Thick Asphalt Research Articles

Random distribution of asphalt and cement mortar in semi-flexible pavement: Implication for cracking resistance

The distribution characteristics of asphalt and cement mortar play a crucial role in determining the cracking resistance of semi-flexible pavements (SFP). In this study, digital image processing technology was utilized to quantify the thickness distribution of asphalt and cement mortar within SFP, and a parallel line segmentation method was employed to characterize the random distribution of mortar thickness (RDMT) of asphalt and cement mortar. Variance, coefficient of variation (COV), and interquartile range were proposed to characterize RDMT, examining the effects of air voids (AV) content, nominal maximum aggregate size (NMAS), and compaction method on RDMT, and its variation along the specimen height. Semi-circular bending test was conducted to evaluated the cracking resistance of SFP, exploring the relationship between RDMT and cracking resistance. Results indicate that parallel line segmentation method with 36 parts and 25 parts is recommended to characterize the RDMT of asphalt and cement mortar, respectively. An increase in AV content led to a higher RDMT for asphalt mortar but a lower RDMT for cement mortar. Conversely, as NMAS increased, the average thickness of asphalt and cement mortar increased, while the RDMT of asphalt mortar decreased and that of cement mortar increased. RDMT of asphalt and cement mortar is higher under Superpave gyration compaction method compared to the Marshall compaction method. Additionally, RDMT increased from the top to the bottom of the specimen height. Based on the RDMT, a random distribution index (RDI) was proposed, which exhibited a strong linear relationship with the cracking resistance of SFP. When the RDI decreased by 44.4 %, the corresponding cracking resistance of SFP increased by 84.0 %. Improving the distribution of asphalt and cement mortar could enhance the cracking resistance of SFP.

Održivost u gradnji cesta – Dvije studije slučaja

Sustainability in the field of traffic route construction should be considered against the background of intelligent resource conservation. A holistic assessment of sustainability, including life cycle considerations and assessments, is essential. In the first case study, the pavement structure at the heavily loaded Daimler intersection was constructed using concrete in the direct intersection area and using both concrete and asphalt in the connecting areas of all four axes. A 31 cm thick unbound frost protection layer with a grain size of 0/45 mm was placed over the existing road surface. This was followed by an 8 cm thick asphalt base course (AC 32), applied using an asphalt paver. Finally, a 26 cm thick concrete layer was placed over the asphalt base course. In the second case study, a horizontal hybrid construction method is presented using the German motorway BAB A61 in the Koblenz area as an example. Horizontal hybrid construction combines the advantages of asphalt and concrete to create a durable and resistant pavement surface capable of withstanding high traffic loads. This innovative construction method enables the optimum use of different materials and ensures efficient rehabilitation and improvement of motorway infrastructure.

Determination of Permanent Deformations of Non-Cohesive Soils in Pavement Structures under Repeated Traffic Load

One of the main types of distress in pavement structures is rutting, which may also reduce serviceability significantly. Most design methods typically attribute rutting to the asphalt layer alone, proposing that it can be managed by controlling vertical deformation or stress at the subgrade’s top. Furthermore, these methods frequently lack precise measurements for rut depth. On-site measurements show that the majority of permanent deformation occurs in the unbound layers beneath the asphalt; attention should be directed towards these layers. In recent literature, there are calculation methods that account for accumulating strains in soils. However, further investigation is needed regarding the effect of soil properties and the significance of the pavement cross-section. The literature is also somewhat contractionary regarding the origin of permanent deformations. In this research, the residual settlement of soils (base, subbase, and subgrade) under flexible pavement systems was analyzed due to the repeated traffic loads. Rut depths were calculated and analyzed using the High-Cycle Accumulation (HCA) model. The different behaviour in each course of the pavement system is discussed to reveal their contribution to total ruts. The effect of the grain size distribution of the subgrade was analyzed, and its significance on the rutting depths was demonstrated. Standardized pavement cross-sections with different asphalt thicknesses were evaluated, and the calculated settlements of the pavement originating from the ground during the design lifetime are also presented. It is shown that, with the same number of repetitions, the settlements calculated in each traffic load class are proportional to the thickness of the asphalt course. The contribution of the base, subbase, and subgrade courses to the total settlement is also presented for different subgrade types and traffic load classes.

Investigation of The Effect of Asphalt Layer and Sub-Base Layer Thicknesses on Pavement Surface Deformation by The Finite Element Method

The layer thickness of flexible pavements is one of the important parameters affecting permanent deformations. In this study, the effect of sub-base thickness and asphalt concrete thickness on the deformations under axial loads was investigated using the Plaxis 2D finite element software. In the study, 100, 200, 300, 400, 500, and 600 kPa pressure were applied to the asphalt pavement surface. In the first case, the base layer of 30 cm thickness was kept constant, and the asphalt layer was changed from 9 to 14 cm. In the second case, the asphalt thickness at 9 cm and the base layer thickness of 30 cm were kept constant, and the sub-base layer thickness was selected at 30, 40, and 50 cm. As a result of the completed analysis, it was observed that the deformations on the pavement surface decreased with increasing thickness of the layers; furthermore, the use of a sub-base layer in the pavement considerably increases the deformation resistance.

Effect of the experimental parameters on the ice-asphalt adhesion strength and corresponding theoretical model based on the energy-balanced principle

Ice adhesion behavior plays a crucial role in assessing ice-phobic materials. However, the variation of ice-adhesion strength across different laboratories is attributed to the neglect of ice/substrate characteristics. In this work, ice-adhesion strength under different geometric parameters of ice/substrate on asphalt surfaces was examined through an optimized methodology. Additionally, a theoretical model was developed based on the energy balance principle to analyze the influence of various parameters on ice-adhesion strength and to make predictions. The results suggested that the ice-adhesion strength was positively correlated with the substrate thickness and negatively correlated with both the asphalt thickness and the ice diameter. The detachment process of the ice-asphalt interface was divided into two regions, the non-linear mechanical response region, and the linear damage region. The slope of the linear damage region can be defined as the interfacial stiffness of the ice-asphalt system, which was the main reason leading to the change in ice adhesion strength. The increase in ice-adhesion strength was observed as the interfacial stiffness increased. Moreover, the theoretical model of ice-adhesion strength illustrated the effect of ice/substrate geometric characteristics on ice-adhesion strength and was verified by the experiments. This study may provide some new references and a theoretical basis for further research on ice adhesion behavior in different laboratories.

Machine Learning Modeling of Wheel and Non-Wheel Path Longitudinal Cracking

Roads degrade over time due to various factors such as traffic loads, environmental conditions, and the quality of materials used. Significant investments have been poured into road construction globally, necessitating regular evaluations and the implementation of maintenance and rehabilitation (M&R) strategies to keep the infrastructure performing at a satisfactory level. The development and refinement of performance prediction models are essential for forecasting the condition of pavements, especially to address longitudinal cracking distress, a major issue in thick asphalt pavements. This research leverages multiple machine learning methods to create models predicting non-wheel path (NWP) and wheel path (WP) longitudinal cracking using data from the Long-Term Pavement Performance (LTPP) program. This study highlights the marked differences in distress conditions between WP and NWP, underscoring the importance of precise models that cater to their unique features. Aging trends for both types of cracking were identified through correlation analysis, showing an increase in WP cracking with age and a higher initial International Roughness Index (IRI) linked to NWP cracking. Factors such as material characteristics, kinematic viscosity, pavement thickness, air voids, particle size distribution, temperature, KESAL, and asphalt properties were found to significantly influence both WP and NWP cracking. The Exponential Gaussian Process Regression (GPR) emerged as the best model for NWP cracking, showcasing exceptional accuracy with the lowest RMSE of 89.11, MSE of 7940.72, and an impressive R-Squared of 0.63. For WP cracking, the Squared Exponential GPR model was most effective, with the lowest RMSE of 12.00, MSE of 143.93, and a high R-Squared of 0.62. The GPR models, with specific kernels for each cracking type, proved their adaptability and efficiency in various pavement scenarios. A comparative analysis highlighted the superiority of our new machine learning model, which achieved an R2 of 0.767, outperforming previous empirical models, demonstrating the strength and precision of our machine learning approach in predicting longitudinal cracking.

Geotechnical Properties of Sand-Asphalt for Use in Road Base Layer Instead of Conventional Materials

The study of the geotechnical properties of sand-asphalt for use in road base layer instead of conventional materials, for low to moderate traffic volume, is based on standardized laboratory tests. Sand samples were taken from Kayar, Darou Alpha and Keur Morry (Thies region, Senegal). Identification tests (particle size distribution and sand equivalent) made it possible to select Kayar and Darou Alpha sands which have respective sand equivalent (ES) values of 92 % and 62 %. These values are higher than the minimum required (40 %), for sand-asphalt, with different 35/50 bitumen dosages. The results of the Hubbard-Field test carried out showed that Darou Alpha sands are suitable, with an optimum dosage of 7 % bitumen : the stability at 18 °C, at dry conditions, is 2 732 kg and is greater than the minimum required value of 2 300 kg; the immersion/compression ratio is 85 % and is greater than the minimum required limit of 75 %; water absorption reaches 5.4 % and remains below the maximum required limit of 7 %; the volumetric swelling is 2.6 % and is below the maximum required limit of 5 %. The compactness is 84 % and is acceptable because it exceeds the minimum required limit of 80 %. The sizing carried out with the Alizé©3 software gives deformation values lower than the permissible values. The pavement structure that is placed on a PF3 type platform is therefore correctly sized to support a traffic of 150 heavy vehicles per day for a service life of 15 years. This roadway structure, which is devoid of a foundation layer, includes a 23 cm thick sand-asphalt base layer on which rests a 5 cm thick asphalt concrete. Keywords: sand-asphalt, Hubbard-Field test, Kayar, Darou Alpha, Keur Morry.

Investigation of the influence of the variable-intensity ultraviolet aging on asphalt properties

Currently, researches related to asphalt UV aging mostly based on constant-intensity UV radiation, which is not consistent with its real service condition. In this study, the variable-intensity and constant-intensity UV radiation conditions were proposed on the principle of constant total radiation. The UV-aged neat asphalt film thickness was 1 mm and 0.5 mm. The temperature sweep, frequency sweep, and multiple stress creep recovery tests were conducted to reveal the rheological properties of UV-aged neat asphalt. Moreover, the Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) tests were performed to reveal the microscopic properties of UV-aged asphalt. Regarding the constant-intensity UV-aged asphalt with 1 mm film thickness, the rutting parameter aging index (RPAI) of 9 cycles was smaller than that of RTFO-aged asphalt. The variable-intensity UV aging induced a much larger aging effect than constant-intensity UV aging for UV-aged asphalt. The ISO and ICO under variable-intensity UV aging conditions is higher than constant-intensity UV aging for the same cycles. Besides, the asphalt thickness is an essential element affecting UV aging. The aging effect increased significantly as the film thickness decreased. SEM test results have indicated that the variable-intensity UV aging caused more microscopic morphology damage to asphalt than constant-intensity UV aging. Variable-intensity UV aging test condition proposed in this study is more accurately simulate the real service conditions. It is meaningful to propose the variable-intensity UV aging process. This study provides new ideas for simulating UV aging conditions and methods in the laboratory.

Case Study: Validation of the Spectral-Analysis-of-Surface-Waves Method for Concrete Pavement Condition Evaluation

Before conducting pavement rehabilitation, the concrete pavement performance evaluation is more than necessary to assess the strength and analyze the internal condition. To evaluate the modulus performance of concrete pavement and provide theoretical support for subsequent maintenance and rehabilitation, the SASW (Spectral-Analysis-of-Surface-Waves) method, a non-destructive testing method based on surface waves, was used to determine the modulus of three in-service concrete pavements. At the same time, FWD (Falling Weight Deflectometer) testing and on-site coring were carried out at the measuring points. Through the analysis of modulus results measured in the field, it was found that the SASW method can accurately obtain the elastic modulus of the concrete layer and base layer. Compared with the concrete modulus measured by FWD, the modulus measured by the SASW method was closer to the uniaxial compressive modulus. Moreover, since the SASW method has the ability to provide gradient modulus varying with depth and determine the pavement thicknesses, it is reasonable to recognize the location of the inherent distress based on the region where the modulus suddenly decreases within one layer. The comprehensive evaluation of concrete pavement condition, including the modulus value and internal distress recognition, has the capability to assist in determining the design of overlay asphalt thickness and material.

Impact of Heavy Vehicle Axle Overload and Axle Configuration on Road Damage in Afghanistan

Aim: This study aimed to determine the percentage of heavy vehicles, overloaded vehicles, and heaviest vehicle that leads to deterioration of asphalt roads. Furthermore, the study aimed to assess the impact of ESALs and axle configuration on asphalt thickness and subgrade.
 Methods: This research used quantitative truck traffic data collected for three months in the Laghman WIM (Weigh in Motion) station located on the Kabul Torkham National Highway. Observed axle loads and GVW (Gross Vehicle Weight) data collected from April/ 10/2022, up to June/7/2022 average daily truck traffic of 73895 was estimated axle loads converted to the standard axle ESALs (Equivalent Single Axle Loads).
 Results: The percentage of heavy overweight vehicles is 13% while the heaviest vehicle is T2-Bedford truck vehicles at 20.17%. The T2 Bedford truck percentage was 42.17% in overall volume; this truck was overloaded 20.17%, compared to the general trend of Ministry of Public Works (MoPW), the T3 Hino truck contribution was 1.24% in total volume and 18.64% was overloaded, the ST112 semi-trailer contributed 3.02% in total volume, and 8.52% were overloaded. The ST113 5-axle and TT123 6-axle trailers percentile in entire volume was 9.6 % and 40.85% whereas, the overloading degree for these heavy vehicles was 2.29% and 6.72% respectively.
 Conclusion: Overloading percentage is 13% while, T2 Truck is the heaviest vehicle The variation of ESALs and axle configuration on asphalt thickness and subgrade modulus illustrates the higher thickness of asphalt decreases the damage to asphalt roads.
 Recommendations: Afghanistan Weigh-in-Motion stations need professional employees, internet facilities, electricity, and proper databases to record the data. By applying tight legislation and fines overloading will be minimized. Researchers may also research heavy vehicle tires' impact on road damage. Furthermore, it needs to determine the level of fatigue cracking and rutting failures by overloading.

Interface horizontal shear strength requirement in Ultra-thin whitetopping (UTW) pavement.

Ultra-thin whitetopping (UTW) is a thin (5-10 cm) layer of concrete overlaying over an existing asphalt pavement. This concrete layer is bonded on the asphalt surface. Although UTW has been successful in strengthening asphalt pavement, in some cases, the composite pavement may debond at the interface under the concrete slab due to high shear stress. This paper aims to determine the interface shear strength required for UTW pavement systems using theoretical methods and calibrated numerical models (FEMs). The study analysed 432 pavement scenarios with different slab sizes, thicknesses, and material properties. The results show that higher shear stress occurs when the asphalt modulus is stiffer. The size of the slab, the modulus of the supporting layer, and the asphalt thickness have only a minor effect on the shear stress value. However, the thickness of the concrete slab significantly affects the maximum shear stress in the UTW, with an increase of up to 36.6% when reducing the slab thickness from 100mm to 50 mm. The FEM calculations can be used to determine the required interface horizontal shear strength for UTW.

Relationship between Fatigue Condition of Asphalt Pavements and Deflection Lag from Traffic Speed Deflectometer

The phase angle is a good indicator of the current fatigue condition of asphalt concrete (AC) layers, but estimating phase angles from drilled core samples is destructive, expensive, and unsuitable for large-scale applications. Similar to the phase angle, a lag between load and response has recently been observed in the deflection basin of the nondestructive traffic speed deflectometer (TSD), i.e., the lag between the loading point and the maximum deflection point, and the deflection lag may be closely related to the phase angle. This study investigated the potential of TSD deflection lag as a nondestructive indicator of pavement fatigue conditions. The relationship between AC phase angle and TSD deflection lag was investigated using the 3D-Move program, and the effects of pavement structure, TSD speed, and test temperature on the deflection lag were also investigated. Field TSD data collected in Tennessee were used to verify the relationship between deflection lag and fatigue cracking. The results show that the lag distance increases uniformly with increasing fatigue levels (phase angles) until fatigue failure occurs. However, the lag distance is also closely related to the asphalt thickness and subgrade modulus; therefore, only the lag distance of pavements with the same structures can be compared to identify the fatigue sections. Overall, the lag distance can be used as an implicit indicator of the fatigue condition of the pavement to predict the initiation and growth of fatigue cracks. Fatigue cracking is expected to occur where the lag distance is relatively large.

The effect of an emulator inductive power transfer pad on the temperature of an asphalt pavement

This paper utilises experimental and numerical methods of determining the temperature distribution in an asphalt slab containing an energized embedded emulator Inductive Power Transfer (IPT) pad. In the series of experiments, five different ambient temperatures from 15 to 50 °C, four IPT pad power loss values from 5 to 30 W, and three pavement layer configurations were considered. A thermal camera was used to measure the surface temperature distribution of the asphalt slab, while a Fiber Bragg Grating (FBG) sensor and six thermistors were used for point temperature measurements. A numerical simulation, using a combination of experimentally determined, and literature-sourced thermal properties was developed for the experimental set-up, which was validated against the measured temperatures. Results show good agreement between the experimental and the simulation models with a maximum point-wise differential of 12.9%. It was found that increasing the ambient temperature from 15 to 50 °C, at a constant pad power loss of 30 W, increased the simulated asphalt surface temperature (25 mm offset from the pad) from approximately 32 to 65 °C. Moreover, varying the IPT pad power loss from 5 to 30 W, at an ambient temperature of 50 °C, increased the simulated asphalt surface temperature at the same location from approximately 53 to 65 °C. However, changing the asphalt thickness from 43 mm to 86 mm, or adding a compacted aggregate base course layer below the IPT-asphalt slab, only marginally decreased the temperature values. An increase in the thermal conductivity of the asphalt from 1 to 3 Wm−1°K−1, results in the temperatures reducing by a maximum of approximately 21% at the bottom of the pad. However, changing the specific heat has a very small effect on the heating and cooling rates. A numerical study of an in-service New Zealand asphalt pavement, containing an emulator IPT pad and utilising a three-day period of recorded New Zealand summer climatic data is presented. The method can be used to identify IPT-pavement hotspots and aid in the design of thermally robust IPT-pavement systems.

Research on thickness problem of pavement asphalt layer based on energy dissipation

Aiming at the problems of low calculation accuracy and poor calculation effect of the traditional calculation method of the optimal thickness of pavement asphalt, a thickness calculation method of pavement asphalt layer based on energy dissipation was proposed. First, according to the energy dissipation theory, the changes of pavement in creep deformation zone and plastic damage zone under different temperature conditions were concretely analyzed, which laid the foundation for subsequent research. Then, proposing to take the lowest energy dissipation control of asphalt pavement as the principle, the energy consumption of asphalt layer was proposed and the optimal thickness of asphalt layer was calculated under the premise of considering the environment and other parameters. Finally, the method was verified by analyzing the changing conditions of the actual failure zone. The verification results show that when the thickness of the asphalt layer reaches the optimal thickness, the main creep deformation zone in the asphalt layer does not change significantly, and the scope of the actual plastic failure zone will be reduced to the minimum. At this time, the energy dissipation of pavement is the lowest, which conforms to the principle that the energy dissipation of the pavement is controlled at the lowest. Moreover, by analyzing the peak value of plastic dissipated energy and the changing situation of the actual plastic failure zone, it is concluded that the most suitable thickness range of asphalt layer is 10 cm∼20 cm. Therefore, through the research of the above, the amount of asphalt used is greatly reduced, and the energy consumption is reduced.

Multiscale performance of composite modified cold patch asphalt mixture for pothole repair

Cold patch asphalt mixture (CPAM) has become primary material for pothole repair of asphalt pavement due to its advantages of convenient storage, strong seasonal adaptability and timely repair, as well as green maintenance feature. However, insufficient adhesion of CPAM due to the asphalt binder diluted by diesel may lead to secondary distress in a short term. In this paper, composite modified CPAM is developed and its multiscale interfacial morphology and mechanical properties are explored. Three kinds of CPAM with different gradations are designed. Based on indirect tensile test, scanning electron microscope and nanoindentation experiment, the multiscale interface morphology characteristics and mechanical properties are analyzed. The results showed that elastic modulus of CPAM increases with the increase of aged time because of the evaporation of light diesel; with the optimal cold patch asphalt (CPA) content 4.3%, the aggerates are all coated and the mixture is loose; with redundant CPA content 5.0%, too thick asphalt film and agglomerate CPA paste can also be observed; with insufficient CPA content 3.5%, aggregates are not completely coated by CPA. By comparing interface microstructure, with the optimal CPA content 4.3%, redundant CPA content 5.0% and insufficient CPA content 3.5%, smooth morphology, redundant CPA spilling and inadequate asphalt attachment are observed in the interface of CPAM, respectively. In addition, the evaporation of light diesel, age of asphalt polymer and composite additive cause microcracks and micro-holes in the interface between CPA mortar and aggregates.

Development of Prediction Model for Rutting Depth Using Artificial Neural Network

One of the most common pavement distresses in flexible pavement is rutting, which is mainly caused by heavy wheel load and various other factors. The prediction of rutting depth is important for safe travel and the long-term performance of pavements. Factors that are considered in this paper for the prediction of rut depth are Temperature, Equivalent Single Axle Load, Resilient modulus, and Thickness of hot mixed asphalt. The input data for all factors are collected from the Long-Term Pavement Performance Information Management System for the state of Texas. Regression analysis is performed for dependent and independent variables to obtain the empirical relationship. In various fields of civil engineering, artificial neural networks have recently been utilized to model the qualities and behavior of materials and to determine the complicated relationship between various properties. An Artificial Neural Network is used to develop a predictive model to predict the rutting depth. A total number of 70 observations were considered for the predictive model. A mathematical relation is developed and verified between rut depth and variable input data.

Bitumen stabilized materials as pavement overlay: Laboratory and field study

Bitumen stabilized materials applied as base courses have been used in many countries. In the southern Brazilian State of Rio Grande do Sul, two test sections were built using such mixes (in 2011 and 2015) as overlay asphalt emulsion base courses and have been under monitoring since then. In 2019, a new test section was built to evaluate mixes of granular materials and reclaimed asphalt pavement stabilized with emulsion and Portland cement as overlay base courses. The main objective of the research reported here was to evaluate the mechanical behavior of such materials and to assess their performance in the field during the years after construction. Indirect tensile strength, monotonic triaxial, and repeated load triaxial tests were carried out to obtain the necessary mechanical parameters. The data collected during the monitoring of the test sections were used to analyze the field performance in terms of permanent deformation. Besides, in situ moduli were obtained using backanalysis of deflection basins. The number of equivalent single axle loads (ESALs) for the analysis period was computed using data collected in toll plazas near the test sections. Using field and laboratory results and the South African TG2 performance model, it was possible to estimate the life of the studied pavements regarding permanent deformation. The TG2 model seems to underestimate the life of pavements with thick (>40 mm) asphalt layers. A model relating rutting to the number of ESALs was obtained, allowing the prediction of rutting of asphalt pavements overlaid by a bitumen stabilized base layer and asphalt wearing courses. Considering the test sections’ performances, the mixes proved to be efficient to improve structural capacity, also reducing the influence of the water level variation on pavement performance.

A prediction model for top-down cracking in asphalt pavements with open-graded friction course

Thick asphalt pavements with open-graded friction course (OGFC) are subjected to top-down cracking (TDC), a distress consisting of cracks that initiate on the pavement surface close to the wheelpath and propagate downwards. However, road agencies do not yet have adequate tools to predict TDC. The objective of this study was to develop a practical and reliable model to predict TDC depth evolution in such pavements. The proposed model provides a maximum TDC depth that can potentially occur in a pavement characterized by certain mechanical properties and traffic at the time of interest. The model was calibrated and validated considering several Italian motorway pavements affected by TDC based on limited data, therefore more data should be collected in the future. This model can be used in a pavement management system (PMS) to plan timely surface maintenance against TDC.

Evaluating structural characteristics of asphalt pavements by using deflection slopes from traffic speed deflectometer

The Traffic Speed Deflectometer (TSD) overcomes the limitations of the Falling Weight Deflectometer (FWD) in terms of traffic interruption and testing inefficiency, making it an efficient tool for network-level pavement structural evaluation. The current research on TSD is inherited from FWD and mainly focuses on deflection itself. However, the raw measurement of TSD is the deflection slope, i.e., the first-order derivative of deflection. TSD deflection is calculated by curve fitting and integration, and the extensive use of deflection itself may accumulate the error caused by curve fitting. This study explores the potential use of TSD deflection slope in assessing the strength of pavement structures, estimating the location of the inflection point, and calculating the lag distance. The results show that both the TSD deflection and slope are effective in detecting weak asphalt layers and subgrades, but they are not sensitive to damage to the base layer. It is recommended that TSD slopes at 5″ and 60″ (in front of the load center) be used to indicate the strength of the asphalt layer and subgrade, respectively. The location of the inflection point is related to the asphalt thickness and subgrade modulus, and it can be approximated by the location of the maximum slope measurement. In addition, the lag distance can be calculated by determining the maximum deflection point or by determining the zero-slope point. Since the maximum deflection method accumulates errors in curve-fitting and is more sensitive to interpolation methods, the zero-slope method is recommended to calculate the lag distance.

Development of a Road Pavement Structure Diagnostic Procedure Based on the Virtual Inertial Point Method

Falling weight deflectometers (FWD) are utilised worldwide to analyse the condition and the load-bearing capacity of road pavement structures. One of the FWD measurement results, the deflection bowl, may provide surplus information that is suitable for better road pavement structure diagnostics, based on the novel approach presented in this paper. This study presents a computational method that can calculate the layer thicknesses from the deflection data recorded by the non-destructive FWD device. The motivation for this research is that FWD and GPR equipment are often not available at the same time. However, the back-calculation of the pavement layer moduli from the deflections requires knowledge of the exact thicknesses. The developed method is based on the inertia point principle and provides not only the total pavement thickness but also the total asphalt thickness at each FWD drop point. From 25,200 linear elastic layered pavement models, 350 virtual inertia points could be identified. To describe the relationship between the structural model characteristics of the pavement (thickness and subgrade modulus) and the virtual inertia points, we chose the Gaussian process regression, a widely used method in machine learning. In addition to the thicknesses, the point of inertia can also be used to calculate the bearing modulus of the subgrade with high accuracy. Based on the data from the experimental road section, the radius value of the inertia point rc is not sensitive to the stiffness of the layers that compose the pavement structure, depending only on the total pavement thickness and the bearing capacity of the subgrade. The calculation was compared with the AASHTO (1993) procedure, and very similar values for the subgrade-bearing capacity were obtained. Moreover, in the near future, the method can be further developed to provide an estimation of layer thicknesses, together with a deflection measurement, especially adapted to continuous deflection measurement devices (Curviameter and Rolling Wheel Deflectometer).