At the start of the plastering work, the moisture release of the concrete in the surface zone must be complete and the rendering base capable of absorbing. This condition, under favourable conditions (e.g. continuous summer weather), can be achieved after four weeks and, with unfavourable weather conditions (e.g. to high humidity), eight weeks at the earliest (at least 60 frost-free days) following removal of shuttering.
In order that a secure bonding of the plaster is
ensured the mass fraction of the moisture in the concrete may not exceed 3 %.
If these constraints are not met, the bonding of the plaster can be prejudiced
- the concrete pores contain moisture which prevents the clawing of the plaster
- the too fresh concrete is still shrinking (drying shrinkage) and through this stresses occur between concrete and plaster
- in the process of further drying of the concrete, salts and alkaline pore fluid come to the surface and the adhesive bonding of the plaster is disrupted
- the gypsum joint is damaged through recrystallisation.