Fiber reinforcement is one of the methods used to improve cracking resistance of quasi-brittle materials such as asphalt concrete (AC). Despite the widespread use of fibers in AC, there is a lack of fundamental understanding of fracture behavior of fiber reinforced asphalt concrete (FRAC). The main goal of this research is comprehensive fracture characterization of low dosage applications of two synthetic fibers (polyethylene-aramid and nylon) and understand the performance bias of low dosage fiber reinforcement at mixture level in different test conditions. Two laboratory prepared mixtures (fine and coarse-graded) were designed with consistent volumetrics. Two crack mouth opening displacement (CMOD) – controlled experiments (SCB-CMOD and DCT) were performed to facilitate a controlled crack propagation at low and intermediate temperatures. Alternative load-line displacement (LLD) control cracking tests (I-FIT and IDEAL-CT) with different geometries, testing parameters, and loading modes were also evaluated. It was observed that the mixes with nylon fibers had a distinctive and better fracture response in all of the experiments. Some improvements were observed with aramid fiber configuration dependent on the test performed and mixture type. The key test features that resulted in more distinction between the low dosage fiber mixes and the control appeared to be ligament length, affected damage area, and cracking patterns observed in the experiments. The tests with larger ligament length (i.e., larger affected damage zones) were sometimes visible with more tortuous cracking patterns with many branches resulted in more consistent and noticeable outcomes with the fibers. Further research is needed to identify potential test bias and develop an ideal test protocol and test geometry that can objectively evaluate the performance of low dosage fibers.

Cracking is one of the most common distresses in asphalt pavement. Because asphalt concrete is relatively weak in tension, synthetic fibers have shown to increase its tensile strength and, therefore, reduce the chance of cracking. An approach was used in this study with the aim of evaluating the interaction between fibers and asphalt mastic and the fiber distribution in asphalt concrete. Three types of aramid fibers and two types of nylon fibers were used. A pullout test was used to determine the typical shear bond strength between fibers and the asphalt mastic. The bond strength obtained from the pullout test was then used to calculate the minimum fiber embedded length on each side of the crack in order for the fiber to reach its full capacity before being pulled out. Of course, increasing fiber length increases the chance of bridging cracks considering the random distribution of fibers in fiber-reinforced asphalt concrete (FRAC) and the random orientation of fibers relative to cracks. On the other hand, increasing fiber length may result in uneven distribution of fibers in the FRAC. Fiber extraction and recovery tests were then used to determine the dispersion of aramid fibers in the FRAC with different fiber lengths. The study showed that aramid fibers in the order of 20 mm would provide a good bond with the asphalt mastic and result in reasonable dispersion in the FRAC. The uniaxial fatigue test and flow number test were also performed on FRAC with different aramid fiber lengths. The 19-mm fibers also showed better performance test results than the 10- and 38-mm fibers. A similar length is recommended for the Nylon 1 fibers based on the bond properties only. Longer Nylon 2 fibers are recommended, but caution needs to be taken to avoid uneven fiber dispersion in the FRAC.

Fiber-reinforced asphalt concrete (FRAC) was tested using limestone, PG 64-22 binder, and 20% reclaimed asphalt pavement (RAP). After mixing fibers with different lengths and dosages, they were extracted and recovered to evaluate their dispersion in the FRAC. The uniaxial fatigue test, IDEAL CT test, and flow number test were performed on FRAC with different fiber lengths and asphalt contents. The balanced mix design (BMD) approach was then used to analyze the uniaxial and flow number test results in order to evaluate the effect of aramid fibers on fatigue and rutting resistance of the pavement. The dispersion test showed that the 19 mm and 10 mm aramid fibers at a dosage rate of 0.5 g/kg provided the best dispersion. The 19 mm fibers showed better performance test results than the 10 mm and 38 mm fibers. The BMD approach provided ranges of asphalt contents to produce mixes with certain resistances to fatigue and rutting. The BMD approach also demonstrated the effect of fibers with different lengths on increasing the resistance to fatigue and rutting. The study concluded that the 19 mm fibers with a dosage of 0.5 g/kg produce best results. The BMD approach is a good tool that can be used to refine the mix ingredients, including additives such as fibers, in order to optimize pavement resistance to various distresses such as fatigue cracking and rutting.

Pavement distresses directly affect ride quality, and indirectly contribute to driver distraction, vehicle operation, and accidents. In this study, analysis was performed on highways in the states of Arizona, North Carolina, and Maryland to investigate the relationship between accident rate and pavement ride quality (roughness) and rut depth. Two main types of data were collected: crash data from the accident records and International Roughness Index (IRI) and rut depth data from the pavement management system database in each state. Crash rates were calculated using the U.S. Department of Transportation method, which is the number of accidents per 100 million vehicle-miles of travel. Sigmoidal function regression analysis was performed to study the relationship between crash rate and both IRI and rut depth. In all cases, the crash rate did not show substantial increases until an IRI value of 210 inches/mile or a critical rut depth of 0.4 inches. When the IRI or rut depth increased above these values the crash rate increased. This is a key conclusion that provides empirically derived thresholds for IRI and rut depth to reducing the accident rate.

The mechanical dynamic modulus test was used to determine the dynamic moduli of hot-mix asphalt (HMA) with and without different types of fibers, and master curves were developed. The ultrasonic pulse velocity (UPV) test was then conducted on the same specimens to estimate the dynamic moduli across multiple temperatures, and master curves were developed. The Poisson's ratios required for UPV modulus prediction were estimated for different reduced frequencies, and a sensitivity analysis on their effect on the modulus values was performed. The master curves developed using the UPV moduli were superimposed on the mechanical test master curves and matched closely. Witczak's predictive equation and the Hirsch model were also used to predict the dynamic modulus master curves. The UPV moduli closely matched the mechanical dynamic moduli better than those predicted from the Witczak or Hirsch models at intermediate and high reduced frequencies. An attempt was made to combine the UPV modulus prediction with the Witczak and Hirsch models to improve moduli estimation at low reduced frequencies. Typical mechanical test methods are time consuming and require an expensive load frame and instrumentation setup. The UPV test requires less time, and the equipment needed is relatively inexpensive. Using the UPV test in combination with either the Witczak or Hirsch model is recommended as a lower-cost alternative to develop the master curves of the HMA dynamic moduli. (C) 2018 American Society of Civil Engineers.

Although roundabouts have been used at many locations around the world, the safety of roundabouts under different conditions has not been fully understood. In this study, 17 roundabouts in five cities in Arizona were evaluated, out of which 11 are single lane and six are double lane. Most of the intersections of single-lane roundabouts were controlled by two-way stop signs before roundabout conversion, whereas most of the intersections of double-lane roundabouts were controlled by traffic signals. Accident data were collected and broken down into five categories: damage without injury, minor injury, nonincapacitating injury, incapacitating injury, and fatality. Equal number of years were used before and after the roundabout conversion at each location. The average rates of accidents, damages without injury, injuries and fatalities per year and per million vehicles were evaluated. It was found that single-lane roundabouts reduced the accident rate, whereas double-lane roundabouts increased the accident rate. The results also showed that single- and double-lane roundabout conversions reduced the severity levels of accidents. Considering accident rate and severity level, warrants need to be developed for roundabout conversion and number of roundabout lanes under different traffic conditions.

Load associated fatigue cracking is one of the major distress types occurring in flexible pavements. Flexural bending beam fatigue laboratory test has been used for several decades and is considered an integral part of the Superpave advanced characterization procedure. One of the most significant solutions to prolong the fatigue life for an asphaltic mixture is to add flexible materials such as rubber or polymers to the asphalt mixture. A laboratory testing program was performed on three gap-graded mixtures: unmodified, asphalt rubber (AR), and polymer-modified. Strain controlled fatigue tests were conducted according to the AASHTO T321-14 procedure. The results from the beam fatigue tests indicated that the AR and polymer-modified gap graded mixtures would have much longer fatigue lives compared to the reference (unmodified) mixture. In addition, a mechanistic analysis using 3D-Move software coupled with a cost-effectiveness analysis study based on the fatigue performance on the three mixtures were performed. Overall, the analysis showed that the AR and polymer-modified asphalt mixtures exhibited significantly higher costeffectiveness compared to unmodified HMA mixture. Although AR and polymer-modification increases the cost of the material, the analysis showed that they are more cost effective than the unmodified mixture.

Designing sustainable, long-lasting asphalt pavement without accumulation of fatigue cracking is an important goal of transportation agencies. If the tensile strain at the bottom of asphalt layer is kept below a certain value (endurance limit), fatigue damage either does not occur or can be healed during the time period between load applications. The objective of this study is to introduce a simple approach that can be used to determine the minimum asphalt layer thickness that would avoid fatigue cracking accumulation. In this approach, the endurance limit model previously developed by a National Cooperative Highway Research Program (NCHRP) study was used with certain simplified assumptions. A wide range of subgrade stiffness, hot-mix asphalt (HMA) stiffness, and traffic volume variations, covering typical roadway conditions was used. The study concluded that the minimum HMA layer thickness required to avoid accumulation of fatigue cracking ranges between 140 mm (5.5 in.) and 240 mm (9.5 in.) under the stated conditions and assumptions. Increasing traffic volume decreases the rest period between load applications and increases the minimum asphalt layer thickness required to avoid accumulation of fatigue cracking. Increasing asphalt concrete stiffness decreases the endurance limit and decreases the minimum asphalt layer thickness required to avoid accumulation of fatigue cracking. Subgrade stiffness does not have much effect on the minimum required surface layer. The results of this study should not be used directly to design specific pavement sections. Rather, the approach introduced here is intended to be used as a guide for a more detailed design procedure by the user incorporating actual traffic loads and volume distributions, specific environmental conditions, and more realistic material properties. Designing pavement to avoid accumulation of fatigue cracking should produce good long-term performance and should have significant design and economic implications. (C) 2017 American Society of Civil Engineers.

Various laboratory testing methods have been developed to characterize the fatigue response of asphalt concrete mixtures. These test methods attempt to simulate in-service conditions or formulate constitutive models. Experiments such as the four-point beam fatigue test have attempted to simulate in-service conditions. The prediction accuracy of such experiments depends on their effectiveness in simulating actual field conditions such as loading, support, stress state, and environment. Constitutive modeling experiments have aimed at measuring fundamental stress–strain relationships so that rigorous constitutive models can be formulated. The results from these experiments are used as input for field performance prediction algorithms. The uniaxial fatigue test is a promising method in this category because of the constant stress state across the specimen section. A few documents have focused on standard test methods for the uniaxial fatigue test; however, there are no AASHTO or ASTM protocols available that include the healing of asphalt concrete mixtures. The main objective of this study was to report on the development of a uniaxial fatigue test protocol that measures fatigue damage and healing of asphalt concrete mixtures. The documented work includes surrogate studies to identify appropriate sample fabrication procedures, gluing materials and procedures, alignment, machine compliance, type of strain wave shape, and strain-control mode of loading. It was found that the use of a gluing jig and 180-mm compaction height was essential to achieve successful mid-specimen failures. In addition, the sinusoidal strain wave shape and on-specimen strain-controlled mode of loading are appropriate test conditions for fatigue damage and healing characterization of asphalt concrete mixtures.

Load associated fatigue cracking is one of the major distress types occurring in flexible pavements. Flexural bending beam fatigue laboratory test has been used for several decades and is considered an integral part of the Superpave advanced characterization procedure. One of the most significant solutions to sustain the fatigue life for an asphaltic mixture is to add sustainable materials such as rubber or polymers to the asphalt mixture. A laboratory testing program was performed on three gap-graded mixtures: unmodified, Asphalt Rubber (AR) and polymer-modified. Strain controlled fatigue tests were conducted according to the AASHTO T321 procedure. The results from the beam fatigue tests indicated that the AR and polymer-modified gap graded mixtures would have much longer fatigue lives compared to the reference (unmodified) mixture. In addition, a mechanistic analysis using 3D-Move software coupled with a cost-effectiveness analysis study based on the fatigue performance on the three mixtures were performed. Overall, the analysis showed that the AR and polymer-modified asphalt mixtures exhibited significantly higher cost-effectiveness compared to unmodified HMA mixture. Although AR and polymer-modification increases the cost of the material, the analysis showed that they are more cost effective than the unmodified mixture.

Beam fatigue testing of hot-mix asphalt (HMA) in the laboratory has been used for several decades by many researchers around the world. A total of two standard procedures are currently available: one that controls constant haversine deflection and another that controls constant sinusoidal deflection. The literature shows a large variability in shift factors between laboratory and field fatigue results. Also, recent literature shows inconsistent waveforms when a haversine deflection is used, which could be one of the reasons causing this large variability. The paper examines the results of the standard procedures and provides a rational explanation of the differences between them. Because of the inconsistency of the ASTM flexural fatigue test standard, it incorrectly produces a larger number of cycles to failure than the AASHTO test, with ratios of approximately 10, which varies depending on the stiffness of the material. Also, specimens tested according to ASTM flexural fatigue test standard, with twice the input tensile strain of specimens tested according to AASHTO flexural fatigue test standard at the same temperature, produced approximately the same number of cycles to failure. An approximate method is recommended to correct old ASTM flexural fatigue test standard results by assuming that the obtained number of cycles to failure corresponds to approximately 55-65% of the tensile strain that was inputted to the machine. More studies need to be conducted to verify the results using other materials and test conditions. (C) 2015 American Society of Civil Engineers.

One of the main requirements of designing perpetual pavements is to determine the endurance limit of asphalt mixtures. The endurance limit is the strain below which no fatigue damage occurs or can be healed during unloading. If the pavement thickness is controlled so that the strain at the bottom of the asphalt layer is kept below the endurance limit, the pavement would endure indefinite load repetitions and would not experience bottom-up fatigue cracking. Field observation shows that an endurance limit for hot mix asphalt (HMA) does exist. The endurance limit values were previously determined in the laboratory in the NCHRP Project 9-44A for conventional HMA at different conditions. The purpose of this paper was to determine the endurance limit values for asphalt rubber (AR) mixtures using laboratory beam fatigue tests. The paper discusses the results of a study that produced a preliminary estimation of the endurance limit for an asphalt rubber mixture placed in Sweden. This study included 24 beam fatigue laboratory tests conducted according to the AASHTO T321-03 test procedure with rest periods between loading cycles. Two factors that affect the fatigue response of asphalt mixtures were evaluated, which are the applied strain and the rest period between loading cycles. A model was developed to determine the stiffness ratio as a function of strain and rest period. The endurance limit was determined using the developed model by setting the stiffness ratio as one, indicating no accumulated damage or complete healing. Endurance limit values for the AR mixture ranged from 150 to 175 microstrain at 20 °C, which are significantly higher than those of conventional HMA. This indicates that a thinner asphalt rubber layer can be used to reach the endurance limit as compared to the HMA layer. Determining the endurance limit of asphalt rubber has significant design and economic implications.

Perpetual pavements, if properly designed and rehabilitated, last longer than 50years without major structural rehabilitation. The fatigue endurance limit (EL) is a key parameter for designing perpetual pavements to mitigate bottom-up fatigue cracking. The endurance limit has not been completely implemented in the Mechanistic Empirical Pavement Design Guide software, currently known as AASHTOWare Pavement ME Design software. This study was conducted as part of the National Cooperative Highway Research Program (NCHRP) project 9-44A to develop a framework and mathematical methodology to determine the EL for hot mix asphalt (HMA) using the uniaxial tension-compression fatigue test. In this unique procedure, the EL is defined as the allowable tensile strains at which a balance takes place between the fatigue damage during loading and the healing during rest periods between loading pulses. The viscoelastic continuum damage model was applied into the data analysis. This study also included the development of a uniaxial fatigue test method and the associated data acquisition computer programs to conduct the test with and without rest period. The laboratory testing program consisted of dynamic modulus testing to estimate the viscoelastic properties of the asphalt mixtures and a uniaxial fatigue test experiment conducted with and without rest periods. Five factors that affect the fatigue and healing behavior of asphalt mixtures were evaluated: asphalt content, air voids, temperature, rest period, and tensile strain. On the basis of the test results, a pseudo stiffness ratio (PSR) regression model was developed as a function of the five factors and the number of loading cycles. The EL was defined when PSR is equal to 1.0 (net damage is equal to zero). The results from the sensitivity analysis showed rational relationships between the EL and investigated factors. The EL value was observed to increase by increasing temperature, asphalt content, and rest periods, whereas it decreased when air voids increased. (C) 2014 American Society of Civil Engineers.

The concept of an endurance limit assumes a strain value below which the net fatigue damage that occurs during a load cycle is zero. The fact that real traffic loads are separated by rest periods may allow for partial or full healing of the microcracks, which can affect this endurance limit. If the asphalt layer thickness is controlled to keep strains below the endurance limit, the fatigue life of the pavement can be extended considerably. In the study reported in this paper, it was hypothesized that the endurance limit in asphalt concrete developed from the interaction and balance of damage and healing during a load cycle. This hypothesis formed the basis of the testing and analysis program, which evaluated the effects of air voids, asphalt content, rest periods, and temperature on the endurance limit. Two types of fatigue tests were conducted: beam (flexural) and uniaxial. A regression model also was developed on the basis of the results of each test and used to obtain the endurance limit values. This paper compares fatigue damage, healing, and endurance limit results from the two tests under similar conditions. The comparison shows that the beam fatigue test yields less overall fatigue damage and less healing than the uniaxial fatigue test. Beam fatigue yields 8 to 14 times longer fatigue lives, while uniaxial fatigue yields higher healing (10.4 times for the only available case). Because damage and healing combined to govern the endurance limit, the two tests produced close values in which the overall uniaxial endurance limit values were 12% less than the beam fatigue endurance limit values.

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

Fatigue endurance limit (FEL) is a key parameter for designing perpetual pavements to mitigate bottom-up fatigue cracking. This study was conducted as part of the National Cooperative Highway Research Program (NCHRP) Project 9-44A to develop a framework and mathematical methodology to determine the FEL using the uniaxial fatigue test. In this unique procedure, the FEL is defined as the allowable tensile strains at which a balance takes place between the fatigue damage during loading, and the healing during the rest periods between loading pulses. The viscoelastic continuum damage model was used to isolate time dependent damage and healing in hot mix asphalt from that due to fatigue. The laboratory testing program consisted of dynamic modulus testing to estimate the viscoelastic properties of the asphalt mixtures, and uniaxial fatigue test experiment conducted with and without rest periods. Five factors that affect the fatigue and healing behavior of asphalt mixtures were evaluated: asphalt content, air voids, temperature, rest period and tensile strain. Based on the test results, a Pseudo Stiffness Ratio (PSR) regression model was developed that is a function of the five factors and the number of loading cycles. The FEL was defined when PSR is equal to 1.0 (net damage is equal to zero). The determined FEL values were rational compared to historical literature ranges. The results from the sensitivity analysis showed rational relationships between the FEL and investigated factors.

A long lasting flexible pavement has been the goal of the pavement community for many years. Designing perpetual pavements requires knowledge of the stresses or strains below which damage does not occur or can be healed during unloading. The endurance limit, as applied to hot mix asphalt (HMA) and flexible pavement design, is the strain or stress level below which the HMA would endure indefinite load repetitions and the pavement would not experience bottom-up fatigue cracking. The purpose of this study is to determine the endurance limit for HMA under different conditions using laboratory beam fatigue tests. The approach used in this study assumes that if stresses or strains are kept below a certain level, fatigue damage may not occur or can be healed during unloading. Relating healing to the endurance limit makes this approach unique compared to previous studies that investigated these concepts separately. An extensive laboratory beam fatigue testing program covering a wide range of asphalt mixtures was conducted according to the AASHTO T 321-03 test procedure as a part of the NCHRP 9-44A project. The endurance limit was determined when no accumulated damage occurred indicating complete healing during the rest period after each load application. The endurance limit varied between 22 and 264 micro-strains (mu s) depending on binder grade, binder content, air void, temperature and the rest period between load applications when 0.1 sec loading cycles are used. The results of this study can be used to design perpetual pavements that can sustain a large number of load applications if traffic volume and vehicle weights are controlled.

National Cooperative Highway Research Program (NCHRP) Project 9-44A extended the results and findings of NCHRP Project 9-38, Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavement. Project 9-44A paid particular attention to the influence of asphalt binder and mixture properties on the fatigue endurance limit and to the relationship of the endurance limit to the phenomenon of healing hypothesized to occur in asphalt mixtures during the rest period between load applications in the laboratory and in pavements. The research investigated the relationship of the fatigue endurance limit to factors such as asphalt binder rheology, air voids, asphalt content, temperature, strain level, number of load cycles, and rest period between load cycles. Both beam fatigue (AASHTO T321) and uniaxial compression-tension testing were conducted according to a factorial design that permitted statistical analysis of the main factor and up to three-factor interactions. Robust regression models were developed that described the effect of the main factors and factor interactions on the stiffness ratio (SR). Testing was conducted with rest periods of 0, 1, 5, and 10s between load cycles. The endurance limit can then be determined for any mixture initial stiffness as the strain at SR = 1, i.e., for the condition in which complete healing of the fatigue damage takes place after each load cycle. The research reaffirmed the existence of the HMA fatigue endurance limit and demonstrated that the endurance limit is the result of a balance between loading damage and the healing, i.e., damage recovery, that happens during rest periods and that the value of the limit varies with the mixture initial stiffness (acting as a surrogate for binder rheology, air voids, asphalt content, and temperature) and the duration of the rest period. It was also found that for a load cycle of 0.1s, a rest period greater than 5 to 10s (from beam fatigue testing) or greater than 3s (from uniaxial testing) will not produce additional healing of the fatigue damage in the laboratory. This report fully documents the research and discusses incorporation of the endurance limit derived from the SR regression model formalism as an algorithm in Pavement Mechanistic Empirical Design software and other design methods.