The objective of this study is to explain the relationship between climate variation and flexible pavement structure. Properties of flexible pavements such as stiffness of asphalt layer fluctuate due to variations in environmental conditions. Seasonal variations of temperature and moisture cause considerable changes in the load carrying capacity of pavements in geographical areas subjected to extreme freeze/thaw conditions. Temperature and rainfall are the two major factors, which contribute to developing stress and strain in pavement layer moduli for a given region.
There are three major types of distress induced into the flexible pavement structure due to the environmental impact; fatigue cracks, low temperature cracking, and moisture sensitivity and stripping are the common distresses. These distresses effect on the performance and age of the facility, and user cost. Measurement of pavements performance directly relates to the ride quality, safety and vehicle operating cost (VOC).
The study will consider the three types of pavements, their applications and methodologies. Perpetual pavement has a long term pavement performance, which minimizes the environmental related distresses and increase useful service life of the pavement. Life cycle cost analysis (LCCA) will be carried out to quantify the benefits that can be obtained through the use of perpetual pavement as a pavement structure.
2. INTRODUCTION
Today's transport system [U.S climate, 2008] includes marine, highway, rail, air, and pipeline transportation. Of these, only marine and pipeline transportation do not make use of pavements.
Pavements are the major structural load-carrying element of the highway system. For air travel, pavements are required in the form of runways, taxiways, and parking aprons. The function of the pavement varies with the specific user. In modern highways, airports, and rail facilities the purpose of the pavement is to serve traffic safely, comfortably, and efficiently, at minimum or reasonable cost.
Pavement is considered as the upper portion of the road, airport, or parking area structure and includes all the layers resting on the sub-grade. Additionally, the pavement is considered to have a bound surface and includes the load-carrying capacity of the sub-grade. Many so-called types of pavements discussed in modern technology, such as rigid pavement, flexible pavement, composite pavement, asphalt pavement, concrete pavement and others. Perhaps the most straight forward terminology is the definition of pavement by its structural function or response. Two basic types can be considered:
2.1. Flexible Pavement
A variety of techniques are commonly used for constructing flexible pavements with base and sub-base layers. Flexible pavements are constructed with a range of plant and road manufactured products composite of asphalt binder and aggregate. The granular base and sub-base layers may or may not be bound. Also the shoulder may or may not be surfaced with asphalt concrete.
2.2. Rigid Pavement
A rigid pavement generally consists of plain, composite or reinforced concrete slabs placed on a base or sub-base, or sometime directly on sub-grade. Portland cement concrete (PCC) is some what unique paving material in that it has a high modulus of elasticity, which makes it very rigid.
Calculating the structural adequacy of the pavement structure under the existing traffic and environmental conditions is important as it helps to predict the performance of anticipated pavement designs and evaluates existing pavements. However, the more significant impacts associated with changes in climate may well be realized on the less important networks of provincial and municipal agencies where weak pavement structures happen together with too much traffic loads.
Considerable improvement has been completed over the past 50 years in the field of pavement engineering towards providing economical pavements. Where as, comparatively little research has been done to examine the possible impact of climate change on pavement roads regardless of the dependence of Canadian Economic and Social Movement on Road Transport, and the influence of climate and other environmental factors on the deterioration of pavements.
3. ENVIRONMENTAL IMPACT ON THE FLEXIBLE PAVEMENTS
There is an increasing body of evidence that the earth's climate is changing with some of the changes attributable to human activities. Climate change can have direct and indirect impacts on road infrastructure, which are due to the effects of the environment. A pavement must be able to work well under the different environment, due to its seasonal variation, which differ greatly at any one time and it can also vary greatly throughout time at any one place. These seasonal variations can have a significant impact on pavement materials and the underlying sub-grade, which in turn can hugely affect pavement performance. The environmental changes have a great impact on the design, construction, safety, operations and maintenance of the transportations, infrastructures and systems. The key environmental parameters are typically temperature, frost action and moisture [Hass, 1994].
Figure 1: Environmental variable that can affect flexible pavement behaviour and performance
The effect of seasonal variation on flexible pavement structure is due to variation in moisture content, pavement temperature, frost thaw depth, ground water depth, and climate measurement (air temperature and rainfall).The researches shows that the maximum stiffness for the pavement layers occurs in the winter, and the minimum stiffness occurs at different periods in a typical year for the different layers. The asphalt concrete modulus is at a minimum in the summer when temperature is high. The seasonal variations in pavement layer moduli are used to establish seasonal factors for each layer which describe the modulus cycling in a typical year. A change in environment is critical to the regional and national transportations systems.
Therefore, the significant of these changes for the transportation systems are:
Temperature and moisture/precipitation are fundamental variables in all problems of pavement construction, design, behaviour, and performance.
3.1. Temperature
Temperature is one of the most significant factors affecting the design and performance of flexible pavement. These frequent temperature changes are considered to be a potential issue for performance of highways, especially in the northern Ontario, where there are more chances of frequent seasonal variation. Temperature variations within pavement structure contribute in a many different ways to distress and possible failure of that structure.
Pavements, like all other materials, will expand as they rise in temperature and contract as the fall in temperature. Small amounts of expansion and contraction are typically accommodated without excessive damage, however extreme temperature variations can lead to catastrophic failures.
The figure 2 [Zhong, 2009] show that the magnitude and direction of thermal stresses of asphalt pavements changes with time and temperature. As the temperature increases, the shrinkage stresses occurs, while due to temperature decrease, the tensile stress occurs. By considering the two Conditions where in
Conditions 1: Material characteristics of asphalt surface are temperature dependent
Conditions 2: Material characteristics of asphalt are supposed to be independent of temperature.
The figure 3 shows that the magnitude of thermal stress, in condition1 is smaller when the temperature increases but larger when the temperature decreases than in condition 2.The difference value of two conditions is the most remarkable at the surface, and decreases as the depth increases. In a word, the temperature-dependent material characteristics have great influence on thermal stresses of asphalt pavement, especially at the surface and when temperature decreases.
Temperature can affect the aging of bitumen resulting in an increase in embrittlement of the surface chip seals that represent more than 90% of the rural sealed roads. Brittlement of the bitumen causes the surface to crack, with a resultant loss of waterproofing of the surface seal. These suddenly increase or decrease in temperature will cause the cracking, potholes and bleeding. Flexible pavements can suffer longitudinal cracks as a result of excessive contraction in cold weather. An increase in the frequency of extremely high temperatures in areas already experiencing a hot climate could result in road surface damage. This combined effect of an increase in mean and extreme high temperature effect on the pavements life .The damage includes pavement softening, traffic-related rutting and melting of older pavements.
Thermal condition, if not addressed, can lead to significant problems, including the following:
In order to maintain the infrastructure at certain level requires more frequent maintenance, milling out ruts and the laying of more heat resistance asphalt. These problems suggest for research on material properties and technology, which is more tolerant to high temperature. More frequent reseal treatments will improve the problem.
3.2. Precipitation
Rainfall changes can modify moisture balances with in the pavement structure and influence on the process of pavement deterioration. Therefore, increase in precipitation will likely effect on infrastructure in both cold and warm weather in a different way. Excessive water content in the pavement base, sub-base, and sub-grade soils can cause early distress and lead to structural or functional failure of pavement if counteractive measures are not undertaken. Pavement deterioration [TRB, 1967] due to excess moisture or improper drainage can cause one or more of the following forms:
Figure 4: Precipitation data throughout the year
Frost action [FHA-2008] can be quite detrimental to pavements because of its effect on the underlying sub-grade, which can be divided into "frost heave" and "thaw weakening". "Frost heave" is an upward movement of the sub-grade resulting from the expansion of accumulated soil moisture as it freezes, while "thaw weakening" is a weakened sub-grade condition resulting from soil saturation as ice within the soil melts.
Thawing weakening occurs when the ice contained within the sub-grade melts. As the ice melts and turns to liquid it cannot drain out of the soil fast enough and thus the sub-grade becomes substantially weaker (less stiff) and loses bearing capacity. Therefore, loading that would not normally damage a given pavement but may cause significant damage during spring thaw.
Thawing can proceed from the top downward, or from the bottom upward, or both, mainly depending upon the pavement surface temperature. During a sudden spring thaw, melting will proceed almost entirely from the surface downward. This type of thawing leads to extremely poor drainage conditions. The frozen soil beneath the thawed layer can trap the water released by the melting ice lenses so that lateral and surface drainage are the only paths the water can take.
Permafrost is the most important consideration during the seasonal variation. Since much of our infrastructure exist on permafrost foundation. Therefore, when the permafrost thaws the infrastructure become instable, in results this will effect on the road structural stability. Since the rainfall is distributed throughout the year with portions of the fall being dryer than other month. Therefore, the moisture is trapped in pavement layer through out much of the year. This excess moisture causes to reduce the strength of aggregate, and it disintegrates into parts. These problems generally exist in new construction asphalt overlays over existing flexible and Portland cement concrete pavements.
3.3. Effect of Freeze and thaw on IRI
Roughness [FHA, 1998] is most important factor to measure the pavements performance. Measurement of pavements performance directly relates to the ride quality, safety and vehicle operating cost. The pavements structure which lies on more susceptible soil of fine grained soil has a higher value of international roughness index. Changes in roughness for flexible pavements can be classified into the following three categories:
Seasonal variation and sub-grade conditions showed strong relations with roughness in flexible pavements. Roughness also varies with time due to variation in profile path, seasonal effect, and maintenance activities. The value of IRI will be higher in a section or zone, which is experienced a higher freezing index, or a higher number of freeze/ thaw cycle. This suggests that adequate frost protection is very important for the long term pavement performance ( Figure 5-8, shows pavement age versus roughness for environmental zones).
The plot shows the behaviour of roughness of pavement due to environmental effect on LTPP program for different Zones
4. DISTRESS ASSOCIATED WITH ENVIRONMENTAL IMPACT
There are three types of distress [PTC] induced into the flexible pavement structure due to the environmental impact. The distresses are as follows:
4.1. Fracture
Fracture could be in the form of cracking, resulting from thermal changes, contraction or slippage, moisture damages and excessive loading or fatigue
4.1.1. Alligator cracking
http://training.ce.washington.edu/WSDOT/Modules/09_pavement_evaluation/Images/flexible_distress/WSDOT065.jpgInterconnected cracks caused by failure of the top surface under repeated traffic loads, changes in the characteristics of material due to freeze and thaw or poor drainage system. In thin pavement the cracks start from the bottom of the layer, where the tensile stress is maximum, due to repetition of loading and move in upward direction.
In thick pavement, theses cracks starts at the top surface due to tensile stress, which is created by the vehicle tire interaction (based on the tire pressure, shape of tire (studded) and aging of the binder.
The exact cause of the problem can be investigated by the following means:
4.1.2.
http://training.ce.washington.edu/WSDOT/Modules/09_pavement_evaluation/Images/flexible_distress/block_cracking.jpgBlock Cracking
Block cracks appears on the top surface of the pavements mainly due to the incompatibility of the material characteristics with the seasonal variations in temperature. The following are the causes:
4.2. Distortions
4.2.1. Depression
http://training.ce.washington.edu/WSDOT/Modules/09_pavement_evaluation/Images/flexible_distress/WSDOT139.jpgDepression or irregularities in the surface elevation of pavements surface are noticeable after the rain, when these depressions are filled with water. The main causes of theses depression is due to frost heave or settlement of sub grade due to improper compaction during construction.
4.3. Disintegration
4.3.1. Potholes
http://training.ce.washington.edu/WSDOT/Modules/09_pavement_evaluation/Images/flexible_distress/WSDOT067.jpgPotholes are small, bowl-shaped depression in the pavements surface, which are usually, occurs on the thin pavement surface. Potholes are end results of the fracture surface (i.e alligator crackling), When the pavement starts disintegrating into different asphalt concrete pieces.
4.3.2. Stripping
Surface stripping is a sign of the structural failure at the bottom surface. The striping typically starts from the bottom of the asphalt pavements and progressed upward. If the stripping start at the top surface, which is due to the effect of loss of bond between aggregate and asphalt binder, is called “Revelling”.
It is very hard to define the exact causes of the stripping by visual inspection. The exact cause of the stripping can be evaluated by detail analysis of the pavement structure; core sampling is the best way to define the following
5. LIFE CYCLE COST ANALYSIS
LCCA [FHA, 1998] is an analysis tool pack, which is build up on well-founded economic principles and used to evaluating the total economic worth of a usable project segment by analyzing its initial costs and discounted future cost, such as maintenance, user, reconstruction, rehabilitation, restoring, and resurfacing costs. These costs are presented in the form of present worth or base year by using the following expression:
Net present worth = Initial cost +
Where i= discount rate
n= year of expenditure
5.1. User cost
User costs are the delay, vehicle operating, and crash cost, which inure due to the use of facility during the period of construction, maintenance, and rehabilitation. These costs largely depend upon the conditions of the pavement surface and capacity of the facility.
5.1.1. Delay cost
When vehicle demand on the facility exceeds work zone capacity, the facility operates under forced flow conditions and user cost can be immerse. The crawling through long, slow-moving queues accounts for more than 95 % of the work zone user cost.
5.1.2. Crash cost
The crash cost is associated with work zone, alternative/detour routes and capacity of the facility. Overall crash rates for the various functional classes of the roadway are well documented but the authenticity of the crash data in work zone is uncertain. Since there is no research data is available, therefore the crash cost in LCCA are neglected.
5.1.3. Vehicle operating cost
The normal operation cost occurred during the period of free construction, maintenance, or rehabilitation, while the work zone operation happened during the operation of construction, maintenance, and rehabilitation, which usually reduce the capacity of facility and create the disruption in normal flow.
During the normal operation, there is a little cost associated in terms of crash cast and user delay, but major part is the vehicle operating cost mainly due to the roughness index (Pavement performance).
Figure 9: Performance curve vs Rehabilitation strategy
In the figure 5, Alternative A represent the performance curve for longer term strategy with 15 year rehabilitation implementation, while the Alternative B represents a short term strategy with 5- year treatment. The difference between two strategies is graphically represented in performance level curves. This difference in performance level, directly effect on the vehicle operating cost. This slight difference in VOC rates caused by difference in pavements performance could result a huge VOC differentials over the life of design strategy when multiplied by several years vehicle miles travelled.
Vehicle costs5 are often divided into variable costs (also called vehicle operating costs or out of pocket expenses) such as fuel, oil, tire wear, which increase with vehicle use, and fixed costs which are not considered affected by how much a vehicle is driven.
Figure 10: Vehical operating cost vs Rougness index
The above Figure 10 shows the effect of road roughness on the user cost. It shows that additional operating cost or variable cost (as compared to a smooth road baseline) start increases around an IRI equal to 1.5 m/km.
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5.1.
5.2. New construction cost
The highway network is a major contributor to social and economic development, but it cost to expand and maintain. Typically agencies use mitigation strategies to minimize the user cost associated with new construction.
5.3. Routine maintenance cost
Minor or routine roadway maintenance includes activities such as joint and cracks sealing and patch repairs. The cost for these maintenance treatments for both types of pavements is not included in LCCA due to the following reasons.
5.4. Salvage value
It is value of an investment alternative at the end of the analysis period of a facility. The residual value and serviceable life are the two major component of any facility.
5.5. Rehabilitation cost
5.5.1. Rigid Pavements
For the Ontario [CAC-2006] rigid Portland cement concrete arterial and high volume highway designs, rehabilitation includes diamond grinding at years 18 and 28, and load transfer restoration at year 28. For the high volume highway only, rehabilitation also includes, at year 38, the addition of an 80mm asphalt concrete overlay covering both the roadway and shoulders.
Figure 10: Typical 50- year Rehabilitation plan for rigid pavement
5.5.2. Flexible Pavements
5.5.2.1. Conventional pavements
For arterial and high[Becca Lane-2008] volume highways, asphalt overlay is required for the Ontario flexible asphalt concrete pavement designs removal and application of 80mm asphalt on roadway at years 19 and 31, and at year 42, both on roadway and shoulder
Figure 11: Typical 50-year Rehabilitation plan for conventional pavement
5.5.2.2. Perpetual pavements
Perpetual pavements [APA-2009] can be easily maintained and their useful service life can be enhanced without breaking and hauling to landfill, since the structure is still long lasting and durable. The only pavement rehabilitation needed would be surface replacement at about 20- year interval.
Figure 12: Typical 50-year Rehabilitation plan for perpetual pavement
5.6. LCCA Model
The primary purpose of an LCCA is to quantify the long-term implication of initial pavement design decisions on the future cost of maintenance and rehabilitation activities necessary to maintain some pre established minimum acceptable level of service for some specified time. A pavement design strategy is a key step in conducting the LCCA. Pavement design strategy is a combination of initial pavement design and set of maintenance and rehabilitation activities for a defined analysis period. The analysis period of 50 year is set up to reflect the long term cost differences associated with design strategies ( Initial design life, periodic maintenance, set of rehabilitation along with scope, timing, and cost of activities).
5.6.1. Flexible LCCA
5.6.1.1. Conventional pavements
The Ontario Highways, flexible asphalt concrete pavement freeway design rehabilitation schedule includes asphalt removal and application of an 80mm asphalt overlay to the roadway surface at years 19 and 31. At year 42, there is another removal and an 80mm overlay is applied to the complete roadway and shoulders. This final overlay is assumed to have a life expectancy of at least 12 years; at year 42, however, there are only eight years left before the end of the first 50-year planning horizon — as a result, only 67% of the last rehabilitation overlay is considered in the analysis calculations
*67% cost is used to calculate the NPV
5.6.1.2.Perpetual pavements
*50% is considered towards NPV
5.6.2.Rigid LCCA
The Ontario Highways, rigid Portland cement concrete design will also undergo minor rehabilitation activities and, again, these are ignored in this analysis. According to Ontario's Ministry of Transportation life cycle cost analysis procedures, the only major, material intensive, rehabilitation activity is the application of an 80mm flexible asphalt concrete overlay over the entire road and shoulder surfaces at year 38. This analysis assumes that the overlay has a life expectancy of about 12 years, which takes it to the end of the 50-year planning cycle
5.6.3.Graph for LCCA Model
5.6.4.Life cycle cost analysis
The results of LCCA show that perpetual pavement has the lower agency cost and lower maintenance and rehabilitation cost, that is why it is the most suitable pavement in terms of environmental impact.
5.6.5. Assumptions
In order to calculate the LCCA, the following assumptions were made:
6. ALTERNATIVE TO PREVENT THE ENVIRONMENTAL DAMAGE
6.1. Perpetual Pavement
Perpetual pavements [APA-2009] are the effect of recent efforts on material selection, mixture design, performance testing, and mechanistic-empirical design approach. These pavements offer a service life of 50 year or more with periodically only replacing the pavement top surface and the recycled old pavement material.
Perpetual pavements are engineered with three multi-viscoelastic layer with the following distinctive feature, which
Perpetual pavement is engineered so that any distress that occurs is confined to the upper pavement layer. The top surface is a renewable surface layer, which can be designed for specific applications. In a low volume facility, the conventional dense-graded superpave mixture is adequate, where as in high volume facility; the SMA will be attractive, depending upon the material availability. Open graded friction courses (OGFC) on the surface can be implemented in the rainstorms area to reduce the splash and spray and top provide better skid resistance. The only pavement rehabilitation needed would be the surface replacement at about 20-year intervals.
6.1.1. Advantages
6.2. Portland concrete pavements
Conventional concrete pavements [Norbert, 2008] are being used for high volume facility roads, airports, street, local roads, parking lots, industrial facilities, and other type of infrastructure. Concrete pavements are generally provides times the service life of asphalt pavements designed and built to similar standards. The life cycle cost of concrete pavements depends mainly upon the material cost at the time of construction and quality of construction, but generally the concrete pavements have the higher initial cost but significantly low future maintenance cost.
The environmental impact on the concrete pavements especially the frost heave, can be minimised by providing the good quality construction of the supporting sub-grade, sub-base and base. The following are the main consideration during construction:
6.2.1. Advantages
6.3. Permeable pavements
Permeable pavements [Permeable pavements] are alternatives that may be used to reduce imperviousness. While there are many different materials commercially available, permeable pavements may be divided into three basic types:
Permeable pavements typically consist of a porous surface course and uniformly graded stone or sand drainage system. Storm water drains through the surface course, is captured in the drainage system, and infiltrates into the surrounding soils. Permeable pavements significantly reduce the amount of impervious cover, provide water quality and groundwater recharge benefits, and may help mitigate temperature increases. Permeable pavements are effective for reducing imperviousness in parking lots, driveways, plazas, and access roads in both new and redevelopment applications in residential, commercial, and industrial projects. They are particularly useful in high-density areas where space is limited. Rainwater passes through the permeable surface, is temporarily stored in the sub-base material, and slowly infiltrates into the underlying soils.
7. Recommendation
8. Conclusion
The purpose of this study is to evaluate the effect of environment on the flexible pavements structure. Environmental conditions can have a particularly significant impact on the performance of the low volume roads.
Properties of the flexible pavements layer are explained in terms of material characteristics, which changes with the change in environmental conditions. The seasonal variation in temperature and moisture (precipitation) can cause to develop the stress and strain in the pavement layer for an area subjected to extreme freeze and thaw conditions. The development of these stresses has a detrimental impact on the structural capacity of the pavement, roughness index, and useful service life of the pavements. A detail preview of distresses associated with change in environments are described, their cause and possible corrective action.
A 50-year LCCA has been generated to quantify the suitability of pavements. In LCCA, a detail description is provided about the initial design life, set of routine maintenance and rehabilitation activities & cost analysis of the alternatives. A preview of the proactive strategy to mitigate the environmental impact in terms of perpetual, Portland concrete and pervious pavements is included.
Acknowledgement
We are pleased and thankful to Prof. Susan Tighe for her guidance, coaching, teaching, training and instruction to complete this group assignment successfully within stipulated time period.
Also, we appreciate lesson planning and course designing that incorporated every aspect of Pavement management. The course message is conveyed successfully to student through lectures, presentations, case studies, group works, individual assignments and class readings. All these practices effectively contributed to learning experience. This course enabled us to manage pavement network successfully through considering business, economics, engineering and management values.
We are also grateful to Vimy Handerson for his assistance and guidance throughout the course.
Reference:
Thermal stress of asphalt under dependence of material characteristics on reference temperature
