MODARES TECHNICAL AND ENGINEERING
Year:2004 | Volume:- | Issue:15|Papers Count:12

The unsaturated hydraulic permeability for expansive soils, as one of the most important properties of unsaturated soils, has been estimated by employing a new proposed indirect method which uses pressure plate test (PPT) apparatus [1]. This paper gives a brief review on direct and indirect methods of measuring unsaturated hydraulic permeability parameter on one hand, and explains different empirical models on the other hand. The theory behind the proposed method is extensively discussed, and a mathematical formula is derived according to unsaturated flow rules, and the physical model, which has been employed. Three different soil samples have been chosen as per the magnitude of their overloads and their unsaturated permeability have been estimated. Accordingly on the other hand, the unsaturated permeability has been estimated according to Van Genuchten [2] empirical model for non-expansive soil and saturated permeability - void ratio relationship [3]. The results obtained from the new method and those of selected empirical model have been compared. Comparison shows a close compatibility between both results and it has been concluded that the new method is more appropriate in predicting unsaturated hydraulic conductivity for swelling soils, specially those with very low water content.

The addition of steel fibers to concrete helps improving post -cracking tensile strength and hence leads to a significant increase in shear strength of reinforced concrete beams. Using steel fiber in structural elements such as pre-cast concrete beams or prestressed beams in road and railway bridges ductility, increase the energy absorption, gives better performance in fatigue, increase shear capacity and incorporates less stirrups spacing. In this paper, the shear performance of steel fiber in monoblock priestess’s concrete sleepers is investigated. From the analyses result, equations for determination of shear capacity of steel fiber used in concrete are determined. Monoblock priestess’s concrete sleepers are in fact priestess’s concrete beams with critical loading compared to conventional concrete beams (bridge beams in road and railway), where the beams have no stirrups even in area where the prestressing is applied. According to the results when steel fibers are adde d to the Prestressed concrete beams, of shear capacity, a decrease in stirrups amount, a better crack control, and some increase in stiffness and an increase ultimate strength of such beams can be gained. Hence, it can be said that in Prestressed concrete sleepers instead of increasing the prestressing parameter (which reduces the ultimate stiffness of the sleepers), addition of fibers is useful increase the beam stiffness after cracking and to increase the ultimate shear capacity of the beams. The results of experimental study also showed that the presented formulae for calculation of shear capacity of steel fiber in concrete have been are overestimated.

Determination of modulus of elasticity of various layers of pavements helps in determining the pavement condition and drawing up an effective rehabilitation strategy. In this research 18 Km long section of Tehran - Qom freeway was structurally evaluated by using a PHOENIX FWD. Part of the testing was carried out on the existing pavement and part of these was performed on a section that had been recently recycled using a cement and foam bitumen stabilization technique. Data were analysed by using two programs; namely KENLA YER and Back Calculation Analysis, which are two pre-defined methods that determine elastic modulus of the various layers. From the above analysis it was revealed that there is no strong correlation between the elastic modulus values obtained by the above two methods. This discrepancy was recognized to be mainly due to difference between two major parameters, namely, the reference temperature and the equivalent single axle loads, which are different in the two methods. Calculating the above differences and other minor discrepancies such as geophone positioning in FWD equipment, it was revealed that the overlay design and pavement life predictions achieved in these systems could not be reliable. Alternatively, a Back Calculation Compromise method was proposed. This resulted in more reliable values of the modulus of elasticity of the various layers.

One of the main parameters of road management system is the process of prioritizing repair and maintenance alternatives and allocating resources for their implementation. In this research, after a comprehensive overview of prioritizing and allocation approaches, the Analytical Hierarchy Process (AHP) was utilized to develop a series of road repair and maintenance models. By using the AHP, it was possible to develop some of the basic data to build the parameters of models. The paired comparison tables, which were completed by groups of experts, were used in the prioritization and resource allocation models. The main sub models developed for a road section are for pavement, infrastructure and safety devices. Moreover; the infrastructure model includes, bridges, tunnels and retaining walls. Using the developed models and prioritization of alternatives and resource allocation can be preformed conveniently. The models were evaluated by comparison of models" results with the manual prioritization of four provinces by a group of experts and managers and the results were proved to be satisfactory.

Absorption of heavy metal ions from aqueous solutions by biomass wastes is a proper and low-cost method for heavy metals removal from industrial waste water, since these materials are abundant in nature. The use of ash and carbon prepared from deal ability to absorb Cr (VI) ions from aqueous solutions are studied. The Cr(VI)-absorption experiments were carried out to analyze the influence of pH, initial Cr(VI) concentration and absorbent dosage, on the efficiency of Cr(VI) removal by the absorbents. Our results showed that the Cr(VI) - absorption mainly was dependent on both final pH of solution and initial Cr(VI) concentration. At optimum conditions, while initial Cr(VI) concentration and initial pH were 10 (mg/L) and 2 respectively, by using of 2.5 (g/L) deal carbon efficiency of Cr(VI)-absorption is 37%. Also it was known that adsorption of Cr(VI) ions well matched the Langmuir isotherm.

This paper outlines a simple technique for introducing displacements of discontinuities into finite element computations. This method considers displacement discontinuities existing in a continuum and could be easily added to available elastic codes. The modification induced by this technique can be contained in the load vector, so that the stiffness matrix is not altered. Besides, this method does not increase the number of degrees of freedom and, no net forces or moments are induced on the finite element mesh either.

Plastic shrinkage is one of the most important parameter, which must be considered in hot weather concreting. If plastic shrinkage is not prevented, cracking will be significant, especially if silica fume is used in the mix. In this paper, the effect of silica fume in bleeding and evaporation was investigated in laboratory. The results showed that in restrained shrinkage, beside relative humidity, temperature and wind velocity, the sun radiation is also a very important factor in cracking characteristic. But the effect of sun radiation is not considered in ACI 305 R-96 nomogram. The results showed that in terms of crack initiation time, crack width and total cracking area, concrete containing silica fume is more severe than concrete with no silica fume. Reduction of water-cement ration in concrete with silica fume makes the concrete more sensitive towards cracking. The results of this project also showed that the severity of the cracking is not related only to compressive strength or bleeding of the concrete but all environmental factors like sun radiation and also silica fume in mix, have important role.

This paper presents the laboratory results of the effect of inhomogeneity of concrete quality in initiation period of corrosion and rate of corrosion of reinforcement. Two types of reinforced concrete including columns and slabs were investigated. Each specimen was made of two different concrete qualities by changing water-cement ratio. Specimens were subject to a series of laboratory tests, including corrosion potential, rate of corrosion and resistivity. The results showed that inhomogeneity of concrete quality is an important factor in reducing of initiation time of corrosion and increasing rate of corrosion. Direction of reinforcement and direction of pouring concrete are important parameters, which have effects on potential differences in two parts of the reinforced concrete members. In other words, duration of initiation corrosion was less in columns compared to that of slabs, due to differences in direction of reinforcement.

Soil composition have detrimental effects on the resulting bulk density and hydraulic conductivity of clay soils because of the rearrangement of the solid particles and chemical stabilization. In this study aimed at the simulation of natural soil, kaolinite and kaolinite with calcium carbonate (KC), silica gel (KS), and both calcium carbonate and silica gel (KSC) were compacted at different moisture contents according to ASTM standards. These mixtures of compacted clays were then exposed to leaching by distilled water, followed by a solution of heavy metals. Experimental results indicate that the dry density and coefficient of hydraulic conductivity significantly influenced by soil constituents. Specifically kaolinite exhibits a low dry density and a high coefficient of permeability, compared to other soils (KS, KC and KSC). As permeate flow approaches a stationary regime, greater pore volumes of effluents result, and an increase in the coefficient of permeability are observed in all types of soils. Tests confirm that chemical reactions are responsible for the permeability increases up to a constant value. Expressly, kaolinite mixtures with silica gel or calcium carbonate exhibit a coefficient of permeability almost one orders less than that of kaolinite. Results indicate the importance of silica gel as an additive for the reduction of the permeability in clay soils. Soil waste physicochemical chemical interaction was simulated through a coupled geo-chemical and transport model.