Influence of concretes compressive strength on superficial wettability

ABSTRACT Concrete surfaces are one of the substrates on which the mortar coating, widely used in buildings, can be applied. Frequently, when there are incompatibilities between the substrate and the mortar, the coating system’s durability can be compromised, causing pathological manifestations such as detachments and displacements. To reach a better mortar coating adhesion and adherence, would be ideal that the substrate presents a hydrophilic surface, which allows a greater coating spreading. In this context, the present research aims to evaluate the superficial wettability of concretes with different compressive strength, in order to infer the performance from the point of view of adherence of a mortar coating layer on these substrates. Five concretes were dosed with different proportions and water/binder ratios, resulting in different values of compressive strength at 28 days. Concrete characterization in terms of compressive strength was done according to ABNT NBR 5739:2007, and water absorption by capillarity according to ABNT NBR 9779:2012. The concretes surface wetness test was evaluated through a sessile drop profile capture equipment. It was identified that the concretes compressive strenght has an influence on the wettability of its surfaces. Concretes with higher compressive strength have, beyond a lower absorption, a lower wettability. The mortar coating applied on a concrete substrate with higher compressive strength and lower capillary porosity can result in a lower mortar paste absorption by substrate pores and a lower contact extension, hampering the development of the mechanical interlocking and the adherence between the materials.