Leonids Pakrastinsh

Utilization of Composite Materials in Saddle-Shaped Cable Roofs

A saddle-shaped cable roof formed by two orthogonal groups of cables joined with a compliant support contour is considered. The kinematic invariability of the roof obtained by prestabilizing the cables makes it possible to avoid heavy slabs of reinforced concrete in the construction of roofs and to use light composite materials. The main geometric characteristics for a cable roof with CFRP load-carrying and contour cables and steel stabilizing cables are determined by a numerical experiment.

Crack Propagation in Concrete with Silica Particles

The propagation of cracking in concrete is a mechanism governing many physical and mechanical properties of the material. The aim of this study was to experimentally investigate the crack propagation of new concrete compositions using image analysis. Several concrete mixes containing microsilica and nanosilica were made. For each composition, Compact Tension (CT) specimens were prepared with dimensions 150x150x12mm. Specimens were subjected to a tensile load. The formation and propagation of the tensile cracks was traced on the surface of the specimens using a high resolution digital camera with 60 mm focal length. Images were captured during testing with a time interval of one second. The compression strength and modulus of elasticity were also determined for reference. The results obtained with this method have shown that it is possible to monitor relatively small displacements on the specimen surface regardless of the scale of the representative area of interest and to evaluate the influence of filler on the cracking properties of concrete.

Prestress Losses in the Stabilizing Cables of a Composite Saddle-Shaped Cable Roof

The paper deals with a square composite saddle-shaped cable roof 30 × 30 m in the plan, which is formed by two orthogonal cable groups joined with a compliant support contour. The kinematic invariability of the roof is achieved by prestressing the cable net. From the viewpoint of material consumption, the cable roof has rational geometrical characteristics. A hybrid composite cable on the basis of CFRP, GFRP, and steel is considered as a material for contour cables. The load-carrying and the stabilizing cables are made of steel. Prestress losses in the stabilizing cables are determined for three variants of prestressing. The possibility of reducing the consumption of cable materials by prestressing each cable of the net by an individual force is revealed.

Prestressed Cladding Elements for Tensioned Structures

Two types of prestressed cladding elements for the long span cable roofs were investigated. The first considered type of prestressed cladding element was pneumatic and formed by the generally woven by the basket weave fabric from the Vectra (LCP) yarns, which is covered by the PoliTetraFluoroEthylene (PTFE) copolymer foil, and compliant contour, consisting from the several glued together layers of the mentioned covered fabric. The second considered type of prestressed cladding element was formed by the steel cables steel pipes and the same type of covered fabric as the first type. Rational from the point of view of materials consumption heights and levels of prestressing for two considered types of cladding elements were evaluated.

Time-Dependent Behavior of High Performance Fiber-Reinforced Concrete

This research deals with experimentally studied time-dependent deformations of polyvinyl alcohol (PVA) fiber reinforced high performance concrete (HPFRC). The creep in compression and crack propagation of the new concrete compositions were determined. Several kind of concrete mixes adding microsilica and nanosilica have been made. For each composition cylinders with dimensions 190x47mm and Compact Tension (CT) specimens with dimensions 150x150x12mm were prepared. Concrete specimens were tested in a controlled constant temperature and with a constant level of moisture. Creep specimens were put into a creep lever test stand and subjected to a uniform, constant compressive load but CT specimens were subjected to a tension load. Deformations and crack mouth opening displacements were measured with extensometers. The compression strength and modulus of elasticity also were determined. The results have shown that nanosilica dont have significant influence on the high performance fiber reinforced concrete time-dependent behavior.

Crack Formation in Cement-Based Composites

The cracking properties in cement-based composites widely influences mechanical behavior of construction structures. The challenge of present investigation is to evaluate the crack propagation near the crack tip. During experiments the tension strength and crack mouth opening displacement of several types of concrete compositions was determined. For each composition the Compact Tension (CT) specimens were prepared with dimensions 150×150×12 mm. Specimens were subjected to a tensile load. Deformations and crack mouth opening displacement were measured with extensometers. Cracks initiation and propagation were analyzed using a digital image analysis technique. The formation and propagation of the tensile cracks was traced on the surface of the specimens using a high resolution digital camera with 60 mm focal length. Images were captured during testing with a time interval of one second. The obtained experimental curve shows the stages of crack development.

Friction of Rubber on the Concrete Surface Treated with a Remedy against Drying out

Concrete pavements and airfields in the curing process of protection against desiccation, particularly, the application of the wax-polymer emulsion which forms a solid film, which persists for a long time in the initial period of operation of roads and airfields. The presence of the film changes the nature of the friction on the surface that could adversely affect traffic and aircraft. This paper investigates the friction of rubber on the surface of the treated concrete with an aqueous emulsion of paraffin series Emcoril. It was established that the friction on the paraffin film, unlike friction on the surface of the concrete does not respect the law of linear Coulomb. The degree of deviation from linearity, the higher the greater the thickness of the film. The frictional force on the film increases with the slip velocity. At high speed, friction in the film is greater than the frictional force on the concrete, and at low speed is much lower than the previous one