Showing papers by "Imad L. Al-Qadi published in 2008"

Dynamic Analysis and In Situ Validation of Perpetual Pavement Response to Vehicular Loading

Abstract: A three-dimensional (3-D) finite element (FE) model was developed to predict pavement responses to vehicular loading. The model incorporates measured tire-pavement contact stresses, continuous moving wheel loading, and hot-mix asphalt (HMA) viscoelastic characteristics. The model was fine-tuned using implicit-dynamic analysis and validated using pavement response from accelerated loading. Two tire configurations (dualtire assembly and wide-base 455 tire) and three full-depth flexible pavement designs (HMA 152 mm, 254 mm, and 420 mm) were used in both FE modeling and accelerated loading tests. The predicted and calculated strain responses at the bottom of HMA were in agreement. Most important, the study shows that vertical shear strain in the upper 76 to 100 mm of the pavement surface is critical for thick pavement and is influenced by the 3-D tire-pavement contact stresses under each tire rib. However, the tensile strain at the bottom of HMA is affected mainly by the total wheel load. The vertical shear s...

Automatic detection of multiple pavement layers from GPR data

Abstract: One of the problems encountered in the nondestructive testing of pavements with ground penetrating radar (GPR) is the detection of multiple-layer reflections within the GPR return. Detecting reflections is especially problematic when the pavement layers are thin with respect to the probing pulse width, in which case overlapping between the reflected pulses occurs, causing the weak reflections to be masked by the stronger reflections in their vicinity. In this study, the problem is solved by iteratively detecting the strong reflections present within the GPR signal using either a threshold or a matched filter detector. The detected pulses are then used in a reflection model to synthesize a signal “similar” to the measured GPR signal in the least-squares sense. The synthesized signal is then subtracted from the measured signal to reveal the masked weak reflections, which are later detected iteratively using the same method. This technique was successfully applied to field GPR data collected from an experimental pavement site: the Virginia Smart Road.

Geogrid in flexible pavements validated mechanism

Abstract: Full-scale accelerated testing was used to provide new insight into quantifying the effectiveness of geogrids on low-volume flexible pavement performance Although several previous studies report that geogrids improve pavement performance by enhancing structural capacity and reducing distress potential, the new study addresses how to maximize the benefits and cost-effectiveness of geogrid To perform full-scale testing, three cells of flexible pavements, each having three pavement sections, were constructed The granular base and hot-mix asphalt (HMA) layer thicknesses varied, and each cell had at least one control and one geogrid-reinforced pavement section Instruments were embedded during construction to measure stress, strain, deflection, moisture, pore-water pressure, and temperature and were used to monitor pavement response to moving load A moving dual-tire at 8 km/h and 44 kN was used to apply accelerated traffic loading The performance of the various pavement sections when exposed to accelerated loading is presented On the basis of pavement measured response as well as visual observation of the pavement cross section after excavation, the study showed that geogrid is very effective in reducing the horizontal shear deformation of the aggregate layer, especially in the traffic direction Hence, the effectiveness of geogrid is clear for aggregate base layers with thicknesses ranging from 203 to 457 mm, and geogrid is expected to show similar effectiveness for greater base thickness given that thin HMA layer is used The study also found that the optimal geogrid location in a thin aggregate layer is at the unbound aggregate-subgrade interface For a thicker base layer, it is optimal to install a single geogrid at the upper third of the layer; the addition of another geogrid at the subgrade-base layer interface may be needed for stability

Validity of Asphalt Binder Film Thickness Concept in Hot-Mix Asphalt:

Abstract: Despite the possible benefits of implementing asphalt binder film thickness into current specifications to address durability problems, most of the related research has been theoretical and only a few attempts have been made to measure this property experimentally. The objective of this study was to investigate the concept of asphalt binder film thickness experimentally on the basis of measurements obtained by image analysis techniques, reflective light microscopy, and scanning electron microscopy. The results of the experimental program were used to gain insight into the concept of asphalt binder film thickness and its validity. Experimental results indicated that asphalt binder films coating large aggregates do not actually exist in hot-mix asphalt. Instead, what are referred to as asphalt binder films surrounding large aggregates are actually asphalt mastic films. These films are highly irregular in shape and have a thickness greater than 100 μm in the mixture considered in this study. The asphalt bind...

Interface bonding between hot-mix asphalt and various portland cement concrete surfaces laboratory assessment

Abstract: Interface bonding between hot-mix asphalt (HMA) overlays and portland cement concrete (PCC) pavements can be one of the most significant factors affecting overlay service life. In this study, a direct shear test device was built and used to investigate the characteristics of the HMA-PCC interface and to determine the interface shear strength. Several parameters affecting the interface performance were included in this study, such as HMA type, tack coat type, tack coat application rate, PCC surface texture, and temperature. The test results showed that the use of SS-1hP emulsion resulted in a greater interface bonding strength than the use of RC-70 cutback. Surface mix SM-9.5 was found to have a better interface strength than binder mix IM-19.0. Among the four residual application rates considered in this study, 0.05 gal/yd2 was found to be the optimum rate, independent of test parameters. The direction of tining in the PCC surface was found to have no effect on interface shear strength at 20°C. At the opt...

Scattering analysis of ground-penetrating radar data to quantify railroad ballast contamination

Abstract: This paper will evaluate ground-penetrating radar (GPR) as a non-destructive method to rapidly, effectively, and continually assess the conditions of railroad ballast. Compared to uniformly graded, clean ballast, fouled ballast has a finer, well-graded particle size with fewer air voids. Ballast under different conditions generates various GPR electromagnetic scattering patterns. A field GPR survey with multiple sets of 1 and 2 GHz air-horn antennae was conducted in summer 2005 at the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado. The 2 GHz antenna was found to be more sensitive to the change in scattering pattern. Appropriate data processing was used to remove the effects of the rails to obtain clear GPR images of the subsurface layers. From the image analysis, ballast thickness, ballast fouling condition, and trapped water can be assessed.

Frequency determination from vehicular loading time pulse to predict appropriate complex modulus in MEPDG

Abstract: The complex modulus is used by the Mechanistic-Empirical Pavement Design Guide (MEPDG) for hot-mix asphalt (HMA) time and temperature dependency simulation. The correctness of the design process essentially depends on the accuracy of the conversion from the time domain to the frequency domain since the complex modulus is measured in the frequency domain while vehicular loading is applied in the time domain. That the frequency is calculated as the inverse of the loading time is assumed by the current MEPDG approach. That the frequency can be determined from the angular frequency is suggested by another approach. These two techniques are analyzed in the paper, and it is demonstrated that they only represent approximations for specific cases not encountered in pavement systems. There is presentation of a novel approach for accurate determination of vehicular loading frequency based on a detailed Fourier analysis. A direct and reliable technique to obtain moving load frequency spectrum based on the time pulse data is provided by the proposed Fourier approach. Data from the Virginia Smart Road were used to evaluate and validate this approach. There was analysis of the field loading frequency spectra at different depths and under various vehicle speeds. Analysis results indicated that the MEPDG approach is associated with a 40 to 140% error range in frequency estimation depending on depth of calculation and the vehicle speed. There is estimation of the natural frequency of an HMA layer and investigation of the delayed-response association with HMA viscous properties during the unloading phase based on this method.

Accuracy of Current Complex Modulus Selection Procedure from Vehicular Load Pulse: NCHRP Project 1-37A Mechanistic-Empirical Pavement Design Guide

Abstract: The Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the complex modulus to simulate the time and temperature dependency of hot-mix asphalt (HMA). To account for the time dependency of HMA, MEPDG recommends calculation of the frequency of the applied load as a function of the vehicle speed and the pavement structure. By this approach, the Odemark method of thickness equivalency is first used to transform the pavement structure into a single-layer system, and it is then assumed that the stress distribution occurs at a constant slope of 45° in the equivalent pavement structure. Concerns were raised that the current MEPDG methodology may be overestimating the frequency, which would result in underconservative distress predictions. Therefore, to evaluate the MEPDG methodology for calculation of the loading time, the results of the MEPDG procedure were compared with those of an advanced three-dimensional (3-D) finite element (FE) approach that simulates the approaching-leaving rolling wheel at a specif...

The Truth and Myth of Fatigue Cracking Potential in Hot-Mix Asphalt: Numerical Analysis and Validation (With Discussion)

Abstract: The study aim is to use implicit-dynamic analyses and three dimensional finite element (FE) models for dynamic flexible pavement responses to various tire loadings quantification. This research goes beyond presenting conclusions to satisfy this objective-it raises valid questions and presents theories about "bottom-up" fatigue cracking and its initiations. Study considerations included three wheel loads (35.5 kN, 45.5 kN, and 53.5 kN), a tire pressure of 720 kPa, a wide-base tire (455/55R22.5), a dual-tire assembly (75/80R22.5), and two tire configurations. The study used pavement designs including three designs with hot-mix asphalt (HMA) thicknesses of 76, 152, and 305 mm and a typical interstate highway. There was simulation of two vehicle speeds (eight and 105 km/hr). Field-measured responses at the Virginia Smart Road were compared to calculated pavement responses. Study conclusions included that critical pavement response prediction occurs at low vehicle speed (8 km/hr) at high (40 degrees C) or intermediate (25 degrees C) temperatures, given that vehicle dynamics are not changing and pavement surface is smooth. Responses included vertical shear strain in HMA and tensile strains at the bottom of the HMA. For tensile strains at the bottom of HMA (38 mm from the surface) and vertical shear strains near the HMA surface under high speed (105 km/hr) at low HMA temperature (5 degrees C) resulted in greater response magnitudes than at low speed (8 km/hr). The transient moving loading high loading frequency (high speed), may exert, at low temperature (5 degrees C) higher inertia forces and acceleration to the pavement system responses than those at the low speed. Research shows that what is referred to as "bottom-up" fatigue cracking may be the result of vertical shear strains near the surface, while top-down cracking could be the result of surface tensile strains near the tire-edge rib. The study questions "bottom-up" fatigue cracking validity in relatively thick pavements as a result, and suggests that there may be initiation of these cracks at 50-100 mm below the HMA surface. This study's most important implication is that a fresh investigation of where these cracks are actually initiated is encouraged, and suggests that a primary pavement cracking distress cause may be "near-surface" cracking.

Fracture-Based Friction Model for Pavement Interface Characterization

Abstract: This paper presents a friction model that characterizes pavement layer interfaces. This model has been built on the basis of three major parameters, shear strength, interface reaction modulus, and friction, to define the behavior of tack coat interfaces. The most important property of this model is its ability to capture the entire range of interface responses, from the fully bonded (initial elastic) interface response to the response in the fully debonded state. A fracture-based elastoplastic constitutive relationship has been implemented for a frictional interface model. Debonding, which can occur in various modes (pure tensile, pure shear, shear with tension, and shear with compression), was formulated by use of a nonlinear softening model integrated into an elastoplastic constitutive model. The pressure dependency of the interface shear strength and dilation because of the surface irregularities are two of the frictional properties examined by the model. The paper includes several numerical examples t...

The truth and myth of fatigue cracking potential in hot-mix asphalt: Numerical analysis and validation

Abstract: The study aim is to use implicit-dynamic analyses and three dimensional finite element (FE) models for dynamic flexible pavement responses to various tire loadings quantification. This research goes beyond presenting conclusions to satisfy this objective-it raises valid questions and presents theories about "bottom-up" fatigue cracking and its initiations. Study considerations included three wheel loads (35.5 kN, 45.5 kN, and 53.5 kN), a tire pressure of 720 kPa, a wide-base tire (455/55R22.5), a dual-tire assembly (75/80R22.5), and two tire configurations. The study used pavement designs including three designs with hot-mix asphalt (HMA) thicknesses of 76, 152, and 305 mm and a typical interstate highway. There was simulation of two vehicle speeds (eight and 105 km/hr). Field-measured responses at the Virginia Smart Road were compared to calculated pavement responses. Study conclusions included that critical pavement response prediction occurs at low vehicle speed (8 km/hr) at high (40 degrees C) or intermediate (25 degrees C) temperatures, given that vehicle dynamics are not changing and pavement surface is smooth. Responses included vertical shear strain in HMA and tensile strains at the bottom of the HMA. For tensile strains at the bottom of HMA (38 mm from the surface) and vertical shear strains near the HMA surface under high speed (105 km/hr) at low HMA temperature (5 degrees C) resulted in greater response magnitudes than at low speed (8 km/hr). The transient moving loading high loading frequency (high speed), may exert, at low temperature (5 degrees C) higher inertia forces and acceleration to the pavement system responses than those at the low speed. Research shows that what is referred to as "bottom-up" fatigue cracking may be the result of vertical shear strains near the surface, while top-down cracking could be the result of surface tensile strains near the tire-edge rib. The study questions "bottom-up" fatigue cracking validity in relatively thick pavements as a result, and suggests that there may be initiation of these cracks at 50-100 mm below the HMA surface. This study's most important implication is that a fresh investigation of where these cracks are actually initiated is encouraged, and suggests that a primary pavement cracking distress cause may be "near-surface" cracking.

Tack Coat Optimization for HMA Overlays: Laboratory Testing

Abstract: Interface bonding between hot-mix asphalt (HMA) overlays and Portland cement concrete (PCC) pavements can be one of the most significant factors affecting overlay service life. Various factors may affect the bonding condition at the interface, including HMA material, tack coat material, tack coat application rate, PCC surface texture, temperature, and moisture conditions. The objective of this study is to quantify the impact of these parameters on the permanent deformation of the HMA overlay. This study includes three major components to achieve the objective: laboratory testing, numerical modeling, and accelerated pavement testing. This report presents and analyzes the laboratory testing results. A direct shear test device was built and utilized to investigate the characteristics of the HMA-PCC interface and to determine the interface shear strength in the lab. Tests were run in monotonic mode at a constant loading rate of 0.47 in/min (12 mm/min). Test specimens were prepared using field PCC cores, laboratory prepared HMA, and tack coat materials provided by the supplier. Parameters affecting the interface performance that were evaluated include HMA material type (SM-9.5 surface mix and IM-19.5A binder mix), tack coat type (SS-1h and SS-1hP emulsions, and RC-70 cutback), tack coat application rate, PCC surface texture, temperature, and moisture conditions. Test results showed that the asphalt emulsions SS-1h and SS-1hP produced greater interface bonding strength than the cutback asphalt RC-70. The SM-9.5 surface mix was found to have better interface strength than the IM-19.0A binder mix. The HMA tested produced the same trend of interface shear strength with tack coat application rate for various tack coat types. The optimum residual tack coat application rate for the SS-1hP emulsion using IM-19.0A binder mix was 0.04 gal/yd2 (0.18 L/m2) in the lab. The direction of tining on the PCC produced no effect on interface shear strength at 20 oC. However, the milled concrete surface provided greater interface shear strength than both tined and smooth PCC surfaces for the same tack coat application rate. At the optimum tack coat application rate, the smooth PCC surface produced higher interface shear strength than the tined surface. As temperature increased, interface bonding strength decreased. Moisture conditioning significantly decreased the interface shear strength. This reduction was more pronounced when a stripping-vulnerable binder mix IM-19.0B was used.

Time-Frequency Approach for Ground Penetrating Radar Data Analysis to Assess Railroad Ballast Condition

Abstract: Railroad ballast plays an important role in supporting heavy rail loading, preventing the deformation of track, and providing drainage of water from the track structure. However, over time, ballast is fouled by the breakdown of ballast aggregate and/or the infiltration of fines, which undermine ballast functions. This may result in damage to the rail system, such as track settlement. Ground penetrating radar (GPR), a nondestructive method, can be used to rapidly, effectively, and continuously assess railroad track substructure conditions. Ballast under various fouling conditions generates various electromagnetic (EM) scattering patterns. In this study, air-coupled 2 GHz antenna was found to be sensitive to the scattering pattern change. Appropriate data processing was used to remove the effects of ties and rails to obtain clear GPR images of the subsurface layers. Then, the amplitude envelope and time-frequency approaches were implemented to characterize the signal in time and frequency domains simultaneo...

Pavement Cracking : Mechanisms, Modeling, Detection, Testing and Case Histories

Abstract: Internationally, much attention is given to causes, prevention, and rehabilitation of cracking in concrete, flexible, and composite pavements. The Sixth RILEMInternational Conference on Cracking in Pavements (Chicago, June 16-18, 2008) provided a forum for discussion of recent developments and research results.This book is a collection of papers fr

Finite Element Modeling of Reflective Cracking under Moving Vehicular Loading: Investigation of the Mechanism of Reflective Cracking in Hot-Mix Asphalt Overlays Reinforced with Interlayer Systems

Abstract: The fracture mechanism of reflective cracking in hot-mix asphalt (HMA) overlay was investigated using a three-dimensional finite element model utilizing the cohesive zone model. The effectiveness of interlayer systems to abate reflective cracking in overlays was evaluated. A fractured area due to potential reflective cracking was calculated at a region in which cohesive elements were inserted. A represent fracture area (RFA) is introduced as a weighted average value of degradation to calculate the fractured area. Utilizing global and local RFAs, the mechanism of reflective cracking is investigated for HMA overlay with interlayer systems. For various interlayer systems, this study found that different reflective cracking patterns occurred in terms of quantity and distribution. The sand mix interlayer reduces reflective cracking in leveling binder at the beginning; however, steel netting interlayer retards reflective cracking in wearing surface as well as leveling binder.

Effect of Bituminous Material Rheology on Adhesion

Abstract: Bituminous materials are used in many civil engineering applications in which adhesion to a substrate is essential for good performance. Yet it is not possible to predict the adhesion of these materials. The particular case of bituminous crack sealants is of interest; the effect of sealant viscosity, aging, test temperature, and loading rates was investigated by means of a blister test. This test provided the bonding characteristics to a model aggregate in relation to interfacial fracture energy (IFE). From testing of several sealants, it was found that pouring viscosity affects adhesion and that higher viscosities help to attain higher IFEs. Temperature was found to play a key role on bonding characteristics and failure mechanism because it affected the viscoelastic properties of the sealant. The glass transition temperature (Tg) was found to have a governing role on bonding characteristics. At temperatures above Tg, bond strength was found to be affected by sealant flow such that failure was flow relate...

Accelerated aging of bituminous sealants: small kettle aging

Abstract: Bituminous sealants used in the maintenance of roadways are installed hot and heated to 150–200°C during installation. High temperatures can degrade polymers in sealants, but there is no standard method to account for this possible degradation. In an attempt to find such a method, the aging of two sealants in large kettles during field applications was compared to that obtained in the laboratory by heating in a small kettle. The results indicate that 4 h of small kettle aging at the highest suggested sealant application temperature (HiSAT), or about 2 h at HiSAT + 10°C, provided as much copolymer aging as that found in sealants sampled midway through installation.

Characterization of Low Temperature Mechanical Properties of Crack Sealants Utilizing Direct Tension Test

Abstract: Crack sealing has been widely used as a routine preventative maintenance practice. Given its proper installation, crack sealants can extend pavement service life by three to five years. However, current specifications for the selection of crack sealants correlate poorly with field performance. The purpose of this research was to develop performance guidelines for the selection of hot-poured bituminous crack sealants at low temperature. This was accomplished by measuring the mechanical properties of crack sealant at low temperature and then developing performance criteria for material selection. The modified direct tension test (DTT), crack sealant direct tension test (CSDTT), simulates the in-situ loading behavior of crack sealants in the laboratory. A modified dog-bone specimen geometry, which allows specimens to be stretched up to 95%, is recommended. This new specimen geometry also facilitates sample preparation. Tensile force is applied to the dog-bone specimen, with its effective gauge length of 20.3mm, and is pulled at a deformation rate of 1.2mm/min. Fifteen sealants were tested at various temperatures, and three performance parameters are suggested as indicators of sealant performance: extendibility, percent modulus reduction, and strain energy density. Extendibility, which is used to assess the degree of deformation undergone by a sealant at low temperature before it ruptures or internal damage is observed, is recommended as a measured parameter to be included in the performance-based guidelines for the selection of hot-poured crack sealants. Extendibility thresholds were defined as function of low service temperatures. The CSDT is conducted at +6oC above the lowest in service temperature because of the relatively high test loading rate compared to in-situ crack sealant movement rate.

In Situ Assessment of Interlayer Systems to Abate Reflective Cracking in Hot-Mix Asphalt Overlays

Abstract: A method to investigate the effectiveness of interlay systems in hot-mix asphalt (HMA) overlays to retard reflective cracking is introduced. Significant research has been conducted on developing techniques to reduce reflective cracking in HMA overlays. Many interlayer systems have been used in the field; some showed no structural or economical benefits, while others could reduce reflective cracking in the short term. However, longterm and quantitative benefits for many of these techniques have not been realized. Hence, this research fulfills the need to quantify the effectiveness of interlayer systems in delaying or abating reflective cracking—it proposes a systematic approach to identify reflective cracking and a conceptual index to evaluate interlayer systems. Ground-penetrating radar and a video integration system are used to detect surface cracks in overlays as well as joint opening and dowel bars in existing portland cement concrete pavements. Transverse cracks that exist over a joint are classified ...

Interfacial Fracture Energy: An Indicator of Bituminous Material Adhesion

Abstract: That an effective binder-aggregate bonding prediction method is the pressurized blister test is demonstrated in this paper. The blister test recently has been introduced as a reliable approach to bituminous sealant and aggregate bonding prediction. There can be test application on any bituminous material, from the most brittle binder to the softest bituminous crack sealant, since this test measures a geometry-independent parameter that is an inherent property interface. Cohesive failures becomes a concern with very brittle material. An increase in thickness of the adhesive specimen can easily prevent such a failure. A specimen's thickness increase, however, also gives rise to shear forces that this analysis cannot neglect. This paper presents shear force effect on the interfacial fracture energy (IFE) of adhesive bituminous materials by utilizing theoretical and experimental analyses. Shear force effect on blister deflection is shown as a material thickness function. Additionally, bituminous material IFE dependence on temperature and loading rate was investigated through laboratory testing. For each material where IFE is optimized, an optimum temperature and loading rate can be identified. For improved performance under defined environmental conditions, this may help select appropriate binder/sealant.

Adhesion Testing Procedure for Hot-Poured Crack Sealants

Abstract: Crack sealing is a common pavement maintenance treatment because it extends pavement service life significantly. However, crack sealant often fails prematurely due to a loss of adhesion. Because current test methods are mostly empirical and only provide a qualitative measure of bond strength, they cannot predict sealant adhesive failure accurately. Hence, there is an urgent need for test methods based on bituminous sealant rheology that can better predict sealant field performance. This study introduces three laboratory tests aimed to assess the bond property of hot-poured crack sealant to pavement crack walls. The three tests are designed to serve the respective needs of producers, engineers, and researchers. The first test implements the principle of surface energy to measure the thermodynamic work of adhesion, which is the energy spent in separating the two materials at the interface. The work of adhesion is reported as a measure of material compatibility at an interface. The second test is a direct adhesion test, a mechanical test which is designed to closely resemble both the installation process and the crack expansion due to thermal loading. This test uses the Direct Tensile Test (DTT) machine. The principle of the test is to apply a tensile force to detach the sealant from its aggregate counterpart. The maximum load, Pmax, and the energy to separation, E, are calculated and reported to indicate interface bonding. The third test implements the principles of fracture mechanics in a pressurized circular blister test. The apparatus is specifically designed to conduct the test for bituminous crack sealant, asphalt binder, or other bitumen-based materials. In this test, a fluid is injected at a constant rate at the interface between the substrate (aggregate or a standard material) and the adhesive (crack sealant) to create a blister. The fluid pressure and blister height are measured as functions of time; the data is used to calculate Interfacial Fracture Energy (IFE), which is a fundamental property that can be used to predict adhesion. The stable interface debonding process makes this test attractive. This test may also provide a means to quantify other factors, such as the moisture susceptibility of a bond. In addition, the elastic modulus of the sealant and its residual stresses can be determined analytically. While the direct adhesion test is proposed as part of a newly developed performance-based guideline for the selection of hot-poured crack sealant, the blister test can be used to estimate the optimum annealing time and installation temperature.

Airfield and Highway Pavements : Efficient Pavements Supporting Transportation's Future

Abstract: Proceedings of the 2008 Airfield and Highway Pavements Conference, held in Bellevue, Washington, October 15-18, 2008. Sponsored by the Transportation and Development Institute of ASCE. This collection contains 54 peer-reviewed papers exploring the newest advances and challenges in pavement design and construction. Today’s pavement engineers are responsible for designing and building durable and cost-effective, yet environmentally friendly pavement systems. Topics include: pavement modeling; characterization of pavement materials; pavement management, evaluation, and rehabilitation; performance of stabilized and unbound pavement layers; pavement design and analysis; pavement performance studies. Papers are contributed by both practitioners and researchers.

Deformation and tracking of bituminous sealants in summer temperatures: pseudo-field behaviour

Abstract: The bituminous sealants used in the maintenance of roadways sometimes fail because of the excessive deformation or flow under the shearing action of tires in summer temperatures. This issue has yet to be studied and no data on sealant deformation and flow are available. To address this issue, a small-scale tracking test was developed and applied to 21 sealants held at temperatures between 46 and 82°C, typical pavement temperature maximums in North America. It was found that sealant shear deformation increased linearly or non-linearly with temperature, and that past a temperature threshold, sealants deformed excessively and tracked. The tracking failure temperature was readily identified. Some sealants showed a tracking failure at fairly low temperatures, 46–64°C, but most only failed at 76°C or beyond. The sealant flow characteristics, acquired under pseudo-field conditions, provide the basis for developing a performance-based method to assist in the selection of sealants with resistance to tracking.

Characterization of low temperature creep properties of crack sealants using bending beam rehometry

Abstract: Crack sealing has been widely used as a routine preventative maintenance practice. Given its proper installation, crack sealants can extend pavement service life by three to five years. However, current specifications for the selection of crack sealants correlate poorly with field performance. The purpose of this research was to develop performance guidelines for the selection of hot-poured bituminous crack sealants at low temperature. In this part of the research, the creep behavior of crack sealant at low temperature is measured and performance criteria for material selection were developed. Because various pavement and State agencies are well acquainted with and own the Bending Beam Rheometer (BBR), which was developed during the Strategic Highway Research Program (SHRP), an attempt was made to utilize the same setup to test hot-poured bituminous-based crack sealants. Testing conducted in this research project indicated that the standard BBR was inappropriate for testing soft bituminous-based hot-poured crack sealant, even at -40°C. The measured deflection exceeded the BBR limit, for some sealants, after only a few seconds of loading. To address this issue, the moment of inertia of the tested beam was increased by doubling its thickness (from 6.35mm to 12.7mm). For the new beam dimensions, it was found that only 4% of the beam center deflection is due to shear, a value deemed acceptable for sealant evaluation and comparison. In an effort towards developing a robust testing procedure, 15 sealants from various manufacturers were included in the study and tested between –4°C and –40°C. In addition, five sealants, which have known field performance, were tested to validate the laboratory results and establish specification thresholds for the selection guidelines. Since stiffness calculation in the BBR test method requires that measurements be made within the linear region of viscoelastic behavior, validation of this theory was conducted for crack sealants. This was found to be generally the case with crack sealants, which allowed for the use of the time-temperature superposition. If the temperature-superposition principle is applied, the stiffness at 240s at a given temperature can be used to predict the stiffness after 5hr of loading at a temperature that is 6°C lower. With the assumption of linear viscoelastic behavior, sealants performance can be characterized through stiffness, average creep rate, and dissipated energy ratio. Stiffness was found to be sensitive to temperature changes and could be used to differentiate between sealants. The measurements of the average creep rate and the dissipated energy ratio were also found to be valuable in differentiating between sealants. In addition, numerical modeling was used to simulate the mechanical response of crack sealants at low temperatures. Parameters that may be used for evaluating crack sealant cohesive performance using the crack sealant BBR (CSBBR) are the stiffness at 240s, average creep rate, and the dissipated energy ratio. For simplicity, the first two parameters, stiffness at 240s and average creep rate, are recommended for implementation in the sealant performance grade. The recommended thresholds are maximum stiffness of 25MPa and minimum average creep rate of 0.31.

A fracture-based constitutive model for pavement interface characterization

Abstract: This paper presents a new fracture-based constitutive model to characterize pave- ment interfaces. The constitutive model captures the entire range of interface responses from fully bonded (initial elastic) to fully debonded. Failure (debonding) mechanisms can occur in the fol- lowing states which the model also captures: pure shear, pure tensile, shear with compression, and shear with tension. Additionally, the developed model shows promise for characterizing cracking problems. In this model, an interface constitutive relationship has been implemented with a frictional Mohr-Coulomb plasticity model. The constitutive model and interface elements were implemented by means of the user element (UEL) of ABAQUS finite element (FE) software. An algorithmi- cally consistent tangent operator for the interface elasto-plastic modulus was used to facilitate convergence. Non-associative plasticity was implemented to have a better control of volumetric expansion (dilation) at the interface element. Debonding was formulated by a nonlinear softening model integrated into an elasto-plastic constitutive model. The model addresses frictional proper- ties, including pressure dependency of the interface shear strength and dilation due to roughness of interfaces. This paper provides numerical examples and discusses the experiments conducted for characteri- zation of hot-mix asphalt (HMA) overlay and Portland cement concrete (PCC) interfaces, including tests on direct shear at various temperatures, tack coat materials, tack coat application rates, and concrete surface texture. As part of the experiments, three-dimensional interface elements were inserted on a predefined failure plane. Load displacement responses obtained from direct shear tests were in agreement with the finite element simulations. The developed model with its inherent frictional nature is ideal for use with interface problems especially under various loading conditions (pure tensile, pure shear, shear with compression, and shear with tension). The model can also be further advanced to be used in the finite element analysis of pavement cracking problems under mixed-mode loading.

Full-depth Flexible Pavement Fatigue Response under Various Tire and Axle Load Configurations

Abstract: The goal of this study was to evaluate the effect of vehicle parameters, mainly tire configuration and axle load, on full-depth flexible pavement response using accelerated pavement testing. The Advanced Transportation Loading ASsembly (ATLAS), housed at the University of Illinois at Urbana-Champaign, was used in this study. The tests were performed on 3 full-depth flexible pavement sections of 152, 254, and 429mm HMA placed on 300mm of lime-stabilized subgrade. The test program included 3 tire types (dual-tire assembly, wide-base 425 tire, and wide-base 455 tire); 5 loading levels (26, 35, 44, 53, and 62kN); 3 tire inflation pressures (550, 690, and 760kPa); and 2 speeds (8 and 16km/h). The measured longitudinal strain at the bottom of the HMA was compared under various loading conditions. The study concluded that the wide-base 425 tire, originally introduced for pavement testing only, exhibited the highest fatigue potential caused by high tensile strain at the bottom of HMA; the dual-tire assembly exhibited the lowest tensile strain at the bottom of HMA. However, the difference in strain responses due to wide-base tires and dual-tire assembly diminishes as HMA thickness increases. The effect of load on fatigue life is expressed as an exponential function. The load damage exponents were found to be in the range of 1.77-3.29 for full-depth flexible pavement. The parameter was found to depend on pavement thickness and tire configuration. However, the authors believe that near-surface cracking caused by shear is more critical than tensile strain at the bottom of HMA for perpetual pavements.

Tack Coat Type and Application Rate Optimization to Enhance HMA Overlay-PCC Interface Bonding: Laboratory and APT Testing

Abstract: Interface bonding between hot-mix asphalt (HMA) overlays and Portland Cement Concrete (PCC) pavements is one of the most significant factors affecting overlay service life. The quality of tack coat and its application rate have long been postulated as primary factors in overlay performance. However, a laboratory test or field validated study did not exist to quantify the effects of these factors. Therefore, the aims of this study were to conduct laboratory testing to optimize the tack coat application rate and validate the laboratory results using accelerated pavement testing. Laboratory direct shear tests were performed on specimens prepared with field extracted PCC cores and laboratory compacted HMA. The laboratory investigation was validated by conducting accelerated full-scale testing at the University of Illinois Advanced Transportation Research and Engineering Lab. Two types of tack coats were applied to the test sections: SS-1hP and RC-70; each at 3 residual rates (0.09, 0.18, and 0.41 L/m2). The HMA overlay thickness was 57 mm. In total, 24 various sections were included in the full-scale testing. Test sections were loaded by the Accelerated Transportation Loading Assembly at the centerline. The tensile strains at the bottom of HMA, to quantify potential interface slippage, were measured for selective sections. In addition, primary HMA rutting was analyzed. The study concluded that SS-1hP provided better interface shear strength than RC-70. Additionally, the optimum residual tack coat application rate was determined to be 0.18 L/m2 based on the field validated laboratory testing results.