What is elastic deformation of concrete

Baunetz_Knowledge

Fresh and hardened concrete are not volume-stable building materials, but change their shape when exposed to loads and external influences. This happens in the event of temperature changes, dehydration, hydration and static stress due to short or long exposure to loads. As a result, there are different changes in shape that create more or less pronounced cracks in the concrete.

Shrink
The shrinkage is a consequence of the volume reduction associated with hydration. The volume of the hydration products in the cement paste is greater than the volume of cement and water in the cement paste. There is an uneven volume distribution. The shrinkage is statically irrelevant.

Dwindling
As the fresh concrete dries out to form hardened concrete, the volume of the unloaded concrete is reduced. The degree of shrinkage depends on the drying conditions, the component dimensions, the water-cement ratio and the cement stone volume. With slow drying, normal concrete shrinkage rates of 0.2 mm / m to 0.5 mm / m apply.

swell
The opposite of shrinkage is swelling, an increase in the volume of the material; this takes place e.g. through water absorption.

Float
Driving phenomena are due to chemical-mineralogical reactions that take place in hardened concrete with an increase in volume, e.g. sulphate drifting.

Crawl
The permanent and / or time-dependent changes in shape of hardened concrete under permanent load are referred to as creep. The creep is attributed to the movement and redistribution of water in the cement stone. It is a property of concrete that manifests itself through a structural transformation and reduction in volume, especially when subjected to pressure. The increase in creep deformations becomes less and less over time and only comes to a standstill after several years.
Creep involves two types of deformation:

  • The reversible deformation component - after a possible relief with a time delay, the component forms back into its original state (creep back). The age of the concrete is little influenced by this and reaches its final value after a short time.
  • The irreversible deformation component - remains after the load is removed (flowing), depends on the age of the concrete and only reaches its value after a long time. Under unfavorable boundary conditions, the final creep index can reach a value of around 3.0, i.e. the concrete deformations due to creep are three times as large as from elastic deformation.
The factors influencing creep include the cement stone volume, the water-cement ratio, the air humidity, the cross-sectional geometry of the component, the hardening speed of the cement and the concrete compressive strength.

The creep coefficients are determined in the laboratory using the creep test. The information in the DIN 1045-1 structures made of concrete, reinforced concrete and prestressed concrete apply to linear creep under compressive stress, i.e. the creep coefficients are independent of the load level. This applies up to a tension of approximately 45% of the cylinder strength of the concrete. At higher concrete compressive stresses, non-linear creep occurs as a result of increased microcracking of the concrete. The creep deformations increase disproportionately with increasing load.

When calculating pre-stressed concrete parts (prestressed concrete), the creep of the concrete is an important parameter that must be taken into account, as the pre-stressing always results in large concrete compressive stresses. The resulting creep strains of the prestressed concrete component reduce the prestressing steel strain and thus also the prestressing force. However, the creep of the concrete can also be decisive when verifying the load-bearing capacity of slender reinforced concrete columns or when verifying the deformation of slender ceilings

Elastic deformation
The modulus of elasticity (modulus of elasticity) indicates the ratio of tension to the associated elastic deformation, which is completely reduced after relief.

Planning aid
The topic Change in shape of concrete is also a section in the Cement paperback Dedicated to the German cement industry. The causes and types of deformation are described in detail on 26 pages. The document is available for free download as a PDF file from the website named under Surf Tips.

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