مجله مهندسی زیر ساخت های حمل و نقل
سال دوم شماره 3 (پیاپی 7، پاییز 1395)

Tie lateral resistance is one of the key parameters ensuring track lateral resistance and track stability in horizontal plane. Single tie push test (STPT) is the most common method to determine the static lateral resistance (SLR) of a single tie in ballasted tracks. The most important weakness of this test is evaluation of the SLR. Since the transferred force by train wheel to the rails has a dynamic nature, evaluation of SLR by STPT seems questionable; hence, for obviating the mentioned deficiency in STPT, in the present study, an innovative pendulum loading test device (PLTD) was designed and developed for assessing the dynamic lateral resistance (DLR) of single tie. By using the PLTD, a hammer is released with adjustable weights and triggering angels to impact on lateral side of sleeper and consequently, the tie lateral displacement as well as its acceleration are recorded. To prevent concrete tie breakage due to hammer impact, an elastomeric pad is attached at the end of tie. The tests’ results with different impact weights and triggering angels are presented as load-displacement diagrams corresponding to real wheel load as a design tool. Also, the results of this study were compared to some other STPT tests under similar conditions. The results of this comparison showed that despite constant lateral resistance of 6.5 kN in the STPT test, the corresponding force of 2 mm displacement in pendulum test varies from 2 to 31 kN. Finally, it was proved that the DLR/SLR ratio is in the range of 0.3-4.9, which can be used for practical purposes.

Today, the construction and development of low cost and smart concrete pavement is considered. Smart concrete pavement provides the ability to measure of load and damage detection at the same time when it withstands under the traffic loading. concrete sensor which is manufactured by mixing conductive fibers (such as carbon nanotubes (CNTs)) with concrete can measure the moving vehicle loads or detect damage propagation by variation in electrical response from two heads of sensor. Two main factors affecting the performance of concrete sensors are the amount of CNTs and their dispersion quality in the mixture with regard to the combined effects of the surfactant composition and content and of the CNTs dispersed with different level of energy. in this research different concrete sensors containing different amounts of CNTs (0.1,0.125,0.15,0.2) percent of cement weight with different levels of dispersion energy levels (one hour of ultrasonic bath, two hours of ultrasonic bath and one hour of ultrasonic bath with 90 minutes of probe ultrasonic) are manufactured. The goal of this study is to evaluate the effects of the main parameters affecting concrete sensor performance using various criteria in dynamic loading regime such as sensitivity of the sensor (Se), standard deviation of the prediction error, repeatability, cross-correlation and hysteresis (SSE). the results shows that, the higher energy levels in dispersion of CNTs in aqua phase lead to proper dispersion in cement phase and this parameter is more effective than the amount of CNTs to improve the concrete sensor performance. Also, in a constant energy level, with an increase in carbon nanotubes, sensor performance improves.

Convenient and efficient design of tunnel ventilation system for increased safety in case of fire is required. One of the main purposes of designing a ventilation system is fire and smoke flow control to create a safe passage to escape the travelers. Longitudinal ventilation system, using jet fan and axial fan, is most commonly used in tunnels. But, efficiency of longitudinal ventilation system decreases with increasing the length of the tunnel. In this paper, the effect of extraction shaft combined with a longitudinal ventilation system, in emergency situations, in a relatively long tunnel, has been studied. In a ventilation system with an extraction shaft, the aim is limiting the flow of fire and smoke in the distance between the fire source and the shaft. In this paper, FDS code, which is a computational fluid dynamic model, was used to study the smoke flow and temperature distribution in the tunnel. The effects of changes in parameters such as flow rate of the output shaft, input speed, slope of tunnel, shaft dimensions and heat release rate on temperature distribution under the tunnel ceiling was investigated. Results showed that efficiency of the longitudinal ventilation system using extraction shaft was increased and change of the tunnel slope, and its corresponding change in the direction of smoke flow in the tunnel, was under control. The results also revealed that increasing the outflow cross section of the ventilation shaft does not have much effect on the system performance. Considering this parameter, different scenarios could be defined and possible control of fire and smoke flow could be created

Nowadays, the use of recycled waste materials as modifier in asphalt mixtures could have several economic and environmental benefits. This research evaluates the use of waste polyethylene terephthalate (PET) in hot mix asphalt mixtures. For this purpose, plastic bottles were cut to 10×2.5, 20×2.5 and 30×2.5 mm pieces and were added to the mixture, in dry conditions, as 0.25, 0.5, 0.75 and 1 % of the aggregates weight. In this research, performance properties of the modified asphalt mixture were evaluated through several experimental tests such as:Marshall Stability, resilient modulus, moisture susceptibility and dynamic creep test. Results indicated that addition of PET to the asphalt mixtures improved the resistance to rutting, but also reduced Marshall Stability and tensile strength of the mixtures. Also, it was revealed that PET fibers with 10 mm length and 0.25, 0.5 and 0.75 % substitution weight have better results than other mixtures.

This paper investigates the details of two-dimensional finite elements modeling of geogrid mechanically stabilized earth wall in plain strain conditions. In this research, Mohr-Coulomb model was used for backfill soil, and geogrid layers were modeled as an isoperimetric linear elastic material. Measured data from an instrumented wall in Tucson, AZ, USA, were used to verify the model. Horizontal strains in geogrid layers and horizontal and vertical strains in individual elevations of backfill soil were obtained from the software. Results were in good agreement with the reference ones. Then, the finite elements model was used to investigate the effects of length and location of geogrid layers on wall deformation and loads in reinforcement layers. Based on the results, an economic design can be successfully performed by changing the length and location of geogrid and this is only possible by finite elements analysis. Moreover, it was achieved that lower geogrid layers had the highest effect on displacement of wall and designers should use more layers at the bottom of the wall. This research showed that finite elements method is an appropriate approach for modeling of MSE walls, predicting of their behavior and resolving defects of the measured data from installation of sensors.

Factors such as bitumen volatile materials’ evaporation and oxidation during mixing with stone grains, its transport to the project site and also during pavement servicing will result in bitumen aging and could lead to some failures in asphalt pavements. As a consequence of aging, the bitumen will become brittle and some cracks can be observed on the pavement which lowers the driving quality and increases the repair and maintaining costs. Rutting is also one of failures which will result in decrease of travel safety, and increase of roads’ maintenance costs. Knowing about the characteristics of asphalt composition (bitumen and stone material) could be useful in design of pavements with longer life. Regarding this subject and also this fact that a few studies have addressed the effect of mixture’s physical properties on the resistance parameters of asphalt, in this study, the impact of stone material’s type on creep abilities of aged warm asphalt mixture is investigated. In this context, asphalt samples with two types of limestone and silica stone materials were fabricated and put under aging trend based on standard methods. Then, the samples underwent dynamic creep test in 100 and 300 kPa and temperature of 50 and 60 oC and their stable strains were measured. Finally, the results were analyzed using MATLAB software and an empirical model for creep behavior of aged asphalt concrete with stone material type consideration was presented. The results showed that lime samples, in comparison with silica samples, are more resistant against time-dependent deformation.

In order to reduce the cost and time of seismic analyses, as well due to lack of deep understanding of structural characteristics and the acting forces, usually, simplified assumptions are applied in structural modeling that could lead to unrealistic responses. Some of these simplified assumptions in bridge modeling are ignoring the pounding phenomenon and soil-structure interaction. Therefore, in the present study, by modeling three 3-span bridges, with different periods of frequency, the pounding effect, which happens in the gap between two parts of the deck and/or the deck and the abutments, on seismic responses of bridges under the near and far field accelerograms, was investigated. Also, the effect of dynamic properties of bridge and soil-structure interaction on pounding phenomenon was studied by calculating the parameters of maximum pounding force and number of impacts. Results showed that considering the pounding effect reduces the maximum displacement due to limiting the movements of the deck such that some of the piers remain within the elastic phase. Minimum and maximum reduced-displacements in different bridges were 31% and 48%, respectively. On the other hand, modeling the soil-structure interaction effect increased the values of maximum displacement (40%) and maximum pounding force (45%), as compared to that of fixed-base model. Also, the analysis of results in terms of the type of earthquake indicated that the near field earthquakes lead to greater responses in the bridges than the far field earthquakes.