Проблема термонапруженого стану в масивах бетону монолітних прогонових будов мостів

Abstract

Висвітлено проблему термонапруженого стану при будівництві монолітних попередньо напружених залізобетонних прогонових будов мостів. Пропонуються регулювання напружень, які виникають у процесі набуття міцності в масиві бетону. This paper describes the problem of thermo stress state at building of monolithic prestressed concrete spans of bridges. The methods of regulation stresses that occur in the recruitment strength in the array of concrete. Experience of monolithic span bridges abroad building shows that cracking problem of these structures constructing are relevant as they significantly affect on their endurance. The problem of cracking acquires a special importance in the heat of hydration in large arrays of reinforced concrete. In bridge construction, it applies not only to arrays of foundations and supports, but also to the monolithic spans that have begun to build in Ukraine more recently. The purpose of this paper is to describe and analyze the crack formation process during concrete monolithic spans bridges hardening and give proposals to warning crack formation in these structures. At the chemical level heat generation depends on the mineral composition of cement, its chemical composition, cement consumption in 1m②concrete, from chemical supplements introduced to the concrete mix, the initial temperature of the enclosed mixture, temperature at places. Technological causes of cracks appearance include: adjacency structure of the new concrete to old, jamming concrete new construction in the old concrete, limiting movement in the concrete mass (supporting elements rigid connection to the geometric difference in terms of size and array), the availability of additional external sources heat. Heating concrete occurs in the first 1–5 days. Interaction cement powder with water accompanied by heat, which at the conclusion of concrete large masses can cause significant heating in concrete, compared with the temperature of concrete mixture at the conclusion. When the internal temperature of concrete increases rapidly concluded by the heat of hydration, increasing its internal volume but small concrete surface deformation due to ambient temperature. In the subsequent cooling of the outer surface of the concrete arrays cools faster than the concrete inside, and reduced in volume there are so-called compressive and tensile thermal stresses, causing hair cracks that may eventually expand and become centers of progressive corrosion of concrete. Considering the features of hardening concrete and proper use them in some cases allows to substantially increase the value of the specified allowable temperature differences during construction cooling and purposefully to reach increase of fracture toughness of concrete structures. For determining the formation of its own thermostressed state in reinforced concrete structures necessary to conduct thermal calculations hardening concrete using automated software build and temperature fields zero stress. Since there are large distances greater tension, then the significant geometrical dimensions of the design heat cements massive structures is quite undesirable. That is thermostressed state in monolithic arrays affecting two interrelated factors: the geometric dimensions and the temperature difference in a concrete mixture at its conclusion. Adjusting these factors and accurate understanding of thermal processes in the array will allow to effectively control temperature and, ultimately, save time, effort, costs and avoid cracks in the concrete. To prevent cracking, the following chemical and technological solutions. Adjust the time and speed of heat dissipation of concrete. Adjusting the geometric characteristics of the design section and length of the junction adjacent sections. Creating a particular stress in the construction of monolithic prestressed spans. Forced cooling by internal and external pipelines. Adding structural reinforcement share of 15% span on both sides of the poles and finished butt sections; 0.15% reinforcement stretched most of the section, or 0.6% of foreignsection reinforcing strip to a depth of 400 mm. Laying concrete with one or two classes higher than accepted at the ends spans a distance of 3–5 meters.