Correction for the asphalt overlay thickness of flexible pavements considering pavement conditions

Pavement overlays represent a common technique used for pavement rehabilitation and maintenance and to increase the structural support of the pavements. In the Department of Defense, the methodology for the design of flexible pavement overlays is contained in the Unified Facilities Criteria 03-260-02 criteria and involves the use of an empirically derived formulation. The overlay design of flexible pavements is based on the thicknesses of the existing asphalt, base and subbase layers and the required minimum thickness for the asphalt layer. However, this formulation does not take into account the quality or the structural condition of the existing surface layers. The current formulation considers the materials to have full structural strength and no deterioration. This study proposes an improved methodology for calculating the required flexible overlay thickness of a flexible pavement by taking into account the structural condition of the existing asphalt layer. An asphalt thickness correction factor is introduced to quantify the amount of the existing asphalt layer thickness that can still offer structural support, and therefore influence the overlay thickness. The asphalt correction factor is based on the existing load-related distresses affecting the asphalt surface. The implementation of this new approach showed that an asphalt layer in poor condition requires up to 60% more in thickness than an asphalt layer in good condition. The proposed methodology aims to standardise the design and evaluation of flexible pavements overlaid with asphalt layers and account for existing structural conditions. Moreover, allocation of maintenance funding can be optimised, thus limiting pavement overdesign.     

Understanding the impact of climate change on pavements with CMIP5, system dynamics and simulation

There is an increasing awareness regarding the impact of climate change on performance and durability of pavements. The objective of this paper is to present a framework of using the global climate forecasts, system dynamics and Monte Carlo analyses to evaluate the rate of change in climate change parameters and understand the impact of climate change on pavements. Changes in maximum air temperature and annual precipitation levels were determined for seven cities across the US, for RCP 4.5 and RCP 8.5 scenarios, using downscaled CMIP5 model output. A system dynamics model was utilised to link the changes in climate change-related parameters to deterioration and life of pavements. Percent of roads that need rehabilitation at different times, up to a time span of 100 years were predicted with simulations of the system dynamics model. Using regression equations developed on the basis of the output data, Monte Carlo analyses were then conducted to obtain distribution and 90% confidence intervals for percent of roads requiring rehabilitation at 50 and 100 years, for no climate change, climate change and climate change with different mitigation scenarios. The results clearly show the significant increase in deterioration of roads as a result of predicted climate change, compared to a ‘no climate change’ scenario. The conclusions are that the CMIP5 model output can be utilised successfully to obtain statistical data regarding climate change parameters that are relevant for pavement design and that a sequential use of the tool, system dynamic and Monte Carlo simulation can be utilised by public agencies to estimate climate change-related parameters for different scenarios, the risk of negative impact of such change on pavement lives and evaluate the effectiveness of various mitigation approaches. This can help them in making justifiable decisions regarding the consideration of climate changes in design of pavements.     

Hierarchical asphalt pavement deterioration model for climate impact studies

Quantification of the impacts of projected climate change on road pavement performance is possible using predictive models that correctly consider key causal factors of pavement deterioration. These factors include climate, traffic, properties of materials and the design of pavements. This paper presents a new model developed to predict rutting in asphalt surfacing. In addition to the key causal factors of road deterioration, the developed model takes into account several sources of uncertainties, particularly those inherent in future climate change predictions and model input parameters. The asphalt surfacing rut depth progression model was developed from a hierarchical road network data structure using a Bayesian regression approach resulting in a model for each surfacing group. The model was applied within a Monte Carlo simulation framework to derive probabilistic outputs of pavement rut depth progression and maintenance costs under the pre-determined future climate scenarios. This model is useful for application at both the network and project levels to develop road management strategies and policies.     

Risk based probabilistic pavement deterioration prediction models for low volume roads

Accurate prediction of pavement performance is important for efficient management of road infrastructure. Pavement performance prediction models developed for low-volume roads are mainly based on deterministic approach. The deterministic prediction models are inadequate to completely capture the deterioration mechanism. Uncertainties may occur in pavement behaviour under changing traffic loads and environment conditions, which may not be realistically represented by deterministic model. The objective of this paper is to develop pavement deterioration prediction models by probabilistic approach, for various distresses observed on low-volume roads in the state of Kerala in India, with the help of existing deterministic models. The major distresses observed on low-volume roads were ravelling, pothole and edge failure. Load-associated distresses were rarely observed on these roads as the maximum cumulative standard axle observed was only one million standard axle (msa). Hence, lack of proper drainage and construction quality (CQ) could be attributed as the major reasons for the pavement deterioration. Progression of deterioration of pavements with age has been studied and the intensity of distresses along with corresponding probabilities was arrived at. The distresses predicted by probabilistic models were compared with those predicted by deterministic models and the actual distress values observed in the field. The prediction models were validated using Mean Absolute Percentage Error, a statistical method for accuracy measurement of forecasting models. A risk analysis was then conducted to select the critical percentile value for each type of distress corresponding to varying pavement age. A sensitivity analysis was also carried out to study the effect of pavement age and CQ on the progression of pavement deterioration.     

Rutting progression models for light duty pavements

Deterioration models allow road managers to assess current condition and to predict future conditions of their road networks. Due to heavy vehicle axle repetitions and the effect of environmental factors, permanent deformation (rutting) develops gradually in the wheel paths and impacts on structural and surface performance of flexible pavements. This paper reports the approach adopted to develop absolute deterministic models for permanent deformation of low volume roads. A representative large sample network (23 highways) of light duty pavements was selected. For each section, time series data from four consecutive condition surveys were collected. Multiple regression analysis was carried out to develop models to predict pavement rutting progression over time as a function of a number of contributing variables. They include traffic loading, pavement strength, climate and drainage condition. For more powerful prediction, family group data-fitting approach was utilised to estimate future rutting progression based on the average rut depth curve for a series of pavements with similar characteristics. This study highlighted that separate family deterioration models are preferred and needed for more realistic results. The paper concludes that the analysis approach used for developing the models confirmed their accuracy and reliability by well-fitting to the validation data with low standard error values. Also, study results show that higher traffic loading, lower pavement strength, poor drainage and climates with high seasonal variation contribute to increasing rutting progression rate.     

Evaluating pavement management treatment selection utilising continuous deflection measurements in flexible pavements

The objective of this study was to develop a new structural index based on rolling wheel deflectometer (RWD) deflection data to describe pavement structural capacity and to improve pavement management treatment selection. To achieve this objective, pavement conditions including surface cracking, rutting, roughness and asphalt layer thicknesses were categorised and sorted according to their AC layer thickness and divided into various subgroups. The cumulative distribution function of the new RWD index in each subgroup was generated so that various percentiles were calculated and used to define the boundary between structural and functional rehabilitation. Results showed that the Louisiana criteria may recommend structural rehabilitation for pavements with sound pavement structure (Type I error) and functional rehabilitation to pavements with weak pavement structure (Type II error). Therefore, the state pavement management system should consider both pavement structural indices and surface distress indices currently available when making recommendations for pavement preservation and rehabilitation. RWD testing technology and indices derived from its data are one of the most promising candidates to fulfil this need.     

A case study on diurnal and seasonal variation in pavement temperature

An empirical numerical model was established for analysing the temperature variation of pavements in Xi'an area of China. This model can consider the influence of solar radiation, atmospheric temperature and humidity. It was verified by comparing the calculated and measured temperature of the natural ground surface. The effect of seasonal variations of temperature on asphalt and concrete pavement surfaces were then calculated. The temperature distribution of both asphalt and concrete pavements in January and July was investigated using the model. For each type of pavement, four groups of different pavement materials were considered to investigate the influence of thermal parameters for pavement materials on the temperature distribution. Furthermore, the diurnal variation of pavement temperature was analysed and discussed based on the normal climate characteristics in Xi'an. The results of the analysis showed that the diurnal variation of pavement temperature is very significant and must be considered in the design of a pavement.     

Numerical simulation tool for paving block structures assessed by means of full-scale accelerated pavement tests

Block pavements are an attractive alternative to asphalt and concrete pavements, especially in communal areas. Architects and urban planners would like to take advantage of various shapes, colours and textures of paving blocks in order to achieve a higher quality of urban space if the performance of block pavements could be better predicted to avoid large horizontal displacements of chipped stone corners and rutting. Unfortunately, the computational performance prediction of paving block structures is more complex than that for flexible pavements with homogeneous surface layers of asphalt concrete. The influence of the large number of vertical joints between paving blocks on the overall mechanical performance has not been considered sufficiently within computational tools yet. The proposed numerical simulation tool is able to take into account the complex mechanical behaviour of sand-filled joints as well as the non-linear mechanical behaviour of the underlying base courses. Joints are modelled using a Mohr–Coulomb-type friction model with the normal stresses non-linearly related with joint opening. Three different experimental set-ups were developed for the identification of the model parameters. The base behaviour was modelled using the Drucker–Prager cap model. The paper shows that the proposed tool predicts reasonable deformations and stresses in block pavements. The results of the simulations were compared with measured stresses from the full-scale accelerated pavement testing and a good agreement was observed.     

A mechanistic approach to evaluate the potential of the debonding distress in asphalt pavements

The debonding distress in asphalt pavement structures is a critical problem that affects the performance of asphalt concrete pavements. It occurs at the layer interface due to the poor bond quality between adjacent asphalt concrete layers and/or when stresses at the layer interface exceed the strengths of the material at the interface. The debonding of the adjacent layers, especially the top surface layer of an asphalt pavement, is a contributing factor to the premature cracking of pavements. Hence, the debonding distress can lead to a reduction in the life of the pavement. This paper presents an analytical and experimental framework to evaluate the potential for debonding at the layer interface of asphalt concrete pavements. Computational analysis was performed to determine the critical stress and strain states in layered asphalt pavements under moving vehicle loads using the Layered ViscoElastic pavement analysis for Critical Distresses (LVECD) computer program developed at North Carolina State University. This computational analysis enables a greater understanding of the critical stress that is involved in debonding and the ways that such stress is affected by pavement design parameters and environmental conditions. In addition, a prediction model was developed that can determine the shear bond strength at the interface of asphalt concrete layers with different tack coat materials at various temperatures, loading rates and normal confining stresses. The systematic and mechanistic framework developed in this study employs the maximum shear ratio concept as a shear failure criterion and provides a tool to evaluate the effects of various loading, environmental and pavement factors on the debonding potential of asphalt pavements. The overall advantages of the mechanistic framework and approach using the LVECD analysis tool will help lead to better understanding of the debonding mechanism, proper selection of the tack coats, and economic benefit in highway pavement maintenance and rehabilitation costs.     

水泥混凝土路面板湿度翘曲形成机理及变形计算

混凝土板内湿度梯度引起的板向上翘曲在车辆荷载耦合作用下可以导致路面板产生早期断裂,所以研究湿度梯度对板变形及应力的影响一直都是道路工程领域亟待解决的问题。但由于缺乏对混凝土路面板内部湿度梯度形成机理的统一认识,及混凝土湿度与变形本构关系的缺失,上述问题一直悬而未决。目前混凝土路面板的变形和应力计算,常常忽略湿度梯度的影响。因此,不能反映处于自然环境中的路面板的实际变形、受力状态。该研究基于水泥基胶凝材料本身特性,并考虑混凝土中水分扩散、吸附的影响,辨别、讨论影响水泥混凝土路面内部湿度状态的三个主要因素,提出混凝土路面板湿度梯度形成机理,给出湿度与变形的本构关系,建立简化的水泥混凝土路面湿度翘曲及应力计算方法。     

Network-level flexible pavement structural evaluation

The Kansas Department of Transportation has a comprehensive pavement management system known as network optimisation system (NOS). Annual condition surveys are conducted for NOS. Currently, the structural number (SN) of flexible pavements is computed using the American association of state highway and transportation officials equation based on the centre and fifth sensor deflections of a falling weight deflectometer (FWD). However, a rolling wheel deflectometer (RWD) can be used to collect deflection data at the network-level. This study was conducted to see whether the SN of flexible pavements can be obtained from this RWD deflection and NOS condition survey results. In this study, FWD deflection data, collected from 1998 to 2006, were analysed. Multiple regression analysis was done. The results showed that there is a negative relationship between SN and centre deflection. Equations can be used to calculate SN based on FWD (or RWD) centre deflections and network-level condition survey results. The SN is more sensitive to centre deflection than the total pavement thickness.     

Choice of aggregates for permeable pavements based on laboratory tests and DEM simulations

Permeable pavement plays an important role in the sustainable urban drainage system as they have the potential to reduce splash and spray. While the pavements need to be permeable for the water to drain into the ground, it must also be able to carry the traffic load without altering the permeability characteristics. This paper aims to investigate the permeability performance and serviceability of pavements constructed with aggregates having a range of crushing strength (crushability) and grading (particle size distribution). Laboratory tests consist of modelling a pavement and carrying out permeability and sieving tests before and after the application of the loading. Experimental investigations showed that particles having higher crushing strength will lead a lower reduction in permeability of the pavement without compromising the performance. Furthermore, it was observed that reduction of permeability depends on the particle breakage pattern. Numerical study was conducted using discrete element modelling to understand the underlying mechanism of this phenomenon, and it was found that a wide particle size range could distribute the load uniformly and reduce the particle breakages. It is suggested that all these factors may be considered while choosing the aggregate for permeable pavement.     

改性沥青对多孔性沥青混合料性能影响的研究

随着汽车工业的发展和城市道路交通量的迅速增长,造成城市道路噪声的增加,给人民群众的日常生活和工作带来严重影响,城市道路沥青路面的降噪研究已为当务之急。为保证沥青路面的降噪效果,要求其路面的空隙率为15%-20%。因此对材料的级配组成、沥青结合料的性能、混合料的油石比、物理性能指标等都有严格的要求。采用试验的方法在不同的级配范围内进行多孔性沥青路面材料的配合比设计,为确定多孔性沥青路面材料的降噪性能提供依据。     

Mechanistic-empirical methodology for the selection of cost-effective rehabilitation strategy for flexible pavements

A well-planned rehabilitation approach helps agencies to optimise the allocation of annual investment in pavement rehabilitation programs. Currently, many agencies are struggling with the selection of an optimal time-based and cost-effective rehabilitation solution to address the long-term needs of pavements. This study offers the use of a mechanistic-empirical methodology to develop a series of time-based rehabilitation strategies for high traffic volume flexible pavements located in Oklahoma. Six different pavement family groups are identified in the state, and comprehensive evaluation of existing pavements are conducted through analysis of falling weight deflectometer data and performance measures available in Oklahoma Pavement Management System database. The inadequacy of performance measures to fully characterise the condition of existing pavements are indicated, and damage factor determined from FWD data are suggested as trigger factor to select rehabilitation candidates. Three levels of rehabilitation activities including light, medium and heavy are considered as potential alternatives for rehabilitation candidates. A mechanistic-empirical methodology is employed to obtain an estimate of the performance of rehabilitation and extension in service lives of pavements. Also, an assessment output matrix is developed, which can be served as a supplemental tool to help the decision-makers in the highway agency with the rehabilitation related decision-making process. Cost-effectiveness of rehabilitation alternatives is determined through life cycle cost analysis, and three time-based renewal solutions are developed for pavement family groups that are in need of rehabilitation.     

Immediate effects of some corrective maintenance interventions on flexible pavements

Different maintenance interventions have different ability to address distresses on flexible pavements. Understanding the maintenance effects can benefit pavement maintenance decision-making. In this study, the immediate maintenance effects on roughness and rutting of three interventions including overlay, overlay with an additional base layer and mill and fill were studied and compared. A method was introduced to validate maintenance effect models, using the pavement management information from Virginia Department of Transportation. The method included a data mining process to extract data and apply regression analysis of maintenance effect models. The outliers in the analysis were detected and removed using the method of Cook’s distance. It was found that the immediate maintenance effects of overlay with base layer were greatest and mill and fill was least when treating pavements with moderate roughness (50–100 in/mi (≈ 0.8–1.6 m/km)). However, mill and fill was more useful for treating pavements with high roughness (>100 in/mi (≈1.6 m/km)). Furthermore, suggestions were proposed on data collection for road authorities to improve the prediction of maintenance effects.     

Implementation of mechanistic-empirical pavement analysis in the State of Qatar

The State of Qatar is experiencing tremendous growth in infrastructure including road network and highways. The current methods used in design of asphalt pavements in the State of Qatar are empirical and might not be suitable for the design of long-lasting pavements. Given the significant increase in traffic, road authorities in the State of Qatar have been considering the use of mechanistic-empirical methods in the design and analysis of asphalt pavements. This study documents the results of a study in which the mechanistic-empirical pavement design guide (M-E PDG) software was used in the design of asphalt pavements with input parameters that were carefully selected to reresent local materials and climatic conditions. The selection of material properties was based primarily on specifications and design guides in the State of Qatar and on published literature about these materials. The mechanistic-empirical method was also used to assess the benefits of adopting the concepts of perpetual pavement design and also to compare the performance of pavement structures in which various bitumen grades, granular bases and chemically stabilised sub-base were used. A life-cycle cost analysis was carried out to determine the design with the highest net present value among the various options investigated. It is expected that the outcomes of this study would promote the use of mechanistic-empirical methods in the State of Qatar and the region. Inevitably, this will require significant efforts to calibrate material and damage prediction models used in the M-E PDG for more accurate representation of material properties and measured pavements performance.     

Investigation of ride comfort limits on urban asphalt concrete pavements

It is well known that pavement distress negatively affects the drivers and passengers of vehicles. Many studies report that foremost among these negative effects is the vibrations that form within the vehicle. Ride comfort depends on the human response to vibration and vehicle response to the road. The goal of this study was to investigate the effect of pavement condition index on ride comfort and to determine the threshold comfort limits for passenger cars on urban asphalt concrete pavements. The pavement condition index (PCI) was determined for pavement sections subject to different surface distress using the PAVER system. Ride (driving) speeds of 20, 30, 40 and 50 km/h were assessed on the same pavement sections to measure vibrational effects inside the vehicle and on the passenger seat. These measurements were then evaluated using the ISO 2631-1 standard in order to determine the a _wz values. Using the logistic regression technique, predictive model that took into account linguistic concepts for estimating ride comfort levels based on PCI values was developed. With the aid of this mathematical model, comfort threshold values were determined for each driving speed within an interval of 0–100 PCI. The study results indicated that increasing driving speed was generally associated with higher PCI comfort thresholds.     

Light-coloured grey asphalt pavements: from theory to practice

This paper presents the technological development and application of hydrated lime in treating the surface of asphalt concrete to develop light-coloured, grey asphalt pavements. When appropriately applied on the surface of fresh asphalt concrete, hydrated lime makes the surface grey, significantly increases its albedo and effectively reduces the pavement's temperature caused by hot weather. Two application case studies are presented, focusing on how to ensure hydrated lime's long-term effectiveness on the surface of asphalt pavements and take into account the effect of the subsequent reduced temperature on the resilient modulus of asphalt concrete in the design of long-life flexible pavements. The increased asphalt concrete modulus, owing to lowered temperature, can reduce the design thickness of the asphalt concrete without sacrificing pavement performance. This also has a positive influence on reduced pavement heat island effects. It is concluded that the appropriate use of hydrated lime on asphalt pavement surfaces is an effective and economical method to produce light-coloured, grey asphalt pavements.     

Improvements to the structural condition index (SCI) for pavement structural evaluation at network level

Structural evaluation provides valuable information about the expected behaviour and response of pavements and can be used at the network level of pavement management to prioritise projects. The falling weight deflectometer (FWD) can be used to identify the beginning and end of management sections and group pavement sections with similar structural capacities. The structural condition index (SCI) was developed as a screening tool for the pavement network-level evaluation, and the FWD data are used to determine the SCI. For the successful implementation of the SCI concept at the network level, one of the critical issues is the accuracy of the index. This article evaluates the accuracy of the SCI and also discusses a concept and procedure how to improve the SCI and its algorithm for low-volume flexible pavements. A case study (Texas) illustrates that the original SCI algorithm underestimates the existing structural condition, resulting in overestimated treatments in the pavement maintenance and rehabilitation.     

Verification of effectiveness of chip seal as a pavement preventive maintenance treatment using LTPP data

It is hypothesised that maintenance treatments should be applied in the preventive mode before pavements display significant amounts of distress in order to be more cost-effective. The objective of this study was to verify the concept of preventive maintenance by examining the long-term effectiveness of chip seal treatment in four climatic zones in the USA using the long-term pavement performance database. Pavement sections were categorised into smooth, medium and rough pavements, based upon initial condition (IC) as indicated by the international roughness index. Pavement performance of treated and untreated sections was collectively modelled using exponential regression analysis. Effectiveness was evaluated in terms of life extension, relative benefit and benefit–cost ratio. The results showed that preventive maintenance is cost-effective. The life extension, relative benefit and benefit cost ratio were highest for sections whose IC was smooth at the time of treatment. Chip seal treatment effectiveness showed no correlation to climatic conditions or to traffic levels.