The influence of new and used formwork coated with different release agents on the appearance of the formed concrete surface

Abstract

The influence of silicon oil and liquid wax based release agents on the formation of surface voids and colour variation in cast-in-place concrete was studied. The performance of the said release agents was compared to conventional release agents based on water-soluble emulsion and mineral oil. In addition, new and used formwork panels made of plywood and steel were used in the research to determine the effect of release agents on contact angle values. The quality of surface finish was evaluated in terms of concrete surface void ratio using the standard image analysis methodology, and in terms of colour variation using a new digital colour reading method. The formed concrete surface category was evaluated according to CIB, ACI, DBV and ÖVBB standards in order to know the applicability limits of liquid wax and silicon oil based release agents. The results revealed that the amount of release agent used and the contact angle values on the facings analysed depend on the formwork surface roughness, i.e. the release agents can lodge inside the formwork surface asperities and trap air bubbles at the formwork/concrete interface. Furthermore, the results showed that plywood coated with silicon oil and liquid wax limited the achievement of the formed concrete surface category that is higher than the surface finish satisfying normal requirements. Steel coated with liquid wax did not produce the formed concrete surface of higher than normal quality either, while the surface voids ratio of concrete cast against steel formwork coated with silicon oil did not satisfy the requirements even of the lowest classification according to ACI, DBV and ÖVBB standards. Colour variation analysis revealed that the formwork with higher absorbing capacity produced a darker colour with visual blackness varying in the range from 45% to 60%, whereas the formwork with low absorbing capacity produced a lighter colour with visual blackness varying in the range from 25% to 35%. Rougher surfaces of the formwork absorb more hydrophilic release agents. This has an effect on concrete surface colour variation and on concrete’s hue, mainly on the yellow hue with redness or the green hue with yellowness.