In a majority of the specimens, fracturing occurred in the interface between gypsum plaster and primer (adhesion fracture). In the case of some primers with high adhesive strength (primers 2 and 7), the fracture occurred partly in the interface between primer and mortar, and partly in the mortar itself (adhesion-cohesion fracture).
Plaster adhesion of non-stored concrete units was very high with some primers (2, 7 and 11). A lack of adhesion during the application of gypsum plaster without primer, as also was the case with primer 9, could be determined. Primers 1 and 8 showed low adhesive values.
It can be assumed that five-week storage leads to superficial drying and partial carbonation of the surface. Due to these reasons, adhesion characteristics of the plasters changed. All specimens that originally showed low or average adhesive qualities increased their adhesive strength significantly. Only the reference test (specimen without primer) showed absolutely no adhesive characteristics even in this case.
The test results could prove the differences in quality of the 10 primers used. We there fore recommend that users get appropriate information from manufacturers as a form of reassurance. Sufficient adhesive strength (0.2 N/mm2) should be achieved by primers in the concrete/primer/gypsum plaster interface. In order to determine the efficiency of a primer it is not only necessary to determine adhesive strength - fracture patterns are equally important.