The influence of concrete shrinkage and temperature stress on structure and the measures taken


The shrinkage deformation of concrete will be produced during the setting and hardening process. The temperature change will cause the thermal expansion and contraction of the material. When these two kinds of deformation are constrained, the shrinkage stress and the temperature stress will be produced in the structure. If these stresses are not properly released, the "weak" part of the concrete structure will be damaged to varying degrees. Therefore, we must attach great importance to this in the process of construction.
1 shrinkage stress and temperature stress
1.1 the formation of shrinkage stress and temperature stress
Shrinkage stress: concrete is a composite material made of cement slurry bonded with sand and stone. During the hardening process, when the cement slurry loses the moisture retained by the hydrostatic tension in the pores, it will cause the shrinkage and deformation of the concrete. Shrinkage stress is produced when concrete is restrained during shrinkage.
Temperature stress: the change of temperature can cause the material to expand and contract, and the stress caused by temperature deformation is called thermal stress. When the temperature stress exceeds the tensile strength of concrete, the concrete structure will produce temperature cracks. The size of the temperature change is called the temperature difference. The temperature difference that causes temperature deformation is seasonal temperature difference, internal and external temperature difference, and sunshine temperature difference.
1.2 effect of shrinkage stress and temperature stress on structure
The influence of shrinkage stress and temperature stress on structure is mainly manifested in two directions, namely length direction and height direction.
Length: when the length of concrete structure is longer, the deformation caused by shrinkage and temperature changes of longitudinal continuous members such as floors will increase. If the deformation of these longitudinal members is constrained by the vertical member, the tensile stress or compressive stress will be produced in the longitudinal member, and the horizontal thrust and tension will also be correspondingly affected in the vertical member, and the cracks will appear in the member. In multi-storey and high-rise buildings, the hazards of shrinkage and temperature stress are more obvious at the top and bottom. On the top floor, as the roof layer of the building contact with the atmospheric environment directly, the roof temperature changes violently relative to the underlying layers, so it is considered that the top layer is relatively constrained by its lower floor, so the cracks at the end of the top layer and the roof panel are the most easy to appear. The middle floors are basically the same, with no constraint on each other, and the effect of temperature stress is also small. The foundation of the house is buried in the ground, and its shrinkage and temperature deformation will be constrained by the rigid foundation. Therefore, in the first floor of the house walls are prone to cracks.
Height direction: temperature difference will also affect the height of the building. With the increase of storeys in high-rise buildings, the temperature changes continue to accumulate, reaching the maximum value at the top level. For 20 storeys above, the impact may be obvious. If the stiffness of the beam is small, the displacement difference will be restrained very little, so the temperature stress is not large. However, this deformation will cause obvious deformation and cracks on the top of several stories. On the contrary, if the stiffness of the beam is larger, the displacement difference between the inner and outer columns will be greatly suppressed, the column under the temperature cold contraction is pulled, the column under the heat expansion is subjected to pressure, and the beam is subjected to bending and shearing stress. Under sunlight, the pillars of the sunny side extend. If the side column is exposed, the temperature difference between indoor and outdoor will cause the bending deformation of the column.
2 structural measures
In order to reduce and eliminate the effect of shrinkage stress and temperature stress on structure, some necessary structural measures are taken in structural design.
2.1 setting expansion joints: "code for design of concrete structures" stipulates that expansion joints should be set when the length of buildings exceeds the prescribed limits. The two sides of the expansion joint are respectively provided with walls or columns, and the structure is disconnected, and the structure is divided into an independent temperature section to meet the requirements of the free expansion length of the structure. The expansion joint is different from the settlement seam. It is required to break all the above ground components, such as the wall, floor, roof, etc. of the building, and the foundation is less affected by the temperature change, and does not need to be broken. The spacing of its setting (the allowable length of the building) is closely related to the material, structure type, construction mode and the geographical location and environment in which the structure is used. The width of the gap is generally 20 ~ 30mm. And the settlement seam is to avoid the uneven settlement of the foundation, the additional stress will be generated in the structure, so that some cracks in the weak parts of the building will be set up by the vertical dislocation. It requires that all the components from the base to the roof have to be split apart to make the buildings on both sides of the settlement joints become independent units, and the units are free to settle down in the vertical and unrestrained. The width of the settlement joint is related to the nature of the foundation and the height of the building. Expansion joints can be divided into wall expansion joints, floor expansion joints and roof expansion joints according to their location.
Wall expansion joints: according to the thickness of the wall, expansion joints can be made into flat joints, wrong joints and groove joints. In order to avoid the influence of external natural factors on the interior, the outer outer wall of the outer wall should be filled or covered with elastic materials with waterproof, thermal insulation and anticorrosion properties, such as bitumen yarn, foam plastic strip, rubber strip, and ointment. When the expansion joint and the settlement joint are set wider, the galvanized iron sheet, lead skin and other metal adjustment sheets are also used. If the wall is plastered to prevent plastering from falling off, the wire mesh can be added to the metal sheet before plastering. Joint and sealant materials and structures shall ensure free expansion of the structure in horizontal direction. The inner side of the outer wall and the inner wall are usually covered with wooden joints or metal sheets with decorative effect.

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