Effect of typical formworks and release agents on aesthetical surface quality of concrete

Abstract

This work involved investigating formwork surface characteristics and concrete surface quality. The results show that release agent types and formwork materials have a great impact on the wettability of a formwork–concrete interface, although all the formwork surfaces, with or without release agents, are hydrophilic. Remarkably, inhomogeneous roughness characteristics are only observed for a steel formwork. Plywood and steel formworks have almost the same roughnesses; however, the roughness profile of plywood is sharper than that of steel. Mineral oil always correlates with larger surface voids and higher surface void ratios (SVRs). Steel formworks significantly increase SVR, compared with plywood and plastics formworks. When SVR < 1‰ and the proportion of small voids (<500 μm) > 50%, the concrete surface appears to be bughole-free. Rusty stains left by a steel formwork aggravate concrete discoloration, especially when water-soluble release agents are applied. To improve concrete surface quality, plywood formwork is more favourable, although release agents and vegetable-oil-based emulsion are more suitable for plastics formworks. The surface area roughness of concretes is mainly determined by that of formworks. The typical microgroove feature on a formwork surface leaves no trace on the formed concrete surface; this might be owing to the difference in dimension between microgrooves and cement particles and the presence of release agents.