Fédération Internationale du Béton Proceedings of the 2nd International Congress June 5-8, 2006 – Naples, Italy
The role of demoulding oils is to prevent the concrete adhering to the steel walls . Because of its extremely fine structure and high lubricity at the very beginning of casting, the cement paste contained in the concrete spreads over the formwork surface and builds up in the asperities. The adhesion of the oil is therefore an important parameter. Another feature of these oils is that they improve the appearance of the facings and the resistance to corrosion of the surfaces of the steel mould and formwork.
This article is by:
Libessart, L., Djelal, C.
Laboratoire d’Artois Mécanique Thermique et Instrumentation, Unviversité d’Artois, TechnoParc Futura – 62400 Béthune, France
De Caro, P.
Laboratoire de Chmie Agro-industriel, UMR1010, INA/INP, ENSIACET, 118 route de Narbonne – 31077 Toulouse Cedex
CHRYSO S.A., ZI - 7 rue de l’Europe – 45300 Sermaises
Currently the most popular demoulding oils are of mineral origin. However, these products are not very environmentally friendly and cause inconvenience to the users (skin and respiratory tract irritations, inhalation of volatile organic components). Prolonged exposure can inflict irreversible damage recognized as occupational diseases. To resolve these problems, vegetable-based formulations have been developed. A European study  was conducted in 1999 to make these agents better known in the building world (SUMOREVA project).
A lack of knowledge of the mechanisms acting at the concrete/wall interface, when demoulding agents are present, means that the formulation of these lubricants remains totally empirical. Manufacturers can but fall back on their experience and site tests which are restrictive and expensive, in order to establish a formulation which best meets users’ requirements.
The aim of this study is to link the physical-chemical properties of mineral or vegetable oil compositions to their mechanical properties when the concrete is laid. Keywords: concrete, release agents, interface, formwork
ESTABLISHING THE CHARACTERISTICS OF THE OILS
The study was conducted on one formulation of mineral origin and one formulation of vegetable origin. The properties of the oils are given in Tab. 1:
The mineral composition comprises a mineral base of paraffin origin. The vegetable composition is formulated from a complex plant ester and an acidifier.
The acid index corresponds to the number of milligrams of potassium (KOH) required to neutralise a gram of oil. In our case, the acid index enables the content to be quantified in COOH carboxylic functions. These functions correspond to free acids, i.e. non saponified. The complex plant esters are prepared by sterification from a fatty acid of plant origin.
Tab. 1. Properties of the oils
The fatty acids (Fig.1) in contact with an alkaline hydroxide (in this case, lime) are transformed into alkaline carboxylates (soaps). They also catalyse the saponification reaction which generates soaps from an ester and an alkaline hydroxide. In this case, the soap obtained is a calcium salt 2(RCOO)Ca2+. A soap is formed from a long carbonated chain (R) and a carboxylate group (–COO-) called polar head.
CHARACTERISTICS OF THE OIL FILM
The demoulding agent must be chosen in relation to the nature of the formwork and the period of use , but it is mainly the conditions of application which are the determining factor. Each time a demoulding agent is used, the formwork surfaces must first be thoroughly cleaned. In addition, the demoulding agent must be applied in a fine and continuous film. It is imperative for the demoulding product to cling to the formwork because if it rides up with the concrete during filling, it may cause flaws and air bubbles. A study conducted by a North American demoulding manufacturer shows that the thicker the oil, the more the surface quality of the concrete deteriorates. According to this study, the best results are obtained with a thickness of 10 μm .
The optimum quantities for mineral-based oils vary from 0.12 to 0.15 litre per m2  and are about 5 to 10 times lower for vegetable-based oils . This is explained by the fact that in the case of vegetable-based oils, the thinner the film, the better it is organized, and consequently its effectiveness is improved. Excessive oil disorganizes the film and impairs its effectiveness.
The most suitable application method is spraying, in some cases together with spreading by rubber scraper to remove the excess oil from the single layer.
The quality of the oil film was evaluated on the basis of wettability and under an optical microscope.
The mineral oil studied has low viscosity and a good rheological resistance to temperature variations. Moreover, it also has high wettability. Despite these favorable characteristics, more oil has to be sprayed to obtain an even film on the formwork surface. Vegetable oil has a higher viscosity and lower wettability than mineral oil. In addition, at temperatures of below 20°C, its viscosity increases considerably, and this change is detrimental to the application of the oil under winter conditions. On the other hand, its film offers a homogeneous appearance for a small volume of sprayed oil. The friction of the concrete on the formwork is also reduced for vegetable oil because of an organized soap/oil structure at the interface. It is a combination between a chemical effect (saponification reaction) and a physical effect (thickness of the film) which leads to a significant reduction in the friction stresses.
Release agents are significant source of pollution in the construction and prefabrication industries. Conventional product are known to be toxic for humans and the environment. New formulations based on vegetable esters have appeared on the market to mitigate against this drawback, but more knowledge must be required about the properties and behaviour of these products, and their industry profile is raised. Understanding the phenomena that occur over formwork-oil-concrete interface is a real challenge. This paper begins with a physicochemical survey of release agents by defining the formwork-oil and oil-concrete interfaces. In then characterizes the formwork-oil-concrete interfaces by dynamic studies (tribometer). Physicochemical mechanisms are proposed.
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