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One important research question related to walking for transportation is whether people like to walk more or not. Past research on walking-related travel behavior has mainly considered obj...

The mortar component plays an essential role in forming different asphalt mixture distresses. The fine aggregate size affects the viscoelastic behavior of mortar. In this study, three cutting sizes of 3.35, 2.36, and 1.7 mm, on the aggregate grading curve were considered to construct the mortar scales, namely mortar (I), (II), and (III), respectively. With a unique loading mode, a frequency sweep test was performed on four different length scales, including bitumen, asphalt mastic, mortar, and asphalt mixture, to make a direct comparison through a multi-scale approach. Also, a stress sweep test was conducted to find the mortar scale's linear viscoelastic domain. The results indicate that different mortar types have variable linear viscoelastic domains. The master curves of dynamic modulus (E*) and phase angle (δ) show that adding filler to bitumen has more effect on the E* than the δ. Although scaling up from the mastic to mortar (II) causes a significant difference between the viscoelastic parameters, the results remained approximately the same. The main gap between the mixture and mentioned sub-scales is filled with mortar (I). While the Cole-Cole diagrams demonstrate that the bitumen to mortar (II) scales have the same behavior, the closest scale to the mixture is the mortar (I). Therefore, using a cutting size of 3.35 mm is recommended to fabricate mortar (for asphalt mixture with a nominal maximum aggregate size of 19 mm) instead of 2.36 mm, which was suggested in previous studies.

Similar to other construction materials, asphaltic materials used for pavements are susceptible to environmental conditions. Subsequently, the environmental factors such as aging should be consider...

Asphalt binder resistance to permanent deformation considerably influences total pavement resistance to rutting at the high service temperatures. To improve asphalt shear strength against rutting resistance, researchers have been used various modifiers and recently the use of waste materials as binder rheological enhancement agents is paid lots of attention. In this study, waste toner was adopted as an anti-rutting enhancement agent, and then neat asphalt binder was modified by waste toner at four dosages: 4, 8, 12 and 16% based on asphalt binder’s weight. The effectiveness of binder modification by waste toner to improve asphalt binder and mixture’s resistance to permanent deformation has been assessed by means of different met hods: the Superpave specification parameter, G*/sinδ, and the multiple stress creep and recovery (MSCR) test for asphalt binder and wheel track test for asphalt mixture to determine the rut depth for a certain number of loadings. Results showed that adding toner to neat asphalt binder can meaningfully enhance the anti-rutting performance of asphalt binder and subsequently asphalt mixture. Moreover, increasing the dosage of toner up to 12% in the modified asphalt binder amplifies the trend of rutting resistance improvement.

While transit exclusive lanes are statically introduced for an entire day (or for the peak period), the total number and distribution of trips vary from one hour to another during a day. In this st...

Exploring the factors affecting walking behavior has become an interesting issue among sustainable transportation researchers. However, walking studies which have examined gender in relation to tri...

Hydroplaning speed can be affected by pavement texture depth, thickness of water film, tire pressure and tread depth. In this study, to understand the influence of pavement texture on the hydroplaning speed, a new lab-scale apparatus has been designed and manufactured. The lack of proportion between linear movement of vehicle shaft and the wheel rotation was found to be a good index to determine hydroplaning threshold. A 5% drop in the ratio of wheel-to-axle rotation has been assumed as an index to determine hydroplaning threshold. Based on the measures, a simplified model was developed that is able to predict the hydroplaning speed depending on pavement’s texture characteristics. The results indicated that a 77% increase in mean texture depth cause 9% increase in hydroplaning threshold speed.

This study investigated the effect of waste toner as a cost-effective enhancement modifier on short-term aging and moisture resistance of asphalt binders and mixtures. Moreover, the mechani...

Moisture damage is one of the common causes of asphalt pavement failure in moisture presence. One of the convenient approaches to decreasing moisture sensitivity in hot-mix asphalt is coating the aggregate surface with a suitable agent. In this study, the effects of polyethylene terephthalate on moisture damage of asphalt mixtures were evaluated by applying indirect tensile strength and resilient modulus tests. The asphalt specimens were prepared with two types of aggregates (granite and limestone) and neat asphalt binder of 60/70 penetration grade. The results showed that the ratio of wet/dry values of indirect tensile strength and resilient modulus for mixtures containing limestone was higher than those of the samples with granite aggregate. Furthermore, the results of the laboratory tests indicate that polyethylene terephthalate improves resistance to moisture susceptibility. Because polyethylene terephthalate increases the wettability of asphalt binder over the aggregate and the adhesion between the asphalt binder and aggregate, especially in the mixtures containing acidic (granite) aggregate prone to moisture damage.

Asphalt binders generally behave in a linear viscoelastic manner under conventional stress and strain level. There are several researches and technical protocols consistent with linear viscoelastic theorem. However, two main reasons can affect this well-known behavior of asphalt binders: 1) modification of asphalt binder with some additives causing a change in its microstructure and 2) higher order stress and strain level induced to asphalt binder under heavy traffic or climate loading. It is important to consider non-linear viscoelastic behavior of asphalt binder and its effect on conventional characteristics of asphalt especially two constitutive functions of creep compliance and relaxation modulus. In this research, nonlinear viscoelastic properties of asphalt binders modified with crumb rubber, styrene-butadiene-styrene and polyphosphoric acid are investigated to evaluate the variation in non-linear viscoelastic parameters due to addition of these modifiers. For this purpose, a specified research program was defined to consider non-linear behavior of one neat asphalt binder as well as nine modified ones. Based on a robust literature review and some prior experiences, three different extents selected for each modifier and then specimens were made. Implementing a simple creep-recovery test with creep loading time of 1 s and 100 s and 999 s unloading to simulate traffic load by using a dynamic shear rheometer at different stress levels of 0.1, 1, 10, 20 and 30 kPa, non-linear viscoelastic behavior of neat and modified asphalt binder was captured. The generalized fractional nonlinear viscoelastic model is introduced and successfully implemented to simulate creep and recovery behavior of neat and modified asphalt binders. Addition of all modifiers to the neat asphalt binder alters its non-linear viscoelastic behavior and properties. For short time creep loading, crumb rubber and polyphosphoric acid amplify non-linear behavior while SBS control it and for long creep loading time all asphalt modifiers increases non-linearity of asphalt binder’s viscoelastic behavior.

This paper examines the effect of waste toner on fatigue resistance of asphalt binder. Fatigue resistance of toner-modified asphalt binders was studied implementing the linear amplitude sweep test,...

Application of geosynthetics is known as an important way to reduce and delay emergence of reflective cracks. However, reliance on these materials can have some disadvantages, including reduction o...

Though geosynthetics materials have been proved effective in delaying reflective cracks, they cause a reduction in shear bonding between asphalt layers, reducing their efficiency and introducing new distresses to overlay. While several devices have been developed for measuring the shear bonding between the overlay and the old asphalt pavement, they are not able to apply normal load or are limited in normal load application and cannot be used for thick overlays over geosynthetic. In this work, a new method and apparatus is developed in order to measure the shear bonding between asphalt layers without the aforementioned limitations. This apparatus, named Amirkabir University Shear Field Test, is also capable of measuring the interlayer tensile strength. The obtained results showed acceptable repeatability and good alignment with the results reported in previous studies. Thus, the device was subsequently employed in evaluation of geocomposite-strengthened specimen. The most influential factors on shear bondi...

Geocomposites can be employed with an asphalt overlay to improve the service life of pavements and reduce reflective cracks. However, similar to other geosynthetic solutions, geocomposites can cause de-bonding between asphalt layers, thus reducing pavement longevity. This research focuses on factors that influence the initiation and propagation of shear bonding and reflective cracks in an asphalt overlay. Laboratory double-shear and pull-out tests are conducted, and the shear bonding and pull-out strength of geocomposites in asphalt specimens are calculated. The performance of each specimen in terms of reflective crack initiation and propagation are investigated. A power law model that relates crack propagation to the number of loadings is developed and verified. Observations of geocomposite sliding on the asphalt layer reveal that this phenomenon contributes less than 40% to the overall shear bonding. Analyses reveal that the number of required loading cycles to initiate reflective cracking and crack propagation varies exponentially with the shear bonding between the overlay and the existing layer. Therefore, overlay longevity declines rapidly with decreasing shear bond strength. The exponential relationship is valid irrespective of whether or not the normal load is applied on the interlayer surface. Moreover, the results indicate that temperature is the parameter with most influence on the shear bonding between the layers and the initiation and propagation of reflective cracks, while the tack-coat application rate and geocomposite grid size are less critical. Finally, this study reveals that the decrease in shear bonding between layers both increases the stress relief ability in the interlayer zone and decreases the reinforcement capacity of geocomposite grids. Increased shear bonding between the overlay and existing pavement increases the resistance to reflective cracking.

Stone matrix asphalt (SMA) is known to have high resistance to permanent deformation and reflective cracking due to their stone structure but similar to other hot mixes it has a high energy intake during its production. Warm mix asphalt (WMA) has recently gained a lot of popularity worldwide due to environmental concerns. Moisture susceptibility as one of the drawbacks of warm mixes has been investigated by many researchers but the extent and interactive effects of different parameters affecting moisture susceptibility particularly for SMA has not yet been reported. The aim of the present study was to examine the effects of grading, bitumen content, Zycosoil content, Sasobit content and mixing temperature on moisture susceptibility of SMA warm mix asphalt (WSMA), as well as their interactions, using response surface methodology. The results indicated that the influence of Sasobit content and grading was more than the influence of other factors on tensile strength ratio -used as an index of moisture susceptibility- respectively. Also, interaction plots between factors affecting tensile strength ratio were generated that can help to understand the interactive relationship between investigated factors.

Although walking behavior in school trips has been the subject of debate in developed countries in recent years, much remains to be learned about this subject in developing countries. Moreover, despite exploring effects of various factors, research on walking mode choice behavior across genders remains quite limited. The main purpose of this study is to understand reasons behind male-female differences in choosing walking as a mode of transportation in trips to school in the city of Rasht, Iran. Separate binary logit models were developed for boys and girls in trips to school using the data from Rasht household travel survey in 2007. Results show that high school girls are less likely to walk relative to elementary and middle aged girls; but elementary and middle aged boys dislike walking to school relative to high school boys. Results show that regardless of gender, an individual who has a car in his/her household is less likely to walk to school. In this study, different distance intervals (increasing by 0.25 miles) were defined. Results show that both boys and girls are sensitive to trips longer than 0.25 miles in choosing walking. However, the decrease in likelihood of walking is greater among boys than girls. For example, if the distance is between 0.25 to 0.5 miles, the probability of walking to school decreases by 14.8% points for boys and 10.5% points for girls with respect to trips less than 0.25 miles. Altogether, our findings suggest that gender differences need to be addressed if policy makers hope to increase rates of walking in children׳s trips to school.

Although walking behavior has been the subject of debate in developed countries, much remains to be learned about this subject in developing countries. Furthermore, it seems likely that the effect of different factors on choosing walking as a commute mode varies across different job groups. Therefore, this research investigated and compared walking behaviors across two job groups with different schedule patterns in the city of Rasht, Iran. The first group is related to those who are flexible schedule (i.e., FLS) workers and the second group refers to people who identify as fixed schedule (FXS) workers. FXS workers attend/leave their workplace during specific period of time which is usually invariant and makes them more time constraint than the second group. Separate behavioral choice models are developed for trips to work using a range of explanatory variables including individual; household; travel; and environmental variables. Results show that females are more likely to walk than males in both groups. Another significant factor is the travel distance which was found with different effects across the studied job groups. Furthermore, it was found that land use features are directly related to walking choice probability of FLS workers.

Application of geosynthetics is known as an effective way to reduce and delay reflective cracks. Although these materials can improve pavement resistance against reflective cracking, they can reduce shear bonding between the overlay and the old pavement. Lack of this resistance can cause some defects, especially in zones in which cars accelerate, decelerate or turn. There are many parameters affecting the quality of shear bonding between layers. In this research, a double shear bonding test denoted as Amirkabir University of Technology Shear Lab Tester was developed and was used for conducting dynamic shear loading experiments on geocomposite reinforced specimens. In order to simulate the weight of passing vehicles, a normal loading system was designed and added to the double shear test device. This normal loading system applies constant stiffness on the specimen, thus simulating real conditions with high fidelity. Seven factors, namely temperature, tack-coat application rate, tack-coat penetration value,...

Due to an increase in motorized trips and a decline in individuals’ physical activity, studying factors that increase active transportation modes in school trips have captured significant attention in recent years. However, research on walking mode choice behavior across different age groups remains limited. The main objective in this study was to understand reasons behind differences in choosing walking as a mode of transportation in trips to school across different age groups and inside the urban areas of Rasht. Three separate behavioral choice models were developed for elementary, middle and high school students in trips to school using a range of explanatory variables including individual; household; travel; and environmental variables. Results show that the effect of different factors on walking behaviors in trips to school is different across age and gender groups. For example girls are less motivated to walk relative to boys in all age groups. Results show that regardless of age, individual who has a car in his/her household is less motivates to walk to school. Time of day only motivates elementary and middle aged students’ to choose walking for transportation. Another significant factor is the travel distance which diversely affects walking in all models. More explanations for differences between age groups in choosing walking in trips to school are discussed in the article; and some issues are suggested for future research.

International audience

After a comprehensive experimental program on asphalt mastic modified with styrene-butadiene-styrene (SBS) and polyphosphoric acid (PPA) with two filling ratios of 18% and 35%, in this pape...

Recycling agents are commonly used to enhance the properties of aged asphalt binder. The aged asphalt binder found in reclaimed asphalt pavement is considerably oxidized and has lost a significant amount of its volatiles during its service life. Therefore, its performance characteristics need to be improved before it can fully function in new–pavement construction. Recycling agents are intended to modify performance characteristics; however, typical recycling agents are prone to oxidation, which renders them ineffective. It is hypothesized that the efficacy of recycling agents can be enhanced by simultaneous addition of antioxidants to the binder system. Therefore, in this paper, the synergy between three types of recycling agents (paraffinic oil, triglycerides and fatty acids, and tall oil) and zinc diethyldithiocarbamate (ZnDEC) as an antioxidant is examined using rheological characterization at high, ambient, and sub-zero temperatures. The study results show that the efficacy of recycling agents was improved with the addition of ZnDEC; however, the efficacy of the ZnDEC varied depending on the recycling agent used. The latter was attributed to the susceptibility of recycling agents to oxidation as well as the extent of synergy between a recycling agent and ZnDEC.

Fatigue cracking is the main form of structural damage in flexible pavements. One of the convenient approaches for decreasing this type of distress in hot mix asphalt (HMA) is use of nanomaterials as additives in asphalt mixtures. In this study, the effects of using nano zinc oxide (ZnO) on fatigue cracking of asphalt mixtures have been evaluated by determining the mechanism that affects the asphalt cement's (AC’s) cohesion and the adhesive bond between aggregate and AC using the surface free energy (SFE) method and indirect tensile fatigue test. The HMA mixtures containing 1%, 3%, 5%, and 7% of nano ZnO are plant prepared with one source of granite aggregate, and one type of AC with 85/100-penetration grade. The results showed that fatigue lives of mixtures containing nano ZnO were higher than those of the control mixtures. This is attributed to the fact that adding nano ZnO leads to decrease of the acid component of SFE and increase of the basic component in SFE of the AC that enhances the adhesion betw...

Skid resistance of asphalt pavements is one of the most important road criteria for users' safety. In a rainy condition the skid resistance is largely compromised and when the road surface is flooded, vehicles can experience hydroplaning at a certain critical speed. Tires under hydroplaning conditions become separated from the pavement surface and the ability of tires to increase braking force or cornering traction for stopping or controlling vehicle motion is almost entirely lost. In this study, a new lab-scale apparatus was designed and manufactured, which can stimulate hydroplaning and evaluate the impact of different asphalt mixtures on this phenomenon. To evaluate various mixes, three types of asphalt mixtures with different grading are made and mean texture depth and British pendulum number are measured by six repetitions. By using this apparatus in dry and wet conditions of pavement surface, five different frequencies are tested in terms of hydroplaning. The results show that there is 33% i...

One of the basic parameters upon which the occurrence of distress in hot mix asphalt depends is the amount of adhesion at the contact surface between asphalt binder and aggregate. Therefore, a number of researches have been conducted throughout the last decade to improve the adhesion between asphalt binder and aggregate using different additives. In the present study, the effects of using nano-TiO2 on the adhesion between the asphalt binder and aggregate are assessed using the surface free energy (SFE) method. The SFE components of the aggregates are determined by universal sorption device test, while SFE components of the asphalt binders are calculated using Wilhelmy plate test. The results of the SFE method indicate that nano-TiO2 increases the wettability of the asphalt binder on the aggregate and promotes the adhesion between the asphalt binder and the aggregate. Furthermore, adding nano-TiO2 leads to the decrease in the acid component of SFE and the increase in the basic component of SFE of the aspha...

Monotonic Indirect Tensile tests were performed on asphalt concrete mixtures made of two different types of aggregates. Since the environmental condition is a determining factor in propagation of moisture damage, the effects of temperature and aging were also observed. Three levels of temperature including 4.4 °C, 21.1 °C and 37.8 °C were used in testing. In order to consider the effect of aging, part of the samples were aged according to AASHTO R 30-02 (2010). The parameters of fracture energy, elastic energy and dissipated creep strain energy were obtained for dry and moisture conditioned samples. The sensitivity of the responses to testing parameters was investigated using analysis of variance methods and visual inspections. According to the results, common energy responses cannot be used as realistic criteria to evaluate asphalt mixtures’ moisture damage. To overcome the inadequacy of common energy based responses, modi­fied energy responses were defined using conventional energy based responses normalized by specimens’ failure strain. The analysis of variance showed that the modified energy responses can effectively evaluate the mixtures’ moisture and temperature susceptibility. First published online:01 Jul 2016

Fatigue cracking is one of the major distresses of hot mix asphalt (HMA) at moderate temperatures. Two of the main properties of asphalt mixtures affecting fatigue are the cohesive bond energy of asphalt binder and the adhesive bond energy between asphalt binder and aggregate. These two parameters were calculated using surface free energy (SFE) theory. Furthermore, the fatigue life of asphalt mixtures was measured by indirect tensile fatigue test. The results showed that asphalt mixtures with limestone aggregates, with the most specific surface area, and magnitude of adhesion had the highest fatigue life. Moreover, asphalt mixtures made with asphalt binder having the highest penetration had greater fatigue life than the other mixtures. The enhanced fatigue life was attributed to the greater cohesion energy and higher resistance to fatigue cracking in asphalt film. Also, these mixtures had the highest adhesion energy on the contact surface between asphalt binders and aggregates, which increased the...

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.

The pollution caused by oil and gas drilling waste is among the major problems affecting the oil industry. In this study, drill cuttings are used as an additive in the asphalt mixture for constructing road pavements. The materials used for manufacturing asphalt samples were neat bitumen with penetration grade 60/70 (PG 64-16), granite aggregates, and drill cuttings in both unwashed and washed states. After determining the optimum bitumen percentage based on Marshall Design, five mixes were considered for combining aggregates, bitumen, and drill cuttings. Marshall Stability, resilient modulus (Mr), indirect tensile strength (ITS), and dynamic creep tests were carried out on all asphalt mixture samples. The test results were applied to evaluate the efficiency and moisture damage susceptibility of drill-cuttings-modified asphalt mixture. The results indicated that the addition of washed drill cuttings in the asphalt mixture increases Marshall Stability, as well as decreasing Flow and thus Marshall Quotient (MQ). Also, the asphalt mixture containing washed drill cuttings has a higher resilient modulus and indirect tensile strength than the control and unwashed mixtures. Also, these results indicated a reduction in the thickness of the asphalt layer (increasing in the layer life with the same thickness) and moisture damage. According to the results of the one-way ANOVA analysis, using unwashed drill cuttings in the asphalt mixture has a reverse effect on the above parameters and the outcome. Besides, the dynamic creep results showed that unwashed and control mixtures have the same resistance against the permanent deformation while using washed drill cutting increased the rutting resistance of asphalt mixture. Therefore, it is recommended using washed drill cuttings in the asphalt mixture to make a suitable pavement. The results suggest that the feasibility of the application of washed drill cuttings in road construction not only could enhance the physical and mechanical properties of asphalt mixture but could also relieve the environmental concerns regarding landfilling of such waste.

International audience

In this paper, the combined effect of adding polyphosphoric acid (PPA) and distillate aromatic extracts (DAE) oil on rheological and mechanical characteristics of neat asphalt binder at hig...

Aging, as an integral part of organic materials like asphalt binder, imposes a significant effect on chemical, physical, and performance-related properties of asphalt mixtures. One of the m...

The main objective of this study was to predict the tensile relaxation modulus of asphalt mixes, without having to perform the common relaxation modulus tests, by developing a predictive model based on the mix characteristics, ageing condition, temperature and loading time. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with four replicates. Uniaxial tensile relaxation modulus tests were conducted on the specimens at four temperatures using the trapezoidal loading pattern at a low level of strain. Tensile relaxation modulus master curves of all the experimental combinations were constructed by the sigmoidal model. Statistical analysis of variance and regression analysis was performed on the test data and a predictive model was developed. Finally, the predictive model was verified using a group of measured ...

Spontaneous damage recovery or healing is an intrinsic behavior of asphalt mixtures. This characteristic plays an important role on extending the service life of asphalt concrete. In the present work, the cyclic controlled-stress IDT tests have been conducted on the asphalt concrete specimens to study their recovery potential using a newly developed energy based parameter named normalized dissipated creep strain energy (NDCSE). It is believed that this parameter shows an average energy demand for a material to withstand 1 μm/m. During the research, the effect of environmental and external factors such as aging, temperature, rest time ratio, and stress level on the damage recovery of asphalt concrete has been studied using the NDCSE. The conventional DCSE parameter was also calculated for the test results. The NDCSE showed a good reverse correlation with the conventional cumulative dissipated creep strain energy parameter. The observations on the stress level in the cyclic IDT test, suggested that very high or very low stress levels might reduce the effect of rest time on asphalt concrete specimens recovery potential. A recovery index was also developed based on the NDCSE. This index testified that aging and rest time are the most effective factors on recovery potential of asphalt concrete. According to the index, increasing the temperature can enhance the recovery potential of un-aged mixtures while it was almost ineffective on that of aged mixtures. However, the overall effect of temperature on mixtures’ recovery index was not statistically meaningful based on the analysis of variance.

In this study, response surface methodology was utilized for creating a continuous response function and optimization of effective variables to improve the tensile strength ratio, as an indicator of stripping potential, using factorial experiment with center composite design. The polynomial regression model equations were proposed based on the regression coefficients calculated for the indirect tensile strength in dry and saturated condition as well as tensile strength ratio. Statistical analyses indicated that all first and second orders plus interactive terms were statistically significant with 90 % confidence level except for interactive terms of bitumen–Sasobit content in tensile strength ratio, and grading-Sasobit content in dry indirect tensile strength. Furthermore, it was found that all first, second and interactive terms were statistically significant for saturated indirect tensile strength.

This study examines the influence of the most important parameters on delaying reflection cracking in geogrid reinforced overlay in bending mode, employing a large scale experimental set up. The objective was to investigate the effects of grid stiffness, tensile strength, coating type, amount of tack coat, overlay thickness, crack width and stiffnesses of asphalt overlay and existing asphalt concrete on reinforced asphalt overlay performance with geogrid interlayer. Results indicate that the cracked layer and overlay moduluses have strongest influence on reduction of reflective cracking. Also, two fatigue life estimation models for reflection cracking were developed, one based on initial strain (fatigue model) and the other by modifying Paris law for fracture mechanic model in mode I. It was found that the coefficient of determination of predicted fatigue lives and measured lives for the two developed models were sufficient. Therefore, both service life models can be used to estimate fatigue life of reflection cracking in bending mode (I).

Relaxation modulus as an important viscoelastic property of asphalt mixes is usually determined using time-consuming and expensive test methods. Therefore, this research was carried out to develop a predictive model for the time-dependent compressive relaxation modulus of asphalt mixtures based on the mix specifications, volumetric properties, ageing condition, and temperature. To this end, crushed stone aggregates together with 60/70 penetration asphalt binder (corresponding to PG 64-22) were used to prepare the asphalt mixture specimens at two aggregate gradations, two binder contents, two air void levels, and three ageing conditions with four replications. Direct compression relaxation modulus tests were conducted at four different temperatures, and the sigmoidal model was used together with the numerical temperature-shifting technique to construct the compressive relaxation modulus master curves for all the experimental combinations. Statistical analysis of variance and regression analysis were then p...

This study examines the influence of the most important parameters on delaying reflection cracking in geogrid reinforced overlay in bending mode, employing a large scale experimental set up. The effects of grid stiffness, tensile strength, coating type, amount of tack coat, overlay thickness, crack width and stiffnesses of overlay and existing asphalt concrete on reinforced asphalt overlay was investigated. Results indicate that the cracked layer and overlay moduluses have strongest influence on reduction of reflective cracking. Also, a fatigue fracture model for reflection cracking based on modified Paris law for fracture mechanic model with linear viscoelastic correspondence principle in mode I was developed. It was found that this engineering fracture mechanics model was an efficient method of determining the fatigue damage process. Based on Schapery’s theory, regression equations were developed to estimate fracture parameters. These equations can be used to predict the crack growth rate in the design of asphalt overlay against reflective cracking.

The influence of the most important parameters on the service life of reinforced asphalt overlay with geogrid materials in bending mode was examined by employing the Taguchi method and analysis of variance techniques. The objectives of this experiment was to investigate the effects of grid stiffness, tensile strength, coating type, amount of tack coat, overlay thickness, crack width and stiffnesses of asphalt overlay and existing asphalt concrete on propagation of the reflection cracking. Results indicate that the stiffnesses of cracked layer and overlay are the main significant factors that can directly improve the service life of an overlay against the reflection cracking. Generally, glass grid is more effective in reinforced overlay than polyester grid. Effect of crack width of the existing layer is significant when its magnitude increases from 6 to 9 mm.

Numerous factors affect long term resistance of asphalt mixtures against stripping, therefore, proposing a mathematical model between the stripping failure and these factors is essential. The rehabilitation time of pavements, a very pivotal issue, can be predicted if the developed model comprises time as an independent factor. Apart from the capability of such model for prediction of rehabilitation time, the proposed model reveals the effect of each individual factor on the stripping process and it discloses the interrelationship between the pertinent factors. In this study, Response Surface Methodology (RSM) was successfully employed to establish the time dependent models between Tensile Strength Ratio (TSR) as the response parameter and independent factors such as time and anti-stripping additives (namely hydrated lime and Zycosoil). The results obtained through the modeling showed that both the short and long term performance of the Zycosoil is superior to the hydrated lime. The proposed models can be solved to find the time that asphalt mixtures take to reach certain terminal TSR; e.g. for TSR = 80. A mathematical model is developed that can predict the proper time of rehabilitation of pavement before stripping failure.

Stripping – the most important mode of Hot Mix Asphalt (HMA) failure – is a problematic issue in the pavement performance and its maintenance. Identification and quantification of the pertinent parameters influencing on stripping process is a prerequisite for dealing with it effectively. Previous studies on stripping potential of HMA have focused on this issue based on empirical/semi-empirical models. In the present work Response Surface Methodology (RSM) was used for the study and optimization of pertinent factors, namely bitumen content employed, grading and lime content, in the stripping process of HMA. Tensile Strength Ratio (TSR) was chosen as the response for the evaluation of stripping potential of HMA using a Central Composite Design (CCD). The statistical analyses showed that second order polynomial models can successfully describe the relationship between the response and factors and the maximum Tensile Strength Ratio (TSR) is achieved at 5.73% bitumen content, 1.84% lime content and 65.30% of the materials passing sieve size 4.75 mm.

This study was to compare the relative ability of seven common fitting models, i.e. Pure Power Law (PPL), Generalized Power Law (GPL), Modified Power Law (MPL), Modified Power Law Series (MPLS), Standard Sigmoid (SS), Generalized Logistic Sigmoid (GLS) and Prony Series (PS), to construct the tensile relaxation modulus master curve of dense graded asphalt mixes. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with three replicates. Direct tension relaxation modulus tests were conducted on the specimens at four different temperatures using the trapezoidal loading pattern at a low level of input strain. Tensile relaxation modulus master curves were constructed using all the fitting models together with the numerical shifting technique. Finally, both the graphical and statistical comparisons were made among the fitting models, and the best one was found to be PS, followed by MPLS, GLS, MPL, SS, GPL and PPL.