Permanent Deformation Behavior Research Articles

Performance characteristics of the open-graded asphalt concrete filled with a special cement grout

This paper presents a performance of the open-graded asphalt concrete filled with a special cement grout in the road structures, introduces practical examples of the usage of this technology and defines the main properties. In addition, laboratory design of asphalt mixtures filled with special grout is researched. Finally, the results obtained from various laboratory tests are evaluated. The paper contains the open-graded asphalt concrete filled with a special cement grout mix design procedure and results of stiffness measurement, low temperature properties, permanent deformation and skid resistance. Open-graded asphalt concrete filled with a special cement grout mixture is compared to the commonly used asphalt mixes. The open-graded asphalt concrete filled with a special cement grout showed better results than the commonly used asphalt mixes by the stiffness and resistance to the permanent deformation characteristics but behavior at the low temperatures is slightly problematic.

A viscoplastic model for the confined permanent deformation of asphalt concrete in compression

This paper presents a viscoplastic model for the permanent deformation behavior of asphalt concrete in compression. Triaxial repeated load permanent deformation (TRLPD) tests with haversine-shaped load pulses and rest periods were used in the experimental investigations as they are able to simulate real traffic loading patterns. For the first time, the viscoelastic–viscoplastic coupling phenomenon in asphalt concrete is illustrated using experimental data, which motivates the direct collecting and modeling of permanent deformation history in the present work instead of modeling the viscoelastic and viscoplastic responses in a separate and uncoupled fashion as in the traditional scheme. By applying a long rest period (100 s) to allow the viscoelastic deformation to recover sufficiently, the permanent strain data can be collected directly for viscoplastic model development. The proposed viscoplastic model features a convolution integral enclosed in Macaulay brackets. A single viscoplastic relaxation spectrum is assumed as the material's intrinsic property, and the nonlinear stress effects are captured through the variable E∞, which is the infinite modulus expressed as a logarithmic function of the triaxiality ratio. Material hardening (or softening) is described via the increase (or decrease) of the internal stress during loading (or unloading and rest period). The model was characterized using TRLPD test data at three levels of deviatoric stress and confining pressure. Finally, the calibrated model was verified by applying it to random loading tests conducted at other confining levels. The proposed model provides an efficient and convenient approach that is able to determine the material's macroscopic deformation as well as to capture the material's internal hardening/softening mechanisms.

Permanent deformation behaviour of reinforced flexible pavements built on soft soil subgrade

This study focuses on evaluating the effectiveness of using various geogrid products to improve permanent deformation resistance in soft subgrade soils commonly encountered during roadway construction in Pennsylvania. Permanent deformation behaviours of the soft soils both with and without the inclusion of geogrids were investigated. Cyclic moving wheel loads were applied through a reduced-scale accelerated pavement testing (APT) device, a one-third-scale model mobile load simulator (MMLS3). Tests were conducted on two soil types, each modified with three different biaxial geogrids placed at the base–subgrade interface. The total permanent deformation/surface rutting of the pavement and the permanent deformation of the subgrade were measured at selected intervals of the wheel loading applications. The pavement sections were trenched upon completion of the accelerated testing to measure the deformed profiles of the cross sections from which the permanent deformations in the asphalt layer and the base layer were determined. Sections modified with geogrids were found to have similar performance with the control section in terms of the total permanent deformation. While the geogrids did not show significant effects on the asphalt layer permanent deformation, sections with geogrids consistently showed a significantly higher base layer permanent deformation as compared to the control sections. Measurements of the subgrade permanent deformation showed that two of the geogrids consistently reduced the permanent deformation of subgrade built with the two types of soft soil. The relative layer contribution to the total permanent deformation suggested a base layer failure in both sets of the accelerated tests, most likely due to the inadequate compaction of the base layer during construction. Sections modified with geogrids exhibited a significantly higher base layer contribution, along with a significantly lower subgrade contribution, to the total permanent deformation, whereas the control section showed the opposite of the layer contributions.

Characterisation of unbound aggregate materials considering physical and morphological properties

The objective of this paper is to evaluate the factors affecting resilient and permanent deformation behaviour of unbound granular materials, with a focus on the aggregate physical and morphological characteristics. To evaluate the behaviour of base course, repeated load triaxial testing is commonly used to establish the stress-dependent resilient modulus properties of unbound aggregate base and subbase materials. Although resilient modulus of aggregates is a critical input into mechanistic-empirical pavement design methods, the resilient modulus of unbound base material is often estimated from empirical correlations with index properties in the AASHTOWare Pavement ME design procedure for its simplicity. Since actual field stress conditions and resilient modulus stress states are generally quite different from those generated in the empirical test methods, use of an empirical correlation could lead to an unreliable prediction of resilient modulus and permanent deformation. In order to properly assess the stability of an unbound aggregate layer, it is necessary to establish a proper process to understand the factors affecting fundamental and performance-related properties of unbound granular materials. In this study, aggregate samples from four different sources were tested for resilient modulus and Poisson’s ratio measurements using the Precision Unbound Material Analyzer equipment. Morphological or shape properties of aggregate samples were also measured using an image analysis device. The results demonstrate that aggregate physical and morphological properties affect aggregate resilient and permanent deformation. Further, it is suggested that the resilient modulus of the aggregate should not be used as the sole indicator of rutting performance of aggregate base.

Permanent deformation characteristics of silty sand subgrades from multistage RLT tests

Rutting is one of the main forms of distresses in thin flexible pavement structures, often associated with accumulation of permanent deformation in unbound granular layers and subgrade soils under traffic loading. Realistic prediction of surface rutting requires models that can reliably capture the cumulative plastic deformation of pavement unbound layers under repeated loads. This study presents an evaluation of three models that incorporate the time-hardening concept for prediction of permanent deformation of silty sand subgrade materials. A series of multistage repeated load triaxial (RLT) tests, in which the material underwent a wide range of continuous stress conditions, were carried out on two silty sand subgrades. The RLT tests were conducted at four different moisture contents in which pore suctions were measured throughout the test. In the modelling of the permanent deformations, the effective stress approach was used taking into account the effects of soil suctions. The material parameters of the predictive models were optimised using the RLT test data and the effect of moisture content (matric suction) on the permanent deformation characteristics of the materials and the predictive model parameters were investigated. Generally, it was observed that the modified models that are based on the shakedown approach performed reasonably well in capturing the permanent deformation behaviour of the selected subgrade materials with minor discrepancies between the models. This indicates that using multistage RLT tests can be an efficient approach for characterising the permanent deformation behaviour of subgrade soils.

Evaluation of permanent deformation of geogrid reinforced asphalt concrete using dynamic creep test

Permanent deformation (rutting) is one of the distresses that can adversely affect the bituminous surface of pavement structures, particularly in hot climates. The geosynthetics reinforcement of hot mix asphalt is one of the means to combat rutting. In this study, a dynamic creep test was performed on asphalt concrete samples reinforced with four different types of fiberglass grid as well as on unreinforced samples. The fiberglass grids used in this study contained two different sizes of grid openings and two tensile strengths, allowing us to test for the mesh size and tensile strength effects of the grids on the permanent deformation behavior of double layered asphalt concrete. In addition, we tested a recently developed creep curve model has been verified and used this to study the creep behavior of the samples in the primary and secondary regions of the creep curve, as well as determining the boundary point of the regions. The results suggest that not only grid tensile strength, but also grid mesh size is of great importance in combatting permanent deformation of fiberglass grid reinforced asphalt concrete within the conditions and grids used in this study. In a nutshell, higher tensile strength and/or smaller mesh size grids lead to overall better performance of grid reinforced samples. Moreover, great care must be taken when the creep curves are not reached in the tertiary region, and the creep rate must be taken into account to avoid any misinterpretation of the results.

Permanent deformation behavior of unbound granular base materials with varying moisture and fines content

Specifications for unbound granular base materials vary among transportation agencies based on the availability of materials, climatic conditions, and function. Specifications aim to provide durable materials that meet design requirements and achieve the target design life with cost effective materials. This paper investigates the effect of the percentage fines (particles passing sieve No. 200) and moisture content on the plastic behavior of unbound granular materials. Plastic response of six gradations representing two types of material, 100% crushed limestone and gravel, was evaluated at two levels of moisture content. The shakedown concept was used to classify the deformation behavior of the materials. Results showed that limestone with an optimum fines content of 4.5% and gravel with an optimum fines content of 9% had better resistance to plastic deformation than other fines contents.

Characterization of geogrid reinforced ballast behavior at different levels of degradation through triaxial shear strength test and discrete element modeling

Recent research efforts at the University of Illinois have aimed at studying geogrid applications in railroad track structures, specifically focusing on ballast and subballast reinforcement. Ballast, typically comprising large sized aggregate particles with uniform gradation, is an essential layer in the railroad track substructure to facilitate load distribution and drainage. The primary mechanism of load transfer within the ballast layer involves inter-particle contact between ballast particles. Similarly, the effectiveness of ballast reinforcement with geogrids is primarily governed by the geogrid-aggregate interlock. Such interaction and the effectiveness thereof can change significantly as the level of grain size and shape degradation or fouling increases in the ballast layer with accumulation of train traffic. Although several studies in the past have investigated the effects of geogrid reinforcement on ballast shear strength and permanent deformation behavior, the effectiveness of geogrid reinforcement at different levels of ballast degradation needs to be further understood. In this study, monotonic triaxial shear strength tests were conducted on both new and degraded ballast materials with and without geogrid reinforcement. Two geogrid types, with square- and triangular-shaped apertures, were used in the laboratory to calibrate an aggregate imaging-based Discrete Element Method (DEM) modeling approach, which is capable of creating actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. The DEM model was observed to adequately capture the shear strength behavior of geogrid-reinforced triaxial ballast specimens prepared using both new and degraded ballast samples.

A Three-Dimensional-Printed Home-Based Flexible Nasopharyngeal Explorer With Diamond Cuts1

A Three-Dimensional-Printed Home-Based Flexible Nasopharyngeal Explorer With Diamond Cuts1

A model for predicting permanent deformation of unbound granular materials

A simple model has been proposed to characterise the accumulation of permanent deformation (PD) in unbound granular materials (UGMs) under cyclic loading of variable magnitudes. The model was developed based on multi-stage (MS) repeated-load triaxial (RLT) tests. The material parameters of this model can be evaluated using an MS RLT test. The model was validated by calibrating it for a few UGMs with a range of grain size distributions, moisture contents and degrees of compaction. The calibrated model was further validated by predicting the PD behaviour of some of these UGMs for different stress conditions. Generally, quite satisfactory predictions were obtained with this model with the advantage of reduced effort required for its calibration compared to some existing models. Additionally, the sensitivity of the parameters of this model to moisture, degree of compaction and grain size distribution was investigated with the aim of incorporating them into the model in future.

Addressing the permanent deformation behavior of hot mix asphalt by triaxial cyclic compression testing with cyclic confining pressure

Rutting or permanent deformation is one of the major distress modes of hot mix asphalt in the field. Triaxial cycle compression testing (TCCT) is a standardized and scientifically accepted test method to address this distress mode in the lab and to characterize the resistance to permanent deformation. In most labs and according to EN 12697-25, standard TCCTs are carried out with cyclic axial loading and a constant confining pressure. In road pavements on the other hand, dynamic traffic loading due to passing wheels leads to cyclic confining pressure. In order to bring the TCCT closer to reality, the radial reaction and its phase lag to axial loading in standard TCCTs are analyzed and an enhanced TCCT with cyclic confining pressure is introduced. The cyclic confining pressure takes into account the viscoelastic material response by the radial phase lag to axial phase loading. In a subsequent test program, TCCTs with different confining pressure amplitudes were carried out on two hot mix asphalts. Results from standard and enhanced TCCTs were analyzed, compared and discussed. It is shown that the resistance to permanent deformation increases significantly when the viscoelastic material response is taken into account in the TCCT by introducing cyclic confining pressure.

Long-term resilient and permanent deformation behaviour of Controlled Low-Strength Materials for pavement applications

The paper deals with the long-term stiffness characterisation of Controlled Low-Strength Materials (CLSMs) for pavement applications in substitution of granular fill materials. Three alternative CLSM mixtures, two with ordinary Portland cement and a third one with an ultra-rapid sulpho-aluminate cement, were examined. Two different sample aspect ratios were considered and the samples were subjected to different testing conditions in terms of saturation, loading time and repetition. The investigated CLSMs are insensitive to variations of loading frequency and to water saturation, and sensitive to sample aspect ratio. They exhibit a significant increase in stiffness under repeated load triaxial testing and a low permanent strain accumulation. Finally, they exhibit an increase in resilient modulus when the deviatoric stress increases.

DEVELOPMENT OF ASPHALT BINDER PERFORMANCE GRADES

Asphalt plays a significant role in a pavement’s ability to withstand thermal and fatigue cracking that contributes to permanent deformation behavior. Temperature is a significant factor that affects asphalt binder and thus the performance and life span of the whole pavement. This paper presents research developing asphalt binder performance grade requirements, according to Superpave, suitable for different climatic conditions all over Egypt. Twenty one weather stations covering Egypt were selected, then after analysis their air temperature data was converted to pavement temperatures using LTPP and performance models. Finally, the converted pavement data were used to propose asphalt binder performance grades (PG) for the various regions of Egypt. The proposed grades range from PG52-10 to PG76-10 depending on the location, the applied model, and the degree of project reliability.

Predicting permanent deformation behaviour of unbound granular materials

To reliably predict the permanent deformation behaviour of unbound granular materials (UGM) in a pavement structure, the material parameters of the constitutive models used in design should be evaluated using a multi-stage (MS) loading approach. This paper investigated the prediction of the accumulation of permanent strain in UGM using some current models, extended applying the time-hardening approach, based on MS repeated load triaxial tests (RLTTs). The material parameters of these models were optimised for five different UGM used in pavement construction using the MS RLTT data with a specific set of stress levels. With these models, the accumulations of permanent strain in the same materials for MS RLTTs with a different set of stress levels were predicted. Using this approach, three out of the four models performed very well, which may be further developed for field conditions for better prediction of rutting.

Laboratory evaluation of asphalt concrete mixtures to permanent deformation

Several laboratory tests have been developed in order to detect the resistance of asphalt concrete mixtures to permanent deformation. Among them, the most commonly used are the wheel-tracking and the cyclic compression test. The purpose of this study was to compare the permanent deformation behaviour of asphalt concrete, under different testing procedures, when gradation and binder content was varied but within the tolerance limits. The asphalt concrete used was an AC 20 with 50/70 penetration grade bitumen. For the wheel-tracking test, three different procedures at two testing temperatures (45 and 60°C), were used: procedure A and B, in air, in accordance with EN 12697-22, and the procedure specified in BS 598-110. For the triaxial cyclic compression test, method B per EN 12697-25, was used at a testing temperature of 45°C. The results of the study showed that the aggregate gradation and the binder content even when both varied within the tolerance limits affect the performance in terms of permanent deformation. The ranking of the mixtures was found to be the same regardless of the testing procedure employed. As far as the acceptability of the mixtures was concerned it was found that by applying the limiting values set by PD 6691:2010, the BS 598-110 procedure and the small device, procedure B per EN 12697-22 gave different outcomes.

Application of time–temperature superposition to develop master curves of cumulative plastic strain in repeated load permanent deformation tests

The repeated load permanent deformation (RLPD) test is used to capture the permanent deformation characteristics of asphalt mixtures for prediction of pavement rutting. The full characterisation of permanent deformation behaviour as required by the National Cooperative Highway Research Program (NCHRP) Project 9-30A enhanced rutting model requires time-consuming RLPD tests at three temperatures – low, intermediate and high. This study investigates the application of time–temperature superposition (TTS) to reduce the material characterisation testing effort in the context of the NCHRP 9-30A models. Dynamic modulus and RLPD test data were collected for 27 mixtures, including mixtures from long-term pavement performance (LTPP) programme. The temperature shift functions obtained from dynamic modulus data were used to shift the associated RLPD test results at various temperatures to develop a master curve of cumulative permanent strain versus reduced number of loading cycles. The cumulative permanent strain curves at different temperatures collapsed fairly well into a single RLPD master curve for the evaluated mixtures, confirming the validity of TTS for the RLPD test. The TTS procedure was also applied individually to the test results at a single temperature to predict the permanent strains at the other temperatures. The results from the 27 mixtures suggest that the RLPD master curve obtained by testing only at the high temperature (LTPPBind high temperature at 50% reliability – 5°C) permits accurate and precise predictions of the permanent deformations at all other temperatures. RLPD testing at a single temperature has the real practical benefit of significantly streamlining the asphalt material characterisation testing programme.

Implementation and verification of a mechanistic permanent deformation model (shift model) to predict rut depths of asphalt pavement

The shift model is implemented in the layered viscoelastic asphalt pavement analysis for critical distresses (LVECD) program to predict the rut depth of asphalt pavements. The rut depth measurements taken at the National Center for Asphalt Technology (NCAT) test track and the Federal Highway Administration Accelerated Facility (FHWA ALF) test sections are evaluated using the model. The model can successfully evaluate rut depth, which proves the capability of the model implemented in the LVECD program. The slight over-prediction of the NCAT sections can be explained by ageing in the field that increases the pavement's resistance to rutting. The simulation results support the hypothesis that triaxial stress sweep tests with confinement can represent the permanent deformation behaviour of asphalt concrete in the field. In this regard, excessive shear flow may be the reason for the under-prediction of the FHWA ALF mixtures. For better predictions, a correction factor (i.e. a transfer function) is suggested, which is quantified via the ratio of shear stress to shear resistance. After applying individual transfer functions, the permanent deformation model in the LVECD can evaluate the growth of the rut depth. Therefore, even though the shift model is a uniaxial model, the model can predict the rut depth of asphalt concrete by employing the transfer function.

Performance of cold recycling materials with foamed bitumen and increasing percentage of reclaimed asphalt pavement

The increasing use of bitumen-stabilised materials (BSMs) in the world and especially in Southern Africa necessitates further research into the fundamental properties and behaviour of BSMs. In particular, bitumen stabilisation is increasingly being used to rehabilitate base layers of pavements incorporating thick asphalt layers, resulting in higher percentages of reclaimed asphalt pavement (RAP) in the mixes. There is a significant need for improved understanding of these materials, which is the main objective of the study being undertaken. The extended knowledge gained from this research is intended to improve current mix design and structural design practices. The project's aim is to investigate the feasibility and suitability of producing BSMs that include RAP with relatively high blending ratios of recycled graded crushed stone. In this study, the RAP used in each mix design contains materials with relatively fresh binder. The proportions range from 100:0 to 0:100 of graded crushed stone: RAP. Tri-axial tests at different temperatures are carried out to determine shear properties, resilient modulus and permanent deformation behaviour of the mix. The findings show that the RAP type and percentage influence the rutting resistance, indirect tensile strength (ITS) and moisture susceptibility of the BSMs. A portion of this study is dedicated to the characterisation of the permanent deformation behaviour and to the moisture damage simulation.

Framework for Development of an Improved Unbound Aggregate Base Rutting Model for Mechanistic–Empirical Pavement Design

This paper presents findings from an ongoing research study at the University of Illinois that aims to develop and calibrate improved models for unbound aggregate rutting through laboratory characterization of aggregate materials used for unbound base and subbase applications in the state of North Carolina. Extensive triaxial laboratory testing was performed to establish a robust link between the number of load applications, stress levels, shear stress and sheer strength ratios, and permanent deformation responses. A framework was established for considering the strong correlation that commonly exists between permanent deformation and shear strength characteristics, as opposed to resilient modulus properties, in the laboratory characterization of the permanent deformation behavior of various types of aggregate materials. Trends of permanent strain accumulations from repeated load triaxial tests were adequately captured in a new rutting model whose development took into account the shear stresses applied at 25%, 50%, and 75% of the shear strength properties of these materials under similar field loading confinement conditions. The research shows that this model is an improvement on the rutting damage model for unbound aggregate currently used in AASHTO's mechanistic–empirical pavement design approach because it offers better material characterization and rutting prediction of the unbound base or subbase layer.

An approach to triangular induction heating in final precision forming of thick steel plates

An automatic high-frequency (HF) induction-heating (IH)-based triangular heating process was introduced in order to investigate the influence of designing heating patterns on the permanent deformation behavior of an SS400 thick plate. Temperature distribution and permanent plate deformation during triangular heating were predicted based on electromagnetic–thermal and thermal–structural analyses, respectively. Both analytical and experimental permanent deformation values obtained by zigzag-type triangular heating were significantly higher than those by fan-shaped triangular heating, presumably because of the different temperature gradient along the thickness. Proper design of triangular heating pattern appeared to be the most important factor in determining the final shape of the thick plate. All predicted results were in good agreement with the experimentally observed permanent deformation.