مجله مهندسی زیر ساخت های حمل و نقل
سال سوم شماره 1 (پیاپی 9، بهار 1396)

Asphalt pavements expose to failure due to various reasons. Moisture failures are the most crucial failures so far. Numerous methods have been carried out to increase asphalt mixtures resistance, one of which is using additives in bitumen or asphalt. This paper intends to determine the effect of amorphous carbon powder on HMAs in order to decrease moisture susceptibility. At the beginning, optimized mixing method of bitumen and carbon powder has evaluated by contact angle test. Carbon powder is utilized in 2 different ways; as additive to bitumen (in weight percentages of 5, 10 and 15) or directly replaced by filler in 25, 50, 75 and 100 percent of mass of filler. Following tests were conducted to study and understand the effect of this powder; proportion of tensile strength in wet and dry conditions based on AASHTO T283, and boiling water test in ASTM D362d. The results of both tests indicate that amorphous carbon powder play an effective role in decreasing the moisture susceptibility of asphalt mixtures. Moreover, replacing this powder by filler shows a much better performance compared to bitumen. Amorphous carbon powder because of its hydrophobicity can be effective to avoid water penetration into asphalt mixture and decrease its moisture susceptibility.

Changes in surface and subsurface moisture content is one of the devastating factors affecting on evaluating performance of soil behavior such as erosion, swelling, shrinkage, differential settlement, freeze and thaw cycles, liquefaction phenomena, landslides and environmental contamination. In the present laboratory research, stabilization of two artificial soil samples with different combinations of sand and low plasticity clay, with 3, 5 and 7 percent of hydrated lime, was investigated and compressive strength of the stabilized specimens after a curing period of 28 days, on the basis of standard proctor compaction, under saturated, unsaturated and dry conditions have been estimated by unconfined compressive strength tests. The filter-paper method, as a measure of equilibrium moisture content and solution of computing the suction, has been selected for estimation of inherent characteristics of soil suction in the presence and absence of lime addition in the unsaturated condition. Also, hydraulic conductivity of the stabilized and cured specimens for 7 and 28 days was measured. Results of the tests during a drying cycle hysteresis, demonstrated that matric suction of clay soil samples was about 11 times of sandy soil samples, and the lime content intensified pore water retention and estimation of soil suction. The stabilized clay and sandy soil samples lost their average unconfined compressive strength by 87 and 83 percent, respectively, because the lime adhesiveness became unstable in the presence of water. Despite the expected results, factors such as heterogeneity in the fabric of soil aggregates during stabilization, weakness of lime adhesion and changes in the gradation situation from fine-grained to coarse-grained particles during the addition of lime, led to permeability increase over the 7- and 28-day curing period.

Analysis of the pavements and their ingredients has always been important due to a better understanding of their behavior under different conditions and leads to better understanding and providing more accurate relations. Due to the extent of asphalt mixture application in the world, assessment of different behaviors of these mixes is very important from the various aspects of performance and safety. Given that the asphalt mixes are inherently very sensitive to temperature changes due to bitumen content, identification and analysis of the viscoelastic and visco-elasto-plastic behavior of the mixes is of particular importance. This research aims at performance evaluation of Burgers model and anew proposed model based on genetic programming techniqe in estimating visco-elastic behavior of asphalt concrete. For this purpose, a number of dynamic creep tests under various temperatures and different stress levels were done. Results showed that performance of the new proposed model based on genetic programming techniqe is quite satisfactory. Also, the new proposed model will help more researchers, willing to perform similar studies, without carrying out destructive tests.

In recent decades, using recycled asphalt materials in pavements is considered by researchers due to their economical costs. The purpose of this laboratory experiment is to introduce optimum mix design including suitable grading limits of the mixture, moisture formula for estimation of the amount of water needed for compaction and finally investigating the effect of reclaimed asphalt pavement (RAP) on unconfined compressive strength (UCS) of different cement-treated base mixtures and determination of optimum RAP content and cement with lowest construction cost. For this purpose, first, grading properties of different materials and RAP were determined and proper grading was selected. For studying the compaction characteristics and determining optimum moisture content, 12 different mixture designs, as a combination of 4 RAP percentage (0, 40, 60 and 80) and 3 cement content (3, 5 and 7 percent) were constructed. Then, 15×30 cm cylindrical samples were constructed to investigate UCS properties at optimum moisture content. Results showed that grading limits in Iran's Road Bulletin No. 101 could be the main reference for cement treated base (CTB) mixtures containing RAP materials. Also, modeling results of optimum moisture content indicated that by adding more recycled asphalt materials to the mixture would increase optimum moisture content and reduce maximum dry density. The UCS tests undertaken on 7-day cured samples at 25 0C indicated that by increasing the RAP materials to the treated mixture would decrease UCS of the samples. The pavement containing 68% of reclaimed asphalt and treated with 5% cement was the most economical mixture considering the 3.8 Mpa limit of UCS index. Finally, it could be expected that instead of compacting the samples with standard hammer, 60 seconds of vibration compression may be used

In recent years, vast extent and severity of permanent deformations has created some concerns regarding the effects of this type of failure on performance of asphalt pavements. To investigate the rutting of asphalt mixtures, it should be noted that these mixtures are multiphase combination of aggregates, binder and voids. Arrangement of aggregate particles (aggregates skeleton) in the internal structure of hot mix asphalt has an important role in mechanical performance of the mixture, especially on its rutting resistance. In most regulations for asphalt mixture's design, only macroscopic parameters such as void content and binder volume, along with mechanical characteristics, such as rutting resistance, are used as the main criteria for quality control. But so far, visual analysis and micro-properties of asphalt mixtures (internal structure of the pavement) have not been considered much. In this article, micro-effectiveness properties of asphalt mixtures such as number of contacts, length of contacts and orientation of contacts for rutting performance of 16 types of mixture (two types of gradation, two types of binder, two types of aggregates and two types of filler) were considered. In this research, the results of rutting resistance (flow number) and micro parameters (as number of contact, length of contact and orientation of contact), determined by image processing (by using iPas2 software), and indirect tensile strength were compared. The analysis showed that rutting performance depends to a large extent on micro characteristics of asphalt mixtures and these properties could be used as a control tool in design of asphalt mixtures.

In this paper, effect of a stabilizing material known as "SSR400 solution", which is obtained from briny waters of Iran's central desert and is rich in calcium chloride and magnesium chloride, on improving the strength and swelling of fine-grained soil (containing silt and clay) in asphalt pavements is investigated. For this purpose, soil samples were taken from various secondary roads in Sistan region, Iran, and by selecting different amounts of SSR400 solution (12.5 to 22.5 percent), some test samples were prepared using standard methods, and they were tested for density, moisture content, California bearing ratio (CBR), unconfined compressive strength (UC), and Atterberg limits (liquid limit and plastic limit). Results showed that using SSR400 solution as a stabilizing material increases bearing capacity of silty and clayey soils, but is not effective in controlling the soil swelling. Moreover, the optimal amount of this solution for mixing with silty and clayey soils was 16.5 and 14.5 percent, respectively.

In the most of the construction and roading projects, the natural subsoil layers do not meet the mechanical requirements such as stiffness and shear strength and need soil improvement. There are several mechanical and chemical soil improvement methods that using reinforcement elements such as geotextiles, geogrids and natural or artificial fibers are among the mechanical methods. Because of the ease of application, low produce casts and short construction time, the use of randomly distributed discrete fibers are increasing in the recent decades. In this research, the shear strength of a silty sand reinforced by randomly distributed discrete glass fibers was studied in the laboratory. The study was focused on the effect of the length and content of the fibers on the shear strength parameters of the soil such as cohesion and internal friction angle. The shear strength of specimens of the improved soil prepared in 28 different configurations using fibers with 5, 10, 20 and 30 mm length and 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 % of weight contents were investigated using direct shear test device. the results of this study show that the inclusion of the fibers increases the shear strengths of the soil. The improvement of the shear strength of the soil resulted from improving of both cohesion and the internal friction angle. Increasing the length and the content of the fibers, increases the shear strength of the soil but further increment from optimum length and content of fibers reduces the shear strengths of the reinforced soil. The shear strength of the fiber reinforced soil is always more than that of unreinforced soil. The internal friction angle and cohesion of the fiber reinforced soil in the optimum condition increased about 30% and 40% respectively. The optimum length and content of the fibers to achieve the maximum shear strength were 10 mm and 0.4% respectively.