فصلنامه مهندسی حمل و نقل
پیاپی 3 (بهار 1389)

Foam bitumen technology has been used in cold mixes in order to reduce bitumen viscosity and to ease mixing of bitumen with cold aggregates. Recently, a new generation of mixes has been developed using foam technology providing the same quality of mixes similar to conventional hot mixes. These are normally produced at lower mixing and compaction temperatures than those of similar HMA. However, there is no proper standard mix design method for these mixes. Regarding to reduce mixing and compaction temperatures, the main challenges of using warm foam mixes in lieu of HMA are their greater moisture susceptibility and rutting potential due to less adhesion properties of foam bitumen to aggregates. In this research, the method of adding two different bitumen types at two stages was adopted. At the first stage, a soft bitumen was added to warm coarse aggregate particles. In the next step, a hard bitumen transformed into foam and was added to the total mix. Warm foam mixes were produced at different mixing and compaction temperatures and their Marshall Parameters were determined and compared with similar HMA. Results indicated that it was possible to reduce mixing temperature of warm foam mixes 30°C below those of similar HMA. The air void content was recognized to be the most effective parameter affecting mixing and compaction temperatures. For investigating the moisture susceptibility of warm foam mixes, indirect tensile test was performed on dry and soaked conditioned samples. The testing results indicated that the moisture susceptibility of warm foam mixes was greater than the specified minimum values of the similar HMA. However, adding anti-stripping agents could compensate this deficiency appreciably. Wheel tracking test results also showed that despite of the larger Marshall Flow values of warm foam mixes compared with similar HMA, by adding hydrated lime powder, it was possible to achieve the same rutting values as for those in HMA. As a result of the above testing results, a mix design method was proposed for designing warm foam mixes.

The undeniable advantages of railway track in comparison to the other modes of transportation have been caused to development of new railways as well as rehabilitation of the old routes all over the world. On the other hand, one of the major concerns in construction of new railway lines and rehabilitation of the old ones on the loose subgrades are arisen from bearing capacity enhancement and control of subgrade settlement in tolerable level. Amongst the various proposed stabilizing methods for railway subgrades including mechanical and chemical methods, utilizing of geosynthetic materials has been grown during the recent decades. Although the considerable amount of positive effects of using of such artificial material will be certainly implemented in increasing ofrailway track stiffness, the stiffness measurement of the stiffened track with geotextile and geogrid material has been investigated in the current study. For this purpose apart of under construction railway route of Ahwaz-Khoramshahrwhich is located on loose clay siltysubgradehas been selected for field test program. In the mentioned railway line, four different sections with 25 meter length have been selected for stiffness measurement. These four sections have been included of reinforced embankmentwith Geotexstile,Geogrid, Geotexstile/Geogrid and normal embankmentfor comparison purposes. In order to measure the track stiffness in the mentioned sections, the track has been loaded byballast container wagons in full and empty conditions.The measured rail deflections have been substituted in Talbot's equation and the track stiffness in various sections have been calculated and compared consequently.In another part of the study a three dimensional FE model have been developed using PLAXIS 3D code and verified by using the av ailable field test data. The sensitivity analysis on geogrid axial stiffness EA in the model has been shown a 31% increase in track stiffness in the best condition.

A large number of traffic accidents are due to the human errors. Advanced driver assistance systems can reduce human errors and thereby the number of accidents. An important issue in developing such systems is the use of a proper warning strategy, which warns the driver when the situation is critical and immediate reaction is required. Although time-to-collision (TTC) has been recently suggested and used, as a cue for decision-making in ADAS, but this indicator is unable to detect potential rear-end collisions. On the other hand, short time headway (THDW) during carfollowing period indicates that there is a high potential risk of collision, as soon as the lead vehicle starts braking. In this paper, a new safety indicator, urgent deceleration index (UDI), is developed. UDI can detect critical situations a few seconds sooner than TTC giving the driver more time for reaction planning, and hence providing a better margin of safety. To compare the sufficiency of UDI, TTC and THDW, as safety indicators, percent time spent unsafe, during following period is computed per lane using the detailed data gathered in the NGSIM project for I-80 freeway. Results indicate that the percentage frequency of time in rear-end conflict is 3, 63 and 76 percent, based on TTC, UDI and THDW, respectively. Also, the average correlation coefficient of critical situations based on UDI and THDW is about 0.644. In all, results indicate that UDI is more capable than TTC in detecting rear-end conflict.

Different driver's perception in routing is one of motivations to different driver's routing models. Drivers routing are usually based on undeterminstic general costs of available routs which the fact of uncenteainity is the background of logit models. Independence of Irrelevant alternatives is the core of logit models which is not satisfied in multinominal logit models, therefore extended logit models have been extended to overhelming this problem. Great mathematical literature review of different logit and extended logit models have been investigated in this paper with its methodlogy and usage on unic Allsop and Charlsworth proposed network. Results of these various logit models have been obtained based on proposed procedure and there are evidences that various logit models tend to have same result in low and also high(about twice of low) demands. Between these levels of demand there are different results by each of these models. Beside proposed algorithm for different logit model's applications, the chance to recognize their effect on link volumes and use of volume-based link-cost models are the distictive effort of this paper. The main innovation of the paper came out by taking into account the intersections effects on rout costs and their delays as a cost to network's performance index.

In the last two decades, decreasing wear rate beside of increasing axel loads and trains velocities have changed the main failure of steel wheels from wear to rolling contact fatigue (RCF). RCF causes surface and subsurface cracks. The former usually disappeared contrasting with wear while the latter propagates and brings about deeper and more dangerous fractures. In any maintenance process of railway systems, having criteria of threshold and critical crack length is a vital tool to determine possibility of propagation and critical condition of identified cracks in wheels. In this paper, stress intensity factor range of a subsurface crack has been calculated by FEM analysis of monobloc R7T wheel in contact with UIC60 rail. Then, threshold and critical length of subsurface crack in the wheel of Iranian railway system have been determined based on fracture mechanics and some approximate relations about mechanical properties. Findings show that a subsurface crack longer than about 3 mm can gradually propagates and before reaching the critical length can make the wheel fracture because of geometrical limitation of the wheel.

Using Inter-city bus system with regard to high flexibility and low cost for the users, especially in large cities and in developing countries, is inevitable. So transportation system improvement is necessarydue to its range of activity. One of the ways in improving this system is optimizing the allocation of buses to active lines that compared with present status, carries more number of passengers and also its transporting cost will be reduced. In this study, a new approach for optimizing the allocation of buses to the existing bus network lines is used. In this method, a model is employed which can determine the number of required buses for line by considering various parameters. For this purpose, artificial neural networks were used. Artificial neural networks due to the ability of learning with examples can be used for non-linear modeling in which exact solution is hard to achieve. After completing the training process, network performance was investigated and when satisfying the constraint conditions, it couldsimulate new cases. In this investigation, the optimized network (NN 11-13-1) with maximum R-value (R=0.996) and minimum squared error (MSE=0.553) was used. Finally, the model obtained on thebus network in Mashhad was tested. The results obtained using neural networks in comparison with regression shows its high level of accuracy which is a valuable approach.

Bitumen aging is an irreversible process that reduces deformability and increases hardening and stiffness. Aging process is due to changes in type and size of very widely and different molecules of asphalt cement. It is considerable at two stages: short-term aging and long-term aging. In this paper a regression model is proposed based on LTPP data to predict complex shear modulus (G*) and phase angle (δ) of bitumen during service time. This model is prepared from ten different types of bitumen data. Using proposed model, by just one DSR test on origin bitumen at a temperature, we can determine DSR results on aged bitumen at each time and each temperature. Model show the most effective time for aging process is earliest three years. Hardening due to aging at temperature under 25oc is clearly but at upper temperature there is no significant different between original and aged bitumen. The model accuracy for prediction of long term bitumen performance and for use in pavement distress models is in good.