Surface microstructure and scaling resistance of concrete

Abstract

Three different types of surfaces were tested to investigate the influence of the microstructure of the surface layers on the resistance of concrete to freezing in the presence of deicer salts: troweled surfaces prepared in two different ways, and sawed surfaces. In order to perform the investigation on various types of concrete, six different mixtures were prepared using two ordinary portland cements and one fly ash. The water to binder ratio was fixed at 0,40, and the replacement level of cement by fly ash was 20% and 40% (by mass). The scanning electron microscope observations carried out clearly indicate that the first millimeters below the surface of troweled laboratory concrete specimens can have a microstructure different than that of the bulk of the concrete. In all concretes tested, an extremely porous layer (i.e. with a very high water/binder ratio) was observed at the surface. The scaling test results show that the higher porosity of the surface layers tends to markedly reduce the deicer salt scaling durability of wood troweled laboratory samples during the first cycles of freezing and thawing. The use of fly ash was found to increase the thickness and the porosity of the surface layer.