dc.description.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. |
en_US |