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.     

Empirical model of roughness effect on vehicle speed

Vehicle speed–roughness relationship has a significant research gap in life cycle assessment model. The current available models describing the roughness effect on vehicle speed are very limited and outdated. In this paper, 32 individual pavement sections, each of which has roughness data of up to 8 years, were selected to develop the model. The roughness data cover a wide range, and the selected pavement sections contain both flexible and rigid pavement types and various numbers of lanes. Involved regression variables include the following: vehicle speed, roughness, volume–capacity ratio, pavement type, number of lanes and speed limit. Analysis of variance was first performed, indicating that pavement type and speed limit are not significant factors influencing the average vehicle speed. Following, strict statistical technique was used to correct the unobserved heterogeneity during the regression using a one-way fixed random model. The obtained regression model reveals that the average vehicle speed decreases 0.0083 mph with every 1 in/mi increase of the roughness ( ? 0.84 km/h per m/km).     

International roughness index models for HMA overlay treatment of flexible and composite pavements

Timely rehabilitation and preservation of pavement systems are imperative to minimising agency's costs and maximising benefits. Reasonable estimates of treatment life and pavement life extension can be made possible by developing reliable treatment performance models. Louisiana Department of Transportation and Development initiated a three-phased study to develop pavement treatment performance models in support of cost-effective selection of pavement treatment type and the time of treatment. As a result of the study, international roughness index (IRI) models for overlay treatment of composite and flexible pavements were developed. Various factors affecting the IRI of overlay treatment were identified. Climatic indices pertaining to Louisiana were developed which exhibited strong statistical significance along with the other variables as used in the IRI models. The developed IRI models provided good agreement between the measured and predicted IRI values with the majority of data within 5% of prediction error. The models could be used as a good pavement management tool for pavement maintenance and rehabilitation actions.     

Analysis of inverted base pavements with thin-asphalt layers

Inverted base pavements are flexible pavement structures built by placing a top quality compacted granular aggregate base between a rigid cement-treated base and a thin-asphalt surface layer. The proximity of the granular base to the load makes its behaviour critical to the pavement response. Three-dimensional finite-element simulations are conducted to assess the mechanical performance of different inverted base pavement structures, with emphasis placed on pavements that feature thin-asphalt surface layers. A nonlinear constitutive model captures the anisotropic stress-dependent stiffness of the granular base. Results show that the stress distribution within inverted base pavements is markedly different from that of conventional pavements due to the stiffness contrast between successive layers. Thin-asphalt layers deform more uniformly and experience lower tension than thick layers. However, in the presence of combined shear and vertical contact loads, the benefits of a membrane response in thin asphalt concrete layers may be overwhelmed by the increased tensile strain at the load edge. The transition from beam to membrane asphalt response depends on the relative stiffness between the asphalt layer and the aggregate base. In most cases, the transition takes place at an asphalt layer thickness between 25 mm and 50 mm.     

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 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.     

Strain energy release rate at interface of concrete overlaid pavements

A new method for calculating energy release rate (ERR) at the interface of concrete overlaid pavements is proposed using crack closure and the nodal force technique. This method transforms a 3D pavement system into a 2D interfacial crack model via a theoretical conversion. The interfacial ERRs of steel fibre-reinforced, roller-compacted, polymer-modified concrete overlay pavement subjected to vehicular load were calculated and compared with the measured interfacial fracture toughness of the bi-material. It was found that the ERRs considerably decrease with the increase in overlay thickness and elastic modulus of foundation. Thin overlays (less than 100 mm) should not be considered in overlay pavement design to avoid interfacial delamination induced by heavy vehicular loading. For a typical overlay pavement system subjected to complex vehicular loads, an interfacial crack suffers mainly from damage due to mode-I, opening, compared to mode-II, sliding, while mode-III, tearing damage is negligible.     

Development of degradation model for urban asphalt pavement

There is a lack of a profound understanding of urban pavement deterioration pattern. This is due to the complexity of traffic conditions and the variety of pavement structures in urban roads. The lack of a suitable deterioration model for the urban pavements limits the possibility of making any scientific and cost-effective repair and maintenance strategy. There is a need for a better understanding of the long-term behaviour of urban pavements by which predictive pavement condition models can be derived and consequently a suitable maintenance management system can be built. In response to this need, a comprehensive field study was performed in three Iranian cosmopolitan cities. Pervasive pavement damages were defined and an urban pavement condition index was established. A deterioration model was developed by monitoring and analysing the conditions of road pavements in a period of four years. This model varies as the structural and loading conditions of the pavement change. The efficiency and practicability of the model in predicting the conditions of the pavements were illustrated.     

Limitations and potential improvement of the aircraft pavement strength rating system to protect airport asphalt surfaces

A pavement strength rating system is internationally adopted in order to protect aircraft pavements from inadvertent overload. The system has two elements. The primary element is designed to protect the pavement against subgrade rutting and the second is intended to protect asphalt pavement surfaces. The surface-protection element is arbitrary and empirical, placing category-based limits on aircraft tyre pressures. In 2008, increases in the tyre pressure limits were proposed by aircraft manufacturers and these were approved in 2013. The research reported in this paper assesses the impact of tyre pressure and individual wheel load increases on calculated flexible pavement stress indicators, as well as identifying an improved surface layer protection element. Stresses were calculated near the surface, at the surface layer interface and at the subgrade. Tyre pressure and wheel load combinations included current (18 t and 1.35 MPa), imminent (33 t and 1.75 MPa) and future (40 t and 2.15 MPa) aircraft. Surface layer stress increased significantly (20–30%) with increases in both tyre pressure and wheel load. The subgrade stress increased near-equally (97%) with wheel load but was insensitive (<1%) to tyre pressure changes. The ability of the current aircraft pavement strength rating system to protect pavements from the increasing demands of aircraft was demonstrated to be limited to the subgrade. It is recommended that the tyre pressure rating be amended to reflect the combined impact of both tyre pressure rating and individual wheel load. It is also recommended that ongoing efforts to incorporate additional asphalt surface failure modes into routine pavement design be given high priority. The importance of these issues is reinforced by the limited availability of remedies to counter any negative impacts of increased surface layer stresses, especially in hot climates.     

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.     

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.     

Multi-scale computational model for design of flexible pavement – part I: expanding multi-scaling

A computational multi-scale procedure for designing flexible pavement is developed in this, the first of a three part series. In this paper, computational analyses are performed on sequentially larger length scales, termed expanding multi-scaling. The model is constructed by the finite element method at each length scale, thereby creating a one-way coupled multi-scale algorithm that is capable of accounting for the effects of variations in design parameters at each length scale on the performance of flexible pavements. For example, the algorithm can be utilised to predict the effects of small-scale design variables such as volume fractions of additives, fines and aggregate, as well as the effects of large-scale design variables such as asphalt concrete thickness and degree of base layer compaction on rutting due to cyclic loading. The computational procedure is briefly herein, including the experimental properties required to deploy the computational scheme for the purpose of pavement design. The paper concludes with several demonstrative examples intended to elucidate the power of this predictive technology for the purpose of designing more sustainable pavements.     

Advancing the design and construction quality control of flexible pavements

The paper presents several advanced features that can be used in the process of pavement design and construction quality control. The models for predicting the pavement performance in the latest version of mechanistic empirical pavement design guide form the basis of the proposed system. In order to add the new features, it was necessary to make simplifications concerning the moisture distribution in the unbound materials and the ageing/oxidation of the upper asphalt concrete (AC) layers. The add-ins to the program include: (1) An analytical reliability analysis where the variability of the performance prediction components is computed from the variability of the pavement parameters; (2) An improved simulation of the temperature distribution in the AC layers during the design period; (3) An option for adding an overlay during the design period, to simulate maintenance or stage construction and (4) The possibility to use a 2D finite element analysis with nonlinear and cross-anisotropic materials, in addition to the well-established linear elastic analysis. The proposed system runs every hour of the design period and computes the fatigue bottom-up cracking, rutting in all pavement layers and international roughness index. It runs very fast, less than one minute in the linear analysis and less than 30 min in the nonlinear case. Few cases are presented to illustrate the benefits of the above add-ins.     

Development of serviceability prediction model for county paved roads

This paper developed a pavement serviceability prediction model for county paved roads. Most county paved roads were built decades ago without following minimum design standards. The recent increase in industrial/mineral activities in the State of Wyoming required developing a pavement management system (PMS) for local paved roads. The developed PMS used the pavement serviceability index (PSI) as a pavement performance parameter. The proposed PSI model for local roads is based on: international roughness index, pavement condition index (PCI) and rut depth for flexible pavements only. Ten panellists from Wyoming rated 30 pavement sections that were randomly selected at different distresses’ levels; using two vehicles (SUV and Sedan). The statistical analysis indicated that the seating position, age and gender are not significant to the rating process. However, the vehicle’s type found to be significant. The newly developed model from this study explains 80% of the variations in the PSI values of county roads (adjusted R² = 0.80). In addition, the new model seems to provide more realistic representation of the conditions of county roads than the statewide model used on the state’s highway system.     

The effect of traffic data source on deterioration rates of heavy-duty flexible pavements

The purpose of this study is to assess the effect of traffic data source (estimated vs. actual) on predicted progression rates of roughness and rutting for heavy-duty flexible pavements of rural freeways. Progression rates are predicted using calibrated HDM-4 models. The assessment is performed in terms of variations in maintenance intervention timing associated with the variations in progression rates. Time series pavement condition data (covering 3–5 years) have been collected for 7 sections of rural freeways for use in calibrating HDM-4 deterioration models. They range in length from 10 to 60.8 km and cover different traffic volumes, climate zones and subgrade soil types. For these sections, estimated annual average daily traffic (AADT), growth factors and assumed loading have been extracted from relevant database. Only six segments of these sections have Weigh-in-Motion (WIM) sites so relevant actual AADT, growth factors and axle load distributions have been extracted from WIM reports. The results of running the calibrated HDM-4 deterioration models using different traffic data show that actual traffic data from WIM sites result in higher rates of deterioration to that of estimated data for four sites, resulting in earlier intervention timing and higher present value agency cost. The other two sites have lower rates with actual data due to lower traffic loading than estimated.     

Mechanical behaviour of surface layer fibreglass-reinforced flexible pavements

The work presented in this paper aims at providing a better understanding of the mechanical response of surface layer fibreglass-reinforced flexible pavements. The surface reinforcement technique consists of installing a fibreglass grid in between the levelling layer (placed on the base course to seal and level the pre-existing distresses) and the wearing course (or overlay). Flexural fracture tests were performed on two-layered reinforced asphalt specimens composed of both levelling and wearing courses to simulate a real overlay structure. Three fibreglass grids characterised by different mechanical and/or geometrical properties were employed. Strain localisation and damage distribution were investigated using an in-house digital image correlation system capable of achieving highly accurate 2D full-field strain maps of the specimens during loading. Finally, an analytical model was developed on purpose to reproduce the mechanical response of the asphalt mixture-interlayer system.     

Coupled thermo-hydraulic modelling of pavement under frost

Frost action is a major factor causing deteriorations of pavements in cold regions. The resultant temperature and moisture redistributions play an important role in determining the mechanical responses of pavement. This paper develops a multi-physical model to analyse the coupled thermo-hydraulic field under pavements, especially those in the unsaturated base and subgrade. This model integrates the Fourier's laws for heat transfer, Richards' equation for fluid transfer and poroelastic constitutive relationships. Various coupled parameters were utilised to transfer information between field variables. Additional relationships, such as the similarity between drying and freezing processes and the Clapeyron equation for ice–water balance, were incorporated to allow for the effects of frost action. The coupled nonlinear partial differential equation system was solved on a multi-physical platform. Two instrumented pavement sections (one asphalt pavement and one concrete pavement) were used to validate the results of the model simulations. The simulation results match reasonably well with the field-monitored data.     

The use of a thermal camera for quality assurance of asphalt pavement construction

The goal of the study was to assess the use of thermal camera to quantify the asphalt pavement homogeneity and to develop the quality assurance criteria to be used in paving contracts. In addition, the benefits of using thermal camera, for both the contractor and the client, were explored. Three test roads were studied in summer 2013 by monitoring paving work, interviewing contractors and taking core samples from the segregated pavement locations. At all sites thermal camera was mounted to the asphalt paver to record temperatures during paving work. Core samples were analyzed to determine pavement properties such as bulk and maximum density, binder content and gradation. Also mechanical properties stiffness and strength were measured. At all sites paving work was mill and fill type overlay with stone mastic asphalt mixture laid less than 40 mm layer. As expected, study confirmed that truck-load segregation can be detected with thermal camera and severity of physical aggregate segregation correlates to temperature differentials. High level of segregation was detected when temperature between truck end and middle load differed more than 24°C. However, not all temperature differentials are caused by physical segregation and certain trends in measured temperatures testified the presence of thermal segregation caused by the long hauling distance. A bonus calculation system provided by the thermal camera vendor was assessed at all three paving contracts. Bonuses were received only by keeping the paver moving as targets for the risk areas and cold spots were not met. No penalties were applied. More research is needed to recommend changes to the temperature assessment in the bonus system, but allowing variable paver speed to prevent paver stops is not recommended. The detrimental effects of truck-load segregation were mitigated by the higher compaction effort, which produced stronger mixture. To quantify this improvement, a follow-up study to monitor distress formation during the coming years is thus warranted.     

An optimum selection strategy of reflective cracking mitigation methods for an asphalt concrete overlay over flexible pavements

Reflective Cracking (RC) has been a daunting challenge in pavement maintenance and rehabilitation (M&R), yet, still, after several decades of research, no exclusive solution prevails. Moreover, RC mitigation methods have shown significant variation in in situ performance. Therefore, a technique tailored to select an effective RC mitigation method is essential for the success of pavement M&R. In this study, a life cycle cost (LCC) and multi-criteria decision-making (MCDM) analyses were conducted to evaluate the effectiveness of currently available RC mitigation methods and to select the optimal method for an asphalt concrete overlay above flexible pavements. The MCDM includes three components: LCC, performance, and materials (recyclability). These criteria determine the selection ranking of each RC mitigation method. In addition, the effects of the priority level including cost, performance, and recyclability on the final decision were evaluated by conducting a series of sensitivity analysis under multiple scenarios; therefore, weight combination of the three criteria were recorded to define the measurements affecting the final decision.