Mechanical & Transportation Engineering Abstracts

Every year, colossal amounts of used and non-biodegradable rubber tyres are accumulated in the world. Experience shows that the most efficient way to increase the concrete fracture energy GF (N/m) is to use metal or polypropylene fibres. The optimal content of fibre increases concrete resistance to stress (especially tensile stress under bending force). Concrete fracture is not brittle; concrete continues deforming after maximum stresses and is able to resist certain stresses, there is no abrupt decrease in loading. The research has proved that crumb rubber can be used in concretes as an alternative to metal and polypropylene fibres. The investigation has found that rubber waste additives, through their specific properties can partly take up tensile stresses in concrete and make the concrete fracture more plastic; besides, such concrete requires a significantly higher fracture energy and concrete samples can withstand much higher residual strength at 500 mm crack mouth opening displacement (CMOD) and deflection.

The paper considers the velocities of air movement in the ventilated air gaps of walls and focuses on pressure fields in both wall models arranged in a climatic chamber and exploited houses. The article investigates the influence of air movement on heat transfer through walls applying numerical modelling methods and conducting experiments in the climatic chamber. The thermal effects of air flows have been described with reference to the Nusselt number defined as the ratio of average convective and conductive heat fluxes and heat flux through still air.

Experimental tests have determined the effect of slag, opoka (silica-calcite sedimentary rock), silica fume (SiO2) suspension, dolomite dust and sodium silicate solution (NaSS) together with the polycarboxylatether based plasticizing admixture on the yield stress and viscosity of Portland cement paste the rheological properties of which have been defined applying a rotational viscometer with co-axial cylinders. The tests have revealed that slag, opoka, silica fume suspension and dolomite dust added to cement paste by replacing 10% of Portland cement (by weight) have an effect on the yield stress and viscosity of the paste subject to the form and fineness of additive particles. When 10wt% of Portland cement is replaced with slag cement, the yield stress of Portland cement paste reduces by about 25.9%, and viscosity increases by about 3.5 times compared with the yield stress and viscosity of reference cement paste. The yield stress of Portland cement paste with 0.5% NaSS admixture increases insignificantly, and viscosity grows approximately twice compared with reference cement paste. The tests have also showed that the modifying admixtures of Portland cement paste enable to control the rheological properties of cement paste.

Numerous alternatives exist for foundation systems and construction technologies. The systems can be described by different criteria values which are incorporated in the conventional design process. Decision on the most suitable construction technology is vital for success and depends on many effectiveness criteria. The business success depends on the right choice. The mandate of a construction management researcher is to use rational, systematic, science-based techniques to inform and improve various decisions. The paper presents multiple criteria decision making model for selection of a pile-column technology. The technological criteria are determined by an experimental study. Based on in-situ investigation of natural soil conditions, criteria values are determined. The decision making model incorporates five different methods and techniques. To solve a problem, it uses three multiple criteria decision making methods. Integrated criteria weights are determined by using the analytic hierarchy process and the expert judgement method. This model could be used to solve complicated problems pertaining to the selection of a construction technology.

The peculiarities of constructing a model of an individual vehicle moving along the urban street are investigated. The aim of the study is to construct a dynamic vehicle-environment-driver model, which enables to implement the situation' an individual vehicle moving along urban streets', taking into consideration a concrete type of a vehicle, its loading, driver's behavior, peculiarities of external characteristics of internal combustion engines run on traditional or alternative fuel as well as the use og hybrid power plants.

The paper addresses one engineering design problem, the optimization of pile placement schemes under grillage-type foundations. Aim of the optimization is to find pile placement scheme where the number of piles is minimal and reactive forces at all piles do not exceed the given allowable reaction. This global optimization problem is solved here using the simulated annealing algorithm which proved to be one of the most promising algorithms for grillage optimization. Optimization of real-life grillages using PC requires usually unacceptably long for engineering practice computation time, therefore present paper investigates the possibilities of employing grid computing for that. Numerical examples presented clearly show potential of grid computing in engineering design problems.

This study presents the research related with abra-sive erosion wear of electric-arc welded hard layers. It is estimated, that when the abrasive particles affect the hard layers surface at oblique angle the wear by microcutting prevails, when they strike perpendicularly – the wear is caused by microfatigue processes. Under abrasive erosion conditions the wear of electric arc welded hard layers is lower if compared to Hardox 400 up to 31% at the abrasive particles impact angle of 30°, and only up to 4% at the perpendicular particles impact. Under the abrasive particles stream impacting the surface at oblique impact angle the most resistant to wear are the layers alloyed with 1.6-1.9% carbon and 4-8% chromium while under the normal impact the most resistant to wear are the low carbon layers alloyed with 0.15% C and high chromium layers alloyed with 15% Cr.

The model of the open queueing network in heavy traffic has been developed. These models are mathematical models of computer networks in heavy traffic. The probability limit theorem has been presented for the extreme virtual waiting time of a customer in heavy traffic in open queueing networks. Finally, we present an application of the theorem - a reliability model from computer network practice.

Widely developed DH systems in Lithuanian towns and small settlements undergo modernisations aimed to increase their efficiency and heat supply quality. Typically, modernisations include the replacement of building heat substations, the reduction of heat losses in pipes and more efficient heat production, including the use of indigenous fuel resources. It was notified that new heat substations equipped with automatic heating control devices have backward impact on the technological parameters of DH networks. Modelling tools for simulation of hydraulic and thermal parameters in the network allow estimating of potential deviation of heat carrier temperature with loading variations not only during the season of the year, but also over day and night. Special technical measures can be taken to avoid unduly low heat carrier supply temperatures and violation of hot water supply quality requirements. Cost estimation of such measures versus the option of full network renovation is presented in the article.

In this work the influence of eight technological factors (clay, non-plastic materials, buming out additive, waste materials, mixing effectiveness in a mixer, degree of compression m fonnation heads, level of vacuuming in a vacuum chamber, duration ofkeeping at the highest buming temperature) on the detennined specific characteristics of structural parameters ofceramic samples: reserve ofpore volume, relative wall tliickness ofthe pores and capillaries, capillary rate of mass flow under vacuum respectively in the direction of freezing and perpendicular to this, degree of structural in homogeneity, capillary rate of mass flow in the direction of freezing, is analysed applying fractional factorial design at two levels. On before-mentioned by structural parameters it is possible to estimate frost resistance ofclay masonry units. Regression equations are derived enabling to predict the values of specific characteristics of structural parameters properties ofclay masonry units or knowing the desirable values ofthe properties to detennuie the optimal effect ofthe technological factors under investigation and thus to produce the clay masonry units of different purpose vvith the desirable properties vvithin certain limits.