffic flows. Despite the measures taken by road maintenance operations, the costs to users in many locations are high and increasing. Hence, there are growing pressures for long-life road infrastructure pavements that require minimal maintenance and future costs to road administrations and users.
The paper summarizes some of the recent European and American achievements in the design, material selection, construction, performance, maintenance, operation and economic issues of the main pavement types, pointing out also the typical research trends.
A European Long-Life Pavement Group (ELLPAG) has been formed under the aegis of the Forum of European National Highway Research Laboratories (FEHRL) to report on the current state of knowledge on long-life pavements in Europe, in particular, on the very practical issue of how to design, build and maintain pavements to give long structural lives [ELLPAG Phase I, 2003; Ferne, 2006; ELLPAG Phase II, 2009]. The findings of the first two phases of the project were summarized in reports reviewing the current state-of-the-art of fully flexible long-life pavements (LLP) and that of semi-rigid pavements based on the combined knowledge of the European pavement community. The reports include chapters on new pavement design, assessment and upgrading, maintenance and treatment design, economics and, finally, identifying knowledge gaps.
Some of the main conclusions of the fully flexible phase of ELLPAG project were:
- Long-life pavements can be built by new construction or by strengthening existing structures
- Just the building of the long-life version of heavily trafficked pavement structures can be economic (with relatively low life cycle costs)
- For long-life, if fully flexible pavement structures, the total thickness of asphalt layers should reach 300 mm
- The maximal tensile strain at the bottom of asphalt layers should not reach 50 µm during the whole life
- Ensuring the continuously high subgrade strength is also among the preconditions
- Eventually, use of special techniques is needed for the construction of pavement layers
- Excellent construction quality is a must with strict, independent quality management
- In case of a “successful” long-life pavement, the deterioration is limited just for the wearing course that should be replaced every 10-12 years.
- The typical European semi-rigid pavement structures considered in the study: 150 - 290mm asphalt layers and 150 – 300mm hydraulically bound layers
- The aggression coefficient of the traffic load considered in the design of semi-rigid pavement structures is much (2-4 times) higher than that of fully flexible pavements (the reason is the difference between the deterioration modes of flexible and rigid pavement layers)
- The following techniques can be effective for the mitigation of reflection cracking in pavement structure: micro-cracking by vibratory compaction, galvanized steel netting, geogrid, geonet, glass-grid, paving fabric, geocomposite, SAMI (stress-absorbing membrane interlayer), fracture slab, NovaChip, saw and seal
- Thicker hydraulically bound base layers by 20mm result in an increase of 50 years in life time expectancy (French research)
- The bearing capacity of a long-life, semi-rigid pavement structure practically does not change during the life
- In France (unlike to other European countries), the appearance of reflection cracks on the surface of pavement surface is not considered as the end of the life time; the cracks are just filled up.
- Jointed, unreinforced cement concrete pavements (can) have longer (20-50 years) duration than asphalt pavements (with 10-15 years); at the same time, asphalt pavements have typically lower construction costs
- The widely used cement concrete pavement types in Europe are: jointed, unreinforced and continuously reinforced cement concrete pavements; (jointed reinforced concrete pavements are more seldom built mainly in the UK)
- The rigid pavement, because of its high stiffness, tends to distribute the load over a relatively wide area of subgrade; so, the eventual changing in subgrade strength has limited influence on the bearing capacity of the pavement structure; min. 25-45 MPa subgrade strength is required in European countries
- However, the durable road base with uniform quality is a must for the creation of long-life rigid pavement structures; almost exclusively bounded base layers are built under the cement concrete pavements in order to avoid the surface erosion of base layers due to penetration of water through cracks or unfilled joints of the pavement
- Constructing continuously reinforced cement concrete pavements (CRCP) allows avoiding constructed transverse contraction or expansion joints except at bridges or at pavement ends; thus the main defect source of concrete pavements can be eliminated (their duration of 40-50 years cannot be taken as unrealistic)
- Steel bars in CRCP are not used to increase the pavement strength but to control pavement surface cracking pattern by the continuously longitudinal reinforcement
- Design methods of cement concrete pavements used in Europe are mainly Empirical or Mechanistic-Empirical types; the combination of experimental sections’ performance data and laboratory test results are utilized
- In the UK, thickness curve series are available for various concrete strengths, base types and traffic sizes
- The maximum allowed (legal) national axle load (between 80 and 130 kN in Europe) is a significant parameter in the design of cement concrete pavements
- A majority of the (Western and Central) European countries design their cement concrete pavements for 40 years, however, the cumulated number of standard (equivalent single) axle loads is also a wide- spread design criterion
- The compressive, bending and splitting tensile strengths are widely selected for the characterization of highway cement concrete pavements (higher strength improves load distribution and fatigue characteristics)
- The long-life cement concrete pavement thickness in various European countries ranges from 220 to 300 mm
- Table 1 presents the design strength of road concrete in some European countries
|Country||Concrete design strength|
|age at testing (days)||design value (MPa)|
|Belgium||90||C = 62.5|
|Czech Republic||28||C = 25-32; B = 3.5-4.5|
|France||28||S = 2.7|
|Germany||28||C = 30 - 37|
|The Netherlands||28||C = 35 - 45|
|Poland||28||B = 4.0 – 6.0|
|Switzerland||28||C = 30; B = 5.5|
|The United Kingdom||28||B= 4.5 -6.0|
|Hungary||28||C = 37; B = 4.0; S = 3.0;|
|Legend: C compressive strength,
B bending strength,
S splitting tensile strength
Long-life surfacings for roads (OECD-project)
Unlike the duration of the pavement structure excluding the wearing course, the main topic of ELLPAG, another international project organized by OECD concentrated on the long life of top layers of heavily trafficked pavement structures. In many nations with mature road networks, new road construction typically accounts for around 50% of the road budget. Much of the remainder of national road budgets is spent on maintenance and rehabilitation of existing roads. Current road construction methods and materials contribute to this outcome, as they lead to recurrent maintenance requirements that can only be met at a relatively high cost.
The Long-life Pavements project of OECD as approved by member countries set out to determine if the costs of future maintenance and repaving, and the resulting road user delays have reached a level on high-traffic roads where long-life pavements are economically justified. For this to be the case, the reduced maintenance and other associated costs (e.g. user costs) would at least need to compensate for higher costs of construction. Based on the co-operative international research undertaken, conclusions were drawn on the availability of suitable materials that can support the development of long-life surface layers for road pavements [OECD, 2005]. Guidelines for a research programme were carried out as part of Phase II of the project. The objective of this further work was to assess the real capacity of candidate materials and their suitability for use as long-life wearing courses.
In recent years, innovation in the road sector has focused on economic and organisational structures, while changes in road paving techniques have been much less dramatic. Rather, they have at best been incremental. Yet, in order to optimise national highway budgets, whole-life costing methods are increasingly used to determine how, where and when to best spend budget funding on road construction and maintenance. Within this framework, the shift to full maintenance contracting has helped reduce costs, and the adoption of long-term contracts has helped establish an environment in which the development of more durable pavement types could be stimulated.
In developing a long-life pavement, it is necessary to consider the performance of the whole pavement, complete from its surfacing down to its foundation. This report focuses on the surface layer of pavements; other studies are currently under way which focus on long-life pavement structures, but not the surface layer.
The economic analysis shows that there could be considerable economic benefit in developing new pavement wearing courses. From a cost viewpoint, long-life pavement surfacing costing around three times that of traditional wearing courses would be economically feasible for a range of high-traffic roads. This would depend on an expected life of 30 years, discount rates of 6% or less, and annual average daily traffic (AADT) of 80,000 or more.
Sensitivity testing was carried out to establish the broad envelope of conditions under which long-life pavement surfacing becomes economically feasible. This work assessed the effect of different discount rates (3-10%), traffic levels (40,000 to 100,000 AADT), durability (30- or 40-year long-life pavements), wearing course cost (three-fold increase or five-fold increase), the proportion of heavy vehicles (5-20%) and the effect of day-time or night-time maintenance schedules. Details are provided in the report.
Such increases in wearing course costs need to be seen in the context of typical pavement construction costs. For the example scheme chosen, a dual three-lane motorway, pavement construction costs would amount to USD 1.8 million to USD 2.25 million per carriageway kilometre. This estimate includes features such as earthworks, drainage, line markings, safety fences, etc., but not other structures such as over or under bridges, gantries, etc.
At present, the surface layer (the wearing course) of such pavements represents around 9-12% of the above indicative pavement construction costs. A three-fold increase in the wearing course cost would imply an increase in overall pavement structure construction costs of up to 24%, and the surface layer would then represent around 30% of the construction costs.
Of course, the total construction costs of high-traffic roads are extremely variable, depending not only on pavement construction costs but also on the number of bridges, tunnels and earthworks actually involved. Overall average costs per kilometre increase to between USD 3.15 million and USD 3.6 million per carriageway kilometre, taking these other costs into account. In this respect, a three-fold increase in the cost of the surface layer of the pavement would have a lower impact in terms of overall motorway construction costs per kilometre, i.e. between 10% and 15%, and the surface layer would represent between 5% and 20% of the total construction cost. If a completely new road scheme were to be examined, this percentage would be even lower when total costs including structures, land purchase, design costs and communications are taken into account.
Long-life wearing courses for which these indicative evaluations have been undertaken are not yet in general use. The cost, the life, the condition and the maintenance arrangements included in the analysis of the advanced surfacing are targets and assumed to be achievable.
A review of advanced surfacing materials, currently under research or in limited use in small-scale projects, indicated that there are indeed materials that could be feasible for long-life surfacing of the standard assumed in the analysis. From the review of materials, the study concluded that two types of materials in particular had the potential to fulfil the requirements. These were:
- Epoxy asphalt: Considerable field data and performance histories exist on epoxy asphalt, which has been used on various bridge decks. Of particular note is that the epoxy asphalt placed on the San Mateo bridge deck in the United States back in 1967 is still performing well.
- High-performance cementitious materials with an epoxy friction course: For high-performance cementitious materials (HPCM), while all of the data stems from laboratory efforts, the properties are quite remarkable, particularly their strength and flexure properties. Possible shortcomings of this product, namely, poor noise and splash reduction and friction properties, can probably be overcome with improvement of its macro texture.
The third phase of the project consisted of full-scale field tests (International, 2017) carried out between 2009 and 2012 in France, New Zealand and the UK. Results obtained on epoxy asphalt technique suggested that the transfer of this technology from bridges to pavements at an industrial scale seems possible.
The HPMC solution is not yet at the same level of maturity. The technique proved difficult to apply on an industrial scale. Due to the noisiness of pavement surface, grooved fibre-reinforced ultrahigh-performance concrete seems to be promising but needs further development to achieve full control of production and placement. Specifically, issues like shrinkage-induced cracking need to be mastered. The short, experimental sections built during Phase 3 (lengths of sections are 60 – 210m) did not allow a sound economic assessment of what could become a mature technique. Even if the economic justification can be only demonstrated through a tender process looking at whole-life costs, it is likely to be recommended.
A Powerful Partnership formed in the United States in 2013, the Pavement Economics Committee (PEC) seeks to develop the scientific data through research and analysis that positions asphalt as the pavement material of choice [The Road, 2018]. The Go to Market (GTM) task group synthesizes and develops supporting materials to communicate PEC research to different stakeholder audiences as part of the deployment efforts of the Asphalt Pavement Alliance (APA), NAPA, and the SAPAs.
A near-complete Pavement Type Selection project is tentatively showing that states may underestimate asphalt pavement’s initial service life in their LCCA inputs. Many states assume an initial service life of 10–15 years for asphalt pavements; however, this project shows an average service life of 17.5 years based on hundreds of pavements analyzed.
PEC support helped update PerRoad software to version 4.4, which now allows the tool to perform a conventional Mechanistic-Empirical (M-E) design to directly compare to Perpetual Pavement designs. The following criteria were identified for the Perpetual Pavements [Perpetual, 2010]:
- Pavement structure lasts 50+ years
- Bottom-up design and construction
- Indefinite fatigue life
- Renewable pavement surface
- High rutting resistance
- Tailored for specific application
- Consistent, smooth and safe driving surface
- Environmentally friendly version
- Avoids costly reconstruction.
The design of perpetual pavements starts with that of the base:
- Bottom-up design and construction
- Foundation of high strength
- Stable paving platform
- Minimized seasonal variability and volume change in service
- Fatigue-resistant lower asphalt layer (high effective asphalt content mixes with modified binders for greater strain capability)
- Rut-resistant upper asphalt layer (aggregate interlock using crushed particles, ensuring stone-on-stone contact; binders: high temperature PG, polymers, fibers, dictated by climate and depth; 4 – 6% in-place air voids; renewable surface, tailored for specific use)
- Tacked layers are of high importance.
Perpetual pavement Award Criteria (in the USA):
- Pavement must be minimum 35 years old
- Pavement must have hot-mix or warmmix asphalt binder and surface layers
- No rehabilitation or series of rehabilitations over the preceding 35 years that has increased the total pavement thickness by more than 4 inches (some 100 mm).
Another typical project: Baltimore Beltway completed in 1993; traffic volume: AADT 175,000 with 19% trucks; pavement structure: 50 mm gap-graded (SMA) + 63 mm + 93mm + 100 mm + 100 mm dense-graded AC; still in perfect condition.
Some of the recent American research into long-life asphalt pavements [Harvey, 2015]:
- PaveM pavement management system developed and implemented (change from reactive to proactive maintenance and rehabilitation; 10-year look ahead of needs; Integrated Caltrans databases; automated statewide pavement condition survey)
- Further development of mechanistic design (use of CalME - a software program developed by Caltrans/UCPRC using the Mechanistic-Empirical (ME) methodologies for analyzing and designing the performance of flexible pavements [Harvey, 2016] - for routine design; improve and expand capability, improve user interface)
- Performance related testing and spec implementation (driven by Superpave and need for performance related specifications for binders and mixes, to be tied to mechanistic design, use of multi-scale testing)
- New “asphalt” materials (faster innovation due to cost and availability of bituminuous binders; more recycled materials; rubberized RAP, RAP in rubber mixes; more forms and uses of recycled tire rubber; methods and additives for “perpetual pavement recycling”; asphalt/cementitious and other hybrid materials; performance evaluation for faster implementation through performance related specs)
- Other environmental and cost impact of designs, management and policies; project level design; network level management; get the science ahead of the policies; joint consideration of agency cost, user cost and environmental impact.
The objective of the DURABROADS project, an EU 7th Framework Programme financed project is the design, development and demonstration of cost-effective, eco-friendly and optimized long-life roads, more adapted to freight corridors and climate change by means of innovative designs and use of greener materials improved by nanotechnology. Optimized asphalt pavement types and rehabilitation procedures for heavily trafficked European roads consider the synergistic effect of extreme climatic and mechanical loads. The European-wide questionnaire survey-based quantification methodology included functional, economic, environmental and social-human aspects using the principles of lifelime engineering [Gáspár, 2016]. After having used several statistical methods, weighted geometric means were applied to aggregate individual opinions into a consensual judgment.
According to the analysis and optimization of candidate asphalt wearing courses, the technical (functional) performance highlighted as the preponderant requirement. This is clearly in agreement with the fact that an adequate mechanical behaviour would have (very likely) good economic and social performances too. The environmental criteria are considered of primary importance among stakeholders probably due to the globally increasing awareness and emphasis on ecological dangers. Likewise, user safety is the most relevant social factor and it slightly overtakes the economic criteria.
Regarding the final ranking of alternatives, the multi-criteria decision methodology presents the SMA mixes as the best wearing course option followed by HRA, BBTM, AC and finally PA. The final results of the methodology will depend on the expected durability of each type of asphalt mixture since this will affect the life cycle and life cycle cost assessment. Although in this case study average data from literature have been used, in a real project, specific case-to-case data should be used in order to reduce uncertainty of the results (according to the literature, PA mixes presents an average service life from 4 to 14 years depending on the source consulted).
The effect of potential climate change in the selection of alternatives is noticeable although SMA maintains its first position as the most suitable mixture. According to the results, in the long-term, the use of thin layers will increase in importance due to its lower resource consumption and therefore its lower CO2 emissions. Besides, in scenarios with increasing rain events, safety might become an even more relevant issue, making PA mixes a better alternative for the user.
The criteria to include the DURABROADS solutions into Green Public Procurement procedures were also defined, along with the recommendations of the next steps to be followed for the future standardization of DURABROADS mixture. [Gáspár et al., 2016; Gáspár et al. 2018].
Based on the research conducted in DURABROADS project, it can be concluded that the addition of slags (up to 30%), RAP (up to 35%), WMA additive and bitumen improved with nanotechnology (actually by carbon black), either separately or combined, had no negative influence on the final properties (e.g. compactibility, stiffness, water sensitivity, resistance to permanent deformation and fatigue) of the asphalt mix. Guidelines were also issued to detail the procedures for the production and utilization for the “DURABROADS mixture” thus, encouraging its implementation and harmonization within the EU road infrastructure sector and neighbour countries (Jato-Espino et al., 2018].
Asset management as a tool for long-life pavements
The development, the maintenance and the operation of the high-valued road network can be considered as an extremely important task, with the whole country needing a lot of money, human resources, machinery, materials, etc. Several sub-systems were developed and being used all over the world to solve the problem mentioned and to allocate economically the necessary resources. However, this task is rather complex and the sum of the best solutions of various subsystems are not identical with the optimum operation of the whole system. That is why intensive research activities started in the topic some 20 years ago: it is called Asset Management System for Road Sector or Road Asset Management System.
One of the most significant relevant basic research institutions is the US Department of Transport Federal Highway Administration, Office of Asset Management [Asset, 1999]. Another important effort in this field has been done by an OECD Committee [Asset, 2001]. There are many definitions for this kind of asset management but each of them refers to a management system, a DSS (Decision Supporting System) and the cost efficiency on road construction, maintenance and operation, besides the model system has both long-term, strategic and short-term, actual elements. In this case, the term “asset” includes not only its actual gross or net value but also the funds needed for its maintenance throughout the service life. The potential users of asset management include decision makers, road users, road proprietors, operators, etc. The Road Asset Management System has several components [Hudson et al., 1997]:
- road pavements
- pavement structures and connected elements
- traffic engineering facilities (traffic signs, road paintings, road lighting)
- traffic census facilities
- information and monitoring systems
- road construction, maintenance and operation machinery
- road vehicles
- parking and rest areas
- roadside building connected with road rehabilitation, maintenance and operation
- materials used and equipment for their production
- organisations in the field
- road staff.
- Information Management Subsystem collects, systematizes, appraises and archives the basic data of modelling. It utilizes the knowledge on data need, data bases and their operation, archiving, hardware and software need, etc.
- Assets Valuation Subsystem deals with a highly important group of basic data needed for the effective operation of the model. It includes the methodology of the collection and evaluation of technical data, as well as the maintenance of the system.
- Condition Evaluation and Performance Modelling Subsystem concentrates on the actual condition of system elements and the modelling of their expected performance. The subsystem includes the condition parameters of each element, scaling of the measurement range of condition parameters, as well as the data storage in close connection with the activity of other subsystems.
- Deterioration Modelling and Defect Analysis Subsystem forecasts the worsening of the condition of various system elements, identifies the probable (expected) defect types. The condition of an element can be characterized by various qualifying parameters or a combined index; the deterioration curves are set accordingly.
- Maintenance, Operation, Rehabilitation and Reconstruction Subsystem defines the types and costs of various intervention techniques. It is a very important supporting element for establishment of the decision strategies.
- Whole Life Cost and Benefit Subsystem also has a significant supporting role for the decision process. Here, among others, the discounted values, the inflation rate, the interest rates are taken into consideration.
- Decision Supporting Models Subsystem determines the use and the applicability of the whole system. Since there are many elements in a system, a complex model creating total optimum for strategic decisions should be extremely aggregated. So, expert models, methods using basis approach, optimization models can be applied here. The already existing system elements (PMS, BMS, systematic condition survey, etc.) should be also included into the system.
- Total Quality Management Subsystem is operational during the whole implementation period of the program. It provides the results and the performance efficiency of the intervention at the end of the period. After feedback, new strategic and tactical objectives are set. When their parameters are set, the whole decision process can be restarted.
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