Shrinkage (concrete)

Shrinkage refers to the shortening or reduction in volume of the concrete over time, due to the release of moisture (drying out) and chemical reactions or structural changes during hardening. Under unfavorable conditions, e.g. B. on ceilings in closed buildings, the linear final shrinkage is up to −0.6 mm / m.

Classification

One distinguishes

• the Early shrinkage by evaporation of the excess water and dehydration in the first few days,
• the chemical or autogenous shrinkage by the removal of water in the hydration and
• the drying shrinkage due to the drying out of the cement paste .

Shrinkage elongation

In the DIN 1045-1, there is a (linear) shrinkage strain of the latter two components:

• the shrinkage elongation during chemical shrinkage, which is important for very solid concretes (f _ck > 50 MPa) and can be up to about −0.3 mm / m. It depends on the type of cement and the strength of the concrete .
• the drying shrinkage expansion , which also depends on the component shape and the average relative humidity of the environment and is decisive for normal-strength concrete.

Both parts grow over time and asymptotically strive towards a limit value.

Causes and manifestations

In particular, insufficient attention to restraints that hinder the shrinkage (shortening) of the concrete is often the cause of shrinkage cracks in reinforced concrete construction. The deformation hindrance causes tensile stresses in the concrete and cracks after the concrete tensile strength is exceeded .

Since the aggregate generally does not shrink, the absolute size of the shrinkage and its development over time depend largely on the volume and composition of the cement paste . In addition, the humidity of the surrounding air and the component dimensions play a major role.

Construction engineering

Shrinkage cracks are undesirable in construction .

It is important to take shrinkage into account when calculating prestressed concrete parts ( prestressed concrete ) because the shrinkage elongation reduces the prestressing steel elongation and thus also the prestressing force.

The process of shrinkage is partially reversible; H. with water absorption , e.g. For example, by immersion in water, the concrete undergoes a volume increase . This is known as sources .