مجله مهندسی زیر ساخت های حمل و نقل
سال سوم شماره 2 (پیاپی 10، تابستان 1396)

Pavement rehabilitation can be accomplished using a variety of methods among them asphalt overlay is the most common ones. Reflective cracking occurs on the overlay surface after a short period of time, which is the most important distress annually threatening the durability of new overlays. Usually, reflective cracking starts from the base layer and continues towards the overlay. Surfacial cracking also causes infiltration of water, salts and other harmful materials which increases the speed of deteriorations. Stress Absorbing Membrane Interlayers (SAMI) can be used to postpone the reflective cracking in the overlays. Nowadays, sand asphalt is a common mixture to be used as a SAMI material. In this research, the performance properties of a sand asphalt interlayer, in an unmodified condition, as well as a crumb rubber modified (CRM) sand asphalt interlayer was evaluated. Indirect tensile strength was selected as an indicator of asphalt layer resistance to cracking. Also, by using the above test, the fracture energy of sand asphalt interlayer was determined and then was compared to that of regular dense graded asphalt used as an overlay. Modified Lottman test and Texas boiling water test were conducted to evaluate the moisture susceptibility of sand asphalt mixtures. A wheel-track test was also performed on the selected samples to evaluate the rutting potential of sand asphalt interlayer mixtures. The test results showed that sand asphalt mixtures can stand higher levels of energy before reflective cracking. Sand asphalt mixture using crumb rubber modified binder was also shown to be more resistance to moisture induced distresses. The wheel-track rutting test results also showed that although the rutting potential of pavement section using an overlay over an unmodified sand asphalt mixture is much higher than the single overlay section, but this can be rectified using a CRM sand asphalt mixture as the interlayer.

To improve the performance and durability of hot mix asphalt against different kinds of distresses, various additives are used. Among these additives, using crumb rubber has showed to have positive effects on increasing the resistance of ashalt mixtures against fatigue and rutting. Also, the use of Sasobit has been effective on decreasing the temperature of mixture and compaction and therby decreasing the aging during the production stage. The purpose of the present study was to investigate the single effects of rubber powder and Sasobit as well as the combined effects of these two additives on parameters such as Marshal stability in dry and wet states, fatigues at different levels of strain, rutting and resilient modulus. Different mixtures were prepared individually with rubber powder (10, 15 and 20 percent) as well as Sasobit (1, 2 and 3 percent) and then mixtures with optimal amount of each additive resulting from different mixtures wer prepared and underwent relevant tests. Results showed that adding the rubber powder and Sasobit, individually and combined, led to an increase in the resistance parameters. The Marshal ratio in dry state (1 and 10 percent) and in wet state (1 and 15 percent) for sasobit and rubber powder, respectively, reached its maximum level. In the beam fatigue test and rutting, the optimum percentages of sasobit and rubber powder were found to be 2% and 15% and the rutting percentage in 2% and 3% sasobit was similar. Increasing the resilient modulus with different amounts of additives was not significant. The highest impact of these two additives was on the fatigue resistance and rutting parameters. The effect of concurrent use of these two additives on increasing fatigue resistance and rutting was remarkable.

Back to back mechanically stabilized earth walls (MSEWs) are the structures usually used in two sides of bridge abutments and ramps. Although this type of MSEW is commonly constructed, but limited number of design guidelines are available in this regard. According to the FHWA recommendations, if the ends of reinforcements at two sides are close enough to each other, the walls are considered as back to back for the design practice; otherwise, each wall is designed independently. In this study, considering a fixed width and height for the walls, variations of the length and vertical spacing of the geogrids, as well as friction angle of the soil, were evaluated through 196 different models built and analyzed using FLAC 2D. The critical slip surface was compared in normal and back to back conditions using limit equilibrium method. Results of modeling include lateral earth pressure behind the wall, maximum force mobilized in geogrids and lateral displacements of the wall for different cases of the study. Accordingly, it was found that maximum lateral displacement of the wall usually occurs in 0.35 to 0.57 of the wall height. Based on the maximum horizontal displacements, the recommended boundary for the two different mechanisms of MSEWs was approved. Using the maximum lateral displacements, the change in wall behavior was studied and a non-linear model with the form of power-law was developed to estimate maximum lateral displacement of the wall with respect to the governing parameters. Coefficients of determination of the analysis results for normal MSEW, back to back MSEW and both cases were calculated as 0.953, 0.969 and 0.947, respectively. This confirms the suitable accuracy of the proposed formulae to estimate maximum lateral displacements as well as the appropriate exhibition of the governing physical trends.

Roads and transportation infrastructures are national assets that play important role in the development of social welfare. During the construction process, maintenance and the whole life cycle of pavements, huge amounts of natural materials, fossil fuels and electricity are consumed to generate energy, which have deteriorating effects on the environment. Therefore, it is necessary to use management tools and methods to manage the use of natural resources, energy consumption and the effects on the environment in an optimal manner. In this respect, this study uses the life cycle assessment as a tool for quantifying and measuring energy consumptions and global warming potentials at some stages of the life cycle of asphalt pavements. This assessment was applied on four stages of pavement life-cycle, including mining and production of raw materials, hot mix asphalt production, transportation of materials and pavement execution, considering separately the electricity and fuel consumption, and Iran’s electricity situation. Energy consumption and production of greenhouse gases at each stage were calculated based on the collected data and previous studies. Results of this study showed that the hot mix asphalt production stage in the asphalt factory has major effect on the environment with a share of 48% in global warming potential. Also, results of the sensitivity analysis on transport distances showed that variations of materials-transport distance can have an important role in the life cycle assessment results.

In the last recent decades, the rail transport system for transit of humans has been expanded as one of the most convenient and low-cost transportation systems. As one of the most important elements of the railway lines is sleeper concrete, thus, the issue of operation and maintenance of this system has special importance. In this research, concrete strength has been improved by simultaneous addition of steel and polypropylene fibers to sleeper concrete (manufactured at Sleeper Manufacturing Factory in Karaj, Iran). A total of 120 cylindrical and prismatic samples were constructed for testing compressive, tensile, and flexural strength in eight different designs. Total percentage of fiber used in each design was 1%. According to the results, the best design for compressive and tensile strength was design number 8 (1% steel fiber, 0% polypropylene fiber) and for flexural strength was design number 6 (0.6% steel fiber, 0.4% polypropylene fiber). Presence of fiber in sleeper concrete can reduce its pre-stressed reinforcement by 40% and prevent occurrence of deep cracks.

The aim of this study was to determine the effects of uncertainty of incident angle on maximum values of engineering demand parameters (EDPs) in highway bridges. Thus, 80 ground motion pairs (40 far-field and 40 near-field ground motion pairs) are imposed on numerical models of a set of skewed bridges (0 to 60 degrees) in 12 various directions (0 to 180 degrees) by performing non-linear dynamic analyses. The components of fragility curves were obtained and compared to the results of the imposed excitations in the principal directions. The difference was up to 65%. Also, using rotation of response axes, the created demands in different directions (0 t0 360 degrees) were calculated to investigate the effect of rotation of response axis on bridge reponse. Results showed that there is 39% between excitation intensities in 50% failure probability in bridge column, when excitation is in different directions on the structure and the response axis is rotated to record maximum response, and the case where direction of imposing the excitation and recording demand correspond to longitudinal and transverse directions of the bridge. Also, this difference has direct relationship with increase of skew angle of the bridge.

As time went on, ballasted tracks have been replaced with slab tracks due to the extreme limitations of ballasted tracks (e.g., speed and capacity limitations) and high costs of repairing these tracks. But, by using the slab tracks, new dimensions have been presented in dynamics of these lines. The aim of this research is to study the effects of dimensional and mechanical parameters of the track components (i.e., thickness and elasticity modulus of layers) on vertical dynamic response of a three-dimensional model using finite element method. The Rheda 2000, as one of the most widely used tracks in the world, is investigated in this study. The finite element analysis is conducted based on two categories: frequency analysis and steady-state dynamic analysis. The natural frequencies and corresponding vibration modes are extracted by using frequency analysis. Also, frequency response functions of the track are extracted by using steady-state dynamic analysis. Results showed that by decreasing the elasticity modulus of track components, the resonance frequencies decreased and the vertical frequency responses of the track increased. Moreover, increasing the thickness of layers led to the increased resonance frequencies and the decreased vertical frequency response of the track. The elasticity modulus of hydraulically bonded layer and the thickness of concrete bearing layer in Rheda 2000 are the most effective parameters on dynamic behavior of this slab track.