A new laboratory method to investigate skin formation

R. Zurbriggen1, M. Herwegh2, U. Pieles3, T. Bühler3 and L. Huwiler1 
1 Nouryon Chemicals AG, Sempach Station
2 Earth Sciences Institute | Bern
3 University of Applied Sciences Northwestern, Switzerland | all Switzerland
As soon as a mortar is applied, it is exposed to its substrate and the environment.
Porous substrate materials as well as hot and windy climate conditions cause the formation of a dry skin at the mortar surface, particularly because of loss of water due to sucking or evaporation. In case of adhesive mortars the skin formation can negatively affect wetting and resulting adhesion properties. In the application test EN1346 the relation between duration of exposure and adhesion strength is measured.
However, such data are difficult to interpret because strength and failure mode are not well correlating with each other.
The paper presents a simple laboratory method (modified EN1347) to observe the skin and its fracturing pattern at the contact between mortar and a glass plate. The principle of the set up is an indirect illumination of the glass plate from the side. Locations where a skin formed appear dark, whereas locations where fresh mortar wetted the glass appearbright. This method was systematically applied to an experimental series where different mortars, substrate materials and application methods were included. In each experiment glass plates were applied after 10, 20 and 30 minutes, which allowed to study the dynamics of skin formation over the first 30 minutes of exposure. The skinning pattern were analysed and quantified by image analysis using Image SXM and the Auto Correlation Function. This kind of data can now be correlated with adhesion strength values and allow to better understand the relation between skin formation and adhesion properties.