مجله مهندسی زیر ساخت های حمل و نقل
پیاپی 1 (بهار 1394)

It is well known that stripping (separation of aggregates and bitumen) is one of the reasons for asphalt-mixture failure in humid conditions. This could be due to moisture diffusion into the asphalt mixtures that is related to bond weakness between aggregate and bitumen in the asphalt mixtures, and consequently, damages the pavement surface. There are several standard laboratory methods for the evaluation of stripping resistance of asphalt mixtures. The modified Lottman indirect tensile test and determination of the TSR parameter is a common method for evaluating stripping resistance of asphalt mixtures. In this study, a method based on image processing is presented to determine the stripping resistance of asphalt mixtures, and a stripping parameter is developed as an indicator for assessing the moisture susceptibility of asphalt mixtures. The specimens in this research were prepared by mixing limestone materials, slag and hydrated lime filler. Comparison between stripping parameter and TSR values showed that the proposed method has sufficient reliability and is suitable for determining the stripping resistance of asphalt mixtures.

Stripping is one of the major distresses that occur in asphalt pavements. This will then results in complete pavement disintegration. Various studies have shown that moisture damage results in some 25% decrease in mix modulus and 60% increase in wheel tracking; plus 30% reduction in fatigue life of HMA mixes. Researchers have shown that hydrated lime, if used in the amount of 1 to 2%, is one of the most effective anti stripping additives in reducing moisture susceptibility of asphalt mixes. This research aimed at studying stripping resistance of asphalt mixes containing various amounts of hydrated lime. The testing method and the testing temperature were the two key factors that were considered in the research. The aggregates, selected from a quarry in west of Tehran were of siliceous type and the binder was a 60-70 penetration grade from Isfahan Refinery. Mixes containing 0, 1, 1.5 and 2% hydrated lime were prepared and were compacted. The testing program consisted of performing boiling water (ASTM D-3625), indirect tensile (AASHTO T-283) and wheel tracking (AASHTO T-324) tests. In addition, Marshall Quotient (ASTM D-1559) of the various mix compositions was determined. The tests were performed at two temperatures of 40 and 60oC. The testing results were analyzed using MiniTab software. These, performed both at dry and saturated conditions, resulted in determination of the optimum amount of hydrated lime in mixes. In addition, it was shown that at increased temperatures moisture susceptibility of mixes were increased.

Since proper functioning of the transportation system is essential in times of crisis, correct design and accurate prediction of earthquake-induced permanent displacement of embankments and trenches have been considered by many engineers. Different methods are available for estimating earthquake-induced permanent displacement of embankments and trenches in roads, such as pseudo-static, numerical, analytical and semi-analytical. Due to its simplicity, Newmarkian rigid block analogy has received considerable attention from geotechnical researchers and practitioners. Since the conventional Newmarkian analogy has many limitations, conservative and non-conservative estimations of sliding displacement would be possible. Therefore, researchers have proposed many modifications to this method. One of these limitations is the effect of rotation. In fact, it is anticipated that the downward rotational (stabilizing) movement of the soil mass, can significantly affect the yielding acceleration of the presumed slip surface. In this paper, keeping in mind the effect of rotation, the conventional formulation of the Newmark approach is modified numerically. The results are presented and compared for several conditions including the conventional rigid block, decoupled assumption of sliding and slope response, coupled consideration of sliding and slope response, and decoupled assumption with the effect of sliding block rotation. According to the results of this study, the period ratio (ratio of natural period of slope to the mean period of input motion) and the length of slip surface, can significantly affect the permanent displacement of sliding mass.

Assessment of the effects of shockwaves caused by explosions near strategic structures such as roads, tunnels, bridges and railways has always been of high importance. The blast loads on structures are dependent on extra atmospheric pressure around the structures. Studies reveal that because of the natural properties of the waves, their behavior is affected by the geometry of the wave propagation domain. These considerations are not included in traditional methods of determining the forces due to blasts on structures. The technical buildings of railways and roads are mostly built in mountainous regions. If an explosion occurs around these structures, the blast waves will be scattered near topographic irregularities and the resulting extra pressure should be considered. Due to reflection of the blast waves near the topographic irregularities, it is not wise to apply common methods of determining forces resulting from explosions imposed on structures to evaluate bridges and embankments in the vicinity of topographic irregularities. The investigations reported in this paper show that this extra pressure imposed on technical structures of roads is much greater than that of flat regions. Furthermore, results demonstrated that with increasing the amount of explosives and decreasing the width-to-depth ratio of the valley, the extra pressure resulting from explosion and its reflection increases as well. Additionally, it was observed that even the applied pressure pattern near the topographic irregularities differs from that of flat regions.

Slab tracks have several benefits respect to traditional tracks especially in high speed tracks. It is necessary to investigate the vertical stiffness effects of railway tracks on dynamic behavior of slab tracks, because of high importance of vertical displacement of railway tracks and also its relationship with stiffness of railway track. In this paper, sensitivity of the railway vertical displacement respect to vertical stiffness is studied with modeling and numerical analyses of railway structure. The suitable range of railway vertical stiffness can be evaluated by considering the allowed values of the railway vertical displacement and the obtained sensitivity analysis diagrams. In this innovative method, maximum and minimum values of railway stiffness are calculated and presented in graphs and tables by using the results of the analyses. According to the obtained results and the allowed values in UIC, the suitable vertical stiffness diagrams are presented and their numerical values are recommended for design of slab tracks.

The objective of this research is to investigate the effects of coal-waste powder as a replacement for the conventional filler material on the mechanical properties of hot-mix asphalt. In this laboratory research, using coal-waste filler, specimens with 0%, 25%, 50%, 75% and 100% filler as replacements for the limestone powder were prepared. The performance of hot-mix asphalts containing coal-waste powder was evaluated using Marshall Stability and indirect tensile strength tests. Marshall Stability, liquidity, tensile strength and durability of asphalt mixtures were determined and the effect of mixture of coal-waste and limestone powders on mixture characteristics was investigated. The coal-waste powder was then burned at 750 oC to produce coal powder ash. This ash was then used as filler in hot-mix asphalt and compared with other fillers. The results revealed that coal-waste powder had superior effects compared to limestone powder and, therefore, could be substituted for conventional filler according to performed tests. Moreover, burned coal-waste powder achieved better results than other tested fillers in this research.

One of the limitations to implementing roller compacted concrete (RCC) pavement in Iran is lack of proper surface texture for high-speed traffic. One method to overcome this issue is to apply a thin asphalt surfacing on RRC pavement. This wearing course is unable to contribute significantly to the increase in stiffness of the structural system; however, if the composite slab system is treated as a beam in bending, the effect of asphalt surfacing could be calculated directly by assessing its effect on the neutral axis. The shift in the neutral axis is translated to a reduction in the concrete stress for a given bending moment as compared to the no-surfacing condition that, consequently, could reduce the concrete slab thickness. In this paper, the finite element method has been used to evaluate the reduction in the thickness of the concrete. In doing so, thin asphalt wearing course with conventional thickness (4 and 5 cm) on RCC pavement of 20 to 25 cm thickness was modeled by using the computer code "ABAQUS". Analysis of vertical stress and tensile strain of concrete substrate showed that reduction in the concrete thickness depends on thicknesses of the concrete pavement and thin bituminous wearing course. Average thickness reduction was calculated as approximately 10% to 15% of the concrete thickness.